WO2024049024A1

WO2024049024A1 - Method and apparatus for video coding based on non-separable secondary transform adaptive to primary transform kernel

Info

Publication number: WO2024049024A1
Application number: PCT/KR2023/011041
Authority: WO
Inventors: 심동규; 변주형; 이민훈; 허진; 박승욱
Original assignee: 현대자동차주식회사; 기아 주식회사; 광운대학교 산학협력단
Priority date: 2022-08-29
Filing date: 2023-07-28
Publication date: 2024-03-07

Abstract

The present embodiment discloses a video coding method and apparatus based on non-separable secondary transform adaptive to a primary transform kernel. In the present embodiment, an image decoding apparatus determines a primary inverse transform kernel in vertical and horizontal directions with respect to a transform block of the current block. The image decoding apparatus acquires dequantized secondary transform coefficients for the transform block. The image decoding apparatus determines a secondary inverse transform kernel of the transform block, on the basis of the size of the transform block, an intra prediction mode of the current block, and the primary inverse transform kernel. The image decoding apparatus generates primary transform coefficients by applying the secondary inverse transform kernel to the secondary transform coefficients, and then generates residual signals by applying the primary inverse transform kernel to the primary transform coefficients.

Description

Method and apparatus for video coding based on non-separable secondary transform adaptive to primary transform kernel

The present disclosure relates to a video coding method and device based on a non-separable secondary transform that is adaptive to a primary transform kernel.

The content described below simply provides background information related to the present invention and does not constitute prior art.

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

Therefore, typically, when storing or transmitting video data, an encoder is used to compress the video data and store or transmit it, and a decoder receives the compressed video data, decompresses it, and plays it. These video compression technologies include H.264/AVC, HEVC (High Efficiency Video Coding), and VVC (Versatile Video Coding), which improves coding efficiency by about 30% or more compared to HEVC.

However, the size, resolution, and frame rate of the image are gradually increasing, and the amount of data that needs to be encoded is also increasing accordingly, so a new compression technology with better coding efficiency and higher picture quality improvement effect than the existing compression technology is required.

LFNST (Low-frequency non-separable transform) technology is used for the low-frequency region among the transform coefficients generated according to the primary transform of the transform block (TU, Transform Unit) during intra prediction. Perform secondary transformation. The LFNST technology performs secondary transformation on L low-frequency primary transformation coefficients among W×H primary transformation coefficients to generate K (where K≤L) secondary transformation coefficients. Afterwards, the encoder can quantize the generated secondary transform coefficient and then encode it. The size of the LFNST conversion kernel is K×L, and the type of kernel can be determined according to the intra prediction mode of the current TU, the size of the TU, and lfnst_idx (LFNST index).

Meanwhile, when the DCT2/DCT2 transformation kernel is applied as the first transformation to the intra predicted TU, LFNST can be applied. Therefore, in order to improve video coding efficiency and improve video quality, it is necessary to consider additionally utilizing the characteristics of the primary transform when applying the secondary transform.

The present disclosure selects a secondary transformation region and transformation kernel by reflecting the characteristics of the primary transformation kernel, and applies a non-separable secondary transformation to the transformation coefficients in the low-frequency region using the selected transformation kernel. The purpose is to provide video coding methods and devices.

According to an embodiment of the present disclosure, a method of restoring a current block performed by an image decoding apparatus includes: determining a first inverse transform kernel in the vertical and horizontal directions for a transform block of the current block; Obtaining inverse-quantized secondary transform coefficients for the transform block; determining a second inverse transform kernel of the transform block based on the size of the transform block, an intra prediction mode of the current block, and the first inverse transform kernel; performing a second-order inversion by applying the second-order inversion kernel to the second-order transformation coefficients to generate first-order transformation coefficients; and performing a first-order inverse transform by applying the first-order inverse transform kernel to the first-order transform coefficients to generate residual signals.

According to another embodiment of the present disclosure, a method of encoding a current block performed by an image encoding apparatus includes: acquiring residual signals for a transform block of the current block; determining a first-order transformation kernel in the vertical and horizontal directions for the transformation block; determining a secondary transform kernel of the transform block based on the size of the transform block, an intra prediction mode of the current block, and the first inverse transform kernel; performing first-order transformation by applying the first-order transformation kernel to the residual signals to generate first-order transformation coefficients; and performing secondary transformation by applying the secondary transformation kernel to the primary transformation coefficients to generate secondary transformation coefficients.

According to another embodiment of the present disclosure, a computer-readable recording medium stores a bitstream generated by an image encoding method, the image encoding method comprising: obtaining residual signals for a transform block of a current block; determining a first-order transformation kernel in the vertical and horizontal directions for the transformation block; determining a secondary transform kernel of the transform block based on the size of the transform block, an intra prediction mode of the current block, and the first inverse transform kernel; performing first-order transformation by applying the first-order transformation kernel to the residual signals to generate first-order transformation coefficients; and performing secondary transformation by applying the secondary transformation kernel to the primary transformation coefficients to generate secondary transformation coefficients.

As described above, according to this embodiment, the secondary transformation region and transformation kernel are selected by reflecting the characteristics of the primary transformation kernel, and an inseparable secondary transformation is performed on the transformation coefficients in the low-frequency region using the selected transformation kernel. By providing an applied video coding method and device, it is possible to improve video coding efficiency and improve video quality.

1 is an example block diagram of a video encoding device that can implement the techniques of the present disclosure.

Figure 2 is a diagram to explain a method of dividing a block using the QTBTTT (QuadTree plus BinaryTree TernaryTree) structure.

3A and 3B are diagrams showing a plurality of intra prediction modes including wide-angle intra prediction modes.

Figure 4 is an example diagram of neighboring blocks of the current block.

Figure 5 is an example block diagram of a video decoding device that can implement the techniques of the present disclosure.

FIG. 6 is a block diagram illustrating in detail a portion of an image decoding device according to an embodiment of the present disclosure.

7A to 7C are exemplary diagrams showing the scanning order of two-dimensional transformation coefficients according to an embodiment of the present disclosure.

Figures 8a and 8b are exemplary diagrams showing the scanning order of one-dimensional transform coefficients according to an embodiment of the present disclosure.

Figure 9 is an example diagram showing scanning positions of one-dimensional transform coefficients, according to an embodiment of the present disclosure.

Figure 10 is an example diagram showing the scanning order of one-dimensional transform coefficients according to another embodiment of the present disclosure.

FIG. 11 is a flowchart showing a method by which an image encoding device transforms a transform block according to an embodiment of the present disclosure.

FIG. 12 is a flowchart showing a method of inversely transforming a transform block by an image decoding device, according to an embodiment of the present disclosure.

Hereinafter, embodiments of the present invention will be described in detail with reference to the exemplary drawings. When adding reference numerals to components in each drawing, it should be noted that identical components are given the same reference numerals as much as possible even if they are shown in different drawings. Additionally, 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 example block diagram of a video encoding device that can implement the techniques of the present disclosure. Hereinafter, the video encoding device and its sub-configurations will be described with reference to the illustration in FIG. 1.

The image encoding device 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. It may be configured to include (160), an inverse transform unit (165), an adder (170), a loop filter unit (180), and a memory (190).

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

One image (video) consists of one or more sequences including a plurality of pictures. Each picture is divided into a plurality of regions and encoding is performed for each region. For example, one picture is divided into one or more tiles and/or slices. Here, one or more tiles can 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 encoded as the syntax of the CU, and information commonly applied to CUs included in one CTU is encoded as the syntax of the CTU. Additionally, information commonly applied to all blocks within one slice is encoded as the syntax of the slice header, and information applied to all blocks constituting one or more pictures is a picture parameter set (PPS) or picture parameter set. Encoded in the header. Furthermore, information commonly referenced by multiple pictures is encoded in a sequence parameter set (SPS). And, information commonly referenced by one or more SPSs is encoded in a video parameter set (VPS). Additionally, information commonly applied to one tile or tile group may be encoded as the 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 division unit 110 determines the size of the CTU. Information about the size of the CTU (CTU size) is encoded as SPS or PPS syntax and transmitted to the video decoding device.

The picture division unit 110 divides each picture constituting the image into a plurality of CTUs with a predetermined size and then recursively divides the CTUs using a tree structure. . The leaf node in the tree structure becomes the CU, the basic unit of encoding.

The tree structure is QuadTree (QT), in which the parent node is divided into four child nodes (or child nodes) of the same size, or BinaryTree, in which the parent node is divided into two child nodes. , BT), or a TernaryTree (TT) in which the parent node is divided into three child nodes in a 1:2:1 ratio, or a structure that mixes two or more of these QT structures, BT structures, and TT structures. 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 and referred to as MTT (Multiple-Type Tree).

Figure 2 is a diagram to explain a method of dividing a block using the QTBTTT structure.

As shown in Figure 2, the CTU can first be divided into a QT structure. Quadtree splitting can be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of the leaf node allowed in QT. The first flag (QT_split_flag) indicating whether each node of the QT structure is split into four nodes of the lower layer is encoded by the entropy encoder 155 and signaled to the image 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 the BT structure or the TT structure. In the BT structure and/or TT structure, there may be multiple division directions. For example, there may be two directions in which the block of the node is divided: horizontally and vertically. As shown in Figure 2, when MTT splitting begins, a second flag (mtt_split_flag) indicates whether the nodes have been split, and if split, an additional flag indicating the splitting direction (vertical or horizontal) and/or the splitting type (Binary). Or, a flag indicating Ternary) is encoded by the entropy encoding unit 155 and signaled to the video decoding device.

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

When QTBT is used as another example of a tree structure, there are two types: a type that horizontally splits the block of the node into two blocks of the same size (i.e., symmetric horizontal splitting) and a type that splits it vertically (i.e., symmetric vertical splitting). Branches may exist. A split flag (split_flag) indicating whether each node of the BT structure is divided into blocks of a lower layer and split type information indicating the type of division are encoded by the entropy encoder 155 and transmitted to the video decoding device. Meanwhile, there may be an additional type that divides the block of the corresponding node into two asymmetric blocks. The asymmetric form may include dividing the block of the corresponding node into two rectangular blocks with a size ratio of 1:3, or may include dividing the block of the corresponding node diagonally.

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

The prediction unit 120 predicts the 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. Typically, prediction of the current block is done using intra prediction techniques (using data from the picture containing the current block) or inter prediction techniques (using data from pictures coded before the picture containing the current block). It can be done. Inter prediction includes both one-way prediction and two-way prediction.

The intra prediction unit 122 predicts pixels within the current block using pixels (reference pixels) located around the current block within the current picture including the current block. There are multiple intra prediction modes depending on 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. The surrounding pixels and calculation formulas to be used are defined differently for each prediction mode.

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

The intra prediction unit 122 can determine the intra prediction mode to be used to encode the current block. In some examples, intra prediction unit 122 may encode the current block using multiple intra prediction modes and select an appropriate intra prediction mode to use from the tested modes. For example, the intra prediction unit 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. You can also select intra prediction mode.

The intra prediction unit 122 selects one intra prediction mode from among a plurality of intra prediction modes and predicts the current block using surrounding pixels (reference pixels) and an operation formula determined according to the selected intra prediction mode. Information about the selected intra prediction mode is encoded by the entropy encoding unit 155 and transmitted to the video decoding device.

The inter prediction unit 124 generates a prediction block for the current block using a motion compensation process. The inter prediction unit 124 searches for a block most similar to the current block in a reference picture that has been encoded and decoded before the current picture, and generates a prediction block for the current block using the searched block. Then, a motion vector (MV) corresponding to the displacement between the current block in the current picture and the prediction block in the reference picture is generated. Typically, motion estimation is performed on the 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 information about the reference picture and information about the motion vector used to predict the current block is encoded by the entropy encoding unit 155 and transmitted to the video decoding device.

The inter prediction unit 124 may perform interpolation on a reference picture or reference block 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. If the process of searching for the block most similar to the current block is performed for the interpolated reference picture, the motion vector can be expressed with precision in decimal units rather than precision in integer samples. The precision or resolution of the motion vector may be set differently for each target area to be encoded, for example, slice, tile, CTU, CU, etc. When such adaptive motion vector resolution (AMVR) is applied, information about the motion vector resolution to be applied to each target area must be signaled for each target area. For example, if the target area is a CU, information about the motion vector resolution applied to each CU is signaled. Information about motion vector resolution may be information indicating the precision of a differential motion vector, which will be described later.

Meanwhile, the inter prediction unit 124 may perform inter prediction using bi-prediction. In the case of bidirectional prediction, two reference pictures and two motion vectors indicating the positions of blocks most similar to the current block within each reference picture are used. The inter prediction unit 124 selects the first reference picture and the 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. Create a first reference block and a second reference block. Then, the first reference block and the second reference block are averaged or weighted to generate a prediction block for the current block. Then, motion information including information about the two reference pictures used to predict the current block and information about the two motion vectors is transmitted to the entropy encoding unit 155. Here, reference picture list 0 may be composed of pictures before the current picture in display order among the restored pictures, and reference picture list 1 may be composed of pictures after the current picture in display order among the restored pictures. there is. However, it is not necessarily limited to this, and in terms of display order, relief pictures after the current picture may be additionally included in reference picture list 0, and conversely, relief pictures before the current picture may be additionally included in reference picture list 1. may be included.

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

For example, if the reference picture and motion vector of the current block are the same as the reference picture and motion vector of the neighboring block, the motion information of the current block can be transmitted to the video decoding device by encoding information that can identify 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.

As shown in FIG. 4, the surrounding blocks for deriving merge candidates include the left block (A0), bottom left block (A1), top block (B0), and top right block (B1) adjacent to the current block in the current picture. ), and all or part of the upper left block (B2) can be used. Additionally, a block located within a reference picture (which may be the same or different from the reference picture used to predict the current block) rather than the current picture where the current block is located may be used as a merge candidate. For example, a block co-located with the current block within the reference picture or blocks adjacent to the co-located block may be additionally used as merge candidates. If the number of merge candidates selected by the method described above is less than the preset number, the 0 vector is added to the merge candidates.

The inter prediction unit 124 uses these neighboring blocks to construct a merge list including a predetermined number of merge candidates. A merge candidate to be used as motion information of the current block is selected from among the merge candidates included in the merge list, and merge index information is generated to identify the selected candidate. The generated merge index information is encoded by the entropy encoding unit 155 and transmitted to the video decoding device.

Merge skip mode is a special case of merge mode. After performing quantization, when all transformation coefficients for entropy encoding are close to zero, only peripheral block selection information is transmitted without transmitting residual signals. By using merge skip mode, relatively high coding efficiency can be achieved in low-motion images, still images, screen content images, etc.

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

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

In AMVP mode, the inter prediction unit 124 uses neighboring blocks of the current block to derive predicted motion vector candidates for the motion vector of the current block. The surrounding blocks used to derive predicted motion vector candidates include the left block (A0), bottom left block (A1), top block (B0), and top right block adjacent to the current block in the current picture shown in FIG. All or part of B1), and the upper left block (B2) can be used. Additionally, a block located within a reference picture (which may be the same or different from the reference picture used to predict the current block) rather than the current picture where the current block is located will be used as a surrounding block used to derive prediction motion vector candidates. It may be possible. For example, a collocated block located at the same location as the current block within the reference picture or blocks adjacent to the block at the same location may be used. If the number of motion vector candidates is less than the preset number by the method described above, the 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, the predicted motion vector is subtracted from the motion vector of the current block to calculate the differential motion vector.

The predicted motion vector can be obtained by applying a predefined function (eg, median, average value calculation, etc.) to the predicted motion vector candidates. In this case, the video decoding device also knows the predefined function. In addition, since the neighboring blocks used to derive predicted motion vector candidates are blocks for which encoding and decoding have already been completed, the video decoding device also already knows the motion vectors of the neighboring blocks. Therefore, the video encoding device does not need to encode information to identify the predicted motion vector candidate. Therefore, in this case, information about the differential motion vector and information about the reference picture 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, information for identifying the selected prediction motion vector candidate is additionally encoded, along with information about the differential motion vector and information about the reference picture used to predict the current block.

The subtractor 130 generates a residual block by subtracting the prediction block generated by the intra prediction unit 122 or the inter prediction unit 124 from the current block.

The transform unit 140 converts the residual signals in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The conversion unit 140 may convert the residual signals in the residual block by using the entire size of the residual block as a conversion unit, or divide the residual block into a plurality of subblocks and perform conversion by using the subblocks as a conversion unit. You may. Alternatively, the residual signals can be converted by dividing them into two subblocks, a transform area and a non-transformation region, and using only the transform region subblock as a transform unit. Here, the transformation area subblock may be one of two rectangular blocks with a size ratio of 1:1 based on the horizontal axis (or vertical axis). In this case, a flag indicating that only the subblock has been converted (cu_sbt_flag), directional (vertical/horizontal) information (cu_sbt_horizontal_flag), and/or position 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 area 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) that distinguishes the corresponding division is additionally encoded by the entropy encoding unit 155 to encode the image. Signaled to the decryption device.

Meanwhile, the transformation unit 140 can separately perform transformation on the residual block in the horizontal and vertical directions. For transformation, various types of transformation functions or transformation matrices can be used. For example, a pair of transformation functions for horizontal transformation and vertical transformation can be defined as MTS (Multiple Transform Set). The conversion unit 140 may select a conversion function pair with the best conversion efficiency among MTSs and convert the residual blocks in the horizontal and vertical directions, respectively. Information (mts_idx) about the transformation function pair selected from the MTS is encoded by the entropy encoder 155 and signaled to the video decoding device.

The quantization unit 145 quantizes the 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 residual block related to a certain block or frame without conversion. The quantization unit 145 may apply different quantization coefficients (scaling values) depending on the positions of the transform coefficients within the transform block. The quantization matrix applied to the quantized transform coefficients arranged in two dimensions may be encoded and signaled to the video decoding device.

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

The rearrangement unit 150 can change a two-dimensional coefficient array into a one-dimensional coefficient sequence using coefficient scanning. For example, the realignment unit 150 can scan from DC coefficients to coefficients in the high frequency region using zig-zag scan or diagonal scan to output a one-dimensional coefficient sequence. . Depending on the size of the transformation unit and the intra prediction mode, a vertical scan that scans a two-dimensional coefficient array in the column direction or a horizontal scan that scans the two-dimensional block-type coefficients in the row direction may be used instead of the zig-zag scan. That is, the scan method to be used among zig-zag scan, diagonal scan, vertical scan, and horizontal scan may be determined depending on the size of the transformation unit and the intra prediction mode.

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

In addition, the entropy encoder 155 encodes information such as CTU size, CU split flag, QT split flag, MTT split type, and MTT split direction related to block splitting, so that the video decoding device can encode blocks in the same way as the video coding device. Allow it to be divided. In addition, the entropy encoding unit 155 encodes information about the prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and generates intra prediction information (i.e., intra prediction) according to the prediction type. Information about the mode) or inter prediction information (coding mode of motion information (merge mode or AMVP mode), merge index in case of merge mode, information on reference picture index and differential motion vector in case of AMVP mode) is encoded. Additionally, the entropy encoding unit 155 encodes information related to quantization, that is, information about quantization parameters and information about the quantization matrix.

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 restores the residual block by converting the transform coefficients output from the inverse quantization unit 160 from the frequency domain to the spatial domain.

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

The loop filter unit 180 restores pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. that occur due to block-based prediction and transformation/quantization. Perform filtering on them. The loop 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. there is.

The deblocking filter 182 filters the boundaries between restored blocks to remove blocking artifacts caused by block-level 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 differences between restored pixels and original pixels caused by lossy coding. The SAO filter 184 improves not only subjective image quality but also coding efficiency by applying an offset in units of CTU. In comparison, the ALF 186 performs filtering on a block basis, distinguishing the edge and degree of change of the block and applying different filters to compensate for distortion. Information about filter coefficients to be used in ALF may be encoded and signaled to a video decoding device.

The restored block filtered through the deblocking filter 182, SAO filter 184, and 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.

The video encoding device can store the bitstream of the encoded video data in a non-transitory recording medium or transmit it to the video decoding device using a communication network.

Figure 5 is an example block diagram of a video decoding device that can implement the techniques of the present disclosure. Hereinafter, the video decoding device and its sub-configurations will be described with reference to FIG. 5.

The image decoding device 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).

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

The entropy decoder 510 decodes the bitstream generated by the video encoding device, extracts information related to block division, determines the current block to be decoded, and provides prediction information and residual signals needed to restore the current block. Extract information, etc.

The entropy decoder 510 extracts information about the CTU size from a Sequence Parameter Set (SPS) or Picture Parameter Set (PPS), determines the size of the CTU, and divides the picture into CTUs of the determined size. Then, the CTU is determined as the highest layer of the tree structure, that is, the root node, and the CTU is divided using the tree structure by extracting the division information for the CTU.

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

As another example, when splitting a CTU using the QTBTTT structure, first extract the CU split flag (split_cu_flag) indicating whether to split the CU, and if the corresponding block is split, extract the first flag (QT_split_flag). It may be possible. During the division process, each node may undergo 0 or more repetitive MTT divisions after 0 or more repetitive QT divisions. For example, MTT division may occur immediately in the CTU, or conversely, only multiple QT divisions may occur.

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

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

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

The reordering unit 515 re-organizes the sequence of one-dimensional quantized transform coefficients entropy decoded in the entropy decoding unit 510 into a two-dimensional coefficient array (i.e., in reverse order of the coefficient scanning order performed by the image encoding device). block).

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

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

In addition, when the inverse transformation unit 530 inversely transforms only a partial area (subblock) of the transformation block, a flag (cu_sbt_flag) indicating that only the subblock of the transformation block has been transformed, and directionality (vertical/horizontal) information of the subblock (cu_sbt_horizontal_flag) ) and/or extracting the position information (cu_sbt_pos_flag) of the subblock, and inversely transforming the transformation coefficients of the corresponding subblock from the frequency domain to the spatial domain to restore the residual signals, and for the area that has not been inversely transformed, the residual signals are set to “0”. By filling in the values, the final residual block for the current block is created.

In addition, when MTS is applied, the inverse transform unit 530 determines a transformation function or transformation matrix to be applied in the horizontal and vertical directions, respectively, using the MTS information (mts_idx) signaled from the video encoding device, and uses the determined transformation function. Inverse transformation is performed on the transformation coefficients in the transformation 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 elements for the intra prediction mode extracted from the entropy decoder 510, and provides a reference around the current block according to the intra prediction mode. Predict the current block using pixels.

The inter prediction unit 544 uses the syntax elements for the inter prediction mode extracted from the entropy decoder 510 to determine the motion vector of the current block and the reference picture to which the motion vector refers, and uses the motion vector and the reference picture to determine the motion vector of the current block. Use it to predict the current block.

The adder 550 restores the current block by adding the residual block output from the inverse transform unit 530 and the prediction block output from the inter prediction unit 544 or intra prediction unit 542. Pixels in the restored 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, a SAO filter 564, and an ALF 566 as an in-loop filter. The deblocking filter 562 performs deblocking filtering on the boundaries between restored blocks to remove blocking artifacts that occur due to block-level 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 pixels and the original pixels caused by lossy coding. do. The filter coefficient of ALF is determined using information about the filter coefficient decoded from the non-stream.

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

This embodiment relates to encoding and decoding of images (videos) as described above. More specifically, the secondary transformation region and transformation kernel are selected by reflecting the characteristics of the primary transformation kernel, and a non-separable secondary transformation is applied to the transformation coefficients in the low-frequency region using the selected transformation kernel. Provides video coding methods and devices.

The following embodiments may be performed by the converter 140 and the inverse converter 165 within a video encoding device. Additionally, it may be performed by the inverse transform unit 530 within a video decoding device.

The video encoding device may generate signaling information related to this embodiment in terms of bit rate distortion optimization when encoding the current block. The video encoding device can encode the video using the entropy encoding unit 155 and then transmit it to the video decoding device. The video decoding device can decode signaling information related to decoding the current block from the bitstream using the entropy decoding unit 510.

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

Additionally, the fact that the value of one flag is true indicates that the flag is set to 1. Additionally, the value of one flag being false indicates a case where the flag is set to 0.

I. 변환 기술 - 1차 변환 기술I. Transformation technology - primary transformation technology

As described above, for efficient video compression, quantization or scaling may be additionally applied to residual signals remaining after prediction according to various prediction techniques. At this time, based on the importance of the cognitive visual information inherent in the residual signals, a transform technique can be applied to gather the residual signals to one side according to the frequency component, and then scaling can be performed. However, for non-natural signals, such as screen contents, these frequency-based conversion techniques may be ineffective. In this case, the conversion technique may be omitted and only scaling may be performed, or encoding/decoding may be performed without applying scaling.

When transformation is applied in HEVC, DCT-II is used as a transform kernel (hereinafter, used interchangeably with transform type) to transform residual signals. However, in order to apply a more appropriate transformation technique according to the diversity of residual signal characteristics, multiple transform selection (MTS) can be used. The MTS determines one or two optimal types among multiple transformation types and then transforms the block according to the determined transformation type. For example, in VVC, as shown in Table 1, two other transformation types, DCT-VIII and DST-VII, are added in addition to DCT-II, allowing residual signals to be converted in various ways.

Here, basis functions constitute a transformation matrix that defines each transformation type. Hereinafter, DCT-II, DCT-VIII and DST-VII are used interchangeably with DCT2, DCT8 and DST7, respectively.

Meanwhile, the flag that determines whether to use MTS can be controlled on a block basis. Additionally, use of MTS may be controlled using an activation flag at the higher SPS level.

When activating MTS in SPS, a CU level flag indicating whether MTS is applied may be displayed. Here, MTS can be applied to the luma component. If both the width and height of the TB are less than or equal to 32 pixels, and the Coded Block Flag (CBF) indicating whether there is a non-zero value among the conversion coefficient levels is true, a CU level flag may be expressed.

When the CU level flag is 0, DCT2 is used as kernels in both horizontal and vertical directions. On the other hand, if the CU level flag is not 0, MTS is applied. MTS can be used in two ways: explicit MTS and implicit MTS.

In explicit MTS, the kernel used for TB is transmitted explicitly. In general, the index of the conversion kernel can be transmitted. For example, mts_idx, a kernel index, may be defined as shown in Table 2.

Here, trTypeHor and trTypeVer represent the horizontal transformation type and the vertical transformation type. Additionally, 0 represents DCT2, 1 represents DST7, and 2 represents DCT8.

Meanwhile, in implicit MTS, for example, in the case of an intra block, the conversion type can be implicitly determined even if the MTS is not explicitly signaled. In VVC, the horizontal and vertical transformation types can be implicitly determined, as shown in Equation 1.

Here, nTbW and nTbH represent the horizontal and vertical lengths of the conversion block, respectively.

As an example, when a specific encoding technology is applied, explicit MTS or implicit MTS may be applied. For example, in the case of Matrix-weighted Intra Prediction (MIP), explicit intra MTS can be used. In the case of ISP (Intra Sub-Partitions) mode, implicit inter MTS is used, and DST7 or DCT2 is used as the conversion type.

Meanwhile, if the conversion block includes at least one non-DC coefficient, mts_idx is signaled. That is, if the position of the last significant coefficient according to the scanning order is greater than 0, mts_idx is signaled. On the other hand, if the conversion block includes only one non-DC coefficient, signaling of mts_idx is omitted, mts_idx = 0 is derived, and DCT2 is applied as the conversion kernel.

The first bin of mts_idx that is signaled indicates whether mts_idx is greater than 0. If mts_idx is greater than 0 (i.e., mts_idx indicates one of 1 to 4), a 2-bit fixed-length code is additionally signaled to indicate the signaled mts_idx among the 4 candidates.

Meanwhile, in the next-generation technology, ECM (Enhanced Compression Model) software, the number and type of MTS kernels are increased, and DST7, DCT8, DCT5, DST4, DST1, and identity transform are added.

II. LFNST(Low-frequency Non-separable Transform) II. Low-frequency Non-separable Transform (LFNST)

LFNST technology performs secondary transform on the low-frequency region among the transform coefficients generated according to the primary transform of the transform block (TU, Transform Unit) during intra prediction. do. In terms of encoding, the LFNST technology performs secondary transformation on L low-frequency primary transformation coefficients among W × H primary transformation coefficients to generate K (where K ≤ L) secondary transformation coefficients. Here, the size of the transformation kernel of LFNST is L×K. That is, the LFNST technology expresses L low-frequency first-order transform coefficients among W Afterwards, the LFNST technology expresses the 1×K vector as a two-dimensional array in the low-frequency region for subsequent processes such as quantization.

Compared to the first-order transformation that applies separate transformation kernels in the horizontal and vertical directions, the LFNST technology performs a non-separable transform that transforms a one-dimensional vector.

Meanwhile, the type of transformation kernel may be determined according to the intra prediction mode of the current TU, the size of the TU, and the LFNST index (lfnst_idx). For example, the transformation kernel set can be determined as shown in Table 2 according to the intra prediction mode (IntraPredMode) of the current TU.

Here, the intra prediction mode (IntraPredMode) follows the example of FIG. 3B. Additionally, lfnstTrSetIdx is an index indicating the kernel set. In Table 3, IntraPredMode of 81, 82, and 83 indicates CCLM (Cross-component Linear Model) prediction modes.

Two types of kernels are defined for each kernel set (lfnstTrSetIdx). Which of the two types of kernels to select can be indicated by the LFNST index. If the LFNST index is 0, it means that LFNST is not performed, and if the LFNST index is 1 or 2, a different LFNST kernel within the same kernel set is applied. Since there is one more kernel set depending on the size of the TU, there are a total of 4×2×2 = 16 transformation kernels. The kernel size of LFNST is defined as 16×16 and 16×48. Additionally, the size of the kernel can be adjusted according to the size of the TU, as shown in Table 4.

Meanwhile, when the DCT2/DCT2 transformation kernel is applied as the first transformation to the intra predicted TU, the LFNST technology can be applied as the second transformation.

The following embodiments are described with a focus on a video decoding device, but may also be implemented in the same or similar manner in a video encoding device.

III. 본 개시에 따른 실시예들III. Embodiments according to the present disclosure

FIG. 6 is a block diagram illustrating in detail a portion of a video decoding device according to an embodiment of the present disclosure.

The video decoding device according to this embodiment determines a prediction and transformation unit, performs prediction and inverse transformation on the current block corresponding to the determined unit using the determined prediction technology and prediction mode, and finally restores the current block to the block. can be created. What is illustrated in FIG. 6 may be performed by the inverse transform unit 530, prediction unit 540, and adder 550 of the image decoding device. Meanwhile, the same operations as illustrated in FIG. 6 may be performed by the inverse transform unit 165, picture division unit 110, prediction unit 120, and adder 170 of the image encoding device. At this time, the video decoding device uses encoding information parsed from the bitstream, but the video encoding device may use encoding information set from a higher level in terms of minimizing bit rate distortion. Hereinafter, for convenience, this embodiment will be described focusing on the video decoding device.

As shown in the example of FIG. 5, the prediction unit 540 includes an intra prediction unit 542 and an inter prediction unit 544 depending on the prediction technology. However, as illustrated in FIG. 6, the prediction unit 540 is in prediction mode. It may include a decision unit 602 and a prediction performance unit 604.

If the color format of the input video is YUV format (YUV420, YUV411, YUV422, YUV444, etc.), the video decoding device can predict and restore the luma component and then predict and restore the chroma component. That is, the luma component and chroma component can be sequentially restored by the components illustrated in FIG. 6. Here, in the case of YUV format, the color format represents the correspondence relationship between luma component pixels and chroma component pixels.

The prediction mode determination unit 602 determines a prediction technology (e.g., intra prediction, inter prediction, or IBC (Intra Block Copy) mode, palette mode, etc.) for the current block. Additionally, the prediction mode determination unit 602 determines a detailed prediction mode for the prediction technology. The prediction performing unit 604 generates a prediction block of the current block according to the determined prediction technology and prediction mode.

The inverse transform unit 530 inversely transforms TUs expressed as inverse quantization signals to generate residual signals.

The adder 550 generates a restored block by adding the prediction block and the residual signals. The restored block is stored in memory and can later be used to predict other blocks.

Hereinafter, TU is used interchangeably with the conversion block.

As shown in the example of FIG. 6, the inverse transform unit 530 includes a first-order inverse transform kernel decision unit 610, a second-order inverse transform performance decision unit 612, a second-order inverse transform kernel decision unit 614, and a second-order inverse transform unit 616. And it may include all or part of the first inverse transform unit 618. The inverse transform unit 530 uses these components to perform adaptive non-separable secondary inverse transformation on the transformation coefficients in the low-frequency region to the primary transformation kernel.

As described above, the first-order inverse transform kernel determination unit 610 may determine the first-order inverse transform kernel in the vertical and horizontal directions based on the prediction technology and prediction mode of the current block, the size of the current transform block, mts_idx, etc.

The first-order inversion kernel can be explicitly determined according to the parsed mts_idx. Alternatively, the first inverse transformation kernel may be implicitly determined based on whether subblock transformation is performed, the size of the TU, etc. At this time, if the current block is predicted according to the intra prediction mode and the first inverse transformation kernel in the vertical and horizontal directions is explicitly determined according to the parsed mts_idx, the kernel indicated by mts_idx is based on the size of the current TU and the intra prediction mode. The type can be determined. In other words, for the combination of possible TU sizes (4 After definition, the first-order inverse transformation kernel determination unit 610 can determine the final inverse transformation kernel by parsing mts_idx.

At this time, the intra prediction mode (IntraPredMode) can be divided into A set of modes centered on the upper left diagonal mode (mode 34). When A=4, the mode set is expressed as Table 5 based on the example in Figure 3b.

Matrix-based prediction modes can be included in the non-directional mode set (mode set 0) or divided into a separate set. Additionally, inter-component prediction using the relationship between chroma and luma components When a chroma block is predicted by mode, the first-order inverse transform kernel decision unit 610 maps the intra prediction mode at the center of the corresponding luma block to the intra prediction mode of the current chroma block. The first-order inverse transform kernel determination unit 610 may set a mode set from the mapped intra prediction modes based on Table 5 and then use the mode set to determine the first-order inverse transform kernel.

If the current block is predicted based on inter prediction, the transformation kernel indicated by mts_idx according to the TU size may be determined according to the agreement between the video encoding device and the video decoding device. Alternatively, the kernel indicated by mts_idx may be fixed.

For example, if the current block is predicted according to CCIP (Combined Inter-intra Prediction) mode, the first inverse transformation kernel of the current TU is determined according to the same method as the method for determining the kernel of the intra prediction block according to the intra prediction mode and TU size. can be decided. Here, CIIP is an example of modes that generate the final prediction block of the current block by weighted summing a block predicted based on inter prediction and a block predicted based on intra prediction. Alternatively, if the current block is predicted according to CCIP mode, the first inverse transformation kernel of the current TU can be determined in the same way as inter prediction.

Depending on the embodiment, when the weight of the intra prediction block is greater than the weight of the inter prediction block, the first inverse transformation kernel of the current TU may be determined in the same manner as intra prediction. At this time, based on the transform block size and intra prediction mode, some kernels among DCT-N (N=1, .... 8), DST-N, and IDT (Identity Transform) are converted into first-order inverse transform kernels in the vertical and horizontal directions. Each can be selected. For example, DCT-2, DCT-5, DCT-8, DST-1, DST-4, DST-7, and IDT kernels can be used as the first inverse transformation kernel.

The secondary inverse transformation performance decision unit 612 determines whether to apply the secondary inverse transformation by parsing the 1-bit flag (lfnst_flag, hereinafter referred to as 'LFNST flag') or the LFNST index (lfnst_idx) indicating whether the secondary inverse transformation is applied. You can. At this time, when parsing the LFNST index, it may be promised in advance that a specific index value (for example, lfnst_idx=0) indicates not to apply the secondary inverse transformation.

As an example, whether to apply the second-order inverse transform may be implicitly determined depending on the type of the first-order transform kernel in the vertical and horizontal directions. If a second-order inversion is applied for a particular first-order transformation kernel, the LFNST flag or Transmission of the LFNST index may be omitted. Additionally, secondary inverse transformation may not be implicitly applied to the remaining transformation kernels except for a specific primary transformation kernel.

When it is decided to perform the secondary inversion, the secondary inverse transformation kernel determination unit 614 may determine the secondary inverse transformation kernel based on the size of the current TU, intra prediction mode, and the first inversion kernel.

As an example, the size of the second-order inversion kernel may depend on a set of kernel sizes (S) with C different sizes depending on the size of the TU.

For example, the size of the kernel may be determined according to the following conditions (size of TU), and the criteria for dividing sets may vary depending on the embodiment.

S ₀ : 4×N/N×4 (N≥4), S ₁ : 8×N/N×8 (N≥8), S ₂ : M×N (M,N≥16)

S ₀ : 4×4, S ₁ : 4×N/N×4 (N≥8), S ₂ : 8×8, S ₃ : M×N (M,N≥8, excluding M=N=8)

The inverse transformation kernel of each set has the size of p _i × q _i (0≤i<C). At this time, p _i refers to the size of the output primary transformation coefficient, q _i refers to the size of the input secondary transformation coefficient, and p _i ≥q _i may be.

As described above, for the kernel size set (S) determined according to the TU size, A (intra prediction mode set) × K (first transformation kernel set) based on the intra prediction mode and first transformation kernel type of the current block = There may be T sets of second-order inversion kernels (or ‘kernel sets’).

As an example in Table 5, the intra prediction mode can be divided into A set of modes centered on the diagonal mode from the top left. Matrix-based prediction modes can be included in the non-directional mode set (mode set 0) or divided into a separate set. In addition, when a chroma block is predicted according to inter-component prediction (e.g., CCLM mode) that performs prediction using the relationship between the chroma component and the luma component, the second-order inverse transform kernel decision unit 614 determines the center position of the corresponding luma block. Map the intra prediction mode of to the intra prediction mode of the current chroma block. The secondary inverse transform kernel decision unit 614 may set a mode set from the mapped intra prediction modes based on Table 5 and then use the mode set to determine the secondary inverse transform kernel.

Depending on the embodiment, K {vertical, horizontal} kernel sets may be classified according to the type of first transformation/inverse transformation kernel. Kernel sets can be classified as follows, and the criteria for classifying the sets may vary depending on the embodiment.

S ₀ : {DCT-2, DCT-2}, S ₁ : {DCT-2, IDT}, S ₂ : {IDT, DCT-2}

S ₀ : {DCT-2, DCT-2}, S ₁ : Other kernels

S ₀ : {DCT-2, DCT-2}, S ₁ : {DST-7, DST-7}, S ₂ : Other kernels

S ₀ : {DCT-2, DCT-2}, S ₁ : {DST-N, DCT-M}, {DCT-M, DST-N},

S ₂ : {DCT-N, IDT}, {IDT, DCT-N}, S ₃ : Other kernels,

N, M={1 and/or 2 and/or, ..., 8}

S ₀ : {DCT-2, DCT-2}, S ₁ : {DST-7, DST-7}, S ₂ : {DCT-N, IDT}, {IDT, DCT-N},

S ₃ : {DST-N, IDT}, {IDT, DST-N}, S ₄ : Other kernels,

N={1 and/or 2 and/or, ..., 8}

Here, the other kernels may be a combination of first-order transformation kernels that are not included in the previous set among the first-order kernel combinations on which secondary transformation/inverse transformation is performed.

The set of C×T secondary kernels determined based on the TU size, intra prediction mode, and first-order inversion kernel may include one or multiple second-order inversion kernels. When multiple kernels are included, the secondary inverse transformation kernel determination unit 614 may parse the LFNST index and determine the secondary inverse transformation kernel from the current kernel set according to the parsed LFNST index. Therefore, as described above, when the second-order inverse transformation is applied to a specific first-order transformation kernel, or when the kernel set includes a plurality of second-order inversion kernels, transmission of the LFNST index cannot be omitted.

At this time, if the LFNST index is 0, it means that the secondary inverse transformation is not performed, and if lfnst_idx≥1, the secondary inverse transformation may be applied based on the LFNST index.

Meanwhile, the entropy decoder 510 restores the quantized secondary transform coefficient when the secondary inverse transform is applied. Alternatively, the entropy decoder 510 may restore the quantized first-order transform coefficient when the second-order inverse transform is not applied. At this time, both the secondary transformation coefficient and the primary transformation coefficient are data in the form of a two-dimensional matrix.

The second-order inverse transform unit 616 receives the inverse-quantized second-order transform coefficient and applies the second-order inverse transform kernel to restore the first-order transform coefficient.

Based on the size and type of the secondary inversion kernel, the secondary inversion unit 616 packs the restored q secondary transformation coefficients into one dimension according to the scanning order to create a secondary transformation coefficient vector (

) can be created. Here, the scanning order may be agreed in advance between the video encoding device and the video decoding device. For example, if the size q of the secondary inverse transform coefficient is 8 (FIG. 7a), M×N (FIG. 7b), or 48 (FIG. 7c), the secondary transform coefficient vector is can be created. Depending on the embodiment, scanning for generating the secondary transformation coefficient vector may be performed in reverse order.

In the example of Figure 7b, when the size of the secondary transformation coefficient vector is 16, M=N=4, and when the size of the secondary transformation coefficient vector is 32, M=8, N=4 or M=4, N= It could be 8.

In the examples of FIGS. 7A to 7C, W _tb represents the width of the conversion block, and H _tb represents the height of the conversion block.

The secondary inversion unit 616 performs inverse transformation using a matrix product between a secondary transformation coefficient vector of size q and a secondary inversion kernel F of size p×q. The secondary inverse transform unit 616 performs the secondary inverse transformation as shown in Equation 2 to obtain a primary transformation coefficient vector of size p (

) can be created.

The secondary inverse transform unit 616 may generate a two-dimensional matrix according to the size of the current TU from the first transform coefficient vector generated according to the secondary inverse transformation. As shown in the examples of FIGS. 8A and 8B, based on the size p of the first transform coefficient vector, the first transform coefficients may be assigned to the upper left corner of the TU in scanning order. Depending on the order in which the image encoding device scans the primary transform coefficients for secondary transformation, the scanning order used by the secondary inverse transform unit 616 may vary.

At this time, the video decoding device can induce the transform coefficient of the remaining TU area in which the first transform coefficient does not exist to be 0.

As an example, when IDT is applied as a first-order transformation kernel in one of the vertical or horizontal directions, the second-order inverse transform unit 616 may allocate first-order transformation coefficients by considering the direction in which the IDT is applied. That is, the secondary inverse transform unit 616 can allocate p primary transform coefficients to the region illustrated in FIG. 9 according to the scanning order using the primary transform coefficient vector. Here, the scanning order can be set in advance according to an agreement between the video encoding device and the video decoding device. When IDT transformation is applied in the horizontal direction, p=W _tb × L is satisfied, and when IDT transformation is performed in the vertical direction, p=L × H _tb is satisfied. At this time, the size of p may be defined according to the secondary inverse transformation kernel determined by the secondary inverse transformation kernel decision unit 614. In the example of FIG. 9, the video decoding device may induce the transform coefficient of the remaining TU area in which p primary transform coefficients are not present to be 0.

As another example, when the primary transformation kernels in the vertical and horizontal directions are different, the scanning order for allocating the primary transformation coefficient vector within the TU may be determined based on the types of kernels in the vertical and horizontal directions. When applying the same secondary transformation kernel to a block to which the B kernel is applied in the vertical direction and the A kernel to the horizontal direction, and to the block to which the A kernel is applied in the vertical direction and the B kernel to the horizontal direction, the secondary inverse transform unit 616 As in the example of FIG. 10, the primary transform coefficient vector can be mapped to the transform coefficients of the TU block by first scanning a specific kernel direction.

The first inversion unit 618 uses the first inversion kernel determined in the first inversion kernel decision unit 610 to vertically and horizontally transform the first transformation coefficient assigned to the pixel value of the TU block according to the second inversion. A first-order inverse transformation can be performed in either direction.

Hereinafter, using the illustrations of FIGS. 11 and 12, a method for converting and inversely converting the conversion block of the current block will be described.

The video encoding device acquires residual signals for the transform block of the current block (S1100).

The video encoding device determines the first transform kernel in the vertical and horizontal directions for the transform block (S1102).

The image encoding device determines the secondary transform kernel of the transform block based on the size of the transform block, the intra prediction mode of the current block, and the first transform kernel (S1104).

The video encoding device selects a mode set that includes the intra prediction mode of the current block among preset mode sets. The video encoding device selects a primary kernel set including a primary transformation kernel from among preset primary kernel sets. The video encoding device selects a kernel size set that includes the size of the transform block from among preset kernel size sets. The video encoding device selects a secondary kernel set according to the selected mode set, the selected primary kernel set, and the selected kernel size set.

When the secondary kernel set includes one secondary kernel, the video encoding device sets the secondary transformation kernel to one secondary kernel. When the secondary kernel set includes multiple secondary kernels, the video encoding device may set the secondary transformation kernel to one of the multiple secondary kernels.

The video encoding device performs primary transformation by applying a primary transformation kernel to the residual signals to generate primary transformation coefficients (S1106).

The video encoding device performs secondary transformation by applying a secondary transformation kernel to the primary transformation coefficients to generate secondary transformation coefficients (S1108).

Based on the size and type of the secondary transform kernel, the image encoding device generates a primary transform coefficient vector by one-dimensionally packing some of the primary transform coefficients according to a preset scanning order. The video encoding device generates a secondary transform coefficient vector by performing matrix multiplication between the primary transform coefficient vector and the secondary transform kernel. The video encoding device can generate secondary transform coefficients by allocating the secondary transform coefficient vector to the upper left of the transform block according to a preset scanning order.

The video encoding device determines the LFNST index based on the first and second transform coefficients (S1110). Here, the LFNST index indicates whether secondary transformation is applied.

In terms of bit rate-distortion optimization, the video encoding device can determine the value of the LFNST index. For example, when the first-order transform coefficients are optimal, the video encoding device can set the LFNST index to 0. On the other hand, when the secondary transformation coefficients are optimal, the LFNST index can be set according to the number of secondary kernels included in the above-described secondary kernel set. If the secondary kernel set includes one secondary kernel, the video encoding device may set the LFNST index to 1. When the secondary kernel set includes multiple secondary kernels, the video encoding device may set the LFNST index to indicate one of the multiple secondary kernels.

The video encoding device encodes the LFNST index (S1112).

The video encoding device checks the LFNST index (S1114).

If the LFNST index is 0 (Yes in S1114), the video encoding device encodes the first transform coefficients (S1116). An image encoding device can generate a bitstream of first-order transform coefficients by quantizing and entropy-encoding the first-order transform coefficients.

On the other hand, if the LFNST index is greater than 0 (No in S1114), the video encoding device encodes the secondary transform coefficients (S1118). An image encoding device can generate a bitstream of secondary transform coefficients by quantizing and entropy coding the secondary transform coefficients.

The video decoding device determines the first inverse transform kernel in the vertical and horizontal directions for the transform block of the current block (S1200).

The video decoding device decodes the LFNST index from the bitstream (S1202). Here, the LFNST index indicates whether the second-order inverse transformation is applied.

The video decoding device checks the LFNST index (S1204).

If the LFNST index is 0 (Yes in S1204), the video decoding device performs the following steps (S1206 and S1208).

The video decoding device obtains the dequantized first transform coefficients for the transform block (S1206).

The video decoding device performs first-order inverse transformation by applying a first-order inverse transformation kernel to the first transformation coefficients to generate residual signals (S1208).

If the LFNST index is greater than 0 (No in S1204), the video decoding device performs the following steps (S1220 to S1224, S1208).

The video decoding device acquires dequantized secondary transform coefficients for the transform block (S1220).

The image decoding device determines the second inverse transform kernel of the transform block based on the size of the transform block, the intra prediction mode of the current block, and the first inverse transform kernel (S1222).

The video decoding device selects a mode set that includes the intra prediction mode of the current block among preset mode sets. The video decoding device selects the first kernel set including the first inverse transform kernel from among the preset first kernel sets. The video decoding device selects a kernel size set that includes the size of the transform block from among the preset kernel size sets. The video decoding device selects a secondary kernel set according to the selected mode set, the selected primary kernel set, and the selected kernel size set.

When the secondary kernel set includes one secondary kernel, the video decoding device sets the secondary transformation kernel to one secondary kernel. At this time, the LFNST index can be decoded to 1. When the secondary kernel set includes a plurality of secondary kernels, the video decoding device can set the secondary transformation kernel to the secondary kernel indicated by the LFNST index.

The video decoding device performs secondary inverse transformation by applying a secondary inverse transformation kernel to the secondary transformation coefficients to generate primary transformation coefficients (S1224).

Based on the size and type of the secondary inverse transform kernel, the image decoding device generates a secondary transform coefficient vector by packing some of the secondary transform coefficients into one dimension according to a preset scanning order. The image decoding device generates a first transform coefficient vector by performing matrix multiplication between the second transform coefficient vector and the second inverse transform kernel. The video decoding device can generate primary transform coefficients by allocating the primary transform coefficient vector to the upper left corner of the transform block according to a preset scanning order.

Afterwards, the video decoding device can generate a restored block of the current block by adding the residual signals and the prediction block of the current block.

In the flowchart/timing diagram of this specification, each process is described as being executed sequentially, but this is merely an illustrative explanation of the technical idea of an embodiment of the present disclosure. In other words, a person skilled in the art to which an embodiment of the present disclosure pertains may change the order described in the flowchart/timing diagram and execute one of the processes without departing from the essential characteristics of the embodiment of the present disclosure. Since the above processes can be applied in various modifications and variations by executing them in parallel, the flowchart/timing diagram is not limited to a time series order.

It should be understood from the above description that the example embodiments may be implemented in many different ways. The 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 herein are labeled as "...units" to particularly emphasize their implementation independence.

Meanwhile, various functions or methods described in this embodiment may be implemented with 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 that store data in a form readable by a computer system. For example, non-transitory recording media include storage media such as erasable programmable read only memory (EPROM), flash drives, optical drives, magnetic hard drives, and solid state drives (SSD).

The above description is merely an illustrative explanation of the technical idea of the present embodiment, and those skilled in the art will be able to make various modifications and variations without departing from the essential characteristics of the present embodiment. Accordingly, the present embodiments are not intended to limit the technical idea of the present embodiment, but rather to explain it, and the scope of the technical idea of the present embodiment is not limited by these examples. The scope of protection of this embodiment should be interpreted in accordance with the claims below, and all technical ideas within the equivalent scope should be interpreted as being included in the scope of rights of this embodiment.

(Explanation of symbols)

140: conversion unit

165: Inverse conversion unit

530: Inverse conversion unit

610: First-order inverse transformation kernel decision unit

612: Secondary inverse transformation performance decision unit

614: Second-order inverse transform kernel decision unit

616: Secondary inverse transform unit

618: First-order inverse transform unit

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims priority over Patent Application No. 10-2022-0108550, filed in Korea on August 29, 2022, and Patent Application No. 10-2023-0097617, filed in Korea on July 26, 2023. and all of its contents are incorporated into this patent application by reference.

Claims

In the method of restoring the current block performed by the video decoding device,

determining a first-order inverse transform kernel in the vertical and horizontal directions for the transform block of the current block;

Obtaining inverse-quantized secondary transform coefficients for the transform block;

determining a second inverse transform kernel of the transform block based on the size of the transform block, an intra prediction mode of the current block, and the first inverse transform kernel;

performing a second-order inversion by applying the second-order inversion kernel to the second-order transformation coefficients to generate first-order transformation coefficients; and

Performing first-order inverse transformation by applying the first-order inverse transformation kernel to the first-order transformation coefficients to generate residual signals

A method comprising:
According to paragraph 1,

Decoding a Low Frequency Non-separable Transform (LFNST) index from a bitstream, where the LFNST index indicates whether to apply secondary inverse transform; and

Steps to check the LFNST index

Including more,

When the LFNST index is greater than 0, the method is characterized in that performing the steps of obtaining the secondary transformation coefficients and performing the secondary inverse transformation.
According to paragraph 2,

When the LFNST index is 0, further comprising obtaining inverse quantized first-order transform coefficients for the transform block, determining the first-order inverse transform kernel, and performing the first-order inverse transform. A method, characterized in that.
According to paragraph 1,

The step of determining the first inverse transformation kernel is,

Decoding an index indicating the first inverse transformation kernel; and

Characterized in that it comprises the step of setting the first inverse transform kernel according to the index.
According to clause 4,

The first inverse transformation kernel is,

A method characterized in that the type of transform kernel indicated by the index is defined in advance based on the intra prediction mode of the current block and the size of the transform block.
According to paragraph 2,

The step of determining the second inverse transformation kernel is,

selecting a mode set including the intra prediction mode of the current block from among preset mode sets; and

Selecting a first-order kernel set including the first-order inverse transformation kernel from among preset first-order kernel sets.

A method comprising:
According to clause 6,

The step of determining the second inverse transformation kernel is,

Selecting a kernel size set including the size of the conversion block from among preset kernel size sets; and

Selecting a secondary kernel set according to the selected mode set, the selected primary kernel set, and the selected kernel size set.

A method further comprising:
In clause 7,

The step of determining the second inverse transformation kernel is,

When the secondary kernel set includes one secondary kernel, the method is characterized in that setting the secondary inverse transformation kernel to the one secondary kernel.
In clause 7,

The step of determining the second inverse transformation kernel is,

When the secondary kernel set includes a plurality of secondary kernels, the method is characterized in that setting the secondary inverse transformation kernel to a secondary kernel indicated by the LFNST index.
According to paragraph 1,

The step of performing the second inverse transformation is,

Based on the size and type of the secondary inverse transform kernel, generating a secondary transform coefficient vector by packing some of the secondary transform coefficients into one dimension according to a preset first scanning order;

generating a first-order transform coefficient vector by performing matrix multiplication between the second-order transform coefficient vector and the second-order inverse transform kernel; and

Generating the first transform coefficients by assigning the first transform coefficient vector to the upper left of the transform block according to a preset second scanning order.

A method comprising:
According to clause 10,

The step of generating the first conversion coefficients is,

When an identity transform (IDT) is applied as the primary transformation kernel in one of the horizontal or vertical directions to the transformation block, the primary transformation coefficient vector is assigned in consideration of the direction in which the IDT is applied. How to.
According to clause 10,

The step of generating the first conversion coefficients is,

For the transformation block, when the primary transformation kernels in the vertical and horizontal directions are different, the direction of a specific primary transformation kernel is first scanned to allocate the primary transformation coefficient vector.
In the method of encoding the current block performed by the video encoding device,

Obtaining residual signals for the conversion block of the current block;

determining a first-order transformation kernel in the vertical and horizontal directions for the transformation block;

determining a secondary transform kernel of the transform block based on the size of the transform block, an intra prediction mode of the current block, and the first inverse transform kernel;

performing first-order transformation by applying the first-order transformation kernel to the residual signals to generate first-order transformation coefficients; and

Performing secondary transformation by applying the secondary transformation kernel to the primary transformation coefficients to generate secondary transformation coefficients

A method comprising:
According to clause 12,

Based on the first transform coefficients and the second transform coefficients, determining a Low Frequency Non-separable Transform (LFNST) index, where the LFNST index indicates whether to apply the second transform; and

Encoding the LFNST index

A method further comprising:
According to clause 13,

Further comprising the step of checking the LFNST index,

When the LFNST index is 0, the method further includes encoding the primary transform coefficients, and when the LFNST index is greater than 0, encoding the secondary transform coefficients. .
A computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method comprising:

Obtaining residual signals for the conversion block of the current block;

determining a first-order transformation kernel in the vertical and horizontal directions for the transformation block;

determining a secondary transform kernel of the transform block based on the size of the transform block, an intra prediction mode of the current block, and the first inverse transform kernel;

performing first-order transformation by applying the first-order transformation kernel to the residual signals to generate first-order transformation coefficients; and

Performing secondary transformation by applying the secondary transformation kernel to the primary transformation coefficients to generate secondary transformation coefficients

A recording medium comprising: