JPH07143483A - Adaptive post-processing filter - Google Patents

Abstract

PURPOSE:To reduce block distortion without deteriorating the resolution of an image. CONSTITUTION:A DC component separation circuit 3 obtains a DC component of picture elements in the unit of blocks and replaces all picture elements in the unit of blocks with the obtained DC component to obtain a DC component signal. A subtractor 5 subtracts the DC component signal from a decoded signal to provide an output of a high frequency component signal. A control circuit 2 uses information of a block size of conversion coding obtained from a decoder and a quantization characteristic of each block to adaptively select a band limit characteristic for the control. When the block size is large and the step size of step size quantization characteristics is rough and the high frequency component of a conversion signal is limited in the quantization characteristics, a band limit filter 4 selects space filter characteristics of a narrow frequency band to limit the band of the DC component signal. An adder 6 adds the DC component signal and the high frequency component signal subjected to band limits to obtain an image signal in which block distortion is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention aims at block distortion caused by quantization noise in a decoded image of an encoding device which orthogonally transforms an image signal for each block, quantizes the coefficient and encodes and transmits it. An adaptive post-processing filter for eliminating loss.

[0002]

2. Description of the Related Art As a method of predictively coding and transmitting a moving image, a prediction error signal is converted into a DCT (Discrete Cosine Transfer).
For example, there is a method in which orthogonal transform coding is performed by using the above method, and the transform coefficient is coded and transmitted by switching the quantization characteristic so that the transmission bit rate becomes constant.

In this system, when the amount of generated information is large, the quantization characteristic is roughened to suppress the amount of generated information.
Moreover, the amount of generated information is suppressed by not transmitting the high-order prediction coefficient. For this reason, when coarsely quantized, the image signal reproduced by encoding on the receiving side has noticeable block distortion due to the coarseness of quantization on the screen, resulting in deterioration of image quality.

As one of the methods for making this distortion inconspicuous, there is a block boundary adaptive filter shown in FIG. In this filter, if there is a discontinuous boundary at the block boundary, the filter coefficient is changed according to the discontinuity, and the image signal at the boundary is filtered to reduce the block boundary distortion. Reduce.

The operation of this filter will be described. The image signal decoded by the decoder 1 is the boundary state detection circuit 1
2. For example, in the vertical or horizontal boundary portion of an 8 × 8 block, for a pixel of 2 samples before and after or at the top and bottom, the change of the signal waveform of 4 consecutive samples at the boundary is observed and the boundary The state of presence / absence of block distortion is detected.

If the change in the waveform is detected to be discontinuous, it is considered that the block boundary at that portion has block distortion, and the block boundary filter 13 horizontally detects the image signal of the pixel at the boundary. Or 3 vertically
A low pass filter (LPF) with about taps is directly added to remove block distortion.

In order to prevent the image signal from blurring too much, the coefficient of the filter is changed depending on the strength of the discontinuity of the signal waveform at the boundary to strengthen or weaken the band limitation characteristic. In the switching circuit 14, a block boundary is selected, and thereby, only the boundary portion is locally smoothed by the block boundary filter to reduce block distortion.

[0008]

Block boundary distortions are more likely to be noticeable in the low frequency components, which is 8 × 8.
It will be more noticeable if the DC components of the blocks are different at adjacent boundaries.

In the conventional method, since the band of the reproduced image signal is directly limited by the filter, the number of taps is increased to remove the block distortion, and the LPF having a strong band limiting characteristic is provided.
However, the image signal becomes blurred, while the LPF having a weak characteristic cannot sufficiently remove the distortion.

That is, the block boundary filter is 8 ×
It is a filter of about 3 taps for 8 blocks,
Even if the block boundary distortion of the high frequency component can be removed to some extent, the block distortion of the DC component of the 8 × 8 block, which is the most prominent, cannot be sufficiently removed.

[0011]

SUMMARY OF THE INVENTION An adaptive post-processing filter according to the present invention is a decoder for decoding a data-compressed image signal using encoding for orthogonally transforming an image signal in block units. Means for obtaining a DC component of a pixel in block units for the decoded signal of the image signal obtained, replacing all the pixels in a block unit with the obtained DC component to obtain a DC component signal, and a DC component signal from the decoded signal Means for subtracting and outputting a high frequency component signal; control means for adaptively switching control of band limiting characteristics using information on the transform coding block size and quantization characteristics for each block obtained from the decoder; , If the block size is large and the quantization characteristic step size is coarse and the quantization characteristic step size is limited and the high frequency component of the converted signal is limited, a narrower frequency band And a filter unit for selecting a spatial filter characteristic to band-limit the DC component signal, and a unit for adding the band-limited DC component signal and the high-frequency component signal to obtain an image signal with reduced block distortion. Is characterized by.

[0012]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a block diagram of a first embodiment of the present invention. The signal input to the decoder 1 is a signal that is coded by DCT of the prediction error signal obtained by inter-frame coding the image signal on the transmission side (not shown) and adaptively quantizing the transform coefficient.

The decoder 1 decodes the transmitted DCT-transformed prediction error signal to decode the image signal. The image signal is sent to the subtractor 5 via the DC western signal separation circuit 3 and the delay circuit 7. Also,
A signal indicating the boundary of the 8 × 8 block on the screen is sent to the control circuit 2.

In the DC component separation circuit 3, 8 is extracted from the image signal.
The DC component of the × 8 block is calculated, and the value of the calculated DC component is output as a DC component signal for all the pixels in the 8 × 8 block.

As a method for obtaining the DC component of the 8 × 8 block, the values of all 8 × 8 pixels are added to obtain the average value. The subtractor 5 subtracts the DC component signal for each block from the image signal supplied via the delay circuit 7 for each pixel to obtain an AC component. The calculated AC component is
It is supplied to the adder 6 via the delay circuit 8.

The band limiting filter 4 is a two-dimensional LPF for sufficiently smoothing a DC component signal for each 8 × 8 block.
Is. The 8 × 8 DC component signal for each block is equivalent to being sampled for each block, and includes 1/8 of the vertical and horizontal signal bands as compared with sampling for each pixel.

That is, in order to sufficiently remove the aliasing distortion from the block by interpolation by re-sampling for each pixel and sufficiently smooth the block boundary, the band limiting filter 4
Has the characteristics of an LPF that limits the band 1/8 in the vertical and horizontal directions to a degree, and outputs a band-limited signal.

As a simple example of the band limiting filter, the coefficients from the -4th order to the 4th order of the Z function are (1, 2, 2, 2,
There is a transversal type filter characteristic of 2, 2, 2, 2, 1). If you use the same characteristics in the vertical direction,
A transversal filter having the same coefficient for each line delay can be used. The two-dimensional band-limiting filter is a product of vertical and horizontal characteristics, and can be easily configured if vertical and horizontal are independent. As another example, the coefficients are (1, 2, 2,
The band can be limited to 1/4 in 2,1) and to 1/2 in (1,2,1).

In the band limiting filter 4, the band-limited DC component signal is supplied to the adder 6 and is added to the AC component signal from the subtractor 5 to output an image signal for each pixel.

As another example of the band limiting filter 4, there is a method of switching the band limiting characteristic according to a control signal from the control circuit. Since the image content is visually complicated and the block distortion is more noticeable in the flat part where the change is small, it is detected whether or not the image is flat, and when it is the flat part, the pass band width of the LPF. Is narrowed, and when it is complicated, it is widened.

The large block distortion is caused by
Since the characteristic of quantizing the DCT coefficient is particularly rough in some cases, the characteristic of the component close to direct current may become coarse. Therefore, if the quantization characteristic is rough, narrow the pass band width of the band limiting filter,
If it is fine, widen the band. The roughness of the quantization is detected from the quantization mode signal sent from the transmission side, and whether the screen is complex is detected from the bit allocation of the quantization of the DCT coefficient or the image signal obtained or reproduced from the coefficient value. The control circuit 2 controls the switching of the filter characteristics based on the above. In order to maintain continuity, these characteristic switches are provided with many characteristics having different bandwidths, and it is possible to obtain a more natural image by gradually changing the characteristics to be wider or narrower.

Referring to FIG. 2, the second embodiment of the present invention is a construction method in which not only 8 × 8 block distortion but also 4 × 4 block distortion is conspicuous. First stage filter 1
In 0, block distortion is removed from 8 × 8 blocks, and in the second-stage filter 20, block distortion is removed from 4 × 4 blocks. The first stage filter 10 is
It operates in the same way as the first embodiment shown in FIG.

The second stage filter 20 is similarly constructed. The DC component separation circuit 23 obtains a DC component for a 4 × 4 block and obtains a DC component signal for each pixel of the block. The band limiting filter 24 is a two-dimensional band limiting filter that removes block distortion of the DC component signal of the 4 × 4 block. According to the control signal from the control circuit 22,
The band limiting characteristic is switched and controlled. This makes 4x
Up to 4 block distortions can be reduced without sacrificing resolution.

The orthogonal transform is not limited to DCT, but Hadamard transform or the like is also effective. The image signal is effective not only for the encoded and transmitted image signal but also for reducing the block distortion of a decoded signal which generally has block distortion due to orthogonal transformation. Depending on the encoding method, the DC component may be obtained directly from the decoding process in the decoder instead of being obtained from the decoded signal.

[0026]

As described above, according to the present invention, the DC component and the high frequency component are separated for each block, and the band limiting filter for removing the distortion of the DC component signal is provided. The block distortion can be reduced without degrading the resolution of.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 2 is a block diagram of a second embodiment of the present invention.

FIG. 3 is a block diagram of a conventional example.

[Explanation of symbols]

 1 Decoder 2,22 Control Circuit 3,23 DC Component Separation Circuit 4,24 Band Limiting Filter 5,25 Subtractor 6,26 Adder 7,8,27,28 Delay Circuit 10 First Stage Filter 12 Boundary State Detection Circuit 13 Block boundary filter 14 Switching device 20 Second stage filter

Claims (3)

[Claims] 1. A block unit for a decoded signal of a decoded image signal from a decoder for decoding a data compression-coded image signal using coding for orthogonally transforming an image signal in block units. Means for obtaining the DC component of the pixel and replacing all the pixels in block units with the obtained DC component to obtain the DC component signal; and means for subtracting the DC component signal from the decoded signal and outputting the high frequency component signal, Control means for adaptively switching and controlling the band limitation characteristic by using the block size of the transform coding obtained from the decoder and the information of the quantization characteristic for each block, and the block size large step size quantization of the quantization characteristic When the characteristic step size is coarse and the converted signal has high-frequency components with limited quantization characteristics, a spatial filter characteristic with a narrower frequency band is selected to control the DC component signal. An adaptive post-processing filter comprising: a limiting filter means; and a means for adding a band-limited DC component signal and a high-frequency component signal to obtain an image signal with reduced block distortion. 2. An adaptive post-processing filter, wherein the adaptive post-processing filters according to claim 1 are cascade-connected in a plurality of stages. 3. The adaptive post-processing filter according to claim 2, wherein the block sizes processed by the cascaded plurality of stages of adaptive post-processing filters are different from each other.