Method and apparatus for encoding / decoding images based on adaptive MTS, and recording medium for storing bitstreams

Abstract

An image encoding / decoding method and device are provided. The image decoding method according to the present disclosure includes: determining whether a multiple transform selection (MTS) is applied to a current block; and performing an inverse transform on the current block based on a predetermined MTS set based on whether the MTS is applied to the current block, and the MTS set can 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, an apparatus, and a recording medium for storing a bitstream, and more particularly, to an image encoding / decoding method and apparatus based on adaptive MTS (multiple transform selection) in CIIP mode, and a recording medium for storing a bitstream generated by the image encoding method / apparatus of the present disclosure. [Background technology]

[0002] Recently, the demand for high-resolution, high-quality images, for example, HD (High Definition) images and UHD (Ultra High Definition) images, is increasing in various fields. As the resolution and quality of image data increases, the amount of information or bits transmitted increases relatively compared to conventional image data. The increase in the amount of information or bits transmitted leads to an increase in transmission costs and storage costs.

[0003] This requires a highly efficient image compression technique for effectively transmitting, storing, and reproducing high-resolution, high-quality image information. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present disclosure is to provide an image encoding / decoding method and apparatus with improved encoding / decoding efficiency.

[0005] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus that adaptively applies MTS in CIIP mode.

[0006] Another object of the present disclosure is to provide an image encoding / decoding method and apparatus for adaptively determining an MTS set in CIIP mode.

[0007] Another object of the present disclosure is to provide a non-transitory computer-readable recording medium for storing a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0008] Another object of the present disclosure is to provide a non-transitory computer-readable recording medium for storing a bitstream that is received by an image decoding device according to the present disclosure, decoded, and used to restore an image.

[0009] Another object of the present disclosure is to provide a method for transmitting a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0010] The technical problems to be solved by the present disclosure are not limited to the above-mentioned technical problems, and other technical problems not described above will be clearly understood by a person having ordinary skill in the art to which the present disclosure pertains from the following description. [Means for solving the problem]

[0011] An image decoding method according to one embodiment of the present disclosure includes a step of determining whether MTS (multiple transform selection) is applied to a current block, and a step of performing an inverse transform on the current block based on a predetermined MTS set based on whether the MTS is applied to the current block, and the MTS set can be determined based on whether a prediction mode of the current block is a CIIP (combined inter and intra prediction) mode.

[0012] An image decoding device according to another aspect of the present invention includes a memory and at least one processor, wherein the at least one processor determines whether or not MTS (multiple transform selection) is applied to a current block, and performs an inverse transform on the current block based on a predetermined MTS set based on whether the MTS is applied to the current block, and the MTS set may be determined based on whether or not a prediction mode of the current block is a CIIP (combined inter and intra prediction) mode.

[0013] An image encoding method according to another aspect of the present disclosure includes a step of determining whether MTS (multiple transform selection) is applied to a current block, and a step of performing a transformation on the current block based on a predetermined MTS set based on the application of the MTS to the current block, where the MTS set can be determined based on whether a prediction mode of the current block is a CIIP (combined inter and intra prediction) mode.

[0014] A computer-readable recording medium according to another aspect of the present disclosure can store a bitstream generated by the image encoding method or image encoding device of the present disclosure.

[0015] A transmission method according to another aspect of the present disclosure can transmit a bitstream generated by the image encoding device or image encoding method of the present disclosure.

[0016] The above-mentioned features of the present disclosure are merely illustrative of aspects of the following detailed description of the present disclosure and are not intended to limit the scope of the present disclosure. Effect of the Invention

[0017] According to the present disclosure, an image encoding / decoding method and apparatus with improved encoding / decoding efficiency can be provided.

[0018] Furthermore, according to the present disclosure, an image encoding / decoding method and apparatus for adaptively applying MTS in CIIP mode can be provided.

[0019] Furthermore, according to the present disclosure, an image encoding / decoding method and apparatus for adaptively determining an MTS set in CIIP mode can be provided.

[0020] According to the present disclosure, a non-transitory computer-readable recording medium can be provided that stores a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0021] According to the present disclosure, a non-transitory computer-readable recording medium can be provided that stores a bitstream that is received by an image decoding device according to the present disclosure, decoded, and used to restore an image.

[0022] According to the present disclosure, a method for transmitting a bitstream generated by an image encoding method or apparatus according to the present disclosure can be provided.

[0023] The effects obtained by the present disclosure are not limited to the effects described above, and other effects not described above will be clearly understood by those having ordinary skill in the art to which the present disclosure pertains from the following description. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram illustrating a video coding system to which an embodiment of the present disclosure can be applied.

[0025] [Diagram 2] 1 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied;

[0026] [Diagram 3] 1 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied;

[0027] [Figure 4a-4d] FIG. 1 is a diagram showing reference samples defined by the PDPC.

[0028] [Diagram 5] FIG. 1 is a diagram showing an example of a template used in TIMD.

[0029] [Figure 6] FIG. 13 is a diagram illustrating an example of neighboring blocks used in calculating CIIP weights.

[0030] [Figure 7] FIG. 13 is a diagram showing a prediction process according to the PDPC blending CIIP mode.

[0031] [Figure 8] FIG. 13 is a diagram for explaining CIIP weights applied to sub-blocks.

[0032] [Figure 9] FIG. 1 is a diagram for explaining a transformation and inverse transformation process.

[0033] [Figure 10] 1 is a flowchart showing an encoding process in which AMT is performed.

[0034] [Figure 11] 11 is a flowchart showing a decoding process in which AMT is performed.

[0035] [Figure 12-17] 1 is a flow chart illustrating a method for applying MTS according to an embodiment of the present disclosure.

[0036] [Figure 18] 1 is a flowchart illustrating an image encoding method according to an embodiment of the present disclosure.

[0037] [Figure 19] 1 is a flowchart illustrating an image decoding method according to an embodiment of the present disclosure.

[0038] [Figure 20] FIG. 1 is a diagram illustrating an example of a content streaming system to which an embodiment of the present disclosure can be applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The present disclosure will be described in detail below with reference to the accompanying drawings so that those skilled in the art can easily implement the present disclosure. However, the present disclosure may be embodied in various different forms and is not limited to the embodiments described herein.

[0040] In describing the embodiments of the present disclosure, if it is determined that a specific description of a known configuration or function may make the gist of the present disclosure unclear, the detailed description thereof will be omitted. In addition, in the drawings, parts that are not related to the description of the present disclosure are omitted, and similar parts are denoted by similar reference numerals.

[0041] In the present disclosure, when a certain component is "connected," "coupled," or "connected" to another component, this includes not only a direct connection relationship, but also an indirect connection relationship in which another component exists between them. Furthermore, when a certain component is described as "including" or "having" another component, this does not mean that the other component is excluded, but that the other component can be further included, unless otherwise specified.

[0042] In this disclosure, terms such as "first" and "second" are used only for the purpose of distinguishing one component from another component, and do not limit the order or importance of the components unless otherwise specified. Therefore, within the scope of this disclosure, a first component in one embodiment may be called a second component in another embodiment, and similarly, a second component in one embodiment may be called a first component in another embodiment.

[0043] In this disclosure, components that are distinguished from one another are used to clearly describe the characteristics of each component, and do not necessarily mean that the components are separate. In other words, multiple components may be integrated and configured as a single hardware or software unit, or one component may be distributed and configured as multiple hardware or software units. Thus, even if not otherwise stated, such integrated or distributed embodiments are also included in the scope of the present disclosure.

[0044] In the present disclosure, the components described in the various embodiments are not necessarily essential components, and some may be optional components. Therefore, an embodiment consisting of a subset of the components described in one embodiment is also included in the scope of the present disclosure. In addition, an embodiment including other components in addition to the components described in the various embodiments is also included in the scope of the present disclosure.

[0045] The present disclosure relates to image encoding and decoding, and terms used in this disclosure may have ordinary meanings in the technical field to which the present disclosure belongs, unless they are newly defined in this disclosure.

[0046] In this disclosure, "video" may refer to a collection of a series of images over time.

[0047] In this disclosure, a "picture" generally means a unit indicating any one image in a particular time period, a slice / tile is a coding unit constituting a part of a picture, and one picture may be composed of one or more slices / tiles. Also, a slice / tile may include one or more coding tree units (CTUs).

[0048] In this disclosure, a "pixel" or a "pel" may refer to the smallest unit that constitutes one picture (or image). A "sample" may also be used as a term corresponding to a pixel. A sample may generally indicate a pixel or a pixel value, may indicate only a pixel / pixel value of a luma component, or may indicate only a pixel / pixel value of a chroma component.

[0049] In this disclosure, a "unit" may refer to a basic unit of image processing. A unit may include at least one of a specific region of a picture and information related to the region. A unit may be used interchangeably with terms such as "sample array," "block," or "area," depending on the case. In a general case, an M×N block may include a set (or array) of samples or transform coefficients consisting of M columns and N rows.

[0050] In the present disclosure, a "current block" may refer to any one of a "current coding block," a "current coding unit," a "block to be coded," a "block to be decoded," or a "block to be processed." If prediction is performed, a "current block" may refer to a "current predicted block" or a "block to be predicted." If transformation (inverse transformation) / quantization (inverse quantization) is performed, a "current block" may refer to a "current transformed block" or a "block to be transformed." If filtering is performed, a "current block" may refer to a "block to be filtered."

[0051] Additionally, in this disclosure, unless explicitly stated as a chroma block, a "current block" may refer to a block including both a luma component block and a chroma component block, or a "luma block of the current block." The luma component block of the current block may be explicitly expressed as including an explicit description of the luma component block, such as a "luma block" or a "current luma block." Additionally, the chroma component block of the current block may be explicitly expressed as including an explicit description of the chroma component block, such as a "chroma block" or a "current chroma block."

[0052] In the present disclosure, " / " and "," can be interpreted as "and / or." For example, "A / B" and "A, B" can be interpreted as "A and / or B." Also, "A / B / C" and "A, B, C" can mean "at least one of A, B, and / or C."

[0053] In this disclosure, "or" can be interpreted as "and / or." For example, "A or B" can mean 1) only "A," 2) only "B," or 3) "A and B." Alternatively, in this disclosure, "or" can mean "additionally or alternatively."

[0054] In this disclosure, "at least one of A, B, and C" can mean "A only," "B only," "C only," or "any and all combinations of A, B, and C." Also, "at least one of A, B, or C" or "at least one of A, B, and / or C" can mean "at least one of A, B, and C."

[0055] Parentheses used in the present disclosure may mean "for example." For example, when "prediction (intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction." In other words, "prediction" in the present disclosure is not limited to "intra prediction," and "intra prediction" may be proposed as an example of "prediction." Also, when "prediction (i.e., intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction."

[0056] Video Coding System Overview

[0057] FIG. 1 is a schematic diagram of a video coding system to which embodiments of the present disclosure can be applied.

[0058] A video coding system according to an embodiment may include an encoding device 10 and a decoding device 20. The encoding device 10 may transmit encoded video and / or image information or data to the decoding device 20 in a file or streaming format via a digital storage medium or a network.

[0059] The encoding device 10 according to an embodiment may include a video source generating unit 11, an encoding unit 12, and a transmitting unit 13. The decoding device 20 according to an embodiment may include a receiving unit 21, a decoding unit 22, and a rendering unit 23. The encoding unit 12 may be referred to as a video / image encoding unit, and the decoding unit 22 may be referred to as a video / image decoding unit. The transmitting unit 13 may be included in the encoding unit 12. The receiving unit 21 may be included in the decoding unit 22. The rendering unit 23 may include a display unit, which may be configured as a separate device or an external component.

[0060] The video source generating unit 11 can obtain videos / images through a process of capturing, synthesizing, or generating videos / images. The video source generating unit 11 can include a video / image capture device and / or a video / image generation device. The video / image capture device can include, for example, one or more cameras, a video / image archive containing previously captured videos / images, etc. The video / image generation device can include, for example, a computer, a tablet, a smartphone, etc., and can (electronically) generate videos / images. For example, a virtual video / image can be generated through a computer, etc., in which case the video / image capture process can be replaced by a process in which related data is generated.

[0061] The encoder 12 may encode the input video / image. The encoder 12 may perform a series of steps such as prediction, transformation, quantization, etc. for compression and encoding efficiency. The encoder 12 may output the encoded data (encoded video / image information) in a bitstream format.

[0062] The transmitting unit 13 may transmit the encoded video / image information or data output in a bitstream format to the receiving unit 21 of the decoding device 20 via a digital storage medium or a network in a file or streaming format. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmitting unit 13 may include an element for generating a media file through a predetermined file format and may include an element for transmitting via a broadcasting / communication network. The receiving unit 21 may extract / receive the bitstream from the storage medium or network and transmit it to the decoding unit 22.

[0063] The decoding unit 22 can decode the video / image by performing a series of steps such as inverse quantization, inverse transformation, and prediction corresponding to the operations of the encoding unit 12.

[0064] The rendering unit 23 can render the decoded video / images. The rendered video / images can be displayed via a display unit.

[0065] Overview of the image encoding device

[0066] FIG. 2 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied.

[0067] As shown in Fig. 2, the image coding device 100 may include an image division unit 110, a subtraction unit 115, a transformation unit 120, a quantization unit 130, an inverse quantization unit 140, an inverse transformation unit 150, an addition unit 155, a filtering unit 160, a memory 170, an inter prediction unit 180, an intra prediction unit 185, and an entropy coding unit 190. The inter prediction unit 180 and the intra prediction unit 185 may be collectively referred to as a "prediction unit." The transformation unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transformation unit 150 may be included in a residual processing unit. The residual processing unit may further include a subtraction unit 115.

[0068] All or at least some of the components constituting the image encoding device 100 may be realized by a single hardware component (e.g., an encoder or a processor) depending on the embodiment. Also, the memory 170 may include a decoded picture buffer (DPB) and may be realized by a digital storage medium.

[0069] The image division unit 110 may divide an input image (or picture, frame) input to the image encoding device 100 into one or more processing units. As an example, the processing units may be called coding units (CUs). The coding units may be obtained by recursively dividing a coding tree unit (CTU) or a largest coding unit (LCU) according to a QT / BT / TT (Quad-tree / Binary-tree / Ternal-tree) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad-tree structure, a binary-tree structure, and / or a ternary-tree structure. For dividing the coding units, a quad-tree structure may be applied first, and a binary-tree structure and / or a ternary-tree structure may be applied later. A coding procedure according to the present disclosure may be performed based on a final coding unit that is not further divided. The maximum coding unit may be used as the final coding unit, and a lower depth coding unit obtained by dividing the maximum coding unit may be used as the final coding unit. Here, the coding procedure may include procedures such as prediction, transformation, and / or restoration, which will be described later. As another example, a processing unit of the coding procedure may be a prediction unit (PU) or a transform unit (TU). The prediction unit and the transform unit may be divided or partitioned from the final coding unit, respectively. The prediction unit may be a unit of sample prediction, and the transform unit may be a unit for deriving transform coefficients and / or a unit for deriving a residual signal from the transform coefficients.

[0070] The prediction unit (inter prediction unit 180 or intra prediction unit 185) may perform prediction on a block to be processed (current block) and generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied in units of a current block or a CU. The prediction unit may generate various information related to prediction of the current block and transmit it to the entropy encoding unit 190. The information related to prediction may be encoded by the entropy encoding unit 190 and output in a bitstream format.

[0071] The intra prediction unit 185 may predict the current block by referring to samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located away from the current block according to an intra prediction mode and / or an intra prediction technique. The intra prediction mode may include a plurality of non-directional modes and a plurality of directional modes. The non-directional mode may include, for example, a DC mode and a planar mode. The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes depending on the fineness of the prediction direction. However, this is merely an example, and more or less directional prediction modes may be used depending on the setting. The intra prediction unit 185 may also determine a prediction mode to be applied to the current block using prediction modes applied to neighboring blocks.

[0072] The inter prediction unit 180 may derive a predicted block for a current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of a block, a sub-block, or a sample based on the correlation of motion information between a neighboring block and a current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block present in the current picture and a temporal neighboring block 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 from each other. The temporal neighboring block may be called a collocated reference block, a collocated CU (colCU), etc. The reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, the inter prediction unit 180 may generate information indicating which candidate is used to derive a motion vector and / or a reference picture index of the current block by forming a motion information candidate list based on neighboring blocks. Inter prediction may be performed based on various prediction modes, and for example, in the case of a skip mode and a merge mode, the inter prediction unit 180 may use motion information of neighboring blocks as motion information of the current block. In the case of the skip mode, unlike the merge mode, a residual signal may not be transmitted.In the case of a motion vector prediction (MVP) mode, the motion vector of the current block can be signaled by using the motion vector of a neighboring block as a motion vector predictor and encoding a motion vector difference and an indicator for the motion vector predictor. The motion vector difference can mean the difference between the motion vector of the current block and the motion vector predictor.

[0073] The prediction unit may generate a prediction signal based on various prediction methods and / or prediction techniques, which will be described later. For example, the prediction unit may apply intra prediction or inter prediction for prediction of the current block, and may simultaneously apply intra prediction and inter prediction. A prediction method that simultaneously applies intra prediction and inter prediction for prediction of the current block may be called combined inter and intra prediction (CIIP). The prediction unit may also perform intra block copy (IBC) for prediction of the current block. Intra block copy can be used for content image / video coding such as games, for example, as in screen content coding (SCC). IBC is a method of predicting a current block using an already restored reference block in a current picture that is located a predetermined distance away from the current block. When IBC is applied, the position of the reference block in the current picture may be coded as a vector (block vector) corresponding to the predetermined distance. IBC is basically performed in the current picture, but may be performed similarly to inter prediction in that a reference block is derived in the current picture. That is, the IBC may use at least one of the inter prediction techniques described in this disclosure.

[0074] The prediction signal generated by the prediction unit may be used to generate a restored signal or a residual signal. The subtraction unit 115 may subtract the prediction signal (predicted block, prediction sample array) output from the prediction unit from the input image signal (original block, original sample array) to generate a residual signal (residual block, residual sample array). The generated residual signal may be transmitted to the conversion unit 120.

[0075] The transform unit 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-Loeve transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). Here, the GBT refers to a transform obtained from a graph when the relationship information between pixels is expressed as a graph. The CNT refers to a transform obtained based on a predicted signal generated using all previously reconstructed pixels. The transform process may be applied to pixel blocks having the same square size, or may be applied to non-square, variable-sized blocks.

[0076] The quantization unit 130 may quantize the transform coefficients and transmit the quantized transform coefficients to the entropy coding unit 190. The entropy coding unit 190 may code the quantized signal (information on the quantized transform coefficients) and output the coded signal in a bitstream format. The information on the quantized transform coefficients may be referred to as residual information. The quantization unit 130 may rearrange the quantized transform coefficients in a block format into a one-dimensional vector format based on a coefficient scan order, and may generate information on the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector format.

[0077] The entropy coding unit 190 may perform various coding methods, such as exponential Golomb, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC). The entropy coding unit 190 may also code information required for video / image restoration (e.g., values ​​of syntax elements, etc.) together or separately in addition to the quantized transform coefficients. The coded information (e.g., coded video / image information) may be transmitted or stored in a network abstraction layer (NAL) unit unit in a bitstream format. The video / image information may further include information on various parameter sets, such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). The video / image information may further include general constraint information. The signaling information, transmitted information and / or syntax elements referred to in this disclosure may be encoded through the above-mentioned encoding procedures and included in the bitstream.

[0078] The bitstream may be transmitted via a network or may be stored in a digital storage medium. Here, the network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as a USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. A transmitting unit (not shown) for transmitting the signal output from the entropy encoding unit 190 and / or a storing unit (not shown) for storing the signal may be provided as an internal / external element of the image encoding device 100, or the transmitting unit may be provided as a component of the entropy encoding unit 190.

[0079] The quantized transform coefficients output from the quantization unit 130 can be used to generate a residual signal. For example, the quantized transform coefficients are subjected to inverse quantization and inverse transformation via the inverse quantization unit 140 and the inverse transformation unit 150, so that a residual signal (residual block or residual sample) can be restored.

[0080] The adder 155 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the reconstructed residual signal to a prediction signal output from the inter prediction unit 180 or the intra prediction unit 185. When there is no residual for the current block to be processed, such as when a skip mode is applied, a predicted block may be used as a reconstructed block. The adder 155 may be referred to as a reconstruction unit or a reconstructed block generation unit. The generated reconstructed signal may be used for intra prediction of the next current block to be processed in the current picture, and may also be used for inter prediction of the next picture after filtering as described below.

[0081] Meanwhile, luma mapping with chroma scaling (LMCS) may be applied during picture encoding and / or reconstruction.

[0082] The filtering unit 160 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 160 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 170, specifically, in the DPB of the memory 170. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. The filtering unit 160 may generate various information related to filtering, as will be described later in the description of each filtering method, and transmit the information to the entropy encoding unit 190. The information related to filtering may be coded by the entropy encoding unit 190 and output in a bitstream format.

[0083] 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 through this, the image encoding device 100 may avoid a prediction mismatch between the image encoding device 100 and the image decoding device, and may also improve encoding efficiency.

[0084] The DPB in the memory 170 may store modified reconstructed pictures to be used as reference pictures in the inter prediction unit 180. The memory 170 may store motion information of blocks from which motion information in the current picture is derived (or coded) and / or motion information of already reconstructed intra-picture blocks. The stored motion information may be transmitted to the inter prediction unit 180 to be used as motion information of spatial surrounding blocks or motion information of temporal surrounding blocks. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra prediction unit 185.

[0085] Overview of the image decoding device

[0086] FIG. 3 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied.

[0087] 3, the image decoding device 200 may include an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265. The inter prediction unit 260 and the intra prediction unit 265 may be collectively referred to as a "prediction unit." The inverse quantization unit 220 and the inverse transform unit 230 may be included in a residual processing unit.

[0088] All or at least some of the components constituting the image decoding device 200 may be realized by one hardware component (e.g., a decoder or a processor) depending on the embodiment. Also, the memory 170 may include a DPB and may be realized by a digital storage medium.

[0089] The image decoding device 200, which receives a bitstream including video / image information, can reconstruct an image by executing a process corresponding to the process performed by the image encoding device 100 of Fig. 2. For example, the image decoding device 200 can perform decoding using a processing unit applied in the image encoding device. Thus, the processing unit for decoding can be, for example, a coding unit. The coding unit can be obtained by dividing a coding tree unit or a maximum coding unit. Then, the reconstructed image signal decoded and output by the image decoding device 200 can be reproduced by a reproduction device (not shown).

[0090] The image decoding apparatus 200 may receive a signal output from the image encoding apparatus of FIG. 2 in the form of a bitstream. The received signal may be decoded via the entropy decoding unit 210. For example, the entropy decoding unit 210 may derive information (e.g., video / image information) required for image restoration (or picture restoration) by parsing the bitstream. The video / image information may further include information on various parameter sets such as an adaptation 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 further include general constraint information. The image decoding apparatus may further use information on the parameter set and / or the general constraint information to decode an image. The signaling information, received information, and / or syntax elements referred to in the present disclosure may be obtained from the bitstream by being decoded via the decoding procedure. For example, the entropy decoding unit 210 may decode information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, and output values ​​of syntax elements required for image restoration and quantized values ​​of transform coefficients related to the residual. More specifically, the CABAC entropy decoding method may receive bins corresponding to each syntax element from the bitstream, determine a context model using syntax element information to be decoded and decoded information of neighboring blocks and a block to be decoded, or information of a symbol / bin decoded in a previous step, predict the occurrence probability of the bin based on the determined context model, and perform arithmetic decoding of the bin to generate a symbol corresponding to the value of each syntax element. In this case, the CABAC entropy decoding method may update the context model using information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model.Among the information decoded by the entropy decoding unit 210, information related to prediction is provided to a prediction unit (inter prediction unit 260 and intra prediction unit 265), and residual values ​​entropy-decoded by the entropy decoding unit 210, i.e., quantized transform coefficients and related parameter information, may be input to the inverse quantization unit 220. Also, among the information decoded by the entropy decoding unit 210, information related to filtering may be provided to the filtering unit 240. Meanwhile, a receiving unit (not shown) for receiving a signal output from the image encoding device may be further provided as an internal / external element of the image decoding device 200, or the receiving unit may be provided as a component of the entropy decoding unit 210.

[0091] Meanwhile, the image decoding device according to the present disclosure may be called a video / image / picture decoding device. The image decoding device may include an information decoder (video / image / picture information decoder) and / or a sample decoder (video / image / picture sample decoder). The information decoder may include an entropy decoding unit 210, and the sample decoder may include at least one of an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265.

[0092] The inverse quantization unit 220 may inverse quantize the quantized transform coefficients to output transform coefficients. The inverse quantization unit 220 may rearrange the quantized transform coefficients in a two-dimensional block format. In this case, the rearrangement may be performed based on a coefficient scan order performed in the image encoding device. The inverse quantization unit 220 may perform inverse quantization on the quantized transform coefficients using a quantization parameter (e.g., quantization step size information) to obtain transform coefficients.

[0093] The inverse transform unit 230 can inversely transform the transform coefficients to obtain a residual signal (residual block, residual sample array).

[0094] The prediction unit may perform prediction on a current block and generate a predicted block including a prediction sample for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied to the current block based on the prediction information output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode (prediction technique).

[0095] The prediction unit can generate a prediction signal based on various prediction methods (techniques) described below, as has been described in the explanation of the prediction unit of the image encoding device 100.

[0096] 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 may be similarly applied to the intra prediction unit 265.

[0097] The inter prediction unit 260 may derive a predicted block for the current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on correlation of motion information between neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further 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 for the current block based on the received candidate selection information. Inter prediction may be performed based on various prediction modes (techniques), and the prediction information may include information indicating a mode (technique) of inter prediction for the current block.

[0098] The adder 235 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the acquired residual signal to a prediction signal (predicted block, predicted sample array) output from a prediction unit (including the inter prediction unit 260 and / or the intra prediction unit 265). When there is no residual for the current block, such as when a skip mode is applied, the predicted block can be used as the reconstructed block. The description of the adder 155 may also be applied to the adder 235. The adder 235 may also be referred to as a reconstruction unit or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of the next current block in the current picture, and may also be used for inter prediction of the next picture via filtering, as described below.

[0099] The filtering unit 240 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 240 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may 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, an adaptive loop filter, a bilateral filter, etc.

[0100] The (modified) reconstructed picture stored in the DPB of the memory 250 may be used as a reference picture in the inter prediction unit 260. The memory 250 may store motion information of a block from which motion information in the current picture is derived (or decoded) and / or motion information of a block in an already reconstructed picture. The stored motion information may be transmitted to the inter prediction unit 260 to be used as motion information of a spatial surrounding block or motion information of a temporal surrounding block. The memory 250 may store a reconstructed sample of a reconstructed block in the current picture and transmit it to the intra prediction unit 265.

[0101] In this specification, the embodiments described for the filtering unit 160, inter prediction unit 180 and intra prediction unit 185 of the image encoding device 100 can also be applied in a similar or corresponding manner to the filtering unit 240, inter prediction unit 260 and intra prediction unit 265 of the image decoding device 200, respectively.

[0102] Meanwhile, the prediction unit of the above-mentioned image encoding / decoding devices 100, 200 can derive a reference sample according to the intra prediction mode of the current block from among the surrounding reference samples of the current block, and can generate a prediction sample of the current block based on the reference sample.

[0103] For example, (i) a prediction sample may be derived based on an average or an 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 with respect to the prediction sample among the neighboring reference samples of the current block. In the case of (i), it may be called a non-directional mode or a non-angular mode, and in the case of (ii), it may be called a directional mode or an angular mode. In addition, the prediction sample may be generated through an interpolation between the second neighboring sample and the first neighboring sample that are located in a direction opposite to the prediction direction of the intra prediction mode of the current block based on the prediction sample of the current block among the neighboring reference samples. The above case may be called a linear interpolation intra prediction (LIP). Also, a temporary prediction sample of the current block may be derived based on the filtered surrounding reference samples, and a prediction sample of the current block may be derived by weighting the temporary prediction sample and at least one reference sample derived according to the intra prediction mode among the existing surrounding reference samples, i.e., unfiltered surrounding reference samples. The above case may be called Position Dependent Intra Prediction (PDPC). Also, a reference sample line with the highest prediction accuracy may be selected from the surrounding multiple reference sample lines of the current block, and a prediction sample may be derived using a reference sample located in a prediction direction from the selected line, and the reference sample line used may be signaled to a decoding device. The above case may be called multi-reference line intra prediction (MRL) or MRL-based intra prediction.Also, the current block is divided into vertical or horizontal sub-partitions, and intra prediction is performed based on the same intra prediction mode, but neighboring reference samples can be derived and used in units of the sub-partitions. That is, in this case, the intra prediction mode for the current block is uniformly applied to the sub-partitions, but neighboring reference samples can be derived and used in units of the sub-partitions, thereby improving intra prediction performance as needed. Such a prediction method is sometimes called intra sub-partitions (ISP) or ISP-based intra prediction. The details will be described later. Also, when a prediction direction based on a prediction sample points between neighboring reference samples, that is, when the prediction direction points to a fractional sample position, a value of the prediction sample can be derived through interpolation of a plurality of reference samples located in the vicinity of the prediction direction (the vicinity of the fractional sample position).

[0104] The above-mentioned intra prediction method may be called an intra prediction type, distinguished from an intra prediction mode. The intra prediction type may be called various terms such as an intra prediction technique or an additional intra prediction mode. For example, the intra prediction type (or the additional intra prediction mode, etc.) may include at least one of the above-mentioned LIP, PDPC, MRL, and ISP. Information on the intra prediction type may be encoded by an encoding device, included in a bitstream, and signaled to a decoding device. The information on the intra prediction type may be realized in various forms such as flag information indicating whether each intra prediction type is applied or index information indicating one of various intra prediction types.

[0105] Position dependent intra prediction (PDPC) ))

[0106] The PDPC may indicate an intra prediction method for deriving a filtered reference sample by filtering based on a filter for the PDPC, deriving a temporary predicted sample of the current block based on an intra prediction mode of the current block and the filtered reference sample, and deriving a predicted sample of the current block by performing a weighted sum of at least one reference sample derived according to the intra prediction mode among the existing reference samples, i.e., unfiltered reference samples, and the temporary predicted sample. Here, the predefined filter may be one of five 7-tap filters. Or, the predefined filter may be one of a 3-tap filter, a 5-tap filter, and a 7-tap filter. The 3-tap filter, the 5-tap filter, and the 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.

[0107] For example, the prediction results of the intra-planar mode can be further modified by PDPC.

[0108] Or, as an example, the PDPC may be applied to an intra planar mode, an intra DC mode, a horizontal intra prediction mode, a vertical intra prediction mode, a bottom left intra prediction mode (i.e., intra prediction mode No. 2), and eight directional intra prediction modes adjacent to the bottom left intra prediction mode, a top right intra prediction mode, and eight directional intra prediction modes adjacent to the top right intra prediction mode without any separate signaling.

[0109] Specifically, when the PDPC is applied, a predicted sample of (x, y) coordinates predicted based on a linear combination of an intra prediction mode and a reference sample can be derived as shown in Equation 1 below.

[0110]

number

[0111] In the above formula 1, the left term pred(x, y) indicates a predicted sample value of the (x, y) coordinate, and the right term pred(x, y) indicates a temporary (first order) predicted sample value of the (x, y) coordinate. (x、-1) and R (-1、y) are the upper and left reference samples located above and to the left of the current sample at (x, y) coordinates, respectively, and R (-1、-1) where wL denotes an upper-left reference sample located at the upper-left corner of the current block, wT denotes a weight applied to the left reference sample, wT denotes a weight applied to the upper reference sample, and wTL denotes a weight applied to the upper-left reference sample.

[0112] On the other hand, when the 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 necessary.

[0113] The temporary (first) predicted sample may be generated as a result of performing intra prediction based on an intra prediction mode of the current block and a reference sample. When PDPC is applied to the current block, a final predicted sample of the current block may be generated, for example, according to Equation 1. When PDPC is not applied to the current block, the temporary (first) predicted sample may be used as a final predicted sample of the current block.

[0114] 4a to 4d are diagrams showing reference samples defined by the PDPC.

[0115] In FIG. 4a to FIG. 4d, pred(x, y) denotes a predicted sample obtained through intra prediction (the above-mentioned temporal predicted sample), and R (x、-1) and R (-1、y) denote the upper and left reference samples located above and to the left of the current sample at (x, y) coordinates.

[0116] Figure 4a shows the reference sample R when the prediction mode is the diagonal top-right mode. x、-1 , R -1、y , R -1、-1 FIG. 4b shows the reference sample (R x、-1 , R -1、y , R -1、-1 4c shows the reference sample R x、-1 , R -1、y , R -1、-1 FIG. 4d shows the reference sample R x、-1 , R -1、y , R -1、-1 The weights of the PDPC can be derived based on a prediction mode. The weights of the PDPC (wT, wL, wTL) can be derived as shown in Table 1 below.

[0117] [Table 1]

[0118] Position dependent intra prediction combination (PDPC) generates a prediction sample using a reference sample according to a prediction mode, and then improves the prediction sample using a surrounding reference sample. Instead of being applied to all intra prediction modes, PDPC can be applied to Planar mode, DC mode, 2nd mode (downward right direction mode), VDIA mode (upward left direction mode), Hor mode (horizontal direction mode), Ver mode (vertical direction mode), surrounding modes of 2nd mode (3rd mode to 10th mode), and surrounding modes of VDIA mode (58th mode to 65th mode) based on 65 directional intra prediction modes. In addition, instead of being applied to all prediction samples in the block to be currently encoded, PDPC can be applied variably in consideration of the size of the block.

[0119] Template-based intra mode derivation (TIMD)

[0120] For each intra prediction mode of the most probable modes (MPM), the sum of absolute differences (SATD) between the predicted and reconstructed samples of a given template can be calculated. The template is a left template T L and the upper template T A The size of the template may vary depending on the width and / or height of the current block. Then, the first two intra prediction modes having the smallest SATD may be selected as the TIMD mode. The two TIMD modes may be fused with a weight, and such weighted intra prediction may be used to code the current CU. The TIMD mode derivation process may include the above-mentioned PDPC.

[0121] The costs of the two selected modes may be compared to a predefined threshold, and if the condition of Equation 2 below using a cost factor of 2 is met, the above combination may be applied. Alternatively, if the condition of Equation 2 is not met, only the first mode (e.g., mode1) may be used.

[0122]

number

[0123] (When the above-mentioned combination is applied) The weight for each mode can be calculated using the SATD cost of each mode as shown in Equation 3 below.

[0124]

number

[0125] Combined inter and intra prediction (CIIP)

[0126] In order to generate a prediction sample of the current block, the above-mentioned CIIP mode may be applied to the current block. Also, a certain additional flag (e.g., ciip_flag) may be signaled to indicate whether the CIIP mode is applied to the current CU. For example, if a CU is coded in merge mode, the CU may include at least 64 luma samples (i.e., the width×height of the CU is 64 or more). Also, if the width and height of the CU are smaller than 128 luma samples, the additional flag may be signaled to indicate whether the CIIP mode is applied to the current CU.

[0127] According to the CIIP mode, the inter prediction signal (P inter ) and intra prediction signal (P intra ) can be combined with the inter-prediction signal (P inter) can be derived by the same inter prediction process as in the regular merge mode, and the intra prediction signal (P intra ) can be derived by a general intra prediction process using Planar mode. Then, the inter prediction signal (P inter ) and the intra-prediction signal (P intra ) can be combined using weighted averaging as shown in Equation 4 below.

[0128]

number

[0129] In Equation 4, the weight wt may be calculated depending on the coding modes of the top peripheral block (top) and the left peripheral block (left) shown in FIG. 6. For example, the variable isIntraTop may be set to 1 if the top peripheral block (top) is available, and may be set to 0 if not. Also, the variable isIntraLeft may be set to 1 if the left peripheral block (left) is available, and may be set to 0 if not. If the sum of the set variables isIntraTop and isIntraLeft is 2, the weight wt may be set to 3. Conversely, if the sum of the set variables isIntraTop and isIntraLeft is 1, the weight wt may be set to 2. Otherwise, the weight wt may be set to 1.

[0130] (1) PDPC Blending CIIP

[0131] In the enhanced compression model (ECM), the CIIP mode can be extended based on position dependent intra prediction (PDPC). The extended CIIP mode (CIIP_PDPC) may be referred to as a PDPC blended CIIP mode. In the PDPC blended CIIP mode (CIIP_PDPC), the prediction of the general merge mode is based on the upper (R x、-1 ) and the left side (R -1、y ) position. The refinement can inherit the PDPC scheme. The prediction process according to the PDPC blending CIIP mode (CIIP_PDPC) is shown in FIG. 7. In FIG. 7, the variable W T and W L can indicate weights that vary depending on the sample position within the block, as defined in PDPC.

[0132] Meanwhile, information about the PDPC blending CIIP mode (CIIP_PDPC) may be signaled together with information about the regular CIIP mode. For example, if a CIIP flag (e.g., ciip_flag) is 1 (i.e., the CIIP mode is currently applied to the CU), the CIIP_PDPC flag may be further signaled to indicate whether the PDPC blending CIIP mode (CIIP_PDPC) is used.

[0133] (2) Combination of TIMD and TM merge with CIIP

[0134] In ECM, the CIIP mode can be extended based on template-based intra mode derivation (TIMD) and template matching (TM) merging. For example, a prediction sample can be generated by weighting an inter prediction signal predicted using a CIIP-TM merging candidate and an intra prediction signal predicted using an intra prediction mode induced by TIMD. The method can only be applied to coding blocks with an area of ​​1024 or less.

[0135] The TIMD deriving method may be used to derive an intra prediction mode in a CIIP. Specifically, an intra prediction mode having the smallest Sum of Absolute Difference (SATD) value in a TIMD mode list may be selected, and the selected intra prediction mode may be mapped to one of 67 intra prediction modes. If the derive intra prediction mode is an angular mode, a weight set (wIntra, wInter) applied to the intra prediction signal and the inter prediction signal may be modified. For example, for a near-horizontal mode (e.g., 2≦angular mode index<34), the current block may be horizontally divided into four sub-blocks (indexes 0 to 3) as shown in (a) of FIG. 8. Also, for a near-vertical mode (e.g., 34≦angular mode index≦66), the current block may be vertically divided into four sub-blocks (indexes 0 to 3) as shown in (b) of FIG. 8. And, a different weight set (wIntra, wInter) may be applied to each sub-block (indexes 0 to 3) as shown in Table 2.

[0136] [Table 2]

[0137] Referring to Table 2, a weight set of (6,2) may be applied to the sub-block of index 0, a weight set of (5,3) may be applied to the sub-block of index 1, a weight set of (3,5) may be applied to the sub-block of index 2, and a weight set of (2,6) may be applied to the sub-block of index 3.

[0138] Meanwhile, in CIIP-TM mode, a list of CIIP-TM merge candidates can be built. The CIIP-TM merge candidates can be refined by template matching. The CIIP-TM merge candidates can be reordered in a predetermined manner (e.g., adaptive reordering of merge candidates (ARMC)) like general merge candidates. In one example, the maximum number of CIIP-TM merge candidates can be two.

[0139] The transform / inverse transform according to the present disclosure is described in detail below.

[0140] Transform / Reverse Transform Overview

[0141] As described above, the encoding apparatus may derive a residual block (residual sample) based on a block (prediction sample) predicted through intra / inter / IBC prediction, etc., and may derive a quantized transform coefficient by applying transform and quantization to the derived residual sample. Information (residual information) about the quantized transform coefficient may be included in a residual coding syntax, encoded, and then output in a bitstream format. The decoding apparatus may obtain information (residual information) about the (quantized) transform coefficient from the bitstream, and decode the information to derive a quantized transform coefficient. The decoding apparatus may derive a residual sample through inverse quantization / inverse transform based on the quantized transform coefficient. As described above, at least one of the quantization / inverse quantization and / or transform / inverse transform may be omitted. When the quantization / inverse quantization is omitted, the quantized transform coefficient may be referred to as a transform coefficient. When the transform / inverse transform is omitted, the transform coefficient may be referred to as a coefficient or a residual coefficient, or may still be referred to as a transform coefficient for uniformity of expression. Whether the transform / inverse transform is skipped can be signaled based on transform_skip_flag.

[0142] Also, in this disclosure, the quantized transform coefficients and the transform coefficients may be referred to as transform coefficients and scaled transform coefficients, respectively. In this case, the residual information may include information on the transform coefficients, and the information on the transform coefficients may be signaled via a residual coding syntax. Transform coefficients may be derived based on the residual information (or the information on the transform coefficients), and scaled transform coefficients may be derived through an inverse transform (scaling) on ​​the transform coefficients. Residual samples may be derived based on an inverse transform (transform) on the scaled transform coefficients. This may be similarly applied / expressed in other parts of this disclosure.

[0143] The transform / inverse transform may be performed based on a transform kernel. For example, according to the present disclosure, a multiple transform selection (MTS) scheme may be applied. In this case, a part of a set of multiple transform kernels may be selected and applied to the current block. The transform kernel may be referred to by various terms such as a transform matrix or a transform type. For example, the transform kernel set may indicate a combination of a vertical transform kernel (vertical transform kernel) and a horizontal transform kernel (horizontal transform kernel).

[0144] For example, MTS index information (or mts_idx syntax element) may be generated / encoded in the encoding device and signaled to the 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 derived as shown in Table 3.

[0145] [Table 3]

[0146] Table 4 shows the values ​​of tyTypeHor and trTypeVer according to tu_mts_idx[x0][y0].

[0147] The transformation kernel set may be determined, for example, as shown in Table 4 based on cu_sbt_horizontal_flag and cu_sbt_pos_flag.

[0148] [Table 4]

[0149] Table 4 shows the values ​​of tyTypeHor and trTypeVer according to cu_sbt_horizontal_flag and cu_sbt_pos_flag. Here, cu_sbt_horizontal_flag such as 1 may indicate that the current coding unit is divided horizontally into two transform blocks. In contrast, cu_sbt_horizontal_flag such as 0 may indicate that the current coding unit is divided vertically into two transform blocks. In addition, cu_sbt_pos_flag such as 1 may indicate that the syntax elements tu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of the first transform unit in the current coding unit are not present in the bitstream. In contrast, cu_sbt_pos_flag such as 0 may indicate that the syntax elements tu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of the second transform unit in the current coding unit are not present in the bitstream.

[0150] Meanwhile, in Tables 3 and 4, trTypeHor may indicate a horizontal transform kernel, and trTypeVer may indicate a vertical transform kernel. A trTypeHor / trTypeVer value of 0 may indicate DCT2, a trTypeHor / trTypeVer value of 1 may indicate DST7, and a trTypeHor / trTypeVer value of 2 may indicate DCT8. However, this is merely an example, and other values ​​may be mapped to other DCT / DST by convention.

[0151] Table 5 shows exemplary basis functions for the above-mentioned DCT2, DCT8, and DST7.

[0152] [Table 5]

[0153] In the present disclosure, the MTS-based transform may be 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 called a reduced secondary transform (RST). For example, the w and / or h may be 4 or 8. In the transform, the primary transform and the secondary transform may be sequentially applied to the residual block, and in the inverse transform, the inverse secondary transform and the inverse primary transform may be sequentially applied to the transform coefficients. The secondary transform (RST transform) may be called a low frequency coefficient transform (LFCT) or a low frequency non-seperable transform (LFNST). The inverse secondary transform may be called an inverse LFCT or an inverse LFNST.

[0154] 9 is a diagram for explaining a conversion and inverse conversion process. In FIG. 9, a conversion unit 910 may correspond to the conversion unit 120 in FIG. 2, and an inverse conversion unit 920 may correspond to the inverse conversion unit 150 in FIG. 2 or the inverse conversion unit 230 in FIG.

[0155] Referring to FIG. 9, a conversion unit 910 may include a primary conversion unit 911 and a secondary conversion unit 912 .

[0156] The linear transform unit 911 may apply a linear transform to the residual samples A to generate (linear) transform coefficients B. In this disclosure, the linear transform may be referred to as a core transform.

[0157] The first transform may be performed based on the MTS scheme. When the existing MTS is applied, a spatial domain to frequency domain transform 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 first transform coefficients). Here, DCT type 2, DST type 7, DCT type 8, etc. may be referred to as transform types, transform kernels, or transform cores. Examples of basis functions for DCT type 2, DST type 7, and DCT type 8 are as described above with reference to Table 4.

[0158] Meanwhile, the transform / inverse transform may be performed in units of CU or TU. That is, the transform / inverse transform may be applied to a residual sample in a CU or a residual sample in a TU. The CU size and the TU size may be the same, or multiple TUs may exist in a CU region. Meanwhile, the CU size may generally indicate a luma component (sample) CB size. The TU size may generally indicate a luma component (sample) TB size. The chroma component (sample) CB or TB size may be derived based on the luma component (sample) CB or TB size according to a component ratio according to a color format (chroma format, for example, 4:4:4, 4:2:2, 4:2:0, etc.). The TU size may be derived based on maxTbSize. For example, if the CU size is larger than the maxTbSize, multiple TUs (TBs) of the maxTbSize may be derived from the CU, and the transform / inverse transform may be performed in units of the TUs (TBs). The maxTbSize may be taken into consideration in determining whether various intra prediction types such as ISP are applied. The information regarding maxTbSize may be predetermined, or may be generated and encoded in an encoding device and signaled to a decoding device.

[0159] As mentioned above, a transform can be applied to the residual block in order to decorrelate the residual block as much as possible, concentrate the coefficients in low frequencies, and create a zero tail at the end of the block. The transform part in the JEM software includes two main functions: a core transform and a secondary transform. The core transform consists of the DCT (discrete cosine transform) and DST (discrete sine transform) transform families that are applied to every row and column of the residual block. Then, a secondary transform can be additionally applied to the top left corner of the output of the core transform. Similarly, inverse transforms can be applied in the order of a secondary inverse transform and a core inverse transform. Specifically, a secondary inverse transform can be applied to the top left corner of the coefficient block. Then, a core inverse transform is applied to the rows and columns of the output of the secondary inverse transform. The core transform / inverse transform may be referred to as a primary transform / inverse transform.

[0160] Overview of AMT (Adaptive multiple core transform)

[0161] In addition to the existing DCT-2 and 4x4 DST-7, adaptive (or explicit) multiple transform (AMT or EMT) techniques can be used for residual coding of inter- and intra-coded blocks. Hereinafter, AMT and EMT will be used interchangeably. In AMT, multiple transforms selected from the DCT / DST family can be used in addition to the existing transforms. The transform matrices newly introduced 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.

[0162] [Table 6]

[0163] 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 luma coding blocks in a CU to which EMT is applied, two additional flags are signaled to identify the horizontal and vertical transforms to be used. In JEM, the residual of a block can be coded in transform skip mode. For intra residual coding, a mode-dependent transform candidate selection process is used according to other residual statistics of other intra prediction modes. For example, three transform subsets are defined as shown in Table 7, and a transform subset is selected based on the intra prediction mode as shown in Table 8.

[0164] [Table 7]

[0165] [Table 8]

[0166] With the subset concept, a transform subset is first identified based on Table 7 using the intra prediction modes of CUs with CU-level EMT_CU_flag set to 1. Then, for each of the horizontal (EMT_TU_horizontal_flag) and vertical (EMT_TU_vertical_flag) transforms, one of the two transform candidates in the identified transform subset is selected based on explicit signaling using flags according to Table 8.

[0167] Table 9 shows the transform configuration groups to which AMT applies.

[0168] [Table 9]

[0169] Referring to Table 9, the transform setting group is determined based on the prediction mode, and the number of groups may be six (G0 to G5). G0 to G4 correspond to cases where intra prediction is applied, and G5 indicates a transform combination (or transform set, transform combination set) applied to a residual block generated by inter prediction.

[0170] A transformation combination can consist of a horizontal transform (or row transform) applied to the rows of the 2D block and a vertical transform (or column transform) applied to the columns.

[0171] Here, each of the transform setting groups may have four transform combination candidates. The four transform combination candidates may be selected or determined through transform combination indexes of 0 to 3, and the transform combination indexes may be encoded and transmitted from the encoder to the decoder.

[0172] For example, residual data (or residual signals) obtained through intra prediction may have different statistical characteristics depending on the intra prediction mode. Therefore, other transforms than the general cosine transform may be applied for each intra prediction as shown in Table 9. In the present disclosure, the transform type may be expressed as, for example, DCT-Type 2, DCT-II, DCT-2, etc.

[0173] Table 9 shows the cases where 35 intra prediction modes are used and where 67 intra prediction modes are used. A plurality of transform combinations can be applied to each transform setting group divided by each intra prediction mode column. For example, the plurality of transform combinations can be composed of four (row direction transform, column direction transform) combinations. As a specific example, in group 0, DST-7 and DCT-5 can be applied to both the row (horizontal) direction and the column (vertical) direction, so a total of four combinations are possible.

[0174] Since a total of four transform kernel combinations can be applied to each intra prediction mode, a transform combination index for selecting one of them can be transmitted for each transform unit. In this disclosure, the transform combination index may be referred to as an AMT index and may be represented by amt_idx.

[0175] In addition to the transform kernels presented in Table 9, there may be cases where DCT-2 is optimal for both row and column directions due to the characteristics of the residual signal. Therefore, transforms can be applied adaptively by defining an AMT flag for each coding unit. Here, if the AMT flag is 0, DCT-2 is applied to both row and column directions, and if the AMT flag is 1, one of four combinations can be selected or determined via the AMT index.

[0176] As an example, when the AMT flag is 0, if the number of transform coefficients for one transform unit is less than 3, the transform kernel in Table 9 is not applied, and DST-7 can be applied in both the row and column directions.

[0177] For example, if the transform coefficient values ​​are parsed first and the number of transform coefficients is less than 3, the AMT index is not parsed and DST-7 is applied, thereby reducing the amount of additional information transmission.

[0178] As an example, AMT may only be applied if the width and height of the transformation unit are 32 or less.

[0179] As an example, Table 9 can be pre-configured via off-line training.

[0180] For example, the AMT index may be defined as a single index that can simultaneously indicate a combination of a horizontal transform and a vertical transform, or the AM index may be defined as a separate horizontal transform index and a separate vertical transform index.

[0181] FIG. 10 is a flowchart showing an encoding process in which AMT is performed.

[0182] AMT can be applied regardless of the primary transformation or the secondary transformation. That is, there is no restriction that it must be applied to only one of the two, and it can be applied to both. Here, the primary transformation may refer to a transformation for initially transforming a residual block, and the secondary transformation may refer to a transformation for applying a transformation to a block generated as a result of the primary transformation.

[0183] 10, the encoding apparatus may determine a transform group corresponding to a current block (S1010). Here, the transform group may refer to the transform group described above with reference to Table 8, but is not limited thereto and may be composed of other transform combinations.

[0184] The encoding device may perform transforms on available candidate transform combinations in the transform group (S1020).

[0185] As a result of the transformation, the encoding device can determine or select a transformation combination that has the smallest RD (Rate Distortion) cost (S1030).

[0186] The encoding device may encode a transform combination index corresponding to the selected transform combination (S1040).

[0187] FIG. 11 is a flowchart showing a decoding process in which AMT is performed.

[0188] Referring to FIG. 11, the decoding apparatus may determine a transform group for a current block (S1110).

[0189] The decoding apparatus may parse the transform combination index (S1120), where the transform combination index may correspond to one of a plurality of transform combinations in the transform group. The steps of determining the transform group (S1110) and parsing the transform combination index (S1120) may be performed simultaneously.

[0190] The decoding apparatus 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 using other transform combinations is also possible.

[0191] The decoding apparatus may perform an inverse transform on the current block based on the transform combination (S1140). If the transform combination is composed of a row transform and a column transform, the row transform may be applied first, and then the column transform may be applied. However, this process is not limited thereto, and may be applied in reverse.

[0192] Improved MTS for Intracoding

[0193] In the VVC design, in the case of MTS, only DST7 and DCT8 transform kernels are used for intra and inter coding. In contrast, in the enhanced compression mode (ECM), additional primary transforms including DCT5, DST4, DST1 and IDT (identity transform) can be used. Also, the MTS set can be configured according to the transform unit (TU) size and intra mode information. Sixteen different TU sizes are considered, and for each TU size, five different classes are considered according to the intra mode information. Four different transform pairs can be considered for each class, which is the same as in the case of VVC. A total of 80 different classes are considered, but some of these different classes may share exactly the same transform set. Thus, there can be 58 unique entries (less than 80) in the resultant lookup table (LUT).

[0194] Meanwhile, for angular modes, the TU shape and joint symmetry for intra prediction are considered. Thus, mode i (where i>34) with TU shape A×B can be mapped to the same class corresponding to mode j (where j=68-i) with TU shape B×A. However, the order of the horizontal and vertical transform kernels for each transform pair changes. For example, a 16×4 block in mode 18 (horizontal prediction) and a 4×16 block in mode 50 (vertical prediction) are mapped to the same class. In this case, the vertical and horizontal transform kernels are swapped. For wide-angle modes, the closest conventional angular mode is used to determine the transform set. 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.

[0195] Problems with the prior art

[0196] According to the existing MTS (multiple transform set) scheme, four MTS combinations as shown in Table 10 below are used using two types of transform kernels, DCT2 and DST7.

[0197] [Table 10]

[0198] However, since the above-mentioned CIIP (Combined inter and intra prediction) predictor is a combination of an intra predictor and an inter predictor, the existing MTS set as shown in Table 10 may be inappropriate. For example, the combination of DST7 / DCT2 may be suitable for an intra predictor but not suitable for an inter predictor. Also, the combination of DCT2 / DCT2 may be suitable for an inter predictor but not suitable for an intra predictor. Therefore, if the existing MTS set is applied as it is to a CIIP predictor having both intra and inter prediction attributes and a residual signal based thereon, the compression efficiency may be significantly reduced.

[0199] Meanwhile, in ECM (Enhanced compression model), the above-mentioned PDPC (Position dependent intra prediction) blending CIIP mode has been newly introduced as an extended CIIP mode. Also, unlike the existing VVC, which generated intra predictors using only planar modes, intra modes can be derived and used through TIMD (Template-based intra mode derivation), so all intra prediction modes can be applied to the extended CIIP. This means that the intra predictor and inter predictor can be combined in more complex and diverse ways in the extended CIIP mode, and as a result, the compression efficiency caused by the application of the existing MTS set is inevitably further reduced.

[0200] In order to solve this problem, the present disclosure proposes various methods for more efficiently applying the MTS scheme in the CIIP mode. The following examples of the present disclosure are provided to illustrate representative aspects of the present disclosure, and may be applied independently or in combination of two or more.

[0201] Example 1

[0202] According to the first embodiment 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 in Table 10. Also, in the intra MTS set, the MTS combination for the current block may be selected based on the intra prediction mode applied to the CIIP mode. Hereinafter, the first embodiment of the present disclosure will be described in more detail with reference to FIG.

[0203] FIG. 12 is a flow chart illustrating a method for applying MTS according to one embodiment of the present disclosure.

[0204] 12 may be performed by an image encoding device or an image decoding device, and for convenience of explanation, the following description will be given based on the image encoding device.

[0205] 12, the image encoding apparatus may determine whether a prediction mode of a current block is a CIIP mode (S1210). Here, the CIIP mode may be a regular CIIP mode based on a planar mode. Alternatively, the CIIP mode may be an extended CIIP mode of ECM, for example, the above-mentioned PDPC blending CIIP mode or a TIMD-based CIIP mode.

[0206] If the prediction mode of the current block is the CIIP mode ("YES" in S1210), the image encoding apparatus may perform a transform on the current block based on a predetermined intra MTS set (S1220). In one embodiment, the predetermined intra MTS set may further include at least one transform kernel among DCT5, DST4, DST1, or IDT (identity transform) in addition to DCT2 and DST7 in Table 10. For example, the intra MTS set may be composed of nine MTS combinations using three types of transform kernels, DCT2, DST7, and DCT5. Or, the intra MTS set may be composed of sixteen MTS combinations using four types of transform kernels, DCT2, DST7, DCT5, and DST1. Meanwhile, the types and numbers of the additional transform kernels described above are merely exemplary, and the embodiments of the present disclosure are not limited thereto.

[0207] 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 a general CIIP mode, the MTS combination for the current block may be selected based on the planar mode.

[0208] Also, when the CIIP mode is a PDPC blending CIIP mode, the intra prediction mode applied to the CIIP mode may be regarded as a planar mode, and the MTS combination for the current block may be selected based on the planar mode. According to an embodiment, the intra prediction mode applied to the CIIP mode may be regarded as an intra prediction mode other than the planar mode. For example, when the CIIP mode is a PDPC blending mode, the intra prediction mode applied to the CIIP mode may be regarded as a DC mode, and the MTS combination for the current block may be selected based on the DC mode.

[0209] Also, when the CIIP mode is a TIMD-based CIIP mode, the MTS combination for the current block may be selected based on the intra prediction mode induced by the TIMD, where, as described above, the intra prediction mode induced by the TIMD may be any existing intra mode such as the DC mode, the 2nd mode, etc. in addition to the planar mode.

[0210] In contrast, if the prediction mode of the current block is not CIIP mode ('NO' in S1210), the image encoding device may perform conversion on the current block based on an existing MTS set, for example, the inter MTS set described above with reference to Table 10 (S1230).

[0211] As described above, according to the first embodiment of the present disclosure, when the prediction mode of the current block is the CIIP mode, a predetermined intra MTS set further including an additional transform kernel can be applied to the current block, instead of the existing inter MTS set using only DCT2 and DST7. The CIIP mode has a residual signal characteristic that occurs in the intra prediction mode by the combination of intra predictors, so that the compression efficiency can be improved by applying the intra MTS set. In addition, since the MTS index binarization method of the intra MTS set and the existing inter MTS set is the same, there is an advantage that the existing binarization process can be applied in the same way.

[0212] Example 2

[0213] Unlike other types of CIIP modes, the TIMD-based CIIP mode has a larger residual signal characteristic occurring in the intra prediction mode, so it may be more suitable to apply the above-mentioned intra MTS set than the existing inter MTS set. However, in the case of the PDPC blending CIIP mode, a predictor having a different characteristic from that of the existing intra prediction mode is generated, so it may be unsuitable to apply the intra MTS set. Therefore, according to the second embodiment of the present disclosure, the above-mentioned intra MTS set may be applied only when the prediction mode of the current block is the TIMD-based CIIP mode. Hereinafter, the second embodiment of the present disclosure will be described in more detail with reference to FIG. 13.

[0214] FIG. 13 is a flow chart illustrating a method for applying MTS according to one embodiment of the present disclosure.

[0215] 13 may be performed by an image encoding device or an image decoding device, and for convenience of explanation, the following description will be given based on the image encoding device.

[0216] Referring to FIG. 13, the image encoding apparatus may determine whether the prediction mode of the current block is the CIIP mode (S1310).

[0217] If the prediction mode of the current block is the CIIP mode ("YES" in S1310), the image encoding apparatus may determine whether the CIIP mode is the above-mentioned TIMD-based CIIP mode (S1320).

[0218] If it is determined that the CIIP mode is a TIMD-based CIIP mode ("YES" in S1320), the image coding apparatus may perform a transform on the current block based on a predetermined intra MTS set (S1330). Here, the predetermined intra MTS set may further include at least one transform kernel of DCT5, DST4, DST1, or IDT (identity transform) in addition to DCT2 and DST7 in Table 10, as described above in the first embodiment.

[0219] Alternatively, if the CIIP mode is not a TIMD-based CIIP mode (e.g., a PDPC blending CIIP mode) ('NO' in S1320), the image coding device may perform conversion for the current block based on an existing MTS set, for example, the inter MTS set described above with reference to Table 10 (S1340).

[0220] On the other hand, even if the prediction mode of the current block is not the CIIP mode (NO in S1310), the image encoding apparatus may perform conversion on the current block based on the existing MTS set (S1340).

[0221] As described above, according to the second embodiment of the present disclosure, a certain intra MTS set including additional transform kernels other than DCT2 and DST7 can be applied only when the prediction mode of the current block is a TIMD-based CIIP mode, which has the advantage of improving compression efficiency in the CIIP mode and using the same MTS index binarization method as in the first embodiment.

[0222] Example 3

[0223] In the CIIP mode, a residual signal is generated by a combination of an intra predictor and an inter predictor, and therefore may have characteristics different from those of a residual signal generated by only an existing intra predictor or an inter predictor. Therefore, when the MTS scheme is applied to the CIIP mode, compression efficiency may be reduced. Therefore, according to the third embodiment of the present disclosure, the MTS scheme may be applied only when the prediction mode of the current block is not the CIIP mode. Hereinafter, the third embodiment of the present disclosure will be described in more detail with reference to FIG. 14.

[0224] FIG. 14 is a flow chart illustrating a method for applying MTS according to one embodiment of the present disclosure.

[0225] 14 may be performed by an image encoding device or an image decoding device. For convenience of explanation, the following description will be given based on an image encoding device.

[0226] Referring to FIG. 14, the image encoding apparatus may determine whether the prediction mode of the current block is the CIIP mode (S1410).

[0227] 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 (i.e., MTS skip) (S1420). In this case, a predetermined transform kernel (e.g., DCT2) determined by a general transform scheme other than the MTS scheme may be used to transform the current block.

[0228] Alternatively, if the prediction mode of the current block is not the CIIP mode ("NO" in S1410), the image encoding apparatus may apply an MTS scheme to the current block (S1430). In this case, a specific MTS combination selected from a predetermined MTS set (e.g., an inter MTS set or an intra MTS set) may be used for transforming the current block.

[0229] As described above, according to the third embodiment of the present disclosure, the MTS scheme can be applied only when the prediction mode of the current block is not the CIIP mode, thereby improving the compression efficiency in the CIIP mode and saving the number of MTS binarization bits (or the number of signaling bits).

[0230] Example 4

[0231] As in the above-described third embodiment, if the MTS scheme is applied to the CIIP mode without any conditions, the compression efficiency may be reduced. Therefore, according to a fourth embodiment of the present disclosure, the MTS scheme may be applied only when the prediction mode of the current block is not a CIIP mode or is a specific CIIP mode. Hereinafter, the fourth embodiment of the present disclosure will be described in more detail with reference to FIGS. 15 to 17.

[0232] FIG. 15 is a flow chart illustrating a method for applying MTS according to one embodiment of the present disclosure.

[0233] 15 may be performed by an image encoding device or an image decoding device. For convenience of explanation, the following description will be given based on an image encoding device.

[0234] Referring to FIG. 15, the image encoding apparatus may determine whether the prediction mode of a current block is the CIIP mode (S1510).

[0235] If the prediction mode of the current block is a CIIP mode ("YES" in 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 blending CIIP mode or a TIMD-based CIIP mode.

[0236] If the CIIP mode is the extended CIIP mode ("YES" in S1520), the image coding apparatus may not apply the MTS scheme to the current block (S1530). In this case, a predetermined transform kernel (e.g., DCT2) determined by a general transform scheme other than the MTS scheme may be used to transform the current block.

[0237] Alternatively, if the CIIP mode is not the extended CIIP mode ("NO" in S1520), the image coding apparatus may apply an MTS scheme to the current block (S1540). In this case, a specific MTS combination selected from a predetermined MTS set (e.g., an inter MTS set or an intra MTS set) may be used for transforming the current block.

[0238] On the other hand, even if the prediction mode of the current block is not the CIIP mode ("NO" in S1510), the image encoding apparatus can apply the MTS scheme to the current block (S1540).

[0239] FIG. 16 is a flow chart illustrating a method for applying MTS according to another embodiment of the present disclosure.

[0240] 16 may be performed by an image encoding device or an image decoding device, and for convenience of explanation, the following description will be given based on the image encoding device.

[0241] Referring to FIG. 16, the image encoding apparatus may determine whether the prediction mode of the current block is the CIIP mode (S1610).

[0242] If the prediction mode of the current block is the CIIP mode ("YES" in S1610), the image encoding apparatus may determine whether the CIIP mode is a TIMD-based CIIP mode (S1620).

[0243] If the CIIP mode is a TIMD-based CIIP mode ("YES" in S1620), the image encoding apparatus may determine whether the intra prediction mode induced by the TIMD is a predetermined mode (S1630). In one embodiment, the predetermined mode may be a non-directional mode, for example, a planar mode or a DC mode. If the determination result indicates that the intra prediction mode induced by the TIMD is a predetermined mode ("YES" in S1630), the image encoding apparatus may not apply the MTS scheme to the current block (S1640). On the other hand, if the intra prediction mode induced by the TIMD is not a predetermined mode ("NO" in S1630), the image encoding apparatus may apply the MTS scheme to the current block (S1650).

[0244] On the other hand, even if the prediction mode of the current block is not a CIIP mode ('NO' in S1610), or if the prediction mode of the current block is a CIIP mode but not a TIMD-based CIIP mode (e.g., a PDPC blending CIIP mode) ('NO' in S1620), the image encoding device can apply the MTS scheme to the current block (S1650).

[0245] FIG. 17 is a flow chart illustrating a method for applying MTS according to another embodiment of the present disclosure.

[0246] 17 may be performed by an image encoding device or an image decoding device. For convenience of explanation, the following description will be given based on an image encoding device.

[0247] Referring to FIG. 17, the image encoding apparatus may determine whether the prediction mode of the current block is the CIIP mode (S1710).

[0248] If the prediction mode of the current block is the CIIP mode ("YES" in S1710), the image encoding apparatus may determine whether the CIIP mode is a TIMD-based CIIP mode (S1720).

[0249] If the CIIP mode is a TIMD-based CIIP mode ("YES" in S1720), the image encoding apparatus may determine whether the intra prediction mode induced by the TIMD is a predetermined mode (S1730). In one embodiment, the predetermined mode may be a non-directional mode, for example, a planar mode or a DC mode. If the determination result indicates that the intra prediction mode induced by the TIMD is a predetermined mode ("YES" in S1730), the image encoding apparatus may apply the MTS scheme to the current block (S1740). In contrast, if the intra prediction mode induced by the TIMD is not a predetermined mode ("NO" in S1730), the image encoding apparatus may not apply the MTS scheme to the current block (S1750).

[0250] On the other hand, if the prediction mode of the current block is not the CIIP mode ("NO" in S1710), the image coding apparatus can apply the MTS scheme to the current block (S1740).

[0251] Alternatively, if the prediction mode of the current block is a CIIP mode but not a TIMD-based CIIP mode (e.g., a PDPC blending CIIP mode) ('NO' in S1720), the image encoding device may not apply the MTS scheme to the current block (S1750).

[0252] As described above, according to the fourth embodiment of the present disclosure, the MTS scheme can be applied only when the prediction mode of the current block is not a CIIP mode or is a specific CIIP mode, thereby improving the compression efficiency in the CIIP mode and saving the number of MTS binarization bits (or the number of signaling bits).

[0253] Hereinafter, an image encoding / decoding method according to an embodiment of the present disclosure will be described in detail with reference to FIGS.

[0254] FIG. 18 is a flowchart illustrating an image encoding method according to an embodiment of the present disclosure.

[0255] The image coding method in Fig. 18 may be performed by the image coding apparatus in Fig. 2. For example, steps S1810 and S1820 may be performed by the conversion unit 120.

[0256] Referring to FIG. 18, the image coding apparatus may determine whether multiple transform selection (MTS) is applied to a current block (S1810).

[0257] If the MTS is applied to the current block ("YES" in S1810), the image encoding apparatus may perform a transform on the current block based on a predetermined MTS set (S1820).

[0258] The MTS set may be determined based on whether the prediction mode of the current block is a combined inter and intra prediction (CIIP) mode. In one embodiment, based on the prediction mode of the current block being the CIIP mode, the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform kernel (e.g., DCT5, DST4, DST1, and / or IDT, etc.). Also, based on the prediction mode of the current block being a template-based intra mode derivation (TIMD) based CIIP mode, the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform kernel.

[0259] 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 skip).

[0260] In one embodiment, the MTS may not be applied to the current block based on the prediction mode of the current block being an extended CIIP mode, where the extended CIIP mode may include at least one of a position dependent intra prediction (PDPC) blending CIIP mode or a TIMD-based CIIP mode.

[0261] In one embodiment, the MTS may not be applied to the current block if the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode and the intra prediction mode induced by the TIMD is a predetermined prediction mode, where the predetermined prediction mode may be a planar mode or a DC mode.

[0262] In one embodiment, the MTS may not be applied to the current block if the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode and the intra prediction mode induced by the TIMD is a predetermined prediction mode, where the predetermined prediction mode may be a planar mode or a DC mode.

[0263] FIG. 19 is a flowchart illustrating an image decoding method according to an embodiment of the present disclosure.

[0264] The image decoding method of Figure 19 may be performed by the image decoding apparatus of Figure 3. For example, steps S1910 and S1920 may be performed by the inverse transform unit 230 of Figure 3.

[0265] Referring to FIG. 19, the image decoding apparatus may determine whether multiple transform selection (MTS) is applied to a current block (S1910).

[0266] If the MTS is applied to the current block ("YES" in S1910), the image encoding apparatus may perform an inverse transform on the current block based on a predetermined MTS set (S1920).

[0267] The MTS set may be determined based on whether the prediction mode of the current block is a combined inter and intra prediction (CIIP) mode. In one embodiment, based on the prediction mode of the current block being the CIIP mode, the MTS set may be determined as an intra MTS set including DCT2, DST7, and at least one additional transform kernel (e.g., DCT5, DST4, DST1, and / or IDT, etc.). Also, based on the prediction mode of the current block being a template-based intra mode derivation (TIMD)-based CIIP mode, the MTS set may be determined as an intra MTS offset including DCT2, DST7, and at least one additional transform kernel.

[0268] 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 skip).

[0269] In one embodiment, the MTS may not be applied to the current block based on the prediction mode of the current block being an extended CIIP mode, where the extended CIIP mode may include at least one of a position dependent intra prediction (PDPC) blending CIIP mode or a TIMD-based CIIP mode.

[0270] In one embodiment, the MTS may not be applied to the current block if the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode and the intra prediction mode induced by the TIMD is a predetermined prediction mode, where the predetermined prediction mode may be a planar mode or a DC mode.

[0271] In one embodiment, the MTS may not be applied to the current block if the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode and the intra prediction mode induced by the TIMD is a predetermined prediction mode, where the predetermined prediction mode may be a planar mode or a DC mode.

[0272] Although the exemplary method of the present disclosure is expressed as a series of operations for clarity of explanation, this is not intended to limit the order in which the steps are performed, and each step may be performed simultaneously or in a different order, if necessary. To realize the method according to the present disclosure, the steps illustrated may include other steps, some steps may be omitted and the remaining steps may be omitted, or some steps may be omitted and additional steps may be included.

[0273] In the present disclosure, an image encoding device or an image decoding device that performs a predetermined operation (step) can perform the operation (step) to check the execution conditions or circumstances of the operation (step). For example, if it is described that a predetermined operation is performed when a predetermined condition is satisfied, the image encoding device or the image decoding device can perform the predetermined operation after performing an operation to check whether the predetermined condition is satisfied or not.

[0274] The various embodiments of the present disclosure are not intended to enumerate all possible combinations, but are intended to describe representative aspects of the present disclosure, and the matters described in the various embodiments may be applied independently or in combination of two or more.

[0275] Additionally, various embodiments of the present disclosure may be implemented using hardware, firmware, software, or a combination thereof, etc. In the case of a hardware implementation, the implementation may be implemented using one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), general processors, controllers, microcontrollers, microprocessors, etc.

[0276] 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 transmitting / receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video conversation device, a real-time communication device such as a video communication, a mobile streaming device, a storage medium, a camcorder, a custom video (VoD) service providing device, an over-the-top video (OTT) device, an internet streaming service providing device, a three-dimensional (3D) video device, an image telephone video device, and a medical video device, and may be used to process a video signal or a data signal. For example, the over-the-top video (OTT) device may include a game console, a Blu-ray player, an internet-connected TV, a home theater system, a smartphone, a tablet PC, a digital video recorder (DVR), and the like.

[0277] FIG. 20 is a diagram illustrating a content streaming system to which an embodiment of the present disclosure can be applied.

[0278] As shown in FIG. 20, a content streaming system to which an embodiment of the present disclosure is applied may broadly include an encoding server, a streaming server, a Web server, a media storage, a user device, and a multimedia input device.

[0279] The encoding server compresses content input from a multimedia input device such as a smartphone, camera, camcorder, etc. into digital data to generate a bitstream and transmits the bitstream to the streaming server. As another example, if a multimedia input device such as a smartphone, camera, video camera, etc. directly generates a bitstream, the encoding server can be omitted.

[0280] The bitstream may be generated by an image encoding method and / or image encoding device to which an embodiment of the present disclosure is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

[0281] The streaming server transmits multimedia data to a user device based on a user request via a web server, and the web server can act as a medium to inform the user of available services. When a user requests a desired service from the web server, the web server transmits the request to the streaming server, and the streaming server can transmit the multimedia data to the user. At this time, the content streaming system can include a separate control server, and in this case, the control server can control commands / responses between devices in the content streaming system.

[0282] The streaming server may receive the content from a media storage and / or an encoding server. For example, when receiving the content from the encoding server, the content may be received in real time. In this case, the streaming server may store the bitstream for a certain period of time in order to provide a smooth streaming service.

[0283] Examples of the user devices include mobile phones, smart phones, laptop computers, digital broadcasting terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigation systems, slate PCs, tablet PCs, ultrabooks, wearable devices such as smartwatches, smart glass, head mounted displays (HMDs), digital TVs, desktop computers, and digital signage.

[0284] Each server in the content streaming system can be operated as a distributed server, in which case data received from each server can be processed in a distributed manner.

[0285] The scope of the present disclosure includes software or machine-executable commands (e.g., operating systems, applications, firmware, programs, etc.) that cause operations according to the methods of the various embodiments to be performed on a device or computer, and non-transitory computer-readable medium on which such software or commands can be stored and executed on a device or computer. [Industrial Applicability]

[0286] The embodiments of the present disclosure can be used to encode / decode images.

Claims

1. An image decoding method performed by an image decoding device, comprising: The image decoding method includes: determining whether multiple transform selection (MTS) is applied to the current block; and performing an inverse transform on the current block based on a predetermined MTS set based on the MTS being applied to the current block; The MTS set is determined based on whether a prediction mode of the current block is a combined inter and intra prediction (CIIP) mode.

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

3. Since the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode, The image decoding method of claim 1 , wherein 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 the MTS is not applied to the current block based on the prediction mode of the current block being the CIIP mode.

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

6. The image decoding method of claim 1, wherein the prediction mode of the current block is a CIIP mode based on template-based intra mode derivation (TIMD), and the MTS is not applied to the current block based on the intra prediction mode induced by the TIMD being a predetermined prediction mode.

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 of claim 1, wherein the MTS is applied to the current block based on the fact that a prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode and an intra prediction mode induced by the TIMD is a predetermined prediction mode.

9. An image coding method performed by an image coding device, comprising: The image encoding method includes: determining whether multiple transform selection (MTS) is applied to the current block; and performing a transformation on the current block based on a predetermined MTS set based on the MTS being applied to the current block; The MTS set is determined based on whether a prediction mode of the current block is a combined inter and intra prediction (CIIP) mode.

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

11. Since the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode, The image coding method of claim 9 , wherein 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 of claim 9 , wherein the MTS is not applied to the current block based on the prediction mode of the current block being the CIIP mode.

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

14. 10. The image encoding method of claim 9, wherein the prediction mode of the current block is a template-based intra mode derivation (TIMD)-based CIIP mode, and the MTS is not applied to the current block based on the intra prediction mode induced by the TIMD being a predetermined prediction mode.

15. A computer-readable recording medium storing a bitstream generated by an image encoding method, comprising: The image encoding method includes: determining whether multiple transform selection (MTS) is applied to the current block; and performing a transformation on the current block based on a predetermined MTS set based on the MTS being applied to the current block; The MTS set is determined based on whether a prediction mode of the current block is a combined inter and intra prediction (CIIP) mode.

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