JPH089374A - Encoded information quantity controller - Google Patents

Abstract

PURPOSE:To provide temporally stable image quality even in still imaged or dynamic imaged close to the still image in a high efficiency dynamic image encoding system. CONSTITUTION:This controller is provided with a lower limit value limiting part 6 for obtaining and storing the average quantization step of intra-frame encoded pictures and defining the stored value as the lower limit value of the quantization step in inter-frame encoded images followed the intra-frame encoded images. Then, in the case of the intra-frame encoded images, the quantization step inputted from a quantization step calculation part 5 is outputted as it is and the quantization step is stored as the lower limit value. In the case of the following inter-frame encoded images, the lower limit value is outputted when the quantization step inputted from the quantization step calculation part 5 is smaller than the lower limit value and the quantization step from the quantization step calculation part 5 is outputted as it is when it is not smaller. Thus, the quantization step calculated in the quantization step calculation part 5 is limited, the reduction of the quantization step is prevented and the timewise image quality fluctuation of the images is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coded information amount control device used in a highly efficient moving image coding system.

[0002]

[Prior Art] CCITT (International Telegraph and Telephone Consultative Committee) and ISO
(International Standardization Committee) Joint Group ISO-IEC / JTC1 / SC2
The part related to the coded information amount control of the high-efficiency moving picture coding method proposed by 9 / WG11 in "Document, AVC-323, July 1992" and the like has a configuration as shown in FIG.

In this encoding method, an input image is divided into a plurality of blocks, and the data of each block itself or prediction error data by motion compensation of each block is input to the orthogonal transform unit 1 to perform orthogonal transform. Then, the quantizing unit 2 quantizes the transform coefficient, the variable length coding unit 3 performs variable length coding, and the coded signal thus obtained is stored in the output buffer 4 and output as output data.

The quantization step calculation unit 5 obtains the amount of generated information after encoding from the variable length encoding unit 3 so that the time average (bit rate) of this amount of generated information is constant and the image quality is improved as much as possible. Then, the quantization step is calculated and output to the quantization unit 2. That is, when the generated code amount in the variable length coding unit 3 is large, the quantization step is controlled to be large, and the information amount in the quantization unit 2 thereafter is reduced. When the amount of generated code in the variable length coding unit 3 is small, the quantization step is controlled so as to be small, and the subsequent quantization unit 2 operates to increase the amount of information.

[0005]

In the high-efficiency moving picture coding system, intraframe coding in which only spatial correlation in the input image is used for coding and temporal correlation between a plurality of input images are also included. It is used in combination with interframe coding, which is used for coding. Normally, a high compression rate is achieved by continuing a large number of inter-frame coded images with a large compression rate after one intra-frame coded image with a low compression rate.

However, when the coded information amount control according to the conventional method is applied to such a configuration, the image quality temporally fluctuates with respect to a still image or an input image having a small motion close to the still image, which is visually large. The problem of causing adverse effects occurs.

More specifically, in the case of a still image or an input image close to a still image, the temporal correlation between the images is large. Therefore, as shown by the bar graph in FIG. 5, the information generation amount per image is large. Is large in the intra-frame coded image and very small in the inter-frame coded image.

In the conventional coded information amount control, if the generated information amount is small, the quantization step is reduced to increase the generated information amount. However, for a still image or an image close to a still image, the quantizer 2 is used. Since the amount of input information itself is originally small, the amount of generated information hardly increases even if the quantization step is reduced.

Then, when the next quantization step is determined, an attempt is made to increase the amount of information to be generated and a smaller quantization step is set. Therefore, as shown by the solid line graph in FIG. It gets smaller and smaller as the images are repeated.

From the viewpoint of the image quality of the decoded image, the image quality is poor in the portion having a large quantization step, and the image quality is good in the portion having a small quantization step. Therefore, as shown by the broken line graph in FIG. The coded image has poor image quality, and temporal variations occur such that the image quality gradually improves as the interframe coded image is repeated.

It is visually very uncomfortable for a still image, which is supposed to be still and stable, to cause such a temporal image quality change. It should be noted that such a problem is particularly noticeable when the information generation amount of the intra-frame coded image is limited by the buffer capacity of the decoder and the processing capacity of the encoding device. Therefore, an object of the present invention is to make it possible to obtain a temporally stable and visually preferable image quality by suppressing the temporal image quality variation.

[0012]

A coded information amount control apparatus according to the present invention applies a lower limit to an average quantization step determined according to the amount of information generated when an input image is intra-coded. A value limiting means is provided.

Another feature of the present invention is that
The lower limit value limiting means is configured to apply the lower limit to the average quantization step when a scene change image is detected.

[0014]

According to the coded information amount control apparatus of the present invention, the most recent average quantization step stored at the time of coding the intra-frame coded image is the one when the inter-frame coded image is coded. Since it will be the lower limit of the quantization step,
The quantization step calculated by the quantization step calculation means is limited to prevent the quantization step from being reduced, and it is possible to prevent temporal image quality fluctuation of a still image or an image close to a still image.

According to another feature of the present invention, an average quantization step is calculated and stored even when an image in which a scene change has occurred is coded, and the stored average quantization step continues. Since it becomes the lower limit of the quantization step at the time of encoding the inter-frame coded image, it becomes possible to further suppress temporal image quality fluctuation of the still image or an image close to the still image.

[0016]

EXAMPLES An example of the present invention will be described below. As shown in FIG. 1, in the information generation amount control device according to the present embodiment, a lower limit value limiting unit 6 is provided at the next stage of the conventional quantization step calculating unit 5.

When the intra-frame coding is performed on the input image, the lower limit value limiting unit 6 outputs the quantization step input from the quantization step calculation unit 5 to the quantization unit 2 as it is. , The average quantization step of the image is stored as a lower limit value.

When interframe coding is performed subsequent to the intraframe coding, the quantization step input from the quantization step calculator 5 and the stored lower limit value (frame The average quantization step of the inner coded image) is compared. Then, if the quantization step input from the quantization step calculation unit 5 is smaller than the lower limit value, the lower limit value is output to the quantization unit 2, and if not, the quantization step of the quantization step calculation unit 5 is used as it is. Output to the conversion unit 2.

In this way, by inserting the lower limit value control unit 6 between the quantization step calculation unit 5 and the quantization unit 2, as shown in FIG. 2, a still image or an image close to a still image is input. In this case, even if the amount of information generated in the inter-frame coded image is small, the average value of the quantization steps can be kept to the same level as in the case of the intra-frame coded image, and the temporally stable image quality Can be obtained.

Next, FIG. 3 shows a structural example of the lower limit value limiting section 6. In FIG. 3, the intra-frame / inter-frame coded image selection unit 7 is a part for instructing selection of whether the image currently being processed is intra-frame coded or inter-frame coded.

The scene change detecting section 8 detects a scene change image and gives an instruction, though not necessarily required. Even if the scene change image is an inter-frame coded image, a large amount of information is generated as in the intra-frame coded image. Therefore, the scene change image is detected and regarded as an intra-frame coded image. By performing the above processing, it is possible to obtain an even better image quality.

The switches 12, 13 and 14 are controlled according to the results of the determinations by the intra-frame / inter-frame encoded image selection unit 7 and the scene change detection unit 8. Then, in the case of an intra-frame coded image or a scene change image, the switches 12 and 13 are turned on and the switch 14 is on the terminal a side. As a result, the input quantization step is input to the average value calculation unit 9 and the average value is calculated,
The calculated average value is stored in the lower limit memory 10. Further, the input quantization step is directly output to the quantization step output via the terminal a.

On the other hand, in the case of an inter-frame coded image,
The switches 12 and 13 are off, and the switch 14 is on the terminal b side. As a result, the input quantization step is compared with the lower limit value stored in the lower limit value memory 10 by the comparison limiter 11, and the larger of the two is output as the quantization step output via the terminal b. .

Although not shown in FIG. 1, in the case of performing adaptive quantization using the statistical information of the input image,
By performing the adaptive quantization control after the lower limit value limiting unit 6, the same image quality stabilizing effect can be obtained.

[0025]

As described above, according to the coded information amount control apparatus of the present invention, it is possible to obtain a temporally stable and visually good image quality even in a still image or a moving image close to a still image. Becomes

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a coded information amount control device according to an embodiment.

FIG. 2 is a graph for explaining the operation of the coded information amount control according to the present embodiment.

FIG. 3 is a block diagram showing a configuration example of a lower limit value limiting unit.

FIG. 4 is a block diagram showing a configuration of a coded information amount control device according to a conventional method.

FIG. 5 is a graph for explaining the operation of the coded information amount control according to the conventional method.

[Explanation of symbols]

 1 Orthogonal Transform Section 2 Quantization Section 3 Variable Length Coding Section 4 Output Buffer 5 Quantization Step Calculation Section 6 Lower Limit Value Limiting Section 7 In-frame / Inter-frame Coded Image Selection Section 8 Scene Change Detection Section 9 Average Value Calculation Section 10 Lower limit memory 11 Comparison limiter 12 Switch 13 Switch 14 3 contact switch

Claims (2)

[Claims] 1. Encoding for controlling a quantization step in accordance with the amount of coded information generated so that the time average value of the amount of coded information generated by intra-frame coding and inter-frame coding becomes constant. In the information amount control device, the average value of the quantization step of the image by the intra-frame coding is obtained and stored, and the stored value in the image by the inter-frame coding performed subsequent to the intra-frame coding is stored. A coded information amount control device comprising a lower limit value limiting means for setting a lower limit value of a quantization step. 2. A scene change detecting means for detecting a scene change image is provided, and when the scene change image is detected by the scene change detecting means, an average value of the quantization steps is calculated and stored, and this is stored. 2. The coded information amount control device according to claim 1, wherein the lower limit value limiting means is configured such that the determined value is set as the lower limit value of the quantization step.