JPH01183981A - System for compressing image signal band - Google Patents

Abstract

PURPOSE:To reduce the deterioration in picture quality caused by the complicated mixture of a mode by correcting the distortion quantity to be a same value within an area where the difference between the distortion quantity of every block is below a fixed value. CONSTITUTION:For an image signal 60 inputted from an input terminal, the thinning and the interpolation of the mode 2 is executed by a mode 2 thinning and interpolation circuit 1 and the interpolation signal is inputted to a difference circuit 2. Then, the difference between the interpolation signal and an original signal is calculated and inputted to a distortion quantity calculation circuit 3. A total sum in the block of the absolute value of a difference signal is calculated and made to be the distortion quantity D2 of every block. By referring the distortion quantity of every block of one field, (n > zero) is made to be the fixed value and the distortion quantity of the block within the areas being adjacent over two blocks, for example, among the blocks of which distortion quantity value is between (nXk)-(nXk+n-1) (k=0.1...) is corrected to be (2XnXk+n-1), for example. Then, the mode is decided by a mode decision circuit 5.

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, a technology called the TAT method has been known, which is reported in IEICE Communications System Study Group Material C884-7F Time Axis Conversion Band Compression Method for High-Definition Television Signals. ing.

The TAT method first extracts basic pixels representing the basic structure of an image by coarse subsampling, and then selects additional pixels necessary to represent the fineness of the image from among the remaining pixels.

This corresponds to dividing an image into small blocks and changing the sampling pattern as shown in FIGS. 5(a) and 5(b) 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. In determining the mode in the TAT method, the number of modes is as shown in FIG. 5(a).

In the case of the two types shown in (b), the fifth
Using a histogram of the distortion amount S for each block, which is the intra-block sum of the absolute values of the interpolation errors for mode 2 in FIG. 2B, the mode is determined using the 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
As described above, the threshold value is determined as a boundary by looking at the distortion amount value for each block, which is the intra-block summation of the absolute value of the interpolation error. Therefore, in an image region where the amount of distortion for each block is close to the threshold Th, limited types of modes with large differences in image quality are complicated for each block, and this appears as image quality deterioration.

An object of the present invention is to correct the distortion amount of each block so that it becomes the same value in an image area where the difference between the distortion amount of each block and the distortion amount of adjacent blocks is small, and to use this corrected distortion amount. It is an object of the present invention to provide an image signal band compression method that alleviates the above-mentioned problems by determining a mode such that the same mode is assigned to an image area where the difference between distortion amounts for each block is small.

(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 that determines the mode and thins out pixels on the transmitting side and restores the thinned out pixels on the receiving side according to the mode, when determining the mode, each mode is set for each pixel,
Calculate the interpolation error for each block, use at least the interpolation error to calculate the distortion amount for each block, and set the distortion amount to the same value within a region where the difference between the distortion amounts for each block is less than or equal to a predetermined value. The mode is determined using at least the modified distortion.

(Operation) In the present invention, 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 restore the image, the amount of distortion for each block used for mode determination is calculated and The key point is how to fix it.

In the present invention, first, the interpolation error for each mode is calculated, and this is used to calculate, for example, the intra-block sum of the absolute values of the interpolation errors, and this is used as the distortion amount for each block. Furthermore, as shown in Fig. 1, areas where the difference ΔD between the distortion amounts for each block is less than or equal to a predetermined value are detected, or the screen is divided in advance into areas consisting of multiple blocks and the distortion amount within these areas is Detect areas where the difference between the maximum value and the minimum value is ΔD, which is smaller than the default value, replace the distortion amount for each block in these areas with the same value, use this as corrected distortion, and use this corrected distortion to perform the conventional The mode is determined in the same way as the TAT method.

By doing this, if the difference in distortion amount between adjacent blocks is small (below the default value), the corrected distortion will be the same value, and therefore the same mode will be assigned, so each block will have a limited number of types with large differences in image quality. Image quality deterioration caused by complicated modes can be reduced.

(Example) Next, an example of the present invention will be described using FIGS. 1 to 6.

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 172. The figure shows the case of thinning out. Furthermore, there are two modes in this embodiment, mode 1 and mode 2, as shown in FIGS. 5(a) and 5(b), as an example. The compression ratio of each mode is 1 for mode 1 and 1/4 for mode 2, and mode 2 uses intra-field interpolation.

That is, this embodiment allocates two types of modes (a) and (b) in FIG. 5 to each block, performs thinning corresponding to each, and performs band compression.The method for determining the mode will be described below. I will explain about it.

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.

Mode 2 is applied to the image signal 50 input from the input terminal.
The thinning and interpolation circuit 1 performs mode 2 thinning and interpolation, and the interpolated signal is input to the difference circuit 2, where the difference between the interpolated signal and the original signal is calculated.

The difference signal 51 is input to the distortion amount calculation circuit 3, and for 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.

The distortion amount signal 52 is input to the distortion amount correction circuit 4. For example, the distortion amount for each block is corrected in the following manner. That is, looking at the amount of distortion for each block for one field, the value of the amount of distortion is (nXk) ~ (nXk+n-1) with n>0 as the default value.
) (k=0.1.-), the distortion amount of blocks in a region where two or more blocks are adjacent to each other is corrected to, for example, (2XnXk+n-1)/2. The corrected distortion amount is output as a corrected distortion amount signal 53.

The mode determining circuit 5 uses the modified distortion amount signal 53 to determine the mode in the same manner as in the conventional TAT method.

FIG. 3 shows an example of a mode determining circuit. First, the corrected distortion amount signal 53 is input to the histogram creation circuit 8, and the sixth
A histogram of the distortion amount D2 is created in the same manner as shown in the figure. In the threshold value determination circuit 9, the contents of the histogram created in the histogram creation circuit 8 are read out one after another from the larger D2 and added, and the value of D2 when the value of D2 becomes 1/3 or more of the total number of blocks is set as the threshold value. As Th,
It is output as a threshold signal 57. Furthermore, the corrected distortion amount signal 5
3 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 one-character positive distortion amount signal 58 and the threshold signal 57 to determine the mode for all blocks. That is, for each block, mode 1 is assigned to blocks whose distortion amount D2 is greater than or equal to Th, and mode 2 is assigned to blocks whose distortion amount D2 is smaller than Th, and the 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 manually 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 respectively sent from the output terminal to the receiving side. Sent.

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.

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

Further, as a method for determining the threshold value Th in the threshold value determination circuit 9 of FIG. 3, the contents of the histogram may be read from the side with the smaller distortion amount D2. However, in that case, the value of D2 when the addition result exceeds 273, which is the total number of blocks, is T
Let it be h.

In addition, as a method for correcting the amount of distortion for each block in the distortion amount correction circuit 4 shown in FIG. If the difference is less than a predetermined value, the distortion amount of the block included in the region may be corrected to, for example, the average value of the distortion amount within the region. Furthermore, if the difference in the amount of distortion between adjacent blocks is less than a predetermined value, the value of the amount of distortion of the block is corrected so that it becomes the same value as the value of the amount of distortion of the adjacent block, and the corrected distortion is A method may also be used in which the amount of distortion of adjacent blocks is corrected one after another while using the amount.

Further, the type of mode to be used may be different from those shown in FIGS. 5(a) and (b), for example, FIG.
) Mode 2, (c) Mode 3, (d) Mode 4 as shown in 4.
Different modes may be used. (b) and (c) use intra-field interpolation, and (d) inter-frame interpolation using a restoration surface f-image signal from one frame before. As a mode determination method in this case, for example, the mode determination method described in "Study of multi-mode adaptive subsampling method for HDTV signals", Material C386-77r, Communication System Research Group, Institute of Electronics and Communication Engineers, may be used.

According to this embodiment, the signal band of the television signal is halved.
The bandwidth of the transmission path required for transmission is 1/2.
This enables an image signal band compression method with even better image quality. 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, if the difference in the amount of distortion between adjacent blocks is small (less than a predetermined value), the amount of distortion can be set to the same value, thereby allowing the same mode to be set. is allocated, and it is possible to reduce the deterioration in image quality caused by a complicated combination of limited types of modes with large differences in image quality for each block. Therefore, it is possible to perform image band compression with visually higher quality than the conventional TAT method.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a method for correcting the amount of distortion in an embodiment of the present invention, FIG. FIG. 4 shows an example of a mode determining circuit in an embodiment of the invention.
The figure shows a mode pattern in an embodiment of the present invention, FIGS. 5(a) and 5(b) are explanatory diagrams showing sampling patterns of the TAT method, and FIG. 6 is an explanatory diagram showing a mode determination method in the TAT method. It is. In the figure, 1...Mode 2 thinning interpolation circuit, 2...
- Difference circuit, 3... Distortion amount calculation circuit, 4... Distortion amount correction circuit, 5... Mode determining circuit, 6... Delay circuit, 7
... Thinning circuit.

Claims (1)

[Claims] An image is divided into blocks of a predetermined size, a mode indicating a pixel thinning and interpolation method is determined for each block on the transmitting side, and pixels are thinned out on the transmitting side and thinned out on the receiving side according to the mode. In an image signal band compression method that restores pixels by interpolation, when determining the mode, calculate an interpolation error for each mode for each pixel, and calculate a distortion amount for each block using at least the interpolation error, In a region where the difference between the distortion amounts for each block is less than or equal to a predetermined value, the distortion amounts are corrected to be the same value and this is used as a corrected distortion, and at least the corrected distortion is used to determine the mode. An image signal band compression method characterized by: