JP4183993B2 - Filter device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to the field of digital video signal processing, and more particularly to block distortion caused by quantization noise in a decoded image of an encoding device that performs orthogonal transformation on an image signal in units of blocks and quantizes and encodes coefficients thereof. The present invention relates to a post-processing filter for making the image less noticeable.
[0002]
[Prior art]
FIG. 7 is a block diagram showing a configuration of a conventional apparatus for processing a video image signal. Referring to FIG. 7, the video signal recorded on the recording medium 10 such as a DVD passes through the system multiplex decoder 20, is decoded by the video decoder 30, and the decoded image is filtered by the filter device 40. , Encoded by the NTSC / PAL encoder 50 and output to a monitor (not shown).
[0003]
A video image can be represented by a digital signal using a series of information bits. For certain lines with limited bandwidth, such as mobile phone lines and public telephone lines, low bit rate image encoding is particularly useful for image transfer and communication. Therefore, the image quality of the video image generated by the low bit rate coding is a serious problem.
[0004]
In general, the quality of an encoded image is determined by the encoding technique used and the target bit rate. When the target bit rate is lowered, that is, when an image is compressed with high efficiency, image quality degradation unique to various compression coding techniques often occurs. For example, in the case of an encoding method in which image compression is performed in units of blocks using DCT (Discrete Cosine Transform) or the like, block distortion in which block boundaries are discontinuous may occur.
[0005]
Block distortion is visually most noticeable image quality degradation in compression coding technology that performs processing in units of blocks, and it is necessary to reduce or eliminate this in order to improve image quality. For this purpose, a conventionally performed method is to perform filtering processing by smoothing locally or globally on a decoded image including block distortion.
[0006]
FIG. 9 and FIG. 10 are diagrams for explaining block distortion filter processing by smoothing by the filter device 40 shown in FIG. FIG. 9 shows an example in the case where the filtering process by global smoothing is suitable, and FIG. 10 shows an example in the case where the filtering process by local smoothing is suitable. 9A and 10A show the luminance values of the decoded image before the filtering process, and FIGS. 9B and 10B show the luminance values after the filtering process. In these figures, a line orthogonal to the block boundary in the image is shown as a representative, and the luminance level of the pixel is represented by a black circle.
[0007]
As shown in FIGS. 9A and 10A, a difference in luminance level between the left and right of the block boundary, that is, block distortion in which the block boundary becomes discontinuous occurs. If there is such block distortion, the decoded image will look like a mosaic at that location, making it difficult to see visually.
[0008]
Therefore, conventionally, as shown in 9 (B), the difference in the luminance level at the block boundary is blunted by the filtering process by global smoothing, and the block distortion is reduced by eliminating the linear edge of the luminance value, Alternatively, as shown in FIG. 10B, the difference in luminance level at the block boundary is dulled only by pixels near the block boundary by filter processing using local smoothing to reduce block distortion.
[0009]
FIG. 8 shows a configuration of a conventional filter device 40 that performs such processing. Referring to FIG. 8, a conventional filter device 40 receives a decoder 41 for receiving a decoded image signal, a delay circuit 42 for delaying the decoded image, and a decoded image and an image delayed by the delay circuit 42. And a control circuit 43 for controlling the filter 44 in accordance with a control signal from the decoder 41.
[0010]
[Problems to be solved by the invention]
Conventional filter processing has been performed as described above. However, if the two types of smoothing filter processing are performed in the vertical direction on the interlaced decoded image, the contour may appear double when an object with a clear contour moves up and down. Deterioration has occurred. In addition, when the two kinds of smoothing filter processing are performed on all block boundaries, there is a problem in that the high frequency components of the decoded image are removed and the image is blurred as a whole.
[0011]
The present invention has been made in view of such a situation, and block distortion can be reduced or eliminated without impairing the high frequency components of the decoded image. An object of the present invention is to provide a filter device that can reduce or eliminate the above.
[0012]
[Means for Solving the Problems]
The filter device according to the present invention provides a line of a decoded image signal with respect to an interlaced decoded image signal obtained by sequentially decoding an image signal divided into a plurality of blocks and encoded in units of blocks. the first pixel in the direction with delay and delay means for performing such align along a first direction, and a first determining means for determining block distortion in the first direction of the decoded image signal intersect, The first determination means determines whether or not a predetermined number of neighboring pixels arranged in the first direction so as to intersect with the block boundary is a low frequency based on a difference in pixel values between neighboring neighboring pixels. Thus, the image processing apparatus includes a first sub-determination unit that determines block distortion in the first direction, and filters the image signal delayed by the delay unit based on the determination result obtained by the first determination unit in the first direction. First Further comprising a filter means, first filter means, the first sub-determination unit, without filtering when it is determined that the neighboring pixel is not a low frequency, a first image signal output from the first filter means Based on the determination result obtained by the second determination means and the second determination means for determining the block distortion in the second direction intersecting with the direction of the image signal in the second direction And a second filter means for performing a filtering process .
Preferably, the first sub-determination unit determines whether any of the absolute values of the pixel value differences between adjacent neighboring pixels is equal to or less than a predetermined threshold value, and the first filter unit includes the second sub-determination unit. If the determination means determines that any of the absolute values is greater than the predetermined threshold value, the neighboring pixels are not filtered.
[0013]
In the present invention, the presence / absence of distortion is determined in advance in each direction for each second direction that intersects the first direction and the first direction, and then the filter process is performed in each direction. Accordingly, since the filtering process is performed only when there is a lot of distortion, all the high frequency components of the decoded image are not removed, and the filtering process can be performed only in the direction in which the image quality is deteriorated in the interlaced image.
[0014]
As a result, it is possible to provide a filter device that can reduce or remove block distortion without impairing the high-frequency component of the decoded image, and can reduce or remove block distortion with respect to an interlaced decoded image without image quality deterioration.
[0015]
More preferably, the second determination means calculates a difference in pixel values between adjacent second direction neighboring pixels for a predetermined number of second direction neighboring pixels arranged along the second direction so as to cross a block boundary. Based on the second sub-determination means for determining the block distortion in the second direction by determining whether or not the frequency is low. The second filter means includes a second sub-determination means in which the second direction neighboring pixels are low. If it is determined that the frequency is a frequency, filtering is performed on the neighboring pixels in the second direction, and if the second sub-determining means determines that the neighboring pixels in the second direction are not low frequency, the neighboring pixels in the second direction The filtering process is performed on the pixels that are included in the number of pixels that are less than the number of pixels in the second direction.
[0017]
The first and second determination means includes means for comparing the absolute value of the difference between the pixel values of two pixels and the adjacent sides of the their respective block boundary and a predetermined threshold voltage; 2 pixels of the pixel values it may include means for comparing the largest of the absolute value and the absolute value of the difference between pixel values of neighboring pixels adjacent the differential.
[0018]
More preferably, the first direction is a vertical direction and the second direction is a horizontal direction.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0020]
FIG. 1 is a view corresponding to FIG. 8 of the related art showing an embodiment of a filter device for reducing or eliminating block distortion according to the present invention.
[0021]
The configuration and operation of the filter device 100 will be described with reference to FIG. Referring to FIG. 1, filter device 100 includes a line memory 101, a vertical block distortion determination unit 102, a vertical filter 103, a horizontal block distortion determination unit 104, and a horizontal filter 105.
[0022]
Next, the operation will be described with reference to FIGS. Here, a description will be given of processing in a case where a decoded image of an image compressed by an encoding device that performs orthogonal transform on an image signal in block units (for example, an 8 × 8 block) and quantizes and encodes the coefficients is input. .
[0023]
FIG. 2 shows a block boundary and its neighboring pixels in the case of an interlaced image in a field having an input decoded image. Here, since processing is performed for each field in the case of an interlaced image, as shown in FIG. 2, when the input decoded image is an interlaced image, a block boundary exists every 4 pixels in the vertical direction, and in the horizontal direction, There is a block boundary every 8 pixels. FIG. 3 shows the pixels in the vertical direction when displayed as a frame image. In FIG. 3, v0, v1, v2, and v3 in FIG. 2 are pixels of lines 0, 2, 4, and 6, respectively, and in the next field, v0 ′, v1 ′, v2 ′, and v3 ′ are respectively lines1, 3, 5, 7 pixels. That is, v0, v1, v2, v3 and v0 ′, v1 ′, v2 ′, v3 ′ are data at the same position.
[0024]
The decoded image is input to the line memory 101 and sent to the vertical block distortion determination unit 102 and the vertical filter 103 with a predetermined delay.
[0025]
The vertical block distortion determination unit 102 receives a signal delayed by a predetermined delay in the line memory 101, and determines the presence or absence of block distortion according to the flowchart shown in FIG. Here, the predetermined delay means the next time. That is, in the vertical filter, the pixels to be filtered must be at the same time. For example, when filtering by v0, v1, and v2 in FIG. 2, v0 is delayed by two lines and v1 is delayed by one line so as to be in the same position in time.
[0026]
Here, the flowchart of FIG. 4 will be described. First, with respect to pixels near the block boundary (v0 to v7 in FIG. 2), the absolute value of the difference between adjacent pixels is compared with a predetermined threshold thv1 to determine whether the frequency is low (step 11, the following steps) Is omitted). For example, the absolute value of the difference between adjacent pixels in v0 to v7 in FIG. 2 can be expressed by the following d0, d1, d2, d4, d5, and d6.
[0027]
d0 = | v1-v0 |
d1 = | v2-v1 |
d2 = | v3-v2 |
d4 = | v5-v4 |
d5 = | v6-v5 |
d6 = | v7−v6 |
When all the absolute values (d0, d1, d2, d4, d5, d6) of the differences between adjacent pixels are smaller than the predetermined threshold thv1, the vicinity of the block boundary is flat, so the frequency is low. judge.
[0028]
Next, the absolute value d3 = | v4-v3 | of the difference between two pixels adjacent to the block boundary (v3, v4 in FIG. 2) is compared with a predetermined threshold thv2, and further, two pixels adjacent to the block boundary Is compared with the absolute value maxv1 (maximum value of d0, d1, d2, d4, d5, and d6) of the absolute value of the difference between two adjacent pixels in the vicinity of the block boundary and the presence or absence of block distortion. Is determined (S12).
[0029]
That is, when the absolute value d3 of the difference between two pixels adjacent to the block boundary is larger than the threshold value thv2, it is determined that the edge is included in the image. Further, when the absolute value d3 of the difference between two pixels adjacent to the block boundary is smaller than maxv1, it is determined that the edge is included in the image. If it is determined that the edge is not included in the image under the two conditions, it is determined that there is block distortion.
[0030]
Here, the value such as the threshold value thv2 is an empirical value determined by looking at actual image data input to the filter device 100. Therefore, it differs depending on the resolution and bit rate of the input image.
[0031]
In the vertical filter 103, when a signal having a predetermined delay is input from the line memory 101 and the vertical block distortion determination unit 102 determines that there is block distortion, four pixels centering on the block boundary (v 2 in FIG. 2). To v5) are filtered (S13).
[0032]
The signal processed by the vertical filter 103 is input to the horizontal block distortion determination unit 104 and the horizontal filter 105. The horizontal block distortion determination unit 104 determines the presence or absence of block distortion with respect to the input signal according to the flowchart shown in FIG. Here, the flowchart of FIG. 5 will be described. First, for pixels near the block boundary (h0 to h7 in FIG. 2), the absolute value of the difference between adjacent pixels is compared with a predetermined threshold value thh1 to determine whether the frequency is low (S21). For example, the absolute value of the difference between adjacent pixels in h0 to h7 in FIG. 2 can be expressed by the following D0, D1, D2, D4, D5, and D6.
[0033]
D0 = | h1-h0 |
D1 = | h2-h1 |
D2 = | h3-h2 |
D4 = | h5-h4 |
D5 = | h6-h5 |
D6 = | h7−h6 |
When all the absolute values (D0, D1, D2, D4, D5, D6) of the differences between adjacent pixels are smaller than thh1, the block boundary vicinity is flat, so it is determined that the frequency is low. In the case of, it is determined that the frequency is non-low frequency.
[0034]
If it is determined that the frequency is low (YES in S21), the absolute value (D3 = | h4-h3 |) of the difference between two pixels adjacent to the block boundary (h3, h4 in FIG. 2) and a predetermined threshold thh2 Further, the absolute value D3 of the difference between two pixels adjacent to the block boundary and the maximum value maxh1 (D0, D1, D2, D4, D5, D6) of the absolute value of the difference between two adjacent pixels near the block boundary And the presence or absence of block distortion is determined (S22).
[0035]
When the absolute value D3 of the difference between two pixels adjacent to the block boundary is larger than a predetermined threshold value thh2, it is determined that the edge is included in the image. Further, when the absolute value D3 of the difference between two pixels adjacent to the block boundary is smaller than maxh1, it is determined that the edge is included in the image. Therefore, when it is determined that the edge is not included in the image under the two conditions (YES in S22), it is determined that there is block distortion.
[0036]
If it is determined that the pixel row near the block boundary has a non-low frequency (NO in S21), the absolute value (D3) of the difference between two pixels adjacent to the block boundary (h3, h4 in FIG. 2) and a predetermined value The threshold value thh3 is compared, and the absolute value D3 of the difference between the two pixels adjacent to the block boundary is compared with the maximum value maxh1 of the absolute value of the difference between the two adjacent pixels near the block boundary. The presence or absence is determined (S24). When the absolute value D3 of the difference between two pixels adjacent to the block boundary is larger than a predetermined threshold value thh3, it is determined that the edge is included in the image. Further, when the absolute value D3 of the difference between two pixels adjacent to the block boundary is smaller than maxh1, it is determined that the edge is included in the image. Therefore, if it is determined that the edge is not included in the image under the two conditions (YES in S24), it is determined that there is block distortion.
[0037]
In the horizontal filter 105, when the signal processed by the vertical filter 103 is input and the horizontal block distortion determination unit 104 determines that there is a block distortion at a low frequency, eight pixels centered on the block boundary (see FIG. Filter processing is performed on h0 to h7) of 2. Further, when the horizontal block distortion determination unit 104 determines that there is a non-low frequency block distortion, it is considered that a high frequency component is included, so that the entire block is filtered to prevent the image from blurring. Filter processing is performed only on two pixels (h3 and h4 in FIG. 2) centering on the block boundary.
[0038]
Here, it is also conceivable to perform filter processing for the non-low frequency for the vertical filter 103 as well. However, if the interlaced image is filtered for two pixels centered on the block boundary, which is a filtering process in the case of non-low frequencies, the input decoded image is an image in which an object with a clear outline moves up and down. In this case, there is a problem that the image quality is deteriorated, for example, the contour may appear double.
[0039]
This will be described with reference to FIG. FIG. 6A shows an example of an interlaced image before filter processing. The pixels in the first field are V0 to V7, and the pixels in the second field are V0 ′ to V7 ′. When it is determined that the first field in FIG. 6A has a non-low frequency block distortion and the second field has no block distortion, V3 and V4 in FIG. Filter processing is performed. FIG. 6B shows the interlaced image after the filter processing. If V0 to V3 and V0 ′ to V2 ′ in FIG. 6A represent white and V4 to V7 and V3 ′ to V7 ′ represent black, filtering is performed on V3 and V4 in FIG. As a result, V3 and V4 in FIG. When FIG. 6B is displayed in a frame, V2 ′, V3, V3 ′, V4, and V4 ′ become white, gray, black, gray, and black, and the outline looks double. Therefore, when the input decoded image is interlaced, the generation of gray pixels as described above can be prevented and image quality can be improved by not performing non-low frequency filter processing in the vertical direction.
[0040]
As described above, according to the present invention, block distortion can be reduced or removed without damaging the high frequency component of the input decoded image. Further, when the input decoded image is interlaced, a non-low-frequency filter is used in the vertical direction. By not performing the processing, it is possible to improve the image quality.
[0041]
In the present embodiment, the case where the block unit is an 8 × 8 block has been described, but other block units may be used. In FIG. 1, the vertical block distortion is first reduced or removed, and then the horizontal block distortion is reduced or removed. However, this order may be reversed.
[0042]
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
[0043]
【The invention's effect】
As described above, according to the present invention, the presence / absence of distortion is determined in advance in each direction for each second direction that intersects the first direction and the first direction, and then the filter process is performed in each direction. Yes. Accordingly, since the filtering process is performed only when there is a lot of distortion, all the high frequency components of the decoded image are not removed, and the filtering process can be performed only in the direction in which the image quality is deteriorated in the interlaced image. As a result, it is possible to provide a filter device that can reduce or remove block distortion without impairing the high-frequency component of the decoded image, and can reduce or remove block distortion with respect to an interlaced decoded image without image quality deterioration.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of a filter device according to the present invention.
FIG. 2 is a diagram for explaining a block boundary and its neighboring pixels.
FIG. 3 is a diagram for explaining pixels of a frame image.
FIG. 4 is a flowchart for explaining processing of a vertical block distortion determination unit;
FIG. 5 is a flowchart for explaining processing of a horizontal block distortion determination unit;
FIG. 6 is a diagram for describing interlaced image filtering.
FIG. 7 is a block diagram showing an overall configuration of a video image processing apparatus.
FIG. 8 is a block diagram illustrating a configuration of a filter device.
FIG. 9 is a diagram for explaining block distortion filter processing by smoothing;
FIG. 10 is a diagram for explaining block distortion filtering processing by smoothing;
[Explanation of symbols]
100 filter device, 101 line memory, 102 vertical block distortion determination unit, 103 vertical filter, 104 horizontal block distortion determination unit, 105 horizontal filter.

Claims (4)

A first direction that intersects a line of the decoded image signal with respect to an interlaced decoded image signal obtained by sequentially decoding the image signal divided into a plurality of blocks and encoded in units of blocks in units of blocks Delay means for delaying the pixels along the first direction along the first direction;
First determining means for determining block distortion in the first direction of the decoded image signal;
The first determination unit is configured to determine a predetermined number of neighboring pixels arranged along the first direction so as to intersect a boundary of the block , based on a difference in pixel values between neighboring pixels in each block. , Including a first sub-determination unit that determines block distortion in the first direction by determining whether the frequency is low.
Further comprising first filter means for filtering the image signal delayed by the delay means in the first direction based on the determination result obtained by the first determination means;
The first filter means does not perform filter processing when the first sub-determination means determines that the neighboring pixels are not low frequency,
And a second determination means for determining the block distortion in the first second direction intersecting the direction of the image signal output from the first filter means,
The second determination unit is configured to determine a predetermined number of second direction neighboring pixels arranged along the second direction so as to intersect with a boundary of the block, and between the neighboring pixels in the second direction adjacent to each other in the block. Including second sub-determination means for determining block distortion in the second direction by determining whether the frequency is low based on a difference between pixel values;
Based on the determination result obtained by the second judging means further example Bei and second filter means for filtering the image signal output from said first filter means in the second direction,
When the second sub-determination unit determines that the pixels in the second direction vicinity are low frequency, the second filter unit performs a filtering process on the pixels in the second direction vicinity, and A filter that performs a filtering process on pixels that are included in the second direction neighboring pixels and that are smaller than the number of the second direction neighboring pixels when the judging unit judges that the second direction neighboring pixels are not low frequency; apparatus. The first sub-determination means determines whether any of absolute values of pixel value differences between the adjacent neighboring pixels is equal to or less than a predetermined threshold;
The said 1st filter means does not perform a filter process with respect to the said vicinity pixel, when the said 1st sub determination means determines that any of the said absolute value is larger than the said predetermined threshold value. 2. The filter device according to 1. The first and second determining means each compare an absolute value of a difference between pixel values of two adjacent pixels across a block boundary with a predetermined threshold;
And means for comparing the the largest of the absolute values of differences between pixel values of neighboring pixels that the absolute value and the adjacent difference between the pixel values of the two pixels, the filter device according to claim 1 or 2 . The first direction is a vertical direction, the second direction is a horizontal direction, the filter device according to any one of claims 1 to 3.

Images