Image encoding / decoding method and device based on adaptive MTS, and recording medium for storing bit stream

Abstract

An image encoding / decoding method and apparatus are provided. The image decoding method disclosed herein comprises the steps of: determining whether to apply multiple transform selection (MTS) to a current block; and performing an inverse transform on the current block based on a prescribed MTS set based on the MTS being applied to the current block, in which the MTS set may be determined based on whether a prediction mode of the current block is a combined inter and intra prediction (CIIP) mode.

Description

Technical Field

[0001] The present disclosure relates to an image encoding / decoding method and a recording medium storing a bit stream, and more specifically, to an image encoding / decoding method and device based on adaptive MTS in CIIP mode, and a recording medium storing a bit stream generated by the image encoding method / device of the present disclosure. Background Art

[0002] Recently, the demand for high-resolution and high-quality images, such as high-definition (HD) images and ultra-high-definition (UHD) images, is increasing in various fields. As the resolution and quality of image data are improved, the amount of information or bit volume transmitted is relatively increased compared to existing image data. The increase in the amount of information or bit volume transmitted leads to an increase in transmission cost and storage cost.

[0003] Therefore, efficient image compression technology is needed to effectively transmit, store, and reproduce information about high-resolution and high-quality images. Summary of the invention

[0004] Technical issues

[0005] An object of the present disclosure is to provide an image encoding / decoding method and apparatus with improved encoding / decoding efficiency.

[0006] In addition, an object of the present disclosure is to provide an image encoding / decoding method and apparatus for adaptively applying MTS in CIIP mode.

[0007] In addition, an object of the present disclosure is to provide an image encoding / decoding method and device for adaptively determining an MTS set in a CIIP mode.

[0008] In addition, an object of the present disclosure is to provide a non-transitory computer-readable recording medium storing a bit stream generated by the image encoding method or apparatus according to the present disclosure.

[0009] In addition, an object of the present disclosure is to provide a non-transitory computer-readable recording medium storing a bit stream received and decoded by the image decoding apparatus according to the present disclosure and used to reconstruct an image.

[0010] In addition, an object of the present disclosure is to provide a method of transmitting a bit stream generated by the image encoding method or apparatus according to the present disclosure.

[0011] The technical problems solved by the present disclosure are not limited to the above-mentioned technical problems, and other technical problems not described herein will be apparent to those skilled in the art through the following description.

[0012] Technical Solution

[0013] An image decoding method according to aspects of the present disclosure may include determining whether to apply a multi-transform selection (MTS) to a current block, and based on the MTS being applied to the current block, performing an inverse transform on the current block based on a predetermined MTS set. The MTS set may be determined based on whether a prediction mode of the current block is a combined inter-frame and intra-frame prediction (CIIP) mode.

[0014] According to another aspect of the present disclosure, an image decoding device may include a memory and at least one processor. The at least one processor may determine whether to apply a multi-transform selection (MTS) to a current block, and based on the MTS being applied to the current block, perform an inverse transform on the current block based on a predetermined MTS set. The MTS set may be determined based on whether a prediction mode of the current block is a combined inter-frame and intra-frame prediction (CIIP) mode.

[0015] According to another aspect of the present disclosure, an image encoding method may include determining whether to apply a multiple transform selection (MTS) to a current block, and based on the MTS being applied to the current block, performing a transform on the current block based on a predetermined MTS set. The MTS set may be determined based on whether a prediction mode of the current block is a combined inter-frame and intra-frame prediction (CIIP) mode.

[0016] A computer-readable recording medium according to another aspect of the present disclosure may store a bit stream generated by the image encoding method or the image encoding apparatus of the present disclosure.

[0017] A transmission method according to another aspect of the present disclosure may transmit a bit stream generated by the image encoding apparatus or the image encoding method of the present disclosure.

[0018] The features briefly summarized above with respect to the present disclosure are merely exemplary aspects of the following detailed description of the present disclosure and do not limit the scope of the present disclosure.

[0019] Beneficial Effects

[0020] According to the present disclosure, an image encoding / decoding method and apparatus with improved encoding / decoding efficiency can be provided.

[0021] According to the present disclosure, an image encoding / decoding method and apparatus for adaptively applying MTS in a CIIP mode may be provided.

[0022] According to the present disclosure, an image encoding / decoding method and apparatus for adaptively determining an MTS set in a CIIP mode may be provided.

[0023] In addition, according to the present disclosure, a non-transitory computer-readable recording medium storing a bit stream generated by the image encoding method or apparatus according to the present disclosure may be provided.

[0024] According to the present disclosure, there can be provided a non-transitory recording medium storing a bit stream received and decoded by the image decoding device according to the present disclosure and used to reconstruct an image.

[0025] According to the present disclosure, a method of transmitting a bit stream generated by the image encoding method or apparatus according to the present disclosure may be provided.

[0026] Those skilled in the art will understand that the effects that can be achieved by the present disclosure are not limited to the effects that have been specifically described above, and other advantages of the present disclosure will be more clearly understood from the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a diagram schematically showing a video encoding system to which an embodiment of the present disclosure is applicable.

[0028] Figure 2 is a view schematically showing an image encoding device to which an embodiment of the present disclosure is applicable.

[0029] Figure 3 is a view schematically showing an image decoding device to which an embodiment of the present disclosure is applicable.

[0030] Figures 4a to 4d is a diagram showing reference samples defined in PDPC.

[0031] Figure 5 is a diagram showing an example of a template used in TIMD.

[0032] Figure 6 is a diagram showing an example of neighboring blocks used for weight calculation of CIIP.

[0033] Figure 7 is a diagram showing the prediction process according to the PDPC hybrid CIIP model.

[0034] Figure 8 is a diagram illustrating CIIP weights applied to sub-blocks.

[0035] Fig. 9 is a diagram illustrating the transformation and inverse transformation process.

[0036] Fig.10 is a flowchart illustrating the encoding process of performing AMT.

[0037] Fig.11 is a flowchart illustrating a decoding process for performing AMT.

[0038] Figures 12 to 17 is a flowchart illustrating an MTS application method according to one embodiment of the present disclosure.

[0039] Fig.18 is a flowchart illustrating an image encoding method according to one embodiment of the present disclosure.

[0040] Fig.19 is a flowchart illustrating an image decoding method according to one embodiment of the present disclosure.

[0041] Fig. 20 is a diagram illustrating a content streaming system to which an embodiment of the present disclosure is applicable. DETAILED DESCRIPTION

[0042] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the accompanying drawings to facilitate implementation by those skilled in the art. However, the present disclosure may be implemented in various forms and is not limited to the embodiments described herein.

[0043] When describing the present disclosure, if it is determined that the detailed description of related known functions or configurations makes the scope of the present disclosure unnecessarily ambiguous, the detailed description thereof will be omitted. In the drawings, parts irrelevant to the description of the present disclosure are omitted, and like reference numerals are given to like parts.

[0044] In the present disclosure, when a component is "connected", "coupled" or "linked" to another component, it may include not only a direct connection relationship but also an indirect connection relationship with intermediate components. In addition, when a component "includes" or "has" other components, unless otherwise specified, it means that other components may also be included, rather than excluding other components.

[0045] In the present disclosure, the terms first, second, etc. may be used only for the purpose of distinguishing one component from other components, and do not limit the order or importance of the components, unless otherwise specified. Accordingly, within the scope of the present disclosure, a first component in one embodiment may be referred to as a second component in another embodiment, and similarly, a second component in one embodiment may be referred to as a first component in another embodiment.

[0046] In the present disclosure, components that are distinguished from each other are intended to clearly describe each feature and do not mean that the components must be separated. That is, multiple components can be integrated and implemented in one hardware or software unit, or one component can be distributed and implemented in multiple hardware or software units. Therefore, even if not otherwise specified, these embodiments in which the components are integrated or the components are distributed are also included in the scope of the present disclosure.

[0047] In the present disclosure, the components described in each embodiment are not necessarily indispensable components, and some components may be optional components. Therefore, the embodiments consisting of a subset of the components described in the embodiments are also included in the scope of the present disclosure. In addition, the embodiments including other components in addition to the components described in the various embodiments are included in the scope of the present disclosure.

[0048] The present disclosure relates to encoding and decoding of images. Unless otherwise defined in the present disclosure, terms used in the present disclosure may have general meanings commonly used in the technical field to which the present disclosure belongs.

[0049] In this disclosure, a video may refer to a collection of images over time.

[0050] In the present disclosure, a "picture" generally refers to a unit representing an image within a specific time period, and a slice / tile is a coding unit constituting a part of a picture, and a picture may be composed of one or more slices / tiles. In addition, a slice / tile may include one or more coding tree units (CTUs).

[0051] In the present disclosure, "pixel" or "pel" may mean the smallest unit constituting a picture (or image). In addition, "sample" may be used as a term corresponding to a pixel. A sample may generally represent a pixel or a value of a pixel, and may represent only a pixel / pixel value of a luminance component, or only a pixel / pixel value of a chrominance component.

[0052] In the present disclosure, a "unit" may refer to a basic unit of image processing. The unit may include at least one of a specific area of ​​a picture and information related to the area. In some cases, the unit may be used interchangeably with terms such as "sample array", "block" or "area". In general, an M×N block may include M columns and N rows of samples (or sample arrays) or a set (or array) of transform coefficients.

[0053] In the present disclosure, "current block" may mean one of "current coding block", "current coding unit", "coding target block", "decoding target block" or "processing target block". When prediction is performed, "current block" may mean "current prediction block" or "prediction target block". When transform (inverse transform) / quantization (dequantization) is performed, "current block" may mean "current transform block" or "transform target block". When filtering is performed, "current block" may mean "filtering target block".

[0054] In addition, in the present disclosure, unless explicitly stated as a chroma block, "current block" may mean a block including both a luma component block and a chroma component block or a "luminance block of the current block". The luma component block of the current block may be expressed by including an explicit description of the luma component block such as "luminance block" or "current luma block". In addition, "chroma component block of the current block" may be expressed by including an explicit description of the chroma component block such as "chroma block" or "current chroma block".

[0055] In the present disclosure, the terms " / " and "," should be interpreted as indicating "and / or". For example, the expressions "A / B" and "A, B" may mean "A and / or B". In addition, "A / B / C" and "A / B / C" may mean "at least one of A, B, and / or C".

[0056] In the present disclosure, 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, in the present disclosure, the term "or" should be interpreted as indicating "additionally or alternatively".

[0057] In the present disclosure, "at least one of A, B, and C" may mean "only A", "only B", "only C", or "any and all combinations of A, B, and C". In addition, "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".

[0058] The brackets used in the present disclosure may mean "for example". For example, if "prediction (intra-frame prediction)" is indicated, "intra-frame prediction" may be proposed as an example of "prediction". In other words, "prediction" in the present disclosure is not limited to "intra-frame prediction", and "intra-frame prediction" may be proposed as an example of "prediction". In addition, even if "prediction (i.e., intra-frame prediction)" is indicated, "intra-frame prediction" may be proposed as an example of "prediction".

[0059] Overview of Video Coding Systems

[0060] Figure 1 is a diagram showing a video encoding system to which an embodiment of the present disclosure is applicable.

[0061] The video encoding system according to the embodiment may include an encoding device 10 and a decoding device 20. The encoding device 10 may deliver the encoded video and / or image information or data to the decoding device 20 via a digital storage medium or a network in the form of a file or a stream.

[0062] The encoding device 10 according to the embodiment may include a video source generator 11, an encoding unit 12, and a transmitter 13. The decoding device 20 according to the embodiment may include a receiver 21, a decoding unit 22, and a renderer 23. The encoding unit 12 may be referred to as a video / image encoding device, and the decoding unit 22 may be referred to as a video / image decoding device. The transmitter 13 may be included in the encoding unit 12. The receiver 21 may be included in the decoding unit 22. The renderer 23 may include a display and the display may be configured as a separate device or an external component.

[0063] The video source generator 11 can obtain the video / image by the process of capturing, synthesizing or generating the video / image. The video source generator 11 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 generate the video / image (electronically). 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 a process of generating relevant data.

[0064] The encoding unit 12 may encode the input video / image. For compression and encoding efficiency, the encoding unit 12 may perform a series of processes such as prediction, transformation, and quantization. The encoding unit 12 may output the encoded data (encoded video / image information) in the form of a bitstream.

[0065] The transmitter 13 may transmit the encoded video / image information or data output in the form of a bit stream to the receiver 21 of the decoding device 20 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 13 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 21 may extract / receive a bit stream from a storage medium or a network and transmit the bit stream to a decoding unit 22.

[0066] The decoding unit 22 may decode the video / image by performing a series of processes corresponding to the operations of the encoding unit 12, such as dequantization, inverse transformation, and prediction.

[0067] The renderer 23 may render the decoded video / image. The rendered video / image may be displayed through a display.

[0068] Overview of Image Coding Equipment

[0069] Figure 2is a view schematically showing an image encoding device to which an embodiment of the present disclosure is applicable.

[0070] like Figure 2 As shown, the image encoding device 100 may include an image segmenter 110, a subtractor 115, a transformer 120, a quantizer 130, a dequantizer 140, an inverse transformer 150, an adder 155, a filter 160, a memory 170, an inter-frame prediction unit 180, an intra-frame prediction unit 185, and an entropy encoder 190. The inter-frame prediction unit 180 and the intra-frame prediction unit 185 may be collectively referred to as a "prediction unit". The transformer 120, the quantizer 130, the dequantizer 140, and the inverse transformer 150 may be included in a residual processor. The residual processor may also include a subtractor 115.

[0071] In some embodiments, all or at least some of the components configuring the image encoding apparatus 100 may be configured by one hardware component (eg, an encoder or a processor). In addition, the memory 170 may include a decoded picture buffer (DPB) and may be configured by a digital storage medium.

[0072] The image segmenter 110 may segment the input image (or picture or frame) input to the image encoding device 100 into one or more processing units. For example, the processing unit may be referred to as a coding unit (CU). The coding unit may be obtained by recursively segmenting a coding tree unit (CTU) or a maximum coding unit (LCU) according to a quadtree binary tree ternary tree (QT / BT / TT) structure. For example, a coding unit may be segmented into a plurality of coding units of a deeper depth based on a quadtree structure, a binary tree structure, and / or a ternary tree structure. For the segmentation of the coding unit, the quadtree structure may be applied first, and the binary tree structure and / or the ternary tree structure may be applied later. The encoding process according to the present disclosure may be performed based on the final coding unit that is no longer segmented. The maximum coding unit may be used as the final coding unit, and the coding unit of a deeper depth obtained by segmenting the maximum coding unit may also be used as the final coding unit. Here, the encoding process may include the prediction, transformation, and reconstruction processes described later. As another example, the processing unit of the encoding process may be a prediction unit (PU) or a transformation unit (TU). The prediction unit and the transform unit may be divided or partitioned from the final coding unit. The prediction unit may be a sample prediction unit, and the transform unit may be a unit for deriving a transform coefficient and / or a unit for deriving a residual signal from the transform coefficient.

[0073] The prediction unit (inter prediction unit 180 or intra prediction unit 185) may perform prediction on the block to be processed (current block) and generate a prediction block including prediction samples of the current block. The prediction unit may determine whether to apply intra prediction or inter prediction based on the current block or CU. The prediction unit may generate various information related to the prediction of the current block and send the generated information to the entropy encoder 190. The information about the prediction may be encoded in the entropy encoder 190 and output in the form of a bitstream.

[0074] The intra prediction unit 185 can predict the current block by referring to the samples in the current picture. Depending on the intra prediction mode and / or the intra prediction technology, the reference samples can be located in the neighbors of the current block or can be placed separately. The intra prediction mode may include multiple non-directional modes and multiple 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 according to the setting. The intra prediction unit 185 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring blocks.

[0075] The inter prediction unit 180 may derive a prediction block of the current block based on a reference block (reference sample array) specified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information sent in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on the correlation of the motion information between the neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may also include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different. The temporal neighboring block may be referred to as a collocated reference block, a collocated CU (colCU), etc. The reference picture including the temporal neighboring block may be referred to as a collocated picture (colPic). For example, the inter prediction unit 180 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 prediction may be performed based on various prediction modes. For example, in the case of skip mode and merge mode, the inter prediction unit 180 may use the motion information of the neighboring block as the motion information of the current block. In the case of skip mode, unlike the merge mode, a residual signal may not be sent. In the case of motion vector prediction (MVP) mode, the motion vector of the neighboring block may be used as a motion vector predictor, and the motion vector of the current block may be signaled by encoding the motion vector difference and an indicator of the motion vector predictor. The motion vector difference may mean the difference between the motion vector of the current block and the motion vector predictor.

[0076] The prediction unit may generate a prediction signal based on various prediction methods and prediction techniques described below. For example, the prediction unit may apply not only intra prediction or inter prediction, but also both intra prediction and inter prediction at the same time to predict the current block. The prediction method of applying both intra prediction and inter prediction to predict the current block may be referred to as combined inter and intra prediction (CIIP). In addition, the prediction unit may perform intra block copying (IBC) to predict the current block. Intra block copying may be used for content image / video encoding of games, etc., such as screen content coding (SCC). IBC is a method of predicting the current picture using a previously reconstructed reference block in the current picture at a position separated by a predetermined distance from the current block. When IBC is applied, the position of the reference block in the current picture may be encoded as a vector (block vector) corresponding to a predetermined distance. IBC basically performs prediction in the current picture, but may be performed similarly to inter prediction because the reference block is derived within the current picture. That is, IBC may use at least one of the inter prediction techniques described in the present disclosure.

[0077] The prediction signal generated by the prediction unit can be used to generate a reconstruction signal or to generate a residual signal. The subtractor 115 can generate a residual signal (residual block or residual sample array) by subtracting the prediction signal (prediction block or prediction sample array) output from the prediction unit from the input image signal (original block or original sample array). The generated residual signal can be sent to the transformer 120.

[0078] The transformer 120 may generate transform coefficients 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 refers to a transform obtained from a graph when relationship information between pixels is represented by a graph. CNT refers to a transform obtained based on a prediction signal generated using all previously reconstructed pixels. In addition, the transform process may be applied to square pixel blocks of the same size or may be applied to blocks of variable size rather than square.

[0079] The quantizer 130 may quantize the transform coefficients and send them to the entropy encoder 190. The entropy encoder 190 may encode the quantized signal (information about the quantized transform coefficients) and output a bitstream. The information about the quantized transform coefficients may be referred to as residual information. The quantizer 130 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.

[0080] The entropy encoder 190 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 190 may encode information required for video / image reconstruction other than quantized transform coefficients together or separately (e.g., values ​​of syntax elements, etc.). The encoded information (e.g., encoded video / image information) may be transmitted or stored in units 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 adaptive parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video / image information may also include general constraint information. The signaled information, transmitted information, and / or syntax elements described in the present disclosure may be encoded and included in a bitstream through the above-described encoding process.

[0081] The bitstream may be transmitted through a network or may be stored in a digital storage medium. The network may include a broadcast network and / or a communication network, 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 190 and / or a storage unit (not shown) that stores the signal may be included as an internal / external element of the image encoding device 100. Alternatively, a transmitter may be provided as a component of the entropy encoder 190.

[0082] The quantized transform coefficients output from the quantizer 130 may be used to generate a residual signal. For example, the residual signal (residual block or residual sample) may be reconstructed by applying dequantization and inverse transformation to the quantized transform coefficients through the dequantizer 140 and the inverse transformer 150.

[0083] The adder 155 adds the reconstructed residual signal to the prediction signal output from the inter-frame prediction unit 180 or the intra-frame prediction unit 185 to generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array). If the block to be processed has no residual, such as the case where the skip mode is applied, the prediction block can be used as a reconstructed block. The adder 155 can be called a reconstructor or a reconstructed block generator. The generated reconstructed signal can be used for intra-frame prediction of the next block to be processed in the current picture, and can be used for inter-frame prediction of the next picture by filtering as described below.

[0084] Furthermore, during picture coding and / or reconstruction, luminance mapping and chrominance scaling (LMCS) may be applied.

[0085] The filter 160 can improve the subjective / objective image quality by applying filtering to the reconstructed signal. For example, the filter 160 can generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and store the modified reconstructed picture in the memory 170, specifically, in the DPB of the memory 170. Various filtering methods may include, for example, deblocking filtering, sample adaptive offset, adaptive loop filtering, bilateral filtering, etc. The filter 160 can generate various information related to filtering and send the generated information to the entropy encoder 190, as described later in the description of each filtering method. The information related to filtering can be encoded by the entropy encoder 190 and output in the form of a bit stream.

[0086] The modified reconstructed picture transmitted to the memory 170 may be used as a reference picture in the inter prediction unit 180. When inter prediction is applied by the image encoding apparatus 100, prediction mismatch between the image encoding apparatus 100 and the image decoding apparatus may be avoided and encoding efficiency may be improved.

[0087] The DPB of the memory 170 may store the modified reconstructed picture for use as a reference picture in the inter-frame prediction unit 180. The memory 170 may store the motion information of the block for deriving (or encoding) the motion information in the current picture and / or the motion information of the reconstructed block in the picture. The stored motion information may be sent to the inter-frame prediction unit 180 and used as the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory 170 may store the reconstructed samples of the reconstructed blocks in the current picture and may transmit the reconstructed samples to the intra-frame prediction unit 185.

[0088] Overview of Image Decoding Equipment

[0089] Figure 3 is a view schematically showing an image decoding device to which an embodiment of the present disclosure is applicable.

[0090] like Figure 3 As shown, the image decoding device 200 may include an entropy decoder 210, a dequantizer 220, an inverse transformer 230, an adder 235, a filter 240, a memory 250, an inter-frame prediction unit 260, and an intra-frame prediction unit 265. The inter-frame prediction unit 260 and the intra-frame prediction unit 265 may be collectively referred to as a "prediction unit". The dequantizer 220 and the inverse transformer 230 may be included in a residual processor.

[0091] According to an embodiment, all or at least some of the multiple components configuring the image decoding apparatus 200 may be configured by hardware components (eg, a decoder or a processor). In addition, the memory 170 may include a decoded picture buffer (DPB) or may be configured by a digital storage medium.

[0092] The image decoding device 200 having received a bit stream including video / image information may perform the same Figure 2 The image may be reconstructed by a process corresponding to the process performed by the image encoding device 100. For example, the image decoding device 200 may perform decoding using a processing unit applied in the image encoding device. Therefore, the processing unit of decoding may be, for example, a coding unit. The coding unit may be obtained by partitioning the coding tree unit or the maximum coding unit. The reconstructed image signal decoded and output by the image decoding device 200 may be reproduced by a reproduction device (not shown).

[0093] The image decoding device 200 may receive the image in the form of a bit stream from Figure 2The received signal may be decoded by the entropy decoder 210. For example, the entropy decoder 210 may parse the bitstream to derive 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 an adaptive parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video / image information may also include general constraint information. The image decoding device may also decode the picture based on the information about the parameter set and / or the general constraint information. The signaled / received information and / or syntax elements described in the present disclosure may be decoded and obtained from the bitstream through a decoding process. For example, the entropy decoder 210 may decode the information in the bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, and 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 a bin corresponding to each syntax element in the bitstream, use the decoding target syntax element information, the decoding information of the neighboring block and the decoding target block, or the information of the symbol / bin decoded in the previous stage to determine the context model, perform arithmetic decoding on the bin by predicting the probability of occurrence of the bin according to the determined context model, and generate a symbol corresponding to the value of each syntax element. In this case, the CABAC entropy decoding method can update the context model by using the information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model. The information related to prediction among the information decoded by the entropy decoder 210 can be provided to the prediction unit (inter-frame prediction unit 260 and intra-frame prediction unit 265), and the residual value (ie, quantized transform coefficient and related parameter information) for which entropy decoding is performed in the entropy decoder 210 can be input to the dequantizer 220. In addition, the information about filtering among the information decoded by the entropy decoder 210 can be provided to the filter 240. In addition, a receiver (not shown) for receiving a signal output from the image encoding apparatus may be further configured as an internal / external element of the image decoding apparatus 200 , or the receiver may be a component of the entropy decoder 210 .

[0094] In addition, the image decoding device according to the present disclosure may be referred to as a video / image / picture decoding device. The image 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 an entropy decoder 210. The sample decoder may include a dequantizer 220, an inverse transformer 230, an adder 235, a filter 240, a memory 250, an inter-frame prediction unit 260, or at least one of an intra-frame prediction unit 265.

[0095] The dequantizer 220 may dequantize the quantized transform coefficients and output the transform coefficients. The dequantizer 220 may rearrange the quantized transform coefficients in the form of two-dimensional blocks. In this case, the rearrangement may be performed based on the coefficient scanning order performed in the image encoding device. The dequantizer 220 may dequantize the quantized transform coefficients by using a quantization parameter (e.g., quantization step size information) and obtain the transform coefficients.

[0096] The inverse transformer 230 may perform an inverse transformation on the transformation coefficients to obtain a residual signal (residual block, residual sample array).

[0097] The prediction unit may perform prediction on the current block and generate a prediction block including prediction samples of the current block. The prediction unit may determine whether to apply intra prediction or inter prediction to the current block based on the information about prediction output from the entropy decoder 210, and may determine a specific intra / inter prediction mode (prediction technique).

[0098] The same as described in the prediction unit of the image encoding device 100 , the prediction unit can generate a prediction signal based on various prediction methods (techniques) to be described later.

[0099] The intra prediction unit 265 may predict the current block by referring to samples in the current picture. The description of the intra prediction unit 185 is also applicable to the intra prediction unit 265.

[0100] The inter prediction unit 260 may derive a prediction block of the current block based on a reference block (reference sample array) specified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information sent in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on the correlation of the motion information between the neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may also include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. For example, the inter prediction unit 260 may configure a motion information candidate list based on the 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 prediction may be performed based on various prediction modes, and information about the prediction may include information indicating an inter prediction mode of the current block.

[0101] The adder 235 can generate a reconstruction 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 prediction unit (including the inter-frame prediction unit 260 and / or the intra-prediction unit 265). If the block to be processed has no residual, such as when the skip mode is applied, the prediction block can be used as the reconstructed block. The description of the adder 155 also applies to the adder 235. The adder 235 can be called a reconstructor or a reconstructed block generator. The generated reconstruction signal can be used for intra-frame prediction of the next block to be processed in the current picture, and can be used for inter-frame prediction of the next picture by filtering as described below.

[0102] The filter 240 can improve the subjective / objective image quality by applying filtering to the reconstructed signal. For example, the filter 240 can generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and store the modified reconstructed picture in the memory 250, specifically, in the DPB of the memory 250. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, adaptive loop filtering, bilateral filtering, etc.

[0103] The (modified) reconstructed picture stored in the DPB of the memory 250 can be used as a reference picture in the inter-frame prediction unit 260. The memory 250 can store the motion information of the block for deriving (or decoding) the motion information in the current picture and / or the motion information of the reconstructed block in the picture. The stored motion information can be sent to the inter-frame prediction unit 260 to be used as the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory 250 can store the reconstructed samples of the reconstructed block in the current picture and transmit the reconstructed samples to the intra-frame prediction unit 265.

[0104] In the present disclosure, the implementations described in the filter 160, the inter-frame prediction unit 180 and the intra-frame prediction unit 185 of the image encoding device 100 can be equally or correspondingly applied to the filter 240, the inter-frame prediction unit 260 and the intra-frame prediction unit 265 of the image decoding device 200.

[0105] In addition, the prediction unit of the above-mentioned image encoding / decoding device 100 or 200 can derive reference samples from neighboring reference samples of the current block according to the intra-frame prediction mode of the current block, and can generate prediction samples of the current block based on the reference samples.

[0106] For example, the prediction sample may be derived based on an average or interpolation of neighboring reference samples of the current block, and (ii) the prediction sample may be derived based on a reference sample that exists in a specific (prediction) direction relative to the prediction sample among the neighboring reference samples of the current block. Case (i) may be referred to as a non-directional mode or a non-angle mode, and case (ii) may be referred to as a directional mode or an angle mode. In addition, the prediction sample may be generated by interpolating a first neighboring sample and a second neighboring sample located in a direction opposite to the prediction direction of the intra-frame prediction mode of the current block among the neighboring reference samples. The above case may be referred to as linear interpolation intra-frame prediction (LIP). In addition, the temporary prediction sample of the current block may be derived based on the filtered neighboring reference samples, and the prediction sample of the current block may be derived by a weighted sum of at least one reference sample derived according to the intra-frame prediction mode among the existing neighboring reference samples (i.e., the unfiltered neighboring reference samples) and the temporary prediction sample. The above case may be referred to as position-dependent intra-frame prediction (PDPC). In addition, a reference sample line with the highest prediction accuracy is selected from multiple adjacent reference sample lines of the current block to derive a prediction sample using a reference sample located in the prediction direction in the corresponding line, and at this time, intra-frame prediction encoding can be performed by indicating (signaling) the reference sample line used to the image decoding device 200. The above situation may be referred to as multi-reference line (MRL) intra-frame prediction or MRL-based intra-frame prediction. In addition, by dividing the current block into vertical or horizontal sub-partitions, intra-frame prediction is performed based on the same intra-frame prediction mode, and adjacent reference samples may be derived and used in units of sub-partitions. That is, in this case, the intra-frame prediction mode for the current block is equally applied to the sub-partitions, but in some cases, the intra-frame prediction performance may be improved by deriving and using adjacent reference samples in units of sub-partitions. This prediction method may be referred to as intra-frame sub-partitioning (ISP) or intra-frame prediction based on ISP. The specific details will be described later. In addition, if the prediction direction based on the predicted sample indicates between adjacent reference samples, that is, if the prediction direction indicates a fractional sample position, the value of the predicted sample can also be derived by interpolating multiple reference samples located around the corresponding prediction direction (around the corresponding fractional sample position).

[0107] The above-mentioned intra-frame prediction method may be referred to as an intra-frame prediction type to distinguish it from an intra-frame prediction mode. The intra-frame prediction type may be referred to as various terms such as an intra-frame prediction technique or an additional intra-frame prediction mode. For example, the intra-frame prediction type (or additional intra-frame prediction mode, etc.) may include at least one of the aforementioned LIP, PDPC, MRL, or ISP. Information about the intra-frame prediction type may be encoded in the encoding device and included in the bitstream and signaled to the decoding device. Information about the intra-frame prediction type may be implemented in various forms (e.g., flag information indicating whether each intra-frame prediction type is applied or index information indicating one of multiple intra-frame prediction types).

[0108] Position Dependent Intra Prediction (PDPC)

[0109] PDPC may indicate an intra prediction method as follows: deriving a filtered reference sample by performing filtering based on a filter for PDPC, deriving a temporary prediction sample of the current block based on the intra prediction mode of the current block and the filtered reference sample, and deriving a prediction sample of the current block by performing a weighted sum of at least one reference sample derived according to the intra prediction mode among existing reference samples (i.e., unfiltered reference samples) and the temporary prediction sample. Here, the predefined filter may be one of five 7-tap filters. Alternatively, the predefined filter may be one of a 3-tap filter, a 5-tap filter, and a 7-tap filter. A 3-tap filter, a 5-tap filter, and a 7-tap filter may indicate a filter having 3 filter coefficients, a filter having 5 filter coefficients, and a filter having 7 filter coefficients, respectively.

[0110] For example, the prediction result of intra planar mode can be further modified by PDPC.

[0111] Alternatively, for example, the PDPC is an intra-frame planar mode, an intra-frame DC mode, a horizontal intra-frame prediction mode, a vertical intra-frame prediction mode, a lower-left intra-frame prediction mode (i.e., #2 intra-frame prediction mode), eight directional intra-frame prediction modes adjacent to the lower-left intra-frame prediction mode, an upper-right intra-frame prediction mode, and eight directional intra-frame prediction modes adjacent to the upper-right intra-frame prediction mode, without the need for separate signaling.

[0112] Specifically, when PDPC is applied, the predicted sample of the (x, y) coordinate predicted based on the linear combination of the reference sample and the intra prediction mode can be derived as shown in Equation 1 below.

[0113] [Formula 1]

[0114] pred(x,y)=(wL×R -1，y +wT×R x，-1 -wTL×R -1，-1+(64-wL-wT+wTL)×pred(x,y)+32)>>6

[0115] In the above formula 1, pred(x, y) on the left side represents the predicted sample value at the (x, y) coordinate, and pred(x, y) on the right side represents the temporary (primary) predicted sample value at the (x, y) coordinate. (x，-1 ) and R (-1，y) represents the upper reference sample and the left reference sample located on the upper and left sides of the current sample at the (x, y) coordinates, and R (-1，-1) represents the upper left reference sample located at the upper left corner of the current block. In addition, wL represents the weight applied to the left reference sample, wT represents the weight applied to the upper reference sample, and wTL represents the weight applied to the upper left reference sample.

[0116] Furthermore, when PDPC is applied to an intra planar mode, an intra DC mode, a horizontal intra prediction mode, and a vertical intra prediction mode, an additional boundary filter such as a conventional DC mode boundary filter or a vertical / horizontal mode edge filter may not be required.

[0117] The temporary (primary) prediction sample may be generated as a result of performing intra prediction based on the intra prediction mode of the current block and the reference sample. If PDPC is applied to the current block, the final prediction sample of the current block may be generated, for example, based on the above formula 1. If PDPC is not applied to the current block, the temporary (primary) prediction sample may be used as the final prediction sample of the current block.

[0118] Figures 4a to 4d is a diagram showing reference samples defined in PDPC.

[0119] exist Figures 4a to 4d In the example, pred(x, y) represents the prediction sample obtained by intra prediction (the above-mentioned temporary prediction sample), and R (x，-1) and R (-1，y) Represents the upper and left reference samples on the upper and left sides of the current sample located at the (x, y) coordinates.

[0120] Figure 4a The reference samples (R x，-1， R -1，y， R -1，-1 ). Figure 4b The reference samples (R x，-1 , R -1，y , R -1，-1 ). Figure 4c shows the reference samples (R x，-1 , R -1，y, R -i，-1 ). Figure 4d shows the reference samples (R x，-1 , R -1，y , R -1，-1 ). The weights of PDPC can be derived based on the prediction mode. The weights of PDPC wT, wL and wTL can be derived as shown in Table 1 below.

[0121] [Table 1]

[0122] Prediction Model w wxya wxya Diagonal upper right mode 16>>((y'<<1)>>shift) 16>>((x'<<1)>>shift) 0 Diagonal bottom left mode 16>>((y'<<1)>>shift) 16>>((x'<<1)>>shift) 0 Adjacent diagonal upper right mode 32>>((y'<<1)>>shift) 0 0 Adjacent diagonal lower left mode 0 32>>((x'<<1)>>shift) 0

[0123] In position-dependent intra prediction combination (PDPC), a prediction sample is generated using a reference sample according to a prediction mode, and then, the prediction sample is improved using a neighboring reference sample. Instead of applying PDPC to all intra prediction modes, PDPC is restrictively applied to Plane, DC, 2 (lower right mode), VDIA (upper left mode), Hor (horizontal mode), Ver (vertical mode), neighbor modes of mode #2 (modes #3 to #10), and neighbor modes of VDIA mode (modes #58 to #65) based on 65 directional intra prediction modes. In addition, instead of being applied to all prediction samples within the current block to be encoded, it can be variably applied in consideration of the size of the block.

[0124] Template-based Intra Mode Derivation (TIMD)

[0125] For each intra prediction mode of the most probable mode (MPM), the sum of absolute differences (SATD) between the predicted samples and the reconstructed samples of the predetermined template can be calculated. Figure 5 As shown in the example of , the template may include a left template T adjacent to the left side of the current block. L and the upper template T adjacent to the upper side of the current block A , and the size of the template can be changed according to the width and / or height of the current block. Then, the first two intra-frame prediction modes with the smallest SATD can be selected as TIMD modes. The two TIMD modes can be fused together with weights, and this weighted intra-frame prediction can be used to encode the current CU. The derivation process of the TIMD mode can include the above-mentioned PDPC.

[0126] The costs of the two selected modes may be compared with a predetermined threshold, and if the condition of Equation 2 using cost factor 2 is satisfied, the combination may be applied. Conversely, if the condition of Equation 2 is not satisfied, only the first mode (ie, Mode 1) may be used.

[0127] [Formula 2]

[0128] costMode2<2*costMode1

[0129] (When the above combination is applied,) the weight for each mode can be calculated using the SATD cost of each mode, as shown in Equation 3 below.

[0130] [Formula 3]

[0131] weight1=costMode2 / (costMode1+costMode2)

[0132] weight2=1-weight1

[0133] Combined Inter and Intra Prediction (CIIP)

[0134] In order to generate the prediction samples of the current block, the above-mentioned CIIP mode may be applied to the current block. In addition, an additional flag (e.g., ciip_flag) may be signaled to indicate whether the CIIP mode is applied to the current CU. For example, if the CU is encoded in merge mode, the CU may include at least 64 luma samples (i.e., the width×height of the CU is 64 or greater). In addition, if both the width and height of the CU are less than 128 luma samples, an additional flag may be signaled to indicate whether the CIIP mode is applied to the current CU.

[0135] According to the CIIP mode, the inter-frame prediction signal P inter and the intra-frame prediction signal P intra The inter-frame prediction signal P may be derived according to the same inter-frame prediction process as the conventional merge mode, and the intra-frame prediction signal P may be derived according to the conventional intra-frame prediction process using the planar mode. intra Thereafter, the inter-frame prediction signal P can be combined using weighted averaging as shown in Equation 4 below: inter and the intra-frame prediction signal P intra .

[0136] [Formula 4]

[0137] P CIIP =((4-wt)*P inter +wt*P intra +2)>>2

[0138] In formula 4, we can use Figure 6The weight wt is calculated based on the coding mode of the upper neighboring block (upper side) and the left neighboring block (left side) shown in the figure. For example, if the upper neighboring block (upper side) is available, the variable isIntraTop can be set to 1, otherwise it is set to 0. In addition, if the left neighboring block (left side) is available, the variable isIntraLeft can be set to 1, otherwise it is set to 0. When the sum of the set variables isIntraTop and isIntraLeft is 2, the weight wt can be set to 3. In contrast, when the sum of the set variables isIntraTop and isIntraLeft is 1, the weight wt can be set to 2. In other cases, the weight wt can be set to 1.

[0139] (1) PDPC mixed with CIIP

[0140] In ECM (Enhanced Compression Model), the CIIP mode can be extended based on PDPC (Position Dependent Intra Prediction). The extended CIIP mode (CIIP_PDPC) can be called the PDPC hybrid CIIP mode. In the PDPC hybrid CIIP mode (CIIP_PDPC), the prediction of the conventional merge mode can be refined using the reconstructed samples at the upper (Rx, -1) and left (R-1, y) positions. The refinement can inherit the PDPC scheme. The prediction process according to the PDPC hybrid CIIP mode (CIIP_PDPC) is as follows Figure 7 As shown. Figure 7 In the example, variable W T and W L It can represent a weight that changes according to the sample position within the block as defined in PDPC.

[0141] In addition, information about the PDPC hybrid CIIP mode (CIIP_PDPC) can be signaled together with information about the conventional CIIP mode. For example, if the CIIP flag (e.g., ciip_flag) is 1 (i.e., the CIIP mode is applied to the current CU), then the CIIP_PDPC flag can be additionally signaled to indicate whether the PDPC hybrid CIIP mode (CIIP_PDPC) is used.

[0142] (2) Combination of TIMD and TM with CIIP

[0143] In ECM, the CIIP mode can be extended based on intra-frame mode derivation (TIMD) and template matching (TM) merging. For example, the prediction sample can be generated by weighting the predicted inter-frame prediction signal using the CIIP-TM merge candidate and the predicted intra-frame prediction signal using the intra-frame prediction mode derived by TIMD. This method can be applied only to encode blocks with an area of ​​1024 or less.

[0144] The TIMD derivation method can be used to derive the intra prediction mode in CIIP. Specifically, the intra prediction mode with the minimum sum of absolute differences (SATD) value in the TIMD mode list can be selected, and the selected intra prediction mode can be mapped to any of the 67 intra prediction modes. If the derived intra prediction mode is an angle mode, the weight set (wIntra, wInter) applied to the intra prediction signal and the inter prediction signal can be modified. For example, Figure 8 As shown in (a) of FIG. 1 , for a near-horizontal mode (eg, 2≤angle mode index<34), the current block may be horizontally split into four sub-blocks (indexed 0 to 3). Figure 8 As shown in (b), for a nearly vertical mode (e.g., 34≤angle mode index≤66), the current block can be vertically divided into four sub-blocks (index 0 to 3). In addition, for each sub-block (index 0 to 3), a different weight set (wIntra, wInter) can be applied, as shown in Table 2.

[0145] [Table 2]

[0146] Sub-block index (wIntra,wInter) 0 (6,2) 1 (5,3) 2 (3,5) 3 (2,6)

[0147] Referring to Table 2, weight set (6,2) can be applied to a subblock with index 0, weight set (5,3) can be applied to a subblock with index 1, weight set (3,5) can be applied to a subblock with index 2, and weight set (2,6) can be applied to a subblock with index 3.

[0148] In addition, in CIIP-TM mode, a list of CIIP-TM merge candidates can be constructed. In addition, the CIIP-TM merge candidates can be refined by template matching. In addition, the CIIP-TM merge candidates can be reordered like conventional merge candidates by a predetermined method (e.g., adaptive reordering of merge candidates (ARMC)). In one example, the maximum number of CIIP-TM merge candidates can be equal to 2.

[0149] Hereinafter, transformation / inverse transformation related to the present disclosure will be described in detail.

[0150] Overview of Transformation / Inverse Transformation

[0151] As described above, the encoding device may derive a residual block (residual sample) based on a block (prediction block) predicted by intra / inter / IBC prediction, and derive a quantized transform coefficient by applying transform and quantization to the derived residual sample. Information about the quantized transform coefficient (residual information) may be included and encoded in a residual coding syntax, and output in the form of a bitstream. The decoding device may obtain and decode information about the quantized transform coefficient (residual information) from the bitstream to derive the quantized transform coefficient. The decoding device may derive the residual sample by dequantization / inverse transformation based on the quantized transform coefficient. As described above, at least one of quantization / dequantization and / or transform / inverse transformation may be skipped. When the transform / inverse transformation is skipped, 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. Whether the transform / inverse transformation is skipped may be signaled based on a transform skip flag (e.g., transform_skip_flag).

[0152] 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 via 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 inverse transforming (scaling) the transform coefficient. The residual sample may be derived based on the inverse transform (transform) of the scaled transform coefficient. This may also be similarly applied / expressed in other parts of the present disclosure.

[0153] Transformation / inverse transformation can be performed based on a transform kernel. For example, a multi-transform selection (MTS) scheme may be applied according to the present disclosure. In this case, some of a plurality of transform kernel sets may be selected and applied to the current block. The transform kernel may be referred to as various terms such as a transform matrix or a transform type. For example, a transform kernel set may indicate a combination of a vertical direction transform kernel (vertical transform kernel) and a horizontal direction transform kernel (horizontal transform kernel). For example, MTS index information (e.g., a tu_mts_idx syntax element) may be generated / encoded in an image encoding device and signaled to an image decoding device to indicate one of the transform kernel sets. For example, the transform kernel set according to the value of the MTS index information may be shown in Table 3.

[0154] For example, MTS index information (eg, mts_idx syntax element) may be generated / encoded in an encoding device and signaled to a decoding device to indicate one of the transform core sets. For example, as shown in Table 3, a transform core set according to the value of the MTS index information may be derived.

[0155] [Table 3]

[0156] tu_mts_idx[x0][y0] 0 1 2 3 4 trTypeHor 0 1 2 1 2 trTypeVer 0 1 1 2 2

[0157] In Table 4, tu_mts_idx represents MTS index information, and trTypeHor and trTypeVer represent a horizontal transform kernel and a vertical transform kernel, respectively.

[0158] The transform kernel set may be determined based on, for example, cu_sbt_horizontal_flag and cu_sbt_pos_flag.

[0159] [Table 4]

[0160] Cu_sbt_horizontal_flag cu_sbt_pos_flag trTypeHor trTypeVer 0 0 2 1 0 1 1 1 1 0 1 2 1 1 1 1

[0161] The transform core set may be determined based on, for example, cu_sbt_horizontal_flag and cu_sbt_pos_flag. When cu_sbt_horizontal_flag has a value of 1, it indicates that the current block is split into two transform blocks in the horizontal direction, and when it has a value of 0, it indicates that the current block is split into two transform blocks in the vertical direction. When cu_sbt_pos_flag has a value of 1, it indicates that tu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of the first transform block of the current block do not exist in the bitstream, and when it has a value of 0, it indicates that tu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of the second transform block of the current block do not exist in the bitstream. Alternatively, cu_sbt_pos_flag equal to 0 may indicate that the syntax elements tu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of the second transform unit within the current coding unit are not present in the bitstream.

[0162] In addition, in Tables 3 and 4, trTypeHor may represent a horizontal transform kernel, and trTypeVer may represent a vertical transform kernel. A trTypeHor / trTypeVer value of 0 may represent DCT2, a trTypeHor / trTypeVer value of 1 may represent DST7, and a trTypeHor / trTypeVer value of 2 may represent DCT8. However, this is only an example, and other values ​​may be mapped to other DCT / DSTs by convention.

[0163] Table 5 shows the basis functions of DCT2, DCT8, and DCT7 described above as examples.

[0164] [Table 5]

[0165]

[0166] In the present disclosure, an MTS-based transform is applied as a primary transform, and a secondary transform may be further applied. The secondary transform may be applied only to coefficients in the upper left w×h region of the coefficient block to which the primary transform is applied, and may be referred to as a reduced secondary transform (RST). For example, w and / or h may be 4 or 8. In the transform, the primary transform and the secondary transform may be applied sequentially to the residual block, and in the inverse transform, the secondary inverse transform and the primary inverse transform may be applied sequentially to the transform coefficients. The secondary transform (RST transform) may be referred to as a low-frequency coefficient transform (LFCT) or a low-frequency non-separable transform (LFNST). The secondary inverse transform may be referred to as an inverse LFCT or an inverse LFNST.

[0167] Fig. 9 is a diagram illustrating the transformation and inverse transformation process. Fig. 9 In the example, the transform unit 910 may correspond to Figure 2 The transform unit 120, and the inverse transform unit 920 may correspond to Figure 2 The inverse transform unit 150 or Figure 3 The inverse transform unit 230.

[0168] Reference Fig. 9 , the transformation unit 910 may include a primary transformation unit 911 and a secondary transformation unit 912 .

[0169] The primary transform unit 911 may apply a primary transform to the residual samples A to generate (primary) transform coefficients B. In the present disclosure, the primary transform may be referred to as a core transform.

[0170] The primary transform may be performed based on the MTS scheme. When conventional MTS is applied, the transform from the spatial domain to the frequency domain may be applied to the residual signal (or residual block) based on DCT type 2, DST type 7, DCT type 8, etc. to generate transform coefficients (or primary transform coefficients). Here, DCT type 2, DST type 7, DCT type 8, etc. may be referred to as transform types, transform kernels, or transform kernels. Examples of basis functions of DCT type 2, DST type 7, and DCT type 8 are described above with reference to Table 4.

[0171] In addition, the transform / inverse transform can be performed in units of CU or TU. That is, the transform / inverse transform can be applied to the residual samples in the CU or the residual samples in the TU. The CU size and the TU size may be the same, or there may be multiple TUs in the CU area. In addition, the CU size may generally represent the luminance component (sample) CB size. The TU size may generally represent the luminance component (sample) TB size. The chrominance component (sample) CB or TB size may be derived based on the luminance component (sample) CB or TB size based on the component ratio according to the color format (chrominance format, for example, 4:4:4, 4:2:2, 4:2:0, etc.). The TU size may be derived based on maxTbSize. For example, when the CU size is greater than maxTbSize, multiple TUs (TBs) of maxTbSize may be derived from the CU, and the transform / inverse transform may be performed in units of TU (TB). maxTbSize may be considered when determining whether to apply various intra-frame prediction types such as ISP. The information about maxTbSize may be predetermined, or may be generated and encoded in the encoding device and signaled to the decoding device.

[0172] As mentioned above, the transform can be applied to the residual block. This is to decorrelate the residual block as much as possible, concentrate the coefficients in the low frequency, and produce a zero tail at the end of the block. In the JEM software, the transform part includes two main functions (i.e., core transform and secondary transform). The core transform consists of a family of DCT (discrete cosine transform) and DST (discrete sine transform) transforms, which are applied to all rows and columns of the residual block. Thereafter, the secondary transform can be additionally applied to the upper left corner of the output of the core transform. Similarly, the inverse transform can be applied in the order of the secondary inverse transform and the core inverse transform. Specifically, the secondary inverse transform can be applied to the upper left corner of the coefficient block. The core inverse transform is then applied to the rows and columns of the output of the secondary inverse transform. The core transform / inverse transform can be referred to as the primary transform / inverse transform.

[0173] Overview of Adaptive Multi-Core Transformation (AMT)

[0174] In addition to conventional DCT-2 and 4×4DST-7, adaptive (or explicit) multi-transform (AMT or EMT) technology can be used for residual coding for inter-frame coded blocks and intra-frame coded blocks. In the following, AMT and EMT will be used interchangeably. In AMT, in addition to conventional transforms, multiple transforms selected from the DCT / DST family can also be used. The newly introduced transform matrices in JEM are DST-7, DCT-8, DST-1 and DCT-5. The basis functions of DST / DCT used in AMT are shown in Table 6.

[0175] [Table 6]

[0176]

[0177] EMT can be applied to CUs with width and height less than or equal to 64, and whether EMT is applied can be controlled by a CU level flag. For example, if the CU level flag is 0, DCT-2 is applied to the CU to encode the residual. For the luma coding blocks within the CU to which EMT is applied, two additional flags are signaled to identify the horizontal and vertical transforms to be used. In JEM, the residuals of the blocks can be encoded in transform skip mode. For intra residual coding, due to the different residual statistics of different intra prediction modes, a mode-dependent transform candidate selection process is used. For example, three transform subsets are defined as in Table 7, and the transform subsets are selected based on the intra prediction mode as in Table 8.

[0178] [Table 7]

[0179]

[0180]

[0181] [Table 8]

[0182]

[0183] Using the subset concept, the transform subset is first identified based on Table 7 using the intra prediction mode of the CU with CU-level EMT_CU_flag 1. Then, for each horizontal (EMT_TU_horizontal_flag) and vertical (EMT_TU_vertical_flag) transform, one of the two transform candidates in the identified transform subset is selected based on explicit signaling using the flag according to Table 8.

[0184] Table 9 shows a transformation configuration group to which AMT is applied.

[0185] [Table 9]

[0186]

[0187]

[0188] Referring to Table 9, the transform configuration group is determined based on the prediction mode, and the number of groups may be a total of 6 (G0 to G5). In addition, G0 to G4 correspond to the case where intra prediction is applied, and G5 represents a transform combination (or transform set, transform combination set) applied to a residual block generated by inter prediction.

[0189] The transform combination may consist of a horizontal transform (or row transform) applied to rows of the corresponding 2D block and a vertical transform (or column transform) applied to columns.

[0190] Here, each transform configuration group may have four transform combination candidates. The four transform combination candidates may be selected or determined by transform combination indexes from 0 to 3, and the transform combination indexes may be encoded and transmitted from the encoder to the decoder.

[0191] For example, depending on the intra prediction mode, the residual data (or residual signal) obtained by intra prediction may have different statistical characteristics. Therefore, as shown in Table 9, other transforms other than conventional cosine transforms may be applied for each intra prediction. In the present disclosure, transform types may be expressed as, for example, DCT type 2, DCT-II, and DCT-2.

[0192] Table 9 shows the case where 35 intra prediction modes are used and the case where 67 intra prediction modes are used. Multiple transform combinations can be applied for each transform configuration group distinguished in each intra prediction mode column. For example, multiple transform combinations can be composed of four combinations (row direction transform, column direction transform). As a specific example, in group 0, DST-7 and DCT-5 can be applied in both the row (horizontal) direction and the column (vertical) direction, so a total of four combinations are possible.

[0193] Since a total of four transform kernel combinations may be applied for each intra prediction mode, a transform combination index for selecting one of them may be transmitted for each transform unit. In the present disclosure, the transform combination index may be referred to as an AMT index and may be expressed as amt_idx.

[0194] In addition, in addition to the transform kernel given in Table 9, due to the characteristics of the residual signal, there may be a case where DCT-2 is optimal for both the row direction and the column direction. Therefore, the transform can be adaptively applied by defining the AMT flag for each coding unit. Here, if the AMT flag is 0, DCT-2 is applied for both the row direction and the column direction, and if the AMT flag is 1, one of the four combinations can be selected or determined by the AMT index.

[0195] For example, if the AMT flag is 0 and the number of transform coefficients for one transform unit is less than 3, the transform kernel in Table 9 may not be applied and DST-7 may be applied for both row and column directions.

[0196] For example, if the transform coefficient value is parsed first and the number of transform coefficients is less than 3, the AMT index is not parsed and DST-7 is applied, which can reduce the amount of additional information transmitted.

[0197] For example, AMT may be applied only when both the width and height of a transform unit are 32 or less.

[0198] For example, Table 9 may be preset through offline training.

[0199] For example, the AMT index may be defined as a single index that may simultaneously indicate a combination of horizontal transform and vertical transform. Alternatively, the AMT index may be defined as a separate horizontal transform index and vertical transform index.

[0200] Fig.10 is a flowchart illustrating the encoding process of performing AMT.

[0201] AMT can be applied regardless of primary transform or secondary transform. That is, there is no restriction that it should be applied to only one of the two, and it can be applied to both. Here, primary transform may mean a transform used to initially transform a residual block, and secondary transform may mean a transform used to apply a transform to a block generated as a result of primary transform.

[0202] Reference Fig.10 , the encoding device may determine a transform group corresponding to the current block (S1010). Here, the transform group may mean the transform group described above with reference to Table 8, but is not limited thereto and may be composed of other transform combinations.

[0203] The encoding apparatus may perform transform on candidate transform combinations available within the transform group ( S1020 ).

[0204] As a result of performing the transform, the encoding apparatus may determine or select a transform combination having a minimum RD (Rate-Distortion) cost ( S1030 ).

[0205] The encoding apparatus may encode a transform combination index corresponding to the selected transform combination (S1040).

[0206] Fig.11 is a flow chart showing a decoding process in which AMT is performed.

[0207] Reference Fig.11 , the decoding device can determine the transformation group of the current block (S1110).

[0208] The decoding device may parse the transform combination index (S1120). Here, the transform combination index may correspond to any one of a plurality of transform combinations within the transform group. The step of determining the transform group (S1110) and the step of parsing the transform combination index (S1120) may be performed simultaneously.

[0209] The decoding device may derive a transform combination corresponding to the transform combination index (S1130). Here, the transform combination may refer to the transform combination described above with reference to Table 8, but is not limited thereto. That is, configuration by another transform combination is also possible.

[0210] The decoding device may perform inverse transform on the current block based on the transform combination (S1140). If the transform combination consists of a row transform and a column transform, the row transform may be applied first, and then the column transform may be applied. However, the process is not limited thereto and may be applied inversely.

[0211] Enhanced MTS for intra-frame coding

[0212] In the VVC design, in the case of MTS, only DST7 and DCT8 transform cores are used for intra-frame coding and inter-frame coding. In contrast, in the enhanced compression mode (ECM), additional primary transforms including DCT5, DST4, DST1, and unit transform (IDT) can be used. In addition, the MTS set can be configured according to the transform unit (TU) size and intra-frame mode information. Consider 16 different TU sizes, and for each TU size, consider 5 different classes according to the intra-frame mode information. For each class, four different transform pairs can be considered, which is the same as in the VVC case. Consider a total of 80 different classes, but some of these different classes can share exactly the same transform set. Therefore, there can be up to 58 unique entries (less than 80) in the result lookup table (LUT).

[0213] In addition, for angle modes, TU shapes and joint symmetry for intra prediction are considered. Therefore, mode i with TU shape A×B (where i>34) can be mapped to the same class as mode j with TU shape B×A (where j=68-i). However, the order of horizontal and vertical transform kernels is swapped for each transform pair. For example, 16×4 blocks of mode 18 (horizontal prediction) and 4×16 blocks of mode 50 (vertical prediction) are mapped to the same class. In this case, the vertical and horizontal transform kernels are swapped. For wide-angle mode, the transform set is determined using the closest conventional angle mode. For example, mode 2 is used for all modes between -2 and -14. Similarly, mode 66 is used for all modes between modes 67 and 80.

[0214] Problems with existing technology

[0215] According to the conventional MTS (Multiple Transform Set) scheme, as shown in Table 10, two types of transform kernels DCT2 and DST7 are used, and four MTS combinations are used.

[0216] [Table 10]

[0217] Horizontal direction DCT2 DST7 DCT2 DST7 Vertical DCT2 DCT2 DST7 DST7

[0218] However, since the CIIP (Combined Inter and Intra Prediction) predictor is a combination of an intra predictor and an inter predictor, the conventional MTS set shown in Table 10 may not be suitable. For example, the DST7 / DCT2 combination may be suitable for an intra predictor, but not for an inter predictor. In addition, the DCT2 / DCT2 combination may be suitable for an inter predictor, but not for an intra predictor. Therefore, if the conventional MTS set is applied to the CIIP predictor having both intra prediction and inter prediction characteristics and the residual signal based thereon without change, the compression efficiency will be significantly reduced.

[0219] In addition, in ECM (Enhanced Compression Model), the previously mentioned PDPC (position-dependent intra prediction) hybrid CIIP mode is newly introduced as an extended CIIP mode. In addition, unlike conventional VVC that generates intra predictors using only planar modes, intra modes can be derived and used by TIMD (template-based intra mode derivation), so that all intra prediction modes can be applied to extended CIIP. This means that intra predictors and inter predictors can be combined in a more complex and diverse way in the extended CIIP mode, and as a result, the compression efficiency degradation caused by applying the conventional MTS set is destined to be greater.

[0220] To solve these problems, the present disclosure proposes various methods for more efficiently applying the MTS scheme in the CIIP mode.The embodiments of the present disclosure described below are intended to explain representative aspects of the present disclosure and may be applied independently or in combination of two or more.

[0221] Implementation Method 1

[0222] According to Embodiment 1 of the present disclosure, when the prediction mode of the current block is the CIIP mode, a predetermined intra MTS set may be applied to the current block instead of the MTS set of Table 10. Then, an MTS combination for the current block in the intra MTS set may be selected based on the intra prediction mode applied to the CIIP mode. Fig.12 Embodiment 1 of the present disclosure is described in more detail.

[0223] Fig.12 is a flowchart illustrating an MTS application method according to one embodiment of the present disclosure.

[0224] Fig.12 Each step of can be performed by an image encoding device or an image decoding device. In the following, for the convenience of explanation, the description will be based on the image encoding device.

[0225] Reference Fig.12, the image encoding device may determine whether the prediction mode of the current block is a CIIP mode (S1210). Here, the CIIP mode may be a conventional CIIP mode based on a planar mode. Alternatively, the CIIP mode may be an extended CIIP mode of ECM (e.g., the aforementioned PDPC hybrid CIIP mode or a TIMD-based CIIP mode).

[0226] If the prediction mode of the current block is the CIIP mode ("Yes" of S1210), the image encoding device may perform a transform on the current block based on a predetermined intra MTS set (S1220). In one embodiment, in addition to DCT2 and DST7 of Table 10, the predetermined intra MTS set may also include at least one transform core of DCT5, DST4, DST1, or IDT (unit transform). For example, the intra MTS set may be composed of nine MTS combinations using three types of transform cores DCT2, DST7, and DCT5. Alternatively, the intra MTS set may be composed of 16 MTS combinations using four types of transform cores DCT2, DST7, DCT5, and DST1. In addition, the types and quantities of the above-mentioned additional transform cores are exemplary, and therefore, the embodiments of the present disclosure are not limited thereto.

[0227] The MTS combination for the current block may be selected from the intra MTS set based on the intra prediction mode applied to the CIIP mode. For example, if the CIIP mode is the normal CIIP mode, the MTS combination for the current block may be selected based on the planar mode.

[0228] In addition, if the CIIP mode is a PDPC hybrid CIIP mode, the intra-frame prediction mode applied to the CIIP mode is considered to be a plane mode, and the MTS combination for the current block can be selected based on the plane mode. According to this embodiment, the intra-frame prediction mode applied to the CIIP mode can be considered to be another intra-frame prediction mode other than the plane mode. For example, if the CIIP mode is a PDPC hybrid mode, the intra-frame prediction mode applied to the CIIP mode is considered to be a DC mode, and the MTS combination for the current block can be selected based on the DC mode.

[0229] In addition, if the CIIP mode is a CIIP mode based on TIMD, the MTS combination for the current block can be selected based on the intra prediction mode derived by TIMD. Here, as described above, in addition to the planar mode, the intra prediction mode derived by TIMD can be any conventional intra mode (e.g., DC mode, mode 2, etc.).

[0230] In contrast, if the prediction mode of the current block is not the CIIP mode (No of S1210), the image encoding apparatus may perform transformation on the current block based on a conventional MTS set (eg, the inter MTS set described above with reference to Table 10) (S1230).

[0231] According to embodiment 1 of the present disclosure, when the prediction mode of the current block is the CIIP mode, a predetermined intra-frame MTS set further including an additional transform kernel can be applied to the current block instead of a conventional inter-frame MTS set using only DCT2 and DST7. Since the CIIP mode has a residual signal characteristic that appears in the intra-frame prediction mode due to the combination of intra-frame predictors, compression efficiency can be improved by applying the intra-frame MTS set. In addition, since the MTS index binarization method of the intra-frame MTS set and the conventional inter-frame MTS set is the same, there is an advantage in that the conventional binarization process can be applied in the same manner.

[0232] Implementation Method 2

[0233] Since the TIMD-based CIIP mode has a larger residual signal characteristic generated in the intra-frame prediction mode compared to other types of CIIP modes, it may be appropriate to apply the aforementioned intra-frame MTS set instead of the conventional inter-frame MTS set. However, since the PDPC hybrid CIIP mode generates a predictor with different characteristics from the conventional intra-frame prediction mode, it may be inappropriate to apply the intra-frame MTS set. Therefore, according to embodiment 2 of the present disclosure, the aforementioned intra-frame MTS set can be applied only when the prediction mode of the current block is the TIMD-based CIIP mode. In the following, reference will be made to Fig.13 Embodiment 2 of the present disclosure is described in more detail.

[0234] Fig.13 is a flowchart illustrating an MTS application method according to one embodiment of the present disclosure.

[0235] Fig.13 Each step of can be performed by an image encoding device or an image decoding device. In the following, for the convenience of explanation, the description will be based on the image encoding device.

[0236] Reference Fig.13 , the image encoding apparatus may determine whether a prediction mode of a current block is a CIIP mode (S1310).

[0237] If the prediction mode of the current block is the CIIP mode (Yes of S1310 ), the image encoding apparatus may determine whether the CIIP mode is the above-mentioned TIMD-based CIIP mode ( S1320 ).

[0238] As a result of the determination, if the CIIP mode is the TIMD-based CIIP mode ("Yes" of S1320), the image encoding apparatus may perform a transform on the current block based on a predetermined intra MTS set (S1330). Here, as described above in Embodiment 1, in addition to DCT2 and DST7 of Table 10, the predetermined intra MTS set may further include at least one transform kernel of DCT5, DST4, DST1, or IDT (unit transform).

[0239] In contrast, if the CIIP mode is not a TIMD-based CIIP mode (e.g., a PDPC hybrid CIIP mode) ("No" in S1320), the image encoding device may perform a transform on the current block based on a conventional MTS set (e.g., the inter-frame MTS set described above with reference to Table 10) (S1340).

[0240] Furthermore, even if the prediction mode of the current block is not the CIIP mode (No of S1310 ), the image encoding apparatus may perform transformation for the current block based on the normal MTS set ( S1340 ).

[0241] According to Embodiment 2 of the present disclosure, only when the prediction mode of the current block is the CIIP mode based on TIMD, a predetermined intra-frame MTS set including additional transform kernels other than DCT2 and DST7 can be applied. Therefore, there is an advantage in that compression efficiency is improved in the CIIP mode, and the same MTS index binarization method used in Embodiment 1 can be used.

[0242] Implementation 3

[0243] In the CIIP mode, since the residual signal is generated by a combination of an intra-frame predictor and an inter-frame predictor, it may have different characteristics from a residual signal generated by only a conventional intra-frame predictor or an inter-frame predictor. Therefore, when the MTS scheme is applied to the CIIP mode, the compression efficiency may be reduced. Therefore, according to Embodiment 3 of the present disclosure, the MTS scheme may be applied only when the prediction mode of the current block is not the CIIP mode. In the following, reference will be made to Fig.14 Embodiment 3 of the present disclosure is described in more detail.

[0244] Fig.14 is a flowchart illustrating an MTS application method according to an embodiment of the present disclosure.

[0245] Fig.14 Each step of can be performed by an image encoding device or an image decoding device. In the following, for the convenience of explanation, the description will be based on the image encoding device.

[0246] Reference Fig.14, the image encoding apparatus may determine whether a prediction mode of a current block is a CIIP mode (S1410).

[0247] If the prediction mode of the current block is the CIIP mode (Yes in S1410), the image encoding apparatus may not apply the MTS scheme to the current block (ie, MTS skipping) (S1420). In this case, a predetermined transform kernel (e.g., DCT2) determined according to a conventional transform scheme different from the MTS scheme may be used for the transform of the current block.

[0248] In contrast, if the prediction mode of the current block is not the CIIP mode ("No" of S1410), the image encoding apparatus may apply the MTS scheme to the current block (S1430). In this case, a specific MTS combination selected from a predetermined MTS set (e.g., an inter-frame MTS set or an intra-frame MTS set) may be used for transformation of the current block.

[0249] According to Embodiment 3 of the present disclosure, the MTS scheme can be applied only when the prediction mode of the current block is not the CIIP mode. Therefore, the compression efficiency can be improved in the CIIP mode, and the number of MTS binarization bits (or the number of signaling bits) can be reduced.

[0250] Implementation 4

[0251] As in the above-mentioned embodiment 3, if the MTS scheme is unconditionally applied to the CIIP mode, the compression efficiency may be reduced. Therefore, according to the embodiment 4 of the present disclosure, the MTS scheme can be applied only when the prediction mode of the current block is not the CIIP mode or is a specific CIIP mode. Figures 15 to 17 Embodiment 4 of the present disclosure is described in more detail.

[0252] Fig.15 is a flowchart illustrating an MTS application method according to one embodiment of the present disclosure.

[0253] Fig.15 Each step of can be performed by an image encoding device or an image decoding device. In the following, for the convenience of explanation, the description will be based on the image encoding device.

[0254] Reference Fig.15 , the image encoding apparatus may determine whether a prediction mode of a current block is a CIIP mode (S1510).

[0255] If the prediction mode of the current block is the CIIP mode (Yes of S1510), the image encoding apparatus may determine whether the CIIP mode is an extended CIIP mode (S1520). In one embodiment, the extended CIIP mode may include at least one of a PDPC hybrid CIIP mode or a TIMD-based CIIP mode.

[0256] As a result of the determination, if the CIIP mode is the extended CIIP mode ("Yes" of S1520), the image encoding device may not apply the MTS scheme to the current block (S1530). In this case, a predetermined transform kernel (e.g., DCT2) determined according to a conventional transform scheme different from the MTS scheme may be used for the transform of the current block.

[0257] In contrast, if the CIIP mode is not the extended CIIP mode ("No" of S1520), the image encoding device may apply the MTS scheme to the current block (S1540). In this case, a specific MTS combination selected from a predetermined MTS set (e.g., an inter-frame MTS set or an intra-frame MTS set) may be used for the transformation of the current block.

[0258] Furthermore, even if the prediction mode of the current block is not the CIIP mode (No of S1510 ), the image encoding apparatus may apply the MTS scheme to the current block ( S1540 ).

[0259] Fig.16 is a flowchart illustrating an MTS application method according to another embodiment of the present disclosure.

[0260] Fig.16 Each step of can be performed by an image encoding device or an image decoding device. In the following, for the convenience of explanation, the description will be based on the image encoding device.

[0261] Reference Fig.16 , the image encoding apparatus may determine whether a prediction mode of a current block is a CIIP mode (S1610).

[0262] If the prediction mode of the current block is the CIIP mode (Yes of S1610 ), the image encoding apparatus may determine whether the CIIP mode is a TIMD-based CIIP mode ( S1620 ).

[0263] If the CIIP mode is a TIMD-based CIIP mode ("Yes" of S1620), the image encoding device may determine whether the intra-frame prediction mode derived by TIMD is a predetermined mode (S1630). In one embodiment, the predetermined mode may be a non-directional mode such as a plane mode or a DC mode. As a result of the determination, if the intra-frame prediction mode derived by TIMD is a predetermined mode ("Yes" of S1630), the image encoding device may not apply the MTS scheme to the current block (S1640). In contrast, if the intra-frame prediction mode derived by TIMD is not a predetermined mode ("No" of S1630), the image encoding device may apply the MTS scheme to the current block (S1650).

[0264] In addition, even if the prediction mode of the current block is not the CIIP mode ("No" in S1610) or the prediction mode of the current block is the CIIP mode but not the TIMD-based CIIP mode (for example, the PDPC hybrid CIIP mode) ("No" in S1620), the image encoding device can also apply the MTS scheme to the current block (S1650).

[0265] Fig.17 is a flowchart illustrating an MTS application method according to another embodiment of the present disclosure.

[0266] Fig.17 Each step of can be performed by an image encoding device or an image decoding device. In the following, for the convenience of explanation, the description will be based on the image encoding device.

[0267] Reference Fig.17 , the image encoding apparatus may determine whether a prediction mode of a current block is a CIIP mode (S1710).

[0268] If the prediction mode of the current block is the CIIP mode (Yes of S1710 ), the image encoding apparatus may determine whether the CIIP mode is a TIMD-based CIIP mode ( S1720 ).

[0269] If the CIIP mode is a CIIP mode based on TIMD ("Yes" of S1720), the image encoding device may determine whether the intra-frame prediction mode derived by TIMD is a predetermined mode (S1730). In one embodiment, the predetermined mode may be a non-directional mode (e.g., a planar mode or a DC mode). As a result of the determination, if the intra-frame prediction mode derived by TIMD is a predetermined mode ("Yes" of S1730), the image encoding device may apply the MTS scheme to the current block (S1740). In contrast, if the intra-frame prediction mode derived by TIMD is not a predetermined mode ("No" of S1730), the image encoding device may not apply the MTS scheme to the current block (S1750).

[0270] Also, if the prediction mode of the current block is not the CIIP mode ('No' of S1710 ), the image encoding apparatus may apply the MTS scheme to the current block ( S1740 ).

[0271] In contrast, if the prediction mode of the current block is the CIIP mode but not the TIMD-based CIIP mode (eg, the PDPC hybrid CIIP mode) ('No' of S1720 ), the image encoding apparatus may not apply the MTS scheme to the current block ( S1750 ).

[0272] According to Embodiment 4 of the present disclosure, the MTS scheme can be applied only when the prediction mode of the current block is not the CIIP mode or is a specific CIIP mode. Therefore, the compression efficiency in the CIIP mode can be improved, and the number of MTS binarization bits (or the number of signaling bits) can be reduced.

[0273] In the following, reference is made to Fig.18 and Fig.19 , an image encoding / decoding method according to an embodiment of the present disclosure is described in detail.

[0274] Fig.18 is a flowchart illustrating an image encoding method according to an embodiment of the present disclosure.

[0275] Fig.18 The image encoding method can be Figure 2 For example, steps S1810 and S1820 may be performed by the transform unit 120.

[0276] Reference Fig.18 , the image encoding apparatus may determine whether to apply MTS (Multiple Transform Selection) to the current block (S1810).

[0277] If the MTS is applied to the current block (Yes of S1810 ), the image encoding apparatus may perform transformation on the current block based on a predetermined MTS set ( S1820 ).

[0278] The MTS set may be determined based on whether the prediction mode of the current block is a CIIP (combined inter and intra prediction) mode. In one embodiment, based on the prediction mode of the current block being a CIIP mode, the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform core (e.g., DCT5, DST4, DST1, and / or IDT, etc.). In addition, based on the prediction mode of the current block being a CIIP mode based on TIMD (template-based intra mode derivation), the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform core.

[0279] In one embodiment, based on the prediction mode of the current block being the CIIP mode, the MTS may not be applied to the current block (ie, MTS skipping).

[0280] In one embodiment, the prediction mode based on the current block is an extended CIIP mode, and MTS may not be applied to the current block. Here, the extended CIIP mode may include at least one of a PDPC (position-dependent intra prediction) hybrid CIIP mode or a TIMD-based CIIP mode.

[0281] In one embodiment, the prediction mode based on the current block is a CIIP mode of template-based intra mode derivation (TIMD) and the intra prediction mode derived by TIMD is a predetermined prediction mode, and MTS may not be applied to the current block. Here, the predetermined prediction mode may be a planar mode or a DC mode.

[0282] In one embodiment, the prediction mode based on the current block is a CIIP mode of template-based intra mode derivation (TIMD) and the intra prediction mode derived by TIMD is a predetermined prediction mode, and MTS may not be applied to the current block. Here, the predetermined prediction mode may be a planar mode or a DC mode.

[0283] Fig.19 is a flowchart illustrating an image decoding method according to one embodiment of the present disclosure.

[0284] Fig.19 The image decoding method can be Figure 3 For example, steps S1910 and S1920 may be performed by Figure 3 The inverse transform unit 230 performs.

[0285] Reference Fig.19, the image decoding apparatus may determine whether to apply MTS (Multiple Transform Selection) to the current block (S1910).

[0286] If the MTS is applied to the current block (Yes of S1910 ), the image encoding apparatus may perform inverse transform on the current block based on a predetermined MTS set ( S1920 ).

[0287] The MTS set may be determined based on whether the prediction mode of the current block is a CIIP (combined inter and intra prediction) mode. In one embodiment, based on the prediction mode of the current block being a CIIP mode, the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform core (e.g., DCT5, DST4, DST1, and / or IDT, etc.). In addition, based on the prediction mode of the current block being a CIIP mode based on TIMD (template-based intra mode derivation), the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform core.

[0288] In one embodiment, based on the prediction mode of the current block being the CIIP mode, the MTS may not be applied to the current block (ie, MTS skipping).

[0289] In one embodiment, the prediction mode based on the current block is an extended CIIP mode, and MTS may not be applied to the current block. Here, the extended CIIP mode may include at least one of a PDPC (position-dependent intra prediction) hybrid CIIP mode or a TIMD-based CIIP mode.

[0290] In one embodiment, the prediction mode based on the current block is a CIIP mode of template-based intra mode derivation (TIMD) and the intra prediction mode derived by TIMD is a predetermined prediction mode, and MTS may not be applied to the current block. Here, the predetermined prediction mode may be a planar mode or a DC mode.

[0291] In one embodiment, the prediction mode based on the current block is a CIIP mode of template-based intra mode derivation (TIMD) and the intra prediction mode derived by TIMD is a predetermined prediction mode, and MTS may not be applied to the current block. Here, the predetermined prediction mode may be a planar mode or a DC mode.

[0292] Although for the sake of clarity of description, the exemplary method of the present disclosure described above is represented as a series of operations, it is not intended to limit the order of executing the steps, and these steps can be performed simultaneously or in different orders when necessary. In order to implement the method according to the present disclosure, the steps described may further include other steps, may include the remaining steps except some steps, or may include other additional steps except some steps.

[0293] In the present disclosure, an image encoding device or an image decoding device that performs a predetermined operation (step) may perform an operation (step) of confirming an execution condition or situation of the corresponding operation (step). For example, if it is described that a predetermined operation is performed when a predetermined condition is met, the image encoding device or the image decoding device may perform the predetermined operation after determining whether the predetermined condition is met.

[0294] The various embodiments of the present disclosure are not a list of all possible combinations and are intended to describe representative aspects of the present disclosure, and matters described in the various embodiments may be applied independently or in combination of two or more.

[0295] Various embodiments of the present disclosure may be implemented in hardware, firmware, software or a combination thereof. In the case of implementing the present disclosure in hardware, the present disclosure may be implemented in an application specific integrated circuit (ASIC), a digital signal processor (DSP), a digital signal processing device (DSPD), a programmable logic device (PLD), a field programmable gate array (FPGA), a general purpose processor, a controller, a microcontroller, a microprocessor, etc.

[0296] In addition, the image decoding device and the image encoding device to which the embodiments of the present disclosure are applied may be included in a multimedia broadcast sending and receiving device, 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 OTT video (over the top video) device, an Internet streaming service provider, a three-dimensional (3D) video device, a video phone video device, a medical video device, etc., and may be used to process a video signal or a data signal. For example, an OTT video device may 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.

[0297] Fig. 20 is a diagram showing a content streaming system to which an embodiment of the present disclosure is applicable.

[0298] like Fig. 20 As shown, the content streaming system to which the embodiments of the present disclosure are applied may mainly include an encoding server, a streaming server, a network server, a media storage device, a user device, and a multimedia input device.

[0299] The encoding server compresses 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 sends the bitstream to the streaming server. As another example, when a multimedia input device such as a smart phone, a camera, or a camcorder directly generates a bitstream, the encoding server can be omitted.

[0300] A bitstream may be generated by an image encoding method or an image encoding device to which an embodiment of the present disclosure is applied, and a streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

[0301] The streaming server sends multimedia data to the user device based on the user's request through the network server, and the network server is used as a medium to inform the user of the service. When the user requests the required service from the network server, the network server can deliver it to the streaming server, and the streaming server can send the multimedia data to the user. In this case, the content streaming system may include a separate control server. In this case, the control server is used to control the command / response between the devices in the content streaming system.

[0302] The streaming server may receive content from a media storage device and / or an encoding server. For example, when receiving content from an encoding server, the content may be received in real time. In this case, in order to provide a smooth streaming service, the streaming server may store the bitstream within a predetermined time.

[0303] Examples of user devices may include mobile phones, smart phones, laptop computers, digital broadcast terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigation devices, slate PCs, tablet PCs, ultrabooks, wearable devices (e.g., smart watches, smart glasses, head-mounted displays), digital televisions, desktop computers, digital signage, etc.

[0304] Each server in the content streaming system may operate as a distributed server, in which case data received from each server may be distributed.

[0305] The scope of the present disclosure includes software or machine-executable commands (e.g., operating systems, applications, firmware, programs, etc.) for enabling operations of methods according to various embodiments to be performed on a device or computer, and non-transitory computer-readable media having such software or commands stored thereon and executable on a device or computer.

[0306] Industrial Applicability

[0307] The embodiments of the present disclosure may be used to encode or decode an image.

Claims

1. An image decoding method performed by an image decoding device, the image decoding method comprising the following steps: Determine whether to apply a multi-transform selection (MTS) to the current block; as well as Based on the MTS being applied to the current block, performing an inverse transform on the current block based on a predetermined MTS set, The MTS set is determined based on whether the prediction mode of the current block is a combined inter-frame and intra-frame prediction CIIP mode.

2. The image decoding method according to claim 1, wherein: Based on the prediction mode of the current block being the CIIP mode, the MTS set is determined to be an intra MTS set including DCT2, DST7, and at least one additional transform kernel.

3. The image decoding method according to claim 1, wherein: Based on that the prediction mode of the current block is a CIIP mode of template-based intra mode derivation TIMD, the MTS set is determined to be an intra MTS set including DCT2, DST7 and at least one additional transform kernel.

4. The image decoding method according to claim 1, wherein: Based on that the prediction mode of the current block is the CIIP mode, the MTS is not applied to the current block.

5. The image decoding method according to claim 1, in, Based on the prediction mode of the current block being an extended CIIP mode, the MTS is not applied to the current block, and The extended CIIP mode includes at least one of a position-dependent intra prediction (PDPC) hybrid CIIP mode and a TIMD-based CIIP mode.

6. The image decoding method according to claim 1, wherein: The prediction mode based on the current block is a CIIP mode based on a template-based intra mode derivation TIMD and the intra prediction mode derived from the TIMD is a predetermined prediction mode, and the MTS is not applied to the current block.

7. The image decoding method according to claim 6, wherein: The predetermined prediction mode is a planar mode or a DC mode.

8. The image decoding method according to claim 1, wherein: Based on that the prediction mode of the current block is a CIIP mode based on a template-based intra mode derivation TIMD and the intra prediction mode derived from the TIMD is a predetermined prediction mode, the MTS is applied to the current block.

9. An image encoding method performed by an image encoding device, the image encoding method comprising the following steps: Determine whether to apply a multi-transform selection (MTS) to the current block; as well as Based on the MTS being applied to the current block, performing a transform on the current block based on a predetermined MTS set, The MTS set is determined based on whether the prediction mode of the current block is a combined inter-frame and intra-frame prediction CIIP mode.

10. The image encoding method according to claim 9, wherein: Based on the prediction mode of the current block being the CIIP mode, the MTS set is determined to be an intra MTS set including DCT2, DST7, and at least one additional transform kernel.

11. The image encoding method according to claim 9, wherein: Based on that the prediction mode of the current block is a CIIP mode of template-based intra mode derivation TIMD, the MTS set is determined to be an intra MTS set including DCT2, DST7 and at least one additional transform kernel.

12. The image encoding method according to claim 9, wherein: Based on that the prediction mode of the current block is the CIIP mode, the MTS is not applied to the current block.

13. The image encoding method according to claim 9, in, Based on the prediction mode of the current block being an extended CIIP mode, the MTS is not applied to the current block, and The extended CIIP mode includes at least one of a position-dependent intra prediction (PDPC) hybrid CIIP mode and a TIMD-based CIIP mode.

14. The image encoding method according to claim 9, wherein: The prediction mode based on the current block is a CIIP mode based on a template-based intra mode derivation TIMD and the intra prediction mode derived from the TIMD is a predetermined prediction mode, and the MTS is not applied to the current block.

15. A computer-readable recording medium storing a bit stream generated by an image encoding method, the image encoding method comprising the steps of: Determine whether to apply a multi-transform selection (MTS) to the current block; as well as Based on the MTS being applied to the current block, performing a transform on the current block based on a predetermined MTS set, The MTS set is determined based on whether the prediction mode of the current block is a combined inter-frame and intra-frame prediction CIIP mode.

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