WO2018172609A2

WO2018172609A2 - Motion compensation in video encoding and decoding

Info

Publication number: WO2018172609A2
Application number: PCT/FI2018/050193
Authority: WO
Inventors: Jani Lainema
Original assignee: Nokia Technologies Oy
Priority date: 2017-03-21
Filing date: 2018-03-16
Publication date: 2018-09-27
Also published as: WO2018172609A3

Abstract

There are disclosed various methods, apparatuses and computer program products for video encoding and decoding. In some embodiments for encoding.In some embodiments for decoding or encoding a block of a current picture a first motion vector predictor candidate pointing to a first reference block in a first reference picture and a second motion vector predictor candidate pointing to a second reference block in a second reference picture are obtained. A difference vector is added to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate and subtracted from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate. A cost estimate is determined for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate. The addition, subtraction and cost estimate determination may be repeated to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates. A first motion vector predictor is selected from the set of modified first motion vector predictor candidates and a second motion vector predictor is selected from the set of modified second motion vector predictor candidates on the basis of the set of cost estimates.

Description

MOTION COMPENSATION IN VIDEO ENCODING AND DECODING TECHNICAL FIELD

The present application relates generally to encoding and decoding of media presentations.

BACKGROUND

This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.

A video coding system may comprise an encoder that transforms an input video into a compressed representation suited for storage/transmission and a decoder that can uncompress the compressed video representation back into a viewable form. The encoder may discard some information in the original video sequence in order to represent the video in a more compact form, for example, to enable the storage/transmission of the video information at a lower bitrate than otherwise might be needed. SUMMARY

Some embodiments provide a method for encoding and decoding video information. In some embodiments an apparatus, a computer program product, a computer-readable medium for implementing the method are provided. In accordance with an embodiment a bi-directional motion compensation for a current block comprises sliding candidate reference blocks of bi-directional motion compensation with respect to each other by an offset and calculating a measure of similarity between resulting reference blocks. This operation may be repeated within a certain search range and that offset which produces the smallest cost may be selected to the representative motion vectors for bi- directional motion compensation for the current block.

In a method according an example embodiment a horizontal displacement and a vertical displacement of candidate reference sample blocks may be separately analyzed during bi- prediction process and the motion vector components may be updated based on the analysis. This approach can also be applied separately to different linesets or "slices" of a prediction unit which may result in different re-alignment parameters for different linesets. In the end the results of the vertical and horizontal analysis may be compared and a decision to split the prediction unit in either horizontal or vertical linesets may be made. The method may effectively align the structures in the reference blocks and may provide a more consistent prediction blocks resulting into bitrate savings and/or visual quality improvements. Various aspects of examples of the invention are provided in the detailed description.

According to a first aspect, there is provided a method comprising:

obtaining a block from a current picture to be decoded;

obtaining a first motion vector predictor candidate pointing to a first reference block in a first reference picture;

obtaining a second motion vector predictor candidate pointing to a second reference block in a second reference picture;

adding a difference vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate;

subtracting the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate;

determining a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate;

repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

selecting from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates.

According to a second aspect, there is provided an apparatus comprising at least one processor; and at least one memory including computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

obtain a block from a current picture to be decoded;

obtain a first motion vector predictor candidate pointing to a first reference block in a first reference picture;

obtain a second motion vector predictor candidate pointing to a second reference block in a second reference picture; add a difference vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate;

subtract the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate;

determine a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate;

repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

select from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates.

According to a third aspect, there is provided a computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

obtain a block from a current picture to be decoded;

obtain a second motion vector predictor candidate pointing to a second reference block in a second reference picture;

add a difference vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate;

select from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates. According to a fourth aspect, there is provided an apparatus comprising a video decoder configured for encoding a bitstream comprising an image sequence, the video decoder comprising:

an input for receiving a block from a current picture to be decoded;

a motion vector selector for selecting a first motion vector predictor candidate pointing to a first reference block in a first reference picture and for selecting a second motion vector predictor candidate pointing to a second reference block in a second reference picture; a motion vector modifier for adding a difference vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate and for subtracting the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate;

a cost estimator to determine a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate;

wherein the apparatus is adapted to repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

the apparatus further comprises a selector for selecting from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates.

According to a fifth aspect, there is provided an apparatus comprising:

means for obtaining a block from a current picture to be decoded;

means for obtaining a first motion vector predictor candidate pointing to a first reference block in a first reference picture;

means for obtaining a second motion vector predictor candidate pointing to a second reference block in a second reference picture;

means for adding a difference vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate;

means for subtracting the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate;

means for determining a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate; means for repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

means for selecting from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates.

According to a sixth aspect, there is provided a method comprising:

obtain a block from a current picture to be encoded;

select from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates. According to a seventh aspect, there is provided an apparatus comprising at least one processor; and at least one memory including computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

obtain a block from a current picture to be encoded;

obtain a first motion vector predictor candidate pointing to a first reference block in a first reference picture; obtain a second motion vector predictor candidate pointing to a second reference block in a second reference picture;

According to an eight aspect, there is provided a computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

obtain a block from a current picture to be encoded;

repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and select from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates. According to a ninth aspect, there is provided an apparatus comprising a video encoder configured for decoding a bitstream comprising an image sequence, the video encoder comprising

an input for receiving a block from a current picture to be encoded;

According to a tenth aspect, there is provided an apparatus comprising:

means for obtaining a block from a current picture to be encoded;

means for subtracting the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate; means for determining a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate;

means for repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

Figure 1 illustrates a block diagram of a video coding system according to an embodiment;

Figure 2 illustrates a layout of an apparatus according to an embodiment;

Figure 3 illustrates an arrangement for video coding comprising a plurality of apparatuses, networks and network elements according to an embodiment;

Figure 4 illustrates a block diagram of a video encoder according to an embodiment;

Figure 5 illustrates a block diagram of a video decoder according to an embodiment;

Figure 6 shows an example of a picture consisting of two tiles;

Figure 7 illustrates a motion vector determination element as a simplified block diagram, in accordance with an embodiment;

Figure 8 a illustrates bi-directional motion prediction, in accordance with an embodiment; Figure 8b illustrates sliding operation in bi-directional motion prediction, in accordance with an embodiment;

Figure 8c illustrates sliding operation in bi-directional motion prediction, in accordance with another embodiment;

Figure 9 illustrates an encoding method according to an embodiment as a flowchart;

Figure 10 illustrates a decoding method according to an embodiment as a flowchart; and Figure 11 shows a schematic diagram of an example multimedia communication system within which various embodiments may be implemented. DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

Figure 1 shows a video coding system as a schematic block diagram of an apparatus or electronic device 50 according to an embodiment. The electronic device 50 may incorporate a codec according to an embodiment. Figure 2 shows a layout of an apparatus according to an embodiment. The elements of Figs. 1 and 2 will be explained next.

The electronic device 50 may, for example, be a mobile terminal or user equipment of a wireless communication system. However, it is appreciated that embodiments of the invention may be implemented within any electronic device or apparatus which may perform encoding and decoding, or encoding or decoding video images.

The apparatus 50 may comprise a housing 30 for incorporating and protecting the device. The apparatus 50 may further comprise a display 32 in the form of a liquid crystal display. In other embodiments, the display may be any suitable display technology suitable to display an image or video. The apparatus 50 may further comprise a keypad 34. According to an embodiment, any suitable data or user interface mechanism may be employed. For example, the user interface may be implemented as a virtual keyboard or data entry system as part of a touch-sensitive display. The apparatus may comprise a microphone 36 or any suitable audio input which may be a digital or analogue signal input. The apparatus 50 may further comprise an audio output device, which - according to an embodiment - may be any one of: an earpiece 38, speaker, or an analogue audio or digital audio output connection. The apparatus 50 may also comprise a battery 40 (or in an embodiment, the device may be powered by any suitable mobile energy device, such as solar cell, fuel cell or clockwork generator). The apparatus may further comprise a camera 42 capable of recording or capturing images and/or video. According to an embodiment, the apparatus 50 may further comprise an infrared port for short range line of sight communication to other devices. According to an embodiment, the apparatus 50 may further comprise any suitable short range communication solution such as for example a Bluetooth® wireless connection or a USB/firewire wired connection.

The apparatus 50 may comprise a controller 56 or processor for controlling the apparatus 50. The controller 56 may be connected to memory 58 which according to an embodiment may store both data in the form of image and audio data and/or may also store instructions for implementation on the controller 56. The controller 56 may further be connected to codec circuitry 54 suitable for carrying out coding and/or decoding of audio and/or video data or assisting in coding and/or decoding carried out by the controller 56.

The apparatus 56 may further comprise a card reader 48 and a smart card 46, for example a UICC (Universal Integrated Circuit Card) and UICC reader for providing user information and being suitable for providing authentication information for authentication and authorization of the user at a network. The apparatus 50 may further comprise a radio interface circuitry 52 connected to the controller and suitable for generating wireless communication signals for example for communication with a cellular communications network, a wireless communication system or a wireless local area network. The apparatus 50 may further comprise an antenna 44 connected to the radio interface circuitry 52 for transmitting radio frequency signals generated at the radio interface circuitry 52 to other apparatus(es) and for receiving radio frequency signals from other apparatus(es).

According to an embodiment, the apparatus 50 comprises a camera capable of recording or detecting individual frames which are then passed to the codec 54 or controller for processing. According to an embodiment, the apparatus may receive the video image data for processing from another device prior to transmission and/or storage. According to an embodiment, the apparatus 50 may receive either wirelessly or by a wired connection the image for coding/decoding.

Figure 3 shows an arrangement for video coding comprising a plurality of apparatuses, networks and network elements according to an embodiment. With respect to Figure 3, an example of a system within which embodiments of the invention can be utilized is shown. The system 10 comprises multiple communication devices which can communicate through one or more networks. The system 10 may comprise any combination of wired or wireless networks including but not limited to a wireless cellular telephone network (such as a GSM, UMTS, CDMA network etc.), a wireless local area network (WLAN) such as defined by any of the IEEE 802.x standards, a Bluetooth personal area network, an Ethernet local area network, a token ring local area network, a wide area network and the Internet.

The system 10 may include both wired and wireless communication devices or apparatus 50 suitable for implementing embodiments. For example, the system shown in Figure 3 shows a mobile telephone network 1 1 and a representation of the internet 28. Connectivity to the internet 28 may include, but is not limited to, long range wireless connections, short range wireless connections, and various wired connections including, but not limited to, telephone lines, cable lines, power lines, and similar communication pathways.

The example communication devices shown in the system 10 may include, but are not limited to, an electronic device or apparatus 50, any combination of a personal digital assistant (PDA) and a mobile telephone 14, a PDA 16, an integrated messaging device (IMD) 18, a desktop computer 20, a notebook computer 22. The apparatus 50 may be stationary or mobile when carried by an individual who is moving. The apparatus 50 may also be located in a mode of transport including, but not limited to, a car, a truck, a taxi, a bus, a train, a boat, an airplane, a bicycle, a motorcycle or any similar suitable mode of transport.

The embodiments may also be implemented in a set-top box; i.e. a digital TV receiver, which may/may not have a display or wireless capabilities, in tablets or (laptop) personal computers (PC), which have hardware or software or combination of the encoder/decoder implementations, in various operating systems, and in chipsets, processors, DSPs and/or embedded systems offering hardware/software based coding. Some or further apparatuses may send and receive calls and messages and communicate with service providers through a wireless connection 25 to a base station 24. The base station 24 may be connected to a network server 26 that allows communication between the mobile telephone network 1 1 and the internet 28. The system may include additional communication devices and communication devices of various types.

The communication devices may communicate using various transmission technologies including, but not limited to, code division multiple access (CDMA), global systems for mobile communications (GSM), universal mobile telecommunications system (UMTS), time divisional multiple access (TDMA), frequency division multiple access (FDMA) transmission control protocol-internet protocol (TCP-IP), short messaging service (SMS), multimedia messaging service (MMS) email, instant messaging service (IMS), Bluetooth, IEEE 802.1 1 and any similar wireless communication technology. A communications device involved in implementing various embodiments of the present invention may communicate using various media including, but not limited to, radio, infrared, laser, cable connections and any suitable connection.

Video encoder may comprise an encoder that transforms the input video into a compressed representation suited for storage/transmission, and a decoder is able to uncompress the compressed video representation back into a viewable form. A video encoder and/or a video decoder may also be separate from each other, i.e. need not form a codec. The encoder may discard some information in the original video sequence in order to represent the video in more compact form (i.e. at lower bitrate).

Hybrid video codecs, for example ITU-T H.263 and H.264, may encode the video information in two phases. At first, pixel values in a certain picture are (or "block") are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly, the prediction error, i.e. the difference between the predicted block of pixels and the original block of pixels, is coded. This may be done by transforming the difference in pixel values using a specified transform (e.g. Discrete Cosine Transform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation (file size of transmission bitrate). Inter prediction, which may also be referred to as temporal prediction, motion compensation, or motion-compensated prediction, reduces temporal redundancy. In inter prediction the sources of prediction are previously decoded pictures. Intra prediction utilizes the fact that adjacent pixels within the same picture are likely to be correlated. Intra prediction can be performed in spatial or transform domain, i.e., either sample values or transform coefficients can be predicted. Intra prediction is typically exploited in intra coding, where no inter prediction is applied.

One outcome of the coding procedure is a set of coding parameters, such as motion vectors and quantized transform coefficients. Many parameters can be entropy-coded more efficiently if they are predicted first from spatially or temporally neighboring parameters. For example, a motion vector may be predicted from spatially adjacent motion vectors and only the difference relative to the motion vector predictor may be coded. Prediction of coding parameters and intra prediction may be collectively referred to as in-picture prediction. The Advanced Video Coding (H.264/AVC a.k.a. AVC) standard was developed by the Joint Video Team (JVT) of the Video Coding Experts Group (VCEG) of the Telecommunications Standardization Sector of International Telecommunication Union (ITU-T) and the Moving Picture Experts Group (MPEG) of International Organisation for Standardization (ISO) / International Electrotechnical Commission (IEC). The H.264/AVC standard is published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.264 and ISO/IEC International Standard 14496-10, also known as MPEG-4 Part 10 Advanced Video Coding (AVC). There have been multiple versions of the H.264/AVC standard, integrating new extensions or features to the specification. These extensions include Scalable Video Coding (SVC) and Multiview Video Coding (MVC).

Version 1 of the High Efficiency Video Coding (H.265/HEVC a.k.a. HEVC) standard was developed by the Joint Collaborative Team - Video Coding (JCT-VC) of VCEG and MPEG. The standard was published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.265 and ISO/IEC International Standard 23008-2, also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). Version 2 of H.265/HEVC included scalable, multiview, and fidelity range extensions, which may be abbreviated SHVC, MV-HEVC, and REXT, respectively. Version 2 of H.265/HEVC was pre -published as ITU-T Recommendation H.265 (10/2014) and is likely to be published as Edition 2 of ISO/IEC 23008-2 in 2015. There are currently ongoing standardization projects to develop further extensions to H.265/HEVC, including three-dimensional and screen content coding extensions, which may be abbreviated 3D-HEVC and SCC, respectively.

SHVC, MV-HEVC, and 3D-HEVC use a common basis specification, specified in Annex F of the version 2 of the HEVC standard. This common basis comprises for example high-level syntax and semantics e.g. specifying some of the characteristics ofthe layers of the bitstream, such as inter-layer dependencies, as well as decoding processes, such as reference picture list construction including inter-layer reference pictures and picture order count derivation for multi-layer bitstream. Annex F may also be used in potential subsequent multi-layer extensions of HEVC. It is to be understood that even though a video encoder, a video decoder, encoding methods, decoding methods, bitstream structures, and/or embodiments may be described in the following with reference to specific extensions, such as SHVC and/or MV-HEVC, they are generally applicable to any multi-layer extensions of HEVC, and even more generally to any multi-layer video coding scheme.

Some key definitions, bitstream and coding structures, and concepts of H.264/AVC and HEVC are described in this section as an example of a video encoder, decoder, encoding method, decoding method, and a bitstream structure, wherein the embodiments may be implemented. Some of the key definitions, bitstream and coding structures, and concepts of H.264/AVC are the same as in HEVC - hence, they are described below jointly. The aspects of the invention are not limited to H.264/AVC or HEVC, but rather the description is given for one possible basis on top of which the invention may be partly or fully realized.

Similarly to many earlier video coding standards, the bitstream syntax and semantics as well as the decoding process for error-free bitstreams are specified in H.264/AVC and HEVC. The encoding process is not specified, but encoders must generate conforming bitstreams. Bitstream and decoder conformance can be verified with the Hypothetical Reference Decoder (HRD). The standards contain coding tools that help in coping with transmission errors and losses, but the use of the tools in encoding is optional and no decoding process has been specified for erroneous bitstreams. In the description of existing standards as well as in the description of example embodiments, a syntax element may be defined as an element of data represented in the bitstream. A syntax structure may be defined as zero or more syntax elements present together in the bitstream in a specified order. In the description of existing standards as well as in the description of example embodiments, a phrase "by external means" or "through external means" may be used. For example, an entity, such as a syntax structure or a value of a variable used in the decoding process, may be provided "by external means" to the decoding process. The phrase "by external means" may indicate that the entity is not included in the bitstream created by the encoder, but rather conveyed externally from the bitstream for example using a control protocol. It may alternatively or additionally mean that the entity is not created by the encoder, but may be created for example in the player or decoding control logic or alike that is using the decoder. The decoder may have an interface for inputting the external means, such as variable values. A profile may be defined as a subset of the entire bitstream syntax that is specified by a decoding/coding standard or specification. Within the bounds imposed by the syntax of a given profile it is still possible to require a very large variation in the performance of encoders and decoders depending upon the values taken by syntax elements in the bitstream such as the specified size of the decoded pictures. In many applications, it might be neither practical nor economic to implement a decoder capable of dealing with all hypothetical uses of the syntax within a particular profile. In order to deal with this issue, levels may be used. A level may be defined as a specified set of constraints imposed on values of the syntax elements in the bitstream and variables specified in a decoding/coding standard or specification. These constraints may be simple limits on values. Alternatively or in addition, they may take the form of constraints on arithmetic combinations of values (e.g., picture width multiplied by picture height multiplied by number of pictures decoded per second). Other means for specifying constraints for levels may also be used. Some of the constraints specified in a level may for example relate to the maximum picture size, maximum bitrate and maximum data rate in terms of coding units, such as macroblocks, per a time period, such as a second. The same set of levels may be defined for all profiles. It may be preferable for example to increase interoperability of terminals implementing different profiles that most or all aspects of the definition of each level may be common across different profiles. A tier may be defined as specified category of level constraints imposed on values of the syntax elements in the bitstream, where the level constraints are nested within a tier and a decoder conforming to a certain tier and level would be capable of decoding all bitstreams that conform to the same tier or the lower tier of that level or any level below it. In some cases, a conformance point may be defined as a combination of a particular profile and a particular level or a combination of a particular profile, a particular tier, and a particular level. It needs to be understood that a conformance point may be defined in alternative ways, while its intent to specify characteristics and limits of bitstream and/or characteristics and (maximum) resources of decoders may be kept unchanged.

The elementary unit for the input to an H.264/AVC or HEVC encoder and the output of an H.264/ AVC or HEVC decoder, respectively, is a picture. A picture given as an input to an encoder may also referred to as a source picture, and a picture decoded by a decoded may be referred to as a decoded picture.

The source and decoded pictures are each comprised of one or more sample arrays, such as one of the following sets of sample arrays :

Luma (Y) only (monochrome).

Luma and two chroma (YCbCr or YCgCo).

Green, Blue and Red (GBR, also known as RGB).

Arrays representing other unspecified monochrome or tri-stimulus color samplings (for example, YZX, also known as XYZ). In the following, these arrays may be referred to as luma (or L or Y) and chroma, where the two chroma arrays may be referred to as Cb and Cr; regardless of the actual color representation method in use. The actual color representation method in use can be indicated e.g. in a coded bitstream e.g. using the Video Usability Information (VUI) syntax of H.264/ AVC and/or HEVC. A component may be defined as an array or single sample from one of the three sample arrays arrays (luma and two chroma) or the array or a single sample of the array that compose a picture in monochrome format.

In H.264/AVC and HEVC, a picture may either be a frame or a field. A frame comprises a matrix of luma samples and possibly the corresponding chroma samples. A field is a set of alternate sample rows of a frame and may be used as encoder input, when the source signal is interlaced. Chroma sample arrays may be absent (and hence monochrome sampling may be in use) or chroma sample arrays may be subsampled when compared to luma sample arrays. Chroma formats may be summarized as follows:

In monochrome sampling there is only one sample array, which may be nominally considered the luma array.

In 4:2:0 sampling, each of the two chroma arrays has half the height and half the width of the luma array. In 4:2:2 sampling, each of the two chroma arrays has the same height and half the width of the luma array.

In 4:4:4 sampling when no separate color planes are in use, each of the two chroma arrays has the same height and width as the luma array.

In H.264/ AVC and HEVC, it is possible to code sample arrays as separate color planes into the bitstream and respectively decode separately coded color planes from the bitstream. When separate colorplanes are in use, each one ofthem is separately processed (by the encoder and/or the decoder) as a picture with monochrome sampling. When chroma subsampling is in use (e.g. 4:2:0 or 4:2:2 chroma sampling), the location of chroma samples with respect to luma samples may be determined in the encoder side (e.g. as pre-processing step or as part of encoding). The chroma sample positions with respect to luma sample positions may be pre-defined for example in a coding standard, such as H.264/AVC or HEVC, or may be indicated in the bitstream for example as part of VUI of H.264/AVC or HEVC.

A partitioning may be defined as a division of a set into subsets such that each element of the set is in exactly one of the subsets. In H.264/AVC, a macroblock is a 16x16 block of luma samples and the corresponding blocks of chroma samples. For example, in the 4:2:0 sampling pattern, a macroblock contains one 8x8 block of chroma samples per each chroma component. In H.264/AVC, a picture is partitioned to one or more slice groups, and a slice group contains one or more slices. In H.264/ AVC, a slice consists of an integer number of macroblocks ordered consecutively in the raster scan within a particular slice group.

When describing the operation of HEVC encoding and/or decoding, the following terms may be used. A coding block may be defined as an NxN block of samples for some value of N such that the division of a coding tree block into coding blocks is a partitioning. A coding tree block (CTB) may be defined as an NxN block of samples for some value of N such that the division of a component into coding tree blocks is a partitioning. A coding tree unit (CTU) may be defined as a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples of a picture that has three sample arrays, or a coding tree block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A coding unit (CU) may be defined as a coding block of luma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples.

In some video codecs, such as High Efficiency Video Coding (HEVC) codec, video pictures are divided into coding units (CU) covering the area of the picture. A CU consists of one or more prediction units (PU) defining the prediction process for the samples within the CU and one or more transform units (TU) defining the prediction error coding process for the samples in the said CU. Typically, a CU consists of a square block of samples with a size selectable from a predefined set of possible CU sizes. A CU with the maximum allowed size may be named as LCU (largest coding unit) or coding tree unit (CTU) and the video picture is divided into non-overlapping LCUs. An LCU can be further split into a combination of smaller CUs, e.g. by recursively splitting the LCU and resultant CUs. Each resulting CU typically has at least one PU and at least one TU associated with it. Each PU andTU can be further split into smaller PUs and TUs in order to increase granularity of the prediction and prediction error coding processes, respectively. Each PU has prediction information associated with it defining what kind of a prediction is to be applied for the pixels within that PU (e.g. motion vector information for inter predicted PUs and intra prediction directionality information for intra predicted PUs). Each TU can be associated with information describing the prediction error decoding process for the samples within the said TU (including e.g. DCT coefficient information). It is typically signalled at CU level whether prediction error coding is applied or not for each CU. In the case there is no prediction error residual associated with the CU, it can be considered there are no TUs for the said CU. The division of the image into CUs, and division of CUs into PUs and TUs may be signalled in the bitstream allowing the decoder to reproduce the intended structure of these units.

In HEVC, a picture can be partitioned in tiles, which are rectangular and contain an integer number of LCUs. In HEVC, the partitioning to tiles forms a regular grid, where heights and widths of tiles differ from each other by one LCU at the maximum. In HEVC, a slice is defined to be an integer number of coding tree units contained in one independent slice segment and all subsequent dependent slice segments (if any) that precede the next independent slice segment (if any) within the same access unit. In HEVC, a slice segment is defined to be an integer number of coding tree units ordered consecutively in the tile scan order and contained in a single NAL unit. The division of each picture into slice segments is a partitioning. In HEVC, an independent slice segment is defined to be a slice segment for which the values of the syntax elements of the slice segment header are not inferred from the values for a preceding slice segment, and a dependent slice segment is defined to be a slice segment for which the values of some syntax elements of the slice segment header are inferred from the values for the preceding independent slice segment in decoding order. In HEVC, a slice header is defined to be the slice segment header of the independent slice segment that is a current slice segment or is the independent slice segment that precedes a current dependent slice segment, and a slice segment header is defined to be a part of a coded slice segment containing the data elements pertaining to the first or all coding tree units represented in the slice segment. The CUs are scanned in the raster scan order of LCUs within tiles or within a picture, if tiles are not in use. Within an LCU, the CUs have a specific scan order. Figure 6 shows an example of a picture consisting of two tiles partitioned into square coding units (solid lines) which have been further partitioned into rectangular prediction units (dashed lines).

The decoder may reconstruct the output video by applying prediction means similar to the encoder to form a predicted representation of the pixel blocks (using the motion or spatial information created by the encoder and stored in the compressed representation) and prediction error decoding (inverse operation of the prediction error coding recovering the quantized prediction error signal in spatial pixel domain). After applying prediction and prediction error decoding means the decoder may sum up the prediction and prediction error signals (pixel values) to form the output video frame. The decoder (and encoder) may also apply additional filtering means to improve the quality of the output video before passing it for display and/or storing it as prediction reference for the forthcoming frames in the video sequence. The filtering may for example include one more of the following: deblocking, sample adaptive offset (SAO), and/or adaptive loop filtering (ALF). H.264/AVC includes a deblocking, whereas HEVC includes both deblocking and SAO.

Instead, or in addition to approaches utilizing sample value prediction and transform coding for indicating the coded sample values, a color palette based coding may be used. Palette based coding refers to a family of approaches for which a palette, i.e. a set of colors and associated indexes, is defined and the value for each sample within a coding unit is expressed by indicating its index in the palette. Palette based coding can achieve good coding efficiency in coding units with a relatively small number of colors (such as image areas which are representing computer screen content, like text or simple graphics). In order to improve the coding efficiency of palette coding different kinds of palette index prediction approaches can be utilized, or the palette indexes can be run-length coded to be able to represent larger homogenous image areas efficiently. Also, in the case the CU contains sample values that are not recurring within the CU, escape coding can be utilized. Escape coded samples are transmitted without referring to any of the palette indexes. Instead their values are indicated individually for each escape coded sample.

A Decoded Picture Buffer (DPB) may be used in the encoder and/or in the decoder. There are two reasons to buffer decoded pictures, for references in inter prediction and for reordering decoded pictures into output order. As H.264/AVC and HEVC provide a great deal of flexibility for both reference picture marking and output reordering, separate buffers for reference picture buffering and output picture buffering may waste memory resources. Hence, the DPB may include a unified decoded picture buffering process for reference pictures and output reordering. A decoded picture may be removed from the DPB when it is no longer used as a reference and is not needed for output. In many coding modes of H.264/AVC and HEVC, the reference picture for inter prediction is indicated with an index to a reference picture list. The index may be coded with variable length coding, which usually causes a smaller index to have a shorter value for the corresponding syntax element. In H.264/AVC and HEVC, two reference picture lists (reference picture list 0 and reference picture list 1) are generated for each bi-predictive (B) slice, and one reference picture list (reference picture list 0) is formed for each inter-coded (P) slice.

A reference picture list, such as reference picture list 0 and reference picture list 1 , is typically constructed in two steps: First, an initial reference picture list is generated. The initial reference picture list may be generated for example on the basis of frame num, POC, temporal id (or Temporalld or alike), or information on the prediction hierarchy such as GOP structure, or any combination thereof. Second, the initial reference picture list may be reordered by reference picture list reordering (RPLR) commands, also known as reference picture list modification syntax structure, which may be contained in slice headers. In H.264/AVC, the RPLR commands indicate the pictures that are ordered to the beginning of the respective reference picture list. This second step may also be referred to as the reference picture list modification process, and the RPLR commands may be included in a reference picture list modification syntax structure. If reference picture sets are used, the reference picture list 0 may be initialized to contain RefPicSetStCurrO first, followed by RefPicSetStCurrl , followed by RefPicSetLtCurr. Reference picture list 1 may be initialized to contain RefPicSetStCurrl first, followed by RefPicSetStCurrO. In HEVC, the initial reference picture lists may be modified through the reference picture list modification syntax structure, where pictures in the initial reference picture lists may be identified through an entry index to the list. In other words, in HEVC, reference picture list modification is encoded into a syntax structure comprising a loop over each entry in the final reference picture list, where each loop entry is a fixed-length coded index to the initial reference picture list and indicates the picture in ascending position order in the final reference picture list.

Many coding standards, including H.264/AVC and HEVC, may have decoding process to derive a reference picture index to a reference picture list, which may be used to indicate which one of the multiple reference pictures is used for inter prediction for a particular block. A reference picture index may be coded by an encoder into the bitstream in some inter coding modes or it may be derived (by an encoder and a decoder) for example using neighboring blocks in some other inter coding modes.

The motion information may be indicated in video codecs with motion vectors associated with each motion compensated image block. Each of these motion vectors may represent the displacement of the image block in the picture to be coded (in the encoder side) or decoded (in the decoder side) and the prediction source block in one of the previously coded or decodedpictures. In order to represent motion vectors efficiently, those vectors may be coded differentially with respect to block specific predicted motion vectors. In video codecs, the predicted motion vectors may be created in a predefined way, e.g. by calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions is to generate a list of candidate predictions from adjacent blocks and/or co-located blocks in temporal reference pictures and signalling the chosen candidate as the motion vector prediction. In addition to predicting the motion vector values, the reference index of a previously coded/ decoded picture may be predicted. The reference index may be predicted e.g. from adjacent blocks and/or co-located blocks in a temporal reference picture. Moreover, high efficiency video codecs may employ an additional motion information coding/decoding mechanism, called "merging/merge mode", where all the motion field information, which may include motion vector and corresponding reference picture index for each available reference picture list, is predicted and used without any modification correction. Similarly, predicting the motion field information may be carried out using the motion field information of adjacent blocks and/or co-located blocks in temporal reference pictures and the used motion field information is signaled among a list of motion field candidate list filled with motion field information of available adjacent/co- located blocks. Some video codecs enable the use of uni-prediction, where a single prediction block is used for a block being (de)coded, and bi-prediction, where two prediction blocks are combined to form the prediction for a block being (de)coded. Some video codecs enable weighted prediction, where the sample values of the prediction blocks are weighted prior to adding residual information. For example, multiplicative weighting factor and an additive offset may be applied. In explicit weighted prediction, enabled by some video codecs, a weighting factor and offset may be coded for example in the slice header for each allowable reference picture index. In implicit weighted prediction, enabled by some video codecs, the weighting factors and/or offsets are not coded but are derived e.g. based on the relative picture order count (POC) distances of the reference pictures.

In addition to applying motion compensation for inter picture prediction, similar approach can be applied to intra picture prediction. In this case the displacement vector indicates where from the same picture a block of samples can be copied to form a prediction of the block to be coded or decoded. This kind of intra block copying methods may improve the coding efficiency substantially in presence of repeating structures within the frame - such as text or other graphics.

In video codecs, the prediction residual after motion compensation may be first transformed with a transform kernel (e.g. DCT) and then coded. The reason for this is that there may still exist some correlation among the residual and transform may in many cases help reduce this correlation and provide more efficient coding.

Video encoders may utilize Lagrangian cost functions to find optimal coding modes, e.g. the desired macroblock mode and associated motion vectors. This kind of cost function uses a weighting factor λ (lambda) to tie together the (exact or estimated) image distortion due to lossy coding methods and the (exact or estimated) amount of information that is required to represent the pixel values in an image area:

C=D+λR (1)

Where C is the Lagrangian cost to be minimized, D is the image distortion (e.g. Mean Squared Error) with the mode and motion vectors considered, and R is the number of bits needed to represent the required data to reconstruct the image block in the decoder (including the amount of data to represent the candidate motion vectors). Video coding standards and specifications may allow encoders to divide a coded picture to coded slices or alike. In-picture prediction is typically disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture to independently decodable pieces. In H.264/AVC and HEVC, in-picture prediction may be disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture into independently decodable pieces, and slices are therefore often regarded as elementary units for transmission. In many cases, encoders may indicate in the bitstream which types of in-picture prediction are turned off across slice boundaries, and the decoder operation takes this information into account for example when concluding which prediction sources are available. For example, samples from a neighboring macroblock or CU may be regarded as unavailable for intra prediction, if the neighboring macroblock or CU resides in a different slice.

An elementary unit for the output of an H.264/AVC or HEVC encoder and the input of an H.264/AVC or HEVC decoder, respectively, is a Network Abstraction Layer (NAL) unit. For transport over packet-oriented networks or storage into structured files, NAL units may be encapsulated into packets or similar structures. A bytestream format has been specified in H.264/AVC and HEVC for transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte-oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. In order to enable straightforward gateway operation between packet- and stream-oriented systems, start code emulation prevention may always be performed regardless of whether the bytestream format is in use or not. A NAL unit may be defined as a syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. A raw byte sequence payload (RBSP) may be defined as a syntax structure containing an integer number of bytes that is encapsulated in a NAL unit. An RBSP is either empty or has the form of a string of data bits containing syntax elements followed by an RBSP stop bit and followed by zero or more subsequent bits equal to 0.

NAL units consist of a header and payload. In H.264/AVC and HEVC, the NAL unit header indicates the type of the NAL unit. H.264/AVC NAL unit header includes a 2-bit nal_ref_idc syntax element, which when equal to 0 indicates that a coded slice contained in the NAL unit is a part of a non-reference picture and when greater than 0 indicates that a coded slice contained in the NAL unit is a part of a reference picture. The header for SVC and MVC NAL units may additionally contain various indications related to the scalability and multiview hierarchy.

In HEVC, a two-byte NAL unit header is used for all specified NAL unit types. The NAL unit header contains one reserved bit, a six-bit NAL unit type indication, a three-bit nuh_temporal_id_plus 1 indication for temporal level (may be required to be greater than or equal to 1 ) and a six-bit nuh layer id syntax element. The temporal_id_plus 1 syntax element may be regarded as a temporal identifier for the NAL unit, and a zero-based Temporalld variable may be derived as follows: Temporalld = temporal id _plus 1 - 1. Temporalld equal to 0 corresponds to the lowest temporal level. The value of temporal_id_plusl is required to be non-zero in order to avoid start code emulation involving the two NAL unit header bytes. The bitstream created by excluding all VCL NAL units having a Temporalld greater than or equal to a selected value and including all other VCL NAL units remains conforming. Consequently, a picture having Temporalld equal to TID does not use any picture having a Temporalld greater than TID as inter prediction reference. A sub-layer or a temporal sublayer may be defined to be a temporal scalable layer of a temporal scalable bitstream, consisting of VCL NAL units with a particular value of the Temporalld variable and the associated non-VCL NAL units. The nuh layer id syntax element of HEVC may carry information on the scalability hierarchy.

NAL units can be categorized into Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL NAL units are typically coded slice NAL units. In H.264/AVC, coded slice NAL units contain syntax elements representing one or more coded macroblocks, each of which corresponds to a block of samples in the uncompressed picture. In HEVC, VCL NAL units contain syntax elements representing one or more CU.

In H.264/AVC, a coded slice NAL unit can be indicated to be a coded slice in an Instantaneous Decoding Refresh (IDR) picture or coded slice in a non-IDR picture. In HEVC, a coded slice NAL unit can be indicated to be one of the following types:

nal unit type Name of Content of NAL unit and RBSP

nal unit type syntax structure

o, TRAIL N, Coded slice segment of a non-TSA,

1 TRAIL R non-STSA trailing picture

slice_segment_layer_rbsp( ) 2, TSA_N, Coded slice segment of a TSA

3 TSA R picture

slice_segment_layer_rbsp( )

4, STSA_N, Coded slice segment of an STSA

5 STSA R picture

slice_layer_rbsp( )

6, RADL N, Coded slice segment of a RADL

7 RADL R picture

slice_layer_rbsp( )

8, RASL N, Coded slice segment of a RASL

9 RASL R, picture

slice_layer_rbsp( )

10, RSV VCL N10 Reserved// reserved non-RAP non-

12, RSV VCL N12 reference VCL NAL unit types

14 RSV VCL N14

1 1 , RSV VCL Rl 1 Reserved // reserved non-RAP

13, RSV VCL R13 reference VCL NAL unit types

15 RSV VCL R15

16, BLA W LP Coded slice segment of a BLA

17, BLA W DLP (a.k.a. picture

18 IDR W RADL) slice_segment_layer_rbsp( )

BLA N LP

19, IDR W DLP (a.k.a. Coded slice segment of an IDR

20 IDR W RADL) picture

IDR N LP slice_segment_layer_rbsp( )

21 CRA NUT Coded slice segment of a CRA

picture

slice_segment_layer_rbsp( )

22, RS V IRAP VCL22.. Reserved // reserved RAP VCL

23 RSV IRAP VCL23 NAL unit types

24..31 RSV VCL24.. Reserved // reserved non-RAP

RSV VCL31 VCL NAL unit types

In HEVC, abbreviations for picture types may be defined as follows: trailing (TRAIL) picture, Temporal Sub-layer Access (TSA), Step-wise Temporal Sub-layer Access (STSA), Random Access Decodable Leading (RADL) picture, Random Access Skipped Leading (PvASL) picture, Broken Link Access (BLA) picture, Instantaneous Decoding Refresh (IDR) picture, Clean Random Access (CRA) picture.

A Random Access Point (RAP) picture, which may also be referred to as an intra random access point (IRAP) picture, is a picture where each slice or slice segment has nal unit type in the range of 16 to 23, inclusive. An IRAP picture in an independent layer contains only intra-coded slices. An IRAP picture belonging to a predicted layer with nuh layer id value currLayerld may contain P, B, and I slices, cannot use inter prediction from other pictures with nuh layer id equal to currLayerld, and may use inter-layer prediction from its direct reference layers. In the present version of HEVC, an IRAP picture may be a BLA picture, a CRA picture or an IDR picture. The first picture in the bitstream containing a base layer is an IRAP picture. Provided the necessary parameter sets are available when they need to be activated, an IRAP picture at an independent layer and all subsequent non-RASL pictures at the independent layer in decoding order can be correctly decoded without performing the decoding process of any pictures that precede the IRAP picture in decoding order. There may be pictures in a bitstream that contain only intra-coded slices that are not RAP pictures. The IRAP picture belonging to a predicted layer with nuh layer id value currLayerld and all subsequent non-RASL pictures with nuh layer id equal to currLayerld in decoding order can be correctly decoded without performing the decoding process of any pictures with nuh layer id equal to currLayerld that precede the IRAP picture in decoding order, when the necessary parameter sets are available when they need to be activated and when the decoding of each direct reference layer of the layer with nuh layer id equal to currLayerld has been initialized (i.e. when LayerInitializedFlag[ refLayerld ] is equal to 1 for refLayerld equal to all nuh_layer_id values of the direct reference layers of the layer with nuh_layer_id equal to currLayerld).

In HEVC a CRA picture may be the first picture in the bitstream in decoding order, or may appear later in the bitstream. CRA pictures in HEVC allow so-called leading pictures that follow the CRA picture in decoding order but precede it in output order. Some of the leading pictures, so-called RASL pictures, may use pictures decoded before the CRA picture as a reference. Pictures that follow a CRA picture in both decoding and output order may be decodable if random access is performed at the CRA picture, and hence clean random access may be achieved similarly to the clean random access functionality of an IDR picture. A CRA picture may have associated RADL or RASL pictures. When a CRA picture is the first picture in the bitstream in decoding order, the CRA picture is the first picture of a coded video sequence in decoding order, and any associated RASL pictures are not output by the decoder and may not be decodable, as they may contain references to pictures that are not present in the bitstream.

A leading picture is a picture that precedes the associated RAP picture in output order. The associated RAP picture is the previous RAP picture in decoding order (if present). A leading picture may either be a RADL picture or a RASL picture.

All RASL pictures are leading pictures of an associated BLA or CRA picture. When the associated RAP picture is a BLA picture or is the first coded picture in the bitstream, the RASL picture is not output and may not be correctly decodable, as the RASL picture may contain references to pictures that are not present in the bitstream. However, a RASL picture can be correctly decoded if the decoding had started from a RAP picture before the associated RAP picture of the RASL picture. RASL pictures are not used as reference pictures for the decoding process of non-RASL pictures. When present, all RASL pictures precede, in decoding order, all trailing pictures of the same associated RAP picture. In some drafts of the HEVC standard, a RASL picture was referred to a Tagged for Discard (TFD) picture.

All RADL pictures are leading pictures. RADL pictures are not used as reference pictures for the decoding process of trailing pictures of the same associated RAP picture. When present, all RADL pictures precede, in decoding order, all trailing pictures of the same associated RAP picture. RADL pictures do not refer to any picture preceding the associated RAP picture in decoding order and can therefore be correctly decoded when the decoding starts from the associated RAP picture. In some drafts of the HEVC standard, a RADL picture was referred to a Decodable Leading Picture (DLP).

When a part of a bitstream starting from a CRA picture is included in another bitstream, the RASL pictures associated with the CRA picture might not be correctly decodable, because some of their reference pictures might not be present in the combined bitstream. To make such a splicing operation straightforward, the NAL unit type of the CRA picture can be changed to indicate that it is a BLA picture. The RASL pictures associated with a BLA picture may not be correctly decodable hence are not be output/displayed. Furthermore, the RASL pictures associated with a BLA picture may be omitted from decoding.

A BLA picture may be the first picture in the bitstream in decoding order, or may appear later in the bitstream. Each BLA picture begins a new coded video sequence, and has similar effect on the decoding process as an IDR picture. However, a BLA picture may contain syntax elements that specify a non-empty reference picture set. When a BLA picture has nal unit type equal to BLA W LP, it may have associated RASL pictures, which are not output by the decoder and may not be decodable, as they may contain references to pictures that are not present in the bitstream. When a BLA picture has nal unit type equal to BLA W LP, it may also have associated RADL pictures, which are specified to be decoded. When a BLA picture has nal unit type equal to BLA W DLP, it does not have associated RASL pictures but may have associated RADL pictures, which are specified to be decoded. When a BLA picture has nal unit type equal to BLA N LP, it does not have any associated leading pictures. An IDR picture having nal unit type equal to IDR N LP does not have associated leading pictures present in the bitstream. An IDR picture having nal unit type equal to IDR W LP does not have associated RASL pictures present in the bitstream, but may have associated RADL pictures in the bitstream. When the value of nal unit type is equal to TRAIL N, TSA_N, STSA_N, RADL N, RASL N, RS V VCL N 10, RS V VCL N 12, or RS V VCL N 14, the decoded picture is not used as a reference for any other picture of the same temporal sub-layer. That is, in HEVC, when the value of nal_unit_type is equal to TRAIL N, TSA_N, STSA_N, RADL N, RASL N, RSV VCL N 10, RSV VCL N 12, or RSV VCL N 14, the decoded picture is not included in any of RefPicSetStCurrBefore, RefPicSetStCurrAfter and RefPicSetLtCurr of any picture with the same value of Temporalld. A coded picture with nal unit type equal to TRAIL N, TSA N, STSA N, RADL N, RASL N, RSV VCL N10, RSV VCL N12, or RSV_VCL_N14 may be discarded without affecting the decodability of other pictures with the same value of Temporalld.

A trailing picture may be defined as a picture that follows the associated RAP picture in output order. Any picture that is a trailing picture does not have nal unit type equal to RADL N, RADL R, RASL N or RASL R. Any picture that is a leading picture may be constrained to precede, in decoding order, all trailing pictures that are associated with the same RAP picture. No RASL pictures are present in the bitstream that are associated with a BLA picture having nal unit type equal to BLA W DLP or BLA N LP. No RADL pictures are present in the bitstream that are associated with a BLA picture having nal unit type equal to BLA N LP or that are associated with an IDR picture having nal unit type equal to IDR N LP. Any RASL picture associated with a CRA or BLA picture may be constrained to precede any RADL picture associated with the CRA or BLA picture in output order. Any RASL picture associated with a CRA picture may be constrained to follow, in output order, any other RAP picture that precedes the CRA picture in decoding order.

In HEVC there are two picture types, the TSA and STSA picture types that can be used to indicate temporal sub-layer switching points. If temporal sub-layers with Temporalld up to N had been decoded until the TSA or STSA picture (exclusive) and the TSA or STSA picture has Temporalld equal to N+l, the TSA or STSA picture enables decoding of all subsequent pictures (in decoding order) having Temporalld equal to N+l . The TSA picture type may impose restrictions on the TSA picture itself and all pictures in the same sub-layer that follow the TSA picture in decoding order. None of these pictures is allowed to use inter prediction from any picture in the same sub-layer that precedes the TSA picture in decoding order. The TSA definition may further impose restrictions on the pictures in higher sub-layers that follow the TSA picture in decoding order. None of these pictures is allowed to refer a picture that precedes the TSA picture in decoding order if that picture belongs to the same or higher sub-layer as the TSA picture. TSA pictures have Temporalld greater than 0. The STSA is similar to the TSA picture but does not impose restrictions on the pictures in higher sublayers that follow the STSA picture in decoding order and hence enable up-switching only onto the sub-layer where the STSA picture resides. A non-VCL NAL unit may be for example one of the following types: a sequence parameter set, a picture parameter set, a supplemental enhancement information (SEI) NAL unit, an access unit delimiter, an end of sequence NAL unit, an end of bitstream NAL unit, or a filler data NAL unit. Parameter sets may be needed for the reconstruction of decoded pictures, whereas many of the other non-VCL NAL units are not necessary for the reconstruction of decoded sample values.

Parameters that remain unchanged through a coded video sequence may be included in a sequence parameter set. In addition to the parameters that may be needed by the decoding process, the sequence parameter set may optionally contain video usability information (VUI), which includes parameters that may be important for buffering, picture output timing, rendering, and resource reservation. There are three NAL units specified in H.264/AVC to carry sequence parameter sets: the sequence parameter set NAL unit containing all the data for H.264/AVC VCL NAL units in the sequence, the sequence parameter set extension NAL unit containing the data for auxiliary coded pictures, and the subset sequence parameter set for MVC and SVC VCL NAL units. In HEVC a sequence parameter set RBSP includes parameters that can be referred to by one or more picture parameter set RBSPs or one or more SEI NAL units containing a buffering period SEI message. A picture parameter set contains such parameters that are likely to be unchanged in several coded pictures. A picture parameter set RBSP may include parameters that can be referred to by the coded slice NAL units of one or more coded pictures. In HEVC, a video parameter set (VPS) may be defined as a syntax structure containing syntax elements that apply to zero or more entire coded video sequences as determined by the content of a syntax element found in the SPS referred to by a syntax element found in the PPS referred to by a syntax element found in each slice segment header. A video parameter set RBSP may include parameters that can be referred to by one or more sequence parameter set RBSPs.

The relationship and hierarchy between video parameter set (VPS), sequence parameter set (SPS), and picture parameter set (PPS) may be described as follows. VPS resides one level above SPS in the parameter set hierarchy and in the context of scalability and/or 3D video. VPS may include parameters that are common for all slices across all (scalability or view) layers in the entire coded video sequence. SPS includes the parameters that are common for all slices in a particular (scalability or view) layer in the entire coded video sequence, and may be shared by multiple (scalability or view) layers. PPS includes the parameters that are common for all slices in a particular layer representation (the representation of one scalability or view layer in one access unit) and are likely to be shared by all slices in multiple layer representations.

VPS may provide information about the dependency relationships of the layers in a bitstream, as well as many other information that are applicable to all slices across all (scalability or view) layers in the entire coded video sequence. VPS may be considered to comprise three parts, the base VPS, the VPS extension, and the VPS VUI, where the VPS extension and the VPS VUI may be optionally present. In HEVC, the base VPS may be considered to comprise the video_parameter_set_rbsp( ) syntax structure without the vps_extension( ) syntax structure. The video _parameter_set_rbsp( ) syntax structure was primarily specified already for HEVC version 1 and includes syntax elements which may be of use for base layer decoding. In HEVC, the VPS extension may be considered to comprise the vps_extension( ) syntax structure. The vps_extension( ) syntax structure was specified in HEVC version 2 primarily for multi-layer extensions and comprises syntax elements which may be of use for decoding of one or more non-base layers, such as syntax elements indicating layer dependency relations. The VPS VUI comprises syntax elements that may be useful for decoding or other purposes but are not required to be used in the HEVC decoding process. H.264/ AVC and HEVC syntax allows many instances of parameter sets, and each instance is identified with a unique identifier. In order to limit the memory usage needed for parameter sets, the value range for parameter set identifiers has been limited. In H.264/AVC and HEVC, each slice header includes the identifier of the picture parameter set that is active for the decoding of the picture that contains the slice, and each picture parameter set contains the identifier of the active sequence parameter set. Consequently, the transmission of picture and sequence parameter sets does not have to be accurately synchronized with the transmission of slices. Instead, it is sufficient that the active sequence and picture parameter sets are received at any moment before they are referenced, which allows transmission of parameter sets "out-of-band" using a more reliable transmission mechanism compared to the protocols used for the slice data. For example, parameter sets can be included as a parameter in the session description for Real-time Transport Protocol (RTP) sessions. If parameter sets are transmitted in -band, they can be repeated to improve error robustness. Out-of-band transmission, signaling or storage can additionally or alternatively be used for other purposes than tolerance against transmission errors, such as ease of access or session negotiation. For example, a sample entry of a track in a file conforming to the ISO Base Media File Format may comprise parameter sets, while the coded data in the bitstream is stored elsewhere in the file or in another file. The phrase along the bitstream (e.g. indicating along the bitstream) may be used in claims and described embodiments to refer to out-of- band transmission, signaling, or storage in a manner that the out-of-band data is associated with the bitstream. The phrase decoding along the bitstream or alike may refer to decoding the referred out-of-band data (which may be obtained from out-of-band transmission, signaling, or storage) that is associated with the bitstream.

A parameter set may be activatedby a reference from a slice or from another active parameter set or in some cases from another syntax structure such as a buffering period SEI message.

A SEI NAL unit may contain one or more SEI messages, which are not required for the decoding of output pictures but may assist in related processes, such as picture output timing, rendering, error detection, error concealment, and resource reservation. Several SEI messages are specified in H.264/AVC and HEVC, and the user data SEI messages enable organizations and companies to specify SEI messages for their own use. H.264/AVC and HEVC contain the syntax and semantics for the specified SEI messages but no process for handling the messages in the recipient is defined. Consequently, encoders are required to follow the H.264/AVC standard or the HEVC standard when they create SEI messages, and decoders conforming to the H.264/AVC standard or the HEVC standard, respectively, are not required to process SEI messages for output order conformance. One of the reasons to include the syntax and semantics of SEI messages in H.264/AVC and HEVC is to allow different system specifications to interpret the supplemental information identically and hence interoperate. It is intended that system specifications can require the use of particular SEI messages both in the encoding end and in the decoding end, and additionally the process for handling particular SEI messages in the recipient can be specified.

Several nesting SEI messages have been specified in the AVC and HEVC standards or proposed otherwise. The idea of nesting SEI messages is to contain one or more SEI messages within a nesting SEI message and provide a mechanism for associating the contained SEI messages with a subset of the bitstream and/or a subset of decoded data. It may be required that a nesting SEI message contains one or more SEI messages that are not nesting SEI messages themselves. An SEI message contained in a nesting SEI message may be referred to as a nested SEI message. An SEI message not contained in a nesting SEI message may be referred to as a non-nested SEI message. The scalable nesting SEI message of HEVC enables to identify either a bitstream subset (resulting from a sub-bitstream extraction process) or a set of layers to which the nested SEI messages apply. A bitstream subset may also be referred to as a sub-bitstream. A coded picture is a coded representation of a picture. A coded picture in H.264/AVC comprises the VCL NAL units that are required for the decoding of the picture. In H.264/AVC, a coded picture can be a primary coded picture or a redundant coded picture. A primary coded picture is used in the decoding process of valid bitstreams, whereas a redundant coded picture is a redundant representation that should only be decoded when the primary coded picture cannot be successfully decoded. In HEVC, no redundant coded picture has been specified.

In H.264/AVC, an access unit (AU) comprises a primary coded picture and those NAL units that are associated with it. In H.264/ AVC, the appearance order of NAL units within an access unit is constrained as follows. An optional access unit delimiter NAL unit may indicate the start of an access unit. It is followed by zero or more SEINAL units. The coded slices of the primary coded picture appear next. In H.264/ AVC, the coded slice of the primary coded picture may be followed by coded slices for zero or more redundant coded pictures. A redundant coded picture is a coded representation of a picture or a part of a picture. A redundant coded picture may be decoded if the primary coded picture is not received by the decoder for example due to a loss in transmission or a corruption in physical storage medium. In H.264/AVC, an access unit may also include an auxiliary coded picture, which is a picture that supplements the primary coded picture and may be used for example in the display process. An auxiliary coded picture may for example be used as an alpha channel or alpha plane specifying the transparency level of the samples in the decoded pictures. An alpha channel or plane may be used in a layered composition or rendering system, where the output picture is formed by overlaying pictures being at least partly transparent on top of each other. An auxiliary coded picture has the same syntactic and semantic restrictions as a monochrome redundant coded picture. In H.264/AVC, an auxiliary coded picture contains the same number of macroblocks as the primary coded picture.

In HEVC, a coded picture may be defined as a coded representation of a picture containing all coding tree units of the picture. In HEVC, an access unit (AU) may be defined as a set of NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain at most one picture with any specific value of nuh layer id. In addition to containing the VCL NAL units of the coded picture, an access unit may also contain non-VCL NAL units.

A bitstream may be defined as a sequence of bits, in the form of a NAL unit stream or a byte stream, that forms the representation of coded pictures and associated data forming one or more coded video sequences. A first bitstream may be followed by a second bitstream in the same logical channel, such as in the same file or in the same connection of a communication protocol. An elementary stream (in the context of video coding) may be defined as a sequence of one or more bitstreams. The end of the first bitstream may be indicated by a specific NAL unit, which may be referred to as the end of bitstream (EOB) NAL unit and which is the last NAL unit of the bitstream. In HEVC and its current draft extensions, the EOB NAL unit is required to have nuh layer id equal to 0.

In H.264/AVC, a coded video sequence is defined to be a sequence of consecutive access units in decoding order from an IDR access unit, inclusive, to the next IDR access unit, exclusive, or to the end of the bitstream, whichever appears earlier.

In HEVC, a coded video sequence (CVS) may be defined, for example, as a sequence of access units that consists, in decoding order, of an IRAP access unit with NoRaslOutputFlag equal to 1 , followed by zero or more access units that are not IRAP access units with NoRaslOutputFlag equal to 1, including all subsequent access units up to but not including any subsequent access unit that is an IRAP access unit with NoRaslOutputFlag equal to 1. An IRAP access unit may be defined as an access unit in which the base layer picture is an IRAP picture. The value of NoRaslOutputFlag is equal to 1 for each IDR picture, each BLA picture, and each IRAP picture that is the first picture in that particular layer in the bitstream in decoding order, is the first IRAP picture that follows an end of sequence NAL unit having the same value of nuh layer id in decoding order. In multi-layer HEVC, the value of NoRaslOutputFlag is equal to 1 for each IRAP picture when its nuh layer id is such that LayerInitializedFlag[ nuh layer id ] is equal to 0 and LayerInitializedFlag[ refLayerld ] is equal to 1 for all values of refLayerld equal to IdDirectRefLayer[ nuh layer id ][ j ], where j is in the range of 0 to NumDirectRefLayers[ nuh layer id ] - 1 , inclusive. Otherwise, the value of NoRaslOutputFlag is equal HandleCraAsBlaFlag. NoRaslOutputFlag equal to 1 has an impact that the RASL pictures associated with the IRAP picture for which the NoRaslOutputFlag is set are not output by the decoder. There may be means to provide the value of HandleCraAsBlaFlag to the decoder from an external entity, such as a player or a receiver, which may control the decoder. HandleCraAsBlaFlag may be set to 1 for example by a player that seeks to a new position in a bitstream or tunes into a broadcast and starts decoding and then starts decoding from a CRA picture. When HandleCraAsBlaFlag is equal to 1 for a CRA picture, the CRA picture is handled and decoded as if it were a BLA picture.

In HEVC, a coded video sequence may additionally or alternatively (to the specification above) be specified to end, when a specific NAL unit, which may be referred to as an end of sequence (EOS) NAL unit, appears in the bitstream and has nuh layer id equal to 0.

In HEVC, a coded video sequence group (CVSG) may be defined, for example, as one or more consecutive CVSs in decoding order that collectively consist of an IRAP access unit that activates a VPS RBSP firstVpsRbsp that was not already active followed by all subsequent access units, in decoding order, for which firstVpsRbsp is the active VPS RBSP up to the end of the bitstream or up to but excluding the access unit that activates a different VPS RBSP than firstVpsRbsp, whichever is earlier in decoding order.

A group of pictures (GOP) and its characteristics may be defined as follows. A GOP can be decoded regardless of whether any previous pictures were decoded. An open GOP is such a group of pictures in which pictures preceding the initial intra picture in output order might not be correctly decodable when the decoding starts from the initial intra picture of the open GOP. In other words, pictures of an open GOP may refer (in inter prediction) to pictures belonging to a previous GOP. An H.264/AVC decoder can recognize an intra picture starting an open GOP from the recovery point SEI message in an H.264/AVC bitstream. An HEVC decoder can recognize an intra picture starting an open GOP, because a specific NAL unit type, CRA NAL unit type, may be used for its coded slices. A closed GOP is such a group of pictures in which all pictures can be correctly decoded when the decoding starts from the initial intra picture of the closed GOP. In other words, no picture in a closed GOP refers to any pictures in previous GOPs. In H.264/AVC and HEVC, a closed GOP may start from an IDR picture. In HEVC a closed GOP may also start from a BLA W RADL or a BLA N LP picture. An open GOP coding structure is potentially more efficient in the compression compared to a closed GOP coding structure, due to a larger flexibility in selection of reference pictures.

A Structure of Pictures (SOP) may be defined as one or more coded pictures consecutive in decoding order, in which the first coded picture in decoding order is a reference picture at the lowest temporal sub-layer and no coded picture except potentially the first coded picture in decoding order is a RAP picture. All pictures in the previous SOP precede in decoding order all pictures in the current SOP and all pictures in the next SOP succeed in decoding order all pictures in the current SOP. A SOP may represent a hierarchical and repetitive inter prediction structure. The term group of pictures (GOP) may sometimes be used interchangeably with the term SOP and having the same semantics as the semantics of SOP.

The bitstream syntax of H.264/AVC and HEVC indicates whether a particular picture is a reference picture for inter prediction of any other picture. Pictures of any coding type (I, P, B) can be reference pictures or non-reference pictures in H.264/AVC and HEVC.

H.264/AVC specifies the process for decoded reference picture marking in order to control the memory consumption in the decoder. The maximum number of reference pictures used for inter prediction, referred to as M, is determined in the sequence parameter set. When a reference picture is decoded, it is marked as "used for reference". If the decoding of the reference picture caused more than M pictures marked as "used for reference", at least one picture is marked as "unused for reference". There are two types of operation for decoded reference picture marking: adaptive memory control and sliding window. The operation mode for decoded reference picture marking is selected on picture basis. The adaptive memory control enables explicit signaling which pictures are marked as "unused for reference" and may also assign long-term indices to short-term reference pictures. The adaptive memory control may require the presence of memory management control operation (MMCO) parameters in the bitstream. MMCO parameters may be included in a decoded reference picture marking syntax structure. If the sliding window operation mode is in use and there are M pictures marked as "used for reference", the short-term reference picture that was the first decoded picture among those short-term reference pictures that are marked as "used for reference" is marked as "unused for reference". In other words, the sliding window operation mode results into first-in- first-out buffering operation among short-term reference pictures.

One of the memory management control operations in H.264/AVC causes all reference pictures except for the current picture to be marked as "unused for reference". An instantaneous decoding refresh (IDR) picture contains only intra-coded slices and causes a similar "reset" of reference pictures.

In HEVC, reference picture marking syntax structures and related decoding processes are not used, but instead a reference picture set (RPS) syntax structure and decoding process are used instead for a similar purpose. A reference picture set valid or active for a picture includes all the reference pictures used as reference for the picture and all the reference pictures that are kept marked as "used for reference" for any subsequent pictures in decoding order. There are six subsets of the reference picture set, which are referred to as namely RefPicSetStCurrO (a.k.a. RefPicSetStCurrBefore), RefPicSetStCurrl (a.k.a. RefPicSetStCurrAfter), RefPicSetStFoUO, RefPicSetStFolll , RefPicSetLtCurr, and RefPicSetLtFoll. RefPicSetStFoUO and RefPicSetStFolll may also be considered to form jointly one subset RefPicSetStFoll. The notation of the six subsets is as follows. "Curr" refers to reference pictures that are included in the reference picture lists of the current picture and hence may be used as inter prediction reference for the current picture. "Foil" refers to reference pictures that are not included in the reference picture lists of the current picture but may be used in subsequent pictures in decoding order as reference pictures. "St" refers to short-term reference pictures, which may generally be identified through a certain number of least significant bits of their POC value. "Lt" refers to long-term reference pictures, which are specifically identified and generally have a greater difference of POC values relative to the current picture than what can be represented by the mentioned certain number of least significant bits. "0" refers to those reference pictures that have a smaller POC value than that of the current picture. "1" refers to those reference pictures that have a greater POC value than that of the current picture. RefPicSetStCurrO, RefPicSetStCurrl, RefPicSetStFoUO and RefPicSetStFolll are collectively referred to as the short-term subset of the reference picture set. RefPicSetLtCurr and RefPicSetLtFoll are collectively referred to as the long-term subset of the reference picture set.

In HEVC, a reference picture set may be specified in a sequence parameter set and taken into use in the slice header through an index to the reference picture set. A reference picture set may also be specified in a slice header. A reference picture set may be coded independently or may be predicted from another reference picture set (known as inter-RPS prediction). In both types of reference picture set coding, a flag (used_by_curr_pic_X_flag) is additionally sent for each reference picture indicating whether the reference picture is used for reference by the current picture (included in a *Curr list) or not (included in a *Foll list). Pictures that are included in the reference picture set used by the current slice are marked as "used for reference", and pictures that are not in the reference picture set used by the current slice are marked as "unused for reference". If the current picture is an IDR picture, RefPicSetStCurrO, RefPicSetStCurrl, RefPicSetStFollO, RefPicSetStFolll , RefPicSetLtCurr, and RefPicSetLtFoll are all set to empty. Scalable video coding may refer to coding structure where one bitstream can contain multiple representations of the content at different bitrates, resolutions or frame rates. In these cases the receiver can extract the desired representation depending on its characteristics (e.g. resolution that matches best the display device). Alternatively, a server or a network element can extract the portions of the bitstream to be transmitted to the receiver depending on e.g. the network characteristics or processing capabilities of the receiver. A scalable bitstream may consist of a "base layer" providing the lowest quality video available and one or more enhancement layers that enhance the video quality when received and decoded together with the lower layers. In order to improve coding efficiency for the enhancement layers, the coded representation of that layer may depend on the lower layers. E.g. the motion and mode information of the enhancement layer may be predicted from lower layers. Similarly the pixel data of the lower layers may be used to create prediction for the enhancement layer.

A scalable video codec for quality scalability (also known as Signal-to-Noise or SNR) and/or spatial scalability may be implemented as follows. For a base layer, a conventional non- scalable video encoder and decoder are used. The reconstructed/decoded pictures of the base layer are included in the reference picture buffer for an enhancement layer. In H.264/AVC, HEVC, and similar codecs using reference picture list(s) for inter prediction, the base layer decoded pictures may be inserted into a reference picture list(s) for coding/decoding of an enhancement layer picture similarly to the decoded reference pictures of the enhancement layer. Consequently, the encoder may choose a base-layer reference picture as an inter prediction reference and indicate its use with a reference picture index in the coded bitstream. The decoder decodes from the bitstream, for example from a reference picture index, that a base-layer picture is used as an inter prediction reference for the enhancement layer. When a decoded base-layer picture is used as a prediction reference for an enhancement layer, it is referred to as an inter-layer reference picture. In addition to quality scalability, there may also be other scalability modes or scalability dimensions, which may include but are not limited to the following:

- Quality scalability: Base layer pictures are coded at a lower quality than enhancement layer pictures, which may be achieved for example using a greater quantization parameter value (i.e., a greater quantization step size for transform coefficient quantization) in the base layer than in the enhancement layer. Quality scalability may be further categorized into fine-grain or fine-granularity scalability (FGS), medium- grain or medium-granularity scalability (MGS), and/or coarse-grain or coarse- granularity scalability (CGS), as described below.

- Spatial scalability: Base layer pictures are coded at a lower resolution (i.e. have fewer samples) than enhancement layer pictures. Spatial scalability and quality scalability, particularly its coarse-grain scalability type, may sometimes be considered the same type of scalability.

- Bit-depth scalability: Base layer pictures are coded at lower bit-depth (e.g. 8 bits) than enhancement layer pictures (e.g. 10 or 12 bits).

- Chroma format scalability: Base layer pictures provide lower spatial resolution in chroma sample arrays (e.g. coded in 4:2:0 chroma format) than enhancement layer pictures (e.g. 4:4:4 format).

- Color gamut scalability: enhancement layer pictures have a richer/broader color representation range than that of the base layer pictures - for example the enhancement layer may have UHDTV (ITU-R BT.2020) color gamut and the base layer may have the ITU-R BT.709 color gamut.

- View scalability, which may also be referred to as multiview coding. The base layer represents a first view, whereas an enhancement layer represents a second view.

- Depth scalability, which may also be referred to as depth-enhanced coding. A layer or some layers of a bitstream may represent texture view(s), while other layer or layers may represent depth view(s).

- Region-of-interest scalability (as described below).

- Interlaced-to-progressive scalability (also known as field-to -frame scalability): coded interlaced source content material of the base layer is enhanced with an enhancement layer to represent progressive source content. The coded interlaced source content in the base layer may comprise coded fields, coded frames representing field pairs, or a mixture of them. In the interlace-to-progressive scalability, the base-layer picture may be resampled so that it becomes a suitable reference picture for one or more enhancement-layer pictures.

- Hybrid codec scalability (also known as coding standard scalability): In hybrid codec scalability, the bitstream syntax, semantics and decoding process of the base layer and the enhancement layer are specified in different video coding standards. Thus, base layer pictures are coded according to a different coding standard or format than enhancement layer pictures. For example, the base layer may be coded with H.264/AVC and an enhancement layer may be coded with an HEVC multi-layer extension. More generally, in hybrid codec scalability one or more layers may be coded according to one coding standard or specification and other one or more layers may be coded according to another coding standard or specification. For example, there may be two layers coded according to the MVC extension of H.264/AVC (out of which one is a base layer coded according to H.264/AVC), and one or more additional layers coded according to MV-HEVC. Furthermore, the number of coding standard or specifications according to which different layers of the same bitstream are coded might not be limited to two in hybrid codec scalability.

It should be understood that many of the scalability types may be combined and applied together. For example color gamut scalability and bit-depth scalability may be combined.

In the above scalability cases, base layer information may be used to code enhancement layer to minimize the additional bitrate overhead. Scalability may be enabled in two ways. Either by introducing new coding modes for performing prediction of pixel values or syntax from lower layers of the scalable representation or by placing the lower layer pictures to the reference picture buffer (decoded picture buffer, DPB) of the higher layer. The first approach is more flexible and thus can provide better coding efficiency in most cases. However, the second, reference frame based scalability, approach can be implemented very efficiently with minimal changes to single layer codecs while still achieving majority of the coding efficiency gains available. Essentially a reference frame based scalability codec may be implemented by utilizing the same hardware or software implementation for all the layers, just taking care of the DPB management by external means.

In order to be able to utilize parallel processing, images may be split into independently codable and decodable image segments (slices or tiles). Slices may refer to image segments constructed of certain number of basic coding units that are processed in default coding or decoding order, while tiles may refer to image segments that have been defined as rectangular image regions that are processed at least to some extend as individual frames. In the following, a method in an encoder 500, 502 in accordance with an embodiment will be described in more detail with reference to the block diagram of Figure 7, Figures 8a— 8c and the flow diagram of Figure 9. Figure 7 illustrates a motion vector determination element 70 as a simplified block diagram, in accordance with an embodiment. The functionality of the motion vector determination element 70 may be included in the inter predictor 306, 406 (Figure 4) or it may be a separate element of the encoder 500. A coding unit which currently is to be coded 900 may be called as a current coding unit. A block to be currently motion predicted may be called as a current block of a current picture and is depicted with the reference numeral 81 in Figure 8a whereas the current picture is depicted with the reference numeral 80. A motion vector selector 71 may select 902 two motion vector candidates for bi-directional motion prediction which may be called as a first motion vector candidate 82 pointing to a first reference block 83 in a first reference picture 84 and a second motion vector candidate 85 pointing to a second reference block 86 in a second reference picture 87. The first reference picture 84 and the second reference picture 87 may have been stored into the reference frame memory 706, 806 when the first reference picture 84 and the second reference picture 87 have been previously decoded. Information of the selected motion vector candidates 82, 85 may be provided 904 to a motion vector modifier 72, which performs a sliding operation to the selected motion vector candidates in at least one of a horizontal and a vertical direction as will now be explained, starting from the horizontal adjustment. The sliding operation may be limited so that a "slided" motion vector should stay within a certain distance from the selected motion vector candidates 82, 85. However, the allowable distance may be different in the horizontal direction and the vertical direction, or may be the same in both directions. The motion vector modifier 72 adds 906 a difference motion vector 88 to a horizontal component of the first motion vector candidate 82, wherein a modified first motion vector candidate 89, slightly displaced from the first motion vector candidate 82, is obtained. Correspondingly, the motion vector modifier 72 subtracts 908 the difference motion vector 88 from a horizontal component of a second motion vector candidate 85, wherein a modified second motion vector candidate 90 is obtained, which is, respectively, slightly displaced from the second motion vector candidate 85. Information of the modified first motion vector candidate 89 and the modified second motion vector candidate 90 are provided 910 to a cost estimator 73. The cost estimator 73 calculates 912 or evaluates by some other appropriate way a cost value which is indicative of the efficiency of motion estimation if the modified first motion vector candidate 89 and the modified second motion vector candidate 90 were used as motion vectors for the current block 81. The evaluated cost value may be stored 914 to a memory 58. The motion vector determination element 70 may then examine 916 whether a cost for all alternative motion vector candidates for the current block have been evaluated. This may be deduced e.g. so that the motion vector determination element 70 examines whether the distance of the latest modified first motion vector candidate 89 from the originally selected first motion vector candidate 82 is smaller than a maximum value and/ or whether the distance of the latest modified second motion vector candidate 90 from the originally selected second motion vector candidate 85 is smaller than the maximum value. If so, another cost estimation may be performed. In other words, the motion vector modifier 72 of the motion vector determination element 70 adds 906 the difference motion vector 88 to the horizontal component of the modified first motion vector candidate 82, wherein another modified first motion vector candidate 89 is obtained. The motion vector modifier 72 also subtracts 908 the difference motion vector from a horizontal component of the modified second motion vector candidate 85, wherein another modified second motion vector candidate 90 is obtained. The cost estimator 73 calculates 912 or evaluates by some other appropriate way a new cost value using the another modified first motion vector candidate 89 and the another modified second motion vector candidate 90. The evaluated cost value may be stored 914 to the memory 58.

Hence, the above described sliding operation and cost estimation may be repeated until a predetermined condition is fulfilled 916. This predetermined condition may limit the maximum distance between the first motion vector candidate 82 and the modified first motion vector candidate 89 within a certain maximum value and, correspondingly, the distance between the second motion vector candidate 85 and the modified first motion vector candidate 89 within the certain maximum value at least in the horizontal and/or the vertical direction. In accordance with another embodiment, the predetermined condition may be a threshold for the cost value. In other words, the search may be stopped if a small enough cost value (less than the threshold) has been found, wherein those motion vector candidates 82, 85 which resulted the cost value dropping below the threshold may be selected as the motion vectors. When the predetermined condition is fulfilled, those motion vector candidates which produced the smallest cost value may be selected 918 as the motion vectors for encoding the current block.

It should be noted that instead of the horizontal direction the vertical direction may be examined first and after that the horizontal direction. Horizontal and vertical directions can be also examined in interleaved fashion, for example examining first positive values of vertical 88, then positive values of horizontal 88, followed by negative values of vertical 88 and finally negative values of horizontal 88.

The difference motion vector 88 may be selected so that the modified first motion vector candidate and the modified second motion vector candidate are closer to each other compared to the first motion vector candidate and the second motion vector candidate, or vice versa i.e. the modified first motion vector candidate and the modified second motion vector candidate are further away from each other than the first motion vector candidate and the second motion vector candidate. In other words, the modified first motion vector candidate and the modified second motion vector candidate slide towards each other or further away from each other. In accordance with an embodiment, this is done by either vertical or horizontal direction - by applying the difference motion vector to the horizontal components of the original motion vectors where the difference component is added to one of the original motion vectors and deducted from the other. In this way the resulting motion vectors are either bringing the reference blocks closer together or pushing the blocks further away from each other in the selected direction - and effectively aligning the structures in the reference blocks in order to provide a more consistent prediction block when the two predictions are combined into one.

In the above description the sliding operation was performed so that the same difference vector was added to or subtracted from a previous result but the sliding operation may also be performed otherwise. For example, a zig-zag type of operation may be possible where the difference vector is first added to the first motion vector candidate 82 and subtracted from the second motion vector candidate 85. In the next sliding operation round the difference vector is first subtracted from the first motion vector candidate 82 and added to the second motion vector candidate 85. In the subsequent sliding operation the horizontal/vertical component of the difference vector is multiplied by two and added to the first motion vector candidate 82 and subtracted from the second motion vector candidate 85. In a further sliding operation, the multiplied version of the difference vector is subtracted from the first motion vector candidate 82 and added to the second motion vector candidate 85.

In accordance with a yet another embodiment the sliding operation may first advance the modified motion vector candidates to a first direction from the first/second motion vector candidates and after reaching the maximum value the sliding operation advances from the first/second motion vector candidates other side to the other direction.

In order to select the final motion vectors the candidate reference blocks (or motion compensated blocks) are compared to each other using a cost measure and the pair of motion vectors giving the smallest cost may be selected by a motion estimator 708 as the final motion vectors for the block. The cost measure can be for example the sum of absolute differences of the reference samples or the sum of squared differences of the reference samples. Algorithmically the above described process may be described for example as:

// Horizontal analysis:

for x from xMin to xMax

{

mvUpdate[x] = ( x, 0 )

mvCandl = mvOrigl + mvUpdate

mvCand2 = mvOrig2 - mvUpdate

predBlkl = motionCompensate( mvCandl )

predBlk2 = motionCompensate( mvCand2 )

costHor[x] = costFunction( predBlkl, predBlk2 )

}

// Vertical analysis:

for y from yMin to yMax

{

mvUpdate[y] = ( 0, y )

mvCandl = mvOrigl + mvUpdate

mvCand2 = mvOrig2 - mvUpdate

predBlkl = motionCompensate( mvCandl )

predBlk2 = motionCompensate( mvCand2 )

costVer[y] = costFunction( predBlkl, predBlk2 )

}

// Final motion vectors:

mvUpdateBest = mvUpdate WithMinimumCost( costHor[], costVer[] )

mvFinall = mvOrigl + mvUpdateBest

mvFinal2 = mvOrig2 - mvUpdateBest

In the horizontal analysis phase the variable x is controlling the offset between the two predictions. That parameter can be expressed either in full pixel or fractional pixel accuracy. During the process, different values of x are tested. In the example above, the value is ranging from xMin to xMax inclusive. xMin could be selected to be equal to -xMax making the search symmetrical around the original motion vectors mvOrigl and mvOrig2. Different approaches to speed up the search may be used. For example, the search can start with x being equal to 0, proceed to both positive and negative directions and terminate if the search is not giving costs that improve over the previously found costs. The search can also start at certain accuracy, e.g. at full pixel accuracy which can be followed by refinement searches at finer accuracy, such as half pixel accuracy. For each tested value of x, a motion vector update candidate mvUpdate[x] may be generated. This update is further added to one of the original motion vectors mvOrigl and deducted from the other original motion vector mvOrig2 resulting in a motion vector candidate pair mvCandl, mvCand2. Instead of direct addition and deduction, the motion vector update candidate mvUpdate could be also scaled differently for the two cases. This pair is used to generate two candidate prediction blocks predBlkl and predBlk2, respectively. Finally, a selected cost function is evaluated to measure the similarity of the two prediction blocks. As mentioned above, e.g. the sum of absolute or squared differences can be used as a cost function, but other functions could also be used. When it comes to generating the two candidate prediction blocks predBlkl and predBlk2, it should be noted that in the case the range of motion vector update candidates is limited to a certain area it may be more efficient to motion compensate the area containing all samples required by different motion vector candidates before entering the analysis phase. In that way the encoder or decoder may not need to perform a complete motion estimation every time a new cost function call is made, but one can use samples of the already motion compensated area with an offset corresponding to the motion vector update candidate instead in the cost calculation.

In accordance with an embodiment, a similar analysis may be performed for the vertical direction and the motion vector update candidate mvUpdate [x] or mvUpdate [y] yielding the smallest cost is selected as the mvUpdateBest. The final motion vectors for the bi-predicted block is then calculated by adding the mvUpdateBest to the mvOrigl and deducting the mvUpdateBest from the mvOrig2. In another alternative embodiment the horizontal analysis may be done separately for horizontal linesets within a prediction unit and the vertical analysis may be done separately for vertical linesets within a prediction unit. In general a prediction unit of size MxN samples can be split into horizontal linesets of size MxS residing on top of each other and similarly the MxN prediction unit can be split into vertical linesets of size SxN each. The size of the lineset S can be predefined and can be selected to be the minimum motion compensation block size of the codec, which can be, for example, four samples. Thus, for example in the case a prediction unit of size 16x16 samples it could be split into four 16x4 horizontal linesets or four vertical 4x16 linesets. An example of this is depicted in Figure 8c where a prediction unit of size MxN samples can be split into horizontal linesets of size MxS residing on top of each other. According to another example, the size of the lineset S can be selected to be, for example, two samples. Thus, for example in the case a prediction unit of size 16x16 samples it could be split into eight 16x2 horizontal linesets or eight vertical 2x16 linesets.

Once the horizontal and vertical analyses have been performed to different linesets in the prediction unit the sum of the vertical costs and the sum of the horizontal costs for the whole prediction unit can be compared and the final division into linesets can be selected to be the one with minimum sum of costs. In this case the selected motion vector updates can be different for different linesets allowing the codec to automatically split the prediction unit into sub-prediction units with different motion parameters. In order to lower the complexity of the motion refinement search the results of the previous searches may be used to initialize the search for the next lineset. Also the search range can be adjusted based on the results obtained for the linesets search earlier, or the analysis performed to the other dimension.

The decision to split a prediction unit into horizontal or vertical linesets can be done in different ways. For example, the decision can be based on analyzing the samples within one or both reference blocks pointed by mvOrigl and mvOrig2. In such video encoder or decoder could for example calculate the sum of horizontal and vertical gradients of the sample values within these reference blocks and select a split direction by comparing the gradients.

When the motion vectors have been defined, motion compensated prediction may be performed 914, for example by a motion prediction block 306, 406, using said motion vector candidates selected by the motion estimator 73 for a sample or a group of samples. In other words, if the motion vectors have been defined sample-wise, one sample of the current coding unit is selected for prediction and the motion vector defined for this coding unit is used in the prediction. Then, for example, a next sample in the current sample is selected and the motion vector defined for this sample in the field of motion vectors is used in the motion prediction. This procedure may be repeated until motion prediction for all samples in the current coding unit has been performed.

In a case where the motion vectors have been defined block-wise, the same motion vector may be used for each sample of the current block for motion prediction. This procedure may be repeated until motion prediction for all blocks in the current coding unit has been performed. Then, another coding unit may be selected as the current coding unit, if any, and the above described procedures may be repeated if motion prediction is to be used for this coding unit.

Different precisions may be used for representing the final motion vectors and the motion vector components. For example, 1/4 sample accuracy can be used for the motion vector components, but when evaluating the final motion vectors for different locations in a coding unit 1/16 sample accuracy can be used, for example.

Motion vectors may be evaluated for individual pixels in a coding unit or a set of samples. For example, a motion vector can be calculated for each 4x4 set of samples.

Figure 4 shows a block diagram of a video encoder in accordance with an embodiment. Figure 4 presents an encoder for two layers, but it would be appreciated that presented encoder could be similarly extended to encode more than two layers, or only one layer. Figure 4 illustrates an embodiment of a video encoder comprising a first encoder section 500 for a base layer and a second encoder section 502 for an enhancement layer. Each of the first encoder section 500 and the second encoder section 502 may comprise similar elements for encoding incoming pictures. The encoder sections 500, 502 may comprise a pixel predictor 302, 402, prediction error encoder 303, 403 and prediction error decoder 304, 404. Figure 4 also shows an embodiment of the pixel predictor 302, 402 as comprising an inter-predictor 306, 406, an intra-predictor 308, 408, a mode selector 310, 410, a filter 316, 416, and a reference frame memory 318, 418. The pixel predictor 302 of the first encoder section 500 receives 300 base layer images of a video stream to be encoded at both the inter-predictor 306 (which determines the difference between the image and a motion compensated reference frame 318) and the intra-predictor 308 (which determines a prediction for an image block based only on the already processed parts of current frame or picture). The output of both the inter-predictor and the intra-predictor are passed to the mode selector 310. The intra- predictor 308 may have more than one intra-prediction modes. Hence, each mode may perform the intra-prediction and provide the predicted signal to the mode selector 310. The mode selector 310 also receives a copy of the base layer picture 300. Correspondingly, the pixel predictor 402 of the second encoder section 502 receives 400 enhancement layer images of a video stream to be encoded at both the inter-predictor 406 (which determines the difference between the image and a motion compensated reference frame 418) and the intra- predictor 408 (which determines a prediction for an image block based only on the already processed parts of current frame or picture). The output of both the inter-predictor and the intra-predictor are passed to the mode selector 410. The intra-predictor 408 may have more than one intra-prediction modes. Hence, each mode may perform the intra-prediction and provide the predicted signal to the mode selector 410. The mode selector 410 also receives a copy of the enhancement layer picture 400.

Depending on which encoding mode is selected to encode the current coding unit, the output of the inter-predictor 306, 406 or the output of one of the optional intra-predictor modes or the output of a surface encoder within the mode selector is passed to the output of the mode selector 310, 410. The output of the mode selector is passed to a first summing device 321 , 421. The first summing device may subtract the output of the pixel predictor 302, 402 from the base layer picture 300/enhancement layer picture 400 to produce a first prediction error signal 320, 420 which is input to the prediction error encoder 303, 403.

The pixel predictor 302, 402 further receives from a preliminary reconstructor 339, 439 the combination of the prediction representation of the image block 312, 412 and the output 338, 438 of the prediction error decoder 304, 404. The preliminary reconstructed image 314, 414 may be passed to the intra-predictor 308, 408 and to a filter 316, 416. The filter 316, 416 receiving the preliminary representation may filter the preliminary representation and output a final reconstructed image 340, 440 which may be saved in a reference frame memory 318, 418. The reference frame memory 318 may be connected to the inter-predictor 306 to be used as the reference image against which a future base layer picture 300 is compared in inter-prediction operations. Subject to the base layer being selected and indicated to be source for inter-layer sample prediction and/or inter-layer motion information prediction of the enhancement layer according to some embodiments, the reference frame memory 318 may also be connected to the inter-predictor 406 to be used as the reference image against which a future enhancement layer pictures 400 is compared in inter-prediction operations. Moreover, the reference frame memory 418 may be connected to the inter-predictor 406 to be used as the reference image against which a future enhancement layer picture 400 is compared in inter-prediction operations.

Filtering parameters from the filter 316 of the first encoder section 500 may be provided to the second encoder section 502 subject to the base layer being selected and indicated to be source for predicting the filtering parameters of the enhancement layer according to some embodiments.

The prediction error encoder 303, 403 comprises a transform unit 342, 442 and a quantizer 344, 444. The transform unit 342, 442 transforms the first prediction error signal 320, 420 to a transform domain. The transform is, for example, the DCT transform. The quantizer 344, 444 quantizes the transform domain signal, e.g. the DCT coefficients, to form quantized coefficients.

The prediction error decoder 304, 404 receives the output from the prediction error encoder 303, 403 and performs the opposite processes of the prediction error encoder 303, 403 to produce a decoded prediction error signal 338, 438 which, when combined with the prediction representation of the image block 312, 412 at the second summing device 339, 439, produces the preliminary reconstructed image 314, 414. The prediction error decoder may be considered to comprise a dequantizer 361 , 461, which dequantizes the quantized coefficient values, e.g. DCT coefficients, to reconstruct the transform signal and an inverse transformation unit 363, 463, which performs the inverse transformation to the reconstructed transform signal wherein the output of the inverse transformation unit 363, 463 contains reconstructed block(s). The prediction error decoder may also comprise a block filter which may filter the reconstructed block(s) according to further decoded information and filter parameters.

The entropy encoder 330, 430 receives the output of the prediction error encoder 303, 403 and may perform a suitable entropy encoding/variable length encoding on the signal to provide error detection and correction capability. The outputs of the entropy encoders 330, 430 may be inserted into a bitstream e.g. by a multiplexer 508.

In a decoder, similar operations may be performed to reconstruct samples of a block. Figure 5 shows a block diagram of a video decoder in accordance with an embodiment. Figure 5 depicts a structure of a two-layer decoder, but it would be appreciated that the decoding operations may similarly be employed in a single-layer decoder.

The video decoder 550 comprises a first decoder section 552 for base view components and a second decoder section 554 for non-base view components. Block 556 illustrates a demultiplexer for delivering information regarding base view components to the first decoder section 552 and for delivering information regarding non-base view components to the second decoder section 554. Reference P'n stands for a predicted representation of an image block. Reference D'n stands for a reconstructed prediction error signal. Blocks 704, 804 illustrate pixel prediction operations. Blocks 705, 805 illustrate preliminary reconstructed images (I'n). Reference R'n stands for a final reconstructed image. Blocks 703, 803 illustrate inverse transform (T-l). Blocks 702, 802 illustrate inverse quantization (Q-l). Blocks 701, 801 illustrate entropy decoding (E-l). Blocks 706, 806 illustrate a reference frame memory (RFM). Blocks 707, 807 illustrate prediction (P) (either inter prediction or intra prediction). Blocks 708, 808 illustrate filtering (F). Blocks 709, 809 may be used to combine decoded prediction error information with predicted base view/non-base view components to obtain the preliminary reconstructed images 705, 805 (I'n). Preliminary reconstructed and filtered base view images may be output 710 from the first decoder section 552 and preliminary reconstructed and filtered base view images may be output 810 from the first decoder section 554.

In the following, a method in a decoder 550 in accordance with an embodiment will be described in more detail. A motion vector determination element similar to the motion vector determination element 70 illustrated in Figure 7 may also be used in the decoder 550. Also the examples of the current picture, the current block and the first and the second reference picture and block illustrated in Figures 8a— 8c and explained previously in this specification may be used when describing the operation bi-directional prediction in the decoder 550. The functionality of the motion vector determination element 70 may be included in the inter predictor 707, 807 (Figure 5) or it may be a separate element of the decoder 550. A coding unit which currently is to be decoded 1000 may be called as a current coding unit. A block 81 to be currently motion predicted may be called as a current block of a current picture 80. A motion vector selector 71 may select 1002 two motion vector candidates for bi-directional motion prediction which may be called as a first motion vector candidate 82 pointing to a first reference block 83 in a first reference picture 84 and a second motion vector candidate 85 pointing to a second reference block 86 in a second reference picture 87. The first reference picture 84 and the second reference picture 87 may have been stored into the reference frame memory 706, 806 when the first reference picture 84 and the second reference picture 87 have been previously decoded. Information of the selected motion vector candidates 82, 85 may be provided 1004 to a motion vector modifier 72, which performs a sliding operation to the selected motion vector candidates in at least one of a horizontal and a vertical direction as was described in connection with the encoding operation. The sliding operation may be limited within a certain distance from the selected motion vector candidates 82, 85. However, the allowable distance may be different in the horizontal direction and the vertical direction, or may be the same in both directions.

The motion vector modifier 72 adds 1006 a difference motion vector 88 to a horizontal component of the first motion vector candidate 82, wherein a modified first motion vector candidate 89, slightly displaced from the first motion vector candidate 82, is obtained. Correspondingly, the motion vector modifier 72 subtracts 1008 the difference motion vector 88 from a horizontal component of a second motion vector candidate 85, wherein a modified second motion vector candidate 90 is obtained, which is, respectively, slightly displaced from the second motion vector candidate 85. Information of the modified first motion vector candidate 89 and the modified second motion vector candidate 90 are provided 1010 to a cost estimator 73. The cost estimator 73 calculates 1012 or evaluates by some other appropriate way a cost value which is indicative of the efficiency of motion estimation if the modified first motion vector candidate 89 and the modified second motion vector candidate 90 were used as motion vectors for the current block 81. The evaluated cost value may be stored 1014 to a memory 58.

The motion vector determination element 70 may then examine 1016 whether a cost for all alternative motion vector candidates for the current block have been evaluated. This may be deduced e.g. so that the motion vector determination element 70 examines whether the distance of the latest modified first motion vector candidate 89 from the originally selected first motion vector candidate 82 is smaller than a maximum value and/ or whether the distance of the latest modified second motion vector candidate 90 from the originally selected second motion vector candidate 85 is smaller than the maximum value. If so, another cost estimation may be performed. In other words, the motion vector modifier 72 of the motion vector determination element 70 adds 1006 the difference motion vector 88 to the horizontal component of the modified first motion vector candidate 82, wherein another modified first motion vector candidate 89 is obtained. The motion vector modifier 72 also subtracts 1008 the difference motion vector from a horizontal component of the modified second motion vector candidate 85, wherein another modified second motion vector candidate 90 is obtained. The cost estimator 73 calculates 1012 or evaluates by some other appropriate way a new cost value using the another modified first motion vector candidate 89 and the another modified second motion vector candidate 90. The evaluated cost value may be stored 1014 to the memory 58.

Hence, the above described sliding operation and cost estimation may be repeated until a predetermined condition is fulfilled 1016. This predetermined condition may limit the maximum distance between the first motion vector candidate 82 and the modified first motion vector candidate 89 within a certain maximum value and, correspondingly, the distance between the second motion vector candidate 85 and the modified first motion vector candidate 89 within the certain maximum value at least in the horizontal and/or the vertical direction. When the predetermined condition is fulfilled, those motion vector candidates which produced the smallest cost value may be selected 1018 as the motion vectors for decoding the current block. It should be noted that instead of the horizontal direction the vertical direction may be examined first and after that the horizontal direction.

H.265/HEVC screen content test model describes a straight-forward intra block copy implementation. Intra block copy method creates a prediction for a block of samples by copying a certain area of the same image to the predicted area. The location of the source area for the copy operation may be indicated with a block vector that may be coded differentially with respect to a predicted block vector selected from a set of candidate blocks vectors formed by block vectors of some blocks coded earlier.

Figure 11 is a graphical representation of an example multimedia communication system within which various embodiments may be implemented. A data source 1110 provides a source signal in an analog, uncompressed digital, or compressed digital format, or any combination of these formats. An encoder 1 120 may include or be connected with a pre- processing, such as data format conversion and/or filtering of the source signal. The encoder 1120 encodes the source signal into a coded media bitstream. It should be noted that a bitstream to be decoded may be received directly or indirectly from a remote device located within virtually any type of network. Additionally, the bitstream may be received from local hardware or software. The encoder 1 120 may be capable of encoding more than one media type, such as audio and video, or more than one encoder 1120 may be required to code different media types of the source signal. The encoder 1120 may also get synthetically produced input, such as graphics and text, or it may be capable of producing coded bitstreams of synthetic media. In the following, only processing of one coded media bitstream of one media type is considered to simplify the description. It should be noted, however, that typically real-time broadcast services comprise several streams (typically at least one audio, video and text sub-titling stream). It should also be noted that the system may include many encoders, but in the figure only one encoder 1120 is represented to simplify the description without a lack of generality. It should be further understood that, although text and examples contained herein may specifically describe an encoding process, one skilled in the art would understand that the same concepts and principles also apply to the corresponding decoding process and vice versa.

The coded media bitstream may be transferred to a storage 1130. The storage 1 130 may comprise any type of mass memory to store the coded media bitstream. The format of the coded media bitstream in the storage 1130 may be an elementary self-contained bitstream format, or one or more coded media bitstreams may be encapsulated into a container file. If one or more media bitstreams are encapsulated in a container file, a file generator (not shown in the figure) may be used to store the one more media bitstreams in the file and create file format metadata, which may also be stored in the file. The encoder 1 120 or the storage 1130 may comprise the file generator, or the file generator is operationally attached to either the encoder 1 120 or the storage 1130. Some systems operate "live", i.e. omit storage and transfer coded media bitstream from the encoder 1120 directly to the sender 1140. The coded media bitstream may then be transferred to the sender 1140, also referred to as the server, on a need basis. The format used in the transmission may be an elementary self-contained bitstream format, a packet stream format, or one or more coded media bitstreams may be encapsulated into a container file. The encoder 1120, the storage 1 130, and the server 1 140 may reside in the same physical device or they may be included in separate devices. The encoder 1120 and server 1 140 may operate with live real-time content, in which case the coded media bitstream is typically not stored permanently, but rather buffered for small periods of time in the content encoder 1 120 and/or in the server 1 140 to smooth out variations in processing delay, transfer delay, and coded media bitrate.

The server 1140 may send the coded media bitstream using a communication protocol stack. The stack may include but is not limited to one or more of Real-Time Transport Protocol (RTP), User Datagram Protocol (UDP), Hypertext Transfer Protocol (HTTP), Transmission Control Protocol (TCP), and Internet Protocol (IP). When the communication protocol stack is packet-oriented, the server 1 140 encapsulates the coded media bitstream into packets. For example, when RTP is used, the server 1140 encapsulates the coded media bitstream into RTP packets according to an RTP payload format. Typically, each media type has a dedicated RTP payload format. It should be again noted that a system may contain more than one server 1140, but for the sake of simplicity, the following description only considers one server 1 140.

If the media content is encapsulated in a container file for the storage 1 130 or for inputting the data to the sender 1 140, the sender 1140 may comprise or be operationally attached to a "sending file parser" (not shown in the figure). In particular, if the container file is not transmitted as such but at least one of the contained coded media bitstream is encapsulated for transport over a communication protocol, a sending file parser locates appropriate parts of the coded media bitstream to be conveyed over the communication protocol. The sending file parser may also help in creating the correct format for the communication protocol, such as packet headers and payloads. The multimedia container file may contain encapsulation instructions, such as hint tracks in the ISO Base Media File Format, for encapsulation of the at least one of the contained media bitstream on the communication protocol. The server 1 140 may or may not be connected to a gateway 1 150 through a communication network. The gateway may also or alternatively be referred to as a middle -box. It is noted that the system may generally comprise any number gateways or alike, but for the sake of simplicity, the following description only considers one gateway 1 150. The gateway 1 150 may perform different types of functions, such as caching packets, streams or resources, prefetching media data pro -actively, translation of a packet stream according to one communication protocol stack to another communication protocol stack, merging and forking of data streams, and manipulation of data stream according to the downlink and/or receiver capabilities, such as controlling the bit rate of the forwarded stream according to prevailing downlink network conditions. Examples of gateways 1 150 include multipoint conference control units (MCUs), gateways between circuit-switched and packet-switched video telephony, Push-to-talk over Cellular (PoC) servers, IP encapsulators in digital video broadcasting-handheld (DVB-H) systems, or set-top boxes or other devices that forward broadcast transmissions locally to home wireless networks. When RTP is used, the gateway 1150 may be called an RTP mixer or an RTP translator and may act as an endpoint of an RTP connection. Instead of or in addition to the gateway 1 150, the system may include a splicer which concatenates video sequence or bitstreams.

The system includes one or more receivers 1 160, typically capable of receiving, de- modulating, and de-capsulating the transmitted signal into a coded media bitstream. The coded media bitstream may be transferred to a recording storage 1170. The recording storage 1170 may comprise any type of mass memory to store the coded media bitstream. The recording storage 1 170 may alternatively or additive ly comprise computation memory, such as random access memory. The format of the coded media bitstream in the recording storage 1170 may be an elementary self-contained bitstream format, or one or more coded media bitstreams may be encapsulated into a container file. If there are multiple coded media bitstreams, such as an audio stream and a video stream, associated with each other, a container file is typically used and the receiver 1160 comprises or is attached to a container file generator producing a container file from input streams. Some systems operate "live," i.e. omit the recording storage 1170 and transfer coded media bitstream from the receiver 1160 directly to the decoder 1180. In some systems, only the most recent part of the recorded stream, e.g., the most recent 10-minute excerption of the recorded stream, is maintained in the recording storage 1 170, while any earlier recorded data is discarded from the recording storage 1170.

The coded media bitstream may be transferred from the recording storage 1170 to the decoder 1 180. If there are many coded media bitstreams, such as an audio stream and a video stream, associated with each other and encapsulated into a container file or a single media bitstream is encapsulated in a container file e.g. for easier access, a file parser (not shown in the figure) is used to decapsulate each coded media bitstream from the container file. The recording storage 1 170 or a decoder 1 180 may comprise the file parser, or the file parser is attached to either recording storage 1 170 or the decoder 1 180. It should also be noted that the system may include many decoders, but here only one decoder 1 170 is discussed to simplify the description without a lack of generality.

The coded media bitstream may be processed further by a decoder 1170, whose output is one or more uncompressed media streams. Finally, a renderer 1 190 may reproduce the uncompressed media streams with a loudspeaker or a display, for example. The receiver 1160, recording storage 1 170, decoder 1170, and renderer 1 190 may reside in the same physical device or they may be included in separate devices. In the above, some embodiments have been described in relation to terms representation and/or bitstream. It needs to be understood that embodiments may be similarly be described in relation to similar terms, e.g. a representation instead of a bitstream, a bitstream instead of a representation, or an elementary stream instead of a bitstream or a representation. In the above, where the example embodiments have been described with reference to an encoder, it needs to be understood that the resulting bitstream and the decoder may have corresponding elements in them. Likewise, where the example embodiments have been described with reference to a decoder, it needs to be understood that the encoder may have structure and/or computer program for generating the bitstream to be decoded by the decoder.

The embodiments of the invention described above describe the codec in terms of separate encoder and decoder apparatus in order to assist the understanding of the processes involved. However, it would be appreciated that the apparatus, structures and operations may be implemented as a single encoder-decoder apparatus/structure/ operation. Furthermore, it is possible that the coder and decoder may share some or all common elements.

Although the above examples describe embodiments of the invention operating within a codec within an electronic device, it would be appreciated that the invention as defined in the claims may be implemented as part of any video codec. Thus, for example, embodiments of the invention may be implemented in a video codec which may implement video coding over fixed or wired communication paths. Thus, user equipment may comprise a video codec such as those described in embodiments of the invention above. It shall be appreciated that the term user equipment is intended to cover any suitable type of wireless user equipment, such as mobile telephones, portable data processing devices or portable web browsers.

Furthermore elements of a public land mobile network (PLMN) may also comprise video codecs as described above.

In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. The computer software may cause the relevant apparatuses, such as encoder or decoder, to carry out the invention. For example, a device may comprise circuitry and electronics for handling, receiving and transmitting data, computer program code in a memory, and a processor that, when running the computer program code, causes the device to carry out the features of an embodiment. Yet further, a network device like a server may comprise circuitry and electronics for handling, receiving and transmitting data, computer program code in a memory, and a processor that, when running the computer program code, causes the network device to carry out the features of an embodiment.

The various embodiments can be implemented with the help of a non-transitory computer- readable medium encoded with instructions that, when executed by a computer, perform the various embodiments.

If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above- described functions may be optional or may be combined. Furthermore, the present embodiments are disclosed in relation to a method for decoding and to a decoder. However, the teachings of the present disclosure can be applied in an encoder configured to perform encoding of coding units and coding the indication the presence of escape coding within the coding unit.

Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architecture, as non-limiting examples.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

Programs, such as those provided by Synopsys, Inc. of Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

The embodiments may provide some advantages. For example, accuracy of the motion compensated prediction may be improved by providing a bitrate efficient way of signaling additional information required to model higher order motion vector fields.

According to a first example there is provided a method comprising:

obtaining a block from a current picture to be decoded;

In some embodiments the method of the first example comprises:

repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate until at least one of the following conditions is fulfilled: a number of iterations is equal to or greater than a predetermined value;

a distance of the modified first motion vector predictor candidate from the first motion vector predictor candidate or a distance of the modified second motion vector predictor candidate from the second motion vector predictor candidate is equal to or greater than a predetermined distance. In some embodiments the difference vector comprises a horizontal and a vertical component, wherein the method of the first example comprises:

adding the horizontal component of the difference vector to the first motion vector prediction candidate; deducting the horizontal component of the difference vector from the second motion vector prediction candidate; determining the cost estimate and including the cost estimate into the set of cost estimates; and

adding the vertical component of the difference vector to the first motion vector prediction candidate; deducting the vertical component of the difference vector from the second motion vector prediction candidate; determining the cost estimate and including the cost estimate into the set of cost estimates.

In some embodiments the method of the first example comprises:

dividing the current block, first reference block and the second reference block either horizontally or vertically into two or more slices; and

performing the cost analyses for each slice.

In some embodiments the method of the first example comprises:

analyzing the samples within the first reference block, the second reference block or both; calculating the sum of horizontal and vertical gradients of sample values within the first reference block and the second reference block; and

selecting the direction of division by comparing the gradients.

In some embodiments the method of the first example comprises:

advancing the addition, subtraction and the cost estimate determination first in a first direction from the first/second motion vector candidates until reaching a first limit; and then advancing the addition, subtraction and the cost estimate determination in a second, opposite direction from the first/second motion vector candidates until reaching a second limit.

In some embodiments the method of the first example comprises:

selecting the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are closer to each other compared to the first motion vector candidate and the second motion vector candidate.

In some embodiments the method of the first example comprises:

selecting the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are further away from each other compared to the first motion vector candidate and the second motion vector candidate. In some embodiments the method of the first example comprises:

using a sum of absolute differences or a sum of squared differences between motion compensated reference blocks as said cost estimate for bi-directional motion prediction.

In some embodiments the method of the first example comprises:

generating the first modified motion vector prediction candidate by adding a difference vector to the first motion vector predictor candidate; and

generating the second modified motion vector prediction candidate by deducting a scaled version of the difference vector from the second motion vector predictor candidate.

According to a second example there is provided an apparatus comprising at least one processor; and at least one memory including computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

obtain a block from a current picture to be decoded;

In some embodiments the apparatus of the second example said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate until at least one of the following conditions is fulfilled: a number of iterations is equal to or greater than a predetermined value;

a distance of the modified first motion vector predictor candidate from the first motion vector predictor candidate or a distance of the modified second motion vector predictor candidate from the second motion vector predictor candidate is equal to or greater than a predetermined distance.

In some embodiments the apparatus of the second example said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: add the horizontal component of the difference vector to the first motion vector prediction candidate; deduct the horizontal component of the difference vector from the second motion vector prediction candidate; determining the cost estimate and including the cost estimate into the set of cost estimates; and

add the vertical component of the difference vector to the first motion vector prediction candidate; deduct the vertical component of the difference vector from the second motion vector prediction candidate; determining the cost estimate and including the cost estimate into the set of cost estimates.

In some embodiments the apparatus of the second example said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: divide the current block, first reference block and the second reference block either horizontally or vertically into two or more slices; and

perform the cost analyses for each slice.

In some embodiments the apparatus of the second example said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: analyze the samples within the first reference block, the second reference block or both;

calculate the sum of horizontal and vertical gradients of sample values within the first reference block and the second reference block; and

select the direction of division by comparing the gradients. In some embodiments the apparatus of the second example said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: advance the addition, subtraction and the cost estimate determination first in a first direction from the first/second motion vector candidates until reaching a first limit; and then advancing the addition, subtraction and the cost estimate determination in a second, opposite direction from the first/second motion vector candidates until reaching a second limit.

In some embodiments the apparatus of the second example said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following: select the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are closer to each other compared to the first motion vector candidate and the second motion vector candidate.

In some embodiments of the apparatus of the second example the encoding said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

select the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are further away from each other compared to the first motion vector candidate and the second motion vector candidate.

According to a third example there is provided a computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

obtain a block from a current picture to be decoded;

subtract the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate; determine a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate;

According to a fourth example there is provided an apparatus comprising a video decoder configured for encoding a bitstream comprising an image sequence, the video decoder comprising:

an input for receiving a block from a current picture to be decoded;

the apparatus further comprises a selector for selecting from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates. According to a fifth example there is provided an apparatus comprising:

means for obtaining a block from a current picture to be decoded; means for obtaining a first motion vector predictor candidate pointing to a first reference block in a first reference picture;

means for determining a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate;

means for selecting from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates. According to a sixth example there is provided a method comprising:

obtain a block from a current picture to be encoded;

repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and select from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates. In some embodiments the method comprises:

performing the cost analyses for each slice..

In some embodiments the method comprises:

analyzing the samples within the first reference block, the second reference block or both;

calculating the sum of horizontal and vertical gradients of sample values within the first reference block and the second reference block; and

selecting the direction of division by comparing the gradients.

In some embodiments the method comprises:

selecting the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are further away from each other compared to the first motion vector candidate and the second motion vector candidate.

In some embodiments the method of the first example comprises:

using a sum of absolute differences or a sum of squared differences between motion compensated reference blocks as said cost estimate for bi-directional motion prediction. In some embodiments the method of the first example comprises:

According to a seventh example there is provided an apparatus comprising at least one processor; and at least one memory including computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

obtain a block from a current picture to be encoded;

In some embodiments of the apparatus said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

add the horizontal component of the difference vector to the first motion vector prediction candidate; deduct the horizontal component of the difference vector from the second motion vector prediction candidate; determining the cost estimate and including the cost estimate into the set of cost estimates; and add the vertical component of the difference vector to the first motion vector prediction candidate; deduct the vertical component of the difference vector from the second motion vector prediction candidate; determining the cost estimate and including the cost estimate into the set of cost estimates.

divide the current block, first reference block and the second reference block either horizontally or vertically into two or more slices; and

perform the cost analyses for each slice.

analyze the samples within the first reference block, the second reference block or both;

select the direction of division by comparing the gradients.

advance the addition, subtraction and the cost estimate determination first in a first direction from the first/second motion vector candidates until reaching a first limit; and then advancing the addition, subtraction and the cost estimate determination in a second, opposite direction from the first/second motion vector candidates until reaching a second limit. In some embodiments of the apparatus said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

select the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are closer to each other compared to the first motion vector candidate and the second motion vector candidate. In some embodiments of the apparatus said at least one memory includes computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

According to an eighth example there is provided a computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

obtain a block from a current picture to be encoded;

According to a ninth embodiment there is provided an apparatus comprising a video encoder configured for decoding a bitstream comprising an image sequence, the video encoder comprising

means for an input for receiving a block from a current picture to be encoded; a motion vector selector for selecting a first motion vector predictor candidate pointing to a first reference block in a first reference picture and for selecting a second motion vector predictor candidate pointing to a second reference block in a second reference picture; a motion vector modifier for adding a difference vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate and for subtracting the difference vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate;

According to a tenth aspect, there is provided an apparatus comprising:

means for obtaining a block from a current picture to be encoded;

Claims

Claims:

1. A method comprising:

obtaining a block from a current picture to be decoded or encoded;

splitting said block at least into a first line-set and a second line-set, where the splitting direction is a horizontal direction or a vertical direction,

performing a one-dimensional motion vector refinement process for each line-set, where horizontal components or vertical components of motion vectors of said line-set are updated based on the splitting direction.

2. The method according to claim 1 , the one-dimensional motion vector refinement process comprising:

splitting said first reference block and said second reference block into corresponding at least first line-set and a second line-set in the splitting direction;

updating the horizontal components or vertical components of motion vectors of said line- set by:

adding a difference motion vector to the first motion vector predictor candidate to obtain a modified first motion vector predictor candidate; and

subtracting the difference motion vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate.

3. The method according to claim 2 comprising:

determining a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate; repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate for each of the first line-set and the second line-set to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

4. The method according to claim 3 further comprising:

repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate until at least one of the following conditions is fulfilled:

a number of iterations is equal to or greater than a predetermined value;

5. The method according to any of the claims 3 to 4 comprising:

advancing the addition, subtraction and the cost estimate determination first in a first direction parallel to the splitting direction from the first/second motion vector candidates until reaching a first limit; and then advancing the addition, subtraction and the cost estimate determination in a second, opposite direction from the first/second motion vector candidates until reaching a second limit.

6. The method according to any of the claims 2 to 5 comprising:

selecting the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are either closer to each other or further from each other compared to the first motion vector candidate and the second motion vector candidate.

7. The method according to any of the claims 2 to 6 comprising:

using a sum of absolute differences or a sum of squared differences between motion compensated reference blocks as a cost estimate for bi-directional motion prediction.

8. The method according to any of the claims 2 to 7 comprising:

generating the first modified motion vector prediction candidate by adding a difference motion vector to the first motion vector predictor candidate; and

9. An apparatus comprising at least one processor; and at least one memory including computer program code the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to perform at least the following:

obtain a block from a current picture to be decoded or encoded; split said block at least into a first line-set and a second line-set, where the splitting direction is a horizontal direction or a vertical direction,

perform a one-dimensional motion vector refinement process for each line-set, where horizontal components or vertical components of motion vectors of said line-set are updated based on the splitting direction.

10. The apparatus according to claim 9 further comprising computer program code for the one-dimensional motion vector refinement process configured to, with the at least one processor, cause the apparatus to:

split said first reference block and said second reference block into corresponding at least first line-set and a second line-set in the splitting direction;

update the horizontal components or vertical components of motion vectors of said line- set by:

11. The apparatus according to claim 10 further comprising computer program code configured to, with the at least one processor, cause the apparatus to:

determine a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate; repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate for each of the first line-set and the second line-set to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

12. The apparatus according to claim 1 1 further comprising computer program code configured to, with the at least one processor, cause the apparatus to:

repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate until at least one of the following conditions is fulfilled:

a number of iterations is equal to or greater than a predetermined value;

13. The apparatus according to claim 1 1 or 12 further comprising computer program code configured to, with the at least one processor, cause the apparatus to perform at least:

advance the addition, subtraction and the cost estimate determination first in a first direction parallel to the splitting direction from the first/second motion vector candidates until reaching a first limit; and then advance the addition, subtraction and the cost estimate determination in a second, opposite direction from the first/second motion vector candidates until reaching a second limit.

14. The apparatus according to any of the claims 10 to 13 further comprising computer program code configured to, with the at least one processor, cause the apparatus to perform at least: select the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are either closer to each other or further from each other compared to the first motion vector candidate and the second motion vector candidate.

15. The apparatus according to any of the claims 10 to 14 further comprising computer program code configured to, with the at least one processor, cause the apparatus to perform at least: use a sum of absolute differences or a sum of squared differences between motion compensated reference blocks as a cost estimate for bi-directional motion prediction.

16. The apparatus according to any of the claims 10 to 15 further comprising computer program code configured to, with the at least one processor, cause the apparatus to perform at least: generate the first modified motion vector prediction candidate by adding a difference motion vector to the first motion vector predictor candidate; and

generate the second modified motion vector prediction candidate by deducting a scaled version of the difference vector from the second motion vector predictor candidate.

17. A computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

obtain a block from a current picture to be decoded or encoded;

split said block at least into a first line-set and a second line-set, where the splitting direction is a horizontal direction or a vertical direction,

18. The computer readable storage medium according to claim 17 further comprising code thereon for use by the apparatus for the one-dimensional motion vector refinement process, which when executed by the processor, causes the apparatus to:

subtracting the difference motion vector from the second motion vector predictor candidate to obtain a modified second motion vector predictor candidate;

19. The computer readable storage medium according to claim 18 further comprising code thereon for use by the apparatus, which when executed by the processor, causes the apparatus to: determine a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate; repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate for each of the first line-set and the second line-set to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and select from the set of modified first motion vector predictor candidates a first motion vector predictor and from the set of modified second motion vector predictor candidates a second motion vector predictor on the basis of the set of cost estimates.

20. The computer readable storage medium according to claim 19 further comprising code thereon for use by the apparatus, which when executed by the processor, causes the apparatus to: repeat the adding the difference vector, subtracting the difference vector and determining the cost estimate until at least one of the following conditions is fulfilled:

a number of iterations is equal to or greater than a predetermined value;

21. The computer readable storage medium according to claim 19 or 20 further comprising code thereon for use by the apparatus, which when executed by the processor, causes the apparatus to:

22. The computer readable storage medium according to any of the claims 19 to 21 further comprising code thereon for use by the apparatus, which when executed by the processor, causes the apparatus to :

select the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are either closer to each other or further from each other compared to the first motion vector candidate and the second motion vector candidate.

23. The computer readable storage medium according to any of the claims 19 to 22 further comprising code thereon for use by the apparatus, which when executed by the processor, causes the apparatus to :

use a sum of absolute differences or a sum of squared differences between motion compensated reference blocks as a cost estimate for bi-directional motion prediction.

24. The computer readable storage medium according to any of the claims 19 to 23 further comprising code thereon for use by the apparatus, which when executed by the processor, causes the apparatus to :

generate the first modified motion vector prediction candidate by adding a difference motion vector to the first motion vector predictor candidate; and

25. An apparatus comprising:

means for obtaining a block from a current picture to be decoded or encoded;

means for splitting said block at least into a first line-set and a second line-set, where the splitting direction is a horizontal direction or a vertical direction,

means for performing a one-dimensional motion vector refinement process for each line- set, where horizontal components or vertical components of motion vectors of said line-set are updated based on the splitting direction.

26. The apparatus according to claim 25 comprising:

means for splitting said first reference block and said second reference block into corresponding at least first line-set and a second line-set in the splitting direction;

means for updating the horizontal components or vertical components of motion vectors of said line-set by:

27. The apparatus according to claim 26 comprising:

means for determining a cost estimate for bi-directional motion prediction on the basis of the modified first motion vector predictor candidate and the modified second motion vector predictor candidate; means for repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate for each of the first line-set and the second line-set to obtain a set of modified first motion vector predictor candidates, a set of modified second motion vector predictor candidates and a set of cost estimates; and

28. The apparatus according to claim 27 further comprising:

means for repeating the adding the difference vector, subtracting the difference vector and determining the cost estimate until at least one of the following conditions is fulfilled:

a number of iterations is equal to or greater than a predetermined value;

29. The apparatus according to any of the claims 27 to 28 comprising:

means for advancing the addition, subtraction and the cost estimate determination first in a first direction parallel to the splitting direction from the first/second motion vector candidates until reaching a first limit; and means for advancing the addition, subtraction and the cost estimate determination in a second, opposite direction from the first/second motion vector candidates until reaching a second limit.

30. The apparatus according to any of the claims 26 to 29 comprising:

means for selecting the difference motion vector so that the modified first motion vector candidate and the modified second motion vector candidate are either closer to each other or further from each other compared to the first motion vector candidate and the second motion vector candidate.

31. The apparatus according to any of the claims 26 to 30 comprising:

means for using a sum of absolute differences or a sum of squared differences between motion compensated reference blocks as a cost estimate for bi-directional motion prediction.

32. The apparatus according to any of the claims 26 to 31 comprising:

means for generating the first modified motion vector prediction candidate by adding a difference motion vector to the first motion vector predictor candidate; and

means for generating the second modified motion vector prediction candidate by deducting a scaled version of the difference vector from the second motion vector predictor candidate.