JPH0511836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an image signal band compression method that can be applied to transmitting or recording image signals.

(Prior art) Conventionally, regarding band compression technology for image signals, there has been a technology called the TAT method, which is reported in IEICE Communication Systems Research Group Material CS84-7 "Time axis conversion band compression method for high-definition television signals." Are known.

The TAT method first uses coarse subsampling to extract basic pixels that represent the basic structure of the image.
Next, additional pixels necessary to express the fineness of the image are selected from the remaining pixels.

This corresponds to dividing the image into small blocks and changing the sampling pattern as shown in FIGS. 8a and 8b depending on the fineness of the image within each block. That is, a plurality of modes are set with one sampling pattern as one mode, the mode is switched and assigned to each block according to the fineness of the image, and pixels are thinned out and band compression is performed according to the assigned mode. By doing so, it is possible to reduce the interpolation error as a whole by thinning out pixels in flat parts of the image without thinning out pixels in fine parts of the image. TAT
In determining the mode in the method, when the number of modes is two types as shown in Figure 8a and b, as shown in Figure 9, the intra-block sum of the absolute value of the interpolation error for mode 2 in Figure 8b is Distortion amount S for each block
The mode is determined using a histogram of , with a certain value Th of S as a boundary. That is, blocks exceeding Th are assigned mode 1, and blocks below Th are assigned mode 2.

(Problem to be Solved by the Invention) In the conventional TAT method, the mode for each block is determined by looking at the distortion amount value for each block, which is the intra-block summation of the absolute value of the interpolation error, as described above. It is determined using the threshold as the boundary. Therefore, in an image region where the amount of distortion for each block is close to the threshold Th, a limited number of modes with large differences in image quality will be complicated for each block, and this will appear as visual image quality deterioration. . In addition, in this area, even a slight change in the amount of distortion in the time direction due to the influence of noise etc. may cause the mode of blocks at the same position to change randomly from screen to screen, which also causes visual image quality deterioration. Appears with age.

An object of the present invention is to provide an image signal band in which the above-mentioned problems are alleviated by applying a filter to the distortion amount of each block in the spatial or temporal direction and determining the mode using the filtered distortion amount. The objective is to provide a compression method.

(Means for Solving the Problems) The image signal band compression method of the present invention divides an image into blocks of a predetermined size, and sets a mode indicating a pixel thinning and interpolation method for each block on the transmitting side. In an image signal band compression method in which pixels are determined on the transmitting side and restored by interpolating the thinned out pixels on the receiving side according to the mode, when determining the mode, the pixels are An interpolation error is calculated, a distortion amount for each block is calculated using at least the interpolation error, a film is applied to the distortion amount for each block in the spatial or temporal direction, and this is used as a correction distortion, and at least the correction distortion is The mode is determined using the following.

(Function) The present invention calculates the amount of distortion for each block used for mode determination in image band compression in which pixels are thinned out on the transmitting side and the thinned out pixels are interpolated on the receiving side to create an image. The key point is how to fix it.

In the present invention, first, the interpolation error for each mode is calculated, and using this, for example, the total sum within a block of the absolute value of the interpolation error is calculated using the distortion amount calculation circuit 10 of FIG.
0, and use this as the distortion amount for each block. Furthermore, the first
As shown in the figure, a filter circuit 200 uses the distortion amounts of a plurality of blocks located around the screen or at the same position but at different times to produce a corrected distortion. The mode determining circuit 300 is configured in the same way as the TAT method.
The mode is determined in . If, for example, a low-pass spatial filter is used as the filter, the amount of distortion between adjacent blocks will be reduced, and the possibility that limited types of modes with large differences in image quality will be complicatedly mixed together will be reduced. As another example of a filter, if the distortion amount of a predetermined number or more blocks within a predetermined range centered on the relevant block is the same value,
A similar effect can be obtained by replacing the distortion amount of the block with the same value. Furthermore, by applying a filter in the time direction, it is possible to suppress random changes in the mode in the time direction due to the influence of noise and the like.

By doing this, applying a filter in the spatial direction reduces the difference in distortion between adjacent blocks, and applying a filter in the time direction makes it possible to suppress changes in the amount of distortion in the time direction. As a result, the possibility that the same mode will be assigned in the spatial or temporal direction increases, so a limited number of modes with large differences in image quality may become spatially complex, or modes may be randomly assigned in the temporal direction due to noise, etc. Image quality deterioration due to changes can be reduced.

(Example) Next, an example of the present invention will be described using FIGS. 2 to 9.

In this embodiment, as an example, a television signal is input, the field is divided into blocks of 4 pixels x 4 pixels, the mode is determined for each block for each field, and the total number of pixels is The case of thinning out to 1/2 is shown. In addition, the modes in this embodiment are, for example, FIG. 8a,
There are two types, mode 1 and mode 2 shown in b. The compression ratio of each mode is 1 for mode 1 and 1/4 for mode 2.
In Mode 2, intra-field interpolation is used.

That is, in this embodiment, two types of modes a and b shown in FIG. 8 are assigned to each block, and corresponding thinning is performed to perform band compression.The mode determination method will be particularly explained below.

FIG. 2 is a block diagram schematically showing the basic part of a transmitting section of a television signal band compression system according to an embodiment of the present invention.

For the image signal 50 input from the input terminal,
A mode 2 decimation and interpolation circuit 1 performs mode 2 decimation and interpolation, and the interpolated signal is input to a difference circuit 2 to calculate the difference between the interpolated signal and the original signal.

The difference signal 51 is input to the distortion amount calculation circuit 3, and as an example, the sum of the absolute values of the difference signal within a block is calculated and this is taken as the distortion amount D2 for each block. The amount of distortion for each block is output as a distortion amount signal 52. FIG. 3 shows a field block division method in this embodiment.

The distortion amount signal 52 is input to the filter circuit 4,
For example, it is applied with a low-pass spatial filter and output as a modified distortion signal 53. FIG. 4 is a block diagram of an example of a filter circuit for applying a spatial direction filter to the distortion amount signal. Distortion amount signal 52 and delay signal 64 obtained by delaying the distortion amount signal 52 by n blocks.
and distortion amount signal 52 delayed by 2×n blocks, three delay circuits (delay circuits 27 to 3) each delay the delay signal 65 by 1 block.
5) Connect them in series, take out the distortion amounts of 3×3 blocks spatially centered on the relevant block, and apply fixed filters to the output signals of the delay circuits 27 to 35 using fixed coefficient multipliers 36 to 44, respectively. The outputs of the fixed coefficient multipliers 36 to 44 are added by an adder circuit 45 to output a corrected distortion signal 53.

The mode determining circuit 5 uses the modified distortion signal 53 to determine the mode in the same manner as in the conventional TAT method. FIG. 5 shows an example of a mode determining circuit. First, the corrected distortion signal 53 is input to the histogram creation circuit 8, and a histogram of the distortion amount D2 is created in the same manner as shown in FIG. In the threshold determination circuit 9, the contents of the histogram created by the histogram creation circuit 8 are read out one after another from the larger D2 and added, and the value of D2 when the number of blocks becomes 1/3 or more of the total number of blocks is calculated. Let the threshold Th be
It is output as a threshold signal 57. Furthermore, the modified distortion signal 53 is delayed by the delay circuit 11 until the threshold value Th is determined by the threshold value determination circuit 9. The mode assignment circuit 10 uses the delayed modified distortion signal 58 and the threshold signal 57 to determine the mode for all blocks. That is, for each block, the amount of distortion D2 is
For blocks with values greater than or equal to Th, mode 1 is used.
For blocks with smaller values, mode 2 is assigned and mode signal 54 is output.

The input image signal 50 is delayed by the delay circuit 6 from when the input image signal 50 is input until the mode is determined by the mode determining circuit 5. The pixels of the delayed input image signal 55 are thinned out in accordance with the mode signal 54 in the thinning circuit 7, and the thinned out image signal 56 and the mode signal 54 determined by the mode determining circuit 5 are sent from the output terminal to the receiving side. sent to.

A mode determination method based on this embodiment will be described below.

In the embodiment shown in FIG. 2, the mode is determined for each field, but the mode may be determined for a plurality of fields at once.

Further, before thinning out in the mode 2 thinning interpolation circuit 1 and thinning circuit 7 shown in FIG. 2, the signal band may be limited in order to prevent aliasing distortion in correspondence with mode 2.

Further, the threshold value in the threshold value determination circuit 9 in FIG.
As a method for determining Th, the contents of the histogram may be read starting from the one with the smallest distortion amount D2. However, in that case, the addition result is 2/2 of the total number of blocks.
Let the value of D2 when it exceeds 3 be Th.

Furthermore, as a spatial filter in the filter circuit 4 of FIG. 2, a filter circuit as shown in the block diagram of FIG. If the quantities have the same value, the distortion quantity of the block may be replaced with the same value. That is, the counting circuit 22 uses the distortion amount signal 52 to create a histogram of the amount of distortion within a range of, for example, a 3×3 block centered on the block in question, and calculates the value of the amount of distortion having the maximum frequency in this histogram.
Ds and the power with respect to Ds are set as H (Ds) and output as a maximum power distortion amount signal 60 and a maximum power signal 61, respectively. The comparison circuit 23 inputs the maximum frequency signal 61 and the predetermined value signal 62, compares whether H(Ds) is larger or smaller than the predetermined value, and outputs a comparison signal 63. In the selection circuit 21, H
If (Ds) is larger than the predetermined value, the maximum frequency distortion signal 60 may be selected, and if H(Ds) is less than or equal to the predetermined value, the distortion signal 52 may be selected as the modified distortion signal 53.

Furthermore, as a spatial filter in the filter circuit 4 of FIG. 2, a filter circuit as shown in the block diagram of FIG. 7 may be used to apply a low-pass filter to the distortion amount signal 52 in the time direction.
In other words, the distortion amount signal 52 and the delayed signal 66 which is obtained by delaying the distortion amount signal 52 by n×m blocks (one fill).
1 for the delayed signal 67 obtained by delaying the distortion amount signal 52 by 2×n×m blocks (2 fields).
Connect one delay circuit to each block (delay circuits 14 to 16), take out the distortion amount for three blocks centered on the block in the time direction, and apply it to the output signals of the delay circuits 14 to 16. The fixed coefficient multipliers 17 to 19 multiply each fixed filter coefficient by the fixed coefficient multipliers 17 to 19, respectively.
The outputs of 19 may be added by the adder circuit 20 and output as the corrected distortion signal 53.

Further, the types of modes used may be different from those shown in FIGS. 8a and 8b.

According to this embodiment, the signal band of the television signal can be compressed to 1/2, the band of the transmission path required for transmission is reduced to 1/2, and an image signal band compression method with even better image quality is possible. . Note that the compression rate of the image signal can be varied by controlling the compression rate of each mode or the value of Th.

(Effects of the Invention) As described above, according to the present invention, it is possible to reduce the difference in the amount of distortion between adjacent blocks and to suppress the change in the amount of distortion in the time direction. Since the possibility of allocation is increased, it is possible to reduce image quality deterioration caused by limited types of modes with large differences in image quality being complicated for each block or by modes changing randomly in the time direction due to the influence of noise or the like. Therefore, conventional
It enables band compression of visually higher-quality images than the TAT method.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a method for calculating and correcting the amount of distortion in the present invention, Fig. 2 is a block diagram showing an outline of the basic part of a transmitting section in an embodiment of the present invention, and Fig. 3 is an embodiment of the present invention. FIG. 4 is a diagram showing an example of a one-field block division method in an example, FIG. 4 is a diagram showing an example of a filter circuit in an embodiment of the present invention, and FIG.
The figure shows an example of a mode determining circuit according to an embodiment of the present invention, FIG. 6 shows an example of a filter circuit according to an embodiment of the present invention, and FIG. 7 shows a filter circuit according to an embodiment of the present invention. FIGS. 8a and 8b are explanatory diagrams showing sampling patterns of the TAT method, and FIG. 9 is an explanatory diagram showing a mode determination method in the TAT method. In the figure, 100...distortion amount calculation circuit, 200...
Filter circuit, 300...mode determining circuit.

Claims (1)

[Claims] 1 Divide an image into blocks of a predetermined size, determine a mode indicating the pixel thinning and interpolation method for each block on the transmitting side, thin out pixels on the transmitting side, and thin out pixels on the receiving side according to the mode. In an image signal band compression method that restores subtracted pixels by interpolation, when determining the mode, an interpolation error for each mode is calculated for each pixel, and at least the amount of distortion for each block is calculated using the interpolation error. calculate,
An image signal band compression method characterized in that the amount of distortion for each block is filtered in the spatial or temporal direction to obtain a modified distortion, and at least the modified distortion is used to determine the mode.