Image encoding / decoding method and device using maximum size restriction of chroma transform block, and bitstream transmission method

Abstract

To provide an image encoding / decoding method and device for improving efficiency by limiting the maximum size of a chroma conversion block.SOLUTION: An image decoding method performed by an image decoding device includes the steps of: determining a prediction mode of a current block; generating a prediction block of the current block on the basis of inter prediction mode information when the prediction mode of the current block is an inter prediction mode; generating a residual block of the current block on the basis of a transform block of the current block; and restoring the current block on the basis of the prediction block and the residual block of the current block.SELECTED DRAWING: Figure 24

Description

[Technical Field]

[0001] The present disclosure relates to an image encoding / decoding method and apparatus, and more particularly to a method and apparatus for encoding / decoding an image by limiting the maximum size of a chroma transform block, and a method for transmitting a bitstream generated by the image encoding method / apparatus of the present disclosure. [Background technology]

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

[0003] This requires highly efficient image compression techniques 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 improves encoding / decoding efficiency by limiting the maximum size of a chroma transform block.

[0006] 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.

[0007] Another object of the present disclosure is to provide a recording medium 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 recording medium storing a bitstream that is received by an image decoding device according to the present disclosure, decoded, and used to restore an image.

[0009] 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 technical field to which the present disclosure pertains from the following description. [Means for solving the problem]

[0010] An image decoding method performed by an image decoding apparatus according to an aspect of the present disclosure may include determining a prediction mode of a current block, generating a prediction block for the current block based on inter prediction mode information if the prediction mode of the current block is inter prediction mode, generating a residual block of the current block based on a transform block of the current block, and reconstructing the current block based on the prediction block and the residual block of the current block. In this case, a size of the transform block may be determined based on color components of the current block.

[0011] According to another aspect of the present disclosure, an image decoding apparatus may include a memory and at least one processor, wherein the at least one processor determines a prediction mode of a current block, and if the prediction mode of the current block is an inter prediction mode, generates a prediction block for the current block based on inter prediction mode information, generates a residual block for the current block based on a transform block of the current block, and reconstructs the current block based on the prediction block and the residual block of the current block. In this case, a size of the transform block may be determined based on color components of the current block.

[0012] In addition, an image encoding method performed by an image encoding apparatus according to an aspect of the present disclosure may include the steps of: dividing the image to determine a current block; generating an inter-predicted block of the current block; generating a residual block of the current block based on the inter-predicted block; and encoding inter-prediction mode information of the current block. In this case, the residual block may be encoded based on a size of a transform block of the current block, and the size of the transform block may be determined based on a color component of the current block.

[0013] Furthermore, 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.

[0014] Furthermore, 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] The features described above in this brief summary of the present disclosure are merely exemplary aspects of the detailed description of the present disclosure that follows and are not intended to limit the scope of the present disclosure. [Effects of the Invention]

[0016] According to the present disclosure, an image encoding / decoding method and apparatus with improved encoding / decoding efficiency can be provided.

[0017] Furthermore, according to the present disclosure, it is possible to provide an image encoding / decoding method and apparatus that improves encoding / decoding efficiency by limiting the maximum size of a chroma transform block.

[0018] The present disclosure also provides a method for transmitting a bitstream generated by the image encoding method or apparatus according to the present disclosure.

[0019] Furthermore, according to the present disclosure, a recording medium storing a bitstream generated by the image encoding method or apparatus according to the present disclosure can be provided.

[0020] Furthermore, according to the present disclosure, it is possible to provide a recording medium that stores a bitstream that is received by the image decoding device according to the present disclosure, decoded, and used to restore an image.

[0021] 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 explanation of the drawings]

[0022] [Figure 1] 1 is a diagram illustrating a video coding system to which embodiments of the present disclosure can be applied; [Figure 2] 1 is a diagram schematically illustrating an image encoding device to which an embodiment of the present disclosure can be applied. [Figure 3] FIG. 1 is a diagram schematically illustrating an image decoding device to which an embodiment of the present disclosure can be applied. [Figure 4] FIG. 2 is a diagram illustrating a division structure of an image according to an embodiment. [Figure 5] FIG. 10 is a diagram showing an example of block division types using a multi-type tree structure. [Figure 6] FIG. 1 illustrates a signaling mechanism for block partition information in a quadtree with nested multi-type tree structure according to the present disclosure. [Figure 7] FIG. 10 illustrates an embodiment in which a CTU is divided into multiple CUs. [Figure 8] FIG. 10 is a diagram illustrating an example of a redundant division pattern. [Figure 9] 1 is a flowchart illustrating an inter-prediction based video / image coding method according to an embodiment. [Figure 10] 10 is a diagram illustrating an example of a configuration of an inter prediction unit 180 according to an embodiment. [Figure 11] 1 is a flowchart illustrating an inter-prediction based video / image decoding method according to an embodiment. [Figure 12] 2 is a diagram illustrating an example of a configuration of an inter-prediction unit 260 according to an embodiment. [Figure 13] FIG. 10 illustrates neighboring blocks that can be used as spatial merging candidates according to one embodiment. [Figure 14] FIG. 1 is a diagram illustrating a method for constructing a merge candidate list according to an embodiment. [Figure 15] 1 is a diagram illustrating a motion vector predictor candidate list construction method according to an embodiment; [Figure 16] FIG. 10 is a diagram showing a syntax structure for transmitting an MVD from an image encoding device to an image decoding device according to one embodiment. [Figure 17] 1 is a flowchart illustrating an IBC-based video / image encoding method according to one embodiment. [Figure 18] 1 is a diagram illustrating an exemplary configuration of a prediction unit that performs an IBC-based video / image encoding method according to an embodiment. [Figure 19] 1 is a flowchart illustrating an IBC-based video / image decoding method according to an embodiment. [Figure 20]1 is a diagram illustrating an exemplary configuration of a prediction unit that performs an IBC-based video / image decoding method according to an embodiment. [Figure 21] FIG. 10 illustrates a syntax for chroma format signaling according to one embodiment. [Figure 22] FIG. 10 is a diagram illustrating a chroma format classification table according to one embodiment. [Figure 23] FIG. 10 is a diagram illustrating an example of a CU division restriction for virtual pipeline processing. [Figure 24] FIG. 10 is a diagram illustrating an example of division of a CU and a TU according to an embodiment. [Figure 25] FIG. 10 is a diagram illustrating an example of division of a CU and a TU according to an embodiment. [Figure 26] FIG. 10 is a diagram illustrating an example of division of a CU and a TU according to an embodiment. [Figure 27] 10 is a flowchart illustrating inter prediction and intra prediction with maximum transform size applied according to an embodiment. [Figure 28] 10 is a flowchart illustrating inter prediction and intra prediction with maximum transform size applied according to an embodiment. [Figure 29] 1 is a flowchart illustrating a method for encoding an image by an encoding device according to an embodiment. [Figure 30] 10 is a flowchart illustrating a method for decoding an image by a decoding device according to an embodiment. [Figure 31] FIG. 1 illustrates a content streaming system to which an embodiment of the present disclosure can be applied. DETAILED DESCRIPTION OF THE INVENTION

[0023] 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.

[0024] In describing the embodiments of the present disclosure, if it is determined that a detailed description of a known configuration or function may obscure the gist of the present disclosure, the detailed description thereof will be omitted. In addition, in the drawings, parts that are not related to the description of the present disclosure will be omitted, and similar parts will be designated by similar reference numerals.

[0025] In this disclosure, when a component is referred to as being "coupled," "coupled," or "connected" to another component, this includes not only a direct connection, but also an indirect connection where another component exists between them. Furthermore, when a component is referred to as "including" or "having" another component, this does not mean that the other component is excluded, but that the component can further include the other component, unless otherwise specified.

[0026] In this disclosure, terms such as "first" and "second" are used only to distinguish 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.

[0027] 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 into a single hardware or software unit, or a single component may be distributed into multiple hardware or software units. Therefore, even if not otherwise specified, such integrated or distributed embodiments are also included within the scope of this disclosure.

[0028] In this disclosure, the components described in 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 within the scope of this disclosure. Furthermore, an embodiment including other components in addition to the components described in various embodiments is also within the scope of this disclosure.

[0029] The present disclosure relates to image encoding and decoding, and terms used in this disclosure may have their ordinary meaning in the technical field to which the present disclosure belongs unless they are newly defined in this disclosure.

[0030] In this disclosure, a "picture" generally refers to a unit representing any one image in a specific time period, and 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. Furthermore, a slice / tile may include one or more coding tree units (CTUs).

[0031] In this disclosure, "pixel" or "pel" may refer to the smallest unit constituting one picture (or image). Also, "sample" may be used as a term corresponding to 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.

[0032] In this disclosure, the term "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 that region. The term "unit" may be used interchangeably with terms such as "sample array," "block," or "area," depending on the situation. In general, an M×N block may include a set (or array) of samples or transform coefficients consisting of M columns and N rows.

[0033] 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." When prediction is performed, a "current block" may refer to a "current predicted block" or a "block to be predicted." When transformation (inverse transformation) / quantization (inverse quantization) is performed, a "current block" may refer to a "current transformed block" or a "block to be transformed." When filtering is performed, a "current block" may refer to a "block to be filtered."

[0034] Furthermore, in this disclosure, "current block" may mean "luma block of the current block" unless explicitly stated as a chroma block. "Chroma block of the current block" may be expressed explicitly including the explicit description of a chroma block, such as "chroma block" or "current chroma block."

[0035] 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."

[0036] 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."

[0037] Video Coding System Overview

[0038] FIG. 1 is a diagram illustrating a video coding system according to this disclosure.

[0039] A video coding system according to one 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.

[0040] An encoding device 10 according to an embodiment may include a video source generation unit 11, an encoding unit 12, and a transmission unit 13. A decoding device 20 according to an embodiment may include a reception 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 transmission unit 13 may be included in the encoding unit 12. The reception 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.

[0041] The video source generation unit 11 can acquire video / images through a video / image capture, synthesis, or generation process. The video source generation 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 video / images, etc. The video / image generation device can include, for example, a computer, a tablet, a smartphone, etc., and can (electronically) generate video / images. For example, a virtual video / image can be generated via a computer, etc. In this case, the video / image capture process can be replaced with a process in which related data is generated.

[0042] The encoder 12 may encode the input video / image. The encoder 12 may perform a series of steps such as prediction, transformation, and quantization for compression and coding efficiency. The encoder 12 may output the encoded data (encoded video / image information) in a bitstream format.

[0043] The transmitter 13 may transmit the encoded video / image information or data output in a bitstream format to the receiver 21 of the decoding device 20 in a file or streaming format via a digital storage medium or a network. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, and SSD. The transmitter 13 may include elements for generating a media file in a predetermined file format and elements for transmitting via a broadcasting / communication network. The receiver 21 may extract / receive the bitstream from the storage medium or network and transmit it to the decoder 22.

[0044] 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.

[0045] The rendering unit 23 can render the decoded video / images, and the rendered video / images can be displayed via the display unit.

[0046] Overview of the image encoding device

[0047] FIG. 2 is a diagram schematically illustrating an image encoding device to which an embodiment of the present disclosure can be applied.

[0048] 2, the image encoding device 100 may include an image division unit 110, a subtraction unit 115, a transform unit 120, a quantization unit 130, an inverse quantization unit 140, an inverse transform 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 encoding unit 190. The inter prediction unit 180 and the intra prediction unit 185 may be collectively referred to as a "prediction unit." The transform unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transform unit 150 may be included in a residual processing unit. The residual processing unit may further include a subtraction unit 115.

[0049] Depending on the embodiment, 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). Also, the memory 170 may include a decoded picture buffer (DPB) and may be realized by a digital storage medium.

[0050] 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) using a QT / BT / TT (quad-tree / binary-tree / ternary-tree) structure. For example, one coding unit may be divided into multiple coding units at deeper depths based on a quad-tree structure, a binary-tree structure, and / or a ternary-tree structure. To divide the coding units, the quad-tree structure may be applied first, and then the binary-tree structure and / or the ternary-tree structure may be applied later. The coding procedure according to the present disclosure may be performed based on the final coding unit that is not further divided. The maximum coding unit may be used as the final coding unit, or 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 reconstruction, 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.

[0051] The prediction unit (inter prediction unit 180 or intra prediction unit 185) may perform prediction on a current block (current block) to generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied to the current block or CU. The prediction unit may generate various information related to prediction of the current block and transmit it to the entropy coding unit 190. The prediction information may be coded by the entropy coding unit 190 and output in a bitstream format.

[0052] 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 far away from the current block according to the intra prediction mode and / or intra prediction technique. The intra prediction modes may include a plurality of non-directional modes and a plurality of directional modes. The non-directional modes may include, for example, DC mode and Planar mode. The directional modes may include, for example, 33 directional prediction modes or 65 directional prediction modes depending on the granularity of the prediction direction. However, this is merely an example, and more or less directional prediction modes may be used depending on the settings. The intra prediction unit 185 may also determine the prediction mode to be applied to the current block using the prediction modes applied to neighboring blocks.

[0053] 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. To reduce the amount of motion information transmitted in inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on the correlation between the motion information of 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 information on the inter prediction direction (e.g., L0 prediction, L1 prediction, Bi prediction, etc.). In the case of inter prediction, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different. The temporal neighboring block may be 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 predictor 180 may construct a motion information candidate list based on neighboring blocks and generate information indicating which candidate is used to derive a motion vector and / or a reference picture index for the current block. Inter prediction may be performed based on various prediction modes. For example, in the case of skip mode and merge mode, the inter predictor 180 may use motion information of neighboring blocks as motion information for the current block. In the case of skip mode, unlike in merge mode, a residual signal may not be transmitted.In the case of a motion vector prediction (MVP) mode, the motion vector of a neighboring block is used as a motion vector predictor, and the motion vector of the current block can be signaled by encoding a motion vector difference and an indicator for the motion vector predictor. The motion vector difference may mean the difference between the motion vector of the current block and the motion vector predictor.

[0054] The predictor may generate a prediction signal based on various prediction methods and / or prediction techniques, which will be described later. For example, the predictor may apply intra prediction or inter prediction to predict the current block, or may simultaneously apply intra prediction and inter prediction. A prediction method that simultaneously applies intra prediction and inter prediction to predict the current block may be referred to as combined inter and intra prediction (CIIP). The predictor may also perform intra block copy (IBC) to predict the current block. Intra block copy can be used for content image / video coding, such as screen content coding (SCC), for games. IBC is a method of predicting a current block using an already reconstructed 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 essentially performed within the current picture, but may be similar to inter prediction in that a reference block is derived within the current picture. That is, the IBC may use at least one of the inter prediction techniques described in this disclosure.

[0055] 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, predicted sample array) output from the prediction unit from the input image signal (original block, original sample array) to generate a residual signal (residual signal, residual block, residual sample array). The generated residual signal may be transmitted to the conversion unit 120.

[0056] 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 representing inter-pixel relationship information. 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.

[0057] 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 encode the quantized signal (information about the quantized transform coefficients) and output the encoded signal in a bitstream format. The information about 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 about the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector format.

[0058] The entropy coding unit 190 can 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., syntax elements (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 NAL (network abstraction layer) unit units 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 also include general constraint information. The signaling information, information to be transmitted, and / or syntax elements mentioned in the present disclosure may be coded through the above-mentioned coding procedure and included in the bitstream.

[0059] The bitstream may be transmitted via a network or 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) that transmits and / or a storing unit (not shown) that stores the signal output from the entropy encoding unit 190 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.

[0060] The quantized transform coefficients output from the quantization unit 130 can be used to generate a residual signal. For example, the residual signal (residual block or residual sample) can be reconstructed by applying inverse quantization and inverse transform to the quantized transform coefficients via the inverse quantization unit 140 and the inverse transform unit 150.

[0061] The adder 155 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the reconstructed residual signal to the 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, the predicted block may be used as the 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 will be described later.

[0062] 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 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, etc. The filtering unit 160 may generate various information related to filtering and transmit it to the entropy coding unit 190, as will be described later in connection with each filtering method. The filtering information may be coded by the entropy coding unit 190 and output in a bitstream format.

[0063] The modified reconstructed picture transmitted to the memory 170 can be used as a reference picture in the inter prediction unit 180. When inter prediction is applied through this, the image encoding device 100 can avoid a prediction mismatch between the image encoding device 100 and the image decoding device, and can also improve encoding efficiency.

[0064] The DPB in the memory 170 may store modified reconstructed pictures for use as reference pictures in the inter predictor 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 predictor 180 to be used as motion information of spatially surrounding blocks or temporally surrounding blocks. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra predictor 185.

[0065] Overview of the image decoding device

[0066] FIG. 3 is a diagram schematically illustrating an image decoding device to which an embodiment of the present disclosure can be applied.

[0067] 3, the image decoding apparatus 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.

[0068] Depending on the embodiment, all or at least some of the components constituting the image decoding device 200 may be realized by a single hardware component (e.g., a decoder or a processor). Also, the memory 170 may include a DPB and may be realized by a digital storage medium.

[0069] The image decoding device 200, which receives a bitstream including video / image information, can reconstruct an image by performing a process corresponding to the process performed by the image encoding device 100 of FIG. 1. For example, the image decoding device 200 can perform decoding using a processing unit applied in the image encoding device. Therefore, the decoding processing unit can be, for example, a coding unit. The coding unit can be obtained by dividing a coding tree unit or a maximum coding unit. The reconstructed image signal decoded and output by the image decoding device 200 can be reproduced by a reproduction device (not shown).

[0070] The image decoding apparatus 200 may receive a signal output from the image encoding apparatus of FIG. 2 in a bitstream format. The received signal may be decoded via an entropy decoding unit 210. For example, the entropy decoding unit 210 may parse the bitstream to derive information (e.g., video / image information) necessary for image reconstruction (or picture reconstruction). 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 also include general constraint information. The image decoding apparatus may further use the information on the parameter sets 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 residuals. More specifically, the CABAC entropy decoding method receives bins corresponding to each syntax element from the bitstream, determines a context model using information on the syntax element to be decoded and decoded information on neighboring blocks and the block to be decoded, or information on symbols / bins decoded in a previous step, predicts the occurrence probability of the bins based on the determined context model, and performs arithmetic decoding of the bins to generate symbols corresponding to the values ​​of each syntax element. After determining the context model, the CABAC entropy decoding method may update the context model using information on the decoded symbol / bin for the context model of the next symbol / bin.Among the information decoded by the entropy decoding unit 210, information related to prediction is provided to the prediction units (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.

[0071] Meanwhile, the image decoding apparatus according to the present disclosure may be referred to as a video / image / picture decoding apparatus. The image decoding apparatus 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.

[0072] The inverse quantization unit 220 may inverse quantize the quantized transform coefficients and output the 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 the coefficient scanning 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.

[0073] The inverse transform unit 230 can inversely transform the transform coefficients to obtain a residual signal (residual block, residual sample array).

[0074] The prediction unit may perform prediction on a 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 to the current block based on information about the prediction output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode (prediction technique).

[0075] The prediction unit can generate a prediction signal based on various prediction methods (techniques) described below, as described in the description of the prediction unit of the image encoding device 100.

[0076] The intra predictor 265 may predict the current block by referring to samples in the current picture. The description of the intra predictor 185 may also be applied to the intra predictor 265.

[0077] The inter prediction unit 260 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. To reduce the amount of motion information transmitted in inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on correlations between motion information of 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 information on an inter prediction direction (e.g., L0 prediction, L1 prediction, Bi prediction, etc.). 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 construct 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 received candidate selection information. Inter prediction may be performed based on various prediction modes (techniques), and the prediction information may include information indicating the inter prediction mode (technique) for the current block.

[0078] The adder 235 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the obtained 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 to be processed, such as when a skip mode is applied, the predicted block may be used as the reconstructed block. The description of the adder 155 may also be applied to the adder 235. The adder 235 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 undergoing filtering, as will be described later.

[0079] 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 a 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.

[0080] The (modified) reconstructed picture stored in the DPB of the memory 250 can be used as a reference picture in the inter predictor 260. The memory 250 can 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 can be transmitted to the inter predictor 260 to be used as motion information of a spatially surrounding block or a temporally surrounding block. The memory 250 can store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra predictor 265.

[0081] 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.

[0082] Image Segmentation Overview

[0083] The video / image coding method according to the present disclosure may be performed based on the following image partition structure. Specifically, procedures such as prediction, residual processing (e.g., (inverse) transform, (inverse) quantization), syntax element coding, and filtering, which will be described later, may be performed based on CTUs and CUs (and / or TUs and PUs) derived based on the image partition structure. An image may be divided into blocks, and the block partitioning procedure may be performed by the image partitioning unit 110 of the encoding device described above. Partition-related information may be coded by the entropy coding unit 190 and transmitted to the decoding device in the form of a bitstream. The entropy decoding unit 210 of the decoding device may derive a block partition structure of the current picture based on the partition-related information obtained from the bitstream, and perform a series of procedures for image decoding (e.g., prediction, residual processing, block / picture reconstruction, in-loop filtering, etc.) based on the block partition structure.

[0084] A picture can be divided into a sequence of coding tree units (CTUs). Figure 4 shows an example of dividing a picture into CTUs. A CTU can correspond to a coding tree block (CTB). Alternatively, a CTU can include a coding tree block of luma samples and two coding tree blocks of corresponding chroma samples. For example, for a picture containing three sample arrays, a CTU can include an NxN block of luma samples and two corresponding blocks of chroma samples.

[0085] Overview of CTU division

[0086] As described above, a coding unit can 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 / ternary-tree) structure. For example, a CTU can be first divided into a quad-tree structure. Then, the leaf nodes of the quad-tree structure can be further divided according to a multi-type tree structure.

[0087] Quadtree division refers to dividing the current CU (or CTU) into four equal parts. By quadtree division, the current CU can be divided into four CUs with the same width and height. If the current CU is not further divided into a quadtree structure, the current CU corresponds to a leaf node of the quadtree structure. A CU corresponding to a leaf node of the quadtree structure is not further divided and can be used as the final coding unit described above. Alternatively, a CU corresponding to a leaf node of the quadtree structure can be further divided into four parts according to a multi-type tree structure.

[0088] 5 is a diagram showing the types of division of blocks using a multi-type tree structure. Division using a multi-type tree structure can include two divisions using a binary tree structure and two divisions using a ternary tree structure.

[0089] The two divisions based on the binary tree structure can include vertical binary splitting (SPLIT_BT_VER) and horizontal binary splitting (SPLIT_BT_HOR). Vertical binary splitting (SPLIT_BT_VER) refers to a division that divides the current CU into two equal parts vertically. As shown in FIG. 4, vertical binary splitting can generate two CUs with the same height as the current CU and half the width of the current CU. Horizontal binary splitting (SPLIT_BT_HOR) refers to a division that divides the current CU into two equal parts horizontally. As shown in FIG. 5, horizontal binary splitting can generate two CUs with the same height as the current CU and half the width of the current CU.

[0090] The two divisions based on the ternary tree structure can include vertical ternary splitting (SPLIT_TT_VER) and horizontal ternary splitting (SPLIT_TT_HOR). Vertical ternary splitting (SPLIT_TT_VER) divides the current CU vertically at a ratio of 1:2:1. As shown in FIG. 5, vertical ternary splitting can generate two CUs each having the same height as the current CU and a width equal to one-quarter of the current CU's width, and a CU each having the same height as the current CU and a width equal to half the current CU's width. Horizontal ternary splitting (SPLIT_TT_HOR) divides the current CU horizontally at a ratio of 1:2:1. As shown in FIG. 4, horizontal ternary splitting can generate two CUs each having a height equal to one-quarter of the current CU's height and a width equal to the current CU's width.

[0091] FIG. 6 is a diagram illustrating a signaling mechanism for block partition information in a quadtree with nested multi-type tree structure according to the present disclosure.

[0092] Here, the CTU is treated as the root node of the quadtree, and the CTU is first split into a quadtree structure. Information (e.g., qt_split_flag) indicating whether quadtree splitting is performed on the current CU (CTU or quadtree node (QT_node)) can be signaled. For example, if qt_split_flag is a first value (e.g., '1'), the current CU can be split into a quadtree. Also, if qt_split_flag is a second value (e.g., '0'), the current CU is not split into a quadtree but becomes a quadtree leaf node (QT_leaf_node). The leaf nodes of each quadtree can then be further split into a multitype tree structure. That is, the leaf nodes of the quadtree can become multitype tree nodes (MTT_node). In a multi-type tree structure, a first flag (e.g., mtt_split_cu_flag) may be signaled to indicate whether the current node is to be further split. If the node is to be further split (e.g., if the first flag is 1), a second flag (e.g., mtt_split_cu_vertical_flag) may be signaled to indicate the splitting direction. For example, if the second flag is 1, the splitting direction may be vertical, and if the second flag is 0, the splitting direction may be horizontal. Then, a third flag (e.g., mtt_split_cu_binary_flag) may be signaled to indicate whether the splitting type is a binary splitting type or a ternary splitting type. For example, if the third flag is 1, the splitting type may be a binary splitting type, and if the third flag is 0, the splitting type may be a ternary splitting type. The nodes of a multitype tree obtained by binary or ternary splitting can be further partitioned into a multitype tree structure, but the nodes of a multitype tree cannot be partitioned into a quadtree structure.If the first flag is 0, the corresponding node of the multitype tree is not further divided and becomes a leaf node (MTT_leaf_node) of the multitype tree. The CU corresponding to the leaf node of the multitype tree can be used as the final coding unit described above.

[0093] Based on the above mtt_split_cu_vertical_flag and mtt_split_cu_binary_flag, the multi-type tree splitting mode (MttSplitMode) of the CU can be derived as shown in Table 1. In the following description, the multi-type tree splitting mode can be abbreviated as multi-tree split type or split type.

[0094] [Table 1]

[0095] FIG. 7 shows an example in which a CTU is divided into multiple CUs by applying a multi-type tree after applying a quadtree. In FIG. 7, a bold block edge 710 indicates the quadtree division, and the remaining edge 720 indicates the multi-type tree division. A CU may correspond to a coding block CB. In one embodiment, a CU may include a coding block of luma samples and two coding blocks of chroma samples corresponding to the luma samples. The chroma component (sample) CB or TB size may be derived based on the luma component (sample) CB or TB size according to the component ratio according to the color format of the picture / image (chroma format, e.g., 4:4:4, 4:2:2, 4:2:0, etc.). If the color format is 4:4:4, the chroma component CB / TB size may be set to be the same as the luma component CB / TB size. If the color format is 4:2:2, the width of the chroma components CB / TB can be set to half the width of the luma components CB / TB, and the height of the chroma components CB / TB can be set to the height of the luma components CB / TB. If the color format is 4:2:0, the width of the chroma components CB / TB can be set to half the width of the luma components CB / TB, and the height of the chroma components CB / TB can be set to half the height of the luma components CB / TB.

[0096] In one embodiment, when the size of the CTU is 128 based on the luma sample unit, the size of the CU can range from 128 x 128, which is the same size as the CTU, to 4 x 4. In one embodiment, in the case of a 4:2:0 color format (or chroma format), the chroma CB size can range from 64 x 64 to 2 x 2.

[0097] Meanwhile, in one embodiment, the CU size and the TU size may be the same, or multiple TUs may exist within a CU region. The TU size may generally refer to the luma component (sample) TB (Transform Block) size.

[0098] The TU size may be derived based on a preset maximum allowable TB size (maxTbSize). For example, if the CU size is larger than the maxTbSize, multiple TUs (TBs) having the maxTbSize may be derived from the CU, and transform / inverse transform may be performed in units of the TUs (TBs). For example, the maximum allowable luma TB size may be 64x64, and the maximum allowable chroma TB size may be 32x32. If the width or height of a CB divided by the tree structure is larger than the maximum transform width or height, the CB may be automatically (or implicitly) divided until the horizontal and vertical TB size constraints are satisfied.

[0099] Also, for example, when intra prediction is applied, the intra prediction mode / type may be derived in units of the CU (or CB), and the procedure of deriving neighboring reference samples and generating predicted samples may be performed in units of TU (or TB). In this case, one or more TUs (or TBs) may exist within one CU (or CB) region, and in this case, the multiple TUs (or TBs) may share the same intra prediction mode / type.

[0100] Meanwhile, for a quadtree coding tree scheme with a multitype tree, the following parameters can be signaled from the encoding device to the decoding device as SPS syntax elements. For example, at least one of CTUsize, a parameter indicating the size of the root node of a quadtree, MinQTSize, a parameter indicating the minimum allowable size of a leaf node of a quadtree, MaxBTSize, a parameter indicating the maximum allowable size of a root node of a binary tree, MaxTTSize, a parameter indicating the maximum allowable size of a root node of a ternary tree, MaxMttDepth, a parameter indicating the maximum allowed hierarchy depth of a multitype tree split from a leaf node of a quadtree, MinBtSize, a parameter indicating the minimum allowable leaf node size of a binary tree, and MinTtSize, a parameter indicating the minimum allowable leaf node size of a ternary tree, can be signaled.

[0101] In one embodiment using the 4:2:0 chroma format, the CTU size may be set to a 128x128 luma block and two 64x64 chroma blocks corresponding to the luma block. In this case, MinQTSize may be set to 16x16, MaxBtSize may be set to 128x128, MaxTtSzie may be set to 64x64, MinBtSize and MinTtSize may be set to 4x4, and MaxMttDepth may be set to 4. Quadtree division may be applied to the CTU to generate quadtree leaf nodes. The quadtree leaf nodes may be called leaf QT nodes. The quadtree leaf nodes may range in size from 16x16 (e.g., the MinQTSize) to 128x128 (e.g., the CTU size). If the leaf QT node is 128x128, it may not be further divided into a binary tree or a ternary tree. This is because even if the partitioning is performed in this case, it would exceed MaxBtSize and MaxTtszie (e.g., 64 x 64). In other cases, the leaf QT node can be further partitioned into a multitype tree. Thus, the leaf QT node is the root node for the multitype tree, and the leaf QT node can have a multitype tree depth (mttDepth) value of 0. If the multitype tree depth reaches MaxMttDepth (e.g., 4), no further subdivisions can be considered. If the width of the multitype tree node is equal to MinBtSize and equal to or less than 2 x MinTtSize, no further horizontal subdivisions can be considered. If the height of the multitype tree node is equal to MinBtSize and equal to or less than 2 x MinTtSize, no further vertical subdivisions can be considered. If no subdivision is considered in this way, the encoding device can omit signaling of the subdivision information. In such cases, the decoding device can guide the subdivision information to a predetermined value.

[0102] Meanwhile, one CTU may include a coding block of luma samples (hereinafter referred to as a "luma block") and two coding blocks of corresponding chroma samples (hereinafter referred to as "chroma blocks"). The above-mentioned coding tree scheme may be applied equally to the luma blocks and chroma blocks of the current CU, or may be applied separately. Specifically, the luma blocks and chroma blocks in one CTU may be divided into the same block tree structure, which may be referred to as a single tree (SINGLE_TREE). Alternatively, the luma blocks and chroma blocks in one CTU may be divided into separate block tree structures, which may be referred to as a dual tree (DUAL_TREE). In other words, when a CTU is divided into a dual tree, a block tree structure for the luma blocks and a block tree structure for the chroma blocks may exist separately. In this case, the block tree structure for the luma block may be referred to as a dual tree luma (DUAL_TREE_LUMA), and the block tree structure for the chroma block may be referred to as a dual tree chroma (DUAL_TREE_CHROMA). For P and B slices / tile groups, the luma block and the chroma block in one CTU may be restricted to have the same coding tree structure. However, for I slices / tile groups, the luma block and the chroma block may have separate block tree structures. If a separate block tree structure is applied, the luma coding tree block (CTB) may be divided into CUs based on a specific coding tree structure, and the chroma CTB may be divided into chroma CUs based on another coding tree structure. That is, this may mean that a CU in an I slice / tile group to which a separate block tree structure is applied may be composed of a coding block of a luma component or a coding block of two chroma components, and a CU in a P or B slice / tile group may be composed of blocks of three color components (a luma component and two chroma components).

[0103] Although the quadtree coding tree structure with a multi-type tree has been described above, the structure in which a CU is divided is not limited to this. For example, the BT structure and the TT structure may be interpreted as concepts included in a multiple partitioning tree (MPT) structure, and a CU may be interpreted as being divided by a QT structure and an MPT structure. In an example in which a CU is divided by a QT structure and an MPT structure, the division structure may be determined by signaling a syntax element (e.g., MPT_split_type) containing information on whether a leaf node of the QT structure is divided into several blocks and a syntax element (e.g., MPT_split_mode) containing information on whether the leaf node of the QT structure is divided vertically or horizontally.

[0104] In another example, CUs may be divided in a manner different from that of the QT structure, BT structure, or TT structure. That is, unlike the QT structure in which lower-depth CUs are divided into 1 / 4 the size of higher-depth CUs, the BT structure in which lower-depth CUs are divided into 1 / 2 the size of higher-depth CUs, or the TT structure in which lower-depth CUs are divided into 1 / 4 or 1 / 2 the size of higher-depth CUs, lower-depth CUs may be divided into 1 / 5, 1 / 3, 3 / 8, 3 / 5, 2 / 3, or 5 / 8 the size of higher-depth CUs, as the case may be, and the manner in which CUs are divided is not limited thereto.

[0105] In this way, the quadtree coding block structure with the multi-type tree can provide a very flexible block partitioning structure. Meanwhile, due to the partitioning types supported by the multi-type tree, different partitioning patterns can potentially result in the same coding block structure in some cases. By limiting the occurrence of such redundant partitioning patterns, the encoding device and the decoding device can reduce the amount of data for partitioning information.

[0106] For example, FIG. 8 exemplarily illustrates redundant partitioning patterns that may occur in binary tree partitioning and ternary tree partitioning. As shown in FIG. 8, consecutive binary partitions 810 and 820 in one direction at two-step levels have the same coding block structure as the binary partitioning for the center partition after ternary partitioning. In this case, binary tree partitioning for the center blocks 830 and 840 of the ternary tree partitioning can be prohibited. This prohibition can be applied to all CUs of a picture. When such a specific partitioning is prohibited, the signaling of the corresponding syntax element can be modified to reflect this prohibition, thereby reducing the number of bits signaled for the partitioning. For example, as in the example shown in FIG. 8, when binary tree partitioning for the center block of a CU is prohibited, the mtt_split_cu_binary_flag syntax element, which indicates whether the partitioning is binary or ternary, is not signaled and can be set to 0 by the decoder.

[0107] Inter Prediction Overview

[0108] Inter prediction according to the present disclosure will be described below.

[0109] The prediction unit of the image encoding / decoding apparatus according to the present disclosure may perform inter prediction on a block-by-block basis to derive predicted samples. Inter prediction may refer to prediction derived in a manner dependent on data elements (e.g., sample values ​​or motion information) of pictures other than the current picture. When inter prediction is applied to the current block, a predicted block (prediction block or prediction sample array) for the current block may be derived based on a reference block (reference sample array) identified by a motion vector in a reference picture indicated by a reference picture index. In this case, to reduce the amount of motion information transmitted in the inter prediction mode, motion information of the current block may be predicted on a block, sub-block, or sample-by-block basis based on correlation between motion information of 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 information on an inter prediction type (e.g., L0 prediction, L1 prediction, Bi prediction, etc.). When inter prediction is applied, the neighboring blocks may include spatial neighboring blocks in the current picture and temporal neighboring blocks in the reference picture. The reference picture including the reference block and the reference picture including the temporal peripheral block may be the same or different. The temporal peripheral block may be called a collocated reference block, a collocated CU (colCU), a col block, etc., and the reference picture including the temporal peripheral block may be called a collocated picture (colPic), a col picture, etc. For example, a motion information candidate list may be constructed based on the peripheral blocks of the current block, and flag or index information indicating which candidate is selected (used) to derive the motion vector and / or reference picture index of the current block may be signaled.

[0110] Inter prediction may be performed based on various prediction modes. For example, in skip mode and merge mode, the motion information of the current block may be the same as the motion information of a selected neighboring block. Unlike merge mode, in skip mode, a residual signal may not be transmitted. In motion vector prediction (MVP) mode, the motion vector of a selected neighboring block may be used as a motion vector predictor, and a motion vector difference may be signaled. In this case, the motion vector of the current block may be derived using the sum of the motion vector predictor and the motion vector difference. In this disclosure, MVP mode may be used interchangeably with AMVP (Advanced Motion Vector Prediction).

[0111] The motion information may include L0 motion information and / or L1 motion information based on an inter-prediction type (such as L0 prediction, L1 prediction, or Bi prediction). A motion vector in the L0 direction may be referred to as an L0 motion vector or MVL0, and a motion vector in the L1 direction may be referred to as an L1 motion vector or MVL1. Prediction based on an L0 motion vector may be referred to as L0 prediction, prediction based on an L1 motion vector may be referred to as L1 prediction, and prediction based on both the L0 motion vector and the L1 motion vector may be referred to as bi-prediction (Bi) prediction. Here, an L0 motion vector may indicate a motion vector associated with a reference picture list L0 (L0), and an L1 motion vector may indicate a motion vector associated with a reference picture list L1 (L1). The reference picture list L0 may include, as reference pictures, pictures that are earlier in output order than the current picture, and the reference picture list L1 may include pictures that are later in output order than the current picture. The previous picture may be referred to as a forward (reference) picture, and the subsequent picture may be referred to as a backward (reference picture). The reference picture list L0 may further include, as reference pictures, pictures that are subsequent to the current picture in output order. In this case, the previous picture may be indexed first in the reference picture list L0, and the subsequent picture may be indexed next. The reference picture list L1 may further include, as reference pictures, pictures that are prior to the current picture in output order. In this case, the subsequent picture may be indexed first in the reference picture list L1, and the previous picture may be indexed next. Here, the output order may correspond to a picture order count (POC) order.

[0112] FIG. 9 is a flowchart illustrating an inter-prediction based video / image coding method.

[0113] FIG. 10 is a diagram illustrating an example configuration of the inter prediction unit 180 according to the present disclosure.

[0114] The encoding method of FIG. 9 may be performed by the image encoding apparatus of FIG. 2. Specifically, step S610 may be performed by the inter prediction unit 180, and step S620 may be performed by the residual processing unit. Specifically, step S620 may be performed by the subtraction unit 115. Step S630 may be performed by the entropy encoding unit 190. The prediction information of step S630 may be derived by the inter prediction unit 180, and the residual information of step S630 may be derived by the residual processing unit. The residual information is information about the residual sample. The residual information may include information about quantized transform coefficients for the residual sample. As described above, the residual sample may be derived as transform coefficients via the transform unit 120 of the image encoding apparatus, and the transform coefficients may be derived as quantized transform coefficients via the quantization unit 130. Information about the quantized transform coefficients may be coded by the entropy encoding unit 190 through a residual coding procedure.

[0115] The image encoding apparatus may perform inter prediction on a current block (S610). The image encoding apparatus may derive an inter prediction mode and motion information of the current block and generate a prediction sample for the current block. Here, the inter prediction mode determination, motion information derivation, and prediction sample generation procedures may be performed simultaneously, or one procedure may be performed before the other procedures. For example, as shown in FIG. 10, the inter prediction unit 180 of the image encoding apparatus may include a prediction mode determination unit 181, a motion information derivation unit 182, and a prediction sample derivation unit 183. The prediction mode determination unit 181 may determine a prediction mode for the current block, the motion information derivation unit 182 may derive motion information of the current block, and the prediction sample derivation unit 183 may derive a prediction sample for the current block. For example, the inter prediction unit 180 of the image encoding apparatus may search for a block similar to the current block within a certain region (search area) of a reference picture through motion estimation and derive a reference block whose difference from the current block is minimum or equal to or less than a certain criterion. Based on this, a reference picture index indicating a reference picture in which the reference block is located can be derived, and a motion vector can be derived based on a position difference between the reference block and the current block. The image encoding apparatus can determine a mode to be applied to the current block from various prediction modes. The image encoding apparatus can compare rate-distortion (RD) costs for the various prediction modes and determine an optimal prediction mode for the current block. However, the method by which the image encoding apparatus determines a prediction mode for the current block is not limited to the above example, and various methods can be used.

[0116] For example, when a skip mode or a merge mode is applied to a current block, the image encoding apparatus may derive merge candidates from neighboring blocks of the current block and construct a merge candidate list using the derived merge candidates. Furthermore, the image encoding apparatus may derive a reference block whose difference from the current block is minimum or equal to or less than a certain criterion among reference blocks indicated by merge candidates included in the merge candidate list. In this case, a merge candidate associated with the derived reference block may be selected, and merge index information indicating the selected merge candidate may be generated and signaled to the image decoding apparatus. Motion information of the current block may be derived using motion information of the selected merge candidate.

[0117] As another example, when the MVP mode is applied to the current block, the image encoding apparatus may derive motion vector predictor (MVP) candidates from neighboring blocks of the current block and construct an MVP candidate list using the induced MVP candidates. The image encoding apparatus may also use the motion vector of an MVP candidate selected from the MVP candidates included in the MVP candidate list as the MVP of the current block. In this case, for example, a motion vector pointing to a reference block derived by the motion estimation described above may be used as the motion vector of the current block, and the MVP candidate having the smallest difference from the motion vector of the current block may be the selected MVP candidate. A motion vector difference (MVD), which is the difference obtained by subtracting the MVP from the motion vector of the current block, may be derived. In this case, index information indicating the selected MVP candidate and information regarding the MVD may be signaled to the image decoding apparatus. Also, when the MVP mode is applied, the value of the reference picture index may be configured as reference picture index information and separately signaled to the image decoding apparatus.

[0118] The image encoding apparatus may derive residual samples based on the predicted samples (S620). The image encoding apparatus may derive the residual samples by comparing the original samples of the current block with the predicted samples. For example, the residual samples may be derived by subtracting the corresponding predicted samples from the original samples.

[0119] The image encoding apparatus may encode image information including prediction information and residual information (S630). The image encoding apparatus may output the encoded image information in a bitstream format. The prediction information may be information related to the prediction procedure and may include prediction mode information (e.g., a skip flag, a merge flag, or a mode index) and information about motion information. Among the prediction mode information, the skip flag is information indicating whether a skip mode is applied to a current block, and the merge flag is information indicating whether a merge mode is applied to the current block. Alternatively, the prediction mode information may be information indicating one of a plurality of prediction modes, such as a mode index. If the skip flag and the merge flag are both 0, it may be determined that the MVP mode is applied to the current block. The information about the motion information may include candidate selection information (e.g., a merge index, an MVP flag, or an MVP index) that is information for deriving a motion vector. Among the candidate selection information, the merge index may be signaled when a merge mode is applied to the current block, and may be information for selecting one of merge candidates included in a merge candidate list. Among the candidate selection information, the MVP flag or MVP index may be signaled when the MVP mode is applied to the current block, and may be information for selecting one of the MVP candidates included in the MVP candidate list. Furthermore, the information about the motion information may include the above-mentioned information about MVD and / or reference picture index information. Furthermore, the information about the motion information may include information indicating whether L0 prediction, L1 prediction, or Bi-prediction is applied. The residual information is information about the residual sample. The residual information may include information about quantized transform coefficients for the residual sample.

[0120] The output bitstream can be stored in a (digital) storage medium and transmitted to the image decoding device, or can be transmitted to the image decoding device via a network.

[0121] Meanwhile, as described above, the image coding apparatus can generate a reconstructed picture (a picture including reconstructed samples and reconstructed blocks) based on the reference samples and the residual samples. This is because the image coding apparatus derives the same prediction result as that performed in the image decoding apparatus, thereby improving coding efficiency. Therefore, the image coding apparatus can store the reconstructed picture (or reconstructed samples, reconstructed blocks) in a memory and use it as a picture for inter prediction. As described above, an in-loop filtering procedure can be further applied to the reconstructed picture.

[0122] FIG. 11 is a flowchart illustrating an inter-prediction based video / image decoding method.

[0123] FIG. 12 is a diagram illustrating an example configuration of the inter prediction unit 260 according to the present disclosure.

[0124] The image decoding apparatus may perform operations corresponding to those performed by the image encoding apparatus, such as performing prediction on a current block based on received prediction information and deriving predicted samples.

[0125] The decoding method of FIG. 11 may be performed by the image decoding apparatus of FIG. 3. Steps S810 to S830 may be performed by the inter prediction unit 260, and the prediction information of step S810 and the residual information of step S840 may be obtained from a bitstream by the entropy decoding unit 210. The residual processing unit of the image decoding apparatus may derive residual samples for the current block based on the residual information (S840). Specifically, the inverse quantization unit 220 of the residual processing unit may derive transform coefficients by performing inverse quantization on the quantized transform coefficients derived based on the residual information, and the inverse transform unit 230 of the residual processing unit may derive residual samples for the current block by performing inverse transform on the transform coefficients. Step S850 may be performed by the adder 235 or a reconstruction unit.

[0126] Specifically, the image decoding apparatus may determine a prediction mode for the current block based on received prediction information (S810). The image decoding apparatus may determine which inter prediction mode is applied to the current block based on prediction mode information in the prediction information.

[0127] For example, it may determine whether the skip mode is applied to the current block based on the skip flag. Also, it may determine whether the merge mode or the MVP mode is applied to the current block based on the merge flag. Or, it may select one of various inter prediction mode candidates based on the mode index. The inter prediction mode candidates may include skip mode, merge mode, and / or MVP mode, or various inter prediction modes described below.

[0128] The image decoding apparatus may derive motion information of the current block based on the determined inter prediction mode (S820). For example, when a skip mode or a merge mode is applied to the current block, the image decoding apparatus may construct a merge candidate list (described below) and select one of the merge candidates included in the merge candidate list. The selection may be made based on the candidate selection information (merge index) described above. Motion information of the selected merge candidate may be used to derive motion information of the current block. For example, the motion information of the selected merge candidate may be used as motion information of the current block.

[0129] As another example, when the MVP mode is applied to the current block, the image decoding apparatus may construct an MVP candidate list and use a motion vector of an MVP candidate selected from the MVP candidates included in the MVP candidate list as the MVP of the current block. The selection may be made based on the candidate selection information (MVP flag or MVP index). In this case, the MVD of the current block may be derived based on information related to the MVD, and the motion vector of the current block may be derived based on the MVP of the current block and the MVD. Also, the image decoding apparatus may derive a reference picture index of the current block based on the reference picture index information. A picture pointed to by the reference picture index in the associated reference picture list for the current block may be derived as a reference picture referenced for inter-prediction of the current block.

[0130] The image decoding apparatus may generate prediction samples for the current block based on the motion information of the current block (S830). In this case, the reference picture may be derived based on a reference picture index of the current block, and the prediction samples of the current block may be derived using samples of a reference block pointed to in the reference picture by the motion vector of the current block. Depending on the circumstances, a prediction sample filtering procedure may further be performed on all or some of the prediction samples of the current block.

[0131] 12, the inter prediction unit 260 of the image decoding apparatus may include a prediction mode determination unit 261, a motion information derivation unit 262, and a prediction sample derivation unit 263. The inter prediction unit 260 of the image decoding apparatus may determine a prediction mode for the current block based on prediction mode information received from the prediction mode determination unit 261, derive motion information (such as a motion vector and / or a reference picture index) of the current block based on information related to the motion information received from the motion information derivation unit 262, and derive a prediction sample of the current block using the prediction sample derivation unit 263.

[0132] The image decoding apparatus may generate residual samples for the current block based on the received residual information (S840). The image decoding apparatus may generate reconstructed samples for the current block based on the predicted samples and the residual samples, and generate a reconstructed picture based on the reconstructed samples (S850). Thereafter, an in-loop filtering procedure may be further applied to the reconstructed picture, as described above.

[0133] As described above, the inter prediction procedure may include an inter prediction mode determination step, a motion information deriving step according to the determined prediction mode, and a prediction execution step (generation of prediction samples) based on the derived motion information. The inter prediction procedure may be performed in an image encoding device and an image decoding device, as described above.

[0134] The motion information derivation step according to the prediction mode will be described in more detail below.

[0135] As described above, inter prediction can be performed using motion information of a current block. The image coding apparatus can derive optimal motion information for the current block through a motion estimation procedure. For example, the image coding apparatus can search for a similar reference block with high correlation using an original block in an original picture for the current block in a fractional pixel unit within a predetermined search range in the reference picture, thereby deriving motion information. The similarity of blocks can be calculated based on the sum of absolute differences (SAD) between the current block and the reference block. In this case, motion information can be derived based on the reference block with the smallest SAD within the search range. The derived motion information can be signaled to the image decoding apparatus in various ways based on the inter prediction mode.

[0136] When a merge mode is applied to a current block, the motion information of the current block is not directly transmitted, but is derived using the motion information of neighboring blocks. Therefore, the motion information of the current predicted block can be indicated by transmitting flag information indicating that the merge mode is used and candidate selection information (e.g., a merge index) indicating which neighboring blocks are used as merge candidates. In this disclosure, since the current block is a unit of prediction execution, the current block may be used in the same sense as a current predicted block, and the neighboring blocks may be used in the same sense as neighboring predicted blocks.

[0137] The image encoding apparatus may search for merge candidate blocks to be used to derive motion information of the current block to perform the merge mode. For example, but not limited to, up to five merge candidate blocks may be used. The maximum number of merge candidate blocks may be transmitted from a slice header or a tile group header, but is not limited to this. After finding the merge candidate blocks, the image encoding apparatus may generate a merge candidate list and select the merge candidate block with the smallest RD cost as the final merge candidate block.

[0138] The present disclosure provides various embodiments for the merge candidate blocks that make up the merge candidate list. The merge candidate list can use, for example, five merge candidate blocks. For example, four spatial merge candidates and one temporal merge candidate can be used.

[0139] FIG. 13 is a diagram illustrating neighboring blocks that can be used as spatial merging candidates.

[0140] FIG. 14 is a diagram illustrating a method for constructing a merge candidate list according to an example of the present disclosure.

[0141] The image encoding / decoding apparatus may search for spatial neighboring blocks of the current block and insert the derived spatial merge candidates into a merge candidate list (S1110). For example, the spatial neighboring blocks may include a lower-left corner neighboring block A0, a left-side neighboring block A1, a upper-right corner neighboring block B0, an upper-side neighboring block B1, and an upper-left corner neighboring block B2 of the current block, as shown in FIG. 13. However, this is merely an example, and additional neighboring blocks, such as a right-side neighboring block, a lower-side neighboring block, and a lower-right neighboring block, may also be used as the spatial neighboring blocks. The image encoding / decoding apparatus may search the spatial neighboring blocks based on priority to detect available blocks and derive motion information of the detected blocks as the spatial merge candidates. For example, the image encoding / decoding apparatus may search the five blocks shown in FIG. 13 in the order of A1, B1, B0, A0, and B2, and sequentially index the available candidates to construct a merge candidate list.

[0142] The image encoding / decoding apparatus may search for temporal peripheral blocks of the current block and insert the derived temporal merge candidates into the merge candidate list (S1120). The temporal peripheral blocks may be located in a reference picture that is a different picture from the current picture in which the current block is located. The reference picture in which the temporal peripheral blocks are located may be called a collocated picture or col picture. The temporal peripheral blocks may be searched for in the order of the lower right corner peripheral block and the lower right center block of the collocated block with respect to the current block in the col picture. Meanwhile, when motion data compression is applied to reduce memory load, specific motion information may be stored as representative motion information for each predetermined storage unit for the col picture. In this case, it is not necessary to store motion information for all blocks within the predetermined storage unit, thereby achieving the effect of motion data compression. In this case, the predetermined storage unit may be predetermined, for example, in units of 16x16 samples or 8x8 samples, or size information for the predetermined storage unit may be signaled from the image encoding apparatus to the image decoding apparatus. When the motion data compression is applied, the motion information of the temporal peripheral block can be replaced with the representative motion information of the certain storage unit where the temporal peripheral block is located. In other words, in this case, from the viewpoint of implementation, the temporal merge candidate can be derived based on the motion information of the prediction block that covers the position arithmetically shifted left after arithmetically shifting a certain value based on the coordinates (upper left sample position) of the temporal peripheral block, rather than the prediction block located at the coordinates of the temporal peripheral block. For example, if the certain storage unit is 2 n ×2 nWhen it is in the sample unit, if the coordinates of the time neighboring block are (xTnb, yTnb), the motion information of the prediction block located at the corrected position ((xTnb>>n)<<n), (yTnb>>n)<<n)) can be used for the time merge candidate. Specifically, for example, when the fixed storage unit is 16×16 sample units, if the coordinates of the time neighboring block are (xTnb, yTnb), the motion information of the prediction block located at the corrected position ((xTnb>>4)<<4), (yTnb>>4)<<4)) can be used for the time merge candidate. Or, for example, when the fixed storage unit is 8×8 sample units, if the coordinates of the time neighboring block are (xTnb, yTnb), the motion information of the prediction block located at the corrected position ((xTnb>>3)<<3), (yTnb>>3)<<3)) can be used for the time merge candidate.

[0143] Referring to FIG. 14 again, the image encoding device / image decoding device can check whether the number of current merge candidates is smaller than the number of maximum merge candidates (S1130). The number of the maximum merge candidates can be defined in advance or signaled from the image encoding device to the image decoding device. For example, the image encoding device can generate information regarding the number of the maximum merge candidates, encode it, and transmit it to the image decoding device in the form of a bit stream. When all of the number of the maximum merge candidates are satisfied, the subsequent candidate addition process (S1140) can be not performed.

[0144] If, as a result of the check in step S1130, the number of the current merge candidates is smaller than the number of the maximum merge candidates, the image encoding device / image decoding device can induce additional merge candidates based on a predetermined method and then insert them into the merge candidate list (S1140).

[0145] If, as a result of the check in step S1130, the number of current merge candidates is not less than the maximum number of merge candidates, the image encoding device / image decoding device may terminate construction of the merge candidate list. In this case, the image encoding device may select an optimal merge candidate from among the merge candidates constituting the merge candidate list based on the RD cost, and may signal candidate selection information (e.g., a merge index) indicating the selected merge candidate to the image decoding device. The image decoding device may select the optimal merge candidate based on the merge candidate list and the candidate selection information.

[0146] As described above, the motion information of the selected merging candidate may be used as the motion information of the current block, and the predicted sample of the current block may be derived based on the motion information of the current block. The image encoding apparatus may derive residual samples of the current block based on the predicted sample and signal residual information regarding the residual sample to the image decoding apparatus. As described above, the image decoding apparatus may generate reconstructed samples based on the residual samples derived based on the residual information and the predicted sample, and generate a reconstructed picture based on the reconstructed samples.

[0147] When a skip mode is applied to a current block, motion information of the current block can be derived in the same manner as when a merge mode is applied to the previous block, except that when the skip mode is applied, the residual signal for the current block is omitted, and therefore, the predicted samples can be used directly as reconstructed samples.

[0148] When the MVP mode is applied to the current block, a motion vector predictor (MVP) candidate list can be generated using the motion vectors of reconstructed spatial surrounding blocks (e.g., surrounding blocks shown in FIG. 13) and / or motion vectors corresponding to temporal surrounding blocks (or Col blocks). That is, the motion vectors of the reconstructed spatial surrounding blocks and / or motion vectors corresponding to the temporal surrounding blocks can be used as motion vector predictor candidates for the current block. When bi-prediction is applied, an MVP candidate list for deriving L0 motion information and an MVP candidate list for deriving L1 motion information can be generated and used separately. Prediction information (or prediction information) for the current block may include candidate selection information (e.g., an MVP flag or an MVP index) indicating an optimal motion vector predictor candidate selected from the motion vector predictor candidates included in the MVP candidate list. In this case, the prediction unit can select a motion vector predictor for the current block from the motion vector predictor candidates included in the MVP candidate list using the candidate selection information. A prediction unit of an image encoding device can obtain a motion vector differential (MVD) between a motion vector of a current block and a motion vector predictor, encode the obtained MVD, and output the MVD in a bitstream format. That is, the MVD can be obtained by subtracting the motion vector predictor from the motion vector of the current block. A prediction unit of an image decoding device can obtain a motion vector differential included in information related to the prediction, and derive the motion vector of the current block by adding the motion vector differential and the motion vector predictor. The prediction unit of an image decoding device can obtain or derive a reference picture index, etc., indicating a reference picture, from information related to the prediction.

[0149] FIG. 15 is a diagram illustrating a motion vector predictor candidate list construction method according to an example of the present disclosure.

[0150] First, spatial candidate blocks for the current block are searched for and available candidate blocks are inserted into the MVP candidate list (S1210). Then, it is determined whether the MVP candidate list contains less than two MVP candidates (S1220). If there are two, the construction of the MVP candidate list can be completed.

[0151] In step S1220, if there are less than two spatial candidate blocks available, the temporal candidate blocks of the current block can be searched and an available candidate block can be inserted into the MVP candidate list (S1230). If no temporal candidate block is available, the construction of the MVP candidate list can be completed by inserting a zero motion vector into the MVP candidate list (S1240).

[0152] On the other hand, when the MVP mode is applied, the reference picture index may be explicitly signaled. In this case, a picture index (refidxL0) for L0 prediction and a reference picture index (refidxL1) for L1 prediction may be separately signaled. For example, when the MVP mode is applied and bi-prediction (BI prediction) is applied, information about refidxL0 and information about refidxL1 may both be signaled.

[0153] As described above, when the MVP mode is applied, information about the MVD derived from the image coding apparatus can be signaled to the image decoding apparatus. The information about the MVD can include, for example, information indicating x and y components of the MVD absolute value and sign. In this case, information indicating whether the MVD absolute value is greater than 0, whether it is greater than 1, and the rest of the MVD can be signaled in stages. For example, information indicating whether the MVD absolute value is greater than 1 can be signaled only when the value of flag information indicating whether the MVD absolute value is greater than 0 is 1.

[0154] FIG. 16 is a diagram illustrating a syntax structure for transmitting an MVD from an image encoding device to an image decoding device according to an example of the present disclosure.

[0155] In Figure 16, abs_mvd_greater0_flag[0] indicates whether the absolute value of the x component of MVD is greater than 0, and abs_mvd_greater0_flag[1] indicates whether the absolute value of the y component of MVD is greater than 0. Similarly, abs_mvd_greater1_flag[0] indicates whether the absolute value of the x component of MVD is greater than 1, and abs_mvd_greater1_flag[1] indicates whether the absolute value of the y component of MVD is greater than 1. As shown in Figure 16, abs_mvd_greater1_flag can be transmitted only when abs_mvd_greater0_flag is 1. In Figure 16, abs_mvd_minus2 indicates the absolute value of MVD minus 2, and mvd_sign_flag indicates whether the sign of MVD is positive or negative. Using the syntax structure shown in FIG. 16, the MVD can be derived as follows:

[0156] [Number 1]

[0157] MVD[compIdx]=abs_mvd_greater0_flag[compIdx]*abs_mvd_minus2[compIdx]+2*1-2*mvd_sign_flag[compIdx]

[0158] Meanwhile, an MVD (MVDL0) for L0 prediction and an MVD (MVDL1) for L1 prediction may be separately signaled, and information about the MVD may include information about MVDL0 and / or information about MVDL1. For example, when MVP mode and BI prediction are applied to the current block, information about MVDL0 and information about MVDL1 may both be signaled.

[0159] Overview of IBC (Intra Block Copy) prediction

[0160] The IBC prediction according to this disclosure will be described below.

[0161] IBC prediction may be performed in a prediction unit of an image encoding device / image decoding device. IBC prediction may be simply referred to as "IBC." The IBC may be used for content image / video coding, such as games, such as screen content coding (SCC). The IBC basically performs prediction within a current picture, but may be similar to inter prediction in that a reference block is derived within the current picture. That is, the IBC may use at least one of the inter prediction techniques described in this disclosure. For example, the IBC may use at least one of the motion information (motion vector) derivation methods described above. At least one of the inter prediction techniques may be partially modified to take the IBC prediction into consideration. The IBC may refer to the current picture. Therefore, it may also be referred to as current picture referencing (CPR).

[0162] For IBC, an image coding apparatus may derive an optimal block vector (or motion vector) for a current block (e.g., a CU) by performing block matching (BM). The derived block vector (or motion vector) may be signaled to an image decoding apparatus via a bitstream using a method similar to the motion information (motion vector) signaling method in inter prediction described above. The image decoding apparatus may derive a reference block for the current block in the current picture via the signaled block vector (motion vector), thereby deriving a prediction signal (predicted block or predicted sample) for the current block. Herein, the block vector (or motion vector) may indicate a displacement from the current block to a reference block located in an already reconstructed area within the current picture. Therefore, the block vector (or motion vector) may also be referred to as a displacement vector. Hereinafter, the motion vector in IBC may correspond to the block vector or the displacement vector. The motion vector of the current block may include a motion vector for a luma component (luma motion vector) or a motion vector for a chroma component (chroma motion vector). For example, luma motion vectors for IBC-coded CUs may also be in integer sample units (i.e., integer precision). Chroma motion vectors may also be clipped in integer sample units. As described above, IBC can use at least one of inter-prediction techniques, and for example, luma motion vectors may be encoded / decoded using the merge mode or MVP mode described above.

[0163] When the merge mode is applied to the luma IBC block, the merge candidate list for the luma IBC block may be configured similarly to the merge candidate list in the inter mode described with reference to Figure 14. However, in the case of the luma IBC block, the temporal surrounding blocks may not be used as merge candidates.

[0164] When the MVP mode is applied to a luma IBC block, the MVP candidate list for the luma IBC block may be configured in the same manner as the MVP candidate list in the inter mode described with reference to Figure 15. However, in the case of a luma IBC block, a temporal candidate block may not be used as an MVP candidate.

[0165] IBC derives reference blocks from already reconstructed regions in the current picture. In this case, to reduce memory consumption and the complexity of the image decoding device, only predefined areas among the already reconstructed regions in the current picture can be referenced. The predefined areas can include the current CTU containing the current block. By limiting the referenceable reconstructed regions to predefined areas, the IBC mode can be implemented in hardware using local on-chip memory.

[0166] An image coding device that implements IBC can search the previously defined region to determine the reference block with the smallest RD cost and derive a motion vector (block vector) based on the positions of the reference block and the current block.

[0167] Whether IBC is applied to the current block can be signaled as IBC execution information at the CU level. Information regarding the signaling method of the motion vector of the current block (IBC MVP mode or IBC skip / merge mode) can be signaled. The IBC execution information can be used to determine the prediction mode of the current block. Therefore, the IBC execution information can be included in information regarding the prediction mode of the current block.

[0168] In IBC skip / merge mode, a merge candidate index may be signaled to indicate a block vector used to predict the current luma block among block vectors included in the merge candidate list. In this case, the merge candidate list may include neighboring blocks coded using IBC. The merge candidate list may include spatial merge candidates but may not include temporal merge candidates. The merge candidate list may also include history-based motion vector predictor (HMVP) candidates and / or pairwise candidates.

[0169] In the IBC MVP mode, block vector differential values ​​can be coded in the same manner as the motion vector differential values ​​in the inter mode. As with the MVP mode in inter mode, the block vector prediction method can be performed by constructing an MVP candidate list including two candidates as predictors. One of the two candidates can be derived from the left-side neighboring block, and the other can be derived from the upper-side neighboring block. In this case, a candidate can be derived from the left-side or upper-side neighboring block only if the neighboring block is coded in IBC. If the left-side or upper-side neighboring block is unavailable, for example, if the block is not coded in IBC, a default block vector can be included in the MVP candidate list as a predictor. Similarly to the MVP mode in inter mode, information (e.g., a flag) indicating one of the two block vector predictors is signaled and used as candidate selection information. The MVP candidate list can include an HMVP candidate and / or a zero motion vector as the default block vector.

[0170] The HMVP candidates may also be called history-based MVP candidates, and MVP candidates, merge candidates, or block vector candidates previously used in encoding / decoding the current block may be stored in the HMVP list as HMVP candidates. Thereafter, if the merge candidate list or MVP candidate list of the current block does not include the maximum number of candidates, the candidates stored in the HMVP list may be added to the merge candidate list or MVP candidate list of the current block as HMVP candidates.

[0171] The pairwise candidate refers to a candidate derived by selecting two candidates from among the candidates already included in the merge candidate list of the current block according to a predetermined order and averaging the two selected candidates.

[0172] FIG. 17 is a flowchart illustrating an IBC-based video / image coding method.

[0173] FIG. 18 is a diagram illustrating an exemplary configuration of a prediction unit that performs the IBC-based video / image encoding method according to the present disclosure.

[0174] The encoding method of FIG. 17 may be performed by the image encoding apparatus of FIG. 2. Specifically, step S1410 may be performed by a prediction unit, and step S1420 may be performed by a residual processing unit. Specifically, step S1420 may be performed by a subtraction unit 115. Step S1430 may be performed by an entropy encoding unit 190. The prediction information of step S1430 may be derived by the prediction unit, and the residual information of step S1430 may be derived by a residual processing unit. The residual information is information about the residual sample. The residual information may include information about quantized transform coefficients for the residual sample. As described above, the residual sample may be derived as a transform coefficient via a transform unit 120 of the image encoding apparatus, and the transform coefficient may be derived as a quantized transform coefficient via a quantization unit 130. Information about the quantized transform coefficient may be coded by the entropy encoding unit 190 through a residual coding procedure.

[0175] An image encoding apparatus may perform IBC prediction (prediction based on IBC) on a current block (S1410). The image encoding apparatus may derive a prediction mode and a motion vector (block vector) of the current block and generate a predicted sample of the current block. The prediction mode may include at least one of the inter prediction modes described above. Here, the prediction mode determination, motion vector derivation, and predicted sample generation procedures may be performed simultaneously, or one of the procedures may be performed first. For example, as shown in FIG. 18, a prediction unit of an image encoding apparatus performing an IBC-based video / image encoding method may include a prediction mode determination unit, a motion vector derivation unit, and a predicted sample derivation unit. The prediction mode determination unit may determine a prediction mode for the current block, the motion vector derivation unit may derive a motion vector of the current block, and the predicted sample derivation unit may derive a predicted sample of the current block. For example, a prediction unit of an image encoding device may search for a block similar to the current block within a reconstructed region of a current picture (or a certain region (search region) within the reconstructed region) and derive a reference block whose difference from the current block is minimum or equal to or less than a certain criterion. The image encoding device may derive a motion vector based on a displacement difference between the reference block and the current block. The image encoding device may determine a mode to be applied to the current block from various prediction modes. The image encoding device may compare rate-distortion costs (RD costs) for the various prediction modes and determine an optimal prediction mode for the current block. However, the method by which the image encoding device determines a prediction mode for the current block is not limited to the above example, and various methods may be used.

[0176] For example, when a skip mode or a merge mode is applied to a current block, the image encoding apparatus may derive merge candidates from neighboring blocks of the current block and construct a merge candidate list using the derived merge candidates. Furthermore, the image encoding apparatus may derive a reference block whose difference from the current block is minimum or equal to or less than a certain criterion among reference blocks indicated by merge candidates included in the merge candidate list. In this case, a merge candidate associated with the derived reference block may be selected, and merge index information indicating the selected merge candidate may be generated and signaled to the image decoding apparatus. A motion vector for the current block may be derived using the motion vector of the selected merge candidate.

[0177] As another example, when the MVP mode is applied to the current block, the image encoding apparatus may derive motion vector predictor (MVP) candidates from neighboring blocks of the current block and construct an MVP candidate list using the induced MVP candidates. The image encoding apparatus may also use a motion vector of an MVP candidate selected from the MVP candidates included in the MVP candidate list as the MVP of the current block. In this case, for example, a motion vector pointing to a reference block derived by the motion estimation described above may be used as the motion vector of the current block, and the MVP candidate having the smallest difference from the motion vector of the current block may be the selected MVP candidate. A motion vector difference (MVD), which is the difference obtained by subtracting the MVP from the motion vector of the current block, may be derived. In this case, index information pointing to the selected MVP candidate and information regarding the MVD may be signaled to the image decoding apparatus.

[0178] The image encoding apparatus may derive residual samples based on the predicted samples (S1420). The image encoding apparatus may derive the residual samples by comparing the original samples of the current block with the predicted samples. For example, the residual samples may be derived by subtracting corresponding predicted samples from the original samples.

[0179] The image encoding apparatus may encode image information including prediction information and residual information (S1430). The image encoding apparatus may output the encoded image information in a bitstream format. The prediction information may include prediction mode information (e.g., a skip flag, a merge flag, or a mode index) and information about a motion vector as information related to the prediction procedure. Among the prediction mode information, the skip flag is information indicating whether a skip mode is applied to a current block, and the merge flag is information indicating whether a merge mode is applied to the current block. Alternatively, the prediction mode information may be information indicating one of a plurality of prediction modes, such as a mode index. If the skip flag and the merge flag are both 0, it may be determined that the MVP mode is applied to the current block. The information about the motion vector may include candidate selection information (e.g., a merge index, an MVP flag, or an MVP index) that is information for deriving a motion vector. The merge index among the candidate selection information may be signaled when a merge mode is applied to the current block, and may be information for selecting one of merge candidates included in a merge candidate list. The MVP flag or MVP index of the candidate selection information may be signaled when the MVP mode is applied to the current block, and may be information for selecting one of the MVP candidates included in the MVP candidate list. Furthermore, the information about the motion vector may include information about the above-mentioned MVD. Furthermore, the information about the motion vector may include information indicating whether L0 prediction, L1 prediction, or bi-prediction is applied. The residual information is information about the residual sample. The residual information may include information about quantized transform coefficients for the residual sample.

[0180] The output bitstream can be stored in a (digital) storage medium and transmitted to the image decoding device, or can be transmitted to the image decoding device via a network.

[0181] Meanwhile, as described above, the image coding apparatus can generate a reconstructed picture (a picture including reconstructed samples and reconstructed blocks) based on the reference samples and the residual samples. This is because the image coding apparatus derives the same prediction result as that performed by the image decoding apparatus, thereby improving coding efficiency. Therefore, the image coding apparatus can store the reconstructed picture (or reconstructed samples, reconstructed blocks) in a memory and use it as a reference picture for inter prediction. As described above, an in-loop filtering procedure can be further applied to the reconstructed picture.

[0182] FIG. 19 is a flowchart illustrating an IBC-based video / image decoding method.

[0183] FIG. 20 is a diagram illustrating an example of the configuration of a prediction unit that performs the IBC-based video / image decoding method according to the present disclosure.

[0184] The image decoding apparatus may perform operations corresponding to those performed by the image encoding apparatus, such as performing IBC prediction on the current block based on the received prediction information, and deriving predicted samples.

[0185] The decoding method of FIG. 19 may be performed by the image decoding apparatus of FIG. 3. Steps S1610 to S1630 may be performed by a prediction unit, and the prediction information of step S1610 and the residual information of step S1640 may be obtained from a bitstream by the entropy decoding unit 210. The residual processing unit of the image decoding apparatus may derive residual samples for the current block based on the residual information (S1640). Specifically, the inverse quantization unit 220 of the residual processing unit may derive transform coefficients by performing inverse quantization on the quantized transform coefficients derived based on the residual information, and the inverse transform unit 230 of the residual processing unit may derive residual samples for the current block by performing inverse transform on the transform coefficients. Step S1650 may be performed by the adder 235 or a reconstruction unit.

[0186] Specifically, the image decoding apparatus may determine a prediction mode for the current block based on received prediction information (S1610). The image decoding apparatus may determine which prediction mode is applied to the current block based on prediction mode information in the prediction information.

[0187] For example, it may determine whether the skip mode is applied to the current block based on the skip flag. Also, it may determine whether the merge mode or the MVP mode is applied to the current block based on the merge flag. Or, it may select one of various candidate prediction modes based on the mode index. The candidate prediction modes may include skip mode, merge mode, and / or MVP mode, or may include the various inter prediction modes described above.

[0188] The image decoding apparatus may derive a motion vector of the current block based on the determined prediction mode (S1620). For example, when a skip mode or a merge mode is applied to the current block, the image decoding apparatus may construct the merge candidate list described above and select one of the merge candidates included in the merge candidate list. The selection may be performed based on the candidate selection information (merge index) described above. The motion vector of the selected merge candidate may be used to derive a motion vector of the current block. For example, the motion vector of the selected merge candidate may be used as the motion vector of the current block.

[0189] As another example, when the MVP mode is applied to the current block, the image decoding apparatus may construct an MVP candidate list and use a motion vector of an MVP candidate selected from the MVP candidates included in the MVP candidate list as the MVP of the current block. The selection may be made based on the candidate selection information (MVP flag or MVP index). In this case, the MVD of the current block may be derived based on information about the MVD, and the motion vector of the current block may be derived based on the MVP of the current block and the MVD.

[0190] The image decoding apparatus may generate prediction samples for the current block based on the motion vector of the current block (S1630). The prediction samples of the current block may be derived using samples of a reference block to which the motion vector of the current block points in the current picture. Depending on the case, a prediction sample filtering procedure may further be performed on all or some of the prediction samples of the current block.

[0191] For example, as shown in Figure 20, a prediction unit of an image decoding device that performs an IBC-based video / image decoding method may include a prediction mode determination unit, a motion vector derivation unit, and a prediction sample derivation unit. The prediction unit of the image decoding device may determine a prediction mode for the current block based on received prediction mode information in a prediction mode determination unit, derive a motion vector for the current block based on received information about the motion vector in a motion vector derivation unit, and derive a prediction sample for the current block in a prediction sample derivation unit.

[0192] The image decoding apparatus may generate residual samples for the current block based on the received residual information (S1640). The image decoding apparatus may generate reconstructed samples for the current block based on the predicted samples and the residual samples, and generate a reconstructed picture based on the reconstructed samples (S1650). Thereafter, an in-loop filtering procedure may be further applied to the reconstructed picture, as described above.

[0193] As described above, one unit (e.g., a coding unit CU) may include a luma block (luma CB) and a chroma block (chroma CB). In this case, the luma block and its corresponding chroma block may have the same motion information (e.g., motion vector) or different motion information. For example, the motion information of the chroma block may be derived based on the motion information of the luma block, so that the luma block and its corresponding chroma block may have the same motion information.

[0194] Chroma Format Overview

[0195] The following describes a chroma format. An image can be encoded with encoding data including a luma component (e.g., Y) array and two chroma component (e.g., Cb, Cr) arrays. For example, one pixel of the encoded image can include a luma sample and a chroma sample. A chroma format can be used to indicate the configuration format of the luma sample and the chroma sample, and the chroma format can also be called a color format.

[0196] In one embodiment, an image can be encoded in various chroma formats, such as monochrome, 4:2:0, 4:2:2, and 4:4:4. In monochrome sampling, there can be one sample array, which can be a luma array. In 4:2:0 sampling, there can be one luma sample array and two chroma sample arrays, each of which can be half the height and half the width of the luma array. In 4:2:2 sampling, there can be one luma sample array and two chroma sample arrays, each of which can be the same height and half the width of the luma array. In 4:4:4 sampling, there can be one luma sample array and two chroma sample arrays, each of which can be the same height and width as the luma array.

[0197] For example, in the case of 4:2:0 sampling, chroma samples can be located at the bottom of the corresponding luma samples. In the case of 4:2:2 sampling, chroma samples can be located overlapping the corresponding luma samples. In the case of 4:4:4 sampling, both luma and chroma samples can be located overlapping.

[0198] The chroma format used in the encoding device and the decoding device may be predetermined. Alternatively, the chroma format may be signaled from the encoding device to the decoding device for adaptive use in the encoding device and the decoding device. In one embodiment, the chroma format may be signaled based on at least one of chroma_format_idc and separate_colour_plane_flag. At least one of chroma_format_idc and separate_colour_plane_flag may be signaled via a higher-level syntax such as DPS, VPS, SPS, or PPS. For example, chroma_format_idc and separate_colour_plane_flag may be included in the SPS syntax as shown in FIG. 21.

[0199] Meanwhile, Figure 22 shows an example of chroma format classification using signaling of chroma_format_idc and separate_colour_plane_flag. chroma_format_idc may be information indicating the chroma format applied to the coded image. separate_colour_plane_flag may indicate whether the color array is processed separately in a specific chroma format. For example, a first value (e.g., 0) of chroma_format_idc may indicate monochrome sampling. A second value (e.g., 1) of chroma_format_idc may indicate 4:2:0 sampling. A third value (e.g., 2) of chroma_format_idc may indicate 4:2:2 sampling. A fourth value (e.g., 3) of chroma_format_idc may indicate 4:4:4 sampling.

[0200] In 4:4:4 sampling, the following applies depending on the value of separate_colour_plane_flag: If the value of separate_colour_plane_flag is the first value (e.g., 0), each of the two chroma arrays can have the same height and width as the luma array. In this case, the value of ChromaArrayType, which indicates the type of chroma sample array, can be set to the same as chroma_format_idc. If the value of separate_colour_plane_flag is the second value (e.g., 1), the luma, Cb, and Cr sample arrays can be processed separately, so that they can each be processed like a monochrome sampled picture. In this case, ChromaArrayType can be set to 0.

[0201] Intra prediction for chroma blocks

[0202] When intra prediction is performed on the current block, prediction can be performed on the luma component block (luma block) of the current block and prediction can be performed on the chroma component block (chroma block), in which case the intra prediction mode for the chroma block can be set separately from the intra prediction mode for the luma block.

[0203] For example, the intra-prediction mode for a chroma block may be indicated based on intra-chroma prediction mode information, which may be signaled in the form of an intra_chroma_pred_mode syntax element. For example, the intra-chroma prediction mode information may indicate any one of a planar mode, a DC mode, a vertical mode, a horizontal mode, a derived mode (DM), and a cross-component linear model (CCLM) mode. Here, the planar mode may indicate intra-prediction mode 0, the DC mode may indicate intra-prediction mode 1, the vertical mode may indicate intra-prediction mode 26, and the horizontal mode may indicate intra-prediction mode 10. DM may also be referred to as a direct mode. CCLM may also be referred to as a linear model (LM).

[0204] Meanwhile, DM and CCLM are dependent intra prediction modes that predict a chroma block using information of a luma block. DM may indicate a mode in which the same intra prediction mode as the intra prediction mode for the luma component is applied as the intra prediction mode for the chroma component. CCLM may indicate an intra prediction mode in which, in generating a prediction block for a chroma block, reconstructed samples of the luma block are subsampled, and then CCLM parameters α and β are applied to the subsampled samples to use the derived samples as prediction samples for the chroma block.

[0205]

number

[0206] where pred c (i, j) may represent a predicted sample at the (i, j) coordinate of the current chroma block in the current CU. L'(i,j) may represent a reconstructed sample at the (i,j) coordinate of the current luma block in the CU. For example, L '(i,j) may denote the down-sampled reconstructed samples of the current luma block. The linear model coefficients α and β may be signaled or derived from neighboring samples.

[0207] Virtual Pipeline Data Unit

[0208] For pipeline processing within a picture, virtual pipeline data units (VPDUs) can be defined. A VPDU can be defined as a non-overlapping unit within a picture. In a hardware decoding device, successive VPDUs can be processed simultaneously by multiple pipeline stages. The VPDU size in most pipeline stages is roughly proportional to the buffer size. Therefore, keeping the VPDU size small is important when considering buffer size from a hardware perspective. In most hardware decoding devices, the VPDU size can be set to the same as the maximum TB (transform block) size. For example, the VPDU size can be 64x64 (64x64 luma samples). Furthermore, the VPDU size can be changed (increased or decreased) taking into account the ternary tree (TT) and / or binary tree (BT) partitions described above in VVC.

[0209] On the other hand, in order to maintain the VPDU size at a luma sample size of 64x64, the division of CUs as shown in Fig. 23 can be restricted. More specifically, at least one of the following restrictions can be applied:

[0210] Restriction 1: Ternary tree division (TT) is not allowed for CUs whose width or height is 128 or whose width and height are 128.

[0211] Restriction 2: Horizontal binary tree splitting (BT) is not allowed for CUs of 128 × N (where N is an integer less than or equal to 64 and greater than 0) (e.g., horizontal binary tree splitting is not allowed for CUs with width 128 and height less than 128).

[0212] Restriction 3: Vertical binary tree splitting (BT) is not allowed for CUs of N×128 (where N is an integer less than or equal to 64 and greater than 0) (e.g., for a CU whose height is 128 and whose width is less than 128, vertical binary tree splitting is not allowed).

[0213] Chroma block maximum size limitation issue for pipeline processing

[0214] As described above regarding the partitioning structure and the transform process, a CU can be divided to generate multiple TUs. If the size of a CU is larger than the maximum TU size, the CU can be divided into multiple TUs. This allows transform and / or inverse transform to be performed on each TU. In general, the maximum TU size for a luma block can be set to the maximum available transform size that the encoding device and / or decoding device can perform. Examples of CU and TU division according to one embodiment are shown in Figures 24 to 26.

[0215] 24 shows an example of TUs generated by dividing a luma CU and a chroma CU according to an embodiment. In one embodiment, the maximum size of a luma CU may be 64×64, the maximum usable transform size may be 32×32, and non-square TUs may not be allowed. Thus, the maximum size of a luma component transform block may be 32×32. In such an embodiment, the maximum TU size may be set according to the following formula:

[0216] [Number 3]

[0217] maxTbSize=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / max(SubWidthC,SubHeightC)

[0218] In the above formula, maxTbSize is the maximum size of a transform block (TB), and cIdx may be the color component of the block. cIdx0 may indicate the luma component, 1 may indicate the Cb chroma component, and 2 may indicate the Cr chroma component. MaxTbSizeY is the maximum size of a luma component transform block, SubWidthC is the ratio of the width of a chroma block to the width of a luma block, SubHeightC is the ratio of the height of a chroma block to the height of a luma block, and max(A,B) is a function that returns the larger value of A and B as a result.

[0219] According to the above formula, in the above embodiment, for a luma block, the maximum size of the transform block can be set to the maximum size of the luma component transform block, where the maximum size of the luma component transform block is a value set during encoding and can be signaled from the encoding device to the decoding device via a bitstream.

[0220] In addition, in the above embodiment, the maximum size of the transform blocks of the chroma blocks may be set to a value obtained by dividing the maximum size of the luma component transform blocks by the larger value of SubWidthC and SubHeightC. Here, SubWidthC and SubHeightC may be determined based on chroma_format_idc and separate_colour_plane_flag signaled from the encoding device to the decoding device via a bitstream, as shown in FIG.

[0221] According to the above formula, in the above embodiment, the maximum size of the transform block may be determined to be any one of the minimum width and minimum height that the transform block can have. Accordingly, in the above embodiment, the TU division of the luma block and the chroma block may be performed as shown in Figure 24. For example, as shown in Figure 24, in the case of a chroma block having a 4:2:2 format, the maximum size of the transform block is determined to be 16, so that the chroma CU may be divided into a plurality of transform blocks in a form different from the division form of the luma CU into transform blocks.

[0222] 25 shows an example of TUs generated by dividing luma CUs and chroma CUs according to another embodiment. In one embodiment, the maximum size of a luma CU may be 128×128, the maximum usable transform size may be 64×64, and non-square TUs may not be allowed. Thus, the maximum size of a luma component transform block may be 64×64. In this embodiment, the maximum size of a transform block may be set as follows: In the following equation, min(A,B) may be a function that returns the smaller value of A and B.

[0223] [Number 4]

[0224] maxTbSize=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / min(SubWidthC,SubHeightC)

[0225] Meanwhile, according to the above formula, the larger value of the width or height of the block is applied as the maximum size of the transform block, so that the luma CU and chroma CU can be divided into multiple TUs as in the example of Figure 25.

[0226] 24 and 25, when a chroma CU having a 4:2:2 format is divided into TUs, the division is performed in a manner different from the TU division manner of the corresponding luma CU. However, when encoding / decoding a chroma block by referring to the luma block, as in the DM mode or CCLM mode for predicting the chroma block described above, it is efficient to process the encoding (or decoding) of the corresponding chroma block immediately after encoding (or decoding) the luma block corresponding to the chroma block, in order to reduce delay in pipeline processing and save memory.

[0227] However, in the example of Figure 24, after encoding one luma transform block 2411, two chroma transform blocks 2421 and 2423 must be encoded, which requires additional processing in relation to other color formats (4:4:4 or 4:2:0). Also, in the example of Figure 25, after encoding two luma transform blocks 2511 and 2512, one chroma transform block 2521 must be encoded. As such, when using the 4:2:2 format, the above-mentioned TU division method has a problem in that a luma block and its corresponding chroma block do not match, which requires an additional process to perform pipeline processing or makes pipeline processing impossible.

[0228] Maximum size limit for chroma transform blocks for pipeline processing

[0229] Hereinafter, a method for setting the maximum transform block size for a chroma CU so that the above-described conditions for performing VPDU are met will be described.

[0230] 26 shows an example of TUs generated by dividing a luma CU and a chroma CU according to another embodiment. In this embodiment, the maximum size of a luma CU may be 128×128, the maximum usable transform size may be 64×64, and non-square TU division may be permitted. As a result, the maximum size of a luma component transform block may be 64×64.

[0231] For division of a non-square TU, the maximum size of a transform block can be defined for width and height, respectively, as shown in Figure 26. For example, the maximum size of a transform block can be defined by defining the maximum width (maxTbWidth) of a transform block and the maximum height (maxTbHeight) of a transform block as follows:

[0232] [Number 5]

[0233] maxTbWidth=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / SubWidthC

[0234] [Number 6]

[0235] maxTbHeight=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / SubHeightC

[0236] As in the above-described embodiment, by defining the maximum size of a transform block by width and height, even in the case of a chroma CU having a 4:2:2 format, as in the example of Fig. 26, it is possible to divide the chroma CU into TUs in the same manner as the TU division form of the corresponding luma CU. In this way, by dividing the TUs of a chroma CU to correspond to the TUs of the luma CU, it is possible to process the encoding (or decoding) of the corresponding chroma block immediately after encoding (or decoding) the luma block, thereby reducing delay in pipeline processing.

[0237] Maximum size limit for chroma transform blocks in inter and IBC prediction modes

[0238] The following describes the execution of the inter prediction mode and the IBC prediction mode in which the maximum size limit of the chroma transform block for the above-mentioned chroma pipeline processing is applied. The encoding device and the decoding device can perform inter prediction and IBC prediction by limiting the maximum size of the chroma transform block according to the following description, and their operations can be compatible with each other. Furthermore, the following description of inter prediction can be directly applied to the IBC prediction mode. Accordingly, the inter prediction operation of the decoding device according to one embodiment will be described below.

[0239] A decoding apparatus according to an embodiment may perform inter prediction to generate a luma prediction block predSamplesL having a size of (cbWidth) × (cbHeight) and chroma prediction blocks predSamplesCb and predSamplesCr having a size of (cbWidth / SubWidthC) × (cbHeight / SubHeightC), where cbWidth may be the width of the current CU and cbHeight may be the height of the current CU.

[0240] Then, the decoding device can generate a luma residual block resSamplesL having a size of (cbWidth)×(cbHeight) and chroma residual blocks resSamplesCr and resSamplesCb having a size of (cbWidth / SubWidthC)×(cbHeight / SubHeightC). Finally, the decoding device can generate a reconstructed block using the prediction block and the residual block.

[0241] Hereinafter, a method for limiting the maximum size of a chroma transform block in order for a decoding device to generate a residual block of a CU coded in inter prediction mode according to an embodiment will be described. The decoding device can generate a reconstruction block using the residual block generated in this step.

[0242] In order to generate a residual block of size (nTbW) × (nTbH) for a CU coded in inter prediction mode, a decoding device according to an embodiment may obtain the following information directly from the bitstream or derive it from other information obtained from the bitstream, where nTbW and nTbH may be set to the width cbWidth and height cbHeight of the current CU.

[0243] - Sample position (xTb0, yTb0) indicating the position of the top left sample of the current transform block relative to the position of the top left sample of the current picture

[0244] - parameter nTbW indicating the width of the current transformation block

[0245] - parameter nTbH indicating the height of the current transformation block

[0246] - Parameter cIdx indicating the color component of the current CU

[0247] The decoding device can derive the maximum width maxTbWidth and the maximum height maxTbHeight of the transform block from the input information as follows:

[0248] [Number 7]

[0249] maxTbWidth=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / SubWidthC

[0250] [Number 8]

[0251] maxTbHeight=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / SubHeightC

[0252] Furthermore, the decoding device can derive the upper left sample position (xTbY, yTbY) of the current transform block based on whether the current CU is a luma component or a chroma component as follows:

[0253] [Number 9]

[0254] (xTbY,yTbY)=(cIdx==0)?(xTb0,yTb0):(xTb0*SubWidthC,yTb0*SubHeightC)

[0255] As in the above formula, if the current transform block is a chroma block, the maximum width and height of the transform block and the upper left sample position of the current transform block can be determined based on the chroma format to reflect the size of the chroma block determined by the chroma format of the current transform block.

[0256] Hereinafter, the decoding apparatus may generate a residual block by performing the following procedure. This will be described with reference to FIG. 27. First, the decoding apparatus may determine whether to divide a current transform block (S2710). For example, the decoding apparatus may determine whether to divide the current transform block based on whether the width and height of the current transform block are greater than the width and height of the largest transform block. For example, if nTbW is greater than maxTbWidth or nTbH is greater than maxTbHeight, the decoding apparatus may determine to divide the current transform block to generate lower transform blocks.

[0257] When dividing the current transform block into lower transform blocks, as described above, the decoding device can derive the width newTbW and height newTbH of the lower transform blocks as shown in the following equations (S2720).

[0258] [Number 10]

[0259] newTbW=(nTbW>maxTbWidth)?(nTbW / 2):nTbW

[0260] [Number 11]

[0261] newTbH=(nTbH>maxTbHeight)?(nTbH / 2):nTbH

[0262] Next, the decoding device may generate residual blocks using lower-order transform blocks that divide the current transform block (S2730). In one embodiment, as shown in Figure 26, the current transform block may be a transform block having the width and height of a chroma CU in 4:2:2 format, and the lower-order transform blocks may be a first lower-order transform block 2621 to a fourth lower-order transform block 2624 that divide the current transform block into four non-square blocks.

[0263] First, the decoding apparatus may generate a residual block for the first lower-order transform block. Referring to FIG. 26, the first lower-order transform block 2621 may be specified by a sample position (xTb0, yTb0), a width newTbW of the lower-order transform block, and a height newTbH of the lower-order transform block. The decoding apparatus may generate a residual block for the first lower-order transform block 2621 using the color component cIdx of the current CU. The decoding apparatus may generate a reconstructed picture (a modified reconstructed picture) based on the generated residual block. Thereafter, in-loop filtering may be performed on the reconstructed picture.

[0264] Next, if nTbW is greater than maxTbWidth, the decoding device can generate a residual block for the second lower-order transform block. The second lower-order transform block 2622 can be specified by the sample position (xTb0+newTbW, yTb0), the width of the lower-order transform block newTbW, and the height of the lower-order transform block newTbH. The decoding device can generate the residual block of the second lower-order transform block 2622 using the color component cIdx of the current CU. The decoding device can generate a reconstructed picture (a modified reconstructed picture) based on the generated residual block. In-loop filtering can then be performed on the reconstructed picture.

[0265] Next, if nTbH is greater than maxTbHeight, the decoding device can generate a residual block for the third lower-order transform block. The third lower-order transform block 2623 can be specified by the sample position (xTb0, yTb0+newTbH), the width of the lower-order transform block newTbW, and the height of the lower-order transform block newTbH. As before, the decoding device can generate the residual block using the color component cIdx of the current CU.

[0266] Next, if nTbW is greater than maxTbWidth and nTbH is greater than maxTbHeight, the decoding device can generate a residual block for the fourth lower-order transform block. The fourth lower-order transform block 2624 can be specified by the sample position (xTb0+newTbW, yTb0+newTbH), the width of the lower-order transform block newTbW, and the height of the lower-order transform block newTbH. As before, the decoding device can generate a residual block using the color component cIdx of the current CU.

[0267] Meanwhile, if the current transform block is not split, the decoding apparatus may perform inter prediction as follows. For example, if nTbW is smaller than maxTbWidth and nTbH is smaller than maxTbHeight, the current transform block may not be split. In this case, the decoding apparatus may generate a residual block for the inter prediction mode by performing a scaling and transform process using the sample position (xTbY, xTbY), the color component cIdx of the current CU, the transform block width nTbW, and the transform block height nTbH as input. Based on this, the decoding apparatus may generate a reconstructed picture (a modified reconstructed picture). Thereafter, in-loop filtering may be performed on the reconstructed picture.

[0268] Maximum size limit for chroma transform blocks in intra prediction modes

[0269] The following describes the execution of intra prediction mode in which the maximum size limit of a chroma transform block for the above-mentioned chroma pipeline processing is applied. The encoding device and the decoding device can perform intra prediction by limiting the maximum size of a chroma transform block according to the following description, and the operations of the encoding device and the decoding device correspond to each other, so the operation of the decoding device will be described below.

[0270] A decoding device according to an embodiment can generate a reconstructed picture by performing intra prediction. The reconstructed picture can be subjected to in-loop filtering. To perform intra prediction, the decoding device according to an embodiment can obtain the following information directly from the bitstream or can derive it from other information obtained from the bitstream:

[0271] - Sample position (xTb0, yTb0) indicating the position of the top left sample of the current transform block relative to the position of the top left sample of the current picture

[0272] - parameter nTbW indicating the width of the current transformation block

[0273] - parameter nTbH indicating the height of the current transformation block

[0274] - parameter predModeIntra indicating the intra prediction mode of the current CU

[0275] - Parameter cIdx indicating the color component of the current CU

[0276] The decoding device can derive the maximum width maxTbWidth and the maximum height maxTbHeight of the transform block from the input information as follows:

[0277] [Number 12]

[0278] maxTbWidth=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / SubWidthC

[0279] [Number 13]

[0280] maxTbHeight=(cIdx==0)?MaxTbSizeY:MaxTbSizeY / SubHeightC

[0281] Furthermore, the decoding device can derive the upper left sample position (xTbY, yTbY) of the current transform block based on whether the current CU is a luma component or a chroma component as follows:

[0282] [Number 14]

[0283] (xTbY,yTbY)=(cIdx==0)?(xTb0,yTb0):(xTb0*SubWidthC,yTb0*SubHeightC)

[0284] Hereinafter, the decoding apparatus may perform intra prediction by performing the following procedure. This will be described with reference to FIG. 28. First, the decoding apparatus may determine whether to split a current transform block (S2810). For example, the decoding apparatus may determine whether to split the current transform block based on whether the width and height of the current transform block are greater than the width and height of the largest transform block. In addition, the decoding apparatus may determine whether to split the current transform block by further considering whether an ISP (Intra Sub-partition) is applied to the current CU. For example, if nTbW is greater than maxTbWidth or nTbH is greater than maxTbHeight, the decoding apparatus may determine to split the current transform block and perform intra prediction. Even in this case, the decoding apparatus may also determine to split the current transform block and perform intra prediction only if an ISP is not applied to the current CU (e.g., the value of IntraSubpartitionSplitType is NO_ISP_SPLIT, i.e., an ISP is not applied to the current CU).

[0285] When dividing the current transform block into lower transform blocks, the decoding apparatus can derive the width newTbW and height newTbH of the lower transform block according to the following equations (S2820).

[0286] [Number 15]

[0287] newTbW=(nTbW>maxTbWidth)?(nTbW / 2):nTbW

[0288] [Number 16]

[0289] newTbH=(nTbH>maxTbHeight)?(nTbH / 2):nTbH

[0290] This will be described with reference to Figure 26. In one embodiment, the width nTbW of the current transform block may be the width of a chroma CU, and the height nTbH of the current transform block may be the height of the chroma CU. In this embodiment, the width newTbW of the lower transform block and the height newTbH of the lower transform block may be determined as the width and height of the transform block 2621 that divides the chroma CU. That is, in this embodiment, the current transform block may be a transform block having the width and height of a chroma CU in 4:2:2 format, and the lower transform blocks may be the first lower transform block 2621 to the fourth lower transform block 2624 that divide the current transform block into four non-square blocks.

[0291] Next, the decoding device may perform intra prediction using the lower transform block dividing the current transform block (S2830). First, the decoding device may perform intra prediction on the first lower transform block. Referring to FIG. 26, the first lower transform block 2621 may be specified by a sample position (xTb0, yTb0), a width newTbW of the lower transform block, and a height newTbH of the lower transform block. The decoding device may perform intra prediction on the first lower transform block 2621 using the intra prediction mode predModeIntra of the current CU and a color component cIdx of the current CU. As a result, a modified reconstructed picture for the first lower transform block 2621 may be generated.

[0292] For example, the decoding device may generate a prediction sample matrix predSamples of size (newTbW) × (newTbH) by performing an intra sample prediction process. For example, the decoding device may perform the intra sample prediction process using the sample position (xTb0, yTb0), the intra prediction mode predModeIntra, the transform block width (nTbW)newTbW, the transform block height (nTbH)newTbH, the coding block width (nCbW)nTbW, the coding block height (nCbH)nTbH, and the value of the parameter cIdx.

[0293] The decoding device may also perform a scaling and transformation process to generate a residual sample matrix resSamples of size (newTbW) × (newTbH). For example, the decoding device may perform the scaling and transformation process based on the sample position (xTb0, yTb0), the value of the parameter cIdx, the transform block width (nTbW)newTbW, and the transform block height (nTbH)newTbH.

[0294] Furthermore, the decoding device can generate a reconstructed picture by performing a picture reconstruction process for a color component. For example, the decoding device can perform the picture reconstruction process for the color component by setting the transform block position to (xTb0, yTb0), the transform block width (nTbW) to newTbW, the transform block height (nTbH) to newTbH, and using the value of the parameter cIdx and a prediction sample matrix predSamples of size (newTbW) × (newTbH) and a residual sample matrix resSamples of size (newTbW) × (newTbH).

[0295] Next, if nTbW is greater than maxTbWidth, the decoding device may perform intra prediction on the second lower-order transform block. The second lower-order transform block 2622 may be specified by the sample position (xTb0+newTbW, yTb0), the width of the lower-order transform block newTbW, and the height of the lower-order transform block newTbH. The decoding device may perform intra prediction on the second lower-order transform block 2622 using the intra prediction mode predModeIntra of the current CU and the color component cIdx of the current CU. The intra prediction of the second lower-order transform block 2622 may be performed in the same manner as the intra prediction of the first lower-order transform block 2621 for the corresponding sample position. As a result, a reconstructed picture (a modified reconstructed picture) for the second lower-order transform block 2622 may be generated.

[0296] Next, if nTbH is greater than maxTbHeight, the decoding device can perform intra prediction on the third lower-order transform block. The third lower-order transform block 2623 can be specified by the sample position (xTb0, yTb0+newTbH), the width newTbW of the lower-order transform block, and the height newTbH of the lower-order transform block. As described above, the decoding device can perform intra prediction using the intra prediction mode predModeIntra of the current CU and the color component cIdx of the current CU.

[0297] Next, if nTbW is greater than maxTbWidth and nTbH is greater than maxTbHeight, the decoding device can perform intra prediction on the fourth lower-order transform block. The fourth lower-order transform block 2624 can be specified by the sample position (xTb0+newTbW, yTb0+newTbH), the width of the lower-order transform block newTbW, and the height of the lower-order transform block newTbH. As before, the decoding device can perform intra prediction using the intra prediction mode predModeIntra of the current CU and the color component cIdx of the current CU.

[0298] Meanwhile, if the current transform block is not split, the decoding device may perform intra prediction as follows: For example, if nTbW is smaller than maxTbWidth and nTbH is smaller than maxTbHeight, or if ISP is applied to the current CU (for example, the value of IntraSubpartitionSplitType is not NO_ISP_SPLIT), the current transform block may not be split.

[0299] First, the decoding device can derive the parameters nW, nH, numPartsX, and numPartsY as follows:

[0300] [Number 17]

[0301] nW=IntraSubPartitionsSplitType==ISP_VER_SPLIT?nTbW / NumIntraSubPartitions:nTbW

[0302] nH=IntraSubPartitionsSplitType==ISP_HOR_SPLIT?nTbH / NumIntraSubPartitions:nTbH

[0303] numPartsX=IntraSubPartitionsSplitType==ISP_VER_SPLIT?NumIntraSubPartitions:1

[0304] numPartsY=IntraSubPartitionsSplitType==ISP_HOR_SPLIT?NumIntraSubPartitions:1

[0305] In the above formula, IntraSubPartitionsSplitType indicates the ISP split type of the current CU, ISP_VER_SPLIT indicates vertical ISP split, ISP_HOR_SPLIT indicates horizontal ISP split, and NumIntraSubPartitions indicates the number of ISP subpartitions.

[0306] Next, the decoding device can generate a prediction sample matrix predSamples of size (nTbW) × (nTbH) by performing an intra sample prediction process. For example, the decoding device can perform the intra sample prediction process using the sample position (xTb0 + nW * xPartIdx, yTb0 + nH * yPartIdx), the intra prediction mode predModeIntra, the width (nTbW)nW of the transform block, the height (nTbH)nH of the transform block, the width (nCbW)nTbW of the coding block, the height (nCbH)nTbH of the coding block, and the value of the parameter cIdx. Here, the value of the partition index xPartIdx can range from 0 to numPartX-1, and the value of yPartIdx can range from 0 to numPartsY-1.

[0307] Then, the decoding device can perform a scaling and transformation process to generate a residual sample matrix resSamples of size (nTbW) × (nTbH). For example, the decoding device can perform the scaling and transformation process based on the sample positions (xTbY+nW*xPartIdx, yTbY+nH*yPartIdx), the value of the parameter cIdx, the width (nTbW)nW of the transform block, and the height (nTbH)nH of the transform block.

[0308] Next, the decoding device can generate a reconstructed picture by performing a picture reconstruction process for the color component. For example, the decoding device can perform the picture reconstruction process for the color component by setting the transform block position to (xTb0+nW*xPartIdx, yTb0+nH*yPartIdx), setting the transform block width (nTbW) to nW, setting the transform block height (nTbH) to nH, using a predetermined value of cIdx, and using a prediction sample matrix predSamples of size (nTbW)×(nTbH) and a residual sample matrix resSamples of size (nTbW)×(nTbH).

[0309] Encoding method

[0310] Hereinafter, a method for encoding by an encoding device according to an embodiment using the above-described method will be described with reference to Figure 29. The encoding device according to an embodiment includes a memory and at least one processor, and the at least one processor can perform the following encoding method.

[0311] First, the encoding device may divide an image to determine a current block (S2910). Next, the encoding device may generate an inter-predicted block of the current block (S2920). Next, the encoding device may generate a residual block of the current block based on the inter-predicted block (S2930). Next, the encoding device may encode inter-prediction mode information of the current block (S2940). In this case, the residual block is encoded based on the size of a transform block of the current block, and the size of the transform block may be determined based on the color components of the current block.

[0312] More specifically, the position of the upper left sample of the transform block may be determined based on the position and color format of the upper left sample of the luma block corresponding to the current block.

[0313] Furthermore, when the transform block is divided into a plurality of lower transform blocks, the upper left position of the lower transform block may be determined based on the maximum width and height of the transform block. For example, the maximum width of the transform block may be determined based on the maximum size and color format of the transform block of the luma block corresponding to the current block, and the maximum height of the transform block may be determined based on the maximum size and color format of the transform block of the luma block corresponding to the current block.

[0314] Furthermore, if the current block is a chroma block and the width of the transform block is greater than the maximum width of the transform block, the current block may be vertically divided to generate a plurality of lower transform blocks. The plurality of lower transform blocks may include a first lower transform block and a second lower transform block, the width of the first lower transform block may be determined to be the maximum width of the transform block, and the upper left coordinate of the second lower transform block may be determined to be a value spaced to the right from the upper left coordinate of the first transform block by the maximum width of the transform block.

[0315] Furthermore, if the current block is a chroma block and the height of the transform block is greater than the maximum height of the transform blocks, the current block may be horizontally divided to generate a plurality of lower transform blocks. The plurality of lower transform blocks may include a third lower transform block and a fourth lower transform block, the height of the third lower transform block may be determined to be the maximum height of the transform blocks, and the upper left coordinate of the fourth lower transform block may be determined to be spaced downward from the upper left coordinate of the first lower transform block by the maximum height of the transform blocks.

[0316] If the color components of the current block are chroma components, the size of the transform block may be determined based on a color format. More specifically, the width of the transform block may be determined based on a maximum width of transform blocks, and the maximum width of the transform block may be determined based on a maximum size and color format of transform blocks of a luma block corresponding to the current block.

[0317] Also, if the color component of the current block is a chroma component, the height of the transform block is determined based on the maximum height of the transform block, and the maximum height of the transform block can be determined based on the maximum size and color format of the transform block of the luma block corresponding to the current block.

[0318] For example, if the color format of the current block is a format indicating that the width of a chroma block is half the width of the corresponding luma block, the maximum size of the transform block of the chroma block can be determined to be 32x64, since the maximum size of the transform block of the luma block is 64x64.

[0319] Decryption method

[0320] Hereinafter, a method for performing decoding by a decoding device according to an embodiment using the above-described method will be described with reference to Figure 30. The decoding device according to an embodiment includes a memory and at least one processor, and the at least one processor can perform the following decoding method.

[0321] First, the decoding apparatus may determine a prediction mode of a current block (S3010). Next, if the prediction mode of the current block is inter prediction mode, the decoding apparatus may generate a prediction block for the current block based on inter prediction mode information (S3020). Next, the decoding apparatus may generate a residual block of the transform block of the current block based on the transform block of the current block (S3030). Next, the decoding apparatus may reconstruct the current block based on the prediction block and the residual block of the current block (S3040). In this case, the size of the transform block may be determined based on the color components of the current block.

[0322] The position of the upper left sample of the transform block may be determined based on the position and color format of the upper left sample of the luma block corresponding to the current block. Also, if the transform block is divided into a plurality of lower transform blocks, the position of the upper left of the lower transform block may be determined based on the maximum width and height of the transform block. For example, the maximum width of the transform block may be determined based on the maximum size and color format of the transform block of the luma block corresponding to the current block, and the maximum height of the transform block may be determined based on the maximum size and color format of the transform block of the luma block corresponding to the current block.

[0323] Furthermore, if the current block is a chroma block and the width of the transform block is greater than the maximum width of the transform block, the current block may be vertically divided to generate a plurality of lower transform blocks. The plurality of lower transform blocks may include a first lower transform block and a second lower transform block, the width of the first lower transform block may be determined to be the maximum width of the transform block, and the upper left coordinate of the second lower transform block may be determined to be a value spaced to the right from the upper left coordinate of the first transform block by the maximum width of the transform block.

[0324] Furthermore, if the block is a chroma block and the height of the transform block is greater than the maximum height of the transform blocks, the current block may be horizontally divided to generate a plurality of lower transform blocks. The plurality of lower transform blocks may include a third lower transform block and a fourth lower transform block, the height of the third lower transform block may be determined to be the maximum height of the transform blocks, and the upper left coordinate of the fourth lower transform block may be determined to be spaced downward from the upper left coordinate of the first lower transform block by the maximum height of the transform blocks.

[0325] If the color components of the current block are chroma components, the size of the transform block may be determined based on a color format. More specifically, the width of the transform block may be determined based on a maximum width of transform blocks, and the maximum width of the transform block may be determined based on a maximum size and color format of transform blocks of a luma block corresponding to the current block.

[0326] Also, if the color component of the current block is a chroma component, the height of the transform block is determined based on the maximum height of the transform block, and the maximum height of the transform block can be determined based on the maximum size and color format of the transform block of the luma block corresponding to the current block.

[0327] For example, if the color format of the current block is a format indicating that the width of a chroma block is half the width of the corresponding luma block, the maximum size of the transform block of the chroma block can be determined to be 32x64, since the maximum size of the transform block of the luma block is 64x64.

[0328] Application example

[0329] 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 the steps may be performed simultaneously or in a different order if necessary. To achieve the method according to the present disclosure, the steps illustrated may include other steps, or some steps may be omitted and the remaining steps may be included, or some steps may be omitted and additional other steps may be included.

[0330] 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 and 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.

[0331] 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.

[0332] 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 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.

[0333] In addition, an image decoding apparatus and an image encoding apparatus to which an embodiment of the present disclosure is applied may be included in a multimedia broadcast transmitting / receiving apparatus, a mobile communication terminal, a home cinema video apparatus, a digital cinema video apparatus, a surveillance camera, a video conversation apparatus, a real-time communication apparatus such as video communication, a mobile streaming apparatus, a storage medium, a camcorder, a video on demand (VoD) service providing apparatus, an over-the-top (OTT) video apparatus, an internet streaming service providing apparatus, a three-dimensional (3D) video apparatus, an image telephone video apparatus, a medical video apparatus, etc., and may be used to process a video signal or a data signal. For example, an over-the-top (OTT) video apparatus 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), etc.

[0334] FIG. 31 is a diagram illustrating a content streaming system to which an embodiment of the present disclosure can be applied.

[0335] As shown in FIG. 31, a content streaming system to which an embodiment of the present disclosure is applied can broadly include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device.

[0336] The encoding server compresses content input from a multimedia input device such as a smartphone, camera, or camcorder 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, or video camera directly generates a bitstream, the encoding server can be omitted.

[0337] The bitstream can 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 can temporarily store the bitstream during the process of transmitting or receiving the bitstream.

[0338] The streaming server transmits multimedia data to a user device based on a user request via a web server, and the web server serves as an intermediary for informing 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, which then transmits the multimedia data to the user. In this case, the content streaming system may include a separate control server, which may control commands and responses between devices in the content streaming system.

[0339] The streaming server may receive content from a media storage and / or an encoding server. For example, when receiving 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 to provide a smooth streaming service.

[0340] Examples of the user device include a mobile phone, a smartphone, a laptop computer, a digital broadcasting terminal, a personal digital assistant (PDA), a portable multimedia player (PMP), a navigation system, a slate PC, a tablet PC, an ultrabook, a wearable device such as a smartwatch, smart glass, a head mounted display (HMD), a digital TV, a desktop computer, and digital signage.

[0341] 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.

[0342] 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 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]

[0343] Embodiments according to the present disclosure can be used to encode / decode images.

Claims

1. An image decoding method performed by an image decoding device, comprising: obtaining prediction information and residual information for a current block from a received bitstream; determining a prediction mode for the current block based on the prediction information; generating an inter-predicted block of the current block based on the determined prediction mode of the current block being an inter-prediction mode; determining whether to divide the transform block for the current block into the one or more lower-order transform blocks based on a comparison between a size of the transform block and a maximum size of the transform block to obtain one or more lower-order transform blocks, wherein the one or more lower-order transform blocks are obtained based on the residual information; generating a residual block of the current block based on the one or more lower-transform blocks; reconstructing the current block based on the inter-predicted block of the current block and the residual block; the maximum size of the transform block includes a maximum width of the transform block and a maximum height of the transform block; The image decoding method, wherein the maximum width and the maximum height are determined separately based on a color format of the current block.

2. An image coding method performed by an image coding device, comprising: determining a current block by dividing the image; determining a prediction mode of the current block; generating an inter-predicted block of the current block based on the determined prediction mode of the current block being an inter-prediction mode; generating a residual block of the current block based on the inter-predicted block; and encoding prediction information for the prediction mode and residual information for the residual block to generate a bitstream, The residual block is coded by determining whether to divide the transform block for the residual block into the one or more lower-order transform blocks based on a comparison between a size of the transform block and a maximum size of the transform block to obtain one or more lower-order transform blocks, and the one or more lower-order transform blocks are coded based on the residual information; the maximum size of the transform block includes a maximum width of the transform block and a maximum height of the transform block; The image coding method, wherein the maximum width and the maximum height are determined separately based on a color format of the current block.

3. 1. A method for transmitting a bitstream, comprising: determining a current block by dividing the image; determining a prediction mode of the current block; generating an inter-predicted block of the current block based on the determined prediction mode of the current block being an inter-prediction mode; generating a residual block of the current block based on the inter-predicted block; generating the bitstream by encoding prediction information for the prediction mode and residual information for the residual block; transmitting the bitstream; The residual block is coded by determining whether to divide the transform block for the residual block into the one or more lower-order transform blocks based on a comparison between a size of the transform block and a maximum size of the transform block to obtain one or more lower-order transform blocks, and the one or more lower-order transform blocks are coded based on the residual information; the maximum size of the transform block includes a maximum width of the transform block and a maximum height of the transform block; The method, wherein the maximum width and the maximum height are determined separately based on a color format of the current block.

Images