WO2017175898A1

WO2017175898A1 - Method and apparatus for encoding/decoding video signal by using intra-prediction filtering

Info

Publication number: WO2017175898A1
Application number: PCT/KR2016/003668
Authority: WO
Inventors: 유선미; 허진
Original assignee: 엘지전자(주)
Priority date: 2016-04-07
Filing date: 2016-04-07
Publication date: 2017-10-12

Abstract

The present invention provides a method for decoding a video signal, the method comprising the steps of: generating an intra-prediction block related to a current block; discovering a reference pixel having generated an artifact within the intra-prediction block; discovering a pixel to be predicted, which is within the intra-prediction block and corresponds to the reference pixel; and filtering the pixel to be predicted.

Description

Method and apparatus for encoding and decoding video signals using intra prediction filtering

The present invention relates to a method and apparatus for encoding / decoding a video signal, and more particularly, to a technique for processing a video signal using intra prediction filtering.

Compression coding refers to a series of signal processing techniques for transmitting digitized information through a communication line or for storing in a form suitable for a storage medium. Media such as an image, an image, an audio, and the like may be a target of compression encoding. In particular, a technique of performing compression encoding on an image is called video image compression.

Next-generation video content will be characterized by high spatial resolution, high frame rate and high dimensionality of scene representation. Processing such content would result in a tremendous increase in terms of memory storage, memory access rate, and processing power.

Accordingly, there is a need to design coding tools for more efficiently processing next generation video content.

The present invention provides a method of filtering a prediction block.

In addition, the present invention can improve image quality by correcting or removing sharp edges in the prediction block.

In addition, the present invention is to provide a method for determining whether an unnecessary edge occurs in the prediction block.

In addition, the present invention is to provide a method for mitigating the edge by taking low pass filtering when an unnecessary edge occurs in the prediction block.

In addition, the present invention is to provide a method for correcting or removing visual artifacts that appear after intra prediction block generation.

The present invention also provides a method for performing post-processing filtering after generating an intra prediction block.

In order to solve the above technical problem,

The present invention provides a method for filtering a prediction block.

The present invention also provides a method for correcting or removing sharp edges in a predictive block.

The present invention also provides a method for determining whether an unnecessary edge has occurred in a prediction block.

In addition, the present invention provides a method for mitigating edges by taking low pass filtering when an unnecessary edge occurs in a prediction block.

In addition, the present invention provides a method for correcting or removing visual artifacts that appear after intra prediction block generation.

In addition, the present invention provides a method for performing post-processing filtering after generating an intra prediction block.

The present invention can remove or mitigate visual artifacts that appear after intra prediction block generation in still image or video encoding. Therefore, after constructing the intra prediction block, it is possible to contribute to subjective picture quality improvement by correcting or removing strong edges in the prediction block.

In addition, when it is determined that an unnecessary strong edge is generated in the prediction block according to the variation degree and the prediction mode of the neighboring pixel referred to when constructing the prediction block, the edge may be relaxed by performing low pass filtering.

1 is a schematic block diagram of an encoder in which encoding of a video signal is performed as an embodiment to which the present invention is applied.

2 is a schematic block diagram of a decoder in which decoding of a video signal is performed as an embodiment to which the present invention is applied.

3 is a diagram for describing a division structure of a coding unit according to an embodiment to which the present invention is applied.

4 is a diagram for describing a prediction unit according to an embodiment to which the present invention is applied.

5 is a diagram for describing an intra prediction method according to an embodiment to which the present invention is applied.

6 is a diagram for describing a prediction direction according to an intra prediction mode according to an embodiment to which the present invention is applied.

FIG. 7 is a diagram for describing a method of interpolating a reference pixel at a subpixel position according to another embodiment to which the present invention is applied.

8 is a schematic block diagram of performing post filtering on a prediction block according to another embodiment to which the present invention is applied.

FIG. 9 is an embodiment to which the present invention is applied and is a view for explaining a condition for finding a target reference pixel that has caused an artifact.

10 to 11 illustrate embodiments to which the present invention is applied and illustrate target prediction pixels for applying post filtering.

12 is a flowchart illustrating a method of performing post filtering on a prediction block according to an embodiment to which the present invention is applied.

FIG. 13 is a flowchart illustrating a method of performing post filtering on a prediction pixel based on a prediction mode and a presence of a target reference pixel in an embodiment to which the present invention is applied.

The present invention provides a method of decoding a video signal, the method comprising: generating an intra prediction block for a current block; Searching for a reference pixel that caused an artifact in the intra prediction block; Searching for a target prediction pixel in the intra prediction block corresponding to the reference pixel; And performing filtering on the target prediction pixel.

In the present invention, the reference pixel is searched by comparing the reference pixel value with a threshold value.

In the present invention, the threshold value is defined as the difference between the average value of the reference pixel and the average value of the pixels in the prediction block.

In the present invention, the threshold is variably set based on at least one of a prediction mode and a block size.

In the present invention, the threshold is variably set at at least one level of a picture, slice, or block.

In the present invention, the target prediction pixel is searched for at least one of a pixel predicted using the reference pixel and a pixel adjacent thereto.

In the present invention, the target prediction pixel is variably determined based on at least one of a block size or a prediction mode.

According to an aspect of the present invention, there is provided an apparatus for decoding a video signal, comprising: a prediction performer configured to generate an intra prediction block for a current block; And a post filtering unit searching for a reference pixel that has caused an artifact in the intra prediction block, searching for a target prediction pixel in the intra prediction block corresponding to the reference pixel, and performing filtering on the target prediction pixel. It provides a device characterized by.

Hereinafter, the configuration and operation of the embodiments of the present invention with reference to the accompanying drawings, the configuration and operation of the present invention described by the drawings will be described as one embodiment, whereby the technical spirit of the present invention And its core composition and operation are not limited.

In addition, the terminology used in the present invention was selected as a general term widely used as possible now, in a specific case will be described using terms arbitrarily selected by the applicant. In such a case, since the meaning is clearly described in the detailed description of the part, it should not be interpreted simply by the name of the term used in the description of the present invention, and it should be understood that the meaning of the term should be interpreted. .

In addition, terms used in the present invention may be replaced for more appropriate interpretation when there are general terms selected to describe the invention or other terms having similar meanings. For example, signals, data, samples, pictures, frames, blocks, etc. may be appropriately replaced and interpreted in each coding process. In addition, partitioning, decomposition, splitting, and division may be appropriately replaced and interpreted in each coding process.

Referring to FIG. 1, the encoder 100 may include an image splitter 110, a transformer 120, a quantizer 130, an inverse quantizer 140, an inverse transformer 150, a filter 160, and a decoder. It may include a decoded picture buffer (DPB) 170, an inter predictor 180, an intra predictor 185, and an entropy encoder 190.

The image divider 110 may divide an input image (or a picture or a frame) input to the encoder 100 into one or more processing units. For example, the processing unit may be a Coding Tree Unit (CTU), a Coding Unit (CU), a Prediction Unit (PU), or a Transform Unit (TU).

However, the terms are only used for the convenience of description of the present invention, the present invention is not limited to the definition of the terms. In addition, in the present specification, for convenience of description, the term coding unit is used as a unit used in encoding or decoding a video signal, but the present invention is not limited thereto and may be appropriately interpreted according to the present invention.

The encoder 100 may generate a residual signal by subtracting a prediction signal output from the inter predictor 180 or the intra predictor 185 from the input image signal and generate the residual signal. The dual signal is transmitted to the converter 120.

The transform unit 120 may generate a transform coefficient by applying a transform technique to the residual signal. The conversion process may be applied to pixel blocks having the same size as the square, or may be applied to blocks of variable size rather than square.

The quantization unit 130 may quantize the transform coefficients and transmit the quantized coefficients to the entropy encoding unit 190, and the entropy encoding unit 190 may entropy code the quantized signal and output the bitstream.

The quantized signal output from the quantization unit 130 may be used to generate a prediction signal. For example, the quantized signal may restore the residual signal by applying inverse quantization and inverse transformation through an inverse quantization unit 140 and an inverse transformation unit 150 in a loop. A reconstructed signal may be generated by adding the reconstructed residual signal to a prediction signal output from the inter predictor 180 or the intra predictor 185.

Meanwhile, in the compression process as described above, adjacent blocks are quantized by different quantization parameters, thereby causing deterioration of the block boundary. This phenomenon is called blocking artifacts, which is one of the important factors in evaluating image quality. In order to reduce such deterioration, a filtering process may be performed. Through this filtering process, the image quality can be improved by removing the blocking degradation and reducing the error of the current picture.

The filtering unit 160 applies filtering to the reconstruction signal and outputs it to the reproduction apparatus or transmits the decoded picture buffer to the decoding picture buffer 170. The filtered signal transmitted to the decoded picture buffer 170 may be used as the reference picture in the inter predictor 180. As such, by using the filtered picture as a reference picture in the inter prediction mode, not only image quality but also encoding efficiency may be improved.

The decoded picture buffer 170 may store the filtered picture for use as a reference picture in the inter prediction unit 180.

The inter prediction unit 180 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture. Here, since the reference picture used to perform the prediction is a transformed signal that has been quantized and dequantized in units of blocks at the time of encoding / decoding, a blocking artifact or a ringing artifact may exist. have.

Accordingly, the inter prediction unit 180 may interpolate the signals between pixels in sub-pixel units by applying a lowpass filter in order to solve performance degradation due to discontinuity or quantization of such signals. Herein, the subpixels mean virtual pixels generated by applying an interpolation filter, and the integer pixels mean actual pixels existing in the reconstructed picture. As the interpolation method, linear interpolation, bi-linear interpolation, wiener filter, or the like may be applied.

The interpolation filter may be applied to a reconstructed picture to improve the precision of prediction. For example, the inter prediction unit 180 generates an interpolation pixel by applying an interpolation filter to integer pixels, and uses an interpolated block composed of interpolated pixels as a prediction block. You can make predictions.

The intra predictor 185 may predict the current block by referring to samples around the block to which current encoding is to be performed. The intra prediction unit 185 may perform the following process to perform intra prediction. First, reference samples necessary for generating a prediction signal may be prepared. The prediction signal may be generated using the prepared reference sample. Then, the prediction mode is encoded. In this case, the reference sample may be prepared through reference sample padding and / or reference sample filtering. Since the reference sample has been predicted and reconstructed, there may be a quantization error. Accordingly, the reference sample filtering process may be performed for each prediction mode used for intra prediction to reduce such an error.

One embodiment of the present invention provides a method of performing filtering on an intra prediction block.

In one embodiment, a method for determining whether an unnecessary edge in an intra prediction block has occurred is provided. In addition, when an unnecessary edge occurs in an intra prediction block, a method of correcting or removing an unnecessary edge is provided.

As such, after constructing the intra prediction block, it is possible to contribute to subjective picture quality improvement by correcting or removing sharp edges in the intra prediction block.

A prediction signal generated by the inter predictor 180 or the intra predictor 185 may be used to generate a reconstruction signal or to generate a residual signal.

2, the decoder 200 includes an entropy decoding unit 210, an inverse quantizer 220, an inverse transform unit 230, a filtering unit 240, and a decoded picture buffer unit (DPB) 250. ), An inter predictor 260, and an intra predictor 265.

The reconstructed video signal output through the decoder 200 may be reproduced through the reproducing apparatus.

The decoder 200 may receive a signal output from the encoder 100 of FIG. 1, and the received signal may be entropy decoded through the entropy decoding unit 210.

The inverse quantization unit 220 obtains a transform coefficient from the entropy decoded signal using the quantization step size information.

The inverse transformer 230 inversely transforms a transform coefficient to obtain a residual signal.

A reconstructed signal is generated by adding the obtained residual signal to a prediction signal output from the inter predictor 260 or the intra predictor 265.

The filtering unit 240 applies filtering to the reconstructed signal and outputs the filtering to the reproducing apparatus or transmits it to the decoded picture buffer unit 250. The filtered signal transmitted to the decoded picture buffer unit 250 may be used as the reference picture in the inter predictor 260.

In the present specification, the embodiments described by the filtering unit 160, the inter prediction unit 180, and the intra prediction unit 185 of the encoder 100 are respectively the filtering unit 240, the inter prediction unit 260, and the decoder. The same may be applied to the intra predictor 265.

The encoder may split one image (or picture) in units of a rectangular Coding Tree Unit (CTU). In addition, one CTU is sequentially encoded according to a raster scan order.

For example, the size of the CTU may be set to any one of 64x64, 32x32, and 16x16, but the present invention is not limited thereto. The encoder may select and use the size of the CTU according to the resolution of the input video or the characteristics of the input video. The CTU may include a coding tree block (CTB) for a luma component and a coding tree block (CTB) for two chroma components corresponding thereto.

One CTU may be decomposed into a quadtree (QT) structure. For example, one CTU may be divided into four units having a square shape and each side is reduced by half in length. The decomposition of this QT structure can be done recursively.

Referring to FIG. 3, a root node of a QT may be associated with a CTU. The QT may be split until it reaches a leaf node, where the leaf node may be referred to as a coding unit (CU).

A CU may mean a basic unit of coding in which an input image is processed, for example, intra / inter prediction is performed. The CU may include a coding block (CB) for a luma component and a CB for two chroma components corresponding thereto. For example, the size of the CU may be determined as any one of 64x64, 32x32, 16x16, and 8x8. However, the present invention is not limited thereto, and in the case of a high resolution image, the size of the CU may be larger or more diverse.

Referring to FIG. 3, the CTU corresponds to a root node and has the smallest depth (ie, level 0) value. The CTU may not be divided according to the characteristics of the input image. In this case, the CTU corresponds to a CU.

The CTU may be decomposed in QT form, and as a result, lower nodes having a depth of level 1 may be generated. And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of level 1 corresponds to a CU. For example, in FIG. 3 (b), CU (a), CU (b) and CU (j) corresponding to nodes a, b and j are divided once in the CTU and have a depth of level 1. FIG.

At least one of the nodes having a depth of level 1 may be split into QT again. And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a level 2 depth corresponds to a CU. For example, in FIG. 3 (b), CU (c), CU (h), and CU (i) corresponding to nodes c, h and i are divided twice in the CTU and have a depth of level 2. FIG.

In addition, at least one of the nodes having a depth of 2 may be divided into QTs. And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of level 3 corresponds to a CU. For example, in FIG. 3 (b), CU (d), CU (e), CU (f), and CU (g) corresponding to nodes d, e, f, and g are divided three times in the CTU, and level 3 Has a depth of

In the encoder, the maximum size or the minimum size of the CU may be determined according to characteristics (eg, resolution) of the video image or in consideration of encoding efficiency. Information about this or information capable of deriving the information may be included in the bitstream. A CU having a maximum size may be referred to as a largest coding unit (LCU), and a CU having a minimum size may be referred to as a smallest coding unit (SCU).

In addition, a CU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each partitioned CU may have depth information. Since the depth information indicates the number and / or degree of division of the CU, the depth information may include information about the size of the CU.

Since the LCU is divided into QT forms, the size of the SCU can be obtained by using the size and maximum depth information of the LCU. Or conversely, using the size of the SCU and the maximum depth information of the tree, the size of the LCU can be obtained.

For one CU, information indicating whether the corresponding CU is split may be delivered to the decoder. For example, the information may be defined as a split flag and may be represented by a syntax element "split_cu_flag". The division flag may be included in all CUs except the SCU. For example, if the split flag value is '1', the corresponding CU is divided into four CUs again. If the split flag value is '0', the CU is not divided any more and the coding process for the CU is not divided. Can be performed.

In the embodiment of FIG. 3, the division process of the CU has been described as an example, but the QT structure described above may also be applied to the division process of a transform unit (TU) which is a basic unit for performing transformation.

The TU may be hierarchically divided into a QT structure from a CU to be coded. For example, a CU may correspond to a root node of a tree for a transform unit (TU).

Since the TU is divided into QT structures, the TU divided from the CU may be divided into smaller lower TUs. For example, the size of the TU may be determined by any one of 32x32, 16x16, 8x8, and 4x4. However, the present invention is not limited thereto, and in the case of a high resolution image, the size of the TU may be larger or more diverse.

For one TU, information indicating whether the corresponding TU is divided may be delivered to the decoder. For example, the information may be defined as a split transform flag and may be represented by a syntax element "split_transform_flag".

The division conversion flag may be included in all TUs except the TU of the minimum size. For example, if the value of the division conversion flag is '1', the corresponding TU is divided into four TUs again. If the value of the division conversion flag is '0', the corresponding TU is no longer divided.

As described above, a CU is a basic unit of coding in which intra prediction or inter prediction is performed. In order to code an input image more effectively, a CU may be divided into prediction units (PUs).

The PU is a basic unit for generating a prediction block, and may generate different prediction blocks in PU units within one CU. The PU may be divided differently according to whether an intra prediction mode or an inter prediction mode is used as a coding mode of a CU to which the PU belongs.

The PU is divided differently according to whether an intra prediction mode or an inter prediction mode is used as a coding mode of a CU to which the PU belongs.

FIG. 4A illustrates a PU when an intra prediction mode is used, and FIG. 4B illustrates a PU when an inter prediction mode is used.

Referring to FIG. 4 (a), assuming that a size of one CU is 2Nx2N (N = 4,8,16,32), one CU may be divided into two types (ie, 2Nx2N or NxN). Can be.

Here, when divided into 2N × 2N type PU, it means that only one PU exists in one CU.

On the other hand, when divided into N × N type PU, one CU is divided into four PUs, and different prediction blocks are generated for each PU unit. However, the division of the PU may be performed only when the size of the CB for the luminance component of the CU is the minimum size (that is, the CU is the SCU).

Referring to FIG. 4 (b), assuming that a size of one CU is 2N × 2N (N = 4,8,16,32), one CU has 8 PU types (ie, 2Nx2N, NxN, 2NxN). , Nx2N, nLx2N, nRx2N, 2NxnU, 2NxnD).

Similar to intra prediction, PU splitting in the form of NxN may be performed only when the size of the CB for the luminance component of the CU is the minimum size (that is, the CU is the SCU).

In inter prediction, 2NxN type partitioning in the horizontal direction and Nx2N type PU partitioning in the vertical direction are supported.

In addition, it supports PU partitions of nLx2N, nRx2N, 2NxnU, and 2NxnD types, which are Asymmetric Motion Partition (AMP). Here, 'n' means a 1/4 value of 2N. However, AMP cannot be used when the CU to which the PU belongs is a CU of the minimum size.

In order to efficiently encode an input image within one CTU, an optimal partitioning structure of a coding unit (CU), a prediction unit (PU), and a transformation unit (TU) is subjected to the following process to perform a minimum rate-distortion. It can be determined based on the value. For example, looking at an optimal CU partitioning process in a 64x64 CTU, rate-distortion cost can be calculated while partitioning from a 64x64 CU to an 8x8 CU. The specific process is as follows.

1) The partition structure of the optimal PU and TU that generates the minimum rate-distortion value is determined by performing inter / intra prediction, transform / quantization, inverse quantization / inverse transform, and entropy encoding for a 64x64 CU.

2) Partition the 64x64 CU into four 32x32 CUs and determine the optimal PU and TU partitioning structure that generates the minimum rate-distortion value for each 32x32 CU.

3) The 32x32 CU is subdivided into four 16x16 CUs, and a partition structure of an optimal PU and TU that generates a minimum rate-distortion value for each 16x16 CU is determined.

4) Subdivide the 16x16 CU into four 8x8 CUs, and determine the optimal PU and TU partitioning structure that generates the minimum rate-distortion value for each 8x8 CU.

5) Compare the sum of the rate-distortion values of the 16x16 CUs calculated in step 3) with the rate-distortion values of the four 8x8 CUs calculated in step 4) to determine the optimal CU in the 16x16 block. Determine the partition structure. This process is similarly performed for the remaining three 16x16 CUs.

6) Compare the sum of the rate-distortion values of the 32x32 CUs calculated in 2) with the rate-distortion values of the four 16x16 CUs obtained in 5) above, Determine the partition structure. Do this for the remaining three 32x32 CUs.

7) Finally, compare the sum of the rate-distortion values of the 64x64 CUs calculated in 1) with the rate-distortion values of the four 32x32 CUs obtained in 6) above, and then optimize the result within the 64x64 block. Determine the partition structure of the CU.

In the intra prediction mode, a prediction mode is selected in units of PUs, and prediction and reconstruction are performed in units of actual TUs for the selected prediction mode.

TU means a basic unit in which actual prediction and reconstruction are performed. The TU includes a transform block (TB) for a luma component and a TB for two chroma components corresponding thereto.

In the example of FIG. 3, as one CTU is divided into quadtree structures to generate a CU, the TUs are hierarchically divided into quadtree structures from one CU to be coded.

Since the TU is divided into quadtree structures, the TU divided from the CU may be divided into smaller lower TUs. In HEVC, the size of the TU may be set to any one of 32 × 32, 16 × 16, 8 × 8, and 4 × 4.

Referring back to FIG. 3, it is assumed that a root node of the quadtree is associated with a CU. The quadtree is split until it reaches a leaf node, and the leaf node corresponds to a TU.

In more detail, a CU corresponds to a root node and has a smallest depth (that is, depth = 0). The CU may not be divided according to the characteristics of the input image. In this case, the CU corresponds to a TU.

The CU may be divided into quad tree shapes, resulting in lower nodes having a depth of 1 (depth = 1). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 1 corresponds to a TU. For example, in FIG. 3B, TU (a), TU (b), and TU (j) corresponding to nodes a, b, and j are divided once in a CU and have a depth of 1. FIG.

At least one of the nodes having a depth of 1 may be split into a quad tree again, resulting in lower nodes having a depth of 1 (ie, depth = 2). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 2 corresponds to a TU. For example, in FIG. 3B, TU (c), TU (h), and TU (i) corresponding to nodes c, h, and i are divided twice in a CU and have a depth of two.

In addition, at least one of the nodes having a depth of 2 may be divided into quad tree shapes, resulting in lower nodes having a depth of 3 (ie, depth = 3). And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of 3 corresponds to a CU. For example, in FIG. 3 (b), TU (d), TU (e), TU (f), and TU (g) corresponding to nodes d, e, f, and g are divided three times in a CU. Has depth.

A TU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each divided TU may have depth information. Since the depth information indicates the number and / or degree of division of the TU, it may include information about the size of the TU.

For one TU, information indicating whether the corresponding TU is split (for example, split TU flag split_transform_flag) may be delivered to the decoder. This partitioning information is included in all TUs except the smallest TU. For example, if the value of the flag indicating whether to split is '1', the corresponding TU is divided into four TUs again. If the value of the flag indicating whether to split is '0', the corresponding TU is no longer divided.

5 to 7 illustrate embodiments to which the present invention is applied. FIG. 5 is a diagram illustrating an intra prediction method, FIG. 6 is a diagram illustrating a prediction direction according to an intra prediction mode, and FIG. 7 is a subpixel. A diagram for describing a method of interpolating a reference pixel at a position.

Referring to FIG. 5, the decoder may derive an intra prediction mode of the current processing block (S501).

In intra prediction, the prediction direction may have a prediction direction with respect to the position of a reference sample used for prediction according to a prediction mode. In this specification, an intra prediction mode having a prediction direction is referred to as an intra_angular prediction mode or an intra directional mode. On the other hand, as an intra prediction mode having no prediction direction, there are an intra planner (INTRA_PLANAR) prediction mode and an intra DC (INTRA_DC) prediction mode.

Table 1 illustrates an intra prediction mode and related names, and FIG. 6 illustrates a prediction direction according to the intra prediction mode.

Table 1

Intra prediction mode	Associated name
0	Intra Planner (INTRA_PLANAR)
One	Intra DC (INTRA_DC)
2 ... 34	Intra directional 2 ... Intra directional 34 (INTRA_ANGULAR2 ... INTRA_ANGULAR34)

In intra prediction, prediction is performed on the current processing block based on the derived prediction mode. Since the reference sample used for the prediction and the specific prediction method vary according to the prediction mode, when the current block is encoded in the intra prediction mode, the decoder may derive the prediction mode of the current block to perform the prediction.

The decoder may check whether neighboring samples of the current processing block can be used for prediction and configure reference samples to be used for prediction (S502).

In intra prediction, neighboring samples of the current processing block are samples adjacent to the left boundary of the current processing block of size nSxnS and a total of 2xnS samples neighboring the bottom-left, top of the current processing block. A total of 2xnS samples neighboring the top-right sample and a sample adjacent to the boundary and one sample neighboring the top-left side of the current processing block.

However, some of the surrounding samples of the current processing block may not be decoded yet or may be available. In this case, the decoder can construct reference samples for use in prediction by substituting samples that are not available with the available samples.

The decoder may perform filtering of reference samples based on the intra prediction mode (S503).

Whether filtering of the reference sample is performed may be determined based on the size of the current processing block. In addition, the filtering method of the reference sample may be determined by the filtering flag transmitted from the encoder.

The decoder may generate a prediction block for the current processing block based on the intra prediction mode and the reference samples (S504). That is, the decoder predicts the current processing block based on the intra prediction mode derived in the intra prediction mode derivation step S501 and the reference samples obtained through the reference sample configuration step S502 and the reference sample filtering step S503. A block may be generated (ie, predictive sample generation).

In order to minimize the discontinuity of the boundary between the processing blocks when the current processing block is encoded in the INTRA_DC mode, the left boundary sample (ie, the sample in the prediction block adjacent to the left boundary) and the upper side of the prediction block in step S504. (top) boundary samples (i.e., samples in prediction blocks adjacent to the upper boundary) may be filtered.

In addition, in operation S504, filtering may be applied to the left boundary sample or the upper boundary sample in the vertical direction mode and the horizontal mode among the intra directional prediction modes similarly to the INTRA_DC mode.

In more detail, when the current processing block is encoded in the vertical mode or the horizontal mode, the value of the prediction sample may be derived based on a reference sample located in the prediction direction. In this case, a boundary sample which is not located in the prediction direction among the left boundary sample or the upper boundary sample of the prediction block may be adjacent to a reference sample which is not used for prediction. That is, the distance from the reference sample not used for prediction may be much closer than the distance from the reference sample used for prediction.

Thus, the decoder may adaptively apply filtering to left boundary samples or upper boundary samples depending on whether the intra prediction direction is vertical or horizontal. That is, when the intra prediction direction is the vertical direction, the filtering may be applied to the left boundary samples, and when the intra prediction direction is the horizontal direction, the filtering may be applied to the upper boundary samples.

7 is a diagram for describing a method of interpolating a reference pixel at a subpixel position.

There are two ways to predict an intra block by using pixels adjacent to the current block. The directional prediction method for constructing a prediction block by copying reference pixels located in a specific direction and making the most of the referenceable pixels are used. It can be divided into non-directional prediction methods (DC mode, planar mode).

The directional prediction method is designed to express the structure of various directions that can appear on the screen. The directional prediction method may be performed by designating a specific direction as a mode and then copying a reference pixel corresponding to the prediction mode angle around the position of the sample to be predicted. However, when the reference pixel of the unit of integer pixels cannot be referred to, the interpolated pixel is copied by using the distance ratio between the two corresponding pixels and the two pixels obtained by the angle as shown in FIG. Can be.

Referring to FIG. 7, it is assumed that a sub-reference pixel value of a T (Xsub, Y1) position is obtained according to an intra directional prediction mode.

First, the X coordinate Xsub of the sub reference pixel T may be obtained according to Equation 1 below.

Equation 1

Here, d represents a distance between the pixel A and the pixel E, and θ represents an angle along the prediction direction.

Thus, the pixel value of the sub-reference pixel T can be obtained based on the distance ratio between two adjacent integer pixels, pixel B and pixel C. A prediction pixel value corresponding to the pixel E may be determined based on the pixel value of the sub reference pixel T.

When the intra block is to be reconstructed, the prediction block can be reconstructed by using the neighboring pixels of the current block and then combined with the received residual signal. Assuming that the size of the current block is N, available reference pixels for performing intra prediction are 2N pixels adjacent to the top and left sides and the top left corner pixels, respectively.

The reference pixel to be used for prediction may be smoothed according to the size and pixel value of the current block. This is to prevent visual artifacts of the prediction block to be derived due to the difference between the reference pixels.

Intra prediction using a pixel adjacent to the current block can be divided into two methods. For example, a directional prediction method for constructing a prediction block by copying reference pixels located in a specific direction, and a non-directional prediction method that makes the most of the referenceable pixel (eg, an intra DC mode or an intra planar mode). Can be divided.

The directional prediction method is performed by designating a specific direction as a mode as shown in FIG. 6, and then copying a reference pixel corresponding to the prediction mode angle around the position of the sample to be predicted. If the reference pixel of the integer pixel unit cannot be referred to, the prediction block may be constructed by copying the interpolated pixel using the distance ratio between the corresponding two pixels and the two pixels obtained by the angle as shown in FIG. 7. .

One of the non-directional modes, the intra DC mode, is a method of constructing a prediction block using an average value of reference pixels located around a current block. Effective prediction can be expected if the pixels in the block are homogeneous. On the other hand, when the value of the reference pixel varies, discontinuity may occur between the prediction block and the reference sample. In a similar situation, unintended visible contouring may occur when predicting by the directional prediction method. In order to compensate for this, an intra planar prediction mode has been devised. The intra planner prediction mode configures a prediction block by performing horizontal linear prediction and vertical linear prediction by using a reference pixel and then averaging them.

After constructing the prediction block, post-filtering to mitigate discontinuities in the reference sample and block boundaries for blocks predicted in the horizontal direction (180 degrees), the vertical direction (90 degrees), or intra DC mode. (post filtering) can be performed. Subsequently, the intra predicted block may be reconstructed by adding the filtered prediction block and the residual signal.

As an embodiment to which the present invention is applied, a method of encoding and decoding an intra block will be described.

When the current block is encoded in the intra mode, the decoder may decode the residual signal transmitted from the encoder. The probability-based symbolized signal is decoded by the entropy decoding unit, and the residual signal in the pixel domain is restored through inverse quantization and inverse transformation. Meanwhile, the intra prediction unit of the decoder generates a prediction block by using the prediction mode transmitted from the encoder and the neighboring pixel values which are already reconstructed. Thereafter, an intra-coded block may be reconstructed by adding the prediction signal and the residual signal.

The present invention proposes a method of applying post filtering after generating a prediction block. In the previous embodiment, post filtering was performed to remove visual artifacts caused by discontinuities with neighboring pixels after the intra prediction block was generated, but this method is to compensate for discontinuities between the current block and neighboring pixels. Predictions do not compensate for visual artifacts that can occur within prediction blocks.

Although smoothing is performed on the reference sample to prevent the occurrence of visual artifacts due to directional prediction, strong edges may still occur within the block, depending on the angle expected. Since most natural images are often composed of gentle stepped regions except for the boundary of an object, the generation of residual signals can also be reduced by suppressing unnecessary edge generation. Accordingly, there is a need for a method of performing a low pass filter after determining whether a visual artifact element has occurred in a prediction block.

Referring to FIG. 8, the intra prediction unit to which the present invention is applied may include a reference sample smoothing unit 810, a prediction performing unit 820, and a post filtering unit 830.

The reference sample smoothing unit 810 may perform smoothing or filtering on the reference sample based on at least one of the reference sample, the block size information, and the intra prediction mode. For example, in order to minimize the discontinuity of the boundary between blocks when the current block is encoded in intra DC mode, the left boundary sample of the prediction block (ie, the sample in the prediction block adjacent to the left boundary) and the upper side (top) boundary samples (i.e., samples in prediction blocks adjacent to the upper boundary) may be filtered.

In addition, when the current block is encoded in the vertical mode or the horizontal mode, the value of the prediction sample may be derived based on a reference sample located in the prediction direction. Thus, the decoder may adaptively apply filtering to left boundary samples or upper boundary samples depending on whether the intra prediction direction is vertical or horizontal.

The prediction performing unit 820 may generate a prediction block by using the reference samples filtered by the reference sample smoothing unit 810 according to the intra prediction mode.

The post filtering unit 830 may determine whether an unnecessary edge exists in the prediction block, and then perform filtering to mitigate or remove the edge when an unnecessary edge exists.

In one embodiment, the present invention provides a method for determining whether an unnecessary edge exists in a prediction block.

As a first example, the decoder may determine whether an unnecessary edge exists in the prediction block through a flag transmitted from the encoder. For example, the flag may be expressed as prediction_edge_flag, and when prediction_edge_flag = 1, it may mean that an unnecessary edge exists in a prediction block, and when the prediction_edge_flag = 0, it may mean that an unnecessary edge in a prediction block does not exist. have.

As a second example, the decoder may determine whether there is an unnecessary edge in the prediction block based on at least one of a reference pixel and a pixel in the prediction block.

As a specific example, the present invention may use a threshold to determine whether there is an unnecessary edge in the prediction block. Hereinafter, various methods of using the thresholds will be described, and the present invention can be performed by at least one of the following methods.

1) In order to determine whether an unnecessary edge exists in the prediction block, a threshold may be defined as a difference between an average of a reference pixel and an average of pixels in the prediction block. For example, it may be defined as Equation 2 below.

Equation 2

here,

Means the average value of the reference pixel,

Denotes an average value of pixels in the prediction block.

2) The threshold may be defined as a specific value transmitted from the encoder. In this case, the specific value may be set to be changeable in picture, slice, or block units.

3) The threshold may be defined as a specific value already set known to the encoder and the decoder.

4) The threshold may be determined based on the difference between the reference pixel and the pixel in the prediction block. For example, the threshold may be defined as a difference between a minimum value and a maximum value after obtaining a difference value between a reference pixel and a pixel in a horizontal or vertical direction in the prediction block. That is, the threshold may be calculated using minimum and maximum values of differences between adjacent pixels.

5) The threshold may be defined as a reference pixel or a pixel value of a specific position in the prediction block.

6) The threshold may be defined as a minimum, maximum, mode, median, or average value for the difference between the reference pixel or a pixel at a specific location within the prediction block.

7) The threshold may be variably defined according to the block size, and may be determined as a pixel value of a specific position according to the block size, for example.

The post filtering unit 830 may determine whether an unnecessary edge exists in the prediction block by using the threshold value, and then perform post filtering to mitigate or remove an unnecessary edge in the prediction block.

Hereinafter, a method of performing post filtering will be described.

One embodiment of the present invention provides a method for mitigating a large gap between pixel values in a prediction block. Therefore, in the present specification, the intra DC mode filled with the average value of the reference pixel and the intra planner mode performed by linear prediction are not described in detail, but the intra directional prediction mode will be described in detail.

The intra directional prediction mode may be largely divided into a horizontal direction mode 2 to 18 and a vertical direction mode 19 to 34. Referring to FIG. 9, reference pixels may be divided into upper reference pixels Ref _up and left reference pixels Ref _left according to an intra directional prediction mode in order to find a target reference pixel that has caused an artifact.

The encoder or decoder to which the present invention is applied may determine whether the following conditions are satisfied in order to find the target reference pixel that caused the artifact. That is, when there is a pixel p _i or q _i satisfying the following condition, it can be confirmed that the pixel has generated an artifact in the prediction block.

Equation 3

Equation 4

Here, Equations 3 and 4 satisfy p _i ∈Ref _up , q _j ∈Ref _left , 0 ≦ i, j <2S, S represents a block size, and E is an edge or an artifact. In order to mean a predetermined integer. For example, suppose E is 25 if the difference between luma values is 25 or more. If E is 25, the difference between pixels is not an edge, but the artifact or luminance inside the image. It can be determined the difference.

In addition, when the threshold value is larger than E or the threshold value is smaller than a predetermined value, the present invention may not be applied. In addition, the present invention may not be applied even when no pixel satisfying the conditions of Equations 3 and 4 is found.

As described above, when a target reference pixel that causes an artifact is found, predicted pixels predicted by using the target reference pixels may generate unnecessary edges, thereby mitigating the edges by performing post filtering on the predicted pixels. Or need to be removed.

Hereinafter, a method of performing post filtering on the prediction pixels will be described.

As shown in FIG. 9, assume that the target reference pixels that cause the artifacts are p _i and p _j . Referring to FIG. 10, pixels (Pred_Xi and Pred_Xj) indicated by thick hatches represent pixels predicted using p _i pixels and p _j pixels when generating a prediction block.

According to an embodiment of the present invention, the pixels in the prediction block generated by using the target reference pixels p _i and p _j among the reference pixels may be defined as a target of the filtering.

In addition, the target reference pixels p _i and p _j may be used to generate sub pixels according to pixel positions in the prediction block. For example, when a sub pixel is configured through a linear filter, a weight of 0.5 or more may be used. Pixels can be selected for filtering. Here, the pixel having the weight equal to or greater than 0.5 may mean a pixel that is generated by using pixels around p _i and p _j to generate a prediction pixel, but further weights p _i and p _j . For example, if p _i just below the pixels of the first line shown by a bold hatching (row) that, one must be not to copy directly from p _i from a position between p _i and p _i +1 position, p since _i is located on the side close to the weight of the p _i is greater than p _i +1. All pixels marked with a thick hatched line may be copied using p _i , but may refer to a pixel in which more p _i is reflected.

Accordingly, the encoder or the decoder may perform filtering using neighboring pixels on the pixels Pred_Xi and Pred_Xj indicated by the thick hatched in FIG. 10. Here, the filtering may be aimed at smoothing using neighboring pixels such as a one-dimensional low pass filter and a two-dimensional low pass filter. For example, when the prediction is performed in the vertical direction as shown in FIG. 10, the 3-tap low pass filter 1, 2, 1-dimensional in the horizontal direction with respect to the pixel predicted using the p _i , p _j reference pixels 1 filter) can be applied.

According to another embodiment of the present invention, as shown in FIG. 11, a position of a prediction pixel generated by using p _i and p _j satisfying the above condition and a position of a pixel adjacent thereto are defined as filtering targets. Can be.

p _i and p _j may be used to generate sub pixels according to pixel positions in the prediction block. For example, when a sub pixel is constructed through a linear filter, a pixel having a weight of 0.5 or more and adjacent to Pixels can be selected for filtering. Filtering may be performed by using neighboring pixels on the pixel position in the prediction block indicated by the thick hatched block and the pixel indicated by the thin hatched block of FIG. 11.

Here, the filtering may be aimed at smoothing using neighboring pixels such as a one-dimensional low pass filter and a two-dimensional low pass filter. For example, when the prediction is performed in the vertical direction as illustrated in FIG. 11, a 3-tap low pass filter having a horizontal dimension with respect to the pixel predicted using the p _i and p _j reference pixels and adjacent pixels thereof. (1, 2, 1 filter) can be applied.

When a pixel directly copying a p _i , p _j reference pixel and its surrounding pixels are designated as filtering targets, the definition of the surrounding pixels may be specified in one or a combination of the following.

1) The peripheral pixel may be variably determined based on the block size. For example, it may be defined as immediately adjacent pixels for 8x8 and 16x16 blocks, and three pixels adjacent to a reference pixel for 32x32 and 64x64 blocks.

2) The peripheral pixel may be variably determined based on a prediction mode. For example, in +3 and -3 modes based on prediction mode 10 (180 degree horizontal) and prediction mode 26 (90 degree vertical), three adjacent pixels may be defined around the reference pixel. If the mode is more than that, it can be defined as the immediately adjacent pixel.

By using the above method, unnecessary visual artifacts can be reduced by mitigating pixel values that are different from the surroundings by blending with the neighboring blocks in the prediction block by the directional mode. .

Embodiments described herein may be performed identically at the encoder and the decoder.

First, the encoder or the decoder may acquire a threshold in order to determine whether an unnecessary edge exists in the prediction block (S1210). Since various methods of obtaining the threshold have already been described above, the above-described embodiments may be applied. For example, the threshold may be defined as the difference between the average of the reference pixel and the average of the pixels in the prediction block.

As another example, the threshold value may not require a separate acquisition process. In this case, the encoder or the decoder already knows the threshold value, and the threshold value may be a preset value.

The encoder or decoder may search for a reference pixel causing an artifact in a prediction block based on at least one of a prediction mode, a reference pixel value, or the threshold (S1220).

When the reference pixel is found, the encoder or decoder may search for a prediction pixel corresponding to the reference pixel (S1230). Here, since the prediction pixel is generated based on the reference pixel causing the artifact in the prediction block, the prediction pixel may have a large difference from neighboring pixels. Thus, the encoder or decoder searches for prediction pixels to perform post filtering.

As another example, the prediction pixel is predefined by at least one of the block size and the directional prediction mode to define the pixel position and information defined in the form of a look-up table or a look-up table. Can be searched with a calculated method.

As another example, the prediction pixel may be calculated using a formula by using at least one of an angle of the prediction mode and position information of the pixel.

When the prediction pixel is found, the encoder or the decoder may perform filtering on the prediction pixel (S1240).

On the other hand, the processes are not necessarily to be performed, some processes may be omitted as necessary. In addition, the preceding and subsequent steps of the corresponding processes may be changed.

Another embodiment of the present invention provides a method of performing post filtering after generating a prediction block. In the present embodiment, the above-described embodiments may be applied to the method of determining a filtering target pixel (hereinafter, referred to as a 'target prediction pixel') in the prediction block, and when the target prediction pixel is determined, the distance between the pixel and the reference sample is determined. According to the present invention, a filtering method is provided.

The present invention is applicable to one or multiple combinations of the following methods depending on the distance between the target prediction pixel and the reference sample.

1) The type of filter can be variably applied according to the distance from the reference pixel. For example, apply a one-dimensional 5-tap bi-linear filter to a pixel that is close to the reference pixel, and if the distance to the reference pixel increases, then the one-dimensional 3-tap bilinear filter A bi-linear filter can be applied.

2) The filtering coefficient may be variably applied according to the distance from the reference pixel. For example, a pixel close to the reference pixel may apply a larger weight to the neighboring pixel when filtering with the neighboring pixel, while a pixel far from the reference pixel may apply a lower weight to the filtering with the neighboring pixel.

3) Depending on the distance from the reference pixel, reference pixels not referenced in the prediction may be referred to when filtering. For example, when predicted in the vertical mode, the pixel in the prediction block may refer to only the upper reference pixel Ref _up of FIG. 9. As the distance from the upper reference pixel Ref _up increases, the left reference pixel Ref _left corresponding to the filtering target position may be reflected in the filtering. A weight for filtering with reference to the left reference pixel Ref _left may be increased in proportion to the distance from the upper reference pixel Ref _up .

In addition, since the distance to the reference pixel is determined according to the size of the prediction block, the type of filter or the filter coefficient may be adaptively selected.

First, the encoder or the decoder may generate an intra prediction block (S1310).

In operation S1320, the encoder or the decoder may determine whether the generated intra prediction block is predicted in the directional prediction mode.

As a result of the check, when the intra prediction block is predicted in the directional prediction mode, the encoder or the decoder may acquire a threshold to determine whether an unnecessary edge exists in the prediction block (S1330). Here, the above-described various embodiments may be applied to the threshold. For example, the threshold may be defined as the difference between the average of the reference pixel and the average of the pixels in the prediction block.

As another example, step S1330 of obtaining the threshold may be omitted. That is, a process for obtaining the threshold may not be necessary, in which case the encoder or decoder already knows the threshold, and the threshold may be a preset value.

The encoder or decoder may search for a target reference pixel that has caused an artifact in a prediction block based on at least one of a prediction mode, a reference pixel value, or the threshold value (S1340).

The encoder or decoder may determine whether a target reference pixel exists by searching for the target reference pixel (S1350).

As a result of the checking, when the reference pixel exists, the encoder or the decoder may search for a target prediction pixel corresponding to the reference pixel (S1360). Here, the target prediction pixel is a pixel to be filtered, and may be generated based on the reference pixel that causes the artifact in the prediction block.

The encoder or decoder may perform filtering on the target prediction pixel (S1370). Through the above process, it is possible to relax or remove the edge generated by the directional prediction.

As described above, the embodiments described herein may be implemented and performed on a processor, microprocessor, controller, or chip. For example, the functional units illustrated in FIGS. 1, 2, and 8 may be implemented and performed on a computer, a processor, a microprocessor, a controller, or a chip.

In addition, the decoder and encoder to which the present invention is applied include a multimedia broadcasting transmitting and receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video chat device, a real time communication device such as video communication, a mobile streaming device, Storage media, camcorders, video on demand (VoD) service providing devices, internet streaming service providing devices, three-dimensional (3D) video devices, video telephony video devices, and medical video devices, and the like, for processing video signals and data signals Can be used for

In addition, the processing method to which the present invention is applied can be produced in the form of a program executed by a computer, and can be stored in a computer-readable recording medium. Multimedia data having a data structure according to the present invention can also be stored in a computer-readable recording medium. The computer readable recording medium includes all kinds of storage devices for storing computer readable data. The computer-readable recording medium may include, for example, a Blu-ray disc (BD), a universal serial bus (USB), a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage device. Can be. The computer-readable recording medium also includes media embodied in the form of a carrier wave (eg, transmission over the Internet). In addition, the bit stream generated by the encoding method may be stored in a computer-readable recording medium or transmitted through a wired or wireless communication network.

As mentioned above, preferred embodiments of the present invention are disclosed for the purpose of illustration, and those skilled in the art can improve and change various other embodiments within the spirit and technical scope of the present invention disclosed in the appended claims below. , Replacement or addition would be possible.

Claims

In the method for decoding a video signal,

Generating an intra prediction block for the current block;

Searching for a reference pixel that caused an artifact in the intra prediction block;

Searching for a target prediction pixel in the intra prediction block corresponding to the reference pixel; And

Performing filtering on the target prediction pixel

Method comprising a.
The method of claim 1,

And the reference pixel is searched by comparing the reference pixel value with a threshold.
The method of claim 2,

Wherein the threshold is defined as the difference between the mean value of the reference pixel and the mean value of the pixels in the prediction block.
The method of claim 2,

Wherein the threshold is variably set based on at least one of a prediction mode and a block size.
The method of claim 2,

Wherein the threshold is variably set at at least one level of a picture, slice, or block.
The method of claim 1,

And the target prediction pixel is searched for at least one of a pixel predicted using the reference pixel and a pixel adjacent thereto.
The method of claim 1,

And the target prediction pixel is variably determined based on at least one of a block size or a prediction mode.
An apparatus for decoding a video signal,

A prediction performing unit generating an intra prediction block for the current block; And

A post filtering unit searching for a reference pixel that has caused an artifact in the intra prediction block, searching for a target prediction pixel in the intra prediction block corresponding to the reference pixel, and performing filtering on the target prediction pixel

Apparatus comprising a.
The method of claim 8,

And the reference pixel is searched by comparing the reference pixel value with a threshold.
The method of claim 9,

Wherein the threshold is defined as the difference between the mean value of the reference pixel and the mean value of the pixels in the prediction block.
The method of claim 9,

And the threshold is variably set based on at least one of a prediction mode and a block size.
The method of claim 9,

And the threshold is variably set at at least one level of a picture, slice, or block.
The method of claim 8,

And the target prediction pixel is searched for at least one of a pixel predicted using the reference pixel and a pixel adjacent thereto.
The method of claim 8,

And the target prediction pixel is variably determined based on at least one of a block size or a prediction mode.