KR20230088234A - Method And Apparatus for Video Coding Using Secondary MPM List Based on Template Matching - Google Patents

Abstract

As a disclosure of a video coding method and apparatus using a secondary MPM list based on template matching, the present embodiment provides a secondary (gradient) based template matching result to improve video encoding efficiency and video quality. secondary) Provides a video coding method and apparatus for generating a Most Probable Mode (MPM) list and using the secondary MPM list for intra prediction of a current block.

Description

Method and Apparatus for Video Coding Using Secondary MPM List Based on Template Matching

The present disclosure relates to a video coding method and apparatus using a secondary MPM list based on template matching.

The contents described below merely provide background information related to the present embodiment and do not constitute prior art.

Since video data has a large amount of data compared to audio data or still image data, it requires a lot of hardware resources including memory to store or transmit itself without processing for compression.

Therefore, when video data is stored or transmitted, an encoder is used to compress and store or transmit the video data, and a decoder receives, decompresses, and reproduces the compressed video data. Examples of such video compression technologies include H.264/AVC, High Efficiency Video Coding (HEVC), and Versatile Video Coding (VVC), which has improved coding efficiency by about 30% or more compared to HEVC.

However, since the size, resolution, and frame rate of video are gradually increasing, and the amount of data to be encoded accordingly increases, a new compression technology with higher encoding efficiency and higher picture quality improvement effect than existing compression technologies is required.

In the template matching-based intra prediction technology, after searching for the most similar template in the undulation region using a template composed of undulation-reconstructed samples adjacent to the current block, a block corresponding to the similar template (corresponding block) is created as a prediction block of the current block. 6 illustrates intra prediction based on template matching. The encoder and the decoder may generate a prediction block using a template adjacent to the current block based on the same method. At this time, motion vector information corresponding to the displacement between the template and the similar template is not transmitted. In order to improve video encoding efficiency and image quality according to an increase in the amount of data, template matching-based intra prediction technology needs to be further improved.

The present disclosure generates a secondary Most Probable Mode (MPM) list according to a gradient-based template matching result in order to improve video encoding efficiency and video quality, and provides a secondary list for intra prediction of a current block. An object of the present invention is to provide a video coding method and apparatus using an MPM list.

According to an embodiment of the present disclosure, in a method of decoding a current block performed by an image decoding apparatus, a residual block of the current block and a secondary MPM (Most Probable Mode) flag are decoded from a bitstream step, where the secondary MPM flag indicates whether to use a secondary MPM list; and checking the secondary MPM flag, wherein if the secondary MPM flag is true, a similar template is obtained by performing gradient-based template matching in a predefined search area of the current block. Searching for; inducing and arranging intra prediction modes using corresponding blocks of the similar template; constructing the secondary MPM list for the current block using the sorted intra prediction modes; and decoding a secondary MPM index from the bitstream.

According to another embodiment of the present disclosure, a method of encoding a current block, performed by an image encoding apparatus, includes generating a template of the current block and calculating a gradient size of the template; Comparing the size of the gradient with a preset threshold, but when the size of the gradient is greater than the preset threshold, performing gradient-based template matching in a predefined search area of the current block, searching for a similar template to the template; inducing and arranging intra prediction modes using corresponding blocks of the similar template; Constructing a secondary MPM (Most Probable Mode) list for the current block using the sorted intra prediction modes and setting a secondary MPM flag, wherein the secondary MPM flag is the secondary Indicates whether to use the MPM list; and determining a secondary MPM index.

According to another embodiment of the present disclosure, a computer-readable recording medium storing a bitstream generated by an image encoding method, wherein the image encoding method generates a template of a current block and calculates a gradient size of the template. doing; Comparing the size of the gradient with a preset threshold, but when the size of the gradient is greater than the preset threshold, performing gradient-based template matching in a predefined search area of the current block, searching for a similar template to the template; inducing and arranging intra prediction modes using corresponding blocks of the similar template; Constructing a secondary MPM (Most Probable Mode) list for the current block using the sorted intra prediction modes and setting a secondary MPM flag, wherein the secondary MPM flag is the secondary Indicates whether to use the MPM list; and determining a secondary MPM index.

As described above, according to the present embodiment, by providing a video coding method and apparatus that generates a secondary MPM list according to a result of gradient-based template matching and uses the secondary MPM list for intra prediction of a current block, video encoding efficiency There is an effect that it becomes possible to improve the video quality and improve the video quality.

1 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of this disclosure.
2 is a diagram for explaining a method of dividing a block using a QTBTTT structure.
3A and 3B are diagrams illustrating a plurality of intra prediction modes including wide-angle intra prediction modes.
4 is an exemplary diagram of neighboring blocks of a current block.
5 is an exemplary block diagram of a video decoding apparatus capable of implementing the techniques of this disclosure.
6 is a block diagram illustrating intra prediction based on template matching.
7 is a flowchart illustrating a method for generating a secondary MPM list by a video encoding apparatus according to an embodiment of the present disclosure.
8 is a flowchart illustrating a method for generating a secondary MPM list by a video decoding apparatus according to an embodiment of the present disclosure.
9 is a flowchart illustrating a method for generating a secondary MPM list by a video decoding apparatus according to another embodiment of the present disclosure.
10 is a flowchart illustrating a method for generating an MPM list by a video encoding apparatus according to another embodiment of the present disclosure.
11 is a flowchart illustrating a method for generating an MPM list by a video decoding apparatus according to another embodiment of the present disclosure.
12 is an exemplary diagram illustrating a form of a template according to an embodiment of the present disclosure.
13 is an exemplary view illustrating a form of a template based on a prediction mode of an undulation and restoration area according to an embodiment of the present disclosure.
14 is an exemplary diagram illustrating rotation of a template according to an embodiment of the present disclosure.
15 is an exemplary diagram illustrating search areas for template matching according to an embodiment of the present disclosure.
16 is an exemplary diagram illustrating calculation of a gradient size in a search area according to an embodiment of the present disclosure.
17 is an exemplary diagram illustrating a region in which HoG is calculated according to an embodiment of the present disclosure.
18 is an exemplary diagram illustrating subblocks from which a current block is divided.
19 is an exemplary diagram illustrating a configuration of a secondary MPM list according to an embodiment of the present disclosure.
20 is an exemplary diagram illustrating template matching in units of subblocks according to an embodiment of the present disclosure.

DETAILED DESCRIPTION Some embodiments of the present disclosure are described in detail below with reference to exemplary drawings. In adding reference numerals to components of each drawing, it should be noted that the same components have the same numerals as much as possible even if they are displayed on different drawings. In addition, in describing the present embodiments, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present embodiments, the detailed description will be omitted.

1 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of this disclosure. Hereinafter, with reference to the illustration of FIG. 1, an image encoding device and sub-components of the device will be described.

The image encoding apparatus includes a picture division unit 110, a prediction unit 120, a subtractor 130, a transform unit 140, a quantization unit 145, a rearrangement unit 150, an entropy encoding unit 155, and an inverse quantization unit. 160, an inverse transform unit 165, an adder 170, a loop filter unit 180, and a memory 190.

Each component of the image encoding device may be implemented as hardware or software, or as a combination of hardware and software. Also, the function of each component may be implemented as software, and the microprocessor may be implemented to execute the software function corresponding to each component.

One image (video) is composed of one or more sequences including a plurality of pictures. Each picture is divided into a plurality of areas and encoding is performed for each area. For example, one picture is divided into one or more tiles and/or slices. Here, one or more tiles may be defined as a tile group. Each tile or/slice is divided into one or more Coding Tree Units (CTUs). And each CTU is divided into one or more CUs (Coding Units) by a tree structure. Information applied to each CU is coded as a CU syntax, and information commonly applied to CUs included in one CTU is coded as a CTU syntax. In addition, information commonly applied to all blocks in one slice is coded as syntax of a slice header, and information applied to all blocks constituting one or more pictures is a picture parameter set (PPS) or picture coded in the header. Furthermore, information commonly referred to by a plurality of pictures is coded into a Sequence Parameter Set (SPS). Also, information commonly referred to by one or more SPSs is coded into a video parameter set (VPS). Also, information commonly applied to one tile or tile group may be encoded as syntax of a tile or tile group header. Syntax included in the SPS, PPS, slice header, tile or tile group header may be referred to as high level syntax.

The picture divider 110 determines the size of a coding tree unit (CTU). Information on the size of the CTU (CTU size) is encoded as SPS or PPS syntax and transmitted to the video decoding apparatus.

The picture division unit 110 divides each picture constituting an image into a plurality of Coding Tree Units (CTUs) having a predetermined size, and then iteratively divides the CTUs using a tree structure. Divide (recursively). A leaf node in the tree structure becomes a coding unit (CU), which is a basic unit of encoding.

As a tree structure, a quad tree (QT) in which a parent node (or parent node) is divided into four subnodes (or child nodes) of the same size, or a binary tree (BinaryTree) in which a parent node is divided into two subnodes , BT), or a TernaryTree (TT) in which a parent node is split into three subnodes at a ratio of 1:2:1, or a structure in which two or more of these QT structures, BT structures, and TT structures are mixed. there is. For example, a QuadTree plus BinaryTree (QTBT) structure may be used, or a QuadTree plus BinaryTree TernaryTree (QTBTTT) structure may be used. Here, BTTT may be combined to be referred to as MTT (Multiple-Type Tree).

2 is a diagram for explaining a method of dividing a block using a QTBTTT structure.

As shown in FIG. 2, the CTU may first be divided into QT structures. Quadtree splitting can be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of leaf nodes allowed by QT. A first flag (QT_split_flag) indicating whether each node of the QT structure is split into four nodes of a lower layer is encoded by the entropy encoder 155 and signaled to the video decoding device. If the leaf node of QT is not larger than the maximum block size (MaxBTSize) of the root node allowed in BT, it may be further divided into either a BT structure or a TT structure. A plurality of division directions may exist in the BT structure and/or the TT structure. For example, there may be two directions in which blocks of the corresponding node are divided horizontally and vertically. As shown in FIG. 2, when MTT splitting starts, a second flag (mtt_split_flag) indicating whether nodes are split, and if split, a flag indicating additional split direction (vertical or horizontal) and/or split type (Binary or Ternary) is encoded by the entropy encoding unit 155 and signaled to the video decoding apparatus.

Alternatively, prior to coding the first flag (QT_split_flag) indicating whether each node is split into four nodes of a lower layer, a CU split flag (split_cu_flag) indicating whether the node is split is coded. It could be. When the value of the CU split flag (split_cu_flag) indicates that it is not split, the block of the corresponding node becomes a leaf node in the split tree structure and becomes a coding unit (CU), which is a basic unit of encoding. When the value of the CU split flag (split_cu_flag) indicates splitting, the video encoding apparatus starts encoding from the first flag in the above-described manner.

As another example of a tree structure, when QTBT is used, the block of the corresponding node is divided into two blocks of the same size horizontally (i.e., symmetric horizontal splitting) and the type that splits vertically (i.e., symmetric vertical splitting). Branches may exist. A split flag (split_flag) indicating whether each node of the BT structure is split into blocks of a lower layer and split type information indicating a split type are encoded by the entropy encoder 155 and transmitted to the video decoding device. Meanwhile, a type in which a block of a corresponding node is divided into two blocks having an asymmetric shape may additionally exist. The asymmetric form may include a form in which the block of the corresponding node is divided into two rectangular blocks having a size ratio of 1:3, or a form in which the block of the corresponding node is divided in a diagonal direction may be included.

A CU can have various sizes depending on the QTBT or QTBTTT split from the CTU. Hereinafter, a block corresponding to a CU to be encoded or decoded (ie, a leaf node of QTBTTT) is referred to as a 'current block'. Depending on the adoption of the QTBTTT division, the shape of the current block may be rectangular as well as square.

The prediction unit 120 predicts a current block and generates a prediction block. The prediction unit 120 includes an intra prediction unit 122 and an inter prediction unit 124 .

In general, each current block in a picture can be coded predictively. In general, prediction of a current block uses an intra-prediction technique (using data from a picture containing the current block) or an inter-prediction technique (using data from a picture coded before the picture containing the current block). can be performed Inter prediction includes both uni-prediction and bi-prediction.

The intra predictor 122 predicts pixels in the current block using pixels (reference pixels) located around the current block in the current picture including the current block. A plurality of intra prediction modes exist according to the prediction direction. For example, as shown in FIG. 3A, the plurality of intra prediction modes may include two non-directional modes including a planar mode and a DC mode and 65 directional modes. Depending on each prediction mode, the neighboring pixels to be used and the arithmetic expression are defined differently.

For efficient directional prediction of the rectangular current block, directional modes (numbers 67 to 80 and -1 to -14 intra prediction modes) indicated by dotted arrows in FIG. 3B may be additionally used. These may be referred to as “wide angle intra-prediction modes”. In FIG. 3B , arrows indicate corresponding reference samples used for prediction and do not indicate prediction directions. The prediction direction is opposite to the direction the arrow is pointing. Wide-angle intra prediction modes are modes that perform prediction in the opposite direction of a specific directional mode without additional bit transmission when the current block is rectangular. At this time, among the wide-angle intra prediction modes, some wide-angle intra prediction modes usable for the current block may be determined by the ratio of the width and height of the rectangular current block. For example, wide-angle intra prediction modes (67 to 80 intra prediction modes) having an angle smaller than 45 degrees are usable when the current block has a rectangular shape with a height smaller than a width, and a wide angle having an angle greater than -135 degrees. Intra prediction modes (-1 to -14 intra prediction modes) are available when the current block has a rectangular shape where the width is greater than the height.

The intra prediction unit 122 may determine an intra prediction mode to be used for encoding the current block. In some examples, the intra prediction unit 122 may encode the current block using several intra prediction modes and select an appropriate intra prediction mode to be used from the tested modes. For example, the intra predictor 122 calculates rate-distortion values using rate-distortion analysis for several tested intra-prediction modes, and has the best rate-distortion characteristics among the tested modes. Intra prediction mode can also be selected.

The intra prediction unit 122 selects one intra prediction mode from among a plurality of intra prediction modes, and predicts a current block using neighboring pixels (reference pixels) determined according to the selected intra prediction mode and an arithmetic expression. Information on the selected intra prediction mode is encoded by the entropy encoder 155 and transmitted to the video decoding apparatus.

The inter prediction unit 124 generates a prediction block for a current block using a motion compensation process. The inter-prediction unit 124 searches for a block most similar to the current block in the encoded and decoded reference picture prior to the current picture, and generates a prediction block for the current block using the searched block. Then, a motion vector (MV) corresponding to displacement between the current block in the current picture and the prediction block in the reference picture is generated. In general, motion estimation is performed on a luma component, and a motion vector calculated based on the luma component is used for both the luma component and the chroma component. Motion information including reference picture information and motion vector information used to predict the current block is encoded by the entropy encoding unit 155 and transmitted to the video decoding apparatus.

The inter-prediction unit 124 may perform interpolation on a reference picture or reference block in order to increase prediction accuracy. That is, subsamples between two consecutive integer samples are interpolated by applying filter coefficients to a plurality of consecutive integer samples including the two integer samples. When a process of searching for a block most similar to the current block is performed for the interpolated reference picture, the motion vector can be expressed with precision of decimal units instead of integer sample units. The precision or resolution of the motion vector may be set differently for each unit of a target region to be encoded, for example, a slice, tile, CTU, or CU. When such adaptive motion vector resolution (AMVR) is applied, information on motion vector resolution to be applied to each target region must be signaled for each target region. For example, when the target region is a CU, information on motion vector resolution applied to each CU is signaled. Information on the motion vector resolution may be information indicating the precision of differential motion vectors, which will be described later.

Meanwhile, the inter prediction unit 124 may perform inter prediction using bi-prediction. In the case of bi-directional prediction, two reference pictures and two motion vectors representing positions of blocks most similar to the current block within each reference picture are used. The inter prediction unit 124 selects a first reference picture and a second reference picture from reference picture list 0 (RefPicList0) and reference picture list 1 (RefPicList1), respectively, and searches for a block similar to the current block within each reference picture. A first reference block and a second reference block are generated. Then, a prediction block for the current block is generated by averaging or weighted averaging the first reference block and the second reference block. Further, motion information including information on two reference pictures used to predict the current block and information on two motion vectors is delivered to the encoder 150. Here, reference picture list 0 may include pictures prior to the current picture in display order among restored pictures, and reference picture list 1 may include pictures after the current picture in display order among restored pictures. there is. However, it is not necessarily limited to this, and in order of display, ups and downs pictures subsequent to the current picture may be additionally included in reference picture list 0, and conversely, ups and downs pictures prior to the current picture may be additionally included in reference picture list 1. may also be included.

Various methods may be used to minimize the amount of bits required to encode motion information.

For example, when the reference picture and motion vector of the current block are the same as those of the neighboring block, the motion information of the current block can be delivered to the video decoding apparatus by encoding information capable of identifying the neighboring block. This method is called 'merge mode'.

In the merge mode, the inter prediction unit 124 selects a predetermined number of merge candidate blocks (hereinafter referred to as 'merge candidates') from neighboring blocks of the current block.

Neighboring blocks for deriving merge candidates include a left block (A0), a lower left block (A1), an upper block (B0), and an upper right block (B1) adjacent to the current block in the current picture, as shown in FIG. ), and all or part of the upper left block A2 may be used. Also, a block located in a reference picture (which may be the same as or different from a reference picture used to predict the current block) other than the current picture in which the current block is located may be used as a merge candidate. For example, a block co-located with the current block in the reference picture or blocks adjacent to the co-located block may be additionally used as a merge candidate. If the number of merge candidates selected by the method described above is less than the preset number, a 0 vector is added to the merge candidates.

The inter prediction unit 124 constructs a merge list including a predetermined number of merge candidates using these neighboring blocks. Among the merge candidates included in the merge list, a merge candidate to be used as motion information of the current block is selected, and merge index information for identifying the selected candidate is generated. The generated merge index information is encoded by the encoder 150 and transmitted to the video decoding apparatus.

Merge skip mode is a special case of merge mode. After performing quantization, when all transform coefficients for entropy encoding are close to zero, only neighboring block selection information is transmitted without transmitting residual signals. By using the merge skip mode, it is possible to achieve a relatively high encoding efficiency in low-motion images, still images, screen content images, and the like.

Hereinafter, merge mode and merge skip mode are collectively referred to as merge/skip mode.

Another method for encoding motion information is Advanced Motion Vector Prediction (AMVP) mode.

In the AMVP mode, the inter prediction unit 124 derives predictive motion vector candidates for the motion vector of the current block using neighboring blocks of the current block. Neighboring blocks used to derive predictive motion vector candidates include a left block A0, a lower left block A1, an upper block B0, and an upper right block adjacent to the current block in the current picture shown in FIG. B1), and all or part of the upper left block (A2) may be used. In addition, a block located in a reference picture (which may be the same as or different from the reference picture used to predict the current block) other than the current picture where the current block is located will be used as a neighboring block used to derive motion vector candidates. may be For example, a collocated block co-located with the current block within the reference picture or blocks adjacent to the collocated block may be used. If the number of motion vector candidates is smaller than the preset number according to the method described above, a 0 vector is added to the motion vector candidates.

The inter-prediction unit 124 derives predicted motion vector candidates using the motion vectors of the neighboring blocks, and determines a predicted motion vector for the motion vector of the current block using the predicted motion vector candidates. Then, a differential motion vector is calculated by subtracting the predicted motion vector from the motion vector of the current block.

The predicted motion vector may be obtained by applying a predefined function (eg, median value, average value operation, etc.) to predicted motion vector candidates. In this case, the video decoding apparatus also knows the predefined function. In addition, since a neighboring block used to derive a predicted motion vector candidate is a block that has already been encoded and decoded, the video decoding apparatus also knows the motion vector of the neighboring block. Therefore, the video encoding apparatus does not need to encode information for identifying a predictive motion vector candidate. Therefore, in this case, information on differential motion vectors and information on reference pictures used to predict the current block are encoded.

Meanwhile, the predicted motion vector may be determined by selecting one of the predicted motion vector candidates. In this case, along with information on differential motion vectors and information on reference pictures used to predict the current block, information for identifying the selected predictive motion vector candidate is additionally encoded.

The subtractor 130 subtracts the prediction block generated by the intra prediction unit 122 or the inter prediction unit 124 from the current block to generate a residual block.

The transform unit 140 transforms the residual signal in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The transform unit 140 may transform residual signals in the residual block by using the entire size of the residual block as a transform unit, or divide the residual block into a plurality of subblocks and use the subblocks as a transform unit to perform transformation. You may. Alternatively, the residual signals may be divided into two subblocks, a transform region and a non-transform region, and transform the residual signals using only the transform region subblock as a transform unit. Here, the transformation region subblock may be one of two rectangular blocks having a size ratio of 1:1 based on a horizontal axis (or a vertical axis). In this case, a flag (cu_sbt_flag) indicating that only subblocks have been transformed, directional (vertical/horizontal) information (cu_sbt_horizontal_flag), and/or location information (cu_sbt_pos_flag) are encoded by the entropy encoding unit 155 and signaled to the video decoding device. do. In addition, the size of the transform region subblock may have a size ratio of 1:3 based on the horizontal axis (or vertical axis), and in this case, a flag (cu_sbt_quad_flag) for distinguishing the corresponding division is additionally encoded by the entropy encoder 155 to obtain an image It is signaled to the decryption device.

Meanwhile, the transform unit 140 may individually transform the residual block in the horizontal direction and the vertical direction. For the transformation, various types of transformation functions or transformation matrices may be used. For example, a pair of transformation functions for horizontal transformation and vertical transformation may be defined as a multiple transform set (MTS). The transform unit 140 may select one transform function pair having the highest transform efficiency among the MTS and transform the residual blocks in the horizontal and vertical directions, respectively. Information (mts_idx) on a pair of transform functions selected from the MTS is encoded by the entropy encoding unit 155 and signaled to the video decoding device.

The quantization unit 145 quantizes transform coefficients output from the transform unit 140 using a quantization parameter, and outputs the quantized transform coefficients to the entropy encoding unit 155 . The quantization unit 145 may directly quantize a related residual block without transformation for a certain block or frame. The quantization unit 145 may apply different quantization coefficients (scaling values) according to positions of transform coefficients in the transform block. A quantization matrix applied to the two-dimensionally arranged quantized transform coefficients may be coded and signaled to the video decoding apparatus.

The rearrangement unit 150 may rearrange the coefficient values of the quantized residual values.

The reordering unit 150 may change a 2D coefficient array into a 1D coefficient sequence using coefficient scanning. For example, the reordering unit 150 may output a one-dimensional coefficient sequence by scanning DC coefficients to coefficients in a high frequency region using a zig-zag scan or a diagonal scan. . Depending on the size of the transformation unit and intra prediction mode, instead of zig-zag scan, vertical scan that scans a 2D coefficient array in a column direction and horizontal scan that scans 2D block-shaped coefficients in a row direction may be used. That is, a scan method to be used among zig-zag scan, diagonal scan, vertical scan, and horizontal scan may be determined according to the size of the transform unit and the intra prediction mode.

The entropy encoding unit 155 uses various encoding schemes such as CABAC (Context-based Adaptive Binary Arithmetic Code) and Exponential Golomb to convert the one-dimensional quantized transform coefficients output from the reordering unit 150 to each other. A bitstream is created by encoding the sequence.

In addition, the entropy encoding unit 155 encodes information such as CTU size, CU splitting flag, QT splitting flag, MTT splitting type, and MTT splitting direction related to block splitting so that the video decoding apparatus can divide the block in the same way as the video encoding apparatus. make it possible to divide In addition, the entropy encoding unit 155 encodes information about a prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and encodes intra prediction information (ie, intra prediction) according to the prediction type. mode) or inter prediction information (motion information encoding mode (merge mode or AMVP mode), merge index in case of merge mode, reference picture index and differential motion vector information in case of AMVP mode) are encoded. Also, the entropy encoding unit 155 encodes information related to quantization, that is, information about quantization parameters and information about quantization matrices.

The inverse quantization unit 160 inversely quantizes the quantized transform coefficients output from the quantization unit 145 to generate transform coefficients. The inverse transform unit 165 transforms transform coefficients output from the inverse quantization unit 160 from a frequency domain to a spatial domain to restore a residual block.

The adder 170 restores the current block by adding the restored residual block and the predicted block generated by the predictor 120. Pixels in the reconstructed current block are used as reference pixels when intra-predicting the next block.

The loop filter unit 180 reconstructs pixels in order to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. caused by block-based prediction and transformation/quantization. perform filtering on The filter unit 180 is an in-loop filter and may include all or part of a deblocking filter 182, a sample adaptive offset (SAO) filter 184, and an adaptive loop filter (ALF) 186. .

The deblocking filter 182 filters the boundary between reconstructed blocks to remove blocking artifacts caused by block-by-block encoding/decoding, and the SAO filter 184 and alf 186 perform deblocking filtering. Additional filtering is performed on the image. The SAO filter 184 and the alf 186 are filters used to compensate for a difference between a reconstructed pixel and an original pixel caused by lossy coding. The SAO filter 184 improves not only subjective picture quality but also coding efficiency by applying an offset in units of CTUs. In contrast, the ALF 186 performs block-by-block filtering. Distortion is compensated for by applying different filters by distinguishing the edge of the corresponding block and the degree of change. Information on filter coefficients to be used for ALF may be coded and signaled to the video decoding apparatus.

The reconstruction block filtered through the deblocking filter 182, the SAO filter 184, and the ALF 186 is stored in the memory 190. When all blocks in one picture are reconstructed, the reconstructed picture can be used as a reference picture for inter-prediction of blocks in the picture to be encoded later.

5 is an exemplary block diagram of a video decoding apparatus capable of implementing the techniques of this disclosure. Hereinafter, referring to FIG. 5, a video decoding device and sub-elements of the device will be described.

The image decoding apparatus includes an entropy decoding unit 510, a rearrangement unit 515, an inverse quantization unit 520, an inverse transform unit 530, a prediction unit 540, an adder 550, a loop filter unit 560, and a memory ( 570) may be configured.

Like the image encoding device of FIG. 1 , each component of the image decoding device may be implemented as hardware or software, or a combination of hardware and software. Also, the function of each component may be implemented as software, and the microprocessor may be implemented to execute the software function corresponding to each component.

The entropy decoding unit 510 determines a current block to be decoded by extracting information related to block division by decoding the bitstream generated by the video encoding apparatus, and provides prediction information and residual signals necessary for restoring the current block. extract information, etc.

The entropy decoding unit 510 determines the size of the CTU by extracting information about the CTU size from a sequence parameter set (SPS) or a picture parameter set (PPS), and divides the picture into CTUs of the determined size. Then, the CTU is divided using the tree structure by determining the CTU as the top layer of the tree structure, that is, the root node, and extracting division information for the CTU.

For example, when a CTU is split using the QTBTTT structure, a first flag (QT_split_flag) related to splitting of QT is first extracted and each node is split into four nodes of a lower layer. In addition, for a node corresponding to a leaf node of QT, a second flag (MTT_split_flag) related to splitting of MTT and split direction (vertical / horizontal) and / or split type (binary / ternary) information are extracted and the corresponding leaf node is MTT split into structures Accordingly, each node below the leaf node of QT is recursively divided into a BT or TT structure.

As another example, when a CTU is split using the QTBTTT structure, a CU split flag (split_cu_flag) indicating whether the CU is split is first extracted, and when the corresponding block is split, a first flag (QT_split_flag) is extracted. may be During the splitting process, each node may have zero or more iterative MTT splits after zero or more repetitive QT splits. For example, the CTU may immediately undergo MTT splitting, or conversely, only QT splitting may occur multiple times.

As another example, when a CTU is split using a QTBT structure, a first flag (QT_split_flag) related to QT splitting is extracted and each node is split into four nodes of a lower layer. And, for a node corresponding to a leaf node of QT, a split flag (split_flag) indicating whether to further split into BTs and split direction information are extracted.

Meanwhile, when the entropy decoding unit 510 determines a current block to be decoded by using tree structure partitioning, it extracts information about a prediction type indicating whether the current block is intra-predicted or inter-predicted. When the prediction type information indicates intra prediction, the entropy decoding unit 510 extracts syntax elements for intra prediction information (intra prediction mode) of the current block. When the prediction type information indicates inter prediction, the entropy decoding unit 510 extracts syntax elements for the inter prediction information, that is, information indicating a motion vector and a reference picture to which the motion vector refers.

In addition, the entropy decoding unit 510 extracts quantization-related information and information about quantized transform coefficients of the current block as information about the residual signal.

The reordering unit 515 converts the sequence of 1-dimensional quantized transform coefficients entropy-decoded in the entropy decoding unit 510 into a 2-dimensional coefficient array (ie, in the reverse order of the coefficient scanning performed by the image encoding apparatus). block) can be changed.

The inverse quantization unit 520 inverse quantizes the quantized transform coefficients and inverse quantizes the quantized transform coefficients using a quantization parameter. The inverse quantization unit 520 may apply different quantization coefficients (scaling values) to the two-dimensionally arranged quantized transform coefficients. The inverse quantization unit 520 may perform inverse quantization by applying a matrix of quantization coefficients (scaling values) from the image encoding device to a 2D array of quantized transformation coefficients.

The inverse transform unit 530 inversely transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to restore residual signals, thereby generating a residual block for the current block.

In addition, when the inverse transform unit 530 inverse transforms only a partial region (subblock) of a transform block, a flag (cu_sbt_flag) indicating that only a subblock of the transform block has been transformed, and direction information (vertical/horizontal) information (cu_sbt_horizontal_flag) of the transform block ) and/or the location information (cu_sbt_pos_flag) of the subblock, and inversely transforms the transform coefficients of the corresponding subblock from the frequency domain to the spatial domain to restore the residual signals. By filling , the final residual block for the current block is created.

In addition, when the MTS is applied, the inverse transform unit 530 determines transform functions or transform matrices to be applied in the horizontal and vertical directions, respectively, using MTS information (mts_idx) signaled from the video encoding device, and uses the determined transform functions. Inverse transform is performed on the transform coefficients in the transform block in the horizontal and vertical directions.

The prediction unit 540 may include an intra prediction unit 542 and an inter prediction unit 544 . The intra prediction unit 542 is activated when the prediction type of the current block is intra prediction, and the inter prediction unit 544 is activated when the prediction type of the current block is inter prediction.

The intra prediction unit 542 determines the intra prediction mode of the current block among a plurality of intra prediction modes from the syntax element for the intra prediction mode extracted from the entropy decoding unit 510, and references the current block according to the intra prediction mode. The current block is predicted using pixels.

The inter prediction unit 544 determines the motion vector of the current block and the reference picture referred to by the motion vector by using the syntax element for the inter prediction mode extracted from the entropy decoding unit 510, and converts the motion vector and the reference picture. to predict the current block.

The adder 550 restores the current block by adding the residual block output from the inverse transform unit and the prediction block output from the inter prediction unit or intra prediction unit. Pixels in the reconstructed current block are used as reference pixels when intra-predicting a block to be decoded later.

The loop filter unit 560 may include a deblocking filter 562, an SAO filter 564, and an ALF 566 as in-loop filters. The deblocking filter 562 performs deblocking filtering on boundaries between reconstructed blocks in order to remove blocking artifacts generated by block-by-block decoding. The SAO filter 564 and the ALF 566 perform additional filtering on the reconstructed block after deblocking filtering to compensate for the difference between the reconstructed pixel and the original pixel caused by lossy coding. ALF filter coefficients are determined using information on filter coefficients decoded from the non-stream.

The reconstruction block filtered through the deblocking filter 562, the SAO filter 564, and the ALF 566 is stored in the memory 570. When all blocks in one picture are reconstructed, the reconstructed picture is used as a reference picture for inter-prediction of blocks in the picture to be encoded later.

This embodiment relates to encoding and decoding of images (video) as described above. More specifically, a secondary MPM (Most Probable Mode) list is generated according to a gradient-based template matching result, and a video coding method and apparatus using the secondary MPM list for intra prediction of a current block are provided. .

The following embodiments may be performed by the intra prediction unit 122 in a video encoding device. Also, it may be performed by the intra prediction unit 542 in the video decoding device.

The video encoding apparatus may generate signaling information related to the present embodiment in terms of bit rate distortion optimization in intra prediction of the current block. The image encoding device may encode the image using the entropy encoding unit 155 and transmit it to the image decoding device. The video decoding apparatus may decode signaling information related to intra prediction of the current block from a bitstream using the entropy decoding unit 510 .

In the following description, the term 'target block' may be used in the same meaning as a current block or a coding unit (CU, Coding Unit), or may mean a partial region of a coding unit.

Also, a value of one flag being true indicates a case in which the flag is set to 1. In addition, a false value of one flag indicates a case in which the flag is set to 0.

I. Intra prediction techniques

As described above, intra prediction is a method of predicting a current block by referring to samples existing around a block to be currently encoded. In the VVC technique, intra prediction modes have subdivided directional modes (ie, 2 to 66) in addition to non-directional modes (ie, planar and DC), as illustrated in FIG. 3A. Also, as added to the example of FIG. 3B, the intra prediction mode of the luma block has directional modes (-14 to -1 and 67 to 80) according to wide-angle intra prediction (WAIP).

In addition, intra prediction may use prediction techniques such as multiple reference line intra prediction (MRLP), intra sub-partitions (ISP), and most probable mode (MPM).

In multiple reference line intra prediction (MRLP) of intra prediction, an image encoding/decoding apparatus may use more reference lines by using multiple reference lines (MRL). When MRL is applied, the video encoding/decoding apparatus may use two lines added to the top and left sides of the current block in addition to the original reference line. To select a reference line when applying MRL, an index (mrl_idx) indicating a reference line may be signaled to the video decoding apparatus.

In intra prediction, a most probable mode (MPM) technique uses intra prediction modes of neighboring blocks when intra prediction of a current block is performed. The video encoding apparatus generates an MPM list to include intra prediction modes derived from predefined positions spatially adjacent to a current block. When the MPM mode is applied, the video encoding apparatus may transmit MPM_flag, which is a flag indicating whether to use the MPM list, to the video decoding apparatus. In addition, the video encoding apparatus can improve the encoding efficiency of the intra prediction mode by transmitting the MPM index, mpm_idx, instead of the index of the prediction mode.

In the ISP technique, after subdividing a current block into small blocks of the same size, an intra prediction mode is shared across all subblocks, but a transform may be applied to each subblock. At this time, subdivision of blocks may be performed in a horizontal direction or a vertical direction.

In the following description, a large block before being subdivided is referred to as a current block, and each of the subdivided small blocks is referred to as a subblock.

In sub-division of the current block in the horizontal or vertical direction, if the size of the current block is too small, the encoding efficiency of the divided sub-blocks is rather deteriorated or the size of the sub-blocks is smaller than the minimum unit for transformation. Conversion itself may not be possible. To prevent such a case from occurring, ISP application may be limited by referring to the size of a subblock obtained after division. That is, when the number of pixels of a divided subblock is 16 or more, subdivision may be applied. For example, if the size of the current block is 4x4, ISP is not applied. A block having a size of 4×8 or 8×4 can be split into two subblocks having the same shape and size, which is called Half_Split. Blocks having other sizes may be divided into 4 subblocks having the same shape and size, which is called Quarter_Split.

II. Secondary MPM list based on template matching

The following embodiments may be performed by the intra prediction unit 122 and the entropy encoding unit 155 in the image encoding apparatus. In addition, it may be performed by the entropy decoding unit 510 and the intra prediction unit 542 in the video decoding apparatus.

Hereinafter, the aforementioned MPM list is used interchangeably with the primary MPM list. Also, in the following description, an MPM list generated based on template matching is referred to as a secondary MPM list.

7 is a flowchart illustrating a method for generating a secondary MPM list by a video encoding apparatus according to an embodiment of the present disclosure.

The video encoding device determines MPM_flag (S700). Here, MPM_flag indicates whether to use the main MPM list.

The video encoding device checks MPM_flag (S702).

When MPM_flag is false, the video encoding device performs the following steps.

The video encoding apparatus performs gradient-based template matching in a predefined search area of the current block (S704).

The step of performing template matching according to the present embodiment includes setting a template of the current block, calculating the gradient size of the template by applying a differential filter to the template, and predefined search of the current block. Calculating a gradient size for the area, and searching for a similar template by performing gradient-based template matching using the gradient size of the template and the gradient size of the search area.

The video encoding device compares the gradient size of the template with a threshold value (S706).

When the size of the gradient of the template is greater than the threshold value, the video encoding apparatus performs the following steps (S708 to S712).

In terms of reducing computational complexity, only the step of setting the template of the current block and the step of calculating the size of the gradient of the template by applying a differentiation filter to the template among the steps of performing template matching can be performed in step S704. Therefore, if the gradient size is greater than the threshold value, calculating the gradient size for the predefined search area, and performing gradient-based template matching using the gradient size of the template and the search area to find similar templates. A search step may be performed before step S708.

The image encoding apparatus derives and arranges intra prediction modes using a corresponding block of a similar template (S708).

The video encoding apparatus constructs a secondary MPM list using the sorted intra prediction modes and sets secondary_mpm_flag to 1 (S710). Here, secondary_mpm_flag, a secondary MPM flag, indicates whether to use a secondary MPM list.

The video encoding device determines secondary_mpm_idx (S712). Here, secondary_mpm_idx, which is a secondary MPM index, indicates one of intra prediction modes stored in the secondary MPM list. In terms of encoding efficiency optimization, the video encoding device may determine secondary_mpm_idx.

Thereafter, the video encoding device may derive the intra prediction mode of the current block from the secondary MPM list using secondary_mpm_idx. After generating a prediction block using the intra prediction mode, the image encoding apparatus may generate a residual block by subtracting the prediction block from the current block. The video encoding device may encode MPM_flag, secondary_mpm_flag, secondary_mpm_idx, and a residual block.

If the gradient size of the template is less than or equal to the threshold value, the video encoding device sets secondary_mpm_flag to 0 (S720) and determines an MPM reminder (S722). The MPM reminder is an intra prediction mode that is not included in the secondary MPM list or the main MPM list.

Thereafter, the video encoding apparatus may derive an intra prediction mode of the current block according to the MPM reminder. After generating a prediction block using the intra prediction mode, the image encoding apparatus may generate a residual block by subtracting the prediction block from the current block. The video encoding apparatus may encode MPM_flag, secondary_mpm_flag, MPM reminder, and residual block.

Meanwhile, when MPM_flag is true, the video encoding apparatus performs the following steps.

The video encoding device determines mpm_idx (S730).

Thereafter, the video encoding apparatus may derive an intra prediction mode of the current block from the main MPM list using mpm_idx. After generating a prediction block using the intra prediction mode, the image encoding apparatus may generate a residual block by subtracting the prediction block from the current block. The video encoding apparatus may encode MPM_flag, mpm_idx, and a residual block.

8 is a flowchart illustrating a method for generating a secondary MPM list by a video decoding apparatus according to an embodiment of the present disclosure.

The video decoding apparatus decodes the residual block and MPM_flag from the bitstream (S800).

The video decoding apparatus checks MPM_flag (S802).

When MPM_flag is false, the video decoding apparatus performs the following steps.

The video decoding apparatus decodes secondary_mpm_flag from the bitstream (S804). Here, secondary_mpm_flag indicates whether to use the secondary MPM list.

The video decoding device checks secondary_mpm_flag (S806).

If secondary_mpm_flag is true, the video decoding apparatus performs the following steps.

The video decoding apparatus searches for a similar template by performing gradient-based template matching in a predefined search area of the current block (S808).

The step of performing template matching according to the present embodiment includes setting a template of the current block, calculating the gradient size of the template by applying a differential filter to the template, and predefined search of the current block. Calculating a gradient size for the area, and searching for a similar template by performing gradient-based template matching using the gradient size of the template and the gradient size of the search area.

The video decoding apparatus derives and arranges intra prediction modes using corresponding blocks of similar templates (S810).

The video decoding apparatus constructs a secondary MPM list using the sorted intra prediction modes (S812).

The video decoding apparatus decodes secondary_mpm_idx from the bitstream (S814). Here, secondary_mpm_idx, which is a secondary MPM index, indicates one of intra prediction modes stored in the secondary MPM list.

Thereafter, the video decoding apparatus may derive the intra prediction mode of the current block from the secondary MPM list using secondary_mpm_idx. After generating a prediction block using the intra prediction mode, the video decoding apparatus may reconstruct the current block by adding the prediction block and the residual block.

If secondary_mpm_flag is false, the video decoding apparatus decodes the MPM reminder from the bitstream (S820).

Thereafter, the video decoding apparatus may derive an intra prediction mode of the current block according to the MPM reminder. After generating a prediction block using the intra prediction mode, the video decoding apparatus may reconstruct the current block by adding the prediction block and the residual block.

Meanwhile, when MPM_flag is true, the video decoding apparatus performs the following steps.

The video decoding apparatus decodes mpm_idx from the bitstream (S830).

Thereafter, the video decoding apparatus may derive an intra prediction mode of the current block from the main MPM list using mpm_idx. After generating a prediction block using the intra prediction mode, the video decoding apparatus may reconstruct the current block by adding the prediction block and the residual block.

As another example, when the size of the gradient is less than or equal to the threshold value, the video encoding device may set secondary_mpm_flag to 0 and perform template matching as illustrated in FIG. 6 . A method of performing template matching may be set in advance according to an agreement between the video encoding device and the video decoding device.

Thereafter, the video encoding apparatus may generate a residual block by subtracting the prediction block from the current block after generating a block corresponding to the similar template according to template matching as a prediction block of the current block. The video encoding device may encode MPM_flag, secondary_mpm_flag, and a residual block.

9 is a flowchart illustrating a method for generating a secondary MPM list by a video decoding apparatus according to another embodiment of the present disclosure.

Since other steps are the same as the flowchart illustrated in FIG. 8, only the case where secondary_mpm_flag is false is described. If secondary_mpm_flag is false, the video decoding apparatus performs template matching (S920).

Thereafter, the video decoding apparatus may generate a prediction block of the current block using a block corresponding to a similar template according to template matching, and then restore the current block by adding the prediction block and the residual block.

As another embodiment, the video encoding apparatus may determine whether to use the secondary MPM list according to the size of the gradient value without generating the secondary_mpm_flag.

10 is a flowchart illustrating a method for generating a secondary MPM list by a video encoding apparatus according to another embodiment of the present disclosure.

Since the other steps are the same as the flowchart illustrated in FIG. 7, only the parts related to secondary_mpm_flag deletion are described.

When the size of the gradient of the template is greater than the threshold value, the video encoding apparatus performs the following steps (S1008 to S1012).

As described above, if the gradient size is greater than the threshold, calculating the gradient size for the predefined search area of the current block, and performing gradient-based template matching using the gradient size of the template and the gradient size of the search area. By doing so, the step of searching for similar templates can be performed before step S1008.

The image encoding apparatus induces and arranges intra prediction modes using corresponding blocks of similar templates (S1008).

The video encoding apparatus constructs a secondary MPM list using the sorted intra prediction modes (S1010).

The video encoding device determines secondary_mpm_idx (S1012). Here, secondary_mpm_idx, which is a secondary MPM index, indicates one of intra prediction modes stored in the secondary MPM list.

Thereafter, the video encoding device may derive the intra prediction mode of the current block from the secondary MPM list using secondary_mpm_idx. After generating a prediction block using the intra prediction mode, the image encoding apparatus may generate a residual block by subtracting the prediction block from the current block. The video encoding device may encode MPM_flag, secondary_mpm_idx, and a residual block.

When the gradient size of the template is less than or equal to the threshold value, the video encoding apparatus determines an MPM reminder (S1020).

Thereafter, the video encoding apparatus may derive an intra prediction mode of the current block according to the MPM reminder. After generating a prediction block using the intra prediction mode, the image encoding apparatus may generate a residual block by subtracting the prediction block from the current block. The video encoding apparatus may encode MPM_flag, MPM reminder, and residual block.

11 is a flowchart illustrating a method for generating a secondary MPM list by a video decoding apparatus according to another embodiment of the present disclosure.

Since the other steps are the same as the flowchart illustrated in FIG. 7, only the parts related to secondary_mpm_flag deletion are described.

When MPM_flag is false, the video decoding apparatus performs the following steps.

The video decoding apparatus searches for a similar template by performing gradient-based template matching in a predefined search area (S1104).

The video decoding apparatus compares the gradient size with a threshold value (S1106).

When the gradient size is greater than the threshold value, the video decoding apparatus performs the following steps.

In terms of reducing computational complexity, only the step of setting the template of the current block and the step of calculating the size of the gradient of the template by applying a differentiation filter to the template among the steps of performing template matching may be performed in step S1104. Therefore, if the gradient size is greater than the threshold value, calculating the gradient size for the predefined search area of the current block, and performing gradient-based template matching using the gradient size of the template and the gradient size of the search area, A step of searching for a similar template may be performed before step S1108.

The video decoding apparatus derives and arranges intra prediction modes using corresponding blocks of similar templates (S1108).

The video decoding apparatus constructs a secondary MPM list using the sorted intra prediction modes (S1110)

The video decoding apparatus decodes secondary_mpm_idx from the bitstream (S1112). Here, secondary_mpm_idx, which is a secondary MPM index, indicates one of intra prediction modes stored in the secondary MPM list.

Thereafter, the video decoding apparatus may derive the intra prediction mode of the current block from the secondary MPM list using secondary_mpm_idx. After generating a prediction block using the intra prediction mode, the video decoding apparatus may reconstruct the current block by adding the prediction block and the residual block.

If the gradient size is less than or equal to the threshold value, the video decoding apparatus decodes the MPM reminder from the bitstream (S1120).

Thereafter, the video decoding apparatus may derive an intra prediction mode of the current block according to the MPM reminder. After generating a prediction block using the intra prediction mode, the video decoding apparatus may reconstruct the current block by adding the prediction block and the residual block.

Meanwhile, the same method for calculating the size of the gradient may be set according to an agreement between the video encoding device and the video decoding device. In addition, a predetermined value may be set for the threshold value compared with the gradient size according to an agreement between the video encoding apparatus and the video decoding apparatus.

Hereinafter, the gradient-based template matching step, the intra prediction mode derivation and sorting step, and the secondary MPM list construction step will be described, focusing on the video encoding device. The steps described below may be equally applied to an image decoding device. First, the gradient-based template matching step is described.

The image encoding apparatus calculates the gradient size of the reconstruction region and the gradient size of the template. Gradient-based template matching may be performed by searching for similar templates in the relief region based on the calculated values of the gradient size.

The template is composed of a region adjacent to the current block, and may have a form as illustrated in FIG. 12 . In the example of FIG. 12 , TW and TH represent horizontal and vertical lengths of the template, respectively, and may be set in advance according to an agreement between the video encoding device and the video decoding device. The shape of the template may be set in advance according to an agreement between the video encoding device and the video decoding device. Alternatively, the video encoding apparatus may encode an index indicating one of preset template types and then signal the video decoding apparatus.

The shape of the template may vary according to the prediction mode of the relief region. As in the example of FIG. 13 , when the upper block is predicted in the vertical mode and the left block is predicted in the non-directional mode, only the template corresponding to the upper block can be used. Alternatively, when the upper block is predicted in the non-directional mode and the left block is in the horizontal mode, only the template of the left block may be used.

As another example, the video encoding device may find a similar template in the search area after rotating the template. As illustrated in FIG. 14 , a basic template (ⓐ), a template rotated 90 degrees clockwise (ⓑ), a template rotated 180 degrees (ⓒ), and a template rotated 270 degrees (ⓓ) may be used.

A gradient value of the template may be calculated by applying a differential filter to the template. For the horizontal and vertical directions, a Prewitt filter, a Roberts filter, a Sobel filter, or the like may be used as a differential filter. A gradient size may be calculated for each pixel in the template as shown in Equation 1 or Equation 2 using dx and dy values, which are gradient values calculated using a differential filter. The calculated gradient size of the template may be stored and managed for future use.

As described above, when the sum of gradient sizes of pixels included in the template is equal to or less than a preset threshold value, a secondary MPM list is not configured. At this time, secondary_mpm_flag is set to 0, and the MPM reminder is signaled.

15 is an exemplary diagram illustrating search areas for template matching according to an embodiment of the present disclosure.

In the example of FIG. 15 , regions R1, R2, R3, and R4 in the restoration region may be a block in the current CTU, an upper left block of the current CTU, an upper block of the current CTU, or a left block of the current CTU, respectively. These search areas may be set in advance according to an agreement between the video encoding device and the video decoding device. In addition, in the example of FIG. 15, SearchRange_W and SearchRange_H may be set in advance according to an agreement between the video encoding device and the video decoding device.

For the search areas illustrated in FIG. 15 , a gradient value may be calculated by applying the differential filter as described above. A gradient size may be calculated for each pixel in the search areas as shown in Equation 1 or Equation 2 using dx and dy values, which are gradient values calculated using a differential filter. The calculated gradient size can be stored and managed for future use.

The type of the differential filter and the method of calculating the gradient size may be set in advance according to an agreement between the video encoding apparatus and the video decoding apparatus. In addition, the same applies to the type of differentiation filter and the method of calculating the gradient size for the template and the search areas.

16 is an exemplary diagram illustrating calculation of a gradient size in a search area according to an embodiment of the present disclosure.

The video encoding apparatus calculates and stores the gradient size for each K×Q block while performing encoding according to the encoding order. Here, the size of the K×Q block may be a CTU, CU, PU (Prediction Unit) or TU (Transform Unit) size. When the K×Q block for the current block is not included in search areas limited according to SearchRange_W and SearchRange_H, the video encoding apparatus may discard the pre-stored gradient size of the corresponding block. The image encoding apparatus may perform template matching only for blocks having a gradient size greater than the threshold after comparing gradient sizes in units of K×Q blocks stored in search areas according to SearchRange_W and SearchRange_H with a threshold. The complexity of performing template matching may be reduced by not searching for a region where the gradient size is less than or equal to the threshold value. Here, the gradient size in units of K×Q blocks represents the sum of gradient sizes of pixels in the K×Q block.

In the example of FIG. 16 , K and Q are integers greater than or equal to 1. Also, as described above, the method of calculating the differential filter and the gradient size in the template and search areas is equally applied.

After calculating the gradient size of the template, the video encoding apparatus performs gradient-based template matching using K×Q block-based gradient sizes pre-stored in SearchRange_W and SearchRange_H. As a cost function for template matching, SAD (Sum of Absolute Difference), SSD (Sum of Squared Difference), and the like may be used. The cost function may be set in advance according to an agreement between the video encoding apparatus and the video decoding apparatus.

The method of calculating the gradient size for each K×Q block as illustrated in FIG. 16 may be equally applied to the video decoding apparatus.

Hereinafter, intra prediction mode derivation and alignment steps are described.

The video encoding apparatus may use Histogram of Oriented Gradients (HoG) to derive intra prediction modes from similar templates found in the gradient-based template matching step and corresponding blocks adjacent to the similar templates. As illustrated in FIG. 17 , HoG may be applied to a W×H size corresponding block or a (W+TW)×(H+TH) size block including a similar template and a corresponding block. Hereinafter, for convenience, a W×H block or a (W+TW)×(H+TH) block is collectively referred to as a corresponding block.

A histogram can be constructed by calculating the gradient direction and the magnitude of the gradient using HoG for the corresponding block. A differential filter, for example, a Sobel filter, may be applied to calculate the direction of the gradient and the magnitude of the gradient for each pixel of the corresponding block.

Also, the histogram can be calculated as follows. Gradient values dx and dy are calculated for each pixel of the corresponding block using a Sobel filter in the vertical and horizontal directions. The size of the gradient for each pixel may be calculated according to Equation 1 or 2 using dx and dy. In addition, angle_value representing the directionality of each pixel may be calculated by calculating the gradient (dy/dx) using dx and dy according to Equation 3 and then applying a shift operation.

A direction table angleTable may be defined in advance to generate an intra prediction mode, that is, intra_mode_index, an index of the intra prediction mode. In this case, index i of the direction table may indicate an index of an intra prediction mode. As shown in Equation 4, the index of the intra prediction mode of each pixel may be calculated so that the SAD value between angleTable[i] and angle_value is the smallest.

As an example, after generating a histogram of gradient sizes according to intra prediction mode indices of each pixel, prediction mode indices (ie, intra prediction modes) may be sorted in descending order according to cumulative gradient sizes.

As another example, after generating a histogram of occurrence frequencies of intra prediction modes, indexes of prediction modes (ie, intra prediction modes) may be sorted in descending order according to the frequency of occurrence.

The method for generating HoG as described above may be set in advance according to an agreement between the video encoding device and the video decoding device.

18 is an exemplary diagram illustrating subblocks from which a current block is divided.

When ISP (Intra Sub-Partitions) technology is used, as illustrated in FIG. 18, it may be vertically or horizontally partitioned according to the size of the current block. The video encoding apparatus may generate HoG for a prediction block in units of subblocks, and may arrange the prediction modes in descending order according to the magnitude of the cumulative gradient, for example.

The secondary MPM list construction step is described below.

The video encoding apparatus may configure a secondary MPM list with indexes of prediction modes arranged in descending order. If the MPM flag is 0, since a secondary MPM list is constructed, a secondary MPM list may be constructed while removing prediction modes included in the main MPM list to prevent duplication. In the example of FIG. 19, intra prediction mode indices a, b, c, d, e, ... are indexes of intra prediction modes sorted in descending order, and may be candidates of a secondary MPM list. When the intra prediction mode indices c and f are prediction modes included in the main MPM list, they can be removed from the candidates as shown in the example of FIG. 19 .

The secondary MPM list has a predefined size. The size of the secondary MPM list may be set in advance according to an agreement between the video encoding device and the video decoding device.

Candidates of the MPM list may not be filled with only the prediction modes sorted in descending order for the secondary MPM list having a predefined size. In this case, the remaining candidates of the secondary MPM list may be filled using the adjacent indices of the candidates stored in the secondary MPM list. For example, it is assumed that the size of the secondary MPM list is 5 and prediction modes a, b, and e are stored as candidates in the secondary MPM list. The remaining candidates of the secondary MPM list may be filled using adjacent indices a-γ, a+γ, b-γ, b+γ, e-γ, e+γ, etc. for the stored indices. In this case, a corresponding neighbor index may be preferentially used for an intra prediction mode having a higher rank in descending order. Here, γ is an integer greater than or equal to 1.

As an example, when candidates of the MPM list are not filled with only prediction modes sorted in descending order for the secondary MPM list having a predefined size, the secondary MPM list may not be used. If the secondary MPM list is not used, the video encoding device sets secondary_mpm_flag to 0 and determines the MPM reminder. Then, the video encoding device signals the secondary_mpm_flag and the MPM reminder to the video decoding device. After decoding the secondary_mpm_flag and the MPM reminder, the video decoding apparatus may induce the intra prediction mode according to the MPM reminder because the secondary_mpm_flag is false.

20 is an exemplary diagram illustrating template matching in units of subblocks according to an embodiment of the present disclosure.

As described above, when the current block is divided into subblocks, the video encoding apparatus may perform template matching in units of subblocks. As in the example of FIG. 20 , a template may be defined for each subblock.

The video encoding apparatus may perform a gradient-based template matching step, an intra prediction mode derivation and alignment step, and a secondary MPM list constructing step on a sub-block basis. For the first subblock, a gradient-based template matching step, an intra prediction mode derivation and sorting step, and a secondary MPM list construction step are performed. Thereafter, the first subblock may be reconstructed through prediction, transformation, quantization, inverse quantization, and inverse transformation. A part of the reconstructed first subblock may be configured as a new template of the second subblock. Thereafter, a gradient-based template matching step, an intra prediction mode derivation and sorting step, and a secondary MPM list constructing step may be performed on the second subblock.

In the flow chart/timing diagram of the present specification, it is described that each process is sequentially executed, but this is merely an example of the technical idea of one embodiment of the present disclosure. In other words, those skilled in the art to which an embodiment of the present disclosure belongs may change and execute the order described in the flowchart/timing diagram within the range that does not deviate from the essential characteristics of the embodiment of the present disclosure, or one of each process Since the above process can be applied by performing various modifications and variations in parallel, the flow chart/timing chart is not limited to a time-series sequence.

In the above description, it should be understood that the exemplary embodiments may be implemented in many different ways. Functions or methods described in one or more examples may be implemented in hardware, software, firmware, or any combination thereof. It should be understood that the functional components described in this specification have been labeled "...unit" to particularly emphasize their implementation independence.

Meanwhile, various functions or methods described in this embodiment may be implemented as instructions stored in a non-transitory recording medium that can be read and executed by one or more processors. Non-transitory recording media include, for example, all types of recording devices in which data is stored in a form readable by a computer system. For example, the non-transitory recording medium includes storage media such as an erasable programmable read only memory (EPROM), a flash drive, an optical drive, a magnetic hard drive, and a solid state drive (SSD).

The above description is merely an example of the technical idea of the present embodiment, and various modifications and variations can be made to those skilled in the art without departing from the essential characteristics of the present embodiment. Therefore, the present embodiments are not intended to limit the technical idea of the present embodiment, but to explain, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of this embodiment should be construed according to the claims below, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of rights of this embodiment.

122: intra prediction unit
155: entropy encoding unit
510: entropy decoding unit
542: intra prediction unit

Claims (16)

In the method of decoding a current block, performed by an image decoding apparatus,
Decoding a residual block of the current block and a secondary Most Probable Mode (MPM) flag from a bitstream, wherein the secondary MPM flag indicates whether to use a secondary MPM list ; and
Checking the secondary MPM flag
Including,
If the secondary MPM flag is true,
searching for a similar template by performing gradient-based template matching in a predefined search area of the current block;
inducing and arranging intra prediction modes using corresponding blocks of the similar template;
constructing the secondary MPM list for the current block using the sorted intra prediction modes; and
Decoding a secondary MPM index from the bitstream
Characterized in that, the method comprising a. According to claim 1,
When a flag indicating whether to use a primary MPM list is false, the step of decoding the secondary MPM flag is performed. According to claim 1,
Steps to explore similar templates include:
setting a template of the current block;
Calculating the size of the gradient of the template by applying a differential filter to the template;
Calculating a gradient size in units of K×Q blocks for the predefined search area, where K and Q are integers greater than or equal to 1; and
Searching for the similar template by performing the gradient-based template matching using the gradient size of the template and the gradient size in units of K×Q blocks
Characterized in that, the method comprising a. According to claim 3,
The step of searching for the similar template,
And performing the gradient-based template matching when the size of the gradient in units of K×Q blocks is greater than or equal to a predetermined threshold value. According to claim 3,
The sorting step is
Calculating the size of the gradient for each pixel of the corresponding block
calculating an index of an intra prediction mode for each pixel of the corresponding block; and
After generating a histogram of the gradient size according to the intra prediction mode of each pixel, sorting the indexes of the prediction mode in descending order according to the cumulative gradient size
Characterized in that, the method comprising a. According to claim 5,
In the step of calculating the index of the intra prediction mode,
Calculating a gradient value for each pixel of the corresponding block, calculating a directionality using the gradient value, and then calculating an index of the intra prediction mode using the directionality and a predefined directionality table , method. According to claim 5,
The step of constructing the secondary MPM list,
The secondary MPM list is constructed using indexes of the sorted prediction modes while removing prediction modes included in the main MPM list. According to claim 7,
The step of constructing the secondary MPM list,
When the secondary MPM list of a predefined size cannot be filled with only the indexes of the sorted prediction modes, the remainder of the secondary MPM list is filled using adjacent indices of prediction modes stored in the secondary MPM list. How to do it. According to claim 1,
deriving an intra prediction mode of the current block from the secondary MPM list using the secondary MPM index;
generating a prediction block of the current block by using the intra prediction mode; and
restoring the current block by adding the prediction block and the residual block;
Characterized in that it further comprises, the method. According to claim 1,
If the secondary MPM flag is false, further comprising decoding an MPM reminder,
The MPM reminder,
Characterized in that the intra prediction mode is not included in the secondary MPM list or the main MPM list. In the method of encoding a current block, performed by an image encoding apparatus,
generating a template of the current block and calculating a gradient size of the template;
Comparing the magnitude of the gradient with a preset threshold
Including,
When the size of the gradient is greater than a predetermined threshold value,
searching for a similar template to the template by performing gradient-based template matching in a predefined search area of the current block;
inducing and arranging intra prediction modes using corresponding blocks of the similar template;
Constructing a secondary MPM (Most Probable Mode) list for the current block using the sorted intra prediction modes and setting a secondary MPM flag, wherein the secondary MPM flag is the secondary Indicates whether to use the MPM list; and
Determining Secondary MPM Index
Characterized in that, the method comprising a. According to claim 11,
When a flag indicating whether to use a primary MPM list is false, the step of searching for the similar template is performed. According to claim 11,
deriving an intra prediction mode of the current block from the secondary MPM list using the secondary MPM index;
generating a prediction block of the current block by using the intra prediction mode; and
Generating a residual block by subtracting the prediction block from the current block
Characterized in that it further comprises, the method. According to claim 13,
characterized in that it further comprises the step of encoding the secondary MPM flag, the secondary MPM index, and the residual block. According to claim 11,
Further comprising determining an MPM reminder when the size of the gradient is less than or equal to a predetermined threshold,
The MPM reminder,
Characterized in that the intra prediction mode is not included in the secondary MPM list or the main MPM list. A computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method comprising:
generating a template of the current block and calculating a gradient size of the template;
Comparing the magnitude of the gradient with a preset threshold
Including,
When the size of the gradient is greater than a predetermined threshold value,
searching for a similar template to the template by performing gradient-based template matching in a predefined search area of the current block;
inducing and arranging intra prediction modes using corresponding blocks of the similar template;
Constructing a secondary MPM (Most Probable Mode) list for the current block using the sorted intra prediction modes and setting a secondary MPM flag, wherein the secondary MPM flag is the secondary Indicates whether to use the MPM list; and
Determining Secondary MPM Index
Characterized in that it comprises, a recording medium.

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