JPH08317389A - Block distortion remover - Google Patents

Abstract

PURPOSE: To remove block distortion with much higher accuracy by non-linearly changing the removing amount of block distortion corresponding to level difference between the picture elements of a block boundary. CONSTITUTION: The level difference between a concerned picture element and an adjacent reference picture element found from delay circuits 1 and 2 and subtracters 3 and 4 is inputted to non-linear processing circuits 5 and 6. When the level difference is set in a previously decided range, the non-linear processing circuits 5 and 6 judge the presence of block distortion and output the block distortion removing amount. This output value is the product of probability for that level difference to be the block distortion and a correcting amount expressed by a straight line of negative slope passing through the origin and is added to the concerned picture element at adders 7 and 8 so that the levels of both the picture elements can get close each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a block distortion removing apparatus for removing block distortion generated by coding when an image signal converted and coded in block units is decoded.

[0002]

2. Description of the Related Art In coding an image signal, the image signal is divided into a plurality of blocks and a DCT is performed for each block.
A block coding method for coding image data by (discrete cosine transform) or the like is often used. In this method, a discontinuous level difference between adjacent blocks, that is, so-called block distortion occurs, which causes deterioration in image quality.
Conventionally, an apparatus has been proposed that removes block distortion by smoothing block boundaries.

As a conventional block distortion removing apparatus, for example, at a block boundary, it is detected whether or not there is block distortion, and if there is block distortion, smoothing processing is carried out. Some store data. Examples of smoothing processing include moving average and median filter.

[0004]

However, in the above conventional apparatus, the processing such as moving average and median filter, which is performed to remove block distortion, can only perform linear smoothing with respect to block distortion. It could not be removed according to the characteristics of strain.

Therefore, an object of the present invention is to provide a block distortion removing apparatus with higher accuracy by constructing a non-linear noise filter that removes distortion according to the characteristics of block distortion.

[0006]

In order to solve the above-mentioned problems, the block distortion removing apparatus of the present invention, when decoding an image signal divided into a plurality of blocks, has a level difference between pixels at the block boundary. When the magnitude is within the range of the probability distribution in which the block distortion occurs, the product of the probability according to the level difference and the correction amount represented by a straight line with a negative slope passing through the origin is calculated as the block distortion removal amount. The above is characterized in that the levels of both pixels are added so as to be close to each other.

[0007]

According to the above construction, the probability of block distortion is obtained from the pixel level difference at the block boundary. The larger the probability, the larger the block distortion removal amount, and the smaller the probability, the smaller the removal amount. There is. By making the distortion removal characteristic non-linear in this way, the distortion can be removed with higher accuracy.

[0008]

Embodiments of the block distortion eliminating device of the present invention will be described below in detail with reference to the drawings. FIG.
Shows a configuration of a block distortion eliminator in one embodiment of the present invention.

First, the block distortion removal in the horizontal direction will be described. The delay circuits 1 and 2 delay the input signal by one pixel. When the signal of the target pixel is used as the output signal of the delay circuit 1 and the XY coordinates thereof are (x, y), the target pixel and its left and right sides. The level difference between the pixels (x-1, y) and (x + 1, y) can be obtained by the subtracters 3 and 4.

When the level difference is input to the non-linear processing circuits 5 and 6, the block distortion removal amounts calculated in accordance with the magnitudes are summed in the adder 7. Then, the adder 8 removes block distortion from the pixel of interest.

The block boundary detection circuit 9 and the ON / OFF
The off control circuit 10 adds the outputs from the non-linear processing circuits 5 and 6 to the signal of the target pixel when the target pixel is on the block boundary, and does not add otherwise.

Here, in the non-linear processing circuits 5 and 6, when the input level difference is within a predetermined range, it is judged that there is block distortion and the distortion removal data is output, and a large value that is not within the above range is output. If it is, it is determined to be an edge and the edge data is saved.

Specifically, the characteristics of the non-linear processing circuits 5 and 6 are determined as follows. That is, the probability distribution in which the level difference at the block boundary is the block distortion is a normal distribution up to the level difference t as shown in FIG. If all the level differences are block distortions, the correction amount of the pixel of interest for removing the block distortions increases according to the level difference, as shown in FIG. 3, and the level differences are positive values. In the case of, it becomes a negative value, and when the level difference is a negative value, it becomes a positive value.

Here, when the block distortion removal amount is the product of the probability distribution of the block distortion and the correction amount, the block distortion removal amount having a size proportional to the probability that the level difference between pixels is the block distortion is given. It will be. Therefore, the characteristics shown in FIG. 4 are obtained, the block distortion is removed in the range where the level difference is −t to t, 0 is output at the other levels, and the edge is preserved. It is a value that determines whether t is block distortion or an edge.

The block distortion removing process will be specifically described with reference to FIGS. 5A shows the pixel arrangement and the position of the block boundary, and FIG. 5B shows the signal levels of the reference pixels A, B, C, and D near the block boundary.
“A” indicates a block boundary, and “b” indicates a pixel to be processed. The level difference between the A pixel and the B pixel is d1, the level difference between the B pixel and the C pixel is d2, and the level difference between the C pixel and the D pixel is d3. Therefore, in this case, the level difference at the block boundary is d2.

In calculating the block distortion removal amount, not only the level difference d2 between the pixels B and C at the block boundary, but also the level differences d1 and d from the reference pixels A and D on the opposite side of the block boundary.
3 is also used, but this is to remove the block distortion more accurately as described later, so to simplify the explanation,
As shown in FIG. 6A, assuming that d1 = 0 and d3 = 0,
Processing is performed only with the level difference d2 at the block boundary.

First, the processing will be described with the pixel of interest as a B pixel. When the absolute value of the level difference d2 at the block boundary with the C pixel is smaller than the reference value t, it is determined that block distortion has occurred, and as shown in FIG. The block distortion removal amount h is output from the non-linear processing circuit 5, and the value of the B pixel is reduced by the removal amount h. At this time, since the level difference d1 from the A pixel is 0, the output of the nonlinear processing circuit 6 is 0.

When the pixel of interest moves to the C pixel, the block distortion removal amount h corresponding to the level difference d2 between the C pixel and the B pixel is output from the non-linear processing circuit 6, and the value of the C pixel is removed by the removal amount h.
Increase by the amount. Also, the level difference d between the C pixel and the D pixel
Since 3 is 0, the output of the non-linear processing circuit 5 is 0. Therefore, the values of the B pixel and the C pixel are close to each other so that the block distortion is reduced. If the absolute value of the level difference d2 is larger than the reference value t, it is determined that the edge is present and the nonlinear processing circuits 5 and 6 output 0, so the edge is saved.

Next, the processing in which the reference pixel is added will be described. By adding the level difference d1 between the A pixel and the B pixel and the level difference d3 between the C pixel and the D pixel to the calculation, the block distortion can be removed with higher accuracy. For example, as shown in FIG. 7A, when the signal level continuously changes at the block boundary, it is considered to remove the block distortion based on only the level difference d2 at the block boundary as described above.

In this case, since the signal level of the B pixel decreases and the signal level of the point C increases by the block distortion removal amount h2 with respect to the level difference d2 of the block boundary, continuity may be impaired by the processing.

Therefore, the level difference d1, C between the A pixel and the B pixel
The levels h1 and h3 from the non-linear processing circuits 5 and 6 for the level difference d3 between the pixel and the D pixel are used to correct the levels of the B pixel and the C pixel, respectively. Since the outputs h1 and h3 suppress the level change of the B pixel and the C pixel due to the block distortion removal amount h2, the continuity can be maintained.

For removing block distortion in the vertical direction,
Similar to the processing in the horizontal direction, when the target pixel is on the block boundary by the block boundary detection circuit 9, the target pixel (x, y), the target pixel and its upper and lower pixels (x, y−).
1), the difference between (x, y + 1) is obtained by the subtracters 3 and 4, and the above-described block distortion removal processing can be performed from the difference data by the nonlinear processing circuits 5 and 6. In this case, the delay circuits 1 and 2 are 1 line delay circuits.

In the above embodiment, only the processing for removing the block distortion at the block boundary is performed, and when the block boundary has an edge, the original edge of the image is preserved. Therefore, if the characteristics shown in FIG. 8 are added to the above-mentioned non-linear processing circuits 5 and 6, the edge can be emphasized and the sharpness of the image can be increased.

That is, the non-linear processing circuits 5 and 6 are supplied with the level differences between the pixel of interest and the left and right or upper and lower pixels calculated by the delay circuits 1 and 2 and the subtractors 3 and 4. When the pixel of interest is on the block boundary, t can be used as a reference value for determining whether it is block distortion or an edge. If the level difference of the signal at the block boundary is a value between -t and t, it is judged as block distortion and the above-mentioned processing is performed. If the level difference is smaller than -t or greater than t, it is determined to be an edge, and data for increasing the level difference is output. When the pixel of interest is not on the block boundary, the ON / OFF control circuit 10 causes the non-linear processing circuits 5 and 6 to operate in the negative direction.
By setting the output value from t to t to 0, only the contour enhancement can be performed. In this case, since the contour enhancement amount is the same as that at the block boundary, the continuity of data can be maintained.

[0025]

As described above, the block distortion removing apparatus of the present invention has a nonlinear characteristic so that the block distortion removing amount changes according to the probability that the level difference detected between pixels on the block boundary is block distortion. Since it is provided, it is possible to perform highly accurate processing according to the characteristics of block distortion. Further, by correcting the block distortion removal between the pixels on the block boundary from the level difference between the reference pixel adjacent to the pixel on the block boundary, the continuity of the data of the original image can be maintained. Further, between the pixels on the block boundary, not only the block distortion removal but also the edge enhancement is selectively performed, whereby a sharp and distortion-free image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a block distortion eliminating device of the present invention.

[Fig. 2] Probability distribution diagram of block distortion

FIG. 3 is a characteristic diagram showing an example of a block distortion correction amount.

FIG. 4 is an input / output characteristic diagram of a nonlinear processing circuit according to the same embodiment.

FIG. 5 is a pixel array according to the embodiment, and a signal level diagram of pixels at a block boundary and reference pixels.

FIG. 6 is a diagram showing distortion processing between pixels at a block boundary in the embodiment.

FIG. 7 is a diagram showing a distortion process including a pixel at a block boundary and a reference pixel in the embodiment.

FIG. 8 is a characteristic diagram of a non-linear processing circuit that performs block distortion removal and edge enhancement in the same embodiment.

[Explanation of symbols]

 1, 2 Delay circuit 3, 4 Subtractor 5, 6 Non-linear processing circuit 7, 8 Adder 9 Block boundary detection circuit 10 ON / OFF control circuit

Claims (3)

[Claims] 1. When decoding an image signal divided into a plurality of blocks, a level difference between pixels at the block boundary is calculated, and when the magnitude is within a range of a probability distribution in which block distortion occurs, The product of the probability according to the level difference and the correction amount represented by a straight line having a negative slope passing through the origin is used as a block distortion removal amount, and the levels of the both pixels are added so as to approach each other. Block distortion removal device. 2. The block distortion removal amount is corrected by a value calculated in the same manner as the block distortion removal amount between pixels at the block boundary from the level difference between a pixel at the block boundary and a reference pixel adjacent to it. The block distortion removal apparatus according to claim 1, wherein the pixels are subtracted from the pixels at the block boundaries. 3. A block boundary pixel and a reference pixel adjacent thereto have a level difference, and when the level difference is out of a probability distribution range of block distortion, the block boundary pixel and the reference pixel are separated. 3. The block distortion eliminating apparatus according to claim 1, wherein the pixel at the block boundary is corrected so that the level difference between the two is increased and the edge is emphasized.