JP2901105B2 - Motion compensated inter-frame subband coding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-efficiency moving picture coding method using motion compensated inter-frame prediction and subband division.

[0002]

2. Description of the Related Art It is known that motion-compensated inter-frame prediction guarantees high coding efficiency in video coding. Discrete Cosine Transfor
m, hereinafter referred to as DCT) is a typical example, and CCITT is a typical example.
H.261, CCIR Rec.723, etc. are adopted as international standard encoding systems. Although DCT has excellent characteristics for compressing spatial redundancy, it has a problem that block distortion occurs at a low bit rate.

Recently, wavelet and QMF (Quadratur) have been used as spatial redundancy elimination methods that do not give block distortion.
Sub-band coding using a multi-stage filter bank as represented by e-mirror filters) has been studied.

[0004] There are two types of configuration methods of hybrid coding combining motion compensated inter-frame prediction and multi-stage filter banks, a "type for performing band division after difference" and a "type for obtaining difference after band division". .

[0005] The former has a configuration in which DCT is replaced by a multistage filter bank in a conventional hybrid coding system using motion compensation. The motion vector detection is performed using the input image and the previously encoded previous frame image. Motion-compensated inter-frame prediction is performed on the input image, and the difference image is sub-band divided. In this type, motion vector detection / prediction is usually performed in block units using a block matching method. Therefore, when a motion vector changes between adjacent blocks, the prediction signal and the prediction error signal often show a sharp level conversion at the block boundary. This change is DCT
However, sub-band coding using a filter that is not closed in a block does not cause a problem, but the coding efficiency is reduced.

In consideration of this point, the latter type is conventionally used (H. Gharavi: “Subband Coding Algo
rithm for Video Applications: Videophone to HDTV C
onferencing ", IEEE Trans., CAS for Video Technolog
y, Vol.1, No.2, pp.174-182, June 1991).

The latter type has a hybrid configuration in which an input signal is divided into bands and motion compensation inter-frame prediction is performed for each subband. The input image is directly sub-band divided and converted into frequency domain data. Motion vector detection is performed by comparing the already encoded frequency domain data with the subband output of the input image. The motion vector detection is performed independently for each sub-band, or the motion is detected only for the lowest band image, and the obtained motion vector is applied to other bands. Can be In this process, motion vector detection and motion compensation are performed on data that has been subsampled after filter output. Therefore, the motion vector detection accuracy and the inter-frame prediction accuracy are lower than the processing in the original image area. For example, when a three-stage two-dimensional full division filter bank is used, motion detection with one pixel accuracy in an image after division corresponds to eight pixel accuracy in the original image area. Therefore, only coarse motion compensation accuracy can be equivalently obtained, which causes a decrease in inter-frame prediction efficiency.

[0008]

The conventional motion compensated inter-frame prediction sub-band coding has the following problems.

1. In the case of a hybrid configuration in which subband division is performed after the motion compensation inter-frame difference, the conventional motion compensation is processed for each block. For this reason, when a motion vector changes between adjacent blocks, a steep change in level is often seen in a prediction signal and a prediction error signal at a block boundary portion. Normally, the order of the subband filter and the subsampling ratio do not match the motion compensation block size. Therefore, subband filtering converts a discontinuous signal generated by motion compensation into frequency domain data, causing an increase in high frequency components and consequently lowering coding efficiency.

[0010] 2. In the case of a hybrid configuration in which the difference between motion compensation frames is obtained after subband division, a hybrid configuration in which an input signal is divided into bands and motion compensation interframe prediction is performed for each subband. Since a prediction loop is formed for the frequency domain data, there is no problem of converting the discontinuity of motion compensation. However, since the motion vector detection and the motion compensation are performed on the subsampled data after the output of the filter, the motion vector detection accuracy and the inter-frame prediction accuracy are lower than the processing in the original image area. In the case of n-stage sub-band filtering with a multi-stage configuration, the motion compensation accuracy deteriorates to 1/2 ⁿ in the final stage.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is an object of the present invention to provide a motion compensation method in a hybrid coding system in which motion compensated inter-frame prediction and a multistage subband splitting filter are combined. It is possible to obtain a motion-compensated sub-band filter output that removes deterioration of sub-band coding efficiency due to discontinuity at a block boundary and maintains the accuracy of a motion vector detected for an image size without sub-sampling. It is to provide a possible technology.

The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0013]

In order to achieve the above object, the present invention suppresses the temporal redundancy of an input digital image signal sequence using motion compensated inter-frame prediction.
In a high-efficiency video coding method in which spatial redundancy is suppressed by a multi-stage filter bank such as a wavelet transform, an input image is divided into blocks of K × K pixels, and the coded image stored in a frame memory is input to the input image. Between images,
The motion amount between two screens, which is the target of inter-frame prediction, is detected for each block, or the motion amount for each block is detected in advance between the original images, and the motion amount for each block in the frame memory is detected. The first stage band division filtering is performed by using a position shifted by a position corresponding to the amount of motion as a start point, thereby obtaining a filter output coefficient based on the position compensated for motion, and the second and subsequent band divisions are performed. Performs band division on the obtained first-stage filter output irrespective of the amount of motion or the block, and sets the finally obtained filter output coefficients as inter-frame prediction filter coefficients in the frequency domain. The input image is subjected to ordinary multi-stage band division irrespective of the block and the amount of motion to obtain a filter output coefficient, and the inter-frame prediction filter coefficient is subtracted from this coefficient to obtain a frame. Seeking beam interframe difference coefficients, we quantize the difference coefficient between the obtained frames to the most important feature to transmit coded.

[0014]

According to the above-mentioned means, of the multi-stage sub-band division, the filter processing necessary for the sub-band division is executed while performing the motion compensation prediction at the first-stage band division, and the ordinary processing is performed in the second and subsequent stages. By repeating division by filtering, inter-frame prediction or intra-frame encoding is performed on data in the frequency domain, so that a motion-compensated inter-frame sub-band encoding method with high pixel accuracy can be realized. In addition, local intra-frame encoding becomes possible, and a block step of a predicted image due to blocking in motion compensation is eliminated. As a result, the coding efficiency is improved, and the coding noise on the visually obstructive block is eliminated.

That is, Q of wavelet or multi-stage configuration
In the case of the MF filter, in order to perform motion compensation and filtering for each macroblock, a position shifted by a motion vector amount for each macroblock may be used as a reference of a filter operation start point. That is, an offset is added by the amount of the motion vector as compared with the normal case.

The motion compensation output obtained by this method is:
When the data is returned in the frequency domain and is returned to the time domain, an image generated from a filter output of a certain macroblock is superimposed on an image generated from adjacent macroblock data.

For example, in a wavelet in which the division into two is repeated with respect to the low frequency component, the filter order and the motion vector may be shifted at the filter operation start point, and the output data may be integrated every two pixels. Here, a significant difference from the conventional motion compensation after subband division is that a motion vector is used for each macroblock size defined as viewed from the original image. Therefore, unlike the conventional method, a motion compensation filtering output reflecting the motion detection accuracy of the original image is obtained. In addition, since the output data of the first stage has already been motion-compensated, there is no need to consider a motion vector in the division of the second and subsequent stages, and simple re-division may be repeated.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the principle of the present invention will be described. A typical example of a filter bank based on a tree structure in which band division processing is repeated is an octave division filter having a structure in which two divisions shown in FIG. 1 are repeated on a low frequency side. In FIG.
H ₁ , H ₂ , and H ₃ indicate low-pass filters, G ₁ , G ₂ , and G ₃ indicate high-pass filters, and ↓ 2 indicates 2: 1.

A typical example of the band division filter is a QMF. However, the orthogonality of the analysis / synthesis filter, the completeness of the reconstruction,
In terms of feasibility in finite order, CQF (Conjugate Quadrat
ureFilter), SSKF (Symmetric Short Kernel)
There are several variations, such as Filter. Wavelet filters are also included in this class.
If a filter bank having a tree structure based on multi-stage decimation and interpolation is used, not only uniform band division but also non-uniform division is possible. From the viewpoint of the amount of calculation, a filter bank having the same level as the high-speed conversion processing of the uniform filter bank can be formed.

In the conventional motion compensation, the original image is processed in units of macroblocks of K × K pixels. The operation obtained as a result of incorporating the same processing accuracy into the first stage of the multistage subband coding constituted by the multistage filters will be described.

The input signal is represented by x (i) (i = 0, 1,...,
T), the coefficients of the Nth-order subband filter are represented by h (i) (i =
0, 1, ..., N-1). The motion vector belonging to the j-th macroblock is v (j), and the time of the input / output signal is n.
(N∈M (j)). Here, M (j) represents a time index group belonging to macroblock j.

The input signal x (n) is shifted by the number of samples v (j) corresponding to the motion vector and convolved with the filter h (n) to obtain a motion-compensated output signal y (n). This is represented by the following equation (1).

[0023]

(Equation 1)

If the equation (1) is Z-transformed,

[0025]

(Equation 2)

Is obtained. When H (z) is a half-band filter, the signal D (z ² ) decimated 2: 1 is

[0027]

(Equation 3)

## EQU1 ##

On the other hand, in the conventional method, decimation is performed first, and motion compensation is performed in the frequency domain. The motion-compensated data F (z ² ) after decimation is given by a motion vector w (j).

[0030]

(Equation 4)

## EQU1 ## The conditions under which equations (3) and (4) match are:

[0032]

V (j) = 2w (j) (5) When w (j) is a motion vector with one-pixel accuracy, v (j) has two-pixel accuracy. Therefore, in the conventional method, the motion compensation accuracy after the first-stage decimation is only half that of the original signal area, and the efficiency is low. Conversely, it can be seen that the use of the method (Equation (1)) of the present invention can achieve more accurate motion compensation than the conventional method.

The output of the first stage filter of the band division filter is
Since the data is motion-compensated, the normal divide-by-two filtering may be repeated for the low frequency components in the second and subsequent stages. FIG. 2 shows the configuration of the motion compensation filter bank. In FIG. 2, H ₁ , H ₂ , and H ₃ are low-pass filters, G ₁ , G ₂ , and G ₃ are high-pass filters, and ↓ 2 is 2: 1.
Shows that it is jumping.

Here, the case of a one-dimensional signal has been described for simplicity. The actual image signal is a two-dimensional signal,
When band division is performed using a separable filter, it is necessary to perform motion compensation at the first stage of both horizontal separation and vertical separation filters. Usually, the motion (shift amount) is represented by a vector v (j) = (v _h (j), v _v (j)) composed of horizontal and vertical components. However, in each of the first-stage filters, the shift amount is not v _h (j) and v _v (j), but both v _h (j) + Lv _v
Note that (j) (L: number of horizontal pixels).

An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 3 is a block diagram showing a configuration of a motion compensation inter-frame sub-band encoder according to an embodiment of the present invention.

In FIG. 3, 1 is an input image and 2 is 1 to N
Stage subband division filter processing unit, 3 is a prediction filter output coefficient, 4 is a motion compensation frame difference filter coefficient, 5
Is a quantizer, 6 is a variable length encoder, 7 is an inverse quantizer, 8
Is a 1-N stage sub-band synthesis filter processing unit, 9 is a frame memory, 10 is a first stage motion compensation sub-band division filter processing unit, 11 is a 2-N stage sub-band division filter processing unit, and 12 is a motion vector detection. Department.

Next, the operation of the motion-compensated inter-frame sub-band encoder of this embodiment will be described with reference to FIG. The input image 1 is compared with the image of the previous frame stored in the frame memory 9, and a motion vector is calculated in the motion vector detection unit 12. Using this motion vector, the first
In the stage subband division filter processing unit 10, the image of the previous frame stored in the frame memory 9 is filtered in units of macroblocks according to the shift amount. Subsequently, the subband division filter processing unit 11 of the second to Nth stages performs normal subband division. The final stage filter output, that is, the prediction filter output coefficient 3, is compared with the filter output coefficient obtained as a result of subband division of the input image 1 in the 1 to N-stage subband division filter processing unit 2, and the difference between motion compensation frames is calculated. When the power of the filter coefficient 4 is smaller than the power of the input data (filter output), it is treated as an encoding target in the inter-frame difference mode.

In the opposite case, the input data is to be encoded and is passed to the next processing. This switching can be performed for each macroblock. The inter-frame difference or the filter output in the frame is quantized by the quantizer 5. The quantization number is entropy-encoded in the variable length encoder 6 and transmitted to the receiving side. In addition, as in the case of normal inter-frame coding, it is necessary to accumulate data in the coding loop in order to compare with the filter output of the next frame. Therefore, the quantized data is returned to the quantized value in the inverse quantizer 7. The quantized value is obtained by subjecting the image of the previous frame stored in the frame memory 9 to the first-stage motion compensation sub-band division filter processing (10), and further to the second-N-stage sub-band division filter processing (11). It is added to the prediction filter output coefficient 3 obtained by the above. The added coefficient is converted into a coded image in the 1 to N-stage subband synthesis filter processing unit 8 and stored in the frame memory 9.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above-described embodiments, and it goes without saying that various changes can be made without departing from the scope of the invention. Absent.

[0041]

As described above, according to the present invention, a motion compensation inter-frame sub-band coding method with high pixel accuracy is realized in multi-stage sub-band coding such as motion compensation inter-frame prediction and wavelet transform. As a result, local intra-frame encoding becomes possible, and a block level difference in a predicted image due to blocking in motion compensation is eliminated. As a result, the coding efficiency is improved, and the coding noise on the visually obstructive block is eliminated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a three-stage octave bank for explaining the principle of the present invention;

FIG. 2 is a block diagram showing a configuration of a motion compensation filter bank for explaining the principle of the present invention;

FIG. 3 is a block diagram illustrating a configuration of a motion-compensated inter-frame subband encoder according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Input image, 2 ... 1 to N stage sub-band division | segmentation filter processing part, 3 ... Predictive filter output coefficient, 4 ... Motion compensation frame difference filter coefficient, 5 ... Quantizer, 6 ... Variable length encoder, 7 ... Inverse quantizer, 8... 1 to N-stage subband synthesis filter processing unit, 9... Frame memory, 10. Unit, 12 ... Motion vector detection unit.

Continuation of the front page (56) References IEEE TRANSITIONS ON CIRCUITS AND SYSTEMS FOR VIDEO ECHNOLOGY, 1 [2] (1991.6) 174-183 (Subband coding Algorithms for video applications) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04N ^7/ 24-7/68

Claims (1)

(57) [Claims] The present invention suppresses temporal redundancy of an input digital image signal sequence using motion compensated inter-frame prediction.
In a high-efficiency video coding method in which spatial redundancy is suppressed by a multi-stage filter bank such as a wavelet transform, an input image is divided into blocks of K × K pixels, and the coded image stored in a frame memory is input to the input image. Between images,
The motion amount between two screens, which is the target of inter-frame prediction, is detected for each block, or the motion amount for each block is detected in advance between the original images, and the motion amount for each block in the frame memory is detected. The first stage band division filtering is performed by using a position shifted by a position corresponding to the amount of motion as a start point, thereby obtaining a filter output coefficient based on the position compensated for motion, and the second and subsequent band divisions are performed. Performs band division on the obtained first-stage filter output irrespective of the amount of motion or block, and uses the finally obtained filter output coefficient as an inter-frame prediction filter coefficient in the frequency domain. The input image is subjected to ordinary multi-stage band division irrespective of the block and motion amount to obtain the filter output coefficient, and the frame is subtracted from this coefficient by subtracting the inter-frame prediction filter coefficient. Calculated between the difference coefficients, the quantized difference coefficients between the obtained frame motion compensation interframe subband coding scheme, wherein the transmission coding.