WO2015131388A1

WO2015131388A1 - Simplification of depth intra mode coding in 3d video coding

Info

Publication number: WO2015131388A1
Application number: PCT/CN2014/073041
Authority: WO
Inventors: Hongbin Liu; Ying Chen
Original assignee: Qualcomm Incorporated
Priority date: 2014-03-07
Filing date: 2014-03-07
Publication date: 2015-09-11

Abstract

This disclosure is related to depth Intra mode coding in a 3D video coding process, such as 3D-HEVC. The disclosure describes techniques for simplifying the derivation of an initialization value for determining an initialized probability state for CABAC coding a depth intra mode syntax element. In some examples, the techniques may reduce the number of context models and/or buffer size used for storing context information by an encoder or decoder for derivation of the initial probability state.

Description

SIMPLIFICATION OF DEPTH INTRA MODE CODING IN 3D VIDEO CODING

TECHNICAL FIELD

[0001] This disclosure relates to video coding, and more particularly, to depth Intra mode coding in a three-dimensional (3D) video coding process.

BACKGROUND

[0002] Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, tablet computers, smartphones, personal digital assistants (PDAs), laptop or desktop computers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, video teleconferencing devices, set-top devices, and the like. Digital video devices implement video compression techniques, such as those described in standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), High Efficiency Video Coding (HEVC), and extensions of such standards.

[0003] An encoder-decoder (codec) applies video compression techniques to perform spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy in video sequences. For block-based video coding, a video slice may be partitioned into video blocks, which may also be referred to as coded treeblocks (CTBs), coding units (CUs) and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) slice of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures.

[0004] Spatial or temporal prediction results in a predictive block for a block to be coded. Residual data represents pixel differences between the original block to be coded and the predictive block. An inter-coded block is encoded according to a motion vector that points to a block of reference samples forming the predictive block, and the residual data indicating the difference between the coded block and the predictive block. An intra- coded block is encoded according to an intra-coding mode and the residual data. For further compression, the residual data may be transformed from the spatial domain to a transform domain, resulting in residual transform coefficients, which may be quantized.

[0005] A multi-view coding bitstream may be generated by encoding views, e.g., from multiple perspectives. Multi-view coding may allow a decoder to select different views, or possibly render multiple views. In addition, some three-dimensional (3D) video techniques and standards that have been developed, or are under development, make use of multiview coding aspects. For example, in some 3D video coding processes, different views may be used to transmit left and right eye views to support 3D video. Other 3D video coding processes may use multiview-plus-depth coding. In a multiview-plus-depth coding process, such as a process defined by the 3D-HEVC extension to HEVC, a 3D video bitstream may contain multiple views. Each view may comprise both a texture view components and a depth view component. The texture view and depth view components may be used to construct 3D video data.

SUMMARY

[0006] In general, this disclosure describes techniques for simplifying depth Intra mode coding in a 3D video coding process, such as 3D-HEVC. The techniques may be used to simplify the context-adaptive binary arithmetic coding (CAB AC) coding of a syntax element that indicates one or more characteristics of a depth Intra prediction mode in a 3D video coding process, such as 3D-HEVC. For example, the disclosure describes techniques for simplifying the derivation of an initialization value used to derive an initialized probability state for CAB AC coding the syntax element. In some examples, the techniques may reduce context models and/or buffer size used by an encoder or decoder for determination of the initialization value used to derive the initialized probability state.

[0007] In one example, the disclosure describes a method of video decoding, the method comprising receiving an encoded video data bitstream, decoding a syntax element from the encoded video data bitstream using a context-based binary arithmetic coding

(CABAC) process, wherein a value of the syntax element indicates one or more characteristics of a depth intra prediction mode for a depth prediction unit, determining an initialization value for determination of an initialized probability state for decoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and without considering a value of the syntax element for an above neighboring depth block of the depth prediction unit, and decoding the depth prediction unit using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth prediction unit.

[0008] In another example, the disclosure describes A method of video encoding, the method comprising encoding a depth prediction unit using a selected characteristic of a depth intra prediction mode, encoding a syntax element using a context-based binary arithmetic coding (CABAC) process, wherein a value of the syntax element indicates the selected characteristic of the depth intra prediction mode for the depth prediction unit, and determining an initialization value for determination of an initialized probability state for encoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and without considering a value of the syntax element for an above neighboring depth block of the depth prediction unit.

[0009] In another example, the disclosure describes A method of video decoding, the method comprising receiving an encoded video data bitstream, decoding a syntax element from the encoded video data bitstream using a context-based binary arithmetic coding (CABAC) process, wherein a value of the syntax element indicates one or more characteristics of a depth intra prediction mode for a depth prediction unit, determining an initialization value for determination of an initialized probability state for decoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same coding tree unit (CTU), determining the initialization value for determination of the initialized probability state for decoding the syntax element using the CABAC process based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU, and decoding the depth prediction unit using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth prediction unit. [0010] In another example, the disclosure describes a method of video encoding, the method comprising encoding a depth prediction unit using a selected characteristic of a depth intra prediction mode, encoding a syntax element from the encoded video data bitstream using a context-based binary arithmetic coding (CABAC) process, wherein a value of the syntax element indicates the selected characteristic of a depth intra prediction mode for a depth prediction unit, determining an initialization value for determination of an initialized probability state for encoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same coding tree unit (CTU), and determining the initialization value for determination of the initialized probability state for encoding the syntax element using the CABAC process based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU.

[0011] In other examples, the disclosure describes a video coding apparatus including one or more processors configured to perform the methods described above. The disclosure also describes a computer-readable medium comprising instructions that, upon execution, cause one or more processors to perform any of the methods described above.

[0012] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a diagram illustrating intra prediction modes used in high efficiency video coding (HEVC).

[0014] FIG. 2 is a block diagram illustrating an example video coding system that may utilize the techniques of this disclosure. [0015] FIG. 3 is a diagram illustrating an example of a wedgelet partition pattern for use in coding an 8x8 block of pixel samples.

[0016] FIG. 4 is a diagram illustrating an example of a contour partition pattern for use in coding an 8x8 block of pixel samples.

[0017] FIG. 5 is a block diagram illustrating an example video encoder that may perform the techniques of this disclosure.

[0018] FIG. 6 is a block diagram illustrating an example video decoder that may perform the techniques of this disclosure.

[0019] FIG. 7A is a diagram illustrating an example of a depth prediction unit (PU) and left and above neighboring depth blocks within a current coding tree unit (CTU) to be coded.

[0020] FIG. 7B is a diagram illustrating an example of a depth prediction unit (PU) and a left neighboring depth block within a current coding tree unit (CTU) to be coded and adjacent to an above neighboring depth block in a different CTU.

[0021] FIG. 8 is a flow diagram illustrating a video decoding method in accordance with an example of the disclosure.

[0022] FIG. 9 is a flow diagram illustrating a video encoding method in accordance with an example of the disclosure.

[0023] FIG. 10 is a flow diagram illustrating a video decoding method in accordance with another example of the disclosure.

[0024] FIG. 11 is a flow diagram illustrating a video encoding method in accordance with another example of the disclosure.

DETAILED DESCRIPTION

[0025] In general, this disclosure is related to depth Intra mode coding in a 3D video coding (e.g., where coding may refer to encoding or decoding) process, such as 3D- HEVC. The disclosure describes techniques for simplifying the derivation of an initialization value used to determine an initialized probability state for a context model for CAB AC coding a depth intra mode syntax element. In some examples, the techniques may reduce the number of context models and/or buffer size used for storing context information by an encoder or decoder for derivation of the initialization value.

[0026] The syntax element that is coded using the CAB AC process may indicate one or more characteristics of a depth intra prediction mode used for a current depth prediction unit (PU). The initialization value used to derive an initialized probability state for CAB AC coding the syntax element may be selected based on a context index of the syntax element. The techniques described in this disclosure may simplify the context index derivation for the syntax element.

[0027] The value of the syntax element may indicate a prediction characteristic of the depth intra prediction mode. For example, the prediction characteristic may comprise whether a depth modeling mode (DMM) depth intra prediction mode or a non-DMM depth intra prediction mode, such as a regular HEVC intra mode, is to be used for the depth prediction unit. In this case, in 3D-HEVC, the hevc_intra_flag syntax element is an example of this type of syntax element that indicates a prediction characteristic (e.g., DMM or non-DMM) of the depth intra prediction mode.

[0028] Alternatively, the value of the syntax element may indicate a residual

characteristic of the depth intra prediction mode. For example, the residual characteristic may comprise whether a segment-wise DC coding (SDC) depth intra prediction mode or a non-SDC depth intra prediction mode, such as a regular residual coding mode, is to be used for the depth intra prediction unit. In this case, in 3D-HEVC, the sdc_flag syntax element is an example of this type of syntax element that indicates a residual

characteristic (e.g., SDC or non-SDC) of the depth intra prediction mode.

[0029] The techniques described in this disclosure may be applied to determine an initialization value used to derive an initialized probability state for a syntax element, such as hevc_intra_flag, that indicates selection of a DMM depth intra prediction mode or a non-DMM depth intra prediction mode, or a syntax element, such as sdc_flag, that indicates selection of an SDC or non-SDC depth intra prediction mode. In some examples, the techniques may be applied to determine an initialization value used to derive initialized probability states for both an hevc_intra_flag and an sdc_flag, or similar types of syntax elements.

[0030] In 3D-HEVC, the hevc_intra_flag and sdc_flag syntax elements may be used together to indicate both the selection of the DMM or non-DMM prediction characteristic for a depth intra prediction mode and the selection of the SDC or non-SDC residual characteristic for the depth intra prediction mode. In this manner, the hevc_intra_flag indicates predictive characteristic of the depth intra prediction mode relating to how predictive samples are generated for the current depth PU, and the sdc_flag indicates a residual characteristic of the depth intra prediction mode relating to how residual values are generated for the current depth PU.

[0031] In the example of 3D-HEVC and HEVC, the initialization value may be an init Value variable, and the context index may be a ctxldx value. In some examples, the context index value may be used to determine the value of an initialization value, such as the init Value variable, for one or more slice types. The init Value variable may be used as an initialization value to determine the initialized probability states used in a CAB AC process to code the hevc_intra_flag syntax element or sdc_flag syntax element, as applicable, e.g., in the manner defined in HEVC WD 10 for determination of initialized probability states for syntax elements generally. In some examples, the initialized probability state may be defined by two parts: a most probable symbol (MPS) stored in valMps, and a probability of a least probable symbol (LPS) stored in a probability state index pStateldx, e.g., as defined in HEVC WD 10.

[0032] In general, it may be desirable to enhance bit rate performance of CAB AC coding by selecting a context index, for derivation of an initialization value used to determine an initialized probability state, based on correlation between values of a syntax element among neighboring blocks. In some cases, however, exploiting correlation may require buffering of syntax element values for neighboring blocks, particularly when such blocks are not in the same coding tree unit (CTU) as the current depth prediction unit for which the syntax element is to be coded. The CTU may correspond to a largest coding unit, e.g., for a picture or slice.

[0033] In a first example, a video encoder and/or video decoder may determine an initialization value for determination of an initialized probability state for coding (i.e., encoding or decoding) a syntax element using the CAB AC process for the syntax element for a current depth PU based on a value of the syntax element for a left neighboring depth block and without considering a value of the syntax element for an above neighboring depth block of the depth prediction unit. This first example may be applied to determine the initialization value used to derive the initialized probability state for CABAC coding a syntax element that indicates a prediction characteristic (e.g., hevc_intra_flag) of the depth intra prediction mode for the current depth PU and/or a syntax element that indicates a residual characteristic (e.g., sdc_flag) of the depth intra prediction mode for the current depth PU. [0034] In this first example, by not using the above neighboring depth block in the derivation, the number of context index values used to derive the initialization value can be reduced. In addition, the need to buffer the value of the syntax element for the above neighboring depth block can be avoided, particularly where the above neighboring depth block is in a different CTU than the current depth prediction unit (PU).

[0035] The video encoder and/or video decoder, in this first example, may determine the initialization value by determining a context index based on the value of the syntax element for the left neighboring depth block of the depth prediction unit, and determining the initialization value based on the determined context index. The initialized probability state for the context model may be determined by the encoder or decoder based on the initialization value, e.g., in a manner similar to that described in HEVC WD 10. For example, an encoder or decoder may use the initialization value (init Value) to determine an initialized probability value defined by valMps and pStateldx.

[0036] In a second example, as an alternative to the first example, a video encoder and/or video decoder may determine an initialization value for determination of an initialized probability state for decoding the syntax element using the CAB AC process for the hevc_intra_flag syntax element for a current depth prediction unit based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same CTU, and based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU. In this case, a video encoder and/or video decoder, e.g., via an entropy encoding and/or entropy decoding unit, may determine whether an above neighboring depth block is in a different CTU than the current depth PU, or in the same CTU as the current depth PU, and derive the

initialization value as described above based on this determination.

[0037] This second example may be applied to derive the initialized probability state for CABAC coding a syntax element that indicates a prediction characteristic (e.g., hevc_intra_flag) of the depth intra prediction mode for the current depth PU and/or a syntax element that indicates a residual characteristic (e.g., sdc_flag) of the depth intra prediction mode for the current depth PU. As in the first example above, the initialized probability state then may be determined based on the initialization value, e.g., in a manner similar to that described in HEVC WD 10, where init Value may be used to determine an initialized probability value defined by valMps and pStateldx for use in CABAC coding of the syntax element.

[0038] In this second example, by not using the actual value of the syntax element for the above neighboring depth block in the derivation when the above neighboring depth block is in a different CTU than the current depth prediction unit, the need to buffer the value of the syntax element for the above neighboring depth block can be avoided. In this manner, in some examples, this technique may help to conserve buffer space and/or reduce buffer size. When the above neighboring block resides in the same CTU as the current depth prediction unit, however, the value of the syntax element of the above neighboring block can be used, permitting an encoder or decoder to promote coding efficiency by exploiting correlation between neighboring blocks.

[0039] The video encoder and/or video decoder, in this second example, may determine the initialization value for determination of an initialized probability state for coding the syntax element using the CABAC process by determining a context index based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same CTU, determining a context index based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU, and determining the initialization value based on the determined context index. The initialized probability state for the context model may be determined by the encoder or decoder based on the initialization value, e.g., in a manner similar to that described in HEVC WD 10. For example, an encoder or decoder may use the initialization value (init Value) to determine an initialized probability value defined by valMps and pStateldx for use in CABAC coding of the syntax element.

[0040] As described above, the techniques described in the first or second example above may be applied to determine an initialization value for determination of an initialized probability state for decoding the syntax element using the CABAC process for a syntax element that indicates whether a DMM or non-DMM depth intra-prediction is to be used, and/or a syntax element that indicates selection of a SDC depth intra prediction mode or a non-SDC depth intra prediction mode, such as a regular residual coding mode.

Alternatively, in other examples, the techniques described in either the first or second example may be used for a syntax element that indicates whether a DMM or non-DMM intra prediction mode (e.g., hevc_intra_flag) is to be used, but the initialized probability value for coding a syntax element indicating whether SDC or non-SDC depth intra prediction mode is used may be determined based on a single context index value of zero.

[0041] Video coding standards and HEVC techniques related to this disclosure will now be reviewed. Examples of video coding standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual and ITU-T H.264 (also known as ISO/IEC MPEG-4 AVC), including its Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions. The latest joint draft of MVC is described in "Advanced video coding for generic audiovisual services," ITU-T Recommendation H.264, Mar 2010.

[0042] In addition, there is a new video coding standard, namely High Efficiency Video Coding (HEVC), developed by the Joint Collaboration Team on Video Coding (JCT-VC) of ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). A recent draft of the HEVC standard, JCTVC-L1003, Benjamin Bross, Woo-Jin Han, Jens-Ranier Ohm, Gary Sullivan, Ye-Kui Wang, Thomas Wiegand, "High Efficiency Video Coding (HEVC) text specification draft 10 (for FDIS & Last Call)," Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and

ISO/IEC JTC 1/SC 29/WG 11, 12th Meeting: Geneva, CH, 14-23 Jan. 2013 ("HEVC WD 10" or just "HEVC"), is incorporated herein by reference in its entirety, and is available from the following link: http://phenix.it-sudparis.eu/ict/doc end user/documents/ 12 Geneva/wg l 1 /JCTVC- L 1003-v34.zip

[0043] FIG. 1 is a diagram illustrating intra prediction modes used in HEVC. The intra prediction modes defined by HEVC and illustrated in FIG. 1 may be referred to as regular HEVC intra prediction modes, particularly in relation to the use of such intra prediction modes in HEVC extensions such as 3D-HEVC, where such regular HEVC intra prediction modes and other intra prediction modes, such as DMM and SDC modes, may be used. FIG. 1 generally illustrates the prediction directions associated with various directional intra-prediction modes available for intra-coding in HEVC. In the current HEVC, e.g., as described in HEVC WD 10, for the luma component of each Prediction Unit (PU), an intra prediction method is utilized with 33 directional (angular) prediction modes (indexed from 2 to 34), DC mode (indexed with 1) and Planar mode (indexed with 0), as shown in FIG. 1.

[0044] In the Planar mode (indexed with 0), prediction is performed using a so-called "plane" function to determine predictor values for each of the pixels within a block of video data, e.g., PU. According to the DC mode (indexed with 1), prediction is performed using an averaging of pixel values within the block to determine predictor values for each of the pixels within the block. According to a directional prediction mode, prediction is performed based on a neighboring block' s reconstructed pixels along a particular direction (as indicated by the mode). In general, the tail end of each arrow shown in FIG. 1 represents a relative one of neighboring pixels from which a value is retrieved, while the head of each arrow represents the direction in which the retrieved value is propagated to form a predictive block.

[0045] For HEVC intra prediction modes, a video encoder and/or video decoder generates a pixel specific predictor value for each pixel in the PU using the various modes discussed above, e.g., by using neighboring samples of the PU for modes 2 to 34. A video encoder determines residual values for the video block based on the differences between the actual depth values and the predictor values for the pixels of the block, and provides the residual values to a video decoder.

[0046] According to HEVC WD 10, a video encoder transforms the residual values and quantizes the transform coefficients, and may also entropy encode the quantized transform coefficients. A video decoder (e.g., after entropy decoding, inverse quantizing, and inverse transforming) determines reconstructed values for the pixels of the block by adding the residual values to the predictor values. Further details regarding HEVC intra prediction modes are specified in HEVC WD 10.

[0047] The entropy coding process used in HEVC will now be described, including the context adaptive binary arithmetic coding (CABAC) parsing process used in HEVC. The main steps for the CABAC coding process include:

1. Binarization

2. Context modeling 3. Binary arithmetic coding

[0048] For binarization, a CAB AC entropy coder maps a nonbinary valued syntax element to a binary sequence, referred to as a bin string. If the syntax element is already binary valued, binarization is not necessary and can be bypassed. Each bin in the bin string represents a binary decision. The CAB AC entropy coder then codes each bin in the bin string, either using a regular coding engine of the CAB AC coder, where a context model is selected, or a bypass coding engine of the CAB AC coder, where context model selection is not required.

[0049] In the regular (i.e., context- adaptive) coding mode, the CAB AC entropy coder includes a context modeler that performs context modeling prior to the arithmetic coding process for each bin. The regular coding engine of the CABAC entropy coder performs context modeling, by which a probability model is selected for each bin. The probability model may be selected in the CABAC entropy coder such that the context selection depends on previously coded binary syntax elements or bins of syntax elements.

[0050] After context model selection, the regular coding engine of the CABAC entropy coder receives the bin and probability model selected for the bin. The CABAC regular coding engine then applies binary arithmetic coding to the pertinent bin using the context model, and subsequently updates the context model. In particular, the bin value may be fed back to the context modeler to update the context model. Before starting a CABAC encoding/decoding (referred to generally as coding, where coding may comprise encoding or decoding), an entropy coding (e.g., entropy encoding or decoding) unit assigns an initialized probability state to each context.

[0051] As an alternative to context- adaptive coding, the entropy coder selects a bypass coding mode for entropy coding selected bins. A bypass coding engine of the CABAC entropy coder uses a simplified arithmetic coder, without the use of explicitly assigned context models, to code bins. The bypass coding engine is not context- adaptive. That is, in the bypass coding engine, bins are not context coded using an estimated probability obtained from a context model. Instead, bypass coded bins may be coded with a fixed probability model.

[0052] For example, the bypass coding engine may assume an equal probability of 0.5, and does not require selection of a context for coding. Hence, some bins may be coded using the regular binary arithmetic coding engine with the use of context models (i.e., contexts coded in the regular coding engine), while other bins may be coded using a bypass coding without the use of context models (i.e., bypass coded in the bypass coding engine).

[0053] The regular coding engine or bypass coding engine of a CAB AC entropy encoder, as applicable, arithmetically codes the bins for a syntax element to generate coded bits that form a bitstream. The regular coding engine or bypass coding engine of a CAB AC entropy decoder, as applicable, decodes bits in the bitstream to generate bins, and decodes one or more bins to generate syntax element. In some examples, bypass coding may provide increased throughput, and may allow multiple bins to be coded in the same cycle. Accordingly, use of the CAB AC bypass coding engine may be desirable for increased computational throughput, whereas use of the CAB AC regular coding engine may be desirable for high coding efficiency.

[0054] In JCT-3V, two HEVC extensions, the multiview extension (MV-HEVC) and 3D video extension (3D-HEVC) are being developed. A recent version of the reference software, "3D-HTM version 10. Orel," for 3D-HEVC is incorporated herein by reference in its entirety, and can be downloaded from the following link:

[3D-HTM version lO.Orcl]:

https://hevc.hhi.fraunhofer.de/svn/svn 3DVCSoftware/tags/HTM-10.0rcl/

[0055] A recent working draft of 3D-HEVC is presented in JCTVC-GIOOI, Gerhard Tech, Krzysztof Wegner, Ying Chen, and Sehoon Yea, "3D-HEVC Draft Text 3," Joint Collaborative Team on 3D Video Coding Extension Development of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 6th Meeting: Geneva, CH, 25 Oct. - 1 Nov. 2013 (referred to hereinafter as "G1001" or "3D-HEVC WD"), is incorporated herein by reference in its entirety, and is available from the following link: http://phenix.it-sudparis.eu/ict2/doc end user/documents/7 San%20Jose/wgl l/JCT3V- G1001-yl.zip

[0056] In 3D-HEVC, as defined in the 3D-HEVC WD referenced above, each access unit contains multiple pictures, and each of the pictures in each view has a unique view identification (id), or view order index. However, the depth picture and texture picture of the same view may have different layer ids. [0057] Depth coding in 3D video coding will now be described. 3D video data is represented using the multiview video plus depth format, in which captured views (texture) are associated with corresponding depth maps. In 3D video coding, textures and depth maps are coded and multiplexed into a 3D video bitstream. Depth maps are coded as grayscale video where the luma samples represent the depth values, and conventional intra- and inter-coding methods can be applied for depth map coding.

[0058] Depth maps may be characterized by sharp edges and constant areas. Due to the different statistics of depth map samples, different coding schemes are designed for depth maps based on a 2D video codec. In a multiview plus depth coding process, a view may include a texture component and a depth component. Depth coding units (CU's) in the depth component may be inter-coded or intra-coded. The depth CU's may be divided into one or more PU's, and the PU's may be divided into one or more partitions. In 3D- HEVC, the same definition of Intra prediction modes is utilized as for HEVC. Depth Modeling Modes (DMMs) are introduced in 3D-HEVC together with the HEVC Intra prediction modes to code an Intra prediction unit of a depth slice.

[0059] FIG. 2 is a block diagram illustrating an example video encoding and decoding system 10 that may be configured to utilize various techniques of this disclosure, such as the use of simplified intra mode coding in a 3D coding process, such as 3D-HEVC. As shown in FIG. 2, system 10 includes a source device 12 that provides encoded video data to be decoded at a later time by a destination device 14. In particular, source device 12 provides the video data to destination device 14 via a computer-readable medium 16. Source device 12 and destination device 14 may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called "smart" phones, so-called "smart" pads, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some cases, source device 12 and destination device 14 may be equipped for wireless communication.

[0060] Destination device 14 may receive the encoded video data to be decoded via computer-readable medium 16. Computer-readable medium 16 may comprise any type of medium or device capable of moving the encoded video data from source device 12 to destination device 14. In one example, computer-readable medium 16 may comprise a communication medium, such as a transmission channel, to enable source device 12 to transmit encoded video data directly to destination device 14 in real-time. [0061] The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device 14. The communication medium 16 may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium 16 may form part of a packet- based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium 16 may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device 12 to destination device 14.

[0062] In some examples, encoded data may be output from output interface 22 to a computer-readable storage medium, such as a non-transitory computer-readable storage medium, i.e., a data storage device. Similarly, encoded data may be accessed from the storage device by input interface. The storage device may include any of a variety of distributed or locally accessed non-transitory data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, the storage device may correspond to a file server or another intermediate storage device that may store the encoded video generated by source device 12.

[0063] Destination device 14 may access stored video data from the storage device via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting that encoded video data to the destination device 14. Example file servers include a web server (e.g., for a website), an FTP server, network attached storage (NAS) devices, or a local disk drive. Destination device 14 may access the encoded video data through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from the storage device may be a streaming transmission, a download transmission, or a combination thereof.

[0064] The techniques of this disclosure may be applied to video coding in support of any of a variety of wired or wireless multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions, such as dynamic adaptive streaming over HTTP (DASH), digital video that is encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications. In some examples, system 10 may be configured to support one-way or two-way video transmission to support applications such as video streaming, video playback, video broadcasting, and/or video telephony.

[0065] In the example of FIG. 2, source device 12 includes video source 18, video encoder 20, and output interface 22. Destination device 14 includes input interface 28, video decoder 30, and display device 32. In accordance with this disclosure, video encoder 20 of source device 12 may be configured to apply techniques for simplifying the derivation of an initialization value used to determine an initialized probability state for a context model for CAB AC coding a depth intra mode syntax element in a 3D video coding process, such as 3D-HEVC. In other examples, a source device and a destination device may include other components or arrangements. For example, source device 12 may receive video data from an external video source 18, such as an external camera. Likewise, destination device 14 may interface with an external display device, rather than including an integrated display device.

[0066] The illustrated system 10 of FIG. 2 is merely one example. Techniques described in this disclosure may be performed by a digital video encoding and/or decoding device. Although generally the techniques of this disclosure are performed by a video encoder 20 and/or video decoder 30, the techniques may also be performed by a video

encoder/decoder, typically referred to as a "CODEC." Moreover, the techniques of this disclosure may also be performed by a video preprocessor. Source device 12 and destination device 14 are merely examples of such coding devices in which source device 12 generates coded video data for transmission to destination device 14. In some examples, devices 12, 14 may operate in a substantially symmetrical manner such that each of devices 12, 14 include video encoding and decoding components. Hence, system 10 may support one-way or two-way video transmission between video devices 12, 14, e.g., for video streaming, video playback, video broadcasting, or video telephony.

[0067] Video source 18 of source device 12 may include a video capture device, such as a video camera, a video archive containing previously captured video, and/or a video feed interface to receive video from a video content provider. As a further alternative, video source 18 may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer generated video. In some cases, if video source 18 is a video camera, source device 12 and destination device 14 may form so-called smart phones, tablet computers or video phones. As mentioned above, however, the techniques described in this disclosure may be applicable to video coding in general, and may be applied to wireless and/or wired applications. In each case, the captured, pre-captured, or computer-generated video may be encoded by video encoder 20. The encoded video information may then be output by output interface 22 onto a computer-readable medium 16.

[0068] Computer-readable medium 16 may include transient media, such as a wireless broadcast or wired network transmission, or data storage media (that is, non-transitory storage media). In some examples, a network server (not shown) may receive encoded video data from source device 12 and provide the encoded video data to destination device 14, e.g., via network transmission. Similarly, a computing device of a medium production facility, such as a disc stamping facility, may receive encoded video data from source device 12 and produce a disc containing the encoded video data. Therefore, computer-readable medium 16 may be understood to include one or more computer- readable media of various forms, in various examples.

[0069] This disclosure may generally refer to video encoder 20 "signaling" certain information to another device, such as video decoder 30. It should be understood, however, that video encoder 20 may signal information by associating certain syntax elements with various encoded portions of video data. That is, video encoder 20 may "signal" data by storing certain syntax elements to headers or in payloads of various encoded portions of video data. In some cases, such syntax elements may be encoded and stored (e.g., stored to computer-readable medium 16) prior to being received and decoded by video decoder 30. Thus, the term "signaling" may generally refer to the communication of syntax or other data for decoding compressed video data, whether such communication occurs in real- or near-real-time or over a span of time via storage media, such as might occur when storing syntax elements to a storage medium at the time of encoding, which then may be retrieved by a decoding device at any time after being stored to this medium.

[0070] Input interface 28 of destination device 14 receives information from computer- readable medium 16. The information of computer-readable medium 16 may include syntax information defined by video encoder 20, which is also used by video decoder 30, that includes syntax elements that describe characteristics and/or processing of blocks and other coded units, e.g., GOPs. Display device 32 displays the decoded video data to a user, and may comprise any of a variety of display devices such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, a projection device, or another type of display device.

[0071] Although not shown in FIG. 2, in some aspects, video encoder 20 and video decoder 30 may each be integrated with an audio encoder and decoder, and may include appropriate MUX-DEMUX units, or other hardware and software, to handle encoding of both audio and video in a common data stream or separate data streams. If applicable, MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, as one example, or other protocols such as the user datagram protocol (UDP).

[0072] Video encoder 20 and video decoder 30 each may be implemented as any of a variety of suitable encoder or decoder circuitry, as applicable, such as one or more processors. Examples of various processors include microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic circuitry, which may be accompanied by software, hardware, firmware or any combinations thereof. Each of video encoder 20 and video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined video encoder/decoder (CODEC). A device including video encoder 20 and/or video decoder 30 may comprise an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone.

[0073] Video encoder 20 and video decoder 30 may operate according to a video coding standard, such as the HEVC standard and, more particularly, the 3D-HEVC extension of the HEVC standard, as referenced in this disclosure, e.g., by the 3D-HEVC WD. HEVC presumes several additional capabilities of video coding devices relative to devices configured to perform coding according to other processes, such as, e.g., ITU-T

H.264/AVC. For example, whereas H.264 provides nine intra-prediction encoding modes, the HM may provide as many as thirty-five intra-prediction encoding modes, as shown in and discussed above with reference to FIG. 1.

[0074] Some basic aspects of HEVC will now be discussed. In general, as defined in HEVC WD 10, HEVC specifies that a video picture (or "frame") may be divided into a sequence of largest coding units referred to as coding tree units (CTUs). A CTU includes corresponding luma and chroma components, referred to as coded tree blocks (CTB), e.g., luma CTB and chroma CTBs, including luma and chroma samples, respectively. Syntax data within a bitstream may define a size for the CTU, which is a largest coding unit in terms of the number of pixels. A slice may be a coded portion of a picture, and may include a number of consecutive CTBs in coding order. A picture may be partitioned into one or more slices. Each CTB may be split into coding units (CUs) according to a quadtree partitioning structure. In general, a quadtree data structure includes one node per CU, with a root node corresponding to the CTB. If a CU is split into four sub-CUs, the node corresponding to the CU includes four leaf nodes, each of which corresponds to one of the sub-CUs.

[0075] Each node of the quadtree data structure may provide syntax data for the corresponding CU. For example, a node in the quadtree may include a split flag, indicating whether the CU corresponding to the node is split into sub-CUs. Syntax elements for a CU may be defined recursively, and may depend on whether the CU is split into sub-CUs. If a CU is not split further, it is referred as a leaf-CU. Four sub-CUs of a leaf-CU may also be referred to as leaf-CUs even if there is no explicit splitting of the original leaf-CU. For example, if a CU at 16x16 size is not split further, the four 8x8 sub-CUs will also be referred to as leaf-CUs although the 16x16 CU was never split.

[0076] A CU in HEVC has a similar purpose as a macroblock of the H.264 standard, except that a CU does not have a size distinction. For example, a CTB may be split into four child nodes (also referred to as sub-CUs), and each child node may in turn be a parent node and be split into another four child nodes. A final, unsplit child node, referred to as a leaf node of the quadtree, comprises a coding node, also referred to as a leaf-CU. Syntax data associated with a coded bitstream may define a maximum number of times a CTB may be split, referred to as a maximum CU depth, and may also define a minimum size of the coding nodes. Accordingly, in some examples, a bitstream may also define a smallest coding unit.

[0077] A CU includes a coding node and prediction units (PUs) and transform units (TUs) associated with the coding node. This disclosure may use the term "block" to refer to any of a CU, prediction unit (PU), transform unit (TU), or partition thereof, in the context of HEVC, or similar data structures in the context of other standards. A size of the CU corresponds to a size of the coding node. The size of the CU may range from 8x8 pixels up to the size of the CTB with a maximum of 64x64 pixels or greater. Each CU may contain one or more PUs and one or more TUs. Syntax data associated with a CU may describe, for example, partitioning of the CU into one or more PUs. Partitioning modes may differ between whether the CU is skip or direct mode encoded, intra-prediction mode encoded, or inter-prediction mode encoded. PUs may be partitioned to be non- square in shape, or include partitions that are non-rectangular in shape, in the case of depth coding as described in this disclosure. Syntax data associated with a CU may also describe, for example, partitioning of the CU into one or more TUs according to a quadtree. A TU can be square or non-square (e.g., rectangular) in shape.

[0078] The HEVC standard allows for transformations according to TUs, which may be different for different CUs. The TUs are typically sized based on the size of PUs within a given CU defined for a partitioned CTB, although this may not always be the case. The TUs are typically the same size or smaller than the PUs. In some examples, residual samples corresponding to a CU may be subdivided into smaller units using a quadtree structure known as "residual quad tree" (RQT). The leaf nodes of the RQT may be referred to as transform units (TUs). Pixel difference values associated with the TUs may be transformed to produce transform coefficients, which may be quantized.

[0079] A leaf-CU may include one or more prediction units (PUs). In general, a PU represents a spatial area corresponding to all or a portion of the corresponding CU, and may include data for retrieving reference samples for the PU. The reference samples may be pixels from a reference block. In some examples, the reference samples may be obtained from a reference block, or generated, e.g., by interpolation or other techniques. A PU also includes data related to prediction. For example, when the PU is intra-mode encoded, data for the PU may be included in a residual quadtree (RQT), which may include data describing an intra-prediction mode for a TU corresponding to the PU.

[0080] As another example, when the PU is inter-mode encoded, the PU may include data defining one or more motion vectors for the PU. The data defining the motion vector for a PU may describe, for example, a horizontal component of the motion vector, a vertical component of the motion vector, a resolution for the motion vector (e.g., one- quarter pixel precision or one-eighth pixel precision), a reference picture to which the motion vector points, and/or a reference picture list (e.g., RefPicList 0, RefPicList 1) for the motion vector.

[0081] A leaf-CU having one or more PUs may also include one or more transform units (TUs). The transform units may be specified using an RQT (also referred to as a TU quadtree structure), as discussed above. For example, a split flag may indicate whether a leaf-CU is split into four transform units. Then, each transform unit may be split further into further sub-TUs. When a TU is not split further, it may be referred to as a leaf-TU. Generally, for intra coding, all the leaf-TUs belonging to a leaf-CU share the same intra prediction mode. That is, the same intra prediction mode is generally applied to calculate predicted values for all TUs of a leaf-CU. For intra coding, a video encoder 20 may calculate a residual value for each leaf-TU using the intra prediction mode, as a difference between the portion of the CU corresponding to the TU and the original block. A TU is not necessarily limited to the size of a PU. Thus, TUs may be larger or smaller than a PU. For intra coding, a PU may be collocated with a corresponding leaf-TU for the same CU. In some examples, the maximum size of a leaf-TU may correspond to the size of the corresponding leaf-CU.

[0082] Moreover, TUs of leaf-CUs may also be associated with respective quadtree data structures, referred to as residual quadtrees (RQTs). That is, a leaf-CU may include a quadtree indicating how the leaf-CU is partitioned into TUs. The root node of a TU quadtree generally corresponds to a leaf-CU, while the root node of a CU quadtree generally corresponds to a CTB. TUs of the RQT that are not split are referred to as leaf- TUs. In general, this disclosure uses the terms CU and TU to refer to a leaf-CU and leaf- TU, respectively, unless noted otherwise.

[0083] A video sequence typically includes a series of pictures. As described herein, "picture" and "frame" may be used interchangeably. That is, a picture containing video data may be referred to as a video frame, or simply a "frame." A group of pictures (GOP) generally comprises a series of one or more of the video pictures. A GOP may include syntax data in a header of the GOP, a header of one or more of the pictures, or elsewhere, that describes a number of pictures included in the GOP. Each slice of a picture may include slice syntax data that describes an encoding mode for the respective slice. Video encoder 20 typically operates on video blocks within individual video slices in order to encode the video data. A video block may correspond to a coding node within a CU. The video blocks may have fixed or varying sizes, and may differ in size according to a specified coding standard.

[0084] As an example, HEVC supports prediction in various PU sizes. Assuming that the size of a particular CU is 2Nx2N, HEVC supports intra prediction in PU sizes of 2Nx2N or NxN, and inter prediction in symmetric PU sizes of 2Nx2N, 2NxN, Nx2N, or NxN. A PU having a size of 2Nx2N represents an undivided CU, as it is the same size as the CU in which it resides. In other words, a 2Nx2N PU is the same size as its CU. HEVC supports asymmetric partitioning for inter prediction in PU sizes of 2NxnU, 2NxnD, nLx2N, and nRx2N. In asymmetric partitioning, one direction of a CU is not partitioned, while the other direction is partitioned into 25% and 75%. The portion of the CU corresponding to the 25% partition is indicated by an "n" followed by an indication of "Up", "Down," "Left," or "Right." Thus, for example, "2NxnU" refers to a 2Nx2N CU that is partitioned horizontally with a 2Nx0.5N PU on top and a 2Nxl.5N PU on bottom. For depth coding, the 3D-HEVC WD further supports partitioning of PU's according to depth modeling modes (DMMs), including non-rectangular partitions, as will be described.

[0085] In this disclosure, "NxN" and "N by N" may be used interchangeably to refer to the pixel dimensions of a video block in terms of vertical and horizontal dimensions, e.g., 16x16 pixels or 16 by 16 pixels. In general, a 16x16 block will have 16 pixels in a vertical direction (y = 16) and 16 pixels in a horizontal direction (x = 16). Likewise, an NxN block generally has N pixels in a vertical direction and N pixels in a horizontal direction, where N represents a non-negative integer value. The pixels in a block may be arranged in rows and columns. Moreover, blocks need not necessarily have the same number of pixels in the horizontal direction as in the vertical direction. For example, blocks may comprise NxM pixels, where M is not necessarily equal to N.

[0086] Following intra predictive or inter predictive coding using the PUs of a CU, video encoder 20 may calculate residual data for the TUs of the CU. The PUs may comprise syntax data describing a method or mode of generating predictive pixel data in the spatial domain (also referred to as the pixel domain) and the TUs may comprise coefficients in the transform domain following application of a transform, e.g., a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to residual video data. The residual data may correspond to pixel differences between pixels of the unencoded picture and prediction values corresponding to the PUs. Video encoder 20 may form the TUs including the residual data for the CU, and then transform the TUs to produce transform coefficients for the CU.

[0087] Following any transforms to produce transform coefficients, video encoder 20 may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the coefficients, providing further compression. The quantization process may reduce the bit depth associated with some or all of the coefficients. For example, an n-bit value may be rounded down to an m-bit value during quantization, where n is greater than m. For depth coding, the 3D-HEVC WD further supports segment-wise DC coding (SDC) of residual data and DMM coding, where delta DC values represent residual values for PU partitions. Unlike regular HEVC residual values, delta DC residual values may not be transformed or quantized.

[0088] Following quantization, video encoder 20 may scan the quantized transform coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher energy (and therefore lower frequency) coefficients at the front of the array and to place lower energy (and therefore higher frequency) coefficients at the back of the array.

[0089] In some examples, video encoder 20 may utilize a predefined scan order to scan the quantized transform coefficients to produce a serialized vector that can be entropy encoded. In other examples, video encoder 20 may perform an adaptive scan. After scanning the quantized transform coefficients to form a one-dimensional vector, video encoder 20 may entropy encode the one-dimensional vector, e.g., according to context- adaptive binary arithmetic coding (CAB AC), as used in HEVC. Examples of other entropy coding processes include context-adaptive variable length coding (CAVLC), syntax-based context-adaptive binary arithmetic coding (SB AC), and Probability Interval Partitioning Entropy (PIPE) coding. Again, in HEVC and 3D-HEVC, CAB AC is used. Video encoder 20 may also entropy encode syntax elements associated with encoded video data for use by video decoder 30 in decoding video data.

[0090] Video encoder 20 may further send syntax data, such as block-based syntax data, picture-based syntax data, and GOP-based syntax data, to video decoder 30, e.g., in a picture header, a block header, a slice header, or a GOP header. The GOP syntax data may describe a number of pictures in the respective GOP, and the picture syntax data may indicate an encoding/prediction mode used to encode the corresponding picture.

[0091] Video encoder 20 and/or video decoder 30 may perform intra-picture prediction coding of depth data and inter-prediction coding of depth data. In addition, video encoder 20 and/or video decoder 30 may use SDC to code residual data resulting from depth intra prediction coding of video data and/or depth inter prediction coding of video data. In some examples, video encoder 20 and/or video decoder 30 may use DMM, with or without SDC, to generate residual data resulting from depth intra prediction. DMM yields a partition-specific predictor for the pixels in a partition. Residual data may be generated for each of the pixels in the partition. Alternatively, if SDC is used with DMM, a single DC residual value may be generated that applies to the pixels in the partition.

[0092] In HEVC, assuming that the size of a coding unit (CU) is 2Nx2N, video encoder 20 and video decoder 30 may support various prediction unit (PU) sizes of 2Nx2N or NxN for intra-prediction, and symmetric PU sizes of 2Nx2N, 2NxN, Nx2N, NxN, or similar sizes for inter-prediction. A video encoder and video decoder may also support asymmetric partitioning for PU sizes of 2NxnU, 2NxnD, nLx2N, and nRx2N for inter- prediction. For depth coding as provided in 3D-HEVC, a video encoder and video decoder may be configured to support a variety of different depth coding modes for intra prediction and/or inter prediction, including various depth modeling modes (DMMs), as described in this disclosure.

[0093] Video data coded using 3D video coding techniques may be rendered and displayed to produce a three-dimensional effect. As one example, two images of different views (i.e., corresponding to two camera perspectives having slightly different horizontal positions) may be displayed substantially simultaneously such that one image is seen by a viewer's left eye, and the other image is seen by the viewer's right eye.

[0094] A 3D effect may be achieved using, for example, stereoscopic displays or autostereoscopic displays. Stereoscopic displays may be used in conjunction with eyewear that filters the two images accordingly. For example, passive glasses may filter the images using polarized lenses, or different colored lenses, or other optical filtering techniques, to ensure that the proper eye views the proper image.

[0095] Active glasses, as another example, may rapidly shutter alternate lenses in coordination with the stereoscopic display, which may alternate between displaying the left eye image and the right eye image. Autostereoscopic displays display the two images in such a way that no glasses are needed. For example, autostereoscopic displays may include mirrors or prisms that are configured to cause each image to be projected into a viewer's appropriate eyes.

[0096] The techniques of this disclosure relate to techniques for coding 3D video data by coding depth data to support 3D video. In general, the term "texture" is used to describe luminance (that is, brightness or "luma") values of an image and chrominance (that is, color or "chroma") values of the image. In some examples, a texture image may include one set of luminance data (Y) and two sets of chrominance data for blue hues (Cb) and red hues (Cr). For example, a CTU may include luma and chroma CTB's. In certain chroma formats, such as 4:2:2 or 4:2:0, the chroma data is downsampled relative to the luma data. That is, the spatial resolution of chrominance pixels may be lower than the spatial resolution of corresponding luminance pixels, e.g., one-half or one-quarter of the luminance resolution.

[0097] Depth data generally describes depth values for corresponding texture data. For example, a depth image may include a set of depth pixels (or depth values) that each describes depth, e.g., in a depth component of a view, for corresponding texture data, e.g., in a texture component of the view. Each pixel may have one or more texture values (e.g., luminance and chrominance), and may also have one or more depth values. A texture picture and a depth map may, but need not, have the same spatial resolution. For instance, the depth map may include more or fewer pixels than the corresponding texture picture. The depth data may be used to determine horizontal disparity for the

corresponding texture data, and in some cases, vertical disparity may also be used.

[0098] A device that receives the texture and depth data may display a first texture image for one view (e.g., a left eye view) and use the depth data to modify the first texture image to generate a second texture image for the other view (e.g., a right eye view) by offsetting pixel values of the first image by the horizontal disparity values determined based on the depth values. In general, horizontal disparity (or simply "disparity") describes the horizontal spatial offset of a pixel in a first view to a corresponding pixel in the right view, where the two pixels correspond to the same portion of the same object as represented in the two views.

[0099] In still other examples, depth data may be defined for pixels in a z-dimension perpendicular to the image plane, such that a depth associated with a given pixel is defined relative to a zero disparity plane defined for the image. Such depth may be used to create horizontal disparity for displaying the pixel, such that the pixel is displayed differently for the left and right eyes, depending on the z-dimension depth value of the pixel relative to the zero disparity plane. The zero disparity plane may change for different portions of a video sequence, and the amount of depth relative to the zero- disparity plane may also change.

[0100] Pixels located on the zero disparity plane may be defined similarly for the left and right eyes. Pixels located in front of the zero disparity plane may be displayed in different locations for the left and right eye (e.g., with horizontal disparity) so as to create a perception that the pixel appears to come out of the image in the z-direction perpendicular to the image plane. Pixels located behind the zero disparity plane may be displayed with a slight blur, to slight perception of depth, or may be displayed in different locations for the left and right eye (e.g., with horizontal disparity that is opposite that of pixels located in front of the zero disparity plane). Many other techniques may also be used to convey or define depth data for an image.

[0101] Two-dimensional video data is generally coded as a sequence of discrete pictures, each of which corresponds to a particular temporal instance. That is, each picture has an associated playback time relative to playback times of other images in the sequence. These pictures may be considered texture pictures or texture images. In depth-based 3D video coding, each texture picture in a sequence may also correspond to a depth map. That is, a depth map corresponding to a texture picture describes depth data for the corresponding texture picture. Multiview video data may include data for various different views, where each view may include a respective sequence of texture components and corresponding depth components.

[0102] A picture generally corresponds to a particular temporal instance. Video data may be represented using a sequence of access units, where each access unit includes all data corresponding to a particular temporal instance. Thus, for example, for multiview video data plus depth coding, texture images from each view for a common temporal instance, plus the depth maps for each of the texture images, may all be included within a particular access unit. Hence, an access unit may include multiple views, where each view may include data for a texture component, corresponding to a texture image, and data for a depth component, corresponding to a depth map.

[0103] Each access unit may contain multiple view components or pictures. The view components for a particular view are associated with a unique view id or view order index, such that view components of different views are associated with different view ids or view order indices. A view component may include a texture view component as well as a depth view component. The texture and depth view components in the same view may have different layer ids. A texture view component may be coded as one or more texture slices, while the depth view component may be coded as one or more depth slices. Multiview-plus-depth creates a variety of coding possibilities, such as intra- picture, inter-picture, intra-view, inter-view, motion prediction, and the like.

[0104] In this manner, with depth map coding in 3D video coding, 3D video data may be represented using a multiview video plus depth format, in which captured or generated views include texture components associated with corresponding depth maps. Moreover, in 3D video coding, textures and depth maps may be coded and multiplexed into a 3D video bitstream. Depth maps may be coded as grayscale images, where "luma" samples (that is, pixels) of the depth maps represent depth values.

[0105] In general, a block of depth data (a block of samples of a depth map, e.g., corresponding to pixels) may be referred to as a depth block. A depth value may be referred to as a luma value associated with a depth sample. That is, a depth map may generally be treated as a monochrome texture picture, i.e., a texture picture including luminance values and no chrominance values. In any case, conventional intra- and inter- coding methods may be applied for depth map coding.

[0106] In 3D-HEVC, as mentioned above, the same definition of intra prediction modes is utilized as in HEVC. That is, the intra modes used in 3D-HEVC include the regular intra modes of HEVC. Also, in 3D-HEVC, Depth Modeling Modes (DMMs) are introduced together with the HEVC intra prediction modes to code an Intra prediction unit of a depth slice.

[0107] For better representations of sharp edges in depth maps, the current HTM (3D- HTM version lO.Orcl) applies a DMM method for intra coding of the depth map. A depth block is partitioned into two regions specified by a DMM pattern, where each region is represented by a constant value. The DMM pattern can be either explicitly signaled (DMM mode 1), or predicted by a co-located texture block (DMM mode 4).

[0108] There are two types of partitioning models defined in DMM, including Wedgelet partitioning and the Contour partitioning. FIG. 3 is a diagram illustrating an example of a Wedgelet partition pattern for use in coding a block of pixel samples. FIG. 4 is a diagram illustrating an example of a contour partition pattern for use in coding a block of pixel samples. For a Wedgelet partition, as shown in FIG. 3, a depth block is partitioned into two regions by a straight line, where the two regions are labeled with P0 and PI. For Contour partitioning, as shown in FIG. 4, a depth block can be partitioned into two irregular regions.

[0109] Contour partitioning is more flexible than the Wedgelet partitioning, but difficult to be explicitly signaled. In DMM mode 4, in the case of 3D-HEVC, the contour partitioning pattern is implicitly derived using reconstructed luma samples of the co- located texture block. [0110] As one example, FIG. 3 provides an illustration of a Wedgelet pattern for an 8x8 block 40. For a Wedgelet partition, a depth block, e.g., PU, is partitioned into two regions 42, 44 by a straight line 46, with a start point 48 located at (Xs, Ys) and an end point 50 located at (Xe, Ye), as illustrated in FIG. 3, where the two regions 42, 44 are also labeled with P0 and PI, respectively. Each pattern in block 40 consists of an array of size uBxvB binary digit labeling whether the corresponding sample belongs to region P0 or PI where uB and vB represents the horizontal and vertical size of the current PU respectively. The regions P0 and PI are represented in FIG. 3 by white and shaded samples, respectively. The Wedgelet patterns are initialized at the beginning of both encoding and decoding.

[0111] As shown in the example of FIG. 4, a depth block, such as depth block 60, can be partitioned into three irregularly- shaped regions 62, 64A and 64B, using contour partitioning, where region 62 is labeled as P0 and the two regions 64A and 64B are co- labeled as PI, respectively. Regions 64A and 64B are represented by contour lines 66 and 68, respectively. Although pixels in region 64A are not immediately adjacent to pixels in region 64B, regions 64A and 64B may be defined to form one single region, for the purposes of predicting a PU of depth block 60.

[0112] With reference to FIGS. 3 and 4, each individual square within NxN depth blocks 40 and 60 represents a respective individual pixel of depth blocks 40 and 60, respectively. Numeric values within the squares represent whether the corresponding pixel belongs to region 42 (value "0" in the example of FIG. 3) or region 44 (value "1" in the example of FIG. 3). Shading is also used in FIG. 3 to indicate whether a pixel belongs to region 42 (white squares) or region 44 (grey shaded squares).

[0113] As discussed above, each pattern (that is, both Wedgelet and Contour) may be defined by an array of size uB X vB binary digit labeling of whether the corresponding sample (that is, pixel) belongs to region P0 or PI (where P0 corresponds to region 42 in FIG. 3 and region 62 in FIG. 4, and PI corresponds to region 44 in FIG. 3 and regions 64A, 64B in FIG. 4), where uB and vB represent the horizontal and vertical size of the current PU, respectively. In the examples of FIG. 3 and FIG. 4, the PU corresponds to blocks 40 and 60, respectively.

[0114] For HEVC intra prediction modes, a pixel specific intra predictor value is generated for each pixel in the PU by using neighboring samples of the PU, as specified in sub-clause 8.4.2 in HEVC WD 10. [0115] For other depth intra prediction modes, such as DMM, a partition specific DC predictor is calculated for each partition within the PU by using up to two neighboring samples of the PU. Let bPattern[x][y] be the partition pattern of the PU, where x = 0..N - 1, y = O. V-1 and N is the width of the PU. bPattern[x][y] indicates which partition pixel (x, y) belongs to and bPattern[x][y] can be equal to 0 or 1. Let BitDepth be the bit depth of depth samples and let RecSample[x][y] be the reconstructed neighboring samples of the PU, with x = -1 and y = 0..N-1 (corresponds to left neighboring pixels of the PU) or y = -1, x = 0..N-1 (corresponds to above neighboring pixels of the PU). Then, the DC predictor of partition X, namely DCPred[X], with X = 0 or 1 is derived as follows:

• Set bT = ( bPattern[0][0] != bPattern[N-l][0] ) ? 1 : 0

• Set bL = ( bPattern[0] [0] != bPattern[0] [N- 1] ) ? 1 : 0

• If bT equals bL

- DCPred[X] = ( RecSample[-l][0] + RecSample[0][-l] ) » 1

- DCPred[l-X] = bL 7 ( RecSample[-l][N-l] + RecSample[N-l][-l] ) » ^BitDepth-

• Otherwise

- DCPred[X] = bL 7 RecSample[(N-l)»l][-l] : RecSample[-l][(N- D»l]

- DCPred[l-X] = bL 7 RecSample[-l][N-l] : RecSample[N-l][-l]

[0116] A Depth Lookup Table (DLT) maps depth indexes to depth values. The DLT can be constructed by analyzing the frames within the first intra period before encoding the full video sequence. In the current design of 3D-HEVC, all of the valid depth values are sorted in ascending order and inserted to the DLT with increasing indexes.

[0117] DLT is an optional coding tool. In the current HTM (3D-HTM version 9.0), encoder 20 will not use DLT if more than half of the values from 0 to

MAX_DEPTH_VALUE (e.g., 255 for 8-bit depth samples) appear in the original depth map at the analysis step. Otherwise, the DLT will be coded in a sequence parameter set (SPS) and/or video parameter set (VPS). In order for encoder 20 to code DLT, the number of valid depth values is coded with an Exp-Golomb code first. Then, each valid depth value is also coded with an Exp-Golomb code. [0118] Video encoder 20 reads a pre-defined number of frames from the input video sequence to be coded and scans all samples for available depth map values. During this process, encoder 20 generates a mapping table that maps depth values to valid depth values based on the original uncompressed depth map.

[0119] Encoder 20 and/or decoder 30 derive the Depth Lookup Table Idx2Depth(.), the Index Lookup TableDepth2Idx(.), the Depth Mapping Table M(.) and the number of valid depth values d_vaiid using the following algorithm that analyzes the depth map D_t:

1. Initialization

• boolean vector B(d) = FALSE for all depth values d

• index counter i = 0

2. Process each pixel position p in D_t for multiple time instances t:

• Set (B(D_t(p ) = TRUE to mark valid depth values

3. Count number of TRUE values in B(d) d_vaud

4. For each d with B(d) = = TRUE:

• Set Idx2Depth(i) = d

• Set M(d) = d

• Set Depth2Idx(d) = i

• i = i + l

5. For each d with B(d) = = FALSE:

• Find d' = arg min \d - d'\ and B(d') = = TRUE

• Set M(d) = d'

• Set Depth2Idx(d ) = Depth2Idx(d ') .

[0120] Mapping from an index Idx back to a depth value d is as follows: d = Idx2Depth [Idx]. Mapping from a depth value d to an index Idx is as follows: Idx = Depth2Idx [d].

[0121] Segment-wise DC coding (SDC) has been introduced in 3D-HEVC. In SDC, one DC residual value is signaled for each partition of the PU, and no transform or quantization is applied. In HEVC intra prediction modes, the entire PU is considered one partition. SDC can be applied for all depth Intra prediction modes, including the regular HEVC intra prediction modes and the DMM modes, to code an intra PU of a depth slice. In the current 3D-HEVC, SDC is only applied for a 2Nx2N PU partition size.

[0122] To signal the residual value of each partition, two methods can be applied:

1. Directly code the DC residual value of each partition which is calculated by subtracting the predictor, denoted by Pred, generated by neighboring samples from the DC value (i.e., average value, denoted by Aver) of the current partition in the current PU.

2. When DLTs are transmitted, instead of coding the DC residual value, the index difference of the Aver and Pred mapped from the Index Lookup Table is coded. The index difference is calculated by subtracting the index of Pred from the index of Aver. At the decoder side, the sum of decoded index difference and the index of Pred is mapped back to depth values based on the DLT.

[0123] It has been proposed, in JCT3V-F0126, Liu et al., "CE5 related: Generic SDC for all Intra modes in 3D-HEVC," Joint Collaborative Team on 3D Video Coding Extensions of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 6th Meeting: Geneva,

Switzerland, 25 Oct. - 1 Nov. 2013, that, in depth coding, SDC can be applied for all of the additional depth Intra prediction modes and original, regular HEVC Intra prediction modes. SDC may be signaled for HEVC intra prediction modes and additional depth Intra prediction modes. For each depth coding unit coded in intra mode, the

hevc_intra_flag and sdc_flag syntax elements may be signaled as indicated below.

[0124] The hevc_intra_flag syntax element is signaled to indicate whether a regular HEVC Intra prediction mode or an additional depth Intra prediction mode, such as DMM mode 1 or DMM mode 4, is used. The value of hevc_intra_flag can be either 0 or 1. A value of hevc_intra_flag equal to 1 means that a regular HEVC Intra prediction mode is used for the coding unit, and a value of hevc_intra_flag equal to 0 means that an additional depth Intra prediction mode is used for the coding unit. Accordingly, hevc_intra_flag sets the prediction characteristic of the depth intra prediction mode in terms of which type of intra prediction mode is used, i.e., regular HEVC intra mode or additional depth mode such as DMM.

[0125] The sdc_flag syntax element is signaled to indicate whether SDC is used or not. The value of sdc_flag can be either 0 or 1. A value of sdc_flag equal to 1 means that SDC is used for the coding unit, and a value of sdc_flag equal to 0 means that SDC is not used for the coding unit. In this latter case, when sdc_flag is equal to 0, a regular residual coding tree may be used. Accordingly, sdc_flag sets the residual characteristic of the depth intra prediction mode in terms of the type of residual coding that is used, i.e., SDC or regular residual tree as in HEVC. [0126] FIG. 5 is a block diagram illustrating an example video encoder 20 that may be configured to implement the techniques of this disclosure. This disclosure describes video encoder 20 in the context of HEVC coding and, more particularly, 3D-HEVC coding, e.g., as described in the HEVC WD 10 and 3D-HEVC WD and as further modified as described in this disclosure. However, the techniques of this disclosure may be applicable to other coding standards or methods and/or further modifications or extensions of HEVC and 3D-HEVC. Accordingly, FIG. 5 is provided for purposes of explanation and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure.

[0127] Video encoder 20 may be configured to perform techniques for simplified context- adaptive binary arithmetic coding (CAB AC) coding of one or more syntax elements that signal one or more characteristics of a depth Intra prediction mode in a 3D video coding process, such as 3D-HEVC. For example, the value of the syntax element (e.g., hevc_intra_flag) may indicate a prediction characteristic of the depth intra prediction mode. As another example, alternatively, the value of the syntax element (e.g., sdc_flag) may indicate a residual characteristic of the depth intra prediction mode.

[0128] Video encoder 20 may use techniques that simplify the derivation of an initialization value used to determine initialized probability state for CAB AC coding of such a syntax element. In some examples, the techniques performed by video encoder 20 may reduce the number of context models and/or buffer size used for derivation of the initial probability state. In some examples, the syntax element may be the

hevc_intra_flag syntax element and/or the sdc_flag syntax element, e.g., in 3D-HEVC.

[0129] In the example of FIG. 5, video encoder 20 includes prediction processing unit 100, video data memory 101, residual generation unit 102, transform processing unit 104, quantization unit 106, inverse quantization unit 108, inverse transform processing unit 110, reconstruction unit 112, filter unit 114, decoded picture buffer 116, and entropy encoding unit 118. Prediction processing unit 100 includes an inter-prediction processing unit 120 and an intra-prediction processing unit 126. Inter-prediction processing unit 120 includes a motion estimation (ME) unit 122 and a motion compensation (MC) unit 124.

[0130] Video data memory 101 may store video data to be encoded by the components of video encoder 20. The video data stored in video data memory 101 may be obtained, for example, from video source 18. Decoded picture buffer 116 may be a reference picture memory that stores reference video data for use in encoding video data by video encoder 20, e.g., in intra- or inter-coding modes. Video data memory 101 and decoded picture buffer 116 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM),

magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory 101 and decoded picture buffer 116 may be provided by the same memory device or separate memory devices.

[0131] The components of prediction processing unit 100 are described as performing both texture encoding and depth encoding. In some examples, texture and depth encoding may be performed by the same components of prediction processing unit 100 or different components within prediction processing unit 100. For example, separate texture and depth encoders may be provided in some implementations. Also, multiple texture and depth encoders may be provided to encode multiple views, e.g., for multiview plus depth coding.

[0132] In either case, prediction processing unit 100 may be configured to intra- or inter- encode texture data and depth data as part of a 3D coding process, such as a 3D-HEVC process. In particular, in some modes, prediction processing unit 100 may use regular HEVC Intra coding modes or additional depth intra prediction modes, such as DMM modes, to code an intra prediction unit of a depth slice. In addition, prediction processing unit 100 may use non-SDC residual coding or SDC coding, with regular HEVC intra prediction modes or additional depth intra prediction modes.

[0133] In the case of DMM coding, prediction processing unit 100 may generate a partition- specific DC predictor for a partition, and then generate either pixel residual values, or in SDC, a single residual value for the pixels in the partition. In the case of a regular HEVC intra prediction mode, prediction processing unit 100 may generate pixel- specific predictors, then generate pixel- specific residual values, or in SDC, a single residual value for the pixels in the partition.

[0134] The residual value generated for SDC may be a delta DC residual value. SDC also may be used for inter-predicted PUs. The delta DC residual value may represent a difference between an average value of pixels in a PU or partition of the coded PU and an average value of predicted samples in an intra- or inter-predicted PU partition. A PU may have a single partition or multiple partitions, depending on the coding mode. HEVC intra, HEVC inter modes, DMM's or other modes may be used to code a depth PU. [0135] In some examples, prediction processing unit 100 may operate substantially in accordance with 3D-HEVC, e.g., as described in the 3D-HEVC WD, subject to modifications and/or additions described in this disclosure, such as those relating to simplified Intra mode coding. In some examples, video encoder 20 may include more, fewer, or different functional components than shown in FIG. 5. Prediction processing unit 100 may provide syntax information to entropy encoding unit 118. The syntax information may indicate, for example, which prediction modes were used and information relating to such modes, such as a motion vector, prediction direction, and reference picture index, in the case of inter-prediction.

[0136] Video encoder 20 receives video data to be encoded. Video encoder 20 may encode each of a plurality of coding tree units (CTU) in a slice of a picture of the video data. CTU's also may be referred to as LCU's. In 3D-HEVC, video encoder 20 may encode CTU's of texture and depth views. Each of the texture CTUs may have luma and chroma components, and may be associated with equally-sized luma coding tree blocks (CTBs) and corresponding chroma CTBs of the picture. A depth CTU may include one or more depth CTBs. As part of encoding a CTU, prediction processing unit 100 may perform quad-tree partitioning to divide the CTBs of the CTU into progressively-smaller blocks. The smaller block may be coding blocks of CUs. For example, prediction processing unit 100 may partition a CTB associated with a CTU into four equally- sized sub-blocks, partition one or more of the sub-blocks into four equally-sized sub-sub- blocks, and so on.

[0137] Video encoder 20 may encode CUs of a CTB to generate encoded representations of the CUs (i.e., coded CUs). As part of encoding a CU, prediction processing unit 100 may partition the coding blocks associated with the CU among one or more PUs of the CU. Thus, each PU in a texture slice may be associated with a luma component prediction block and corresponding chroma component prediction blocks. Each PU in a depth slice may have a single component.

[0138] Video encoder 20 and video decoder 30 may support PUs having various sizes. As indicated above, the size of a CU may refer to the size of the luma coding block of the CU and the size of a PU may refer to the size of a luma prediction block of the PU. Assuming that the size of a particular CU is 2Nx2N, video encoder 20 and video decoder 30 may support PU sizes of 2Nx2N or NxN for intra prediction, and symmetric PU sizes of 2Nx2N, 2NxN, Nx2N, NxN, or similar for inter prediction. Video encoder 20 and video decoder 30 may also support asymmetric partitioning for PU sizes of 2NxnU, 2NxnD, nLx2N, and nRx2N for inter prediction. In accordance with aspects of this disclosure, video encoder 20 and video decoder 30 also support non-rectangular partitions of a PU for depth inter coding, such as partitions for DMM modes.

[0139] Inter-prediction processing unit 120 may generate predictive data for a PU by performing inter prediction on each PU of a CU. The predictive data for the PU may include predictive sample blocks of the PU and motion information for the PU. Inter- prediction processing unit 120 may perform different operations for a PU of a CU depending on whether the PU is in an I slice, a P slice, or a B slice. In an I slice, all PUs are intra predicted. Hence, if the PU is in an I slice, inter-prediction processing unit 120 does not perform inter prediction on the PU. Thus, for blocks encoded in I-mode, the predicted block is formed using spatial prediction from previously-encoded neighboring blocks within the same picture.

[0140] If a PU is in a P slice, motion estimation (ME) unit 122 may search the reference pictures in a list of reference pictures (e.g., "RefPicListO") for a reference region for the PU. The reference pictures may be stored in decoded picture buffer 116. The reference region for the PU may be a region, within a reference picture, that contains sample blocks that most closely corresponds to the sample blocks of the PU. Motion estimation (ME) unit 122 may generate a reference index that indicates a position in RefPicListO of the reference picture containing the reference region for the PU.

[0141] In addition, for inter-coding, motion estimation (ME) unit 122 may generate a motion vector (MV) that indicates a spatial displacement between a coding block of the PU and a reference location associated with the reference region. For instance, the MV may be a two-dimensional vector that provides an offset from the coordinates in the current decoded picture to coordinates in a reference picture. Motion estimation (ME) unit 122 may output the reference index and the MV as the motion information of the PU. Motion compensation (MC) unit 124 may generate the predictive sample blocks of the PU based on actual or interpolated samples at the reference location indicated by the motion vector of the PU.

[0142] If a PU is in a B slice, motion estimation unit 122 may perform uni-prediction or bi-prediction for the PU. To perform uni-prediction for the PU, motion estimation unit 122 may search the reference pictures of RefPicListO or a second reference picture list ("RefPicListl") for a reference region for the PU. Motion estimation (ME) unit 122 may output, as the motion information of the PU, a reference index that indicates a position in RefPicListO or RefPicListl of the reference picture that contains the reference region, an MV that indicates a spatial displacement between a sample block of the PU and a reference location associated with the reference region, and one or more prediction direction indicators that indicate whether the reference picture is in RefPicListO or RefPicListl. Motion compensation (MC) unit 124 may generate the predictive sample blocks of the PU based at least in part on actual or interpolated samples at the reference region indicated by the motion vector of the PU.

[0143] To perform bi-directional inter-prediction for a PU, motion estimation unit 122 may search the reference pictures in RefPicListO for a reference region for the PU and may also search the reference pictures in RefPicListl for another reference region for the PU. Motion estimation (ME) unit 122 may generate reference picture indexes that indicate positions in RefPicListO and RefPicListl of the reference pictures that contain the reference regions. In addition, motion estimation (ME) unit 122 may generate MVs that indicate spatial displacements between the reference location associated with the reference regions and a sample block of the PU. The motion information of the PU may include the reference indexes and the MVs of the PU. Motion compensation (MC) unit 124 may generate the predictive sample blocks of the PU based at least in part on actual or interpolated samples at the reference region indicated by the motion vector of the PU.

[0144] Intra-prediction processing unit 126 may generate predictive data for a PU by performing intra prediction on the PU. The intra-predictive data for the PU may include predictive sample blocks for the PU and various syntax elements. Intra-prediction processing unit 126 may perform intra prediction on PUs in I slices, P slices, and B slices. To perform intra prediction on a PU, intra-prediction processing unit 126 may use multiple intra prediction modes to generate multiple sets of predictive data for the PU, and then select one of the intra-prediction modes that yields acceptable or optimal coding performance, e.g., using rate-distortion optimization techniques.

[0145] To use an intra prediction mode to generate a set of predictive data for the PU, intra-prediction processing unit 126 may extend samples from sample blocks of spatially neighboring PUs across the sample blocks of the PU in a direction associated with the intra prediction mode. The neighboring PUs may be above, above and to the right, above and to the left, or to the left of the PU, assuming a left-to-right, top-to-bottom encoding order for PUs, CUs, and CTUs. Intra-prediction processing unit 126 may use various numbers of intra prediction modes, e.g., thirty-five directional intra prediction modes, as shown in FIG. 1. In some examples, the number of intra prediction modes may depend on the size of the region associated with the PU.

[0146] Prediction processing unit 100 may select the predictive data for PUs of a CU from among the predictive data generated by inter-prediction processing unit 120 for the PUs or the predictive data generated by intra-prediction processing unit 126 for the PUs. In some examples, prediction processing unit 100 selects the predictive data for the PUs of the CU based on rate/distortion metrics of the sets of predictive data. The predictive sample blocks of the selected predictive data may be referred to herein as the selected predictive sample blocks.

[0147] Residual generation unit 102 may generate, based on the luma, Cb and Cr coding block of a CU and the selected inter- or intra-predictive luma, Cb and Cr blocks of the PUs of the CU, a luma, Cb and Cr residual blocks of the CU. For instance, residual generation unit 102 may generate the residual blocks of the CU such that each sample in the residual blocks has a value equal to a difference between a sample in a coding block of the CU and a corresponding sample, i.e., in luma or chroma pixel value, as applicable, in a corresponding selected predictive sample block of a PU of the CU. Residual data may be generated in a similar manner for depth prediction units, using regular residual coding or SDC techniques.

[0148] For HEVC intra modes, HEVC inter modes and other modes, delta DC coding may be used to generate a delta DC residual value, also referred to as a DC residual value, for a predicted depth PU or depth PU partition. For SDC, or for DMM with SDC, residual generation unit 102 may generate a single delta DC value for each depth PU or PU partition, where the single delta DC value represents a difference between an average value of pixels in the PU or PU partition, and an average value of predicted samples in an intra- or inter-predicted PU or PU partition. For DMM, without SDC, residual generation unit 102 may generate a DC prediction value and a regular residual tree. The delta DC residual value is not transformed or quantized and may be provided by residual generation unit 102 to entropy coding unit 118, e.g., as indicated by line 115 of FIG. 5.

[0149] Transform processing unit 104 may perform quad-tree partitioning to partition the residual blocks associated with a CU into transform blocks associated with TUs of the CU. Thus, a TU may be associated with a luma transform block and two chroma transform blocks, in the case of a texture view. The sizes and positions of the luma and chroma transform blocks of TUs of a CU may or may not be based on the sizes and positions of prediction blocks of the PUs of the CU. A quad-tree structure known as a "residual quad-tree" (RQT) may include nodes associated with each of the regions. The TUs of a CU may correspond to leaf nodes of the RQT. Again, such a RQT structure may not be used for SDC residual coding.

[0150] Transform processing unit 104 may generate transform coefficient blocks for each TU of a CU by applying one or more transforms to the transform blocks of the TU.

Transform processing unit 104 may apply various transforms to a transform block associated with a TU. For example, transform processing unit 104 may apply a discrete cosine transform (DCT), a directional transform, or a conceptually similar transform to a transform block. In some examples, transform processing unit 104 does not apply transforms to a transform block. In such examples, the transform block may be treated as a transform coefficient block.

[0151] Quantization unit 106 may quantize the transform coefficients in a coefficient block. The quantization process may reduce the bit depth associated with some or all of the transform coefficients. For example, an ra-bit transform coefficient may be rounded down to an m-bit transform coefficient during quantization, where n is greater than m. Quantization unit 106 may quantize a coefficient block associated with a TU of a CU based on a quantization parameter (QP) value associated with the CU. Video encoder 20 may adjust the degree of quantization applied to the coefficient blocks associated with a CU by adjusting the QP value associated with the CU. Quantization may introduce loss of information, thus quantized transform coefficients may have lower precision than the original ones.

[0152] Inverse quantization unit 108 and inverse transform processing unit 110 may apply inverse quantization and inverse transforms to a coefficient block, respectively, to reconstruct a residual block from the coefficient block. Reconstruction unit 112 may add the reconstructed residual block to corresponding samples from one or more predictive sample blocks generated by prediction processing unit 100 to produce a reconstructed transform block associated with a TU. By reconstructing transform blocks for each TU of a CU in this way, video encoder 20 may reconstruct the coding blocks of the CU.

[0153] If SDC is used, reconstruction unit 112 may reconstruct a depth CU based on DC residual values for partitions of PU's of the CU and corresponding predicted partitions of the PU's of the CU. For example, the delta DC residual value for each depth PU partition may be added to the pixel values in a corresponding predicted partition to reconstruct the depth PU partition, wherein the DC residual value may represent a difference between an average value of the pixels of the depth PU partition and the average value of the predicted samples of the predicted partition. For SDC, including DMM with SDC, only the DC residual value is used. For DMM, without SDC, the DC predictor and a regular residual tree may be used. In some examples, information representing the DC residual value, such as one or more syntax elements representing delta DC values, may be generated by prediction processing unit 100, received by entropy encoding unit 118, and used by reconstruction unit 112 without inverse quantization or inverse transform processing, e.g., as indicated by line 115.

[0154] Filter unit 114 may perform one or more deblocking operations to reduce blocking artifacts in the coding blocks associated with a reconstructed CU. Decoded picture buffer 116 may store the reconstructed coding blocks after filter unit 114 performs the one or more deblocking operations on the reconstructed coding blocks.

Inter-prediction unit 120 may use a reference picture that contains the reconstructed coding blocks to perform inter prediction on PUs of other pictures. In addition, intra- prediction processing unit 126 may use reconstructed coding blocks in decoded picture buffer 116 to perform intra prediction on other PUs in the same picture as the CU.

[0155] Entropy encoding unit 118 may receive data from various functional components of video encoder 20. For example, entropy encoding unit 118 may receive coefficient blocks from quantization unit 106 and may receive syntax elements from prediction processing unit 100. In addition, entropy encoding unit 118 may receive delta DC residual values from residual generation unit 102. Entropy encoding unit 118 may perform one or more entropy encoding operations on the data to generate entropy- encoded data. For example, entropy encoding unit 118 may perform a CAB AC operation. Video encoder 20 may output an encoded video data bitstream that includes CAB AC entropy-encoded data generated by entropy encoding unit 118. For instance, the bitstream may include bits that represent bins of binary syntax elements or binarized syntax elements.

[0156] Video encoder 20 is an example of a video encoder configured to perform any of the techniques described in this disclosure. Additional 3D processing components may also be included within video encoder 20. In accordance with one or more techniques of this disclosure, one or more units within video encoder 20 may perform the techniques described herein as part of a video encoding process. Similarly, video encoder 20 may perform a video decoding process to reconstruct video data used as reference data for prediction of subsequently coded video data.

[0157] Video encoder 20 may be configured to use techniques that simplify the derivation of an initialzation value for determining an initialized probability state for CAB AC coding of a syntax element that indicates one or more characteristics of a depth intra prediction mode. Examples of such a syntax element include a syntax element, such as hevc_intra_flag, that indicates a predictive characteristic of the depth intra prediction mode, and a syntax element, such as sdc_flag, that indicates a residual characteristic of the depth intra prediction mode. The techniques may reduce the number of context models and/or buffer size used for derivation of an initialization value that may be used to determine the initialized probability state for CAB AC coding the syntax element. In some examples, entropy encoding unit 118 of video encoder 20 may be configured to perform some or all aspects of such techniques.

[0158] FIG. 6 is a block diagram illustrating an example video decoder 30 that is configured to perform the techniques of this disclosure. FIG. 6 is provided for purposes of illustration and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure. This disclosure describes video decoder 30 in the context of HEVC coding and, in particular, 3D-HEVC coding. However, the techniques of this disclosure may be applicable to other 3D video coding standards or methods. Accordingly, FIG. 6 is provided for purposes of explanation and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure.

[0159] Video decoder 30 may be configured to perform techniques for simplified context- adaptive binary arithmetic coding (CAB AC) coding of a syntax element that signals one or more characteristics of a depth Intra prediction mode in a 3D video coding process, such as 3D-HEVC. For example, the value of the syntax element (e.g., hevc_intra_flag) may indicate a prediction characteristic of the depth intra prediction mode. As another example, alternatively, the value of the syntax element (e.g., sdc_flag) may indicate a residual characteristic of the depth intra prediction mode.

[0160] For example, video decoder 30 may use techniques that simplify the derivation of an initialization value for determination of an initial probability state for CAB AC coding of such a syntax element. In some examples, some or all aspects of these techniques may be performed by entropy decoding unit 150. In some examples, the techniques performed by video decoder 30 may reduce the number of context models and/or buffer size used for derivation of the initialization value. In some examples, the syntax element may be the hevc_intra_flag syntax element and/or the sdc_flag syntax element, e.g., in 3D-HEVC.

[0161] In the example of FIG. 6, video decoder 30 includes an entropy decoding unit 150, video data memory 151, a prediction processing unit 152, an inverse quantization unit 154, an inverse transform processing unit 156, a reconstruction unit 158, a filter unit 160, and a decoded picture buffer 162. Prediction processing unit 152 may include a motion compensation (MC) unit 164 for inter-prediction and an intra-prediction processing unit 166.

[0162] Video data memory 151 may store video data, such video data from an encoded video data bitstream, to be decoded by the components of video decoder 30. The video data stored in video data memory 151 may be obtained, for example, from computer- readable medium 16, e.g., from a local video source, such as a camera, via wired or wireless network communication of video data, or by accessing physical data storage media. Video data memory 151 may form a coded picture buffer (CPB) that stores encoded video data from an encoded video data bitstream. Decoded picture buffer 162 may be a reference picture memory that stores reference video data for use in decoding video data by video decoder 30, e.g., in intra- or inter-coding modes. Video data memory 151 and decoded picture buffer 162 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MR AM), resistive RAM (RRAM), or other types of memory devices. Video data memory 151 and decoded picture buffer 162 may be provided by the same memory device or separate memory devices.

[0163] For ease of illustration, the components of prediction processing unit 152 are described as performing both texture decoding and depth decoding. In some examples, texture and depth decoding may be performed by the same components of prediction processing unit 152 or different components within prediction processing unit 152. For example, separate texture and depth decoders may be provided in some implementations. Also, multiple texture and depth decoders may be provided to decode multiple views, e.g., for multiview plus depth coding. In either case, prediction processing unit 152 may be configured to intra- or inter-decode texture data and depth data as part of a 3D coding process, such as a 3D-HEVC process.

[0164] Accordingly, prediction processing unit 152 may operate substantially in accordance with 3D-HEVC, subject to modifications and/or additions described in this disclosure, such as those relating to simplified coding of a depth Intra mode syntax element. Prediction processing unit 152 may obtain residual data from the encoded video data bitstream for intra-decoded or inter-decoded depth data via entropy decoding unit 150. Video decoder 30 may reconstruct CU's using intra-predicted or inter-predicted depth data and the residual data. In some examples, the residual data may be regular residual data or delta DC residual data, which may be generated, for example, by SDC coding. Video decoder 30 may include more, fewer, or different functional components than shown in FIG. 6.

[0165] Video decoder 30 receives an encoded video data bitstream. Entropy decoding unit 150 parses the bitstream to decode entropy-encoded syntax elements from the bitstream. Prediction processing unit 152, inverse quantization unit 154, inverse transform processing unit 156, reconstruction unit 158, and filter unit 160 may generate decoded video data based on the syntax elements extracted from the bitstream. The bitstream may comprise a series of NAL units. The NAL units of the bitstream may include coded slice NAL units. As part of decoding the bitstream, entropy decoding unit 150 may extract and entropy decode syntax elements from the coded slice NAL units.

[0166] Each of the coded slices may include a slice header and slice data. The slice header may contain syntax elements pertaining to a slice. The syntax elements in the slice header may include a syntax element that identifies a PPS associated with a picture that contains the slice. The PPS may refer to an SPS, which may in turn refer to a VPS. Entropy decoding unit 150 may also entropy decode other elements that may include syntax information, such as SEI messages. Decoded syntax elements in any of the slice header, parameter sets, or SEI messages may include information described herein as being signaled in accordance with example techniques described in this disclosure. Such syntax information may be provided to prediction processing unit 152 for decoding and reconstruction of texture or depth blocks.

[0167] Video decoder 30 may perform a reconstruction operation on a non-partitioned CU's and PUs. To perform the reconstruction operation, for non-SDC coding, video decoder 30 may perform a reconstruction operation on each TU of the CU. By performing the reconstruction operation for each TU of the CU, video decoder 30 may reconstruct blocks of the CU. As part of performing a reconstruction operation on a TU of a CU, inverse quantization unit 154 may inverse quantize, i.e., de-quantize, coefficient blocks associated with the TU. Inverse quantization unit 154 may use a QP value associated with the CU of the TU to determine a degree of quantization and, likewise, a degree of inverse quantization for inverse quantization unit 154 to apply. That is, the compression ratio, i.e., the ratio of the number of bits used to represent original sequence and the compressed one, may be controlled by adjusting the value of the QP used when quantizing transform coefficients. The compression ratio may also depend on the method of entropy coding employed.

[0168] After inverse quantization unit 154 inverse quantizes a coefficient block, i.e., for non-SDC regular residual coding, inverse transform processing unit 156 may apply one or more inverse transforms to the coefficient block in order to generate a residual block associated with the TU. For example, inverse transform processing unit 156 may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the coefficient block.

[0169] If a PU is encoded using intra-prediction, intra-prediction processing unit 166 may perform intra prediction to generate predictive blocks for the PU. Intra-prediction processing unit 166 may use an intra prediction mode to generate the predictive luma, Cb and Cr blocks for the PU for texture slices based on the prediction blocks of spatially- neighboring PUs. Intra-prediction processing unit 166 may use an intra prediction mode to generate depth blocks for a depth slice. Intra-prediction processing unit 166 may determine the intra prediction mode for the PU based on one or more syntax elements decoded from the bitstream.

[0170] If a PU is encoded using inter-prediction, MC unit 164 may perform intra prediction to generate an inter-predictive block for the PU. MC unit 164 may use an inter prediction mode to generate the predictive luma, Cb and Cr blocks for the texture PU and/or predictive depth blocks based on the prediction blocks of PUs in other pictures or views. MC unit 164 may determine the inter prediction mode for the PU based on one or more syntax elements decoded from the bitstream, and may receive motion information such as motion vectors, prediction direction, and reference picture indexes. [0171] For inter-prediction, MC unit 164 may construct a first reference picture list (RefPicListO) and a second reference picture list (RefPicListl) based on syntax elements extracted from the bitstream. If a PU is encoded using inter prediction, entropy decoding unit 150 may extract motion information for the PU. MC unit 164 may determine, based on the motion information of the PU, one or more reference blocks for the PU. Motion compensation (MC) unit 164 may generate, based on samples in blocks at the one or more reference blocks for the PU, predictive luma, Cb and Cr blocks for a texture PU and a predictive depth block for a depth PU.

[0172] Reconstruction unit 158 may use the luma, Cb and Cr transform blocks associated with TUs of a CU and the predictive luma, Cb and Cr blocks of the PUs of the CU, i.e., either intra-prediction data or inter-prediction data, as applicable, to reconstruct the luma, Cb and Cr coding blocks of the CU. For example, reconstruction unit 158 may add residual samples of the luma, Cb and Cr transform blocks to corresponding samples of the predictive luma, Cb and Cr blocks to reconstruct the luma, Cb and Cr coding blocks of the CU. Similarly, reconstruction unit 158 may use intra-prediction data or inter- prediction data to reconstruct depth blocks of the CU.

[0173] Filter unit 160 may perform a deblocking operation to reduce blocking artifacts associated with the luma, Cb and Cr coding blocks of the CU. Video decoder 30 may store the luma, Cb and Cr coding blocks of the CU in decoded picture buffer 162.

Decoded picture buffer 162 may provide reference pictures for subsequent motion compensation, intra prediction, and presentation on a display device, such as display device 32 of FIG. 2. For instance, video decoder 30 may perform, based on the luma, Cb and Cr blocks, or depth blocks, in decoded picture buffer 162, intra prediction or inter prediction operations on PUs of other CUs.

[0174] Video decoder 30 is an example of a video decoder configured to perform any of the techniques described in this disclosure. Additional 3D processing components may also be included within video decoder 30. In accordance with one or more techniques of this disclosure, one or more units within video encoder 30 may perform the techniques described herein as part of a video decoding process. Video decoder 30, e.g., via entropy decoding unit 150, may be configured to use techniques that simplify the derivation of an initial probability state for CAB AC coding of a syntax element that indicates one or more characteristics of a depth intra prediction mode. Examples of such a syntax element include a syntax element, such as hevc_intra_flag, that indicates a predictive characteristic of the depth intra prediction mode, and a syntax element, such as sdc_flag, that indicates a residual characteristic of the depth intra prediction mode. The techniques may reduce the number of context models and/or buffer size used for derivation of the initialized probability state for CAB AC coding the syntax element. In some examples, entropy decoding unit 150 of video decoder 30 may be configured to perform such techniques.

[0175] Prediction processing unit 152 and, more particularly, intra-prediction processing unit 166 and motion compensation (MC) unit 164, may determine, based on received syntax information, whether to perform DMM or non-DMM intra prediction for a current depth PU, and whether to perform SDC or non-SDC residual coding techniques for the depth PU. Again, a non-DMM intra prediction mode may use regular HEVC intra modes, and a non-SDC residual coding mode may use regular residual coding, e.g., in a residual quadtree structure. When SDC is used, for example, entropy decoding unit 150 may entropy decode one or more delta DC residual values for PU's or PU partitions of a depth CU, as well as associated syntax information.

[0176] For SDC, entropy decoding unit 150 may provide SDC syntax information for the block to prediction processing unit 152, as indicated in FIG. 6. Entropy decoding unit 150 may provide delta DC residual value to reconstruction unit 158, as indicated by line 157 in FIG. 6. The delta DC residual values received by video decoder 30 are not transformed and quantized. In particular, the delta DC residual value(s) need not be first provided to inverse quantization unit 154 and inverse transform processing unit 156 for inverse quantization and inverse transformation. Instead, entropy decoding unit 150 may decode, from bits in the bitstream, bins for a syntax element representing a delta DC residual value, and provide information representing the delta DC residual value to reconstruction unit 158 for use in reconstructing a coded PU or partition. Reconstruction unit 158 may receive an intra- or inter-predicted PU or PU partition of a depth CU from prediction processing unit 152 and add the delta DC residual value to each of the samples of the predicted PU or PU partition to reconstruct the coded PU or PU partition.

[0177] In this manner, when SDC is used, for example, reconstruction unit 158 may reconstruct a depth CU based on delta DC residual values for partitions of PU's of the CU and corresponding predicted PUs or PU partitions of the CU. Again, the delta DC residual value may represent a difference between an average value of the pixels of the depth PU or PU partition and the average value of the predicted samples of the predicted PU or PU partition. When DMM is used without SDC, a regular residual coding tree may be used. Likewise, when HEVC intra modes are used without SDC, a regular residual coding tree may be used.

[0178] In accordance with various examples of this disclosure, entropy encoding unit 118 and/or entropy decoding unit 150 may perform techniques that include simplified derivation of an initialization value used to determine initial probability state for CAB AC decoding of a depth Intra mode syntax element. In some examples, a CAB AC coder used by entropy encoding unit 118 and/or entropy decoding unit 150 may be configured to reduce the number of context models and/or buffer size used for derivation of the initial probability state for coding of a depth Intra mode syntax element in a 3D video coding process, such as 3D-HEVC.

[0179] As discussed above, in 3D-HEVC, for each depth prediction unit coded in Intra mode, one flag, named hevc_intra_flag, is signaled to indicate whether HEVC Intra prediction mode or DMM mode is used. The value of hevc_intra_flag can be either 0 or 1. The value hevc_intra_flag equal to 1 means that an HEVC intra prediction mode is used for the prediction unit, and the value of hevc_intra_flag equal to 0 means a DMM mode is used for the prediction unit. A depth_intra_mode syntax element may be further signaled to indicate, when DMM is used, whether DMM mode 1 or DMM mode 4 is used. The value of depth_intra_mode equal to 0 means DMM mode 1 is used for the prediction unit, and the value of depth_intra_mode equal to 1 means DMM mode 4 is used for the prediction unit.

[0180] Context-based adaptive binary arithmetic coding (CAB AC) is employed to encode hevc_intra_flag. As described in U.S. provisional patent application no.

61/916,041, filed December 13, 2013, to Ying Chen and Li Zhang, entitled "Techniques for SDC Coding in 3D-HEVC," values of hevc_intra_flag for left and above neighboring blocks can be used to calculate a context index for selection of an initialization value used to derive an initialized probability state for CAB AC coding of the hevc_intra_flag syntax element. A similar process can be performed to derive the initialization value for determination of the initialized probability state for CAB AC coding of the sdc_flag syntax element. For example, values of sdc_flag for left and above neighboring blocks can be used to calculate a context index for selection of an initialization value used to derive an initialized probability state for CAB AC coding of the sdc_flag syntax element. [0181] As described in U.S. provisional patent application no. 61/916,041, in such a process, nbLeft and nbAbove are the left and above neighboring blocks, respectively, relative to the current prediction unit (PU). In this case, the context index of

hevc_intra_flag (used to decide which probability state should be used and updated in CAB AC coding of hevc_intra_flag) for the current prediction unit is calculated as: hevc_intra_flag of nbLeft + hevc_intra_flag of nbAbove

In this example, the context index for hevc_intra_flag can be 0, 1 or 2. Correspondingly, three context models are used for coding hevc_intra_flag.

[0182] When hevc_intra_flag is equal to 1, HEVC Intra prediction mode is used, and the same signaling method as specified in sub-clause 7.3.8.5 and sub-clause 8.4.2 in HEVC WD 10 applies.

[0183] When hevc_intra_flag is equal to 0, DMM modes are used, and one flag, namely depth_intra_mode, is signaled to indicate whether DMM mode 1 or DMM mode 4 is used. The value of depth_intra_mode equal to 0 means DMM mode 1 is used for the prediction unit, and the value of depth_intra_mode equal to 1 means DMM mode 4 is used for the prediction unit. CAB AC coding is employed to encode depth_intra_mode, and the context index is always set equal to 0 regardless of the value of

depth_intra_mode of nbleft and nbAbove, and only one context model is used for coding depth_intra_mode .

[0184] Furthermore, for each depth coding unit coded in Intra mode, when the partition mode of the coding unit is 2Nx2N, sdc_flag is signaled to indicate whether SDC is used or not. A value of sdc_flag equal to 1 means SDC is used for the coding unit, and sdc_flag equal to 0 means SDC is not used for the coding unit.

[0185] In one example, CAB AC coding is employed to encode sdc_flag, and the context index of sdc_flag is always set equal to 0 regardless of the value of sdc_flag of nbleft and nbAbove is, and only one context model is used for coding sdc_flag. This simple approach of always setting the context index equal to 0 has been adopted in 3D-HEVC.

[0186] In an alternative example, however, as discussed above, the context index of sdc_flag (used to decide which probability state should be used and updated in CAB AC coding of sdc_flag) for the current depth prediction unit could be calculated, in a similar manner to the context index for hevc_intra_flag, as: • sdc_flag of nbLeft + sdc_flag of nbAbove

[0187] There has been some recent progress related to depth Intra mode signaling in 3D- HEVC. For CAB AC coding of the hevc_intra_flag syntax element, as described in U.S. provisional patent application no. 61/916,041, the context index of hevc_intra_flag for the current prediction unit can be calculated by the following two methods.

[0188] In a First Method, described in U.S. provisional patent application no. 61/916,041 referenced above, the context index of hevc_intra_flag for the current prediction unit is determined based on the hevc_intra_flag values for the left neighboring block and the above neighboring block, i.e., the blocks that neighbor the current depth prediction unit to the left of and above the current prediction unit. In this example, as described above, the context index of hevc_intra_flag for the current prediction unit is set equal to:

hevc_intra_flag of nbLeft + hevc_intra_flag of nbAbove

In this example of the First Method, the context index can be 0, 1 or 2. The values of initValue (used as an initialization value to derive the initialized probability state in CABAC coding in the same manner as prescribed by HEVC, i.e., in HEVC WD 10) for different context indexes (denoted as ctxidx) of different slice types are shown in Table 1 below. A B slice refers to a slice that may include intra-predicted blocks, undirectionally inter-predicted blocks, and bidirectionally inter-predicted blocks, a P slice refers to a slice that may include intra-predicted blocks and uni-directional inter-predicted blocks, and an I slice refers to a slice that includes intra-predicted blocks.

Table 1. Values of initValue for ctxidx of hevc_intra_flag.

[0189] A similar approach may be used to derive the initialization value for determining an initialized probability state for CABAC coding sdc_flag, as described in U.S.

provisional patent application no. 61/916,041. In particular, the context index for sdc_flag for a current depth PU to be coded may be calculated as: sdc_flag of nbLeft + sdc_flag of nbAbove

In this case, the value initValue for sdc_flag may be selected for the different context indexes as shown in Table 2 below.

Table 2. Values of initValue for ctxidx of sdc_flag.

[0190] In a Second Method, described in U.S. provisional patent application no.

61/916,041, only one initialized probability state is used and updated for CABAC coding of hevc_intra_flag. The value of the context index of hevc_intra_flag is always set equal to 0, regardless of the value hevc_intra_flag of nbLeft and nbAbove. The values of initValue (used to derive initialized probability state in CABAC coding in the manner defined by HEVC WD 10) for different slice types for hevc_intra_flag are shown in Table 3 below. Table 3. Values of init Value for ctxldx of hevc_intra_flag.

[0191] A similar approach may be used to derive the initialization value (used to derive initialized probability state) for CAB AC coding sdc_flag, as described in U.S.

provisional patent application no. 61/916,041. In particular, the context index for sdc_flag may the same, regardless of the value hevc_intra_flag of nbLeft and nb Above. In this Second Method, the values of initValue (used to derive initialized probability state in CAB AC coding) for different slice types for sdc_flag are shown in Table 4 below.

Table 4. Values of initValue for sdc_flag.

[0192] The First Method described above was adopted into 3D-HEVC for coding hevc_intra_flag because it is believed to achieve better coding performance than the Second Method. The Second Method described above was adopted into 3D-HEVC for coding sdc_intra flag.

[0193] In 3D-HEVC (for the First Method described immediately above), to calculate the context index of hevc_intra_flag for the current prediction unit, the value for

hevc_intra_flag of the above neighboring block (nbAbove) is needed, even when the current prediction unit and the above neighboring block are in different coding tree units (CTUs). Therefore, when encoding the current CTU row (i.e., a row in a unit of the CTU), additional buffer space in memory is required to store the value of hevc_intra_flag of all the above neighboring blocks (e.g., above neighboring 4x4 blocks) of the current CTU row. For example, when the picture width is 1920, a buffer with a size of 480 bits is needed to store the values of hevc_intra_flag of the above neighboring blocks of the current CTU row. This buffer issue also arises in the event the First Method is used for the sdc_flag syntax element. [0194] For the Second Method described immediately above, the context index of hevc_intra_flag is always set equal to 0 and no additional buffer space is needed to store hevc_intra_flag. However, because correlation of hevc_intra_flag between neighboring blocks is not exploited, this Second Method may increase bits spent on hevc_intra_flag. Hence, the Second Method may have less efficient coding performance. Again, this efficiency issue may also arise in the event the Second Method is used for the sdc_flag syntax element.

[0195] In accordance with various examples of this disclosure, video encoder 20 and/or video decoder 30, and more particularly entropy encoding unit 118 and entropy decoding unit 150, may be configured to perform techniques for simplifying the derivation of an initial probability state for CAB AC coding of a syntax element that indicates one or more characteristics of a depth intra prediction mode for a current depth PU. CAB AC coding of the hevc_intra_flag syntax and/or sdc_flag element used in 3D-HEVC depth Intra mode signaling and SDC mode signaling may be simplified, for example, by reducing context model number and/or buffer (used for storing values of hevc_intra_flag and/or sdc_flag) size used for derivation of an initialization value used to determine the initialized probability state.

[0196] Various aspects of the techniques for simplified derivation of the initialized probability sate for CAB AC coding of a syntax element that indicates one or more characteristics of a depth intra prediction mode are described below. In this description, for 3D-HEVC, the hevc_intra_flag syntax element is an example of this type of syntax element that indicates a prediction characteristic of the depth intra prediction mode, and the sdc_flag syntax element is an example of this type of syntax element that indicates a residual characteristic of the depth intra prediction mode.

[0197] In the methods described below, the context index of hevc_intra_flag may be calculated as indicated, yielding two or more context indexes depending on the particular example, and the context index of sdc_flag can be calculated as zero so that there is only one context index. Alternatively, instead of setting the context index to zero for sdc_flag, in some examples, the context index for sdc_flag may be calculated in the same way that the context index for hevc_intra_flag is calculated, i.e., based on values of the syntax element for left and/or above neighboring blocks as described below. Accordingly, the methods described below for calculating the context index for hevc_intra_flag may be applied only for hevc_intra_flag, while the context index for sdc_flag is set to zero, or the methods may be applied to calculate the context indexes for both hevc_intra_flag and sdc_flag in a similar way.

[0198] In all of the following methods, the variable initValue may be used to derive an initialized probablity state for a variable coded with a context model, in the same way as prescribed for HEVC, e.g., in HEVC WD 10. The initialized probability state may be updated in the same manner as in HEVC, e.g., in HEVC WD 10. Hence, the initialized probability state for decoding the syntax element using the CAB AC process may be determined based on the initialization value (e.g., initValue) according to the probability state initialization process defined in the High Efficiency Video Coding (HEVC) standard.

[0199] For example, as described in HEVC WD 10, e.g., in Section 9.3, the variable initValue is used in the initialization of context variables that are assigned to syntax elements. For each context variable, two variables pStateldx and valMps are initialized. The variable pStateldx corresponds to a probability state index and the variable valMps corresponds to the value of the most probable symbol as described in subclause 9.3.4.3 of HEVC WD 10.

[0200] As described in HEVC WD 10, an initialized probability value for a context variable assigned to a syntax element to be CAB AC coded can be determined based on initValue. From the table entry initValue, the two variables slopeldx and offsetldx are derived as follows:

slopeldx = initValue » 4

offsetldx = initValue & 15

[0201] The variables m and n, used in the initialization of context variables, are derived from slopeldx and offsetldx as follows:

m = slopeldx * 5 - 45

n = ( offsetldx « 3 ) - 16

The two values assigned to pStateldx and valMps for the initialization are derived from SliceQpY, which is derived in Equation 7-40 in HEVC WD 10. Given the variables m and n, the initialized probability value is specified as follows:

preCtxState = Clip3( 1, 126, ( ( m * Clip3( 0, 51, SliceQpY ) ) » 4 ) + n ) valMps = ( preCtxState <= 63 ) ? 0 : 1

pStateldx = valMps ? ( preCtxState - 64 ) : ( 63 - preCtxState ) [0202] The initialized probability state for CAB AC coding is defined by two parts:

1. Most probable symbol (MPS). The MPS is stored in valMps, and it can be either 0 or 1.

2. Probability of least probable symbol (LPS). The probability of LPS is stored in the probability state index pStateldx, which can range from 0-63. Each value of pStateldx corresponds to a probability value. For example, pStateldx equal to 0 means the probability of the LPS is 50%.

[0203] Note that, if the MPS is equal to 0, then the LPS is equal to 1, and vice versa. Using the initialized probability value, an entropy coding unit encodes or decodes, as applicable, the syntax element with a CABAC process.

[0204] Third Method. In this Third Method, for example, it is proposed that the value of hevc_intra_flag of the left neighboring depth block is used by encoder 20 and/or decoder 30 to derive a context index of hevc_intra_flag of the current depth prediction unit. The left neighboring depth block may be a depth block that is immediately adjacent to the left of the current depth PU, e.g., as shown in FIGS. 7 A and 7B. Hence, instead of deriving the context index based on the value of hevc_intra_flag of the left neighboring block (nbLeft) and above neighboring block, the context index is derived based on only the value of hevc_intra_flag of the left neighboring block, i.e., without considering a value of the syntax element for an above neighboring depth block of the depth PU. In this way, buffering of the value of hevc_intra_flag of the above neighboring block (iibAbove) is not necessary for derivation of the context index of hevc_intra_flag for the current prediction unit. Hence, when encoding or decoding a syntax element, such as hevc_intra_flag, that indicates one or more characteristics of a depth intra prediction mode for a current prediction unit, video encoder 20 and/or video decoder 30 may use the value of hevc_intra_flag for the left neighboring block exclusively, without using the value of hevc_intra_flag for the above neighboring block, to derive the context index for hevc_intra_flag for the current prediction unit. The same approach may be used to derive the context index for sdc_flag or, alternatively, the context index for sdc_flag may be always set to zero. a) In this example Third Method, the context index of hevc_intra_flag for the

current prediction unit is set equal to 0 when hevc_intra_flag of the left neighboring block is equal to 0, and the context index of hevc_intra_flag for the current prediction unit is set equal to 1 when hevc_intra_flag of the left neighboring block is equal to 1. The context index (e.g., ctxldx) values map to different values of the initialization value. Initialized probability states are provided and updated respectively for the above two cases. b) The value of initValue (used to derive initialized probability state) for different context indexes (denoted as ctxldx) or different slice types may be made different, for hevc_intra_flag, as shown in Table 5 below. The Fifth Method below describes an approach similar to this Third Method, in which the context index for sdc_flag is determined in a manner similar to the context index for

hevc_intra_flag .

Table 5. Values of initValue for hevc_intra_flag.

c) As an alternative to item b above, for this Third Method, one same value of

initValue (used to derive initialized probability state) is used for different context indexes (denoted as ctxldx) and different slice types, for hevc_intra_flag, as shown in Table 6 below.

Table 6. Values of initValue for hevc_intra_flag.

[0205] Fourth Method. As an alternative to the Third Method described above, the value of hevc_intra_flag of both the left neighboring block and the above neighboring block may be used by video encoder 20 and/or video decoder 30 to derive the context index of hevc_intra_flag of the current depth prediction unit. However, when the above neighboring block and the current prediction unit are in two different CTU's, the value of hevc_intra_flag of the above neighboring block is considered to be 0. In other words, the value of hevc_intra_flag of the above neighboring block is set to 0 when the above neighboring block is in a different CTU than the current depth PU.

[0206] Hence, when coding hevc_intra_flag, for a current depth PU, video encoder 20 and/or video decoder 30 uses the value of hevc_intra_flag for the left neighboring block and the value of hevc_intra_flag for the above neighboring block to derive the context index for hevc_intra_flag for the current depth PU (context index = hevc_intrajlag of nbLeft + hevc_intrajlag ofnbAbove), but sets the value of hevc_intra_flag to 0 for the above neighboring block when the above neighboring block is in a different CTU than the current prediction unit.

[0207] In this case, it is not necessary to buffer the value of hevc_intra_flag for the above block when the above block is in a different CTU, relative to the current prediction unit, because it is assumed to be zero. In some examples, video encoder 20 and/or video decoder 30 may be configured to not buffer the value of the syntax element for the above neighboring depth block of the depth prediction unit at least when the above neighboring depth block and the depth prediction unit do not reside in the same coding tree unit (CTU). The same approach may be used to derive the context index for sdc_flag as described in the Sixth Method below, or, alternatively, the context index for sdc_flag may be set to zero. a) To calculate the context index for hevc_intra_flag, in this Fourth Method, video encoder 20 and/or video decoder 30 determine whether the above neighboring block and the current prediction unit are in two different CTUs, e.g., as shown in FIG. 7B. If not, i.e., the above neighboring block and current depth PU are in the same CTU, as shown in FIG 7A, the context index is calculated as context index = hevc_intrajlag of nbLeft + hevc_intrajlag ofnbAbove, and the actual value of hevc_intra_flag for nbAbove is used. If so, i.e., the above neighboring block and the current depth PU are in different CTUs, the value of hevc_intra_flag of the above neighboring block is set equal to 0 for purposes of deriving the context index for hevc_intra_flag of the current depth PU. Hence, in this case, it can be considered that the context index is calculated as context index = hevc_intrajlag ofnbLeft + hevc_intra_flag of nb Above, where hevc_intra_flag of nb Above is set to zero, or as context index = hevc_intra_flag ofnbLeft + 0. b) In either case, in this Fourth Method, whether the value of hevc_intra_flag for the above neighboring depth block is set to 0 or not, the context index of

hevc_intra_flag of the current prediction unit is set equal to 0 when the values of hevc_intra_flag of the left neighboring block and the above neighboring block are both equal to 0; the context index of hevc_intra_flag of the current prediction unit is set equal to 1 when hevc_intra_flag of only one neighboring block (either the left neighboring block or the above neighboring block) is equal to 0; and the context index of hevc_intra_flag of the current prediction unit is set equal to 2 when the values of hevc_intra_flag of both the left neighboring block and the above neighboring block are equal to 1. If the value of hevc_intra_flag of the above neighboring depth block is set to zero, because the above neighboring block is in a different CTU than the current depth PU, then only context index 0 or context index 1 are possible. If the above neighboring depth block is in the same CTU as the current depth PU, then context 0, context 1, or context 2 are possible, depending on actual values for the neighboring boocks. Initialized probability states are provided and updated respectively for the above three cases. c) In this Fourth Method, the values of initValue (used to derive initialized

probability state) for different context indexes (denoted as ctxidx) or different slice types may be made different, for hevc_intra_flag, as shown in Table 7 below.

Table 7. Values of initValue for hevc_intra_flag.

d) Alternatively, in this Fourth Method, one same value of initValue (used to derive initialized probability state) may be used by video encoder 20 and/or video decoder 30 for the different context indexes (denoted as ctxidx) and different slice types, for hevc_intra_flag, as shown in Table 8 below. Table 8. Values of init Value for hevc_intra_flag.

[0208] Fifth Method. In an example Fifth Method, it is proposed that the value of sdc_flag (as an example of a syntax element that indicates a residual coding characteristic of a depth intra prediction mode) of the left neighboring block is used by video encoder 20 and/or video decoder 30 to derive the context index of sdc_flag of the current depth prediction unit. In this example, an approach similar to that outlined above in the Third Method for hevc_intra_flag may be used in this Fifth Method for sdc_flag. For exmaple, the context index is derived based on only the value of sdc_flag of the left neighboring block, i.e., without considering a value of the syntax element for an above neighboring depth block of the depth PU. The Third Method and Fifth Method may be used together in some examples to code hevc_intra_flag and sdc_flag. a) In this example Fifth Method, the context index of sdc_flag is set equal to 0 when the value of sdc_flag of the left neighboring block is equal to 0, and the context index of sdc_flag is set equal to 1 when the value of sdc_flag of the left neighboring block is equal to 1. Initialized probability states are provided and updated respectively for the above two cases. b) The value of initValue (used to derive initialized probability state) for different context indexes (denoted as ctxldx) or different slice types is made different, for sdc_flag, as shown in Table 9.

Table 9. Values of initValue for sdc_flag. ctxidx

(context

index) 0 1

B slice 214 229

P slice 215 202

I slice 213 201 c) As an alternative to item b above, one same value of initValue (used to derive initialized probability state) is used for different context indexes (denoted as ctxidx) and different slice types, for sdc_flag, as shown in Table 10 below.

Table 10. Values of initValue for sdc_flag.

[0209] Sixth Method. As an alternative to the Fifth Method above, sdc_flag of both the left neighboring block and the above neighboring block are used to derive the context index of sdc_flag of the current depth prediction unit. As in the Fourth Method above for hevc_intra_flag, however, when the above neighboring block and the current prediction unit are in two different CTUs, the value of sdc_flag of the above neighboring block is considered to be 0. In other words, the value of sdc_flag of the above neighboring block is set to 0 when the above neighboring block is in a different CTU than the current depth PU.

[0210] Hence, when coding the sdc_flag for a current depth PU, video encoder 20 and/or video decider 30 uses the value of sdc_flag for the left neighboring block and the value of sdc_flag for the above neighboring block to derive the context index for sdc_flag for the current depth PU (context index = sdc_flag ofnbLeft + sdc_flag of nb Above), but sets the value of sdc_flag to 0 for the above neighboring block when the above neighboring block is in a different CTU than the current depth prediction unit.

[0211] In this manner, video encoder 20 and/or video decoder 30 can avoid buffering the value of sdc_flag for the above neighboring depth block when the above neighboring depth block is in a different CTU, relative to the current depth prediction unit, because it is assumed to be zero. In some examples, video encoder 20 and/or video decoder 30 may be configured to not buffer the value of the syntax element for the above neighboring depth block of the depth prediction unit at least when the above neighboring depth block and the depth prediction unit do not reside in the same coding tree unit (CTU).

a) To calculate the context index for sdc_flag, in this Sixth Method, video

encoder 20 and/or video decoder 30 determine whether the above neighboring block and the current prediction unit are in two different CTUs. If not, i.e., the above neighboring block and current depth PU are in the same CTU, the context index for sdc_flag is calculated as context index = sdcjlag ofnbLeft + sdcjlag ofnbAbove, and the actual value of sdc_flag for nbAbove is used. If so, i.e., the above neighboring block and the current depth PU are in different CTUs, the value of sdc_flag of the above neighboring block is set equal to 0 for purposes of deriving the context index for sdc_flag of the current depth prediction unit. Hence, in this case, it can be considered that the context index is calculated as context index = sdc_flag ofnbLeft + sdc_flag of nbAbove, where sdc_flag of nbAbove is set to zero, or as context index = sdcjlag ofnbLeft + 0. b) In either case, in this Sixth Method, wherein the value of sdc_flag for the above neighboring depth block is set to 0 or not, the context index of sdc_flag is set equal to 0 when sdc_flag of both the left neighboring block and the above neighboring block are equal to 0; the context index of sdc_flag is set equal to 1 when sdc_flag of only one neighboring block (either the left neighboring block or the above neighboring block) is equal to 0; and the context index of sdc_flag is set equal to 2 when sdc_flag of both the left neighboring block and the above neighboring block are equal to 1. Initialized probability states are provided and updated respectively for the above three cases. c) The value of initValue (used to derive initialized probability state) for different context indexes (denoted as ctxldx) or different slice types may be made different, as shown in Table 11.

Table 11. Values of initValue for sdc_flag. ctxidx

(context

index) 0 1 2

B slice 214 229 230

P slice 215 202 174

I slice 213 201 246 d) Alternatively, one same initValue (used to derive initialized probability state) may be used by video encoder 20 and/or video decoder 30 for sdc_flag for different context indexes (denoted as ctxidx) and different slice types, as shown in Table 12.

Table 12. Values of initValue for sdc_flag.

[0212] The Third Method and Fourth Method for deriving the context index for hevc_intra_flag may be used in conjunction with the Fifth Method and Sixth Method for deriving the context index for sdc_flag. In some examples, the Third Method and Fourth Method for deriving the context index for hevc_intra_flag may be used in conjunction with the Second Method for deriving the context index for sdc_flag, i.e., where the context is set to zero for sdc_flag.

[0213] As discussed above, the coding techniques described in this disclosure for deriving a context index may be performed for both hevc_intra_flag and sdc_flag.

Alternatively, such techniques may be applied for hevc_intra_flag, and a different technique may be used for sdc_flag, such as always setting the context index for sdc_flag to zero per Method 2 above without regard to values of sdc_flag for left or above neighboring depth blocks.

[0214] The various coding techniques described in this disclosure may be performed by video encoder 20 (FIGS. 2 and 5) and/or video decoder 30 (FIGS. 2 and 6), both of which may be generally referred to as a video coder. In addition, video coding may generally refer to video encoding and/or video decoding, as applicable.

[0215] FIG. 7A is a diagram illustrating an example of a depth prediction unit (PU) and left and above neighboring depth blocks within a CTU to be coded. FIG. 7B is a diagram illustrating an example of a depth prediction unit (PU) and a left neighboring depth block within a CTU to be coded and adjacent to an above neighboring depth block in a different CTU. In the examples of FIGS. 7A and 7B, a current depth PU to be coded is an 8x8 sample block that resides with a current CTU. A left neighboring block and an above neighboring block are positioned adjacent to the current depth PU within the current CTU.

[0216] In the examples of FIGS. 7 A and 7B, the above neighboring block for the current depth PU is the block (with size equal to the smallest transform block size specified in an applicable sequence parameter set (SPS), e.g., 4x4 samples) covering corner pixel 164. Also, in this example, the left neighboring block for the current depth PU is the block (with size equal to the smallest transform block size specified in the SPS set, e.g., 4x4 samples) covering corner pixel 166. The current CTU is defined by boundary 168.

[0217] In the example, FIG 7A, the above neighboring depth block and the depth prediction unit reside in the same CTU. In the example of FIG. 7B, the above

neighboring depth block block and the depth prediction unit do not reside in the same CTU. Rather, the above neighboring depth block and the current depth prediction unit are in different CTU's divided by CTU boundary 168. When the above neighboring block is in a different CTU, the value of hevc_intra_flag and/or sdc_flag may need to be buffered for determination of a context index for CAB AC coding. In accordance with some techniques described in this disclosure, the context index may be derived in a manner that reduces or avoids this additional buffering requirement.

[0218] FIG. 8 is a flow diagram illustrating a video decoding method in accordance with an example of the disclosure. As shown in FIG. 8, video decoder 30 may be configured to perform operations 170-176. In some examples, entropy decoding unit 150 of video decoder 30 may be configured to receive an encoded video data bitstream (170) with a syntax element to be decoded for a current depth PU. Video decoder 30 may use the Third Method or Fifth Method, described above, to decode the syntax element.

[0219] The syntax element decoded by video decoder 30 indicates one or more characteristics of a depth intra prediction mode for the depth PU, such as a prediction characteristic or a residual coding characteristic. For example, the syntax element (e.g., hevc_intra_flag) may indicate a prediction characteristic indicating whether a DMM mode or a non-DMM mode, such as a regular HEVC intra mode, is to be used as a depth intra prediction mode for the depth PU.

[0220] Alternatively, the syntax element (e.g., sdc_flag) may indicate a residual coding characteristic indicating whether an SDC mode or non-SDC mode, such as a regular residual quadtree mode, is to be used as an intra prediction mode for the depth PU. As a further alternative, video decoder 30 may decode first and second syntax elements (e.g., hevc_intra_flag and sdc_flag) that respectively provide the indications above.

[0221] Entropy decoding unit 150 may determine an initialization value for derivation of an initialized probability state for a CAB AC process based on a value of the syntax element for a left neighboring depth block (NB_LEFT) of a depth prediction unit and without considering a value of the syntax element for an above neighboring depth block (NB_ABOVE) of the depth prediction unit (172). The initialization value may be derived, for example, using either of the Third Method or Fifth Method, as applicable, for hevc_intra_flag and/or sdc_flag. The initialized probability value may be determined based on the initialization value (e.g., initValue) according to HEVC WD 10. Entropy decoding unit 150 may decode the syntax element for the current depth PU from the encoded video data bitstream using a CAB AC process with the initialized probability state (174).

[0222] Upon decoding the syntax element, prediction processing unit 152 may decode the depth PU using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth prediction unit (176). For example, if the syntax elements indicate a DMM mode and SDC mode, video decoder 30 may use a DMM mode for prediction and an SDC mode for residual coding. The particular DMM mode may be indicated by a further syntax element such as depth_intra_mode in 3D- HEVC.

[0223] In this example, the context index of the syntax element (hevc_intra_flag or sdc_flag) decoded by video decoder 30 for the current PU is set equal to 0 when the value of the syntax element for the left neighboring block is equal to 0, and the context index of the syntax element for the current PU is set equal to 1 when the value of the syntax element for the left neighboring block is equal to 1. [0224] FIG. 9 is a flow diagram illustrating a video encoding method in accordance with an example of the disclosure including operations 178-182. As shown in FIG. 9, video encoder 20 may encode a depth PU using one or more selected characteristics of a depth intra prediction mode (178), such as characteristics specifying whether DMM or non- DMM modes are used, and/or characteristics indicating whether SDC or non-SDC residual coding modes are used. Video decoder 30 may use the Third Method or Fifth Method, as described above, to decode the syntax element.

[0225] Video encoder 20 may determine an initialization value for derivation of the initialized probability state for a CAB AC process to be used to entropy encode the syntax element based on the value of the syntax element for a left neighboring depth block (NB_LEFT), and without considering a value of the syntax element for an above neighboring depth block (NB_ABOVE) (180). Using the CAB AC process with the initialized probability state, video encoder 20 then may encode the syntax element for the current depth PU (182). Video encoder 20 includes the encoded depth PU and the encoded syntax element in an encoded video data bitstream.

[0226] In the example of FIG. 9, the syntax element indicates one or more characteristics of a depth intra prediction mode for the depth PU, such as a prediction characteristic or a residual coding characteristic. For example, the syntax element (e.g., hevc_intra_flag) encoded by video encoder 20 may indicate whether a DMM mode or a non-DMM mode, such as a regular HEVC intra mode, is to be used as a depth intra prediction mode for the depth PU. Alternatively, the syntax element (e.g., sdc_flag) encoded by video encoder 20 may indicate whether an SDC mode or non-SDC mode, such as a regular residual quadtree mode, is to be used as an intra prediction mode for the depth PU. As a further alternative, video encoder 20 may decode first and second syntax elements (e.g., hevc_intra_flag and sdc_flag) that respectively provide the indications above.

[0227] In this example, the context index of the syntax element (hevc_intra_flag or sdc_flag) encoded by video encoder 20 for the current PU is set equal to 0 when the value of the syntax element for the left neighboring block is equal to 0, and the context index of the syntax element for the current PU is set equal to 1 when the value of the syntax element for the left neighboring block is equal to 1.

[0228] FIG. 10 is a flow diagram illustrating a video decoding method in accordance with another example of the disclosure including operations 184-192. As shown in FIG. 10, video decoder 30 receives an encoded video data bitstream (184). Video decoder 30 may use the Fourth Method or the Sixth Method, described above, to decode the syntax element.

[0229] To decode a syntax element for a current depth PU, video decoder 30 determines whether the above neighboring depth block (NB_ABOVE) is in a different CTU than the current depth PU (186). If so, video decoder 30 determines an initialization value for derivation of an initialized probability state for the CAB AC process to be used to code the syntax element based on an actual value of the syntax element for the neighboring left depth block (NB_LEFT) and an assumed value of zero for the syntax element for the neighboring above depth block (NB_ ABOVE) (188). If the above neighboring depth block (NB_ABOVE) is not in a different CTU than the current depth PU (i.e., it is in the same CTU), video decoder 30 determines the initialization value for derivation of the initialized probability state for the CAB AC process to be used to code the syntax element based on an actual value of the syntax element for the neighboring left depth block (NB_LEFT) and an actual value of the syntax element for the neighboring above depth block (NB_ABOVE) (190).

[0230] As further shown in FIG. 10, video decoder 30 decodes the syntax element for the current depth PU using the CAB AC process with the initialized probability state (192), and decodes the depth PU based on one or more characteristics of the depth intra prediction mode indicated by the syntax element (194).

[0231] Again, the syntax element (e.g., hevc_intra_flag) decoded by video decoder 3020 may indicate whether a DMM mode or a non-DMM mode, such as a regular HEVC intra mode, is to be used as a depth intra prediction mode for the depth PU. Alternatively, the syntax element (e.g., sdc_flag) decoded by video decoder 30 may indicate whether an SDC mode or non-SDC mode, such as a regular residual quadtree mode, is to be used as an intra prediction mode for the depth PU. As a further alternative, video decoder 30 may decode first and second syntax elements (e.g., hevc_intra_flag and sdc_flag) that respectively provide the indications above.

[0232] In this example, the context index of the syntax element (e.g., hevc_intra_flag or sdc_flag) decoded by video decoder 30 for the current depth PU is set equal to 0 when the values of the syntax element for the left neighboring block and the above neighboring block are both equal to 0; the context index of the syntax element of the current PU is set equal to 1 when the value of the syntax element of only one neighboring block (either the left neighboring block or the above neighboring block) is equal to 0; and the context index of the syntax element of the current PU is set equal to 2 when the values of the syntax element of both the left neighboring block and the above neighboring block are equal to 1.

[0233] FIG. 11 is a flow diagram illustrating a video encoding method in accordance with another example of the disclosure including operations 196-204. As shown in FIG. 11, video encoder 20 may encode a depth PU with one or more selected characteristics of a depth intra prediction mode (196). Video encoder 20 also may encode a syntax element to indicate the one or more characteristics of the depth intra prediction mode. Video encoder 20 may use the Fourth Method or Sixth Method, described above, to encode the syntax element.

[0234] To encode the syntax element for the current depth PU, video encoder 20 determines whether the above neighboring depth block (NB_ABOVE) is in a different CTU than the current depth PU (198). If so, video encoder 20 determines an

initialization value for derivation of an initialized probability state for the CAB AC process to be used to code the syntax element based on an actual value of the syntax element for the neighboring left depth block (NB_LEFT) and an assumed value of zero for the syntax element for the neighboring above depth block (NB_ABOVE) (200). If the above neighboring depth block (NB_ABOVE) is not in a different CTU than the current depth PU (i.e., it is in the same CTU), video encoder 20 determines the initialization value for derivation of the initialized probability state for the CAB AC process to be used to code the syntax element based on an actual value of the syntax element for the neighboring left depth block (NB_LEFT) and an actual value of the syntax element for the neighboring above depth block (NB_ABOVE) (202). As further shown in FIG. 11, video encoder 20 encodes the syntax element for the current depth PU using the CABAC process with the initialized probability state (204).

[0235] In this example, the context index of the syntax element (e.g., hevc_intra_flag or sdc_flag) encoded by video encoder 20 for the current depth PU is set equal to 0 when the values of the syntax element for the left neighboring block and the above neighboring block are both equal to 0; the context index of the syntax element of the current PU is set equal to 1 when the value of the syntax element of only one neighboring block (either the left neighboring block or the above neighboring block) is equal to 0; and the context index of the syntax element of the current PU is set equal to 2 when the values of the syntax element of both the left neighboring block and the above neighboring block are equal to 1.

[0236] Again, the syntax element (e.g., hevc_intra_flag) decoded by video encoder 20 may indicate whether a DMM mode or a non-DMM mode, such as a regular HEVC intra mode, is to be used as a depth intra prediction mode for the depth PU. Alternatively, the syntax element (e.g., sdc_flag) decoded by video encoder 20 may indicate whether an SDC mode or non-SDC mode, such as a regular residual quadtree mode, is to be used as an intra prediction mode for the depth PU. As a further alternative, video encoder 20 may encode first and second syntax elements (e.g., hevc_intra_flag and sdc_flag) that respectively provide both of the indications above.

[0237] In operation, video encoder 20 and/or video decoder 30 may encode/decode a syntax element using a context-based binary arithmetic coding (CAB AC) process. A value of the syntax element indicates one or more characteristics of a depth intra prediction mode for a depth prediction unit encoded by video encoder 20 or decoded by video decoder 30.

[0238] In implementing the Third Method or Fifth Method, video encoder 20 and/or video decoder 30 may encode/decode a syntax element using a CAB AC process with an initialized probability state for the CAB AC process that is determined based on a value of the syntax element for a left neighboring depth block of a depth PU and without considering a value of the syntax element for an above neighboring depth block of the depth PU, video encoder 20 and/or video decoder 30 encodes/decodes the depth PU using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth PU.

[0239] In implementing the Fourth Method or Sixth Method, video encoder 20 and/or video decoder 30 may encode/decode a syntax element using a CAB AC process with an initialized probability state for the CAB AC process that is determined based on (a) a value of the syntax element for a left neighboring depth block of a depth PU and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth PU reside in the same coding tree unit (CTU), and (b) a value of the syntax element for a left neighboring depth block of a depth PU and an assumed value of zero for the syntax element for the above neighboring depth block of the depth PU when the above neighboring depth block and the depth PU do not reside in the same CTU. [0240] Video encoder 20 and/or video decoder 30 may be configured to performa any of the methods described in this disclosure. For example, video encoder 20 and/or video decoder 30 encodes/decodes the depth PU using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth PU. To determine the initialized probability state, video encoder 20 and/or video decoder 30 may determine a context index based on the value of the syntax element for the left neighboring depth block of the depth PU, and determine an initialization value based on the determined context index. In some examples, video encoder 20 and/or video decoder 30 may determine the initialized probability state for coding the syntax element based on the initialization value. The video encoder 20 and/or video decoder 30 then uses the initialized probability to CAB AC encode/decode the syntax element.

[0241] According to the Third Method or Fifth Method, the context index derived by video encoder 20 and/or video decoder 30 is equal to 0 when the value of the syntax element for the left neighboring block is equal to 0, and equal to 1 when the value of the syntax element for the left neighboring block is equal to 1. According to the Fourth Method and Sixth Method, the context index derived by video encoder 20 and/or video decoder 30 is equal to 0 when the value of the syntax element for the left neighboring block and the value of the syntax element for the above neighboring block are equal to 0, equal to 1 when only the value of the syntax element for the left neighboring block or the value of the syntax element for the above neighboring block is equal to 1, and equal to 2 when both the value of the syntax element for the left neighboring block and the value of the syntax element for the above neighboring block are equal to 1.

[0242] To determine the initialization value, video encoder 20 and/or video decoder 30 may determine different initialization values for different values of the context index. The different initialization values for the different values of the context index may be different for different slice types (e.g., B, P and I) applicable to the depth prediction unit.

[0243] In some examples, the characteristic indicated by the syntax element may comprise a prediction characteristic of the depth intra prediction mode such as whether a DMM or non-DMM Mode is to be used for the depth prediction unit (e.g.,

hevc_intra_flag). In other examples, the characteristic indicated by the syntax element may comprise a residual characteristic such as whether SDC or non-SDC coding is to be used for the depth prediction unit (e.g., sdc_flag). In additional examples, video encoder 20 and/or video decoder 30 30 may encode/decode both a syntax element indicating a prediction characteristic and a syntax element indicating a residual characteristic of a depth intra prediction mode, e.g., hevc_intra_flag and sdc_flag syntax elements.

[0244] While the techniques of this disclosure are generally described with respect to 3D-HEVC, the techniques are not limited in this way. The techniques described above may also be applicable to other current standards or future standards for 3D video coding. For example, the techniques described in this disclosure for entropy coding may also be applicable to other current or future standards involving coding of depth Intra modes for depth partitions, e.g., for 3D video coding or other applications.

[0245] In one or more examples, the functions described herein may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit.

Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.

[0246] By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transient media, but are instead directed to non-transient, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0247] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term "processor," as used herein may refer to any of the foregoing structure or any other structure suitable for

implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements.

[0248] The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware.

[0249] Various examples have been described. These and other examples are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method of video decoding, the method comprising:

receiving an encoded video data bitstream;

decoding a syntax element from the encoded video data bitstream using a context- based binary arithmetic coding (CABAC) process, wherein a value of the syntax element indicates one or more characteristics of a depth intra prediction mode for a depth prediction unit;

determining an initialization value for determination of an initialized probability state for decoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and without considering a value of the syntax element for an above neighboring depth block of the depth prediction unit; and

decoding the depth prediction unit using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth prediction unit.

2. The method of claim 1, wherein determining the initialization value comprises: determining a context index based on the value of the syntax element for the left neighboring depth block of the depth prediction unit; and

determining the initialization value based on the determined context index.

3. The method of claim 2, wherein determining the context index comprises determining the context index to be equal to 0 when the value of the syntax element for the left neighboring block is equal to 0, and determining the context index to be equal to 1 when the value of the syntax element for the left neighboring block is equal to 1.

4. The method of claim 2 or 3, wherein determining the initialization value comprises determining different initialization values for different values of the context index, and wherein the different initialization values for the different values of the context index are different for different slice types applicable to the depth prediction unit.

5. The method of claim 4, wherein the characteristic comprises a prediction characteristic of the depth intra prediction mode, and wherein the prediction characteristic comprises whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit.

6. The method of claim 5, wherein the syntax element comprises an hevc_intra_flag syntax element, and wherein the initialization values for the different values of the context index and the different slice types are 154, 141, and 155 for B, P, and I slices, respectively, for a value of 0 for the context index, and 155, 185 and 170 for B, P and I slices, respectively, for a value of 1 for the context index.

7. The method of claim 4, wherein the characteristic comprises a residual characteristic of the depth intra prediction mode, and wherein the residual characteristic comprises whether segment- wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit.

8. The method of claim 7, wherein the syntax element comprises an sdc_flag syntax element, and wherein the initialization values for the different values of the context index and the different slice types are 214, 215, and 213 for B, P, and I slices, respectively, for a value of 0 for the context index, and 229, 202, and 201 for B, P and I slices, respectively, for a value of 1 for the context index.

9. The method of claim 1, wherein the characteristic comprises a first characteristic indicating whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit and a second characteristic indicating whether segment- wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit, and wherein decoding a syntax element comprises decoding a first syntax element indicating the first characteristic and a second syntax element indicating the second characteristic.

10. The method of any of claims 2-9, further comprising determining the initialized probability state based on the initialization value according to a probability state initialization process as defined in the High Efficiency Video Coding (HEVC) standard.

11. The method of any of claims 1-10, wherein the encoded video data bitstream is a Three-dimensional High Efficiency Video Coding (3D-HEVC) encoded video data bitstream.

12. A method of video encoding, the method comprising:

encoding a depth prediction unit using a selected characteristic of a depth intra prediction mode;

encoding a syntax element using a context-based binary arithmetic coding (CABAC) process, wherein a value of the syntax element indicates the selected characteristic of the depth intra prediction mode for the depth prediction unit; and

determining an initialization value for determination of an initialized probability state for encoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and without considering a value of the syntax element for an above neighboring depth block of the depth prediction unit.

13. The method of claim 12, wherein determining the initialization value comprises: determining a context index based on the value of the syntax element for the left neighboring depth block of the depth prediction unit; and

determining the initialization value based on the determined context index.

14. The method of claim 13, wherein determining the context index comprises determining the context index to be equal to 0 when the value of the syntax element for the left neighboring block is equal to 0, and determining the context index to be equal to 1 when the value of the syntax element for the left neighboring block is equal to 1.

15. The method of claim 13 or 14, wherein determining the initialization value comprises determining different initialization values for different values of the context index, and wherein the different initialization values for the different values of the context index are different for different slice types applicable to the depth prediction unit.

16. The method of claim 15, wherein the characteristic comprises a prediction characteristic of the depth intra prediction mode, and wherein the prediction characteristic comprises whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit.

17. The method of claim 16, wherein the syntax element comprises an

hevc_intra_flag syntax element, and wherein the initialization values for the different values of the context index and the different slice types are 154, 141, and 155 for B, P, and I slices, respectively, for a value of 0 for the context index, and 155, 185 and 170 for B, P and I slices, respectively, for a value of 1 for the context index.

18. The method of claim 15, wherein the characteristic comprises a residual characteristic of the depth intra prediction mode, and wherein the residual characteristic comprises whether segment-wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit,.

19. The method of claim 18, wherein the syntax element comprises an sdc_flag syntax element, and wherein the initialization values for the different values of the context index and the different slice types are 214, 215, and 213 for B, P, and I slices, respectively, for a value of 0 for the context index, and 229, 202, and 201 for B, P and I slices, respectively, for a value of 1 for the context index.

20. The method of claim 12, wherein the selected characteristic comprises a first characteristic indicating whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit and a second characteristic indicating whether segment- wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit, and wherein encoding a syntax element comprises encoding a first syntax element indicating the first characteristic and a second syntax element indicating the second characteristic.

21. The method of any of claims 13-20, further comprising determining the initialized probability state based on the initialization value according to a probability state initialization process as defined in the High Efficiency Video Coding (HEVC) standard.

22. The method of any of claims 12-21, wherein the encoded video data bitstream is a Three-dimensional High Efficiency Video Coding (3D-HEVC) encoded video data bitstream.

23. A video coding apparatus comprising:

a memory storing video data; and

a video coder comprising one or more processors configured to perform the method of any of claims 1-22.

24. A computer-readable medium comprising instructions that, upon execution, cause one of or more processors to perform the method of any of claims 1-22.

25. A video coding apparatus comprising means for performing the method of any of claims 1-22.

26. A method of video decoding, the method comprising:

receiving an encoded video data bitstream;

determining an initialization value for determination of an initialized probability state for decoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same coding tree unit (CTU);

determining the initialization value for determination of the initialized probability state for decoding the syntax element using the CABAC process based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU; and decoding the depth prediction unit using the characteristic of the depth intra prediction mode indicated by the value of the syntax element for the depth prediction unit.

27. The method of claim 26, wherein determining the initialization value comprises: determining a context index based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above

neighboring depth block and the depth prediction unit reside in the same CTU;

determining a context index based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU;

determining the initialization value based on the determined context index.

28. The method of claim 27, wherein determining the context index comprises determining the context index to be equal to 0 when the value of the syntax element for the left neighboring block and the value of the syntax element for the above neighboring block are equal to 0, determining the context index to be equal to 1 when only the value of the syntax element for the left neighboring block or the value of the syntax element for the above neighboring block is equal to 1, and determining the context index to be equal to 2 when both the value of the syntax element for the left neighboring block and the value of the syntax element for the above neighboring block are equal to 1.

29. The method of claim 27 or 28, wherein determining the initialization value comprises determining different initialization values for different values of the context index, and wherein the different initialization values for the different values of the context index are different for different slice types applicable to the depth prediction unit.

30. The method of any of claims 29, wherein the characteristic comprises a prediction characteristic of the depth intra prediction mode, and wherein the prediction

characteristic comprises whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit.

31. The method of claim 30, wherein the syntax element comprises an

hevc_intra_flag syntax element, and wherein the initialization values for the different values of the context index and the different slice types are 154, 141, and 155 for B, P, and I slices, respectively, for a value of 0 for the context index, 155, 185 and 170 for B, P and I slices, respectively, for a value of 1 for the context index, and 156, 214 and 157 for B, P and I slices, respectively, for a value of 2 for the context index.

32. The method of claim 29, wherein the characteristic comprises a residual characteristic of the depth intra prediction mode, and wherein the residual characteristic comprises whether segment-wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit.

33. The method of claim 32, wherein the syntax element comprises an sdc_flag syntax element, wherein the initialization values for the different values of the context index and the different slice types are 214, 215, and 213 for B, P, and I slices,

respectively, for a value of 0 for the context index, 229, 202, and 201 for B, P and I slices, respectively, for a value of 1 for the context index, and 230, 174 and 246 for B, P and I slices, respectively, for a value of 2 for the context index.

34. The method of claim 26, wherein the characteristic comprises a first characteristic indicating whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit and a second characteristic indicating whether segment- wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit, and wherein decoding a syntax element comprises decoding a first syntax element indicating the first characteristic and a second syntax element indicating the second characteristic.

35. The method of any of claims 27-34, further comprising determining the initialized probability state based on the initialization value according to a probability state initialization process as defined in the High Efficiency Video Coding (HEVC) standard.

36. The method of any of claims 26-35, wherein the encoded video data bitstream is a Three-dimensional High Efficiency Video Coding (3D-HEVC) encoded video data bitstream, and the syntax element is an hevc_intra_flag syntax element.

37. A method of video encoding, the method comprising:

encoding a syntax element from the encoded video data bitstream using a context- based binary arithmetic coding (CABAC) process, wherein a value of the syntax element indicates the selected characteristic of a depth intra prediction mode for a depth prediction unit;

determining an initialization value for determination of an initialized probability state for encoding the syntax element using the CABAC process based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same coding tree unit (CTU); and

determining the initialization value for determination of the initialized probability state for encoding the syntax element using the CABAC process based on the value of the syntax element for a left neighboring depth block of a depth prediction unit and an assumed value of zero for the syntax element for the above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit do not reside in the same CTU.

38. The method of claim 37, wherein determining the initialization value comprises: determining a context index based on a value of the syntax element for a left neighboring depth block of a depth prediction unit and a value of the syntax element for an above neighboring depth block of the depth prediction unit when the above neighboring depth block and the depth prediction unit reside in the same CTU;

determining the initialization value based on the determined context index.

39. The method of claim 38, wherein determining the context index comprises determining the context index to be equal to 0 when the value of the syntax element for the left neighboring block and the value of the syntax element for the above neighboring block are equal to 0, determining the context index to be equal to 1 when only the value of the syntax element for the left neighboring block or the value of the syntax element for the above neighboring block is equal to 1, and determining the context index to be equal to 2 when both the value of the syntax element for the left neighboring block and the value of the syntax element for the above neighboring block are equal to 1.

40. The method of claim 38 or 39, wherein determining the initialization value comprises determining different initialization values for different values of the context index, and wherein the different initialization values for the different values of the context index are different for different slice types applicable to the depth prediction unit.

41. The method of claim 40, wherein the characteristic comprises a prediction characteristic of the depth intra prediction mode, and wherein the prediction

42. The method of claim 41, wherein the syntax element comprises an

hevc_intra_flag syntax element, wherein the initialization values for the different values of the context index and the different slice types are 154, 141, and 155 for B, P, and I slices, respectively, for a value of 0 for the context index, 155, 185 and 170 for B, P and I slices, respectively, for a value of 1 for the context index, and 156, 214 and 157 for B, P and I slices, respectively, for a value of 2 for the context index.

43. The method of claim 40, wherein the characteristic comprises a residual characteristic of the depth intra prediction mode, and wherein the residual characteristic comprises whether segment-wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit,.

44. The method of claim 43, wherein the syntax element comprises an sdc_flag syntax element, wherein the initialization values for the different values of the context index and the different slice types are 214, 215, and 213 for B, P, and I slices,

45. The method of claim 37, wherein the selected characteristic comprises a first characteristic indicating whether a depth modeling mode (DMM) or a non-DMM Mode is to be used for the depth prediction unit and a second characteristic indicating whether segment- wise DC coding (SDC) or non-SDC coding is to be used for the depth prediction unit, and wherein encoding a syntax element comprises encoding a first syntax element indicating the first characteristic and a second syntax element indicating the second characteristic.

46. The method of any of claims 38-45, further comprising determining the initialized probability state based on the initialization value according to a probability state initialization process as defined in the High Efficiency Video Coding (HEVC) standard.

47. The method of any of claims 37-46, wherein the encoded video data bitstream is a Three-dimensional High Efficiency Video Coding (3D-HEVC) encoded video data bitstream, and the syntax element is an hevc_intra_flag syntax element.

48. A video coding apparatus comprising:

a memory storing video data; and

a video coder comprising one or more processors configured to perform the method of any of claims 26-47.

49. A computer-readable medium comprising instructions that, upon execution, cause one of or more processors to perform the method of any of claims 26-47.

50. A video coding apparatus comprising means for performing the method of any of claims 26-47.