JPH06133301A - Moving picture compression coding method - Google Patents

Abstract

PURPOSE:To improve picture quality of a decoded picture by implementing intra-coding when a macro block is coded and a variance in a predicted error of the macro block is larger than a threshold level so as to avoid the macro block not predicted excellently. CONSTITUTION:Figure shows a criterion of deciding intra-coding/prediction coding, the X axis indicates a variance (VAR) of a prediction error and the Y axis indicates a variance (VAROR) of an original signal. When the VAR is larger than a threshold level by using the criterion as above, intra-coding is implemented to implement coding without use of prediction not successful. The distribution of the VAR and VAROR is calculated in the frame coding in the setting of an optimum threshold level and the threshold level is decided so that number of macro blocks subject to intra-coding based on the distribution and the threshold level is used for the coding of a succeeding frame.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moving picture compression encoding method for improving the quality of a decoded image.

[0002]

2. Description of the Related Art In recent years, MP has been used as a storage type moving image compression method.
The EG method has been proposed. Encoding and decoding of moving images are performed as follows. FIG. 5 shows the encoding of a moving image,
It is a block block diagram of the system which performs a decoding. The input device 1 inputs video signals of various formats such as Y, Cb, and Cr. The preprocessor 2 converts into a format required by the encoder. The encoder 3 is
A bit stream is generated by reducing the data amount without degrading the input moving image as much as possible.

The storage device 4 is composed of a CD, a DAT, a hard disk, etc., and stores the generated bit stream. The decoder 5 receives the bitstream and creates a reproduced image. The post-processor 6 performs processing such as line interpolation, pixel interpolation, rate conversion, frame field conversion, and pixel aspect ratio conversion according to the specifications of the output display. The output device 7 displays and outputs the reproduced moving image.

FIG. 6 shows a block diagram of the encoder 3. Before describing the encoder 3, the encoding mode will be briefly described. The coding modes are roughly classified into two modes: (1) intra coding mode and (2) non-intra coding mode (predictive coding mode).

In the intra coding mode, the input image is coded as it is. On the other hand, in the non-intra coding mode, that is, the prediction coding mode, the prediction image is referred to by referring to the image already coded in any of the three prediction modes of forward, backward, or their interpolation. Generated, and the difference image from this predicted image is encoded.
At this time, in the case of the prediction mode and the motion compensation prediction, the motion vector is also coded at the same time.

Explaining the encoder 3, the input image data (ID) is subjected to DCT without passing through the difference unit 301 according to the encoding mode and in the intra encoding mode.
In the case of a non-intra coding mode, that is, a mode of performing coding by performing some prediction, the prediction image (PID) according to the prediction mode and motion vector
And the difference image (S
ID) is input to the DCT unit 302. DCT device 302
Transformation coefficient (C) of the result of discrete cosine transformation in
Is quantized by the quantizer 303, variable-length coded by the VLC unit 304, and stored in the buffer 305. The encoded data stored in the buffer 305 is read at a constant rate and sent to DSM (digital storage medium) or the like.

The predicted image in the non-intra coding mode is generated by the predictor from the locally decoded image stored in the frame memory of the frame memory & predictor 309. The locally decoded image is encoded by the encoder at the same time as
It is an image locally decoded by performing reverse decoding processing. This is because at the time of encoding, the data (QC) quantized by the quantizer 303 is inversely quantized by the inverse quantizer 306 and inversely DCT by the inverse DCT unit 307, which is already stored in the frame memory 309. The prediction image generated by the predictor 309 is added to the prediction image generated by the predictor 309 from another locally decoded image that has been generated, is generated, and is stored in the frame memory 309.

The two switches in FIG. 6, that is, the switch 311 between the difference unit 301 and the DCT unit 302.
And the output of the frame memory & predictor 309 and the adder 30
The switch 312 during the input to 8 operates as follows.

In the intra coding mode, the switch 3
11 is connected to the lower side, that is, the ID side, and the switch 312 is connected to the upper side, that is, "0". In the non-intra coding mode, the switch 311 is connected to the upper side, that is, the SID side, and the switch 312 is connected to the lower side, that is, the PID.

FIG. 7 is a diagram showing the configuration of the decoder 5.
The processing of the decoder is basically the same as the local decoder (310 in FIG. 6) in the encoder described above. The encoded data is input to the buffer 501 at a constant rate. The data read from the buffer 501 is the inverse VLC unit 50.
2 and is inversely quantized by the inverse quantizer 503 and inverse D
Inverse DCT is performed by the CT device 504. After that, in the intra coding mode, it is output as it is (“0” is added) through the adder 505, and in the non-intra coding mode, the coding mode and motion that are simultaneously decoded in the frame memory & predictor 506. According to the vector, the predicted image generated from the already decoded image on the frame memory is added by the adder 505 and output.

The switch 507 provided between the output of the frame memory & predictor 506 and the input to the adder 505 is connected and operated as follows.

In the intra coding mode, the switch 5
07 is connected to the left side or "0", and in the non-intra coding mode, the switch 507 is connected to the right side or PI.
Connected to D.

The structure of the I / P / B picture will be described. FIG. 8A is a diagram showing some pictures in display order, and the arrows indicate the dependency relationship between P pictures and B pictures.

There are four types of pictures. That is, the I (intra) picture is encoded without referring to other pictures. P (predicted)
Pictures are coded with motion compensation from previous I or P pictures. B (bidiritiona
Lly predicted) pictures are encoded using motion compensation from previous or subsequent I or P pictures. Then, the D (DC) picture is used in the fast forward reproduction mode (not shown in FIG. 8A). In a typical encoding mechanism, I, P and B pictures are mixed.

Also, because of the picture dependencies, the order of the bitstreams, that is, the order in which the pictures are transmitted, stored and received, is not the order in which they are displayed, but the order required by the decoder to decode the bitstream. Become.

FIG. 8B shows a picture sequence in a typical display order, and FIG. 8C shows a bitstream sequence for the picture sequence. Since the B picture depends on the subsequent P picture in the display order, the P picture is transmitted and decoded before the dependent B picture as shown in FIG. 8C.

The MPEG coding technique has a layer structure corresponding to a hierarchical structure. FIG. 9 shows the data structure of encoded data. The video sequence layer 11 is the top layer of the coding structure and contains the header and some gros.
It comprises a layer of ups of pictures (GOPs) 12. The sequence header initializes the state of the decoder, which allows the decoder to decode any sequence regardless of past decoding history.

The GOP layer 12 is the smallest coding unit that can be individually decoded in the sequence, and is composed of a header and several picture layers 13, and contains at least one I picture. The GOP header contains time and edit information.

The picture layer 13 comprises a header and one or more slice layers 14. If the bitstream becomes unreadable in the middle of a picture, the decoder can wait for the next slice layer 14 to recover and not lose the entire picture.

The slice layer 14 comprises a header and one or more macroblock layers 15. The slice header includes information on position and quantization scale. This is sufficient for recovery from local errors.

The macroblock layer 15 is a basic unit for motion compensation and quantization scale change. Each macroblock layer 15 includes a header and a 6-component block layer 16. That is, 4 blocks of brightness and 1 block of Cb
Color difference is one block of Cr color difference. The macroblock header contains quantization scale and motion compensation information.

The block layer 16 is a basic coding unit, and DCT is applied to this block level. Each block consists of 64 pixels in an 8 × 8 matrix. Pixel values are not individually coded, but are elements of a coded block. Each luminance pixel corresponds to one picture pixel, but since the chrominance information is subsampled 2: 1 both horizontally and vertically, each chrominance pixel corresponds to four picture pixels.

[0023]

As a standard for determining the intra coding and the predictive coding in the above-mentioned MPEG coding, there is one recommended by CCITT. In a severe image, the image quality may be greatly deteriorated.

That is, the "intra-coding / prediction-coding decision criteria" for coding currently used macroblocks are as shown in FIG. Here, the horizontal axis represents the variance of the prediction error (VAR), and the vertical axis represents the variance of the original signal (VAROR). According to this standard, VAR>
In the range of 64, only the magnitude relation between VAR and VAROR is taken into consideration, and even if the prediction is not successful and VAR takes an extremely large value, if the condition of VAR <VAROR (shaded area in the figure) is satisfied. , Predictive coding is performed.

As described above, in the moving picture compression technique of the MPEG system, the P picture and the B picture, which are non-independent frames, are predicted from the pictures preceding or succeeding in time and the prediction error is encoded. However, there is a problem that the distortion of the decoded image becomes large in the macroblock in which the prediction itself is not performed well.

An object of the present invention is to provide a moving picture compression coding method in which, when prediction is not successful, intra coding is carried out without predictive coding and the quality of a decoded picture is improved. Especially.

[0027]

In order to achieve the above object, according to the invention of claim 1, a compression coding of a moving picture for coding a macro block by using either intra coding or predictive coding. The method is characterized in that intra coding is performed when the variance of the prediction error of the macroblock is larger than a predetermined threshold.

The invention according to claim 2 is characterized in that the value of the threshold value is adaptively changed according to the characteristics of the image being encoded.

According to the third aspect of the present invention, the distribution of the prediction error variance is calculated for each frame during the encoding process, and the threshold is adaptively set so that the number of macroblocks to be intra-encoded becomes constant. However, the set threshold value is applied to the frame to be encoded next.

According to the fourth aspect of the invention, P picture and B picture
The feature is that different thresholds are set for the pictures.

[0031]

When a macroblock is encoded, if the prediction error variance (VAR) of the macroblock is larger than a certain threshold, intra-encoding is performed. By this means, it is possible to avoid performing predictive coding on a macroblock that could not be predicted well, so that it is possible to improve the image quality of the decoded image.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be specifically described below with reference to the drawings. As described above, since the distortion of the predicted image is often large where the quality deterioration of the decoded image is large, intra coding is performed on the macro block for which the prediction is not successful in this embodiment.

FIG. 1 is a diagram showing the "intra-coding / prediction-coding decision criterion" of the present invention. When such a criterion is used, intra coding is performed when the variance (VAR) of prediction errors is larger than a certain threshold, and thus coding can be performed without using the prediction that was not performed well.

Since macroblocks having a large value of variance (VAR) of prediction errors have a great influence on the deterioration of image quality of a decoded image, intra-coding of such macroblocks is expected to improve the SN ratio. it can. However, if too many macroblocks are intra-coded, the code amount increases and the amount of data accumulated in the buffer increases, so the quantization width increases due to the effect of the code amount control,
On the contrary, it causes a decrease in the SN ratio. Therefore, how to determine the optimum threshold value becomes a problem.

The distribution of the prediction error variance (VAR) in an image sequence is typically that shown in FIG. In general, the frequency decreases exponentially with respect to the prediction error variance (VAR), but the degree of the decrease varies depending on each image sequence and time. Therefore, it is necessary to adaptively change the threshold value.

The shaded area in FIG. 3 represents the number of macroblocks to be intra-coded. If the threshold value is determined so that this area becomes constant, it is possible to prevent the SN ratio from decreasing due to an abnormal increase in the code amount.

FIG. 4 is a flow chart of the threshold value determination process of the present invention. That is, from the 0th to the last frame (step 101), from the 0th macroblock to the 1320th macroblock in each frame (step 102), the variance of prediction error (VAR) during macroblock coding, VAROR is calculated, and intra coding / prediction coding is determined using the threshold value set at the time of coding the previous frame (step 103),
A threshold value is set from the distribution of the calculated prediction error variance (VAR) (step 104).

That is, in step 103, the distribution of VAR and VAROR is calculated during the coding of the frame, and in step 104, the threshold is determined so that the number of macroblocks to be intra-coded becomes constant from the distribution.
The threshold is used for coding the next frame.

Among these processes, the process that requires a large amount of calculation is "VAR
Calculation of “intra coding /
This is a process that has been conventionally performed even if it is not the “determination criterion for predictive coding”. The newly added processes include a process of “determining intra coding / prediction coding with a threshold” and a process of “V
This is a process of setting a threshold value from the distribution of AR '. In the former, one line of an if sentence is added in the macroblock layer, and in the latter, the process is performed in the picture layer, so the processing time does not change much.

The distribution of the prediction error variance (VAR) is larger in the P picture than in the B picture. This is because, as described in the related art, the best mode among forward prediction / backward prediction / bidirectional prediction can be selected for B pictures, whereas only forward prediction is available for P pictures. Therefore, the threshold value obtained for the B picture is P
When used in picture coding, many macroblocks are intra-coded, resulting in an increase in code amount.
Further, when the threshold value obtained for the P picture is used for encoding the B picture, the number of macroblocks that are intra-coded becomes too small. Therefore, the effect of the decision criterion of the intra-coding / prediction coding of the present invention, that is, the The effect of intra-coding a macroblock that could not be predicted cannot be expected.

Therefore, in another embodiment of the present invention, threshold values are set separately for P pictures and B pictures. This makes it possible to obtain the effect of the intra-prediction / prediction-coding decision criterion of the present invention while avoiding an extreme increase in the code amount for both P-pictures and B-pictures.

[0042]

As described above, according to the first aspect of the present invention, since the macroblock which cannot be predicted well is intra-coded, the image quality of the decoded image can be improved.

According to the second aspect of the invention, by adaptively changing the threshold value, it is possible to stably improve the image quality of all images.

According to the third aspect of the present invention, by making the number of intra-coded macroblocks constant to some extent, it is possible to prevent an extreme increase in the code amount and improve the image quality.

According to the invention as set forth in claim 4, since different thresholds are set for the P picture and the B picture, macroblocks that could not be predicted well are intra-coded while avoiding an extreme increase in the code amount. can do.

[Brief description of drawings]

FIG. 1 is a diagram showing “a decision criterion for intra coding / prediction coding” of the present invention.

[Fig. 2] Fig. 2 is a diagram illustrating "intra-coding / prediction-coding determination criteria" currently used.

FIG. 3 is a diagram showing a distribution of prediction error variances in an image sequence.

FIG. 4 is a processing flowchart of threshold value determination according to the present invention.

FIG. 5 is a block configuration diagram of a system for encoding and decoding a moving image.

FIG. 6 is a block configuration diagram of an encoder.

FIG. 7 is a diagram showing a configuration of a decoder.

FIG. 8A is a diagram showing some pictures in display order, and FIG. 8B shows a picture sequence in a typical display order.
(C) shows a bitstream sequence for it.

FIG. 9 is a diagram showing a data structure of encoded data.

[Explanation of symbols]

 1 Input Device 2 Preprocessor 3 Encoder 4 Storage Device 5 Decoder 6 Postprocessor 7 Output Device

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[Procedure amendment]

[Submission date] October 28, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0024

[Correction method] Change

[Correction content]

That is, “intra-coding / prediction-coding determination criteria” when coding a currently used macroblock are as shown in FIG. Here, the horizontal axis represents the variance of the prediction error (VAR), and the vertical axis represents the variance of the original signal (VAROR). According to this standard, VAR
In the range of> 64, only the magnitude relation between VAR and VAROR is taken into consideration, and even if the prediction is not successful and VAR takes an extremely large value, the condition of VAR <VAROR (white area in the figure) can be satisfied. For example, predictive coding is performed.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

Claims (4)

[Claims] 1. A moving picture compression coding method for coding a macroblock using either intra coding or predictive coding, when the variance of the prediction error of the macroblock is larger than a predetermined threshold value. A moving image compression encoding method characterized by performing encoding. 2. The moving picture compression encoding method according to claim 1, wherein the threshold value is adaptively changed according to the characteristics of the image being encoded. 3. A distribution of prediction error variances is calculated for each frame during the encoding process, and the threshold value is adaptively set so that the number of macroblocks to be intra-encoded is constant, and the set value is set. The moving picture compression coding method according to claim 2, wherein the threshold value is applied to a frame to be coded next. 4. The moving picture compression coding method according to claim 2, wherein different thresholds are set for the P picture and the B picture.