Transformation-based image coding method and device

Abstract

The image decoding method according to the present document includes the step of deriving a modified transform coefficient by applying LFNST to the transform coefficient, wherein the step of deriving the modified transform coefficient includes the following steps: deriving a variable indicating whether a significant coefficient exists in a DC component of a current block; and parsing a LFNST index based on the variable indicating that a significant coefficient exists at a position other than the DC component, wherein the variable can be derived based on respective transform skip flag values ​​of color components of the current block.

Description

Technical Field

[0001] The present disclosure relates to an image encoding technology, and more particularly, to a method and apparatus for encoding an image based on transformation in an image encoding system. Background Art

[0002] Nowadays, the demand for high-resolution and high-quality images / videos such as 4K, 8K or higher ultra-high-definition (UHD) images / videos has been growing in various fields. As image / video data becomes higher resolution and higher quality, the amount of information or bit volume sent increases compared to traditional image data. Therefore, when using a medium such as a traditional wired / wireless broadband line to send image data or using an existing storage medium to store image / video data, its transmission cost and storage cost increase.

[0003] In addition, nowadays, interest and demand for immersive media such as virtual reality (VR) and artificial reality (AR) content or holograms are increasing, and broadcasting of images / videos having image characteristics different from real images such as game images is increasing.

[0004] Therefore, there is a need for an efficient image / video compression technology that effectively compresses and transmits or stores and reproduces information of high-resolution and high-quality images / videos having various characteristics as described above. Summary of the invention

[0005] Technical issues

[0006] A technical aspect of the present disclosure is to provide a method and an apparatus for increasing image encoding efficiency.

[0007] Another technical aspect of the present disclosure is to provide a method and apparatus for increasing the efficiency of LFNST index encoding.

[0008] Yet another technical aspect of the present disclosure is to provide a method and apparatus for increasing encoding efficiency of a LFNST index based on a transform skip flag.

[0009] Technical Solution

[0010] In one aspect, an image decoding method performed by a decoding device is provided. The method includes: deriving a modified transform coefficient by applying LFNST to the transform coefficient; wherein deriving the modified transform coefficient includes: deriving a variable indicating whether a significant coefficient exists in a DC component of a current block; and parsing an LFNST index based on the variable indicating that a significant coefficient exists at a position other than the DC component, and the variable may be derived based on respective transform skip flag values ​​of color components of the current block.

[0011] Based on the transform skip flag value of the color component being 0, the variable may indicate that a significant coefficient exists at a position other than the DC component.

[0012] The variable may be initially set to 1 at the coding unit level of the current block, and if the transform skip flag value is 0, the variable may be changed to 0 at the residual coding level, and the LFNST index may be parsed based on the variable being 0.

[0013] The transform skip flag of the current block may be signaled per color component.

[0014] Deriving the modified transform coefficients may further include setting a plurality of variables for the LFNST based on whether the LFNST index is not zero and whether respective transform skip flag values ​​of the color components are zero.

[0015] If the tree type of the current block is single tree, the variable may be derived based on a transform skip flag value of a luma component, a transform skip flag value of a chroma Cb component, and a transform skip flag value of a chroma Cr component.

[0016] If the tree type of the current block is dual-tree luma, the variable may be derived based on the transform skip flag value of the luma component.

[0017] If the tree type of the current block is dual-tree chroma, the variable may be derived based on a transform skip flag value of the chroma Cb component and a transform skip flag value of the chroma Cr component.

[0018] According to an embodiment of the present disclosure, an image encoding method performed by an encoding device is provided. The method includes: applying LFNST to derive a modified transform coefficient from a transform coefficient, applying a plurality of LFNST matrices to the transform coefficient to derive a variable indicating whether a significant coefficient exists in a DC component of a current block; based on the variable indicating that a significant coefficient exists at a position other than the DC component, selecting an optimal LFNST matrix among LFNST kernels, and deriving the modified transform coefficient based on the selected LFNST matrix, and the variable may be derived based on respective transform skip flag values ​​of color components of the current block.

[0019] According to still another embodiment of the present disclosure, a digital storage medium storing image data including a bit stream generated according to an image encoding method performed by an encoding device and encoded image information may be provided.

[0020] According to yet another embodiment of the present disclosure, a digital storage medium storing image data including encoded image information and a bit stream so that a decoding device performs an image decoding method may be provided.

[0021] Technical Effects

[0022] According to the present disclosure, the overall image / video compression efficiency can be increased.

[0023] According to the present disclosure, the efficiency of LFNST index encoding can be increased.

[0024] According to the present disclosure, the encoding efficiency of the LFNST index may be increased based on the transform skip flag.

[0025] The effects that can be obtained by the specific examples of the present disclosure are not limited to the effects listed above. For example, there may be various technical effects that can be understood by a person of ordinary skill in the relevant field or derived from the present disclosure. Therefore, the specific effects of the present disclosure are not limited to those explicitly described in the present disclosure, and may include various effects that can be understood or derived from the technical features of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 An example of a video / image encoding system to which the present disclosure can be applied is schematically illustrated.

[0027] Figure 2 is a diagram schematically illustrating a configuration of a video / image encoding device to which the present disclosure can be applied.

[0028] Figure 3 is a diagram schematically illustrating a configuration of a video / image decoding device to which the present disclosure can be applied.

[0029] Figure 4 is a diagram exemplarily illustrating a structural diagram of a content streaming system to which the present disclosure is applied.

[0030] Figure 5 FIG. 4 is a diagram schematically illustrating a multi-conversion technique according to an embodiment of the present disclosure.

[0031] Figure 6 FIG. 4 is a diagram schematically illustrating intra-frame directional modes of 65 prediction directions.

[0032] Figure 7 is a diagram for describing an RST according to an embodiment of the present disclosure.

[0033] Figure 8 is a diagram illustrating an order of arranging output data of a forward primary transform into a one-dimensional vector according to an example.

[0034] Fig. 9 is a diagram illustrating an order of arranging output data of a forward quadratic transform into a one-dimensional vector according to an example.

[0035] Fig.10 is a diagram illustrating a wide-angle intra prediction mode according to an embodiment of the present disclosure.

[0036] Fig.11 is a diagram illustrating a block shape to which LFNST is applied.

[0037] Fig.12 is a diagram illustrating an arrangement of output data of a forward LFNST according to an example.

[0038] Fig.13 is a diagram illustrating clearing in a block to which 4×4 LFNST is applied according to an example.

[0039] Fig.14 is a diagram illustrating clearing in a block to which 8×8 LFNST is applied according to an example.

[0040] Fig.15 is a flowchart illustrating the operation of a video decoding apparatus according to an embodiment.

[0041] Fig.16 is a flowchart illustrating the operation of a video encoding apparatus according to an embodiment. DETAILED DESCRIPTION

[0042] Although the present disclosure may be susceptible to various modifications and includes various embodiments, its specific embodiments have been shown by way of example in the accompanying drawings, and will now be described in detail. However, this is not intended to limit the present disclosure to the specific embodiments disclosed herein. The terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the technical ideas of the present disclosure. Unless the context clearly indicates otherwise, the singular form may include the plural form. Terms such as "including" and "having" are intended to indicate the presence of features, numbers, steps, operations, elements, components, or combinations thereof used in the following description, and therefore should not be understood as precluding the possibility of the presence or addition of one or more different features, numbers, steps, operations, elements, components, or combinations thereof.

[0043] In addition, for the convenience of describing the different characteristic functions, each component on the drawings described herein is illustrated independently, however, it is not intended that each component is implemented by separate hardware or software. For example, any two or more of these components can be combined to form a single component, and any single component can be divided into multiple components. Embodiments in which components are combined and / or divided will fall within the scope of the patent rights of the present disclosure as long as they do not depart from the essence of the present disclosure.

[0044] Hereinafter, preferred embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. In addition, in the accompanying drawings, the same reference numerals are used for the same components, and repeated description of the same components will be omitted.

[0045] This document relates to video / image coding. For example, the methods / examples disclosed in this document may relate to the VVC (Versatile Video Coding) standard (ITU-T Rec. H.266), the next generation video / image coding standard after VVC, or other video coding related standards (e.g., HEVC (High Efficiency Video Coding) standard (ITU-T Rec. H.265), EVC (Basic Video Coding) standard, AVS2 standard, etc.).

[0046] In this document, various embodiments related to video / image encoding may be provided, and unless otherwise specified, the embodiments may be combined with each other and performed.

[0047] In this document, video can refer to a collection of a series of images over a period of time. Generally, a picture refers to a unit of an image representing a specific time region, and a slice / tile is a unit that constitutes a part of a picture. A slice / tile may include one or more coding tree units (CTUs). A picture may consist of one or more slices / tiles. A picture may consist of one or more tile groups. A tile group may include one or more tiles.

[0048] A pixel or a picture element (pel) may refer to the smallest unit constituting a picture (or image). In addition, a "sample" may be used as a term corresponding to a pixel. A sample may generally represent a pixel or a pixel value, and may represent only a pixel / pixel value of a luminance component or only a pixel / pixel value of a chrominance component. Alternatively, a sample may mean a pixel value in a spatial domain, or when the pixel value is transformed into a frequency domain, it may mean a transform coefficient in a frequency domain.

[0049] A unit may represent a basic unit of image processing. A unit may include at least one of a specific region and information related to the region. A unit may include a luminance block and two chrominance (e.g., cb, cr) blocks. Depending on the situation, terms such as unit, block, region, etc. may be used interchangeably. In general, an M×N block may include a set (or array) of samples (or sample arrays) or transform coefficients consisting of M columns and N rows.

[0050] In this document, the terms " / " and "," should be interpreted as indicating "and / or". For example, the expression "A / B" may mean "A and / or B". In addition, "A, B" may mean "A and / or B". In addition, "A / B / C" may mean "at least one of A, B, and / or C". In addition, "A / B / C" may mean "at least one of A, B, and / or C".

[0051] In addition, in this document, the term "or" should be interpreted as indicating "and / or". For example, the expression "A or B" may include 1) only A, 2) only B, and / or 3) both A and B. In other words, the term "or" in this document should be interpreted as indicating "additionally or alternatively".

[0052] In the present disclosure, "at least one of A and B" may mean "only A", "only B", or "both A and B". In addition, in the present disclosure, the expression "at least one of A or B" or "at least one of A and / or B" may be interpreted as "at least one of A and B".

[0053] Furthermore, in the present disclosure, “at least one of A, B, and C” may mean “only A”, “only B”, “only C”, or “any combination of A, B, and C”. Furthermore, “at least one of A, B, or C” or “at least one of A, B and / or C” may mean “at least one of A, B, and C”.

[0054] In addition, the brackets used in the present disclosure may mean "for example". Specifically, when the indication is "prediction (intra-frame prediction)", it may mean that "intra-frame prediction" is proposed as an example of "prediction". In other words, the "prediction" of the present disclosure is not limited to "intra-frame prediction", and "intra-frame prediction" is proposed as an example of "prediction". In addition, when the indication is "prediction (ie, intra-frame prediction)", this may also mean that "intra-frame prediction" is proposed as an example of "prediction".

[0055] Technical features described separately in one drawing in the present disclosure may be implemented separately or may be implemented simultaneously.

[0056] Figure 1 An example of a video / image encoding system to which the present disclosure can be applied is schematically illustrated.

[0057] Reference Figure 1 The video / image coding system may include a first device (source device) and a second device (receiving device). The source device may transmit the coded video / image information or data to the receiving device in the form of a file or stream via a digital storage medium or a network.

[0058] The source device may include a video source, an encoding device, and a transmitter. The receiving device may include a receiver, a decoding device, and a renderer. The encoding device may be referred to as a video / image encoding device, and the decoding device may be referred to as a video / image decoding device. The transmitter may be included in the encoding device. The receiver may be included in the decoding device. The renderer may include a display, and the display may be configured as a separate device or an external component.

[0059] The video source can obtain the video / image by capturing, synthesizing or generating the process of the video / image. The video source may include a video / image capturing device and / or a video / image generating device. The video / image capturing device may include, for example, one or more cameras, a video / image archive including previously captured videos / images, etc. The video / image generating device may include, for example, a computer, a tablet computer, and a smart phone, and may (electronically) generate the video / image. For example, a virtual video / image may be generated by a computer, etc. In this case, the video / image capturing process may be replaced by the process of generating the relevant data.

[0060] The encoding device can encode the input video / image. The encoding device can perform a series of processes such as prediction, transformation and quantization for compression and coding efficiency. The encoded data (encoded video / image information) can be output in the form of a bitstream.

[0061] The transmitter may transmit the encoded video / image information or data output in the form of a bit stream to a receiver of a receiving device in the form of a file or stream through a digital storage medium or a network. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmitter may include an element for generating a media file in a predetermined file format, and may include an element for transmitting through a broadcast / communication network. The receiver may receive / extract a bit stream and transmit the received / extracted bit stream to a decoding device.

[0062] The decoding device may decode a video / image by performing a series of processes such as dequantization, inverse transformation, prediction, etc. corresponding to the operation of the encoding device.

[0063] The renderer can render the decoded video / image, and the rendered video / image can be displayed through a display.

[0064] Figure 2 is a diagram schematically illustrating a configuration of a video / image encoding device to which the present disclosure can be applied. Hereinafter, the so-called video encoding device may include an image encoding device.

[0065] Reference Figure 2, the encoding device 200 may include an image divider 210, a predictor 220, a residual processor 230, an entropy encoder 240, an adder 250, a filter 260, and a memory 270. The predictor 220 may include an inter-frame predictor 221 and an intra-frame predictor 222. The residual processor 230 may include a transformer 232, a quantizer 233, a dequantizer 234, and an inverse transformer 235. The residual processor 230 may further include a subtractor 231. The adder 250 may be referred to as a reconstructor or a reconstructed block generator. According to an embodiment, the image divider 210, the predictor 220, the residual processor 230, the entropy encoder 240, the adder 250, and the filter 260 described above may be composed of one or more hardware components (e.g., an encoder chipset or processor). In addition, the memory 270 may include a decoded picture buffer (DPB) and may be composed of a digital storage medium. The hardware component may further include a memory 270 as an internal / external component.

[0066] The image divider 210 may divide the input image (or picture or frame) input to the encoding device 200 into one or more processing units. As an example, the processing unit may be referred to as a coding unit (CU). In this case, starting from a coding tree unit (CTU) or a maximum coding unit (LCU), the coding unit may be recursively divided according to a quadtree binary tree ternary tree (QTBTTT) structure. For example, based on a quadtree structure, a binary tree structure, and / or a ternary tree structure, a coding unit may be divided into a plurality of coding units with a deeper depth. In this case, for example, a quadtree structure may be applied first, and a binary tree structure and / or a ternary tree structure may be applied later. Alternatively, a binary tree structure may be applied first. The encoding process according to the present disclosure may be performed based on a final coding unit that is not further divided. In this case, based on the coding efficiency according to the image characteristics, the maximum coding unit may be directly used as the final coding unit. Alternatively, the coding unit may be recursively divided into coding units of a deeper depth as needed, whereby a coding unit of the best size may be used as the final coding unit. Here, the encoding process may include processes such as prediction, transformation, and reconstruction, which will be described later. As another example, the processing unit may further include a prediction unit (PU) or a transform unit (TU). In this case, the prediction unit and the transform unit may be separated or divided from the above-mentioned final coding unit. The prediction unit may be a unit for sample prediction, and the transform unit may be a unit for deriving a transform coefficient and / or a unit for deriving a residual signal from a transform coefficient.

[0067] Unit and terms such as block, region, etc. may be used to replace each other depending on the situation. In general, an M×N block may represent a set of samples or transform coefficients consisting of M columns and N rows. A sample may generally represent a pixel or a pixel value, and may represent only a pixel / pixel value of a luminance component, or only a pixel / pixel value of a chrominance component. A sample may be used as a term corresponding to a pixel or a pel of a picture (or image).

[0068] The subtractor 231 subtracts the prediction signal (prediction block, prediction sample array) output from the predictor 220 from the input image signal (original block, original sample array) to generate a residual signal (residual block, residual sample array), and the generated residual signal is sent to the transformer 232. The predictor 220 can perform prediction on the processing target block (hereinafter referred to as the "current block"), and can generate a prediction block including the prediction samples of the current block. The predictor 220 can determine whether to apply intra-frame prediction or inter-frame prediction based on the current block or CU. As discussed later in the description of each prediction mode, the predictor can generate various information related to the prediction, such as prediction mode information, and send the generated information to the entropy encoder 240. Information about the prediction can be encoded in the entropy encoder 240 and output in the form of a bitstream.

[0069] The intra-frame predictor 222 can predict the current block by referring to the samples in the current picture. Depending on the prediction mode, the reference sample can be located near the current block or separated from the current block. In intra-frame prediction, the prediction mode may include a variety of non-directional modes and a variety of directional modes. The non-directional mode may include, for example, a DC mode and a plane mode. Depending on the level of detail of the prediction direction, the directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes. However, this is only an example, and more or fewer directional prediction modes may be used depending on the setting. The intra-frame predictor 222 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring blocks.

[0070] The inter-frame predictor 221 may derive a prediction block for the current block based on a reference block (reference sample array) specified by a motion vector on a reference picture. At this time, in order to reduce the amount of motion information sent in the inter-frame prediction mode, the motion information may be predicted based on a block, a sub-block, or a sample based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may also include inter-frame prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter-frame prediction, the neighboring blocks may include spatial neighboring blocks existing in the current picture and temporal neighboring blocks existing in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same as or different from each other. The temporal neighboring block may be referred to as a collocated reference block, a collocated CU (colCU), etc., and the reference picture including the temporal neighboring block may be referred to as a collocated picture (colPic). For example, the inter-frame predictor 221 may configure a motion information candidate list based on the neighboring blocks, and generate information indicating which candidate is used to derive the motion vector and / or reference picture index of the current block. Inter-frame prediction may be performed based on various prediction modes. For example, in the case of the skip mode and the merge mode, the inter-frame predictor 221 can use the motion information of the neighboring block as the motion information of the current block. In the skip mode, unlike the merge mode, the residual signal cannot be sent. In the case of the motion information prediction (motion vector prediction, MVP) mode, the motion vector of the neighboring block can be used as a motion vector predictor, and the motion vector of the current block can be indicated by signaling the motion vector difference.

[0071] The predictor 220 can generate a prediction signal based on various prediction methods. For example, the predictor can apply intra prediction or inter prediction to the prediction of a block, and can also apply intra prediction and inter prediction at the same time. This can be called combined inter and intra prediction (CIIP). In addition, the predictor can be based on an intra-block copy (IBC) prediction mode or a palette mode to perform prediction on the block. The IBC prediction mode or the palette mode can be used for content image / video encoding such as games such as screen content coding (SCC). Although IBC basically performs prediction in the current block, its execution method is similar to inter prediction in that it derives a reference block in the current block. That is, IBC can use at least one of the inter prediction techniques described in the present disclosure.

[0072] The prediction signal generated by the inter-frame predictor 221 and / or the intra-frame predictor 222 can be used to generate a reconstruction signal or to generate a residual signal. The transformer 232 can generate a transform coefficient by applying a transform technique to the residual signal. For example, the transform technique may include at least one of a discrete cosine transform (DCT), a discrete sine transform (DST), a Karhunen-Loève transform (KLT), a graph-based transform (GBT), or a conditional nonlinear transform (CNT). Here, GBT means a transform obtained from a curve graph when the relationship information between pixels is represented by a curve graph. CNT refers to a transform obtained based on a prediction signal generated using all previously reconstructed pixels. In addition, the transform process can be applied to square pixel blocks of the same size, or can be applied to blocks of variable size rather than square blocks.

[0073] The quantizer 233 may quantize the transform coefficients and send them to the entropy encoder 240, and the entropy encoder 240 may encode the quantized signal (information about the quantized transform coefficients) and output the encoded signal in the bitstream. The information about the quantized transform coefficients may be referred to as residual information. The quantizer 233 may rearrange the quantized transform coefficients of the block type into a one-dimensional vector form based on the coefficient scanning order, and generate information about the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector form. The entropy encoder 240 may perform various encoding methods such as, for example, exponential Golomb, context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), etc. The entropy encoder 240 may encode the information required for video / image reconstruction other than the quantized transform coefficients (e.g., the value of the syntax element, etc.) together or separately. The encoded information (e.g., the encoded video / image information) may be transmitted or stored on a unit basis of a network abstraction layer (NAL) in the form of a bitstream. The video / image information may also include information about various parameter sets such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), a video parameter set (VPS), etc. In addition, the video / image information may also include conventional constraint information. In the present disclosure, information and / or syntax elements sent from an encoding device to / signaled to a decoding device may be included in the video / image information. The video / image information may be encoded by the above-mentioned encoding process and included in a bitstream. The bitstream may be transmitted over a network or stored in a digital storage medium. Here, the network may include a broadcast network, a communication network, and / or the like, and the digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. A transmitter (not shown) that transmits a signal output from the entropy encoder 240 or a memory (not shown) that stores it may be configured as an internal / external element of the encoding device 200, or the transmitter may be included in the entropy encoder 240.

[0074] The quantized transform coefficients output from the quantizer 233 can be used to generate a prediction signal. For example, by applying dequantization and inverse transformation to the quantized transform coefficients using the dequantizer 234 and the inverse transformer 235, the residual signal (residual block or residual sample) can be reconstructed. The adder 155 adds the reconstructed residual signal to the prediction signal output from the inter-frame predictor 221 or the intra-frame predictor 222, so that a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) can be generated. When there is no residual for the processing target block as in the case of applying the jump mode, the prediction block can be used as a reconstructed block. The adder 250 can be referred to as a reconstructor or a reconstructed block generator. The generated reconstructed signal can be used for intra-frame prediction of the next processing target block in the target picture, and as described later, can be used for inter-frame prediction of the next picture performed by filtering.

[0075] Furthermore, in the picture encoding and / or reconstruction process, luminance mapping with chroma scaling (LMCS) may be applied.

[0076] The filter 260 can improve the subjective / objective video quality by applying filtering to the reconstructed signal. For example, the filter 260 can generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and the modified reconstructed picture can be stored in the memory 270, especially in the DPB of the memory 270. Various filtering methods may include, for example, deblocking filtering, sample adaptive offset, adaptive ring filter, bilateral filter, etc. As discussed later in the description of each filtering method, the filter 260 can generate various information related to filtering and send the generated information to the entropy encoder 240. Information about filtering can be encoded in the entropy encoder 240 and output in the form of a bitstream.

[0077] The modified reconstructed picture transmitted to the memory 270 may be used as a reference picture in the inter-frame predictor 221. Accordingly, the encoding device can avoid prediction mismatch in the encoding device 100 and the decoding device when applying inter-frame prediction, and can also improve encoding efficiency.

[0078] The memory 270DPB may store the modified reconstructed picture so as to use it as a reference picture in the inter-frame predictor 221. The memory 270 may store the motion information of the block in the current picture from which the motion information has been derived (or encoded) and / or the motion information of the block in the reconstructed picture. The stored motion information may be sent to the inter-frame predictor 221 to be used as the motion information of the neighboring block or the motion information of the temporal neighboring block. The memory 270 may store the reconstructed samples of the reconstructed block in the current picture and send them to the intra-frame predictor 222.

[0079] Figure 3is a diagram schematically illustrating a configuration of a video / image decoding device to which the present disclosure can be applied.

[0080] Reference Figure 3 , the video decoding device 300 may include an entropy decoder 310, a residual processor 320, a predictor 330, an adder 340, a filter 350, and a memory 360. The predictor 330 may include an intra-frame predictor 331 and an inter-frame predictor 332. The residual processor 320 may include a dequantizer 321 and an inverse transformer 322. According to an embodiment, the entropy decoder 310, the residual processor 320, the predictor 330, the adder 340, and the filter 350 described above may be composed of one or more hardware components (e.g., a decoder chipset or a processor). In addition, the memory 360 may include a decoded picture buffer (DPB) and may be composed of a digital storage medium. The hardware component may also include a memory 360 as an internal / external component.

[0081] When a bit stream including video / image information is input, the decoding device 300 can Figure 2 The image is reconstructed correspondingly to the processing of the video / image information in the encoding device. For example, the decoding device 300 can derive the unit / block based on the information related to the block segmentation obtained from the bit stream. The decoding device 300 can perform decoding by using the processing unit applied in the encoding device. Therefore, the decoding processing unit can be, for example, a coding unit, which can be divided into a quadtree structure, a binary tree structure and / or a ternary tree structure using a coding tree unit or a maximum coding unit. One or more transformation units can be derived from the coding unit. And, the reconstructed image signal decoded and output by the decoding device 300 can be reproduced by a reproducer.

[0082] The decoding device 300 may receive the Figure 2The signal output by the encoding device of the encoding device, and the received signal can be decoded by the entropy decoder 310. For example, the entropy decoder 310 can parse the bitstream to derive the information (e.g., video / image information) required for image reconstruction (or picture reconstruction). The video / image information may also include information about various parameter sets such as adaptive parameter set (APS), picture parameter set (PPS), sequence parameter set (SPS), video parameter set (VPS), etc. In addition, the video / image information may also include conventional constraint information. The decoding device may further decode the picture based on the information about the parameter set and / or the conventional constraint information. In the present disclosure, the signaling / receiving information and / or syntax elements to be described later can be decoded and obtained from the bitstream through the decoding process. For example, the entropy decoder 310 can decode the information in the bitstream based on a coding method such as exponential Golomb coding, CAVLC, CABAC, etc., and can output the values ​​of the syntax elements required for image reconstruction and the quantized values ​​of the transform coefficients of the residual. More specifically, the CABAC entropy decoding method can receive the bin corresponding to each syntax element in the bitstream, use the decoding target syntax element information and the decoding information of the neighboring and decoding target blocks or the information of the symbol / bin decoded in the previous step to determine the context model, predict the bin generation probability according to the determined context model and perform arithmetic decoding on the bin to generate the symbol corresponding to each syntax element value. Here, the CABAC entropy decoding method can update the context model using the information of the symbol / bin decoded by the context model for the next symbol / bin after determining the context model. The information about prediction among the information decoded in the entropy decoder 310 can be provided to the predictor (inter-frame predictor 332 and intra-frame predictor 331), and the residual value (ie, quantized transform coefficient) and the associated parameter information that have been entropy decoded in the entropy decoder 310 can be input to the residual processor 320. The residual processor 320 can derive a residual signal (residual block, residual sample, residual sample array). In addition, the information about filtering among the information decoded in the entropy decoder 310 can be provided to the filter 350. In addition, a receiver (not shown) receiving a signal output from the encoding device may also constitute the decoding device 300 as an internal / external element, and the receiver may be a component of the entropy decoder 310. In addition, the decoding device according to the present disclosure may be referred to as a video / image / picture encoding device, and the decoding device may be divided into an information decoder (video / image / picture information decoder) and a sample decoder (video / image / picture sample decoder). The information decoder may include the entropy decoder 310, and the sample decoder may include at least one of the dequantizer 321, the inverse transformer 322, the adder 340, the filter 350, the memory 360, the inter-frame predictor 332, and the intra-frame predictor 331.

[0083] The dequantizer 321 may output the transform coefficient by dequantizing the quantized transform coefficient. The dequantizer 321 may rearrange the quantized transform coefficient into a two-dimensional block form. In this case, the rearrangement may be performed based on the order of coefficient scanning that has been performed in the encoding device. The dequantizer 321 may perform dequantization on the quantized transform coefficient using a quantization parameter (e.g., quantization step size information) and obtain the transform coefficient.

[0084] The inverse transformer 322 obtains a residual signal (residual block, residual sample array) by performing inverse transformation on the transformation coefficients.

[0085] The predictor may perform prediction on the current block and generate a prediction block including prediction samples for the current block. The predictor may determine whether to apply intra prediction or inter prediction to the current block based on information on prediction output from the entropy decoder 310, and may specifically determine an intra / inter prediction mode.

[0086] The predictor can generate a prediction signal based on various prediction methods. For example, the predictor can apply intra prediction or inter prediction to the prediction of a block, and can also apply intra prediction and inter prediction at the same time. This can be called combined inter and intra prediction (CIIP). In addition, the predictor can perform intra-block copying (IBC) for the prediction of the block. Intra-block copying can be used for content image / video coding such as games such as screen content coding (SCC). Although IBC basically performs prediction in the current block, its execution method is similar to inter prediction in that it derives a reference block in the current block. That is, IBC can use at least one of the inter prediction techniques described in the present disclosure.

[0087] The intra-frame predictor 331 can predict the current block by referring to samples in the current picture. Depending on the prediction mode, the reference sample can be located near the current block or separated from the current block. In intra-frame prediction, the prediction mode can include multiple non-directional modes and multiple directional modes. The intra-frame predictor 331 can determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

[0088] The inter-frame predictor 332 may derive a prediction block for the current block based on a reference block (reference sample array) specified by a motion vector on a reference picture. At this time, in order to reduce the amount of motion information sent in the inter-frame prediction mode, the motion information may be predicted based on a block, a sub-block, or a sample based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may also include inter-frame prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter-frame prediction, the neighboring blocks may include spatial neighboring blocks existing in the current picture and temporal neighboring blocks existing in the reference picture. For example, the inter-frame predictor 332 may configure a motion information candidate list based on neighboring blocks, and derive a motion vector and / or a reference picture index of the current block based on the received candidate selection information. Inter-frame prediction may be performed based on various prediction modes, and information about the prediction may include information indicating a mode of inter-frame prediction for the current block.

[0089] The adder 340 can generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the obtained residual signal to the prediction signal (prediction block, prediction sample array) output from the predictor 330. When there is no residual for the processing target block as in the case of applying the skip mode, the prediction block can be used as the reconstructed block.

[0090] The adder 340 may be referred to as a reconstructor or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of the next processing target block in the current block, and as described later, may be output through filtering or used for inter prediction of the next picture.

[0091] Furthermore, in the picture decoding process, luma mapping with chroma scaling (LMCS) may be applied.

[0092] The filter 350 can improve the subjective / objective video quality by applying filtering to the reconstructed signal. For example, the filter 350 can generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and the modified reconstructed picture can be sent to the memory 360, especially to the DPB of the memory 360. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, adaptive ring filter, bilateral filter, etc.

[0093] The (modified) reconstructed picture stored in the DPB of the memory 360 may be used as a reference picture in the inter-frame predictor 332. The memory 360 may store the motion information of the block in the current picture from which the motion information has been derived (or decoded) and / or the motion information of the block in the reconstructed picture. The stored motion information may be sent to the inter-frame predictor 221 to be used as the motion information of the neighboring block or the motion information of the temporal neighboring block. The memory 360 may store the reconstructed samples of the reconstructed block in the current picture and send them to the intra-frame predictor 331.

[0094] In the present specification, the examples described in the predictor 330, the dequantizer 321, the inverse transformer 322, and the filter 350 of the decoding device 300 may be similarly or correspondingly applied to the predictor 220, the dequantizer 234, the inverse transformer 235, and the filter 260 of the encoding device 200, respectively.

[0095] As described above, prediction is performed in order to improve the compression efficiency when performing video encoding. Accordingly, a prediction block including prediction samples for a current block as a coding target block can be generated. Here, the prediction block includes prediction samples in a spatial domain (or a pixel domain). The prediction block can be derived identically in an encoding device and a decoding device, and the encoding device can improve the image coding efficiency by signaling to the decoding device information (residual information) about the residual between the original block and the prediction block rather than the original sample value of the original block itself. The decoding device can derive a residual block including residual samples based on the residual information, generate a reconstructed block including reconstructed samples by adding the residual block to the prediction block, and generate a reconstructed picture including the reconstructed block.

[0096] The residual information may be generated by a transform process and a quantization process. For example, the encoding device may derive a residual block between the original block and the prediction block, derive a transform coefficient by performing a transform process on the residual samples (residual sample array) included in the residual block, and derive a quantized transform coefficient by performing a quantization process on the transform coefficient, so that it can signal the associated residual information to the decoding device (through a bitstream). Here, the residual information may include value information, position information, transform technology, transform kernel, quantization parameter, etc. of the quantized transform coefficient. The decoding device may perform a quantization / dequantization process based on the residual information and derive a residual sample (or residual sample block). The decoding device may generate a reconstructed block based on the prediction block and the residual block. The encoding device may derive a residual block by dequantizing / inverse transforming the quantized transform coefficient so as to serve as a reference for inter-frame prediction of the next picture, and may generate a reconstructed picture based on this.

[0097] Figure 4 The structure of the content streaming system to which the present disclosure is applied is illustrated.

[0098] Furthermore, a content streaming system to which the present disclosure is applied may generally include an encoding server, a streaming server, a web server, a media storage device, a user device, and a multimedia input device.

[0099] The encoding server is used to compress the content input from a multimedia input device such as a smart phone, a camera, a camcorder, etc. into digital data to generate a bitstream, and send it to a streaming server. As another example, in the case where a multimedia input device such as a smart phone, a camera, a camcorder, etc. directly generates a bitstream, the encoding server can be omitted. The bitstream can be generated by applying the encoding method or the bitstream generation method disclosed in the present invention. And the streaming server can temporarily store the bitstream during the process of sending or receiving the bitstream.

[0100] The streaming server sends multimedia data to the user device through the web server based on the user's request, and the web server serves as a tool to inform the user of what services exist. When the user requests the service the user wants, the web server transmits the request to the streaming server, and the streaming server sends the multimedia data to the user. In this regard, the content streaming system may include a separate control server, and in this case, the control server is used to control the command / response between the corresponding devices in the content streaming system.

[0101] The streaming server can receive content from a media storage device and / or an encoding server. For example, in the case of receiving content from an encoding server, the content can be received in real time. In this case, in order to smoothly provide a streaming service, the streaming server can store the bit stream for a predetermined time.

[0102] For example, user devices may include mobile phones, smart phones, laptop computers, digital broadcast terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigators, tablet PCs, tablet PCs, ultrabooks, wearable devices (e.g., watch-type terminals (smart watches), glasses-type terminals (smart glasses), head-mounted displays (HMDs)), digital TVs, desktop computers, digital signage, etc. Each server in the content streaming system may operate as a distributed server, and in this case, data received by each server may be processed in a distributed manner.

[0103] Figure 5 The multi-conversion technology according to the embodiment of the present disclosure is schematically illustrated.

[0104] Reference Figure 5 , the converter can correspond to the aforementioned Figure 2 The converter in the encoding device, and the inverse converter may correspond to the aforementioned Figure 2 an inverse transformer in an encoding device, or Figure 3 An inverse transformer in a decoding device.

[0105] The transformer may derive (primary) transform coefficients by performing a primary transform based on the residual samples (residual sample array) in the residual block (S510). The primary transform may be referred to as a core transform. In this document, a primary transform may be based on a multi-transform selection (MTS), and when a multi-transform is used as a primary transform, it may be referred to as a multi-core transform.

[0106] The multi-core transform may refer to a method of performing transform using discrete cosine transform (DCT) type 2 and discrete sine transform (DST) type 7, DCT type 8, and / or DST type 1 in addition. That is, the multi-core transform may refer to a transform method of transforming a residual signal (or residual block) in the spatial domain into a transform coefficient (or primary transform coefficient) in the frequency domain based on a plurality of transform kernels selected from DCT type 2, DST type 7, DCT type 8, and DST type 1. In this article, the primary transform coefficient may be referred to as a temporary transform coefficient from the perspective of the transformer.

[0107] In other words, when a conventional transform method is applied, a transform coefficient can be generated by applying a transform from the spatial domain to the frequency domain to a residual signal (or residual block) based on DCT type 2. In contrast, when a multi-core transform is applied, a transform coefficient (or a primary transform coefficient) can be generated by applying a transform from the spatial domain to the frequency domain to a residual signal (or residual block) based on DCT type 2, DST type 7, DCT type 8, and / or DST type 1. In this document, DCT type 2, DST type 7, DCT type 8, and DST type 1 may be referred to as a transform type, a transform kernel, or a transform core. These DCT / DST transform types may be defined based on basis functions.

[0108] When performing a multi-core transform, a vertical transform core and a horizontal transform core for a target block may be selected from the transform cores, a vertical transform may be performed on the target block based on the vertical transform core, and a horizontal transform may be performed on the target block based on the horizontal transform core. Here, the horizontal transform may indicate a transform of a horizontal component of the target block, and the vertical transform may indicate a transform of a vertical component of the target block. The vertical transform core / horizontal transform core may be adaptively determined based on a prediction mode and / or a transform index of a target (CU or subblock) including a residual block.

[0109] In addition, according to the example, if a transformation is performed once by applying MTS, the mapping relationship of the transformation kernel can be set by setting a specific basis function to a predetermined value and combining the basis functions to be applied in the vertical transformation or the horizontal transformation. For example, when the horizontal transformation kernel is represented as trTypeHor and the vertical direction transformation kernel is represented as trTypeVer, trTypeHor or trTypeVer with a value of 0 can be set to DCT2, trTypeHor or trTypeVer with a value of 1 can be set to DST7, and trTypeHor or trTypeVer with a value of 2 can be set to DCT8.

[0110] In this case, the MTS index information may be encoded and signaled to the decoding device to indicate any one of the plurality of transform kernel sets. For example, an MTS index of 0 may indicate that both trTypeHor and trTypeVer values ​​are 0, an MTS index of 1 may indicate that both trTypeHor and trTypeVer values ​​are 1, an MTS index of 2 may indicate that both trTypeHor and trTypeVer values ​​are 2, an MTS index of 3 may indicate that both trTypeHor and trTypeVer values ​​are 1, and an MTS index of 4 may indicate that both trTypeHor and trTypeVer values ​​are 2.

[0111] In one example, the transformation core set according to the MTS index information is shown in the following table.

[0112] [Table 1]

[0113] tu_mts_idx[x0][y0] 0 1 2 3 4 trTrpeHor 0 1 2 1 2 trTvpeVer 0 1 1 2 2

[0114] The transformer may perform a secondary transform based on the (primary) transform coefficients to derive modified (secondary) transform coefficients (S520). A primary transform is a transform from the spatial domain to the frequency domain, while a secondary transform refers to a transform into a more compact representation using the correlation between the (primary) transform coefficients. The secondary transform may include an inseparable transform. In this case, the secondary transform may be referred to as a non-separable secondary transform (NSST) or a pattern-dependent non-separable secondary transform (MDNSST). NSST may represent a transform that performs a secondary transform on the (primary) transform coefficients derived by the primary transform based on an inseparable transform matrix to generate a modified transform coefficient (or secondary transform coefficient) for the residual signal. Here, based on the inseparable transform matrix, the transform may be applied once to the (primary) transform coefficients without separating the vertical transform and the horizontal transform (or applying the horizontal / vertical transforms independently). In other words, NSST is not applied separately to (primary) transform coefficients in the vertical and horizontal directions, and can represent, for example, a transform method in which a two-dimensional signal (transform coefficient) is rearranged into a one-dimensional signal through a specific predetermined direction (e.g., a row-first direction or a column-first direction) and then a modified transform coefficient (or secondary transform coefficient) is generated based on an inseparable transform matrix. For example, the row-first order is to arrange the M×N blocks in the order of the first row, the second row, ... and the Nth row, and the column-first order is to arrange the M×N blocks in the order of the first column, the second column, ... and the Mth column. NSST can be applied to the upper left area of ​​a block (hereinafter referred to as a transform coefficient block) configured with (primary) transform coefficients. For example, when the width W and the height H of the transform coefficient block are both 8 or more, the 8×8 NSST can be applied to the upper left 8×8 area of ​​the transform coefficient block. In addition, when both the width (W) and the height (H) of the transform coefficient block are 4 or more, when the width (W) or the height (H) of the transform coefficient block is less than 8, the 4×4 NSST may be applied to the upper left min(8,W)×min(8,H) region of the transform coefficient block. However, the embodiment is not limited thereto, and for example, even if only the condition that the width W or the height H of the transform coefficient block is 4 or more is satisfied, the 4×4 NSST may be applied to the upper left min(8,W)×min(8,H) region of the transform coefficient block.

[0115] Specifically, for example, if a 4×4 input block is used, the non-separable secondary transform may be performed as follows.

[0116] A 4×4 input block X can be represented as follows.

[0117] [Formula 1]

[0118]

[0119] If X is represented as a vector, then the vector It can be expressed as follows.

[0120] [Formula 2]

[0121]

[0122] In formula 2, the vector is a one-dimensional vector obtained by rearranging the two-dimensional block X of Equation 1 according to the row-major order.

[0123] In this case, the non-separable quadratic transform can be calculated as follows.

[0124] [Formula 3]

[0125]

[0126] In this formula, denotes a transform coefficient vector and T denotes a 16×16 (non-separable) transform matrix.

[0127] By using the above formula 3, the 16×1 transform coefficient vector can be derived And the vector can be divided into two parts by scanning order (horizontally, vertically, diagonally, etc.) Reorganized into 4×4 blocks. However, the above calculation is an example, and Hypercube-Givens Transform (HyGT) or the like may also be used for the calculation of the inseparable secondary transform in order to reduce the calculation complexity of the inseparable secondary transform.

[0128] Furthermore, in the inseparable secondary transform, the transform kernel (or transform core, transform type) may be selected to be mode-dependent. In this case, the mode may include an intra prediction mode and / or an inter prediction mode.

[0129] As described above, an inseparable secondary transform may be performed based on an 8×8 transform or a 4×4 transform determined based on the width (W) and height (H) of the transform coefficient block. An 8×8 transform refers to a transform that can be applied to an 8×8 region included in a transform coefficient block when both W and H are equal to or greater than 8, and the 8×8 region may be the upper left 8×8 region in the transform coefficient block. Similarly, a 4×4 transform refers to a transform that can be applied to a 4×4 region included in a transform coefficient block when both W and H are equal to or greater than 4, and the 4×4 region may be the upper left 4×4 region in the transform coefficient block. For example, an 8×8 transform kernel matrix may be a 64×64 / 16×64 matrix, and a 4×4 transform kernel matrix may be a 16×16 / 8×16 matrix.

[0130] Here, in order to select the mode-dependent transform cores, two inseparable secondary transform cores may be configured for each transform set of inseparable secondary transforms for both the 8×8 transform and the 4×4 transform, and there may be four transform sets. That is, four transform sets may be configured for the 8×8 transform, and four transform sets may be configured for the 4×4 transform. In this case, each of the four transform sets for the 8×8 transform may include two 8×8 transform cores, and each of the four transform sets for the 4×4 transform may include two 4×4 transform cores.

[0131] However, as the size of the transform (ie, the size of the region to which the transform is applied) may be, for example, a size other than 8×8 or 4×4, the number of sets may be n, and the number of transform kernels in each set may be k.

[0132] The transform set may be referred to as an NSST set or a LFNST set. A specific set among the transform sets may be selected, for example, based on the intra prediction mode of the current block (CU or subblock). A low frequency non-separable transform (LFNST) may be an example of a reduced non-separable transform, which will be described later and represents a non-separable transform for a low frequency component.

[0133] For reference, for example, the intra prediction mode may include two non-directional (or non-angle) intra prediction modes and 65 directional (or angle) intra prediction modes. The non-directional intra prediction mode may include a plane intra prediction mode No. 0 and a DC intra prediction mode No. 1, and the directional intra prediction mode may include 65 intra prediction modes No. 2 to No. 66. However, this is an example, and this document may be applied even if the number of intra prediction modes is different. In addition, in some cases, intra prediction mode No. 67 may also be used, and intra prediction mode No. 67 may represent a linear model (LM) mode.

[0134] Figure 6 The intra directional mode for 65 prediction directions is schematically shown.

[0135] Reference Figure 6 , based on the intra prediction mode 34 having the upper left diagonal prediction direction, the intra prediction mode can be divided into an intra prediction mode having a horizontal directionality and an intra prediction mode having a vertical directionality. Figure 6In , H and V indicate horizontal directivity and vertical directivity, respectively, and numbers -32 to 32 indicate displacements of 1 / 32 units on sample grid positions. These numbers may represent offsets for mode index values. Intra-prediction modes 2 to 33 have horizontal directivity, and intra-prediction modes 34 to 66 have vertical directivity. Strictly speaking, intra-prediction mode 34 may be considered to be neither horizontal nor vertical, but may be classified as belonging to horizontal directivity when determining a transform set for a secondary transform. This is because the input data is transposed for a vertical orientation mode symmetrical based on intra-prediction mode 34, and an input data alignment method for a horizontal mode is used for intra-prediction mode 34. Transposing the input data means switching rows and columns of two-dimensional M×N block data to N×M data. Intra-prediction mode 18 and intra-prediction mode 50 may represent a horizontal intra-prediction mode and a vertical intra-prediction mode, respectively, and intra-prediction mode 2 may be referred to as an upper right diagonal intra-prediction mode because intra-prediction mode 2 has a left reference pixel and performs prediction in the upper right direction. Similarly, intra-prediction mode 34 may be referred to as a bottom-right diagonal intra-prediction mode, and intra-prediction mode 66 may be referred to as a bottom-left diagonal intra-prediction mode.

[0136] According to an example, four transform sets according to intra prediction modes may be mapped, for example, as shown in the following table.

[0137] [Table 2]

[0138] predModeIntra lfnstTrSetIdx predModeIntra<0 1 0<=predModeIntra<=1 0 2<=predModeIntra<=12 1 13<=predModeIntra<=23 2 24<=predModeIntra<=44 3 45<=predModeIntra<=55 2 56<=predModeIntra<=80 1

[0139] As shown in Table 2, any one of four transform sets, ie, lfnstTrSetIdx, may be mapped to any one of four indexes (ie, 0 to 3) according to the intra prediction mode.

[0140] When it is determined that a specific set is used for an inseparable transform, one of the k transform cores in the specific set can be selected by an inseparable secondary transform index. The encoding device can derive an inseparable secondary transform index indicating a specific transform core based on a rate-distortion (RD) check, and can signal the inseparable secondary transform index to the decoding device. The decoding device can select one of the k transform cores in a specific set based on the inseparable secondary transform index. For example, lfnst index value 0 can refer to a first inseparable secondary transform core, lfnst index value 1 can refer to a second inseparable secondary transform core, and lfnst index value 2 can refer to a third inseparable secondary transform core. Alternatively, lfnst index value 0 can indicate that the first inseparable secondary transform is not applied to the target block, and lfnst index values ​​1 to 3 can indicate three transform cores.

[0141] The transformer can perform an inseparable secondary transform based on the selected transform kernel, and can obtain modified (secondary) transform coefficients. As described above, the modified transform coefficients can be derived as transform coefficients quantized by a quantizer, and can be encoded and signaled to a decoding device, and transmitted to a dequantizer / inverse transformer in an encoding device.

[0142] In addition, as described above, if the secondary transform is omitted, the (primary) transform coefficients that are the output of the primary (separable) transform can be derived as transform coefficients quantized by the quantizer as described above, and can be encoded and signaled to the decoding device and transmitted to the dequantizer / inverse transformer in the encoding device.

[0143] The inverse transformer may perform a series of processes in the reverse order of the order already performed in the above-mentioned transformer. The inverse transformer may receive the (dequantized) transform coefficients, and derive the (primary) transform coefficients by performing a secondary (inverse) transform (S550), and may obtain the residual block (residual sample) by performing a (inverse) transform on the (primary) transform coefficients (S560). In this regard, from the perspective of the inverse transformer, the primary transform coefficients may be referred to as modified transform coefficients. As described above, the encoding device and the decoding device may generate a reconstructed block based on the residual block and the prediction block, and may generate a reconstructed picture based on the reconstructed block.

[0144] The decoding device may further include a secondary inverse transform application determiner (or an element for determining whether to apply a secondary inverse transform) and a secondary inverse transform determiner (or an element for determining a secondary inverse transform). The secondary inverse transform application determiner may determine whether to apply a secondary inverse transform. For example, the secondary inverse transform may be NSST, RST, or LFNST, and the secondary inverse transform application determiner may determine whether to apply a secondary inverse transform based on a secondary transform flag obtained by parsing the bitstream. In another example, the secondary inverse transform application determiner may determine whether to apply a secondary inverse transform based on a transform coefficient of a residual block.

[0145] The secondary inverse transform determiner may determine the secondary inverse transform. In this case, the secondary inverse transform determiner may determine the secondary inverse transform applied to the current block based on the LFNST (NSST or RST) transform set specified according to the intra prediction mode. In an embodiment, the secondary transform determination method may be determined depending on the primary transform determination method. Various combinations of the primary transform and the secondary transform may be determined according to the intra prediction mode. In addition, in an example, the secondary inverse transform determiner may determine the area to which the secondary inverse transform is applied based on the size of the current block.

[0146] In addition, as described above, if the secondary (inverse) transform is omitted, the (dequantized) transform coefficients can be received, a (separable) inverse transform can be performed, and a residual block (residual sample) can be obtained. As described above, the encoding device and the decoding device can generate a reconstructed block based on the residual block and the prediction block, and can generate a reconstructed picture based on the reconstructed block.

[0147] Furthermore, in the present disclosure, a reduced secondary transform (RST) in which the size of a transformation matrix (kernel) is reduced may be applied in the concept of NSST in order to reduce the amount of calculation and storage required for an inseparable secondary transform.

[0148] In addition, the transformation kernel, transformation matrix, and coefficients constituting the transformation kernel matrix described in the present disclosure, that is, the kernel coefficients or matrix coefficients, can be represented by 8 bits. This can be a condition for implementation in a decoding device and an encoding device, and compared with the existing 9 bits or 10 bits, the amount of storage required to store the transformation kernel can be reduced, and performance degradation can be reasonably adapted. In addition, representing the kernel matrix with 8 bits can allow the use of small multipliers and can be more suitable for single instruction multiple data (SIMD) instructions for optimal software implementation.

[0149] In the present specification, the term "RST" may refer to a transformation performed on residual samples of a target block based on a transformation matrix whose size is reduced according to a reduction factor. In the case of performing a reduced transformation, the amount of calculation required for the transformation can be reduced due to the reduction in the size of the transformation matrix. That is, RST can be used to solve the computational complexity problem that occurs when transforming a block of large size or an inseparable transform.

[0150] RST may be referred to as various terms such as reduced transform, reduced secondary transform, reduced transform, simplified transform, and simple transform, and the names that RST may be referred to as are not limited to the listed examples. Alternatively, since RST is mainly performed in a low-frequency region including non-zero coefficients in a transform block, it may be referred to as a low-frequency non-separable transform (LFNST). A transform index may be referred to as a LFNST index.

[0151] In addition, when performing a secondary inverse transform based on RST, the inverse transformer 235 of the encoding device 200 and the inverse transformer 322 of the decoding device 300 may include: an inverse reduction secondary transformer that derives a modified transform coefficient based on an inverse RST of the transform coefficient; and an inverse primary transformer that derives a residual sample of the target block based on an inverse primary transform of the modified transform coefficient. The inverse primary transform refers to an inverse transform of a primary transform applied to the residual. In the present disclosure, deriving a transform coefficient based on a transform may refer to deriving a transform coefficient by applying a transform.

[0152] Figure 7is a diagram illustrating an RST according to an embodiment of the present disclosure.

[0153] In the present disclosure, a “target block” may refer to a current block to be encoded, a residual block, or a transform block.

[0154] In the RST according to the example, an N-dimensional vector can be mapped to an R-dimensional vector located in another space, so that a reduction transformation matrix can be determined, where R is less than N. N may refer to the square of the length of the side of the block to which the transformation is applied, or the total number of transformation coefficients corresponding to the block to which the transformation is applied, and the reduction factor may refer to an R / N value. The reduction factor may be referred to as a reduction factor, a reduction factor, a simplification factor, a simple factor, or other various terms. In addition, R may be referred to as a reduction coefficient, but depending on the situation, the reduction factor may refer to R. In addition, depending on the situation, the reduction factor may refer to an N / R value.

[0155] In the example, the reduction factor or reduction coefficient may be signaled through the bitstream, but the example is not limited thereto. For example, a predetermined value for the reduction factor or reduction coefficient may be stored in each of the encoding device 200 and the decoding device 300, and in this case, the reduction factor or reduction coefficient may not be signaled separately.

[0156] The size of the reduced transform matrix according to an example may be R×N which is smaller than N×N (the size of a conventional transform matrix), and may be defined as in Equation 4 below.

[0157] [Formula 4]

[0158]

[0159] Figure 7 The matrix T in the reduced transform block shown in (a) may refer to the matrix T of Formula 4 R×N .like Figure 7 As shown in (a), when the reduction transformation matrix T R×N When multiplied by the residual samples of the target block, the transform coefficients of the current block can be derived.

[0160] In the example, if the size of the block to which the transform is applied is 8×8 and R=16 (ie, R / N=16 / 64=1 / 4), then according to Figure 7 The RST of (a) can be expressed as a matrix operation shown in the following Equation 5. In this case, the storage and multiplication calculations can be reduced to about 1 / 4 by a reduction factor.

[0161] In the present disclosure, a matrix operation may be understood as an operation of obtaining a column vector by multiplying a column vector by a matrix provided on the left side of the column vector.

[0162] [Formula 5]

[0163]

[0164] In formula 5, r1 to r 64 y may represent the residual sample of the target block, and specifically may be a transform coefficient generated by applying a transform. As a result of the calculation of Equation 5, the transform coefficient c of the target block may be derived. i , and derive c i The process can be shown as formula 6.

[0165] [Formula 6]

[0166]

[0167] As a result of the calculation of Equation 6, the transform coefficients c1 to c R That is, when R=16, the transform coefficients c1 to c 16 . If a conventional transform is applied instead of RST, and a transform matrix of a size of 64×64 (N×N) is multiplied by a residual sample of a size of 64×1 (N×1), only 16 (R) transform coefficients are derived for the target block because RST is applied, although 64 (N) transform coefficients are derived for the target block. Since the total number of transform coefficients for the target block is reduced from N to R, the amount of data transmitted by the encoding device 200 to the decoding device 300 is reduced, and thus the transmission efficiency between the encoding device 200 and the decoding device 300 can be improved.

[0168] When considered from the perspective of the size of the transformation matrix, the size of the conventional transformation matrix is ​​64×64 (N×N), but the size of the reduced transformation matrix is ​​reduced to 16×64 (R×N), so the storage usage in the case of performing RST can be reduced by the R / N ratio compared to the case of performing the conventional transformation. In addition, when compared with the number of multiplication calculations N×N in the case of using the conventional transformation matrix, the use of the reduced transformation matrix can reduce the number of multiplication calculations (R×N) by the R / N ratio.

[0169] In an example, the transformer 232 of the encoding device 200 may derive the transform coefficient of the target block by performing a primary transform and a secondary transform based on RST on the residual samples of the target block. These transform coefficients may be transmitted to the inverse transformer of the decoding device 300, and the inverse transformer 322 of the decoding device 300 may derive the modified transform coefficient based on the inverse reduced secondary transform (RST) for the transform coefficient, and may derive the residual sample of the target block based on the inverse primary transform for the modified transform coefficient.

[0170] According to the example, the inverse RST matrix T N×RThe size of is N×R which is smaller than the size of the conventional inverse transform matrix N×N, and is similar to the reduced transform matrix T shown in Equation 4. R×N Has a transposition relationship.

[0171] Figure 7 The matrix T in the reduced inverse transform block shown in (b) t It can refer to the inverse RST matrix T N×R T (The superscript T refers to transposition). Figure 7 As shown in (b), when the inverse RST matrix T N×R T When multiplied by the transform coefficients of the target block, the modified transform coefficients of the target block or the residual samples of the target block can be derived. R×N T It can be expressed as (T R×N ) T N×R .

[0172] More specifically, when the inverse RST is used as the secondary inverse transform, when the inverse RST matrix T N×R T When multiplied by the transform coefficients of the target block, the modified transform coefficients of the target block can be derived. In addition, the inverse RST can be used as an inverse primary transform, and in this case, when the inverse RST matrix T N×R T When multiplied by the transform coefficients of the target block, the residual samples of the target block can be derived.

[0173] In the example, if the size of the block to which the inverse transform is applied is 8×8 and R=16 (ie, R / N=16 / 64=1 / 4), then according to Figure 7 The RST of (b) can be expressed as the matrix operation shown in the following Equation 7.

[0174] [Formula 7]

[0175]

[0176] In formula 7, c1 to c 16 As a result of the calculation of Equation 7, r representing the modified transform coefficient of the target block or the residual sample of the target block can be derived. i , and derive r i The process can be shown as formula 8.

[0177] [Formula 8]

[0178]

[0179] As a result of the calculation of Equation 8, r1 to r2 representing the modified transform coefficients of the target block or the residual samples of the target block can be derived. N . From the perspective of the size of the inverse transformation matrix, the size of the conventional inverse transformation matrix is ​​64×64 (N×N), but the size of the inverse reduction transformation matrix is ​​reduced to 64×16 (R×N), so the storage usage rate in the case of performing the inverse RST can be reduced by the R / N ratio compared to the case of performing the conventional inverse transformation. In addition, when compared with the number of multiplication calculations N×N in the case of using the conventional inverse transformation matrix, the use of the inverse reduction transformation matrix can reduce the number of multiplication calculations (N×R) by the R / N ratio.

[0180] The transform set configuration shown in Table 2 can also be applied to 8×8 RST. That is, 8×8 RST can be applied according to the transform set in Table 2. Since a transform set includes two or three transforms (kernels) according to the intra prediction mode, it can be configured to select one of up to four transforms including the case where the secondary transform is not applied. In the transform where the secondary transform is not applied, the application of the identity matrix can be considered. Assuming that indexes 0, 1, 2, and 3 are assigned to the four transforms respectively (for example, index 0 can be assigned to the case where the identity matrix is ​​applied, that is, the case where the secondary transform is not applied), the transform index or lfnst index as a syntax element can be signaled for each transform coefficient block, thereby specifying the transform to be applied. That is, for the upper left 8×8 block, the 8×8 NSST in the RST configuration can be specified by the transform index, or the 8×8 lfnst can be specified when LFNST is applied. 8×8lfnst and 8×8RST refer to the transform applicable to an 8×8 region included in a transform coefficient block when W and H of a target block to be transformed are both equal to or greater than 8, and the 8×8 region may be the upper left 8×8 region in the transform coefficient block. Similarly, 4×4lfnst and 4×4RST refer to the transform applicable to a 4×4 region included in a transform coefficient block when W and H of a target block are both equal to or greater than 4, and the 4×4 region may be the upper left 4×4 region in the transform coefficient block.

[0181] According to an embodiment of the present disclosure, for the transformation in the encoding process, only 48 pieces of data can be selected, and a maximum 16×48 transform kernel matrix can be applied thereto, instead of applying a 16×64 transform kernel matrix to 64 pieces of data forming an 8×8 region. Here, "maximum" means that m has a maximum value of 16 in the m×48 transform kernel matrix for generating m coefficients. That is, when RST is performed by applying an m×48 transform kernel matrix (m≤16) to an 8×8 region, 48 pieces of data are input, and m coefficients are generated. When m is 16, 48 pieces of data are input and 16 coefficients are generated. That is, assuming that 48 pieces of data form a 48×1 vector, the 16×48 matrix and the 48×1 vector are multiplied in sequence, thereby generating a 16×1 vector. Here, the 48 pieces of data forming an 8×8 region can be appropriately arranged, thereby forming a 48×1 vector. For example, a 48×1 vector can be constructed based on 48 pieces of data constituting an area other than the lower right 4×4 area among the 8×8 area. Here, when the matrix operation is performed by applying the maximum 16×48 transform kernel matrix, 16 modified transform coefficients are generated and the 16 modified transform coefficients can be arranged in the upper left 4×4 region according to the scanning order, and the upper right 4×4 region and the lower left 4×4 region can be filled with zeros.

[0182] For the inverse transform in the decoding process, the transposed matrix of the aforementioned transform kernel matrix can be used. That is, when the inverse RST or LFNST is performed in the inverse transform process performed by the decoding device, the input coefficient data to which the inverse RST is applied is configured in a one-dimensional vector according to a predetermined arrangement order, and the modified coefficient vector obtained by multiplying the one-dimensional vector with the corresponding inverse RST matrix on the left side of the one-dimensional vector can be arranged in a two-dimensional block according to a predetermined arrangement order.

[0183] In summary, in the transformation process, when RST or LFNST is applied to an 8×8 region, 48 transform coefficients in the upper left region, the upper right region, and the lower left region except the lower right region of the 8×8 region are matrix-operated with a 16×48 transform kernel matrix. For the matrix operation, the 48 transform coefficients are input in a one-dimensional array. When the matrix operation is performed, 16 modified transform coefficients are derived, and the modified transform coefficients can be arranged in the upper left region of the 8×8 region.

[0184] On the contrary, in the inverse transform process, when the inverse RST or LFNST is applied to the 8×8 area, 16 transform coefficients corresponding to the upper left area of ​​the 8×8 area among the transform coefficients in the 8×8 area can be input in a one-dimensional array according to the scanning order, and can undergo a matrix operation with a 48×16 transform kernel matrix. That is, the matrix operation can be expressed as (48×16 matrix)*(16×1 transform coefficient vector)=(48×1 modified transform coefficient vector). Here, the n×1 vector can be interpreted as having the same meaning as the n×1 matrix, and can therefore be expressed as an n×1 column vector. In addition, * represents matrix multiplication. When the matrix operation is performed, 48 modified transform coefficients can be derived, and the 48 modified transform coefficients can be arranged in the upper left area, the upper right area, and the lower left area of ​​the 8×8 area except the lower right area.

[0185] When the inverse secondary transform is based on RST, the inverse transformer 235 of the encoding device 200 and the inverse transformer 322 of the decoding device 300 may include an inverse reduction secondary transformer for deriving modified transform coefficients based on an inverse RST of the transform coefficients and an inverse primary transformer for deriving residual samples of the target block based on an inverse primary transform of the modified transform coefficients. The inverse primary transform refers to an inverse transform of a primary transform applied to the residual. In the present disclosure, deriving transform coefficients based on a transform may refer to deriving transform coefficients by applying a transform.

[0186] The non-separable transform (LFNST) described above will be described in detail as follows: LFNST may include a forward transform performed by an encoding device and an inverse transform performed by a decoding device.

[0187] The encoding device receives as input a result (or a portion of a result) derived after applying a primary (core) transform, and applies a forward secondary transform (secondary transform).

[0188] [Formula 9]

[0189] y=G T x

[0190] In Equation 9, x and y are the input and output of the secondary transform, respectively, G is a matrix representing the secondary transform, and the transform basis vectors consist of column vectors. In the case of inverse LFNST, when the dimension of the transform matrix G is expressed as [number of rows × number of columns], in the case of forward LFNST, the transpose of the matrix G becomes G T Dimension.

[0191] For inverse LFNST, the dimensions of the matrix G are [48×16], [48×8], [16×16], [16×8], and the [48×8] matrix and the [16×8] matrix are partial matrices of 8 transformation basis vectors sampled from the left side of the [48×16] matrix and the [16×16] matrix, respectively.

[0192] On the other hand, for forward LFNST, the matrix G T The dimensions of are [16×48], [8×48], [16×16], [8×16], and the [8×48] matrix and the [8×16] matrix are partial matrices obtained by sampling 8 transformation basis vectors from the upper parts of the [16×48] matrix and the [16×16] matrix, respectively.

[0193] Therefore, in the case of forward LFNST, a [48×1] vector or a [16×1] vector can be used as input x, and a [16×1] vector or a [8×1] vector can be used as output y. In video encoding and decoding, the output of the forward primary transform is two-dimensional (2D) data, so in order to construct a [48×1] vector or a [16×1] vector as input x, it is necessary to construct a one-dimensional vector by appropriately arranging the 2D data as the output of the forward transform.

[0194] Figure 8 is a diagram illustrating an order of arranging output data of a forward primary transform into a one-dimensional vector according to an example. Figure 8 The left figures of (a) and (b) show the order for constructing a [48×1] vector, and Figure 8 The right figures of (a) and (b) show the order for constructing a [16×1] vector. In the case of LFNST, the 2D data can be constructed by Figure 8 The same order as in (a) and (b) is sequentially arranged to obtain a one-dimensional vector x.

[0195] The arrangement direction of the output data of the forward primary transform may be determined according to the intra-frame prediction mode of the current block. For example, when the intra-frame prediction mode of the current block is in the horizontal direction relative to the diagonal direction, the arrangement direction of the output data of the forward primary transform may be determined according to the intra-frame prediction mode of the current block. Figure 8 The output data of the forward primary transform are arranged in the order of (a), and when the intra prediction mode of the current block is in a vertical direction relative to the diagonal direction, the output data of the forward primary transform can be arranged in the order of (a). Figure 8 The output data of the forward primary transformation are arranged in the order of (b).

[0196] According to the example, different from Figure 8 The arrangement order of (a) and (b) is shown in Figure 1, and in order to derive and apply Figure 8If the arrangement order of (a) and (b) is the same result (y vector), the column vectors of the matrix G can be rearranged according to the arrangement order. In other words, the column vectors of G can be rearranged so that each element constituting the x vector is always multiplied by the same transformation basis vector.

[0197] Since the output y derived by Formula 9 is a one-dimensional vector, when two-dimensional data is required as input data in a process using the result of a forward quadratic transform as input (for example, in a process of performing quantization or residual encoding), the output y vector of Formula 9 needs to be properly arranged as 2D data again.

[0198] Fig. 9 is a diagram illustrating an order of arranging output data of a forward quadratic transform into two-dimensional blocks according to an example.

[0199] In the case of LFNST, the output values ​​may be arranged in 2D blocks according to a predetermined scanning order. Fig. 9 (a) shows that when the output y is a [16×1] vector, the output values ​​are arranged at 16 positions of the 2D block according to a diagonal scanning order. Fig. 9 (b) shows that when the output y is a [8×1] vector, the output values ​​are arranged at 8 positions of the 2D block according to a diagonal scanning order, and the remaining 8 positions are filled with zeros. Fig. 9 The X in (b) indicates that it is filled with zeros.

[0200] According to another example, since the order of processing the output vector y when performing quantization or residual encoding can be preset, the output vector y may not be arranged in a sequence such as Fig. 9 However, in the case of residual coding, data encoding may be performed in 2D block (eg, 4×4) units (eg, CG (coefficient group)), and in this case, according to Fig. 9 The data is arranged in a specific order in the diagonal scan order of .

[0201] In addition, the decoding device may configure a one-dimensional input vector y by arranging two-dimensional data output through a dequantization process according to a preset scanning order for inverse transformation. The input vector y may be output as an output vector x through the following equation.

[0202] [Formula 10]

[0203] x=Gy

[0204] In the case of inverse LFNST, the output vector x can be derived by multiplying the input vector y, which is a [16×1] vector or a [8×1] vector, by the G matrix. For inverse LFNST, the output vector x can be a [48×1] vector or a [16×1] vector.

[0205] The output vector x is based on Figure 8 The order shown in is arranged in a two-dimensional block and is arranged as two-dimensional data, and the two-dimensional data becomes input data (or a part of the input data) of an inverse primary transform.

[0206] Thus, the inverse quadratic transform is overall the reverse of the forward quadratic transform process, and in the case of an inverse transform, unlike in the forward direction, the inverse quadratic transform is applied first and then the inverse primary transform.

[0207] In the inverse LFNST, one of 8 [48×16] matrices and 8 [16×16] matrices can be selected as the transformation matrix G. Whether to apply the [48×16] matrix or the [16×16] matrix depends on the size and shape of the block.

[0208] In addition, 8 matrices can be derived from the four transform sets shown in Table 2 above, and each transform set can be composed of two matrices. Which transform set to use among the 4 transform sets is determined according to the intra prediction mode, and more specifically, the transform set is determined based on the value of the intra prediction mode extended by considering wide-angle intra prediction (WAIP). Which matrix is ​​selected from the two matrices constituting the selected transform set is derived by index signaling. More specifically, 0, 1, and 2 can be used as the index values ​​sent, 0 can indicate that LFNST is not applied, and 1 and 2 can indicate any one of the two transform matrices constituting the transform set selected based on the intra prediction mode value.

[0209] Fig.10 is a diagram illustrating a wide-angle intra prediction mode according to an embodiment of this document.

[0210] The general intra prediction mode value may have a value from 0 to 66 and from 81 to 83, and the intra prediction mode value extended due to WAIP may have a value from -14 to 83 as shown. The value from 81 to 83 indicates the CCLM (Cross Component Linear Model) mode, and the value from -14 to -1 and the value from 67 to 80 indicate the intra prediction mode extended due to WAIP application.

[0211] When the width of the current prediction block is greater than the height, the upper reference pixel is generally closer to the position inside the block to be predicted. Therefore, prediction in the lower left direction can be more accurate than in the upper right direction. Conversely, when the height of the block is greater than the width, the left reference pixel is generally closer to the position inside the block to be predicted. Therefore, prediction in the upper right direction can be more accurate than in the lower left direction. Therefore, it can be advantageous to apply remapping (i.e., mode index modification) to the index of the wide-angle intra prediction mode.

[0212] When wide-angle intra prediction is applied, information about the existing intra prediction may be signaled, and after the information is parsed, the information may be remapped to the index of the wide-angle intra prediction mode. Therefore, the total number of intra prediction modes for a specific block (e.g., a non-square block of a specific size) may not be changed, that is, the total number of intra prediction modes is 67, and the intra prediction mode encoding for the specific block may not be changed.

[0213] Table 3 below shows a process of deriving a modified intra mode by remapping the intra prediction mode to the wide-angle intra prediction mode.

[0214] [Table 3]

[0215]

[0216] In Table 3, the extended intra prediction mode value is finally stored in the predModeIntra variable, and ISP_NO_SPLIT indicates that the CU block is not divided into sub-partitions by the intra sub-partitioning (ISP) technology currently adopted in the VVC standard, and the cIdx variable values ​​of 0, 1, and 2 indicate the cases of the luminance component, the Cb component, and the Cr component, respectively. The log2 function shown in Table 3 returns a log value with a base of 2, and the Abs function returns an absolute value.

[0217] The variable predModeIntra indicating the intra prediction mode and the height and width of the transform block are used as input values ​​of the wide-angle intra prediction mode mapping process, and the output value is the modified intra prediction mode predModeIntra. The height and width of the transform block or the coding block can be the height and width of the current block for remapping of the intra prediction mode. At this time, the variable whRatio reflecting the ratio of width to width can be set to Abs(Log2(nW / nH)).

[0218] For non-square blocks, the intra prediction mode can be divided into two cases and modified.

[0219] First, if all of conditions (1) to (3) are satisfied, (1) the width of the current block is greater than the height, (2) the intra prediction mode before modification is equal to or greater than 2, and (3) the intra prediction mode is less than the value derived as (8+2*whRatio) when the variable whRatio is greater than 1 and is less than 8 when the variable whRatio is less than or equal to 1 (predModeIntra is less than (whRatio>1)? (8+2*whRatio): 8), then the intra prediction mode is set to a value 65 greater than predModeIntra [predModeIntra is set equal to (predModeIntra+65)].

[0220] If different from the above, that is, if conditions (1) to (3) are satisfied, (1) the height of the current block is greater than the width, (2) the intra-frame prediction mode before modification is less than or equal to 66, and (3) the intra-frame prediction mode is greater than the value derived as (60-2*whRatio) when whRatio is greater than 1 and is greater than 60 when whRatio is less than or equal to 1 (predModeIntra is greater than (whRatio>1)? (60-2*whRatio):60), then the intra-frame prediction mode is set to a value that is 67 less than predModeIntra [predModeIntra is set equal to (predModeIntra-67)].

[0221] Table 2 above shows how to select a transform set based on the intra prediction mode value extended by WAIP in LFNST. Fig. 9 As shown, modes 14 to 33 and modes 35 to 80 are symmetric about the prediction direction around mode 34. For example, mode 14 and mode 54 are symmetric about the direction corresponding to mode 34. Therefore, the same set of transformations is applied to modes located in mutually symmetric directions, and this symmetry is also reflected in Table 2.

[0222] In addition, it is assumed that the forward LFNST input data of mode 54 is symmetric with the forward LFNST input data of mode 14. For example, for mode 14 and mode 54, according to Figure 8 (a) and Figure 8 The arrangement order shown in (b) rearranges the two-dimensional data into one-dimensional data. In addition, it can be seen that Figure 8 (a) and Figure 8 The pattern of the sequence shown in (b) is symmetrical about the direction indicated by the pattern 34 (diagonal direction).

[0223] Furthermore, as described above, which transform matrix of the [48×16] matrix and the [16×16] matrix is ​​applied to the LFNST is determined by the size and shape of the transform target block.

[0224] Fig.11 is a diagram illustrating a block shape to which LFNST is applied. Fig.11 (a) shows a 4×4 block, Fig.11 (b) shows a 4×8 block and an 8×4 block, Fig.11 (c) shows a 4×N block or an N×4 block, where N is 16 or greater, Fig.11 (d) shows an 8×8 block, Fig.11 (e) shows an M×N block, where M≥8, N≥8 and N>8 or M>8.

[0225] exist Fig.11In , the blocks with thick borders indicate the areas where LFNST is applied. Fig.11 For the blocks (a) and (b), LFNST is applied to the top left 4×4 region, and for Fig.11 In the block (c), LFNST is applied separately to the two upper left 4×4 regions that are arranged consecutively. Fig.11 In (a), (b), and (c), since LFNST is applied in units of 4×4 regions, this LFNST will be referred to as “4×4LFNST” hereinafter. Depending on the matrix dimension of G, a [16×16] or [16×8] matrix may be applied.

[0226] More specifically, the [16×8] matrix is ​​applied to Fig.11 (a) 4×4 block (4×4TU or 4×4CU), and the [16×16] matrix is ​​applied to Fig.11 This is to adjust the worst case computational complexity to 8 multiplications per sample.

[0227] about Fig.11 In (d) and (e), LFNST is applied to the upper left 8×8 region, and this LFNST is hereinafter referred to as "8×8LFNST". As the corresponding transformation matrix, a [48×16] matrix or a [48×8] matrix may be applied. In the case of the forward LFNST, since a [48×1] vector (the X vector in Equation 9) is input as input data, not all sample values ​​of the upper left 8×8 region are used as input values ​​of the forward LFNST. That is, as can be seen from Figure 8 The left order of (a) or Figure 8 As can be seen from the left side sequence of (b), a [48×1] vector can be constructed based on samples belonging to the remaining 3 4×4 blocks while leaving the lower right 4×4 block as is.

[0228] The [48×8] matrix can be applied to Fig.11 The 8×8 block (8×8TU or 8×8CU) in (d) and the [48×16] matrix can be applied to Fig.11 The 8×8 block in (e) is also to adjust the worst-case computational complexity to 8 multiplications per sample.

[0229] Depending on the block shape, when the corresponding forward LFNST (4×4 or 8×8 LFNST) is applied, 8 or 16 output data (Y vector in Eq. 9, [8×1] or [16×1] vector) are generated. In the forward LFNST, since the matrix G T The characteristic is that the amount of output data is equal to or less than the amount of input data.

[0230] Fig.12 is a diagram illustrating arrangement of output data of a forward LFNST according to an example, and shows blocks in which the output data of the forward LFNST is arranged according to a block shape.

[0231] exist Fig.12 The shaded area on the upper left of the block shown corresponds to the area where the output data of the forward LFNST is located, the positions marked with 0 indicate samples filled with the value 0, and the remaining area represents the area not changed by the forward LFNST. In the area not changed by the LFNST, the output data of the forward primary transform remains unchanged.

[0232] As described above, since the size of the applied transformation matrix varies according to the shape of the block, the amount of output data also varies. Fig.12 , the output data of the forward LFNST may not completely fill the upper left 4×4 block. Fig.12 In the cases of (a) and (d) of , the [16×8] matrix and the A[48×8] matrix are applied to the block or partial area inside the block indicated by the bold line, respectively, and an [8×1] vector is generated as the output of the forward LFNST. That is, according to Fig. 9 The scanning order shown in (b) can only fill 8 output data, such as Fig.12 As shown in (a) and (d), the remaining 8 positions can be filled with 0. Fig.11 (d) The case of LFNST application blocks, such as Fig.12 As shown in (d), the two 4×4 blocks on the upper right and lower left adjacent to the upper left 4×4 block are also filled with values ​​of 0.

[0233] As described above, basically, by signaling the LFNST index, it is specified whether to apply LFNST and the transformation matrix to be applied. Fig.12 As shown, when LFNST is applied, since the number of output data of the forward LFNST may be equal to or less than the number of input data, an area filled with zero values ​​appears as follows.

[0234] 1) If Fig.12 As shown in (a), samples from the eighth position and subsequent positions in the scanning order in the upper left 4×4 block, that is, samples from the ninth to sixteenth positions.

[0235] 2) If Fig.12 As shown in (d) and (e), when the [48×16] matrix or the [48×8] matrix is ​​applied, two 4×4 blocks adjacent to the upper left 4×4 block or the second and third 4×4 blocks in the scanning order.

[0236] Therefore, if non-zero data exists by checking areas 1) and 2), it is determined that LFNST is not applied, so that signaling of the corresponding LFNST index can be omitted.

[0237] According to an example, for example, in the case of LFNST adopted in the VVC standard, since the signaling of the LFNST index is performed after the residual coding, the encoding device can know whether there is non-zero data (significant coefficient) at all positions within the TU or CU block through the residual coding. Therefore, the encoding device can determine whether to perform signaling about the LFNST index based on the presence of non-zero data, and the decoding device can determine whether to parse the LFNST index. When non-zero data does not exist in the area specified in 1) and 2) above, the signaling of the LFNST index is performed.

[0238] In addition, for the adopted LFNST, the following simplification method can be applied.

[0239] (i) According to an example, the number of output data of the forward LFNST may be limited to a maximum value of 16.

[0240] exist Fig.11 In the case of (c), 4×4 LFNST can be applied to two 4×4 regions adjacent to the upper left, respectively, and in this case, a maximum of 32 LFNST output data can be generated. When the number of output data of the forward LFNST is limited to a maximum of 16, in the case of a 4×N / N×4 (N≥16) block (TU or CU), 4×4 LFNST is applied only to one 4×4 region in the upper left, and LFNST can be applied only to Fig.11 By doing this, the implementation of image coding can be simplified.

[0241] (ii) According to an example, clearing may be additionally applied to an area to which LFNST is not applied. In this document, clearing may mean filling all positions belonging to a specific area with a value of 0. That is, clearing may be applied to an area that is unchanged due to LFNST, and the result of the forward primary transformation is maintained. As described above, since LFNST is divided into 4×4 LFNST and 8×8 LFNST, clearing may be divided into two types ((ii)-(A) and (ii)-(B)) as follows.

[0242] (ii)-(A) When 4×4 LFNST is applied, an area to which 4×4 LFNST is not applied may be cleared. Fig.13 is a diagram illustrating clearing in a block to which 4×4 LFNST is applied according to an example.

[0243] like Fig.13 As shown, for the block to which 4×4 LFNST is applied, that is, for Fig.12For all blocks in (a), (b), and (c), the entire area where LFNST is not applied can be filled with zeros.

[0244] on the other hand, Fig.13 (d) shows that when the maximum value of the number of output data according to the example forward LFNST is limited to 16, clearing is performed on the remaining blocks to which the 4×4 LFNST is not applied.

[0245] (ii)-(B) When 8×8 LFNST is applied, an area to which 8×8 LFNST is not applied may be cleared. Fig.14 is a diagram illustrating clearing in a block to which 8×8 LFNST is applied according to an example.

[0246] like Fig.14 As shown, for the block with 8×8 LFNST applied, that is, Fig.12 For all blocks in (d) and (e), the entire area where LFNST is not applied can be filled with zeros.

[0247] (iii) Due to the zeroing presented in (ii) above, the area filled with zeros may not be the same as when LFNST is applied. Fig.12 The wider region of the LFNST case performs the zeroing proposed in (ii) to check whether there is non-zero data.

[0248] For example, when (ii)-(B) is applied, in checking Fig.12 After the zero-filled areas in (d) and (e) have non-zero data, additionally check Fig.14 Whether there is non-zero data in the area filled with 0, the signaling for the LFNST index can be performed only when there is no non-zero data.

[0249] Of course, even if the clearing proposed in (ii) is applied, the presence of non-zero data can be checked in the same way as the existing LFNST index signaling. Fig.12 After checking whether there is non-zero data in the block filled with zeros in , LFNST index signaling can be applied. In this case, the encoding device only performs zero clearing and the decoding device does not assume zero clearing, that is, it only checks whether non-zero data exists only in Fig.12 In the regions explicitly marked as 0 in , LFNST index parsing can be performed.

[0250] Various embodiments of the combination of the simplified methods ((i), (ii)-(A), (ii)-(B), (iii)) applying LFNST can be derived. Of course, the combination of the simplified methods described above is not limited to the following embodiments, and any combination can be applied to LFNST.

[0251] Implementation

[0252] -Limit the number of output data of the forward LFNST to a maximum of 16 → (i)

[0253] -When 4×4 LFNST is applied, all regions where 4×4 LFNST is not applied are cleared → (II)-(A)

[0254] - When 8×8 LFNST is applied, all regions where 8×8 LFNST is not applied are cleared → (II)-(B)

[0255] - After checking whether non-zero data also exists in the existing areas filled with zero values ​​and in the areas filled with zeros due to the additional clearing ((ii)-(A), (ii)-(B)), the LFNST index is signaled only if non-zero data does not exist → (iii).

[0256] In the case of an embodiment, when LFNST is applied, the area where non-zero output data can exist is limited to the interior of the upper left 4×4 area. Fig.13 (a) and Fig.14 In the case of (a), the eighth position in the scan order is the last position where non-zero data can exist. Fig.13 (b) and (c) and Fig.14 In the case of (b), the sixteenth position in the scanning order (ie, the position of the lower right edge of the upper left 4×4 block) is the last position in which data other than 0 may exist.

[0257] Therefore, after applying LFNST, after checking whether non-zero data exists at a position that is not allowed by the residual encoding process (at a position beyond the last position), it can be determined whether to signal the LFNST index.

[0258] In the case of the zeroing method proposed in (ii), the amount of calculation required to perform the entire transformation process can be reduced due to the amount of data ultimately generated when both the primary transformation and LFNST are applied. That is, when LFNST is applied, since zeroing is applied to the forward primary transformation output data exists in the area where LFNST is not applied, it is not necessary to generate data for the area that becomes zeroed during the execution of the forward primary transformation. Therefore, the amount of calculation required to generate the corresponding data can be reduced. The additional effects of the zeroing method proposed in (ii) are summarized as follows.

[0259] First, as mentioned above, the amount of computation required to perform the entire transformation process is reduced.

[0260] In particular, when (ii)-(B) is applied, the amount of calculation in the worst case is reduced, so that the transformation process can be made lighter. In other words, in general, a large amount of calculation is required to perform a large-size primary transformation. By applying (ii)-(B), the amount of data derived as a result of performing forward LFNST can be reduced to 16 or less. In addition, as the size of the entire block (TU or CU) increases, the effect of reducing the amount of transformation operations further increases.

[0261] Second, the amount of computation required for the entire transformation process can be reduced, thereby reducing the power consumption required to perform the transformation.

[0262] Third, the delay involved in the transformation process is reduced.

[0263] Secondary transforms such as LFNST add computational effort to the existing primary transform, thus increasing the overall latency involved in performing the transform. In particular, in the case of intra prediction, since reconstructed data of neighboring blocks is used in the prediction process, during encoding, the increase in latency due to the secondary transform results in an increase in latency until reconstruction. This can result in an increase in the overall latency of intra prediction encoding.

[0264] However, if the clearing proposed in (ii) is applied, the delay time for performing one transform can be greatly reduced when LFNST is applied, maintaining or reducing the delay time of the entire transform, so that the encoding device can be implemented more simply.

[0265] In conventional intra prediction, the current block to be encoded is regarded as one coding unit, and encoding is performed without segmentation. However, intra sub-partitioning (ISP) encoding means performing intra prediction encoding by dividing the current block to be encoded in the horizontal direction or the vertical direction. In this case, a reconstructed block can be generated by performing encoding / decoding in units of divided blocks, and the reconstructed block can be used as a reference block for the next divided block. According to an embodiment, in ISP encoding, one coding block may be divided into two or four sub-blocks and encoded, and in ISP, in one sub-block, intra prediction is performed with reference to the reconstructed pixel values ​​of the sub-block located on the adjacent left side or the adjacent upper side. Hereinafter, "encoding" may be used as a concept including both encoding performed by an encoding device and decoding performed by a decoding device.

[0266] ISP is to divide the block predicted as a luma frame into two or four sub-partitions in the vertical or horizontal direction according to the size of the block. For example, the minimum block size to which ISP can be applied is 4×8 or 8×4. When the block size is larger than 4×8 or 8×4, the block is divided into 4 sub-partitions.

[0267] When ISP is applied, subblocks are encoded sequentially from left to right or from top to bottom according to the partition type (e.g., horizontally or vertically), and after performing reconstruction processing via inverse transform and intra prediction for one subblock, encoding of the next subblock can be performed. For the leftmost or topmost subblock, the reconstructed pixels of the already encoded coding block are referenced, as in the conventional intra prediction method. In addition, when each side of the subsequent internal subblock is not adjacent to the previous subblock, in order to derive the reference pixels adjacent to the corresponding side, the reconstructed pixels of the already encoded adjacent coding blocks are referenced, as in the conventional intra prediction method.

[0268] In ISP coding mode, all sub-blocks can be encoded with the same intra prediction mode, and a flag indicating whether ISP coding is used and a flag indicating in which direction to split (horizontally or vertically) can be signaled. At this time, the number of sub-blocks can be adjusted to 2 or 4 according to the shape of the block. When the size (width × height) of a sub-block is less than 16, it can be restricted so that division into corresponding sub-blocks is not allowed or ISP coding itself is not applied.

[0269] In case of the ISP prediction mode, one coding unit is divided into two or four partition blocks (ie, subblocks) and predicted, and the same intra prediction mode is applied to the divided two or four partition blocks.

[0270] As described above, in the division direction, both the horizontal direction (when an M×N coding unit having a horizontal length and a vertical length of M and N, respectively, is divided in the horizontal direction, if the M×N coding unit is divided into two, the M×N coding unit is divided into M×(N / 2) blocks, and if the M×N coding unit is divided into four blocks, the M×N coding unit is divided into M×(N / 4) blocks) and the vertical direction (when the M×N coding unit is divided in the vertical direction, if the M×N coding unit is divided into two, the M×N coding unit is divided into (M / 2)×N blocks, and if the M×N coding unit is divided into four, the M×N coding unit is divided into (M / 4)×N blocks) are possible. When the M×N coding unit is divided in the horizontal direction, the partition blocks are encoded in a top-to-bottom order, and when the M×N coding unit is divided in the vertical direction, the partition blocks are encoded in a left-to-right order. In the case of horizontal (vertical) partitioning, the currently encoded partition block can be predicted by referring to the reconstructed pixel values ​​of the upper (left) partition block.

[0271] A transformation may be applied to the residual signal generated in units of partition blocks by the ISP prediction method. A multi-transform selection (MTS) technique based on a DST-7 / DCT-8 combination and the existing DCT-2 may be applied to a forward-based primary transform (core transform), and a forward low-frequency non-separable transform (LFNST) may be applied to the transform coefficients generated from the primary transform to generate the final modified transform coefficients.

[0272] That is, LFNST can be applied to the partition blocks divided by applying the ISP prediction mode, and the same intra prediction mode is applied to the divided partition blocks, as described above. Therefore, when the LFNST set derived based on the intra prediction mode is selected, the derived LFNST set can be applied to all partition blocks. That is, because the same intra prediction mode is applied to all partition blocks, the same LFNST set can be applied to all partition blocks.

[0273] According to an embodiment, LFNST may be applied only to a transform block having both a horizontal length and a vertical length of 4 or more. Therefore, when the horizontal length or the vertical length of a partition block divided according to the ISP prediction method is less than 4, LFNST is not applied and the LFNST index is not signaled. In addition, when LFNST is applied to each partition block, the corresponding partition block may be regarded as one transform block. When the ISP prediction method is not applied, LFNST may be applied to the coding block.

[0274] A method of applying LFNST to each partition block will be described in detail.

[0275] According to an embodiment, after applying forward LFNST to each partition block, only a maximum of 16 (8 or 16) coefficients are left in the upper left 4×4 area in the transform coefficient scanning order, and then zeroing can be applied, where the remaining positions and areas are all filled with 0.

[0276] Alternatively, according to an embodiment, when the length of one side of the partition block is 4, LFNST is applied only to the upper left 4×4 region, and when the lengths of all sides of the partition block (i.e., width and height) are 8 or greater, LFNST can be applied to the remaining 48 coefficients within the upper left 8×8 region except for the lower right 4×4 region.

[0277] Alternatively, according to an embodiment, in order to adjust the worst-case computational complexity to 8 multiplications per sample, when each partition block is 4×4 or 8×8, only 8 transform coefficients may be output after applying the forward LFNST. That is, when the partition block is 4×4, an 8×16 matrix may be applied as the transform matrix, and when the partition block is 8×8, an 8×48 matrix may be applied as the transform matrix.

[0278] In the current VVC standard, LFNST index signaling is performed in units of coding units. Therefore, in ISP prediction mode and when LFNST is applied to all partition blocks, the same LFNST index value can be applied to the corresponding partition blocks. That is, when the LFNST index value is sent once at the coding unit level, the corresponding LFNST index can be applied to all partition blocks in the coding unit. As described above, the LFNST index value can have values ​​of 0, 1, and 2, where 0 represents the case where LFNST is not applied, and 1 and 2 represent two transform matrices present in one LFNST set when LFNST is applied.

[0279] As described above, the LFNST set is determined by the intra prediction mode, and in the case of the ISP prediction mode, since all partition blocks in a coding unit are predicted in the same intra prediction mode, the partition blocks can refer to the same LFNST set.

[0280] As another example, LFNST index signaling is still performed in units of coding units, but in the case of the ISP prediction mode, it is not determined whether LFNST is uniformly applied to all partition blocks, and for each partition block, whether to apply the LFNST index value signaled at the coding unit level and whether to apply LFNST can be determined by a separate condition. Here, a separate condition can be signaled in the form of a flag for each partition block through the bitstream, and when the flag value is 1, the LFNST index value signaled at the coding unit level is applied, and when the flag value is 0, LFNST may not be applied.

[0281] Hereinafter, a method of maintaining the worst-case computational complexity when applying LFNST to the ISP mode will be described.

[0282] In the case of ISP mode, when LFNST is applied, in order to keep the number of multiplications per sample (or per coefficient, per position) to a certain value or less, the application of LFNST may be limited. Depending on the size of the partition block, the number of multiplications per sample (or per coefficient, per position) can be kept at 8 or less by applying LFNST as follows.

[0283] 1. When both the horizontal length and the vertical length of the partition block are 4 or more, the same method as the worst-case computational complexity control method for LFNST in the current VVC standard may be applied.

[0284] That is, when the partition block is a 4×4 block, an 8×16 matrix obtained by sampling the upper 8 rows from the 16×16 matrix may be applied in the forward direction instead of the 16×16 matrix, and a 16×8 matrix obtained by sampling the left 8 columns from the 16×16 matrix may be applied in the reverse direction. In addition, when the partition block is an 8×8 block, in the forward direction, instead of the 16×48 matrix, an 8×48 matrix obtained by sampling the upper 8 rows from the 16×48 matrix is ​​applied, and in the reverse direction, instead of the 48×16 matrix, a 48×8 matrix obtained by sampling the left 8 columns from the 48×16 matrix may be applied.

[0285] In the case of a 4×N or N×4 (N>4) block, when forward transform is performed, 16 coefficients generated after applying the 16×16 matrix to only the upper left 4×4 block may be set in the upper left 4×4 region, and the other regions may be filled with a value of 0. In addition, when inverse transform is performed, the 16 coefficients located in the upper left 4×4 block are set in a scanning order to form an input vector, and then 16 output data may be generated by multiplying the 16×16 matrix. The generated output data may be set in the upper left 4×4 region, and the remaining regions except the upper left 4×4 region may be filled with a value of 0.

[0286] In the case of an 8×N or N×8 (N>8) block, when performing forward transform, the generated 16 coefficients after applying the 16×48 matrix to the ROI region within only the upper left 8×8 block (except for the remaining regions except the lower right 4×4 block from the upper left 8×8 block) can be set in the upper left 4×4 region, and all other regions can be filled with a value of 0. In addition, when performing inverse transform, the 16 coefficients located in the upper left 4×4 region are set in a scanning order to form an input vector, and then 48 output data can be generated by multiplying the 48×16 matrix. The generated output data can be filled in the ROI region, and all other regions can be filled with a value of 0.

[0287] As another example, in order to keep the number of multiplications for each sample (or each coefficient, each position) at a certain value or less, the number of multiplications for each sample (or each coefficient, each position) based on the ISP coding unit size rather than the size of the ISP partition block can be kept at 8 or less. When only one block among the ISP partition blocks meets the conditions for applying LFNST, the worst-case complexity calculation of LFNST can be applied based on the corresponding coding unit size rather than the size of the partition block. For example, when the luminance coding block of a certain coding unit is divided into four partition blocks of size 4×4 and encoded by ISP, and there are no non-zero transform coefficients for two of the four partition blocks, it can be configured so that instead of eight transform coefficients, 16 transform coefficients are generated for the other two partition blocks (based on the encoder).

[0288] Hereinafter, a method of signaling a LFNST index in the ISP mode will be described.

[0289] As described above, the LFNST index may have any one of the values ​​0, 1, and 2, where the value 0 indicates that LFNST is not applied, and the values ​​1 and 2 respectively indicate two LFNST kernel matrices included in the selected LFNST set. LFNST is applied based on the LFNST kernel matrix selected by the LFNST index. A method for transmitting the LFNST index in the current VVC standard will be described as follows.

[0290] 1. LFNST indexes may be sent once for each coding unit (CU), and in the case of dual-tree type, separate LFNST indexes may be signaled for luma blocks and chroma blocks.

[0291] 2. When the LFNST index is not signaled, the value of the LFNST index is set (inferred) to a default value of 0. The case where the value of the LFNST index is inferred to be 0 is as follows:

[0292] A. The case where a mode that does not apply transform (eg, transform skip, BDPCM, lossless coding, etc.) is used.

[0293] B. A case where one transform is not DCT-2 (DST7 or DCT8), that is, a case where the transform in the horizontal direction or the transform in the vertical direction is not DCT-2.

[0294] C. When the horizontal length or vertical length of the luma block of the coding unit exceeds the maximum luma transformable size, for example, when the maximum luma transform size is 64, LFNST is not applied when the luma block size of the coding block is 128×16.

[0295] In the case of a dual tree type, for each of the coding units of the luma component and the coding units of the chroma component, determine whether the maximum luma transformable size is exceeded. That is, for the luma block, check whether the maximum luma transformable size is exceeded, and for the chroma block, check whether the horizontal / vertical length and the maximum luma transformable size of the corresponding luma block for the color format are exceeded. For example, when the color format is 4:2:0, the horizontal / vertical length of the corresponding luma block is twice the horizontal / vertical length of the chroma block, and the transform size of the corresponding luma block is twice the transform size of the chroma block. In another example, when the color format is 4:4:4, the horizontal / vertical length and transform size of the corresponding luma block are the same as those of the chroma block.

[0296] A 64-length transform or a 32-length transform refers to a transform applied horizontally or vertically with a length of 64 or 32, respectively, and a “transform size” may refer to the corresponding length 64 or 32.

[0297] In the case of a single tree type, it may be checked whether the horizontal length or vertical length of the luma block exceeds the maximum transformable size of the luma transform block, and when the horizontal length or vertical length of the luma block exceeds the maximum transformable size of the luma transform block, signaling of the LFNST index may be omitted.

[0298] D. The LFNST index may be transmitted only when both the horizontal length and the vertical length of the coding unit are greater than or equal to 4, respectively.

[0299] In case of the dual tree type, the LFNST index may be signaled only when both the horizontal length and the vertical length of the corresponding component (ie, the luma component or the chroma component) are greater than or equal to 4, respectively.

[0300] In case of a single tree type, the LFNST index may be signaled when both the horizontal length and the vertical length of the luma component are greater than or equal to 4, respectively.

[0301] E. In the case where the last non-zero coefficient position is not the DC position (which is the position located at the upper left of the block), if the last non-zero coefficient position is not the DC position for the dual-tree type luma block, the LFNST index is sent. In the case of a dual-tree type chroma block, if any one of the last non-zero coefficient position of Cb and the last non-zero coefficient position of Cr is not the DC position, the corresponding LNFST index is sent.

[0302] In case of a single tree type, when the last non-zero coefficient position of any one of the luma component, the Cb component, and the Cr component is not the DC position, the LFNST index is transmitted.

[0303] Here, when a coded block flag (CBF) value indicating whether a transform coefficient of a transform block exists is 0, the last non-zero coefficient position of the corresponding transform block is not checked to determine whether to signal the LFNST index. That is, when the corresponding CBF value is 0, the transform is not applied to the corresponding block, and thus the position of the last non-zero coefficient may not be considered when checking the condition for LFNST index signaling.

[0304] For example, 1) in the case of a dual-tree type luma component, if the corresponding CBF value is 0, the LFNST index is not signaled; 2) in the case of a dual-tree type chroma component, if the CBF value of Cb is 0 and the CBF value of Cr is 1, only the last non-zero coefficient position of Cr is checked and the corresponding LFNST index is sent; 3) in the case of a single-tree type, the last non-zero coefficient position of any component whose CBF value is 1 among the luma component, Cb component, and Cr component is checked.

[0305] F. When it is found that a transform coefficient exists at a position other than the position where the LFNST transform coefficient is allowed to exist, the signaling of the LFNST index can be omitted. In the case of 4×4 transform blocks and 8×8 transform blocks, according to the transform coefficient scanning order in the VVC standard, the LFNST transform coefficient can exist in 8 positions starting from the DC position and all the remaining positions are filled with 0. In addition, in the case other than 4×4 transform blocks and 8×8 transform blocks, according to the transform coefficient scanning order in the VVC standard, the LFNST transform coefficient can exist in 16 positions starting from the DC position and the remaining positions are all filled with 0.

[0306] Therefore, when there are non-zero transform coefficients in a region that should be filled with 0 after residual encoding is performed, LFNST index signaling may be omitted.

[0307] In addition, the ISP mode can be applied only to luma blocks, or can be applied to both luma blocks and chroma blocks. As described above, when ISP prediction is applied, the corresponding coding unit is divided into two or four partition blocks and predicted, and a transform can be applied to each corresponding partition block. Therefore, when determining the conditions for signaling the LFNST index in units of coding units, it is necessary to consider the fact that LFNST is applicable to each of the corresponding partition blocks. In addition, when the ISP prediction mode is applied only to a specific component (e.g., a luma block), the LFNST index should be signaled taking into account the fact that only the corresponding component is divided into partition blocks. The LFNST index signaling methods available in ISP mode can be summarized as follows.

[0308] 1. LFNST indexes may be transmitted once for each coding unit (CU), and in case of dual tree type, separate LFNST indexes may be signaled for luma blocks and chroma blocks, respectively.

[0309] 2. When the LFNST index is signaled, the value of the LFNST index is set (inferred) to a default value of 0. The case where the LFNST index value is inferred to 0 is as follows.

[0310] A. The case where a mode that does not apply transform (eg, transform skip, BDPCM, lossless coding, etc.) is used.

[0311] B. When the horizontal length or vertical length of the luma block of the coding unit exceeds the maximum luma transformable size, for example, when the maximum transformable luma size is 64, LFNST cannot be applied when the size of the luma block of the coding block is 128×16.

[0312] Whether to signal the LFNST index may be determined based on the size of the partition block rather than the size of the coding unit. That is, when the horizontal length or vertical length of the partition block corresponding to the luma block exceeds the maximum luma transformable size, the LFNST index signaling may be omitted and the value of the LFNST index may be inferred to be 0.

[0313] In the case of a dual tree type, it is determined whether the maximum transform block size is exceeded for each of the coding unit or partition block of the luma component and the coding unit or partition block of the chroma component. That is, the horizontal length and vertical length of the coding unit or partition block for luma are compared with the maximum luma transformable size, and if any one of the horizontal length and vertical length is greater than the maximum luma transformable size, LFNST is not applied, and in the case of the coding unit or partition block for chroma, the horizontal / vertical length of the corresponding luma block for the color format and the maximum luma transformable size are compared. For example, when the color format is 4:2:0, the horizontal / vertical length of the corresponding luma block is twice the horizontal / vertical length of the chroma block, and the transform size of the corresponding luma block is twice the transform size of the chroma block. In another example, when the color format is 4:4:4, the horizontal / vertical length and transform size of the corresponding luma block are the same as those of the chroma block.

[0314] In case of a single tree type, it may be checked whether the horizontal length or vertical length of a luma block (coding unit or partition block) exceeds the maximum transformable size of a luma transform block, and if so, LFNST index signaling may be omitted.

[0315] C. When LFNST included in the current VVC standard is applied, the LFNST index may be transmitted only when both the horizontal length and the vertical length of the partition block are greater than or equal to 4, respectively.

[0316] When LFNST for 2×M (1×M) or M×2 (M×1) blocks is applied in addition to LFNST included in the current VVC standard, the LFNST index may be transmitted only when the size of the partition block is equal to or larger than 2×M (1×M) or M×2 (M×1) blocks. Here, if a P×Q block is equal to or larger than an R×S block, this means that P≥R and Q≥S.

[0317] In summary, the LFNST index may be sent only when the partition block is equal to or larger than the minimum size to which the LFNST may be applied. In the case of the dual tree type, the LFNST index may be signaled only when the partition block of the luma or chroma component is equal to or larger than the minimum size to which the LFNST may be applied. In the case of the single tree type, the LFNST index may be signaled only when the partition block of the luma component is equal to or larger than the minimum size to which the LFNST may be applied.

[0318] In this document, if an M×N block is equal to or greater than a K×L block, this means that M is equal to or greater than K and N is equal to or greater than L. If an M×N block is greater than a K×L block, this means that M is equal to or greater than K, N is equal to or greater than L, and either M is greater than K or N is greater than L. If an M×N block is less than or equal to a K×L block, this means that M is less than or equal to K and N is less than or equal to L, and if an M×N block is less than a K×L block, this means that M is less than or equal to K, N is less than or equal to L, and either M is less than K or N is less than L.

[0319] D. In the case where the last non-zero coefficient position is not the DC position (which is the position located at the upper left of the block), if the last non-zero coefficient position in any one of all partition blocks is not the DC position for the dual-tree type luma block, the LFNST index may be transmitted. In the case of the dual-tree type chroma block, if any of the last non-zero coefficient positions of all partition blocks for Cb (when the ISP mode is not applied to the chroma component, the number of partition blocks is considered to be 1) and the last non-zero coefficient positions of all partition blocks for Cr (when the ISP mode is not applied to the chroma component, the number of partition blocks is considered to be 1) is not the DC position, the corresponding LNFST index may be transmitted.

[0320] In case of a single tree type, when the last non-zero coefficient position of any one of all partition blocks of luma components, Cb components, and Cr components is not a DC position, a corresponding LFNST index may be transmitted.

[0321] Here, when a coded block flag (CBF) value indicating whether a transform coefficient exists for each partition block is 0, the last non-zero coefficient position of the corresponding partition block is not checked to determine whether to signal the LFNST index. That is, when the corresponding CBF value is 0, the transform is not applied to the corresponding block, and thus the last non-zero coefficient position of the corresponding partition block is not considered when checking the condition for LFNST index signaling.

[0322] For example, 1) in the case of a dual-tree type luma component, when the corresponding CBF value of each partition block is 0, the corresponding partition block is excluded when determining whether to signal the LFNST index; 2) in the case of a dual-tree type chroma component, when the CBF value of Cb of each partition block is 0 and the CBF value of Cr is 1, only the last non-zero coefficient position of Cr is checked when determining whether to signal the LFNST index; and 3) in the case of a single-tree type, whether to signal the LFNST index can be determined by checking only the last non-zero coefficient position of the block with a CBF value of 1 for all partition blocks of the luma component, Cb component, and Cr component.

[0323] In the case of the ISP mode, the image information may be configured such that the last non-zero coefficient position is not checked, and its implementation is as follows.

[0324] i. In the case of ISP mode, LFNST index signaling can be allowed without checking the last non-zero coefficient position of both the luminance block and the chrominance block. That is, even when the last non-zero coefficient position of all partition blocks is at the DC position or the corresponding CBF value is 0, the corresponding LFNST index signaling can be allowed.

[0325] ii. In the case of ISP mode, the check of the last non-zero coefficient position can be omitted only for the luma block, and the last non-zero coefficient position of the chroma block can be checked in the above manner. For example, in the case of a dual-tree type luma block, LFNST index signaling is allowed without checking the last non-zero coefficient position, and in the case of a dual-tree type chroma block, it can be determined whether to signal the corresponding LFNST index by checking whether there is a DC position of the last non-zero coefficient position in the above manner.

[0326] iii. In the case of ISP mode and single tree type, the above method i or the above method ii can be applied. That is, in the case where method i is applied to the single tree type in ISP mode, the check of the last non-zero coefficient position can be omitted for both the luminance block and the chrominance block, and LFNST index signaling is allowed. Alternatively, in the case of applying method ii, the check of the last non-zero coefficient position can be omitted for the partition block of the luminance component, and for the partition block of the chrominance component (when ISP is not applied to the chrominance component, the number can be considered to be 1), the last non-zero coefficient position can be checked in the above manner to determine whether to signal the corresponding LFNST index.

[0327] E. When a transform coefficient is found to exist at a position other than the position where the LFNST transform coefficient may exist for even one partition block among all partition blocks, LFNST index signaling may be omitted.

[0328] For example, in the case of a 4×4 partition block and an 8×8 partition block, according to the transform coefficient scanning order in the VVC standard, the LFNST transform coefficient may be present in 8 positions starting from the DC position, and all the remaining positions are filled with 0. In addition, when the partition block is equal to or greater than 4×4 and is neither a 4×4 partition block nor an 8×8 partition block, according to the transform coefficient scanning order in the VVC standard, the LFNST transform coefficient may be present in 16 positions starting from the DC position, and all the remaining positions are filled with 0.

[0329] Therefore, when there are non-zero transform coefficients in a region where 0 padding is applied after residual encoding is performed, LFNST index signaling may be omitted.

[0330] On the other hand, in the case of ISP mode, according to the current VVC standard, the horizontal and vertical directions are independently regarded as length conditions, and DST-7 is applied instead of DCT-2 without signaling the MTS index. It is determined whether the horizontal length or the vertical length is greater than or equal to 4 and greater than or equal to 16, and a transform kernel is determined based on the result of the determination. Therefore, for the case where LFNST can be applied in ISP mode, the following transform combinations are feasible.

[0331] 1. When the LFNST index is 0 (including the case where the LFNST index is inferred to be 0), the one-time transformation determination condition in the ISP mode can be followed, which is included in the current VVC standard. That is, if the length condition (which is equal to or greater than 4 and equal to or less than 16) is satisfied separately and independently for the horizontal direction and the vertical direction; if yes, DST-7 can be applied instead of DCT-2, and if not, DCT-2 can be applied.

[0332] 2. When the LFNST index is greater than 0, the following two configurations can be used as a transformation.

[0333] A. DCT-2 can be applied to both horizontal and vertical directions.

[0334] B. The conditions for determining a transformation in ISP mode, which are included in the current VVC standard, may be followed. In other words, check whether the length condition (which is equal to or greater than 4 and equal to or less than 16) is satisfied separately and independently for the horizontal direction and the vertical direction; if yes, DST-7 may be applied instead of DCT-2, and if not, DCT-2 may be applied.

[0335] In ISP mode, the image information may be configured such that: the LFNST index is transmitted for each partition block instead of each coding unit. In this case, in the above-mentioned LFNST index signaling method, considering that there is only one partition block in the unit in which the LFNST index is transmitted, it may be determined whether to signal the LFNST index.

[0336] Furthermore, signaling of the LFNST index and the MTS index will be described below.

[0337] The following table gives the coding unit syntax table, transform unit syntax table, and residual coding syntax table related to the signaling of LFNST index and MTS index according to the example. According to Table 4, the MTS index is moved from the transform unit level to the coding unit level syntax and is signaled after the LFNST index signaling. In addition, the constraint that does not allow LFNST when ISP is applied to the coding unit has been removed. When ISP is applied to the coding unit, the constraint that does not allow LFNST is removed so that LFNST can be applied to all intra-frame prediction blocks. In addition, both the MTS index and the LFNST index are conditionally signaled in the last part of the coding unit level.

[0338] [Table 4]

[0339]

[0340] [Table 5]

[0341]

[0342] [Table 6]

[0343]

[0344] The meanings of the main variables in the table are as follows.

[0345] 1.cbWidth, cbHeight: the width and height of the current encoding block

[0346] 2.log2TbWidth, log2TbHeight: The base 2 logarithm of the width and height of the current transform block, and reflect zeroing to reduce to the upper left area where non-zero coefficients may exist.

[0347] 3. sps_lfnst_enabled_flag: It is a flag indicating whether LFNST is enabled, if the flag value is 0, it indicates that LFNST is not enabled, and if the flag value is 1, it indicates that LFNST is enabled. It is defined in a sequence parameter set (SPS).

[0348] 4. CuPredMode[chType][x0][y0]: A prediction mode corresponding to a variable chType and a position (x0, y0), chType may have values ​​of 0 and 1, where 0 represents a luma component and 1 represents a chroma component. The position (x0, y0) indicates a position on a picture, and MODE_INTRA (intra-frame prediction) and MODE_INTER (inter-frame prediction) may have a CuPredMode[chType][x0][y0] value.

[0349] 5. IntraSubPartitionsSplit[x0][y0]: The content of the position (x0, y0) is the same as in item 4. It indicates which ISP split is applied at the position (x0, y0), and ISP_NO_SPLIT indicates that the coding unit corresponding to the position (x0, y0) is not divided into partition blocks.

[0350] 6. intra_mip_flag[x0][y0]: The content at position (x0, y0) is the same as in item 4 above. intra_mip_flag is a flag indicating whether matrix-based intra prediction (MIP) prediction mode is applied. A flag value of 0 indicates that MIP is not enabled, while a flag value of 1 indicates that MIP is applied.

[0351] 7. cIdx: A value of 0 indicates luma, and values ​​of 1 and 2 indicate Cb and Cr of chroma components, respectively.

[0352] 8.treeType: It indicates single tree and dual tree etc. (SINGLE_TREE: single tree, DUAL_TREE_LUMA: dual tree for luma component, DUAL_TREE_CHROMA: dual tree for chroma component)

[0353] 9. lastSubBlock: It indicates the position of the subblock (coefficient group (CG)) where the last non-zero coefficient is located in the scanning order. 0 indicates a subblock containing a DC component, and if greater than 0, it is not a subblock containing a DC component.

[0354] 10.lastScanPos: It indicates where the last significant coefficient is located in a sub-block in the scan order. If a sub-block consists of 16 positions, it can have a value from 0 to 15.

[0355] 11.lfnst_idx[x0][y0]: LFNST index syntax element to be parsed. If it is not parsed, it is inferred to be a value of 0. That is, the default value is set to 0 and indicates that LFNST is not applied.

[0356] 12.LastSignificantCoeffX, LastSignificantCoeffY: It indicates the x-coordinate and y-coordinate where the last significant coefficient in the transform block is located. The x-coordinate starts from 0 and increases from left to right, and the y-coordinate starts from 0 and increases from top to bottom. If the value of both variables is 0, it means that the last significant coefficient is located at DC.

[0357] 13. cu_sbt_flag: It is a flag indicating whether sub-block transform (SBT) included in the current VVC standard is enabled. If the flag value is 0, it indicates that SBT is not enabled, and if the flag value is 1, it indicates that SBT is enabled.

[0358] 14.sps_explicit_mts_inter_enabled_flag, sps_explicit_mts_intra_enabled_flag: It is a flag indicating whether explicit MTS is applied to inter CU and intra CU respectively. If the corresponding flag value is 0, it indicates that MTS is not applicable to inter CU or intra CU, and if it is 1, it indicates that MTS is applicable.

[0359] 15.tu_mts_idx[x0][y0]: It is the MTS index syntax element to be parsed. If it is not parsed, it is inferred to be a value of 0. That is, the default value is set to 0, which indicates that DCT-2 is applied to both the horizontal and vertical directions.

[0360] As shown in Table 4, several conditions are checked when encoding mts_idx[x0][y0], and tu_mts_idx[x0][y0] is signaled only when the value of lfnst_idx[x0][y0] is 0.

[0361] In addition, tu_cbf_luma[x0][y0] is a flag indicating whether there is a significant coefficient for the luma component. A value of 0 indicates that there is no significant coefficient in the corresponding transform block of the luma component, and 1 indicates that there is a significant coefficient in the corresponding transform block of the luma component.

[0362] According to Table 4, when both the width and height of the coding unit of the luma component are 32 or less, mts_idx[x0][y0](Max(cbWidth,cbHeight)<=32) is signaled, that is, whether MTS is applied is determined by the width and height of the coding unit of the luma component.

[0363] In addition, according to Table 4, lfnst_idx[x0][y0] may be configured to be signaled even in ISP mode (IntraSubPartitionsSplitType!=ISP_NO_SPLIT), and the same LFNST index value may be applied to all ISP partition blocks.

[0364] On the other hand, mts_idx[x0][y0] may be signaled only when not in ISP mode (IntraSubPartitionsSplit[x0][y0] == ISP_NO_SPLIT).

[0365] In the process of determining log2ZoTbWidth and log2ZoTbHeight as shown in Table 6 (where log2ZoTbWidth and log2ZoTbHeight represent the base 2 logarithmic values ​​of the width and height of the upper left area remaining after clearing is performed), the part of checking the mts_idx[x0][y0] value can be omitted.

[0366] Furthermore, according to an example, when log2ZoTbWidth and log2ZoTbHeight are determined in residual encoding, a condition for checking sps_mts_enable_flag may be added.

[0367] If there are valid coefficients at zeroed positions when LFNST is applied, the variable LfnstZeroOutSigCoeffFlag of Table 4 is 0, otherwise it is 1. The variable LfnstZeroOutSigCoeffFlag may be set according to several conditions shown in Table 6.

[0368] According to an example, the variable LfnstDcOnly in Table 4 becomes 1 when all the last significant coefficients are located at the DC position (upper left position) of the transform block having a corresponding coded block flag (CBF) with a value of 1 (1 if there is at least one significant coefficient in the block, otherwise 0), otherwise it becomes 0. More specifically, in the case of dual-tree luma, the position of the last significant coefficient is checked for one luma transform block, and in the case of dual-tree chroma, the position of the last significant coefficient is checked for both the transform block of Cb and the transform block of Cr. In the case of a single tree, the position of the last significant coefficient can be checked for the transform blocks of luma, Cb, and Cr.

[0369] In Table 4, MtsZeroOutSigCoeffFlag is initially set to 1, and this value may change in the residual coding of Table 6. If there are significant coefficients in the region filled with zeros due to zeroing (LastSignificantCoeffX>15||LastSignificantCoeffY>15), the variable MtsZeroOutSigCoeffFlag changes from 1 to 0. In this case, as shown in Table 4, the MTS index is not signaled.

[0370] In addition, as shown in Table 4, when tu_cbf_luma[x0][y0] is 0, mts_idx[x0][y0] encoding can be omitted. That is, if the CBF value of the luma component is 0, since no transform is applied, there is no need to signal the MTS index, so that the MTS index encoding can be omitted.

[0371] According to an example, the technical feature can be implemented with another conditional syntax. For example, after performing MTS, a variable indicating whether there is a valid coefficient in an area other than the DC area of ​​the current block can be derived, and if the variable indicates that there is a valid coefficient in an area other than the DC area, an MTS index can be signaled. That is, the presence of a valid coefficient in an area other than the DC area of ​​the current block indicates that the tu_cbf_luma[x0][y0] value is 1, and in this case, an MTS index can be signaled.

[0372] This variable may be represented as MtsDcOnly, and after the variable MtsDcOnly is initially set to 1 at the coding unit level, the value may be changed to 0 when the residual coding level indicates that there are significant coefficients in an area other than the DC area of ​​the current block. When the variable MtsDcOnly is 0, the image information may be configured such that the MTS index is signaled.

[0373] If tu_cbf_luma[x0][y0] is 0, the variable MtsDcOnly maintains the initial value 1 because the residual coding syntax is not called at the transform unit level of Table 5. In this case, since the variable MtsDcOnly is not changed to 0, the image information may be configured so that the MTS index is not signaled. That is, the MTS index is not parsed and signaled.

[0374] In addition, the decoding device may determine the color index cIdx of the transformation coefficient to derive the variable MtsZeroOutSigCoeffFlag of Table 6. The color index cIdx being 0 indicates a luminance component.

[0375] According to an example, since the MTS may be applied only to the luma component of the current block, the decoding device may determine whether the color index is luma when deriving the variable MtsZeroOutSigCoeffFlag that determines whether to parse the MTS index.

[0376] The variable MtsZeroOutSigCoeffFlag is a variable indicating whether clearing is performed when MTS is applied. It indicates whether there is a transform coefficient in an area other than the upper left area (i.e., in an area other than the upper left 16×16 area) where the last valid coefficient may exist due to clearing after MTS is performed. As shown in Table 4, the variable MtsZeroOutSigCoeffFlag is initially set to 1 (MtsZeroOutSigCoeffFlag=1) at the coding unit level, and if there is a transform coefficient in an area other than the 16×16 area, the value is changed from 1 to 0 at the residual encoding, as shown in Table 6, and can be changed (MtsZeroOutSigCoeffFlag=0). If the value of the variable MtsZeroOutSigCoeffFlag is 0, the MTS index is not signaled.

[0377] As shown in Table 6, at the residual coding level, a non-cleared area where non-zero transform coefficients may exist may be set depending on whether the clearing of the accompanying MTS is performed. Even in this case, when the color index (cIdx) is 0, the non-cleared area may be set to the upper left 16×16 area of ​​the current block.

[0378] In this way, when deriving a variable for determining whether to parse the MTS index, it is determined whether the color component is luma or chroma. However, since LFNST can be applied to both luma and chroma components of the current block, the color component is not determined when deriving a variable for determining whether to parse the LFNST index.

[0379] For example, Table 4 shows a variable LfnstZeroOutSigCoeffFlag, which can indicate that zeroing is performed when LFNST is applied. The variable LfnstZeroOutSigCoeffFlag indicates whether there is a valid coefficient in the second area of ​​the current block other than the first area located in the upper left. The value is initially set to 1, and if there is a valid coefficient in the second area, the value can be changed to 0. The LFNST index can be parsed only when the value of the initially set variable LfnstZeroOutSigCoeffFlag remains 1. When determining and deriving whether the variable LfnstZeroOutSigCoeffFlag value is 1, since LFNST can be applied to both the luminance component and the chrominance component of the current block, the color index of the current block is not determined.

[0380] In addition, a syntax table of a coding unit for signaling a LFNST index according to an example is as follows.

[0381] [Table 7]

[0382]

[0383] In Table 7, lfnst_idx represents the LFNST index and can have values ​​0, 1, and 2, as described above. As shown in Table 7, lfnst_idx is signaled only when the condition (!intra_mip_flag[x0][y0]||Min(lfnstWidth,lfnstHeight)>=16) is satisfied. Here, intra_mip_flag[x0][y0] is a flag indicating whether the matrix-based intra prediction (MIP) mode is applied to the luma block to which the (x0,y0) coordinates belong. If the MIP mode is applied to the luma block, the value is 1, and if not applied, the value is 0.

[0384] lfnstWidth and lfnstHeight indicate the width and height of LFNST applied to the coding block currently being encoded (including both luma coding blocks and chroma coding blocks). When ISP is applied to a coding block, it can indicate the width and height of each partition block divided into two or four.

[0385] In addition, in the above conditions, when Min(lfnstWidth, lfnstHeight)>=16 is equal to or greater than a 16×16 block (for example, both the width and height of the luminance coding block to which MIP is applied are equal to or greater than 16) when MIP is applied, it indicates that LFNST can be applied. The meanings of the main variables contained in Table 7 that are not repeated in the description of Table 4 are briefly introduced below.

[0386] 1. IntraSubPartitionsSplitType: It indicates how the ISP partition is formed for the current coding unit, and ISP_NO_SPLIT indicates that the corresponding coding unit is not a coding unit split into sub-blocks. ISP_VER_SPLIT indicates vertical splitting, and ISP_HOR_SPLIT indicates horizontal splitting. For example, when a W×H (width W, height H) block is split horizontally into n partition blocks, it is split into W×(H / n) blocks, and when a W×H (width W, height H) block is split vertically into n partition blocks, it is split into (W / n)×H blocks.

[0387] 2. SubWidthC, SubHeightC: SubWidthC and SubHeightC are values ​​set according to the color format (or chroma format, such as 4:2:0, 4:2:2, 4:4:4), and more specifically, they indicate the ratio of the width and height of the luminance component and the chroma component, respectively. (See the table below)

[0388] [Table 8]

[0389] Chroma Format SubWidthC SubHeightC monochrome 1 1 4:2:0 2 2 4:2:2 2 1 4:4:4 1 1 4:4:4 1 1

[0390] 3. NumIntraSubPartitions: It indicates how many partition blocks are divided when ISP is applied. That is, it indicates that the partition is divided into NumIntraSubPartitions partition blocks.

[0391] 4. LfnstDcOnly: For all transform blocks belonging to the current coding unit, each last non-zero coefficient position is the DC position (i.e., the upper left position within the corresponding transform block) or when there is no valid coefficient (i.e., when the corresponding CBF value is 0), the value of the LnfstDCOnly variable becomes 1.

[0392] In the case of a luma single tree or luma double tree, the LfnstDcOnly variable value is determined by checking the condition only for the transform block corresponding to the luma component in the corresponding coding unit, and in the case of a chroma single tree or chroma double tree, the LfnstDcOnly variable value can be determined by checking the condition only for the transform block corresponding to the chroma component (Cb, Cr) in the corresponding coding unit. In the case of a single tree, the LfnstDcOnly variable value can be determined by checking the above conditions for all transform blocks corresponding to the luma component and the chroma component (Cb, Cr) in the corresponding coding unit.

[0393] 5. LfnstZeroOutSigCoeffFlag: When LFNST is applied, if significant coefficients exist only in the region where significant coefficients can exist, it may be set to 1; otherwise, it may be set to 0.

[0394] In the case of a 4×4 transform block or an 8×8 transform block, up to 8 significant coefficients may be located starting from the (0,0) position (upper left) in the corresponding transform block according to the scanning order, and the remaining positions in the corresponding transform block may be cleared. In the case of a transform block that is not 4×4 and 8×8 and whose width and height are respectively equal to or greater than 4 (i.e., a transform block to which LFNST can be applied), 16 significant coefficients may be located starting from the (0,0) position (upper left) in the corresponding transform block according to the scanning order (i.e., the significant coefficients may be located only in the upper left 4×4 block), and the remaining positions in the corresponding transform block may be cleared.

[0395] In addition, as shown in Table 7, when encoded in a partition tree or a dual tree, it is not checked whether the MIP is applied to the chroma component when the LFNST index is signaled. In this way, LFNST can be appropriately applied to the chroma component.

[0396] As shown in Table 7, when the condition (treeType==DUAL_TREE_CHROMA||!intra_mip_flag[x0][y0]||Min(lfnstWidth,lfnstHeight)>=16)))) is satisfied, the LFNST index is signaled. This means that when the tree type is a dual tree chroma type (treeType==DUAL_TREE_CHROMA), the MIP mode is not applied (!intra_mip_flag[x0][y0]), or the smaller of the width and height of the block to which LFNST is applied is 16 or more (Min(lfnstWidth,lfnstHeight)>=16)), the LFNST index is signaled. That is, when the coding block is dual tree chroma, the LFNST index is signaled without determining whether the MIP mode is applied or the width and height of the block to which LFNST is applied.

[0397] Additionally, the above condition can be interpreted as if the coding block is not dual-tree chroma and MIP is not applied, then the LFNST index is signaled without determining the width and height of the block to which LFNST is applied.

[0398] Additionally, when the coding block is not dual-tree chroma and MIP is applied, it can be interpreted that the LFNST index can be signaled when the smaller of the width and height of the block to which LFNST is applied is 16 or greater.

[0399] On the other hand, the LFNST index is signaled only when transform skipping is not applied to the luma component as shown in Table 7 (that is, when the condition transform_skip_flag[x0][y0][0] == 0 is satisfied).

[0400] Here, x0 and y0 represent coordinates (x0, y0) when the upper left position in the picture is (0, 0) for the luma component and the horizontal X coordinate increases from left to right and the vertical Y coordinate increases from top to bottom.

[0401] (x0, y0) are coordinates based on the luma component, but can also be used for chroma phase components. In this case, the actual position indicated by the (x0, y0) coordinates can be scaled based on the picture for the chroma component. For example, when the chroma format is 4:2:0, the actual position of the chroma component indicated by (x0, y0) on the picture can be (x0 / 2, y0 / 2). For example, when the chroma format is 4:2:0, the actual position of the chroma component indicated by (x0, y0) on the picture can be (x0 / 2, y0 / 2).

[0402] In transform_skip_flag[x0][y0][0], the last index 0 refers to the luma component. More specifically, in transform_skip_flag[x0][y0][cIdx], cIdx refers to the component it is for, and if the cIdx value is 0, the cIdx value of 0 indicates luma, while cIdx greater than 0 (1 or 2) indicates chroma.

[0403] In addition, the variable LfnstDcOnly is initialized to a value of 1, as shown in Table 7, and can be set to 0 according to a condition in the parsing function for residual coding, as shown in the following table.

[0404] [Table 9]

[0405]

[0406]

[0407] As shown in Table 9, the LfnstDcOnly value can be set to 0 only when the transform_skip_flag[x0][y0][cIdx] value is 0 (that is, only when transform skipping is not applied to the component indicated by cIdx). If it is not ISP mode, as shown in Table 7, the LFNST index is signaled only when the LfnstDcOnly value is 0, and when the LFNST index is not signaled, the LFNST index value can be inferred to be 0.

[0408] For reference, the residual coding function presented in Table 9 is called when the transform_tree called in Table 7 is executed, and for a single tree, the residual coding functions for luma (cIdx=0) and chroma (cIdx=1 or 2, corresponding to Cb component and Cr component) are all called, and for a dual tree, in the case of a luma dual tree (DUAL_TREE_LUMA), only the residual coding function of luma (cIdx=0) is called, and in the case of a chroma dual tree (DUAL_TREE_CHROMA), only the residual coding function of chroma (cIdx=1 or 2, corresponding to Cb and Cr components) is called.

[0409] The conditions for signaling the LFNST index for the case not in ISP mode are summarized as follows (here, it can be assumed that other conditions for signaling the LFNST index are met, for example, assuming that the condition Max(cbWidth, cbHeight)≤MaxTbSizeY is met).

[0410] 1. When transform_skip_flag[x0][y0][0] is 1

[0411] - LFNST index is inferred to be 0 and not signaled

[0412] 2. When transform_skip_flag[x0][y0][0] is 0

[0413] 2-A. When transform_skip_flag[x0][y0][1] is 0 and transform_skip_flag[x0][y0][2] is 0

[0414] - In Table 9, for all cIdx (for cIdx 0, 1, 2), the LfnstDcOnly value may be set to 0

[0415] - If the LfnstDcOnly value is 0, the LFNST index is signaled; otherwise, the LFNST index is not signaled and the value is inferred to be 0.

[0416] 2-B. When transform_skip_flag[x0][y0][1] is 0 and transform_skip_flag[x0][y0][2] is 1

[0417] - In Table 9, the LfnstDcOnly value can be set to 0 only when cIdx is 0 and 1

[0418] - If the LfnstDcOnly value is 0, the LFNST index is signaled; otherwise, the LFNST index is not signaled and the value is inferred to be 0.

[0419] 2-C. When transform_skip_flag[x0][y0][1] is 1 and transform_skip_flag[x0][y0][2] is 0

[0420] - In Table 9, the LfnstDcOnly value can be set to 0 only when cIdx is 0 and 2

[0421] - If the LfnstDcOnly value is 0, the LFNST index is signaled; otherwise, the LFNST index is not signaled and the value is inferred to be 0.

[0422] 2-D. When transform_skip_flag[x0][y0][1] is 1 and transform_skip_flag[x0][y0][2] is 1

[0423] - In Table 9, the LfnstDcOnly value can be set to 0 only when cIdx is 0

[0424] - If the LfnstDcOnly value is 0, the LFNST index is signaled; otherwise, the LFNST index is not signaled and the value is inferred to be 0.

[0425] In the case of a single tree, the values ​​of transform_skip_flag[x0][y0][0], transform_skip_flag[x0][y0][1], and transform_skip_flag[x0][y0][2] are checked for the above cases. In the case of a luma double tree, only transform_skip_flag[x0][y0][0] is checked, and in the case of a chroma double tree, the values ​​of transform_skip_flag[x0][y0][1] and transform_skip_flag[x0][y0][2] are checked.

[0426] In case of ISP mode (IntraSubPartitionsSplitType!=ISP_NO_SPLIT condition in Table 7, ie, horizontal partition or vertical partition), as shown in Table 7, the LfnstDcOnly variable is not checked and the LFNST index is signaled.

[0427] Therefore, in the case of ISP mode under luma dual-tree and single-tree, regardless of the value of the LfnstDcOnly variable, when the transform_skip_flag[x0][y0][0] value is 0 (transform skipping is not applied for the luma component), the LFNST index is signaled (when the LFNST index is not signaled, the LFNST index value can be inferred to be 0).

[0428] In the case of chroma dual tree, based on the fact that ISP prediction is only applied to luma in the current VVC standard, it is considered that ISP is not applied to chroma, and the LFNST index can be signaled by checking the LfnstDcOnly variable in the above method, and as shown in Table 9, the LfnstDcOnly variable can be set to 0 only when the transform_skip_flag[x0][y0][cIdx] value is 0.

[0429] Of course, the application of ISP mode to luma affects even the chroma dual tree, so even in the case of chroma dual tree, regardless of the LfnstDcOnly variable, the LFNST index is signaled when transform_skip_flag[x0][y0][0] value is 0.

[0430] The conditions for signaling the LFNST index when the ISP mode is applied and the transform_skip_flag[x0][y0][0] value is 0 are summarized as follows. If the transform_skip_flag[x0][y0][0] value is 1, the LFNST index is not signaled and the LFNST index is inferred to be 0. Of course, it can be assumed that other conditions required for signaling the LFNST index in Table 7 are met, for example, it can be assumed that conditions such as Max(cbWidth, cbHeight) <= MaxTbSizeY are met.

[0431] 1. In the case of a single tree

[0432] - Signals LFNST index regardless of LfnstDcOnly variable value

[0433] 2. In the case of double tree

[0434] 2-A. In the case of brightness double tree

[0435] - Signals LFNST index regardless of LfnstDcOnly variable value

[0436] 2-B. In the case of chroma dual trees

[0437] -Depending on the values ​​of transform_skip_flag[x0][y0][1] and transform_skip_flag[x0][y0][2], when the LfnstDcOnly variable value is set to 0, that is, the cIdx value in transform_skip_flag[x0][y0][cIdx] is 1, the LfnstDcOnly variable value can be set to 0 only when the transform_skip_flag[x0][y0][1] value is 0, and when the cIdx value is 2, the LfnstDcOnly variable value can be set to 0 only when the transform_skip_flag[x0][y0][2] value is 0.

[0438] - If the LfnstDcOnly value is 0, the LFNST index is signaled; otherwise, the LFNST index is not signaled and the value is inferred to be 0.

[0439] In the above case, the case of chroma dual tree is the same as the case where ISP is not applied.

[0440] Furthermore, according to an example, although transform skipping is allowed for chroma components in the current VVC standard, a transform skip flag corresponding to each chroma component is added as shown in the following table.

[0441] [Table 10]

[0442]

[0443] In Table 10, except for the case of luma double tree, it can be confirmed that transform_skip_flag[xC][yC][1] corresponding to whether transform skip is applied to Cb and transform_skip_flag[xC][yC][2] corresponding to whether transform skip is applied to Cr can be signaled. If the transform_skip_flag[xC][yC][1] value is 1, transform skip is applied to Cb, and if it is 0, transform skip is not applied to Cb, and if the transform_skip_flag[xC][yC][2] value is 1, transform skip is applied to Cr, and if it is 0, transform skip is not applied to Cr.

[0444] Therefore, even if the LFNST index value is greater than 0 (ie, when LFNST is applied), each transform_skip_flag[x0][y0][cIdx] value for the luma component (Y component) and the chroma component (Cb component and Cr component) may be different. According to Table 7, since the LFNST index value can be greater than 0 only when the transform_skip_flag[x0][y0][0] value is 0, when the LFNST index value is greater than 0, transform_skip_flag[x0][y0][0] is always zero.

[0445] Therefore, the cases where LFNST can be applied according to the transform_skip_flag[x0][y0][cIdx] value are summarized as follows. Here, the LFNST index is greater than 0, and the transform_skip_flag[x0][y0][0] value is 0. It can be assumed that other conditions for applying LFNST are met, for example, the width and height of the corresponding block can be greater than or equal to 4.

[0446] 1. Single Tree

[0447] -LFNST is applied to the luma component

[0448] - If the transform_skip_flag[x0][y0][1] value is 0, LFNST is applied to the Cb component, and if it is 1, LFNST is not applied to the Cb component.

[0449] - If the transform_skip_flag[x0][y0][2] value is 0, LFNST is applied to the Cr component, and if it is 1, LFNST is not applied to the Cr component.

[0450] 2. Brightness double tree

[0451] -LFNST is applied to the luma component

[0452] 3. Chroma Dual Tree

[0453] - If the transform_skip_flag[x0][y0][1] value is 0, LFNST is applied to the Cb component, and if it is 1, LFNST is not applied to the Cb component.

[0454] - If the transform_skip_flag[x0][y0][2] value is 0, LFNST is applied to the Cr component, and if it is 1, LFNST is not applied to the Cr component.

[0455] As described above, in order to selectively apply LFNST according to the transform_skip_flag[x0][y0][cIdx] value, the following condition should be added to the specification text of LFNST.

[0456] [Table 11]

[0457]

[0458]

[0459]

[0460] As shown in Table 11, when the LFNST index (lfnst_idx) value is not 0 (ie, when LFNST is applied), by checking the transform_skip_flag[xTbY][yTbY][cIdx] value of the component specified by cIdx (when lfnst_idx is not equal to 0 and transform_skip_flag[xTbY][yTbY][cIdx] is equal to 0 and both nTbW and nTbH are greater than or equal to 4, the following applies), it can be configured so that the subsequent encoding process is performed only when the transform_skip_flag[xTbY][yTbY][cIdx] value is 0 (ie, LFNST is applied).

[0461] Furthermore, the LFNST index may be signaled according to whether transform is skipped for each color component.

[0462] As an example, when compared with Table 7, in Table 12, only when the transform_skip_flag[x0][y0][0] value is 0, the condition for transmitting the LFNST index may be removed.

[0463] [Table 12]

[0464]

[0465] However, since the method of setting the LfnstDcOnly variable value described in Table 12 is the same as that in Table 9, the setting of the LfnstDcOnly variable value is changed according to the transform_skip_flag[x0][y0][cIdx] value, and whether the LFNST index is signaled or not will also change in the end.

[0466] When ISP mode is not applied, how to signal the LFNST index through the transform_skip_flag[x0][y0][cIdx] value is summarized as follows. It can be assumed that other conditions for signaling the LFNST index have been met, for example, conditions such as Max(cbWidth, cbHeight) <= MaxTbSizeY are met.

[0467] 1. In the case of a single tree

[0468] -When the transform_skip_flag[x0][y0][0] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0469] -When the transform_skip_flag[x0][y0][1] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0470] -When the transform_skip_flag[x0][y0][2] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0471] - The LFNST index may be signaled when the LfnstDcOnly value is 0. If the LFNST index is not signaled, it may be inferred to be 0.

[0472] 2. In the case of dual trees for the luminance component

[0473] -When the transform_skip_flag[x0][y0][0] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0474] - The LFNST index may be signaled when the LfnstDcOnly value is 0. If the LFNST index is not signaled, it may be inferred to be 0.

[0475] 3. In the case of dual chrominance component trees

[0476] -When the transform_skip_flag[x0][y0][1] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0477] -When the transform_skip_flag[x0][y0][2] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0478] - The LFNST index may be signaled when the LfnstDcOnly value is 0. If the LFNST index is not signaled, it may be inferred to be 0.

[0479] As shown in Table 12, the LfnstDcOnly value is initialized to 1, and in the case of a dual tree, the LFNST index corresponding to the luma dual tree and the LFNST index corresponding to the chroma dual tree can be separately signaled. This means that different LFNST kernels can be applied to luma and chroma.

[0480] In addition, the dual trees shown in Tables 7 to 12 may include DUAL_TREE_LUMA (corresponding to the luma component) and DUAL_TREE_CHROMA (corresponding to the chroma component) appearing in the current VVC specification document, which may include a syntax parsing tree for luma and a syntax parsing tree for chroma that are different due to the size condition of the coding unit, etc. For example, a single tree case may be included.

[0481] When the ISP mode is applied, transform_skip_flag[x0][y0][0] is not signaled, and transform_skip_flag[x0][y0][0] is inferred to be 0, as shown in Table 10. That is, as shown in Table 10, transform_skip_flag[x0][y0]][0] is signaled only when the condition IntraSubPartitionsSplit[x0][y0]==ISP_NO_SPLIT is satisfied (which is the case where the ISP mode is not applied).

[0482] In addition, as shown in Table 10, whether the ISP mode is applied or not, transform_skip_flag[x0][y0][1] and transform_skip_flag[x0][y0][2] may be signaled.

[0483] Therefore, the LFNST signaling in the case of applying the ISP mode can be summarized as follows: It can be assumed that other conditions for sending the LFNST index have been met, for example, conditions such as Max(cbWidth, cbHeight)≤MaxTbSizeY can be met.

[0484] 1. In the case of a single tree

[0485] - A LFNST index may be signaled. If no LFNST index is signaled, it may be inferred to be 0.

[0486] 2. In the case of dual trees for the luminance component

[0487] - A LFNST index may be signaled. If no LFNST index is signaled, it may be inferred to be 0.

[0488] 3. In the case of dual chrominance component trees

[0489] -When the transform_skip_flag[x0][y0][1] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0490] -When the transform_skip_flag[x0][y0][2] value is 0, the LfnstDcOnly variable value can be set to 0 according to the method shown in Table 9.

[0491] - The LFNST index may be signaled when the LfnstDcOnly value is 0. If the LFNST index is not signaled, it may be inferred to be 0.

[0492] When the ISP mode is applied, the LfnstDcOnly condition is not checked, as shown in Table 12. Therefore, in the case of the first single tree and the second luma component dual tree, the LFNST index can be signaled without checking the LfnstDcOnly condition. In the case of the chroma dual tree, the LFNST index can be signaled according to the same condition as the case where the ISP mode is not applied in the above case 3. That is, the LFNST index can be signaled according to the LfnstDcOnly condition.

[0493] According to the example, since the transform_skip_flag[x0][y0][cIdx] value can be assigned to the luma component and the two chroma components respectively, when the LFNST index value is greater than 0, that is, even when LFNST is applied, LFNST can be applied to the component indicated by cIdx only when the transform_skip_flag[x0][y0][cIdx] value is 0. The corresponding changed normative text content is the same as Table 11.

[0494] According to an example, if the condition of checking whether the transform_skip_flag[x0][y0][0] value is 0 only for the dual tree case compared to Table 7 is removed, LFNST index signaling may be configured as shown in Table 13. The LfnstDcOnly variable shown in Table 13 may be set to 0 according to the condition shown in Table 9.

[0495] [Table 13]

[0496]

[0497] When configured as shown in Table 13, in the case of a single tree, the LFNST index signaling method shown in Tables 7 to 9 may be applied, and in the case of a dual tree, the method shown in Table 12 may be applied. In addition, since transform_skip_flag[x0][y0][cIdx] values ​​are assigned to the luma component and the two chroma components, respectively, as shown in Tables 7 to 9, even if the LFNST index value is greater than 0 (i.e., when LFNST is applied), it may be configured to apply LFNST to the component indicated by cIdx only when the transform_skip_flag[x0][y0][cIdx] value is 0. The correspondingly changed normative text content is the same as that in Table 11.

[0498] The following figures are created to illustrate specific examples of this specification. Since the names of specific devices or the names of specific signals / messages / fields described in the figures are presented by way of example, the technical features of this specification are not limited to the specific names used in the following figures.

[0499] Fig.15 is a flowchart illustrating the operation of a video decoding device according to an embodiment of the present disclosure.

[0500] Fig.15 Each step disclosed in the above is based on Figures 5 to 14 Therefore, some of the contents described in the above will be omitted or simplified Figure 3 as well as Figures 5 to 14 A detailed description of what is described in .

[0501] The decoding apparatus 300 according to an embodiment may receive information about an intra prediction mode, residual information, and a LFNST index from a bitstream ( S1510 ).

[0502] More specifically, the decoding device 300 may decode information about the quantized transform coefficients of the current block from the bitstream, and derive the quantized transform coefficients of the target block based on the information about the quantized transform coefficients of the current block. The information about the quantized transform coefficients of the target block may be included in a sequence parameter set (SPS) or a slice header, and may include information about a simplification factor, information about a minimum transform size to which a simplified transform is applied, information about a maximum transform size to which a simplified transform is applied, and at least one of information about a transform index indicating any one of a transform kernel matrix and a simplified inverse transform size included in a transform set.

[0503] In addition, the decoding device may also receive information about the intra prediction mode of the current block and information about whether ISP is applied to the current block. The decoding device may derive whether the current block is divided into a predetermined number of sub-partition transform blocks by receiving and parsing flag information indicating whether ISP encoding or ISP mode is applied. Here, the current block may be a coding block. In addition, the decoding device may derive the size and number of the divided sub-partition blocks by indicating the flag information in which direction the current block is to be divided.

[0504] The decoding apparatus 300 may induce a transform coefficient by performing inverse quantization on residual information (ie, quantized transform coefficient) about the current block (S1520).

[0505] The derived transform coefficients may be arranged in a reverse diagonal scanning order in units of 4×4 blocks, and the transform coefficients within the 4×4 block may also be arranged in a reverse diagonal scanning order. That is, the transform coefficients that have been inversely quantized may be arranged according to the reverse scanning order applied in a video codec such as in VVC or HEVC.

[0506] The decoding apparatus may derive modified transform coefficients by applying LFNST to the transform coefficients.

[0507] Unlike the first transform that separates and transforms the transform target coefficients in the vertical direction or the horizontal direction, LFNST is a non-separable transform that applies the transform without separating the coefficients in a specific direction. The non-separable transform may be a low-frequency non-separable transform that applies the forward transform only to the low-frequency region instead of the entire block region.

[0508] The LFNST index information may be received as syntax information, and the syntax information may be received as a binarized bin string including 0 and 1.

[0509] The syntax element of the LFNST index according to the present embodiment may indicate whether inverse LFNST or inverse non-separate transform and any one of transform kernel matrices included in the transform set are applied, and when two transform kernel matrices are included in the transform set, the syntax element of the transform index may have three values.

[0510] That is, according to an embodiment, the syntax element value of the LFNST index may include: 0, which indicates that the inverse LFNST is not applied to the target block; 1, which indicates the first transform core matrix among the transform core matrices; and 2, which indicates the second transform core matrix among the transform core matrices.

[0511] Intra prediction mode information and LFNST index information may be signaled at a coding unit level.

[0512] The decoding apparatus may derive a variable indicating whether a significant coefficient exists in a DC component of the current block to determine whether to resolve an LFNST index for the current block, and may derive the variable based on respective transform skip flag values ​​of color components of the current block ( S1530 ).

[0513] A variable indicating whether there is a significant coefficient in the DC component of the current block may be represented as a variable LfnstDcOnly, and becomes 0 when there is a non-zero coefficient at a non-DC component for at least one transform block in one coding unit, and becomes 1 when there is no non-zero coefficient at a position other than the DC component for all transform blocks in one coding unit. In the present disclosure, the DC component refers to the upper left position or (0,0) as a position reference of a 2D component.

[0514] There may be several transform blocks within one coding unit. For example, in the case of chrominance components, there may be transform blocks of Cb and Cr, and in the case of a single tree type, there may be transform blocks of luminance, Cb, and Cr. According to an example, when a non-zero coefficient other than a DC component position is found even in one transform block among the transform blocks constituting the current coding block, the value of the variable LnfstDcOnly may be set to 0.

[0515] In addition, since residual encoding is not performed on the corresponding transform block if there is no non-zero coefficient in the transform block, the variable LfnstDcOnly value is not changed by the corresponding transform block. Therefore, if there is no non-zero coefficient in the non-DC component of the transform block, the variable LfnstDcOnly value is not changed and maintains the previous value. For example, when the coding unit is encoded according to the single tree type and the variable LfnstDcOnly value is changed to 0 due to the luma transform block, when the non-zero coefficient exists only in the DC component in the Cb transform block or there is no non-zero coefficient in the Cb transform block, the variable LfnstDcOnly value remains 0. The variable LfnstDcOnly value is initially initialized to 1, and if no component in the current coding unit updates the variable LfnstDcOnly value to 0, it remains the value 1 as it is, and when one of the transform blocks constituting the coding unit sets the variable LfnstDcOnly value to 0, it finally remains 0.

[0516] In addition, the variable LfnstDcOnly can be derived based on the respective transform skip flag values ​​of the color components of the current block. The transform skip flag of the current block can be signaled for each color component, and if the tree type of the current block is single tree, the transform skip flag value of the luma component, the transform skip flag value of the chroma Cb component, and the transform skip flag value of the chroma Cr component can be derived based on the transform skip flag value of the luma component. Alternatively, if the tree type of the current block is dual-tree luma, the variable LfnstDcOnly is derived based on the transform skip flag value of the luma component, and if the tree type of the current block is dual-tree chroma, the variable LfnstDcOnly can be derived based on the transform skip flag value of the chroma Cb component and the transform skip flag value of the chroma Cr component.

[0517] According to an example, based on the transform skip flag value of the color component being 0, the variable LfnstDcOnly may indicate that a valid coefficient exists at a position other than the DC component. That is, if the tree type of the current block is a single tree, the transform skip flag value of the luma component, the variable LfnstDcOnly may be derived to be 0 based on the fact that at least one of the transform skip flag value of the luma component, the transform skip flag value of the chroma Cb component, and the transform skip flag value of the chroma Cr component is 0. Alternatively, if the tree type of the current block is dual-tree luma, the variable LfnstDcOnly is derived based on the transform skip flag value of the luma component, and if the tree type of the current block is dual-tree chroma, the variable LfnstDcOnly may be derived based on the transform skip flag value of the chroma Cb component and the transform skip flag value of the chroma Cr component.

[0518] As described above, the variable LfnstDcOnly may be initially set to 1 at the coding unit level of the current block, and if the transform skip flag value is 0, the variable LfnstDcOnly may be changed to 0 at the residual coding level.

[0519] The decoding apparatus may parse the LFNST index based on the variable LfnstDcOnly indicating that a significant coefficient exists at a position other than a DC component, that is, based on the variable LfnstDcOnly being 0 ( S1540 ).

[0520] Furthermore, in case of a luma block to which an intra sub-partitioning (ISP) mode may be applied, the LFNST index may be parsed without deriving the variable LfnstDcOnly.

[0521] Specifically, when the ISP mode is applied and the transform skip flag of the luma component (i.e., transform_skip_flag[x0][y0][0] value) is 0, and the tree type of the current block is a single tree or a luma dual tree, the LFNST index can be signaled regardless of the variable LfnstDcOnly value.

[0522] On the other hand, in the case of a chroma component to which the ISP mode is not applied, the variable LfnstDcOnly value may be set to 0 according to transform_skip_flag[x0][y0][1] as a transform skip flag of the chroma Cb component and transform_skip_flag[x0][y0][2] as a transform skip flag of the chroma Cr component. That is, in transform_skip_flag[x0][y0][cIdx], when the cIdx value is 1, the variable LfnstDcOnly value may be set to 0 only when the transform_skip_flag[x0][y0][1] value is 0, and when the cIdx value is 2, the transform_skip_flag[x0][y0][2] value may be set to 0 only when the transform_skip_flag[x0][y0][2] value is 0. If the variable LfnstDcOnly value is 0, the decoding device may parse the LFNST index, otherwise the LFNST index may not be signaled and may be inferred to be a value of 0.

[0523] Thereafter, the decoding apparatus may derive a modified transform coefficient from the transform coefficient based on the LFNST index and the LFNST matrix used for LFNST ( S1550 ).

[0524] The decoding device may set a plurality of variables for LFNST based on whether the LFNST index is not 0 (ie, whether the LFNST index is greater than 0) and respective transform skip flag values ​​of the color components are 0.

[0525] For example, in the step of applying LFNST after parsing the LFNST index, the decoding device may determine again whether the respective transform skip flag values ​​of the color components are 0, and may set various variables for applying LFNST. For example, the intra prediction mode for selecting the LFNST set, the number of transform coefficients output after applying LFNST, the size of the block to which LFNST is applied, etc. may be set.

[0526] In the case of a block encoded with BDPCM, the transform skip flag may be automatically set to 1, and in this case, the transform skip flag may be 1 even if the LFNST index is not 0, so when LFNST is actually applied, the transform skip flag value of each color component may be checked again.

[0527] Alternatively, according to an example, when the flag value indicating whether there is a coding significant coefficient in the transform block is 0, there may be a case where the transform skip flag value is not checked. Also in this case, since the transform skip flag value is not guaranteed to be 0 simply because the LFNST index is not 0, when the LFNST is actually applied, the transform skip flag value of each color component may be checked again.

[0528] That is, the decoding device may check the transform skip flag value of each color component in the LFNST index parsing step, and may check the transform skip flag value of each color component again when LFNST is actually applied.

[0529] The decoding apparatus may determine an LFNST set including an LFNST matrix based on the intra prediction mode derived from the intra prediction mode information, and select any one of a plurality of LFNST matrices based on the LFNST set and the LFNST index.

[0530] In this case, the same LFNST set and the same LFNST index can be applied to the sub-partition transform blocks split from the current block. That is, because the same intra prediction mode is applied to the sub-partition transform blocks, the LFNST set determined based on the intra prediction mode can be equally applied to all sub-partition transform blocks. In addition, because the LFNST index is signaled at the coding unit level, the same LFNST matrix can be applied to the sub-partition transform blocks split from the current block.

[0531] As described above, a transform set may be determined according to an intra prediction mode of a transform block to be transformed, and inverse LFNST may be performed based on any one of the transform kernel matrices (i.e., LFNST matrices) included in the transform set indicated by the LFNST index. A matrix applied to inverse LFNST may be referred to as an inverse LFNST matrix or a LFNST matrix, and such a matrix may have any name as long as it has a transposed relationship with a matrix used for forward LFNST.

[0532] In one example, the inverse LFNST matrix may be a non-square matrix in which the number of columns is less than the number of rows.

[0533] The decoding apparatus may induce residual samples of the current block based on a primary inverse transform of the modified transform coefficient (S1560).

[0534] In this case, as the inverse primary transform, a conventional separate transform can be used, and the above-mentioned MTS can be used.

[0535] Subsequently, the decoding apparatus 300 may generate a reconstructed sample based on the residual sample of the current block and the prediction sample of the current block.

[0536] The following figures are provided to describe specific examples of the present disclosure. Since the specific names of the devices or the names of the specific signals / messages / fields shown in the figures are provided for illustration, the technical features of the present disclosure are not limited to the specific names used in the following figures.

[0537] Fig.16 is a flowchart illustrating the operation of a video encoding device according to an embodiment of the present disclosure.

[0538] Fig.16 Each step disclosed in the above is based on Figures 5 to 14 Therefore, some of the contents described in the above will be omitted or simplified Figure 2 as well as Figures 5 to 14 A detailed description of what is described in .

[0539] The encoding apparatus 200 according to an embodiment may derive a prediction sample of a current block based on an intra prediction mode applied to the current block.

[0540] The encoding apparatus may perform prediction for each sub-partitioned transform block when applying the ISP to the current block.

[0541] The encoding device can determine whether to apply ISP encoding or ISP mode to the current block (ie, the encoding block), and determine in which direction the current block is to be divided according to the determination result, and derive the size and number of the divided sub-blocks.

[0542] The same intra prediction mode is applied to the sub-partition transform blocks divided from the current block, and the encoding device can derive prediction samples for each sub-partition transform block. That is, the encoding device performs intra prediction in sequence according to the division form of the sub-partition transform blocks, such as horizontally or vertically, from left to right or from top to bottom. For the leftmost or topmost sub-block, the reconstructed pixels of the already encoded coding block are referenced in the traditional intra prediction method. In addition, for each edge of the subsequent internal sub-partition transform block, when it is not adjacent to the previous sub-partition transform block, in order to derive the reference pixels adjacent to the corresponding edge, the adjacent coding block that has been encoded as in the traditional intra prediction method refers to the reconstructed pixels.

[0543] The encoding apparatus 200 may induce a residual sample of the current block based on the prediction sample ( S1610 ).

[0544] The encoding apparatus 200 may induce a transform coefficient of a current block by applying at least one of LFNST and MTS to residual samples, and may arrange the transform coefficients according to a predetermined scanning order.

[0545] The encoding apparatus may induce a transformation coefficient of the current block based on the primary transformation of the residual sample (S1620).

[0546] One transform may be performed by a plurality of transform cores like MTS, and in this case, the transform core may be selected based on the intra prediction mode.

[0547] The encoding apparatus 200 may determine whether to perform a secondary transform or a non-separate transform (specifically, LFNST) on a transform coefficient of a current block, and apply LFNST to the transform coefficient to derive a modified transform coefficient.

[0548] Unlike the first transform that separates and transforms the transform target coefficients in the vertical direction or the horizontal direction, LFNST is a non-separable transform that applies the transform without separating the coefficients in a specific direction. The non-separable transform may be a low-frequency non-separable transform that applies the transform only to the low-frequency region instead of the entire target block to be transformed.

[0549] The encoding apparatus may apply a plurality of LFNST matrices to the transform coefficients to derive a variable indicating whether a significant coefficient exists in a DC component of the current block, and may derive the variable based on respective transform skip flag values ​​of color components of the current block (S1630).

[0550] The encoding apparatus may induce the variable after applying LFNST to each LFNST matrix candidate, or in a state where LFNST is not applied when LFNST is not applied.

[0551] Specifically, the encoding device may apply a plurality of LFNST candidates (i.e., LFNST matrices) to exclude corresponding LFNST matrices in which significant coefficients of all transform blocks exist only in the DC position (of course, when CBF is 0, the corresponding variable is excluded from the process), and compare RD values ​​only between LFNST matrices in which the variable LfnstDcOnly value is 0. For example, when LFNST is not applied, it is included in the comparison process because it is irrelevant to the variable LfnstDcOnly value (in this case, since LFNST is not applied, the variable LfnstDcOnly value can be determined based on the transform coefficient obtained as a result of one transform), and the LFNST matrix corresponding to the LfnstDcOnly value of 0 is also included in the comparison process of the RD value.

[0552] A variable indicating whether there is a valid coefficient in the DC component of the current block can be represented as a variable LfnstDcOnly, and for at least one transform block in a coding unit, becomes 0 when there is a non-zero coefficient at a non-DC component, and for all transform blocks in a coding unit, becomes 1 when there is no non-zero coefficient in a position other than the DC component.

[0553] There may be several transform blocks in one coding unit. For example, in the case of chrominance components, there may be transform blocks of Cb and Cr, and in the case of a single tree type, there may be transform blocks of luma, Cb, and Cr. According to an example, when a non-zero coefficient other than the DC component position is found even in one transform block among the transform blocks constituting the current coding block, the variable LnfstDcOnly value may be set to 0.

[0554] In addition, since residual encoding is not performed on the corresponding transform block if there is no non-zero coefficient in the transform block, the variable LfnstDcOnly value is not changed by the corresponding transform block. Therefore, if there is no non-zero coefficient in the non-DC component of the transform block, the variable LfnstDcOnly value is not changed and the previous value is maintained. For example, when the coding unit is encoded according to the single tree type and the variable LfnstDcOnly value is changed to 0 due to the luminance transform block, when the non-zero coefficient exists only in the DC component in the Cb transform block or there is no non-zero coefficient in the Cb transform block, the variable LfnstDcOnly value remains 0. The variable LfnstDcOnly value is initially initialized to 1, and if no component in the current coding unit updates the variable LfnstDcOnly value to 0, it remains the value 1 as it is, and when one of the transform blocks constituting the coding unit sets the variable LfnstDcOnly value to 0, it finally remains 0.

[0555] In addition, the variable LfnstDcOnly can be derived based on the respective transform skip flag values ​​of the color components of the current block. The transform skip flag of the current block can be signaled for each color component, and if the tree type of the current block is single tree, the transform skip flag value of the luma component, the transform skip flag value of the chroma Cb component, and the transform skip flag value of the chroma Cr component, the variable LfnstDcOnly can be derived based on the transform skip flag value of the luma component, the transform skip flag value of the chroma Cb component, and the transform skip flag value of the chroma Cr component. Alternatively, if the tree type of the current block is dual-tree luma, the variable LfnstDcOnly is derived based on the transform skip flag value of the luma component, and if the tree type of the current block is dual-tree chroma, the variable LfnstDcOnly can be derived based on the transform skip flag value of the chroma Cb component and the transform skip flag value of the chroma Cr component.

[0556] According to an example, based on the transform skip flag value of the color component being 0, the variable LfnstDcOnly may indicate that a valid coefficient exists at a position other than the DC component. That is, if the tree type of the current block is a single tree, the transform skip flag value of the luma component, the variable LfnstDcOnly may be derived to be 0 based on the fact that at least one of the transform skip flag value of the luma component, the transform skip flag value of the chroma Cb component, and the transform skip flag value of the chroma Cr component is 0. Alternatively, if the tree type of the current block is dual-tree luma, the variable LfnstDcOnly is derived based on the transform skip flag value of the luma component, and if the tree type of the current block is dual-tree chroma, the variable LfnstDcOnly may be derived based on the transform skip flag value of the chroma Cb component and the transform skip flag value of the chroma Cr component.

[0557] As described above, the variable LfnstDcOnly may be initially set to 1 at the coding unit level of the current block, and if the transform skip flag value is 0, the variable LfnstDcOnly may be changed to 0 at the residual coding level.

[0558] The encoding apparatus may select an optimal LFNST matrix based on the variable indicating that a significant coefficient exists at a position other than a DC component, and may derive a modified transform coefficient based on the selected LFNST matrix ( S1640 ).

[0559] The encoding apparatus may set a plurality of variables for LFNST based on whether respective transform skip flag values ​​of color components are 0 in the step of deriving the modified transform coefficient.

[0560] For example, after determining whether to apply LFNST, the encoding device may determine again in the step of applying LFNST whether the respective transform skip flag values ​​of the color components are 0, and may set various variables for applying LFNST. For example, the intra prediction mode for selecting the LFNST set, the number of transform coefficients output after applying LFNST, the size of the block to which LFNST is applied, etc. may be set.

[0561] In the case of a block encoded by BDPCM, since the transform skip flag may be automatically set to 1, when LFNST is actually applied, the transform skip flag value of each color component may be checked again.

[0562] Alternatively, according to an example, when the flag value indicating that there is a coding significant coefficient in the transform block is 0, there may be a case where the transform skip flag value is not checked. Also in this case, since the transform skip flag value is not guaranteed to be 0 simply because the LFNST index is not 0, when the LFNST is actually applied, the transform skip flag value of each color component may be checked again.

[0563] That is, the encoding device may check the transform skip flag value of each color component in the step of determining whether to apply LFNST, and may check the transform skip flag value of each color component again when LFNST is actually applied.

[0564] Furthermore, in case of a luma block to which an intra sub-partitioning (ISP) mode may be applied, LFNST may be applied without deriving a variable LfnstDcOnly.

[0565] Specifically, when the ISP mode is applied and the transform skip flag for the luma component (ie, transform_skip_flag[x0][y0][0] value) is 0, and the tree type of the current block is a single tree or a luma dual tree, LFNST can be applied regardless of the value of the variable LfnstDcOnly.

[0566] On the other hand, in the case where the chroma component of the ISP mode is not applied, the variable LfnstDcOnly value may be set to 0 according to transform_skip_flag[x0][y0][1] as the transform skip flag of the chroma Cb component and transform_skip_flag[x0][y0][2] as the transform skip flag of the chroma Cr component. That is, in transform_skip_flag[x0][y0][cIdx], when the cIdx value is 1, the variable LfnstDcOnly value may be set to 0 only when the transform_skip_flag[x0][y0][1] value is 0, and when the cIdx value is 2, the transform_skip_flag[x0][y0][2] value may be set to 0 only when the transform_skip_flag[x0][y0][2] value is 0. If the variable LfnstDcOnly value is 0, the encoding device may apply LFNST, otherwise LFNST is not applied.

[0567] The encoding apparatus 200 may determine an LFNST set based on a mapping relationship according to an intra prediction mode applied to a current block, and perform LFNST, ie, an inseparable transform, based on one of two LFNST matrices included in the LFNST set.

[0568] In this case, the same LFNST set and the same LFNST index can be applied to the sub-partition transform blocks split from the current block. That is, since the same intra prediction mode is applied to the sub-partition transform blocks, the LFNST set determined based on the intra prediction mode can also be equally applied to all sub-partition transform blocks. In addition, since the LFNST index is encoded in units of coding units, the same LFNST matrix can be applied to the sub-partition transform blocks split from the current block.

[0569] As described above, the transform set may be determined according to the intra prediction mode of the transform block to be transformed. The matrix applied to LFNST has a transposed relationship with the matrix used for inverse LFNST.

[0570] In one example, the LFNST matrix may be a non-square matrix in which the number of rows is less than the number of columns.

[0571] The encoding device can construct image information so that the LFNST index indicating the LFNST matrix applied to LFNST is parsed based on the following facts: the variable LfnstDcOnly is initially set to 1 in the coding unit level of the current block, and the variable LfnstDcOnly is changed to 0 at the residual coding level when the transform skip flag value is 0, and the variable LfnstDcOnly is 0 (S1650).

[0572] The encoding apparatus may perform quantization based on the modified transform coefficient of the current block to induce a quantized transform coefficient, and encode the LFNST index.

[0573] That is, the encoding device may generate residual information including information about the quantized transform coefficients. The residual information may include the above-mentioned transform-related information / syntax elements. The encoding device may encode the image / video information including the residual information and output the encoded image / video information in the form of a bitstream.

[0574] More specifically, the encoding apparatus 200 may generate information about a quantized transform coefficient and encode the information about the generated quantized transform coefficient.

[0575] The syntax element of the LFNST index according to the present embodiment may indicate whether to apply (inverse) LFNST and any one LFNST matrix included in the LFNST set, and when the LFNST set includes two transform kernel matrices, the syntax element of the LFNST index may have three values.

[0576] According to an embodiment, when the partition tree structure of the current block is a dual tree type, a LFNST index may be encoded for each of the luma block and the chroma block.

[0577] According to an embodiment, the syntax element value of the transform index may be derived as 0, 1, and 2, 0 indicating that (inverse) LFNST is not applied to the current block, 1 indicating the first LFNST matrix in the LFNST matrix, and 2 indicating the second LFNST matrix in the LFNST matrix.

[0578] In the present disclosure, at least one of quantization / dequantization and / or transform / inverse transform may be omitted. When quantization / dequantization is omitted, the quantized transform coefficient may be referred to as a transform coefficient. When transform / inverse transform is omitted, the transform coefficient may be referred to as a coefficient or a residual coefficient, or may still be referred to as a transform coefficient for consistency of expression.

[0579] In addition, in the present disclosure, the quantized transform coefficient and the transform coefficient may be referred to as a transform coefficient and a scaled transform coefficient, respectively. In this case, the residual information may include information about the transform coefficient, and the information about the transform coefficient may be signaled by the residual coding syntax. The transform coefficient may be derived based on the residual information (or information about the transform coefficient), and the scaled transform coefficient may be derived by an inverse transform (scaling) of the transform coefficient. The residual sample may be derived based on an inverse transform (transform) of the scaled transform coefficient. These details may also be applied / expressed in other parts of the present disclosure.

[0580] In the above-mentioned embodiments, the method is explained based on the flowchart with the aid of a series of steps or boxes, but the present disclosure is not limited to the order of the steps, and a certain step may be performed in an order or step different from the above order or step, or a certain step may be performed concurrently with other steps. In addition, it can be understood by those of ordinary skill in the art that the steps shown in the flowchart are not exclusive, and another step may be incorporated or one or more steps in the flowchart may be deleted without affecting the scope of the present disclosure.

[0581] The above method according to the present disclosure may be implemented in a software form, and the encoding device and / or decoding device according to the present disclosure may be included in a device for image processing such as a television, a computer, a smart phone, a set-top box, a display device, and the like.

[0582] When the embodiments in the present disclosure are implemented by software, the above methods can be implemented as modules (steps, functions, etc.) for performing the above functions. These modules can be stored in a memory and can be executed by a processor. The memory can be inside or outside the processor and can be connected to the processor in various well-known ways. The processor may include an application-specific integrated circuit (ASIC), other chipsets, logic circuits and / or data processing devices. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium and / or other storage devices. In other words, the embodiments described in the present disclosure can be implemented and executed on a processor, a microprocessor, a controller or a chip. For example, the functional units shown in each of the accompanying drawings can be implemented and executed on a computer, a processor, a microprocessor, a controller or a chip.

[0583] In addition, the decoding device and encoding device of the present disclosure can be included in a multimedia broadcast transceiver, a mobile communication terminal, a home theater video device, a digital theater video device, a surveillance camera, a video chat device, a real-time communication device (such as video communication), a mobile streaming device, a storage medium, a camera, a video on demand (VoD) service provider, an over-the-top (OTT) video device, an Internet streaming service provider, a three-dimensional (3D) video device, a video phone video device, and a medical video device, and can be used to process a video signal or a data signal. For example, an over-the-top (OTT) video device can include a game console, a Blu-ray player, an Internet access TV, a home theater system, a smart phone, a tablet PC, a digital video recorder (DVR), etc.

[0584] In addition, the processing method of the present disclosure can be produced in the form of a program executed by a computer and can be stored in a computer-readable recording medium. Multimedia data with a data structure according to the present disclosure can also be stored in a computer-readable recording medium. Computer-readable recording media include various storage devices and distributed storage devices that store computer-readable data. Computer-readable recording media may include, for example, a Blu-ray disc (BD), a universal serial bus (USB), a ROM, a PROM, an EPROM, an EEPROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage device. In addition, the computer-readable recording medium includes a medium implemented in the form of a carrier (e.g., transmission on the Internet). In addition, the bit stream generated by the encoding method can be stored in a computer-readable recording medium or transmitted via a wired or wireless communication network. In addition, the embodiments of the present disclosure can be implemented as a computer program product through a program code, and the program code can be executed on a computer according to the embodiments of the present disclosure. The program code can be stored on a computer-readable carrier.

[0585] The claims disclosed herein may be combined in various ways. For example, the technical features of the method claims of the present disclosure may be combined to be implemented or performed in a device, and the technical features of the device claims may be combined to be implemented or performed in a method. In addition, the technical features of the method claims and the device claims may be combined to be implemented or performed in a device, and the technical features of the method claims and the device claims may be combined to be implemented or performed in a method.

Claims

1. An image decoding method performed by a decoding device, the image decoding method comprising the following steps: Receiving residual information via a bitstream; deriving a transform coefficient for a current block based on the residual information; deriving modified transform coefficients by applying a low frequency non-separable transform (LFNST) to the transform coefficients; deriving residual samples for the current block based on an inverse primary transform of the modified transform coefficients; as well as Generate a reconstructed picture based on the residual samples, The step of deriving the modified transform coefficients comprises the following steps: deriving a variable indicating whether a significant coefficient exists only in an upper-left sample position within the current block based on a first transform skip flag for a chroma Cb component and a second transform skip flag for a chroma Cr component in the current block, the first transform skip flag for the chroma Cb component and the second transform skip flag for the chroma Cr component being respectively signaled from the bitstream based on a tree type of the current block being dual-tree chroma; and parsing a LFNST index based on the variable indicating that the significant coefficient exists at a position other than the upper left sample position, wherein, based on the LFNST index being equal to 0, the LFNST is not applied to the transform coefficient for the chroma Cb component, wherein, based on the LFNST index not being equal to 0 and the first transform skip flag for the chroma Cb component being equal to 0, the LFNST is applied to the transform coefficient for the chroma Cb component, and Wherein, based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 1, the LFNST is not applied to the transform coefficient for the chroma Cb component.

2. The image decoding method according to claim 1, wherein: The variable indicating that the significant coefficient exists at the position other than the upper left sample position is derived based on at least one of the first transform skip flag for the chroma Cb component or the second transform skip flag for the chroma Cr component being equal to 0.

3. The image decoding method according to claim 2, in, The variable is initially set to 1 at the coding unit level of the current block, wherein, based on the at least one of the first transform skip flag for the chroma Cb component or the second transform skip flag for the chroma Cr component being equal to 0, the variable is changed to 0 at the residual coding level, and The LFNST index is resolved based on the variable being equal to 0.

4. The image decoding method according to claim 1, in, The step of deriving the modified transform coefficients further comprises the following steps: A plurality of variables for the LFNST are set based on whether the LFNST index is not equal to 0 and whether the first transform skip flag and the second transform skip flag are equal to 0.

5. An image encoding method performed by an image encoding device, the image encoding method comprising the following steps: deriving prediction samples for the current block; deriving residual samples for the current block based on the prediction samples; Deriving transform coefficients for the current block based on a transform for the residual samples; applying a low frequency non-separable transform (LFNST) to derive modified transform coefficients from said transform coefficients; generating residual information based on the modified transform coefficients; as well as encoding the image information including the residual information, The step of applying the LFNST to derive the modified transform coefficients comprises the following steps: deriving a variable indicating whether a significant coefficient exists only in an upper-left sample position within the current block based on a first transform skip flag for a chroma Cb component and a second transform skip flag for a chroma Cr component in the current block, the first transform skip flag for the chroma Cb component and the second transform skip flag for the chroma Cr component being encoded into a bitstream based on a tree type of the current block being dual-tree chroma, respectively; and encoding a LFNST index based on the variable indicating that the significant coefficient exists at a position other than the upper left sample position, wherein, based on the LFNST index being equal to 0, the LFNST is not applied to the transform coefficient for the chroma Cb component, wherein, based on the LFNST index not being equal to 0 and the first transform skip flag for the chroma Cb component being equal to 0, the LFNST is applied to the transform coefficient for the chroma Cb component, and Wherein, based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 1, the LFNST is not applied to the transform coefficient for the chroma Cb component.

6. The image encoding method according to claim 5, wherein: The variable indicating that the significant coefficient exists at a position other than the upper left sample position is derived based on at least one of the first transform skip flag for the chroma Cb component or the second transform skip flag for the chroma Cr component being equal to 0.

7. The image encoding method according to claim 6, in, The variable is initially set to 1 at the coding unit level of the current block, wherein, based on the at least one of the first transform skip flag for the chroma Cb component or the second transform skip flag for the chroma Cr component being equal to 0, the variable is changed to 0 at the residual coding level, and The LFNST index is encoded based on the variable being equal to 0.

8. The image encoding method according to claim 5, in, The step of applying the LFNST further includes the step of setting a plurality of variables for the LFNST based on whether the first transform skip flag and the second transform skip flag are equal to 0.

9. A method for transmitting a bit stream, the method comprising the steps of: deriving prediction samples for the current block; deriving residual samples for the current block based on the prediction samples; Deriving transform coefficients for the current block based on a transform for the residual samples; applying a low frequency non-separable transform (LFNST) to derive modified transform coefficients from said transform coefficients; generating residual information based on the modified transform coefficients; encoding the image information including the residual information to generate the bit stream; as well as sending the bit stream, The step of applying the LFNST to derive the modified transform coefficients comprises the following steps: deriving a variable indicating whether a significant coefficient exists only in an upper-left sample position within the current block based on a first transform skip flag for a chroma Cb component and a second transform skip flag for a chroma Cr component in the current block, the first transform skip flag for the chroma Cb component and the second transform skip flag for the chroma Cr component being encoded into a bitstream based on a tree type of the current block being dual-tree chroma, respectively; and encoding a LFNST index based on the variable indicating that the significant coefficient exists at a position other than the upper left sample position, wherein, based on the LFNST index being equal to 0, the LFNST is not applied to the transform coefficient for the chroma Cb component, wherein, based on the LFNST index not being equal to 0 and the first transform skip flag for the chroma Cb component being equal to 0, the LFNST is applied to the transform coefficient for the chroma Cb component, and Wherein, based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 1, the LFNST is not applied to the transform coefficient for the chroma Cb component.

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