Image coding method and apparatus using motion vectors

Abstract

To provide an image decoding method.SOLUTION: A method includes the steps of: receiving image information from a bitstream, the image information including prediction related information including residual information and information on MVD; deriving an inter prediction mode for a current block in a current picture, on the basis of the prediction related information; deriving motion information for the current block on the basis of the inter prediction mode; generating prediction samples based on the motion information; generating residual samples based on the residual information; and generating reconstructed samples of the current picture on the basis of the prediction samples and the residual samples. Symmetric motion vector difference reference indices are derived based on POC differences between the current picture and each of the short-term reference pictures. MVDL0 for L0 prediction is derived based on the information on MVD. MVDL1 for L1 prediction is derived based on MVDL0.SELECTED DRAWING: Figure 22

Description

[Technical field]

[0001] This document relates to a method and apparatus for image coding using motion vectors. [Background technology]

[0002] In recent years, the demand for high-resolution, high-quality images / videos such as 4K or 8K or higher UHD (Ultra High Definition) images / videos is increasing in various fields. As the image / video data has higher resolution and quality, the amount of information or bits to be transmitted increases relatively compared to existing (conventional) image / video data, so that when image data is transmitted using a medium such as an existing wired or wireless broadband line or when image / video data is stored using an existing storage (storage) medium, the transmission cost and storage cost increase.

[0003] In addition, in recent years, interest in and demand for immersive media such as virtual reality (VR), artificial reality (AR) content, and holograms has increased, and the broadcast of images / videos that have different image characteristics from real images, such as game images, has increased.

[0004] Therefore, there is a demand for highly efficient image / video compression technology to effectively compress and transmit, store, and play back high-resolution, high-quality image / video information having the various characteristics described above.

[0005] In addition, inter-prediction in image / video coding may include a procedure for Symmetric Motion Vector Difference (SMVD) reference indexing and / or a procedure for Merge Motion Vector Difference (MMVD), and there are discussions of techniques for performing the above procedures taking into account reference picture markings (e.g., short-term or long-term references). Summary of the Invention [Means for solving the problem]

[0006] According to one embodiment of the present document, a method and apparatus for increasing image / video coding efficiency is provided.

[0007] According to one embodiment of the present document, a method and apparatus for efficient inter prediction in an image / video coding system is provided.

[0008] According to one embodiment of the present document, a method and apparatus are provided for signaling information regarding motion vector differentials in inter prediction.

[0009] According to one embodiment of this document, a method and apparatus are provided for signaling information about L0 motion vector differential and L1 motion vector differential when bi-prediction is applied to a current block.

[0010] According to an embodiment of the present document, a method and apparatus for signaling an SMVD flag is provided.

[0011] According to one embodiment of this document, a prediction procedure may be performed based on the type of reference picture for bi-prediction.

[0012] According to one embodiment of this document, a procedure regarding SMVD reference indexes may be performed based on reference picture markings.

[0013] According to one embodiment of this document, the procedure for SMVD reference indexes may be performed using short-term reference pictures (pictures marked as being used for short-term referencing).

[0014] According to one embodiment of the present document, there is provided a video / image decoding method performed by a decoding device.

[0015] According to one embodiment of the present document, a decoding device for performing video / image decoding is provided.

[0016] According to one embodiment of the present document, there is provided a video / image encoding method performed by an encoding device.

[0017] According to one embodiment of the present document, an encoding device for performing video / image encoding is provided.

[0018] According to one embodiment of the present document, there is provided a computer-readable digital storage medium having stored thereon encoded video / image information generated by the video / image encoding method disclosed in at least one of the embodiments of the present document.

[0019] According to one embodiment of the present document, there is provided a computer-readable digital storage medium having encoded information or encoded video / image information stored thereon, which enables a decoding device to perform the video / image decoding method disclosed in at least one of the embodiments of the present document. Effect of the Invention

[0020] According to this document, it can improve the overall image / video compression efficiency.

[0021] According to this document, information regarding motion vector differentials can be signaled efficiently.

[0022] According to this document, when bi-prediction is applied to the current block, the L1 motion vector differential can be derived efficiently.

[0023] According to this document, the information used to derive the L1 motion vector differential is signaled based on the type of reference picture, thus reducing coding complexity.

[0024] According to an embodiment of this document, efficient inter prediction can be performed by using short-term reference pictures for the derivation of reference picture indexes for SMVD.

[0025] The effects that can be obtained through a specific example of this document are not limited to the effects listed above. For example, there may be various technical effects that a person having ordinary skill in the related art can understand or derive from this document. Thus, the specific effects of this document are not limited to those explicitly described in this document, but may include various effects that can be understood or derived from the technical features of this document. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 illustrates a schematic diagram of an example of a video / image coding system to which embodiments of the present document can be applied. [Diagram 2] FIG. 1 is a diagram illustrating the configuration of a video / image encoding device that can be applied to an embodiment of the present document. [Diagram 3] FIG. 1 is a diagram illustrating the configuration of a video / image decoding device that can be applied to an embodiment of the present document. [Figure 4] FIG. 1 illustrates an example of an inter-prediction based video / image encoding method. [Diagram 5] FIG. 2 illustrates an example of an inter-prediction based video / image decoding method. [Figure 6] FIG. 13 is a diagram exemplarily illustrating an inter prediction procedure. [Figure 7] FIG. 2 is a schematic diagram illustrating a method for constructing a merge candidate list according to the present document; [Figure 8] FIG. 1 is a schematic diagram illustrating a method for constructing an MVP candidate list according to the present document. [Figure 9] FIG. 1 is a diagram for explaining SMVD. [Figure 10] FIG. 1 is a diagram illustrating a method for deriving a motion vector in inter prediction. [Figure 11] FIG. 1 is a diagram showing the MVD derivation process of the MMVD according to one embodiment of the present document. [Figure 12] FIG. 13 is a diagram showing the MVD derivation process of the MMVD according to another embodiment of this document. [Figure 13] FIG. 13 is a diagram showing the MVD derivation process of the MMVD according to another embodiment of this document. [Figure 14] FIG. 1 illustrates the MVD derivation process of the MMVD according to one embodiment of this document. [Figure 15] FIG. 1 illustrates the MVD derivation process of the MMVD according to one embodiment of this document. [Figure 16] FIG. 1 illustrates SMVD according to one embodiment of the present document. [Figure 17] 1 is a flow chart illustrating a method for deriving the MMVD according to one embodiment of the present document. [Figure 18] 1 is a flow chart illustrating a method for deriving the MMVD according to one embodiment of the present document. [Figure 19] 1 is a flow chart illustrating a method for deriving the MMVD according to one embodiment of the present document. [Figure 20] FIG. 1 illustrates a schematic diagram of an example of a video / image encoding method and associated components according to an embodiment of the present document. [Figure 21] FIG. 1 illustrates a schematic diagram of an example of a video / image encoding method and associated components according to an embodiment of the present document. [Figure 22] FIG. 1 illustrates a schematic diagram of an example of an image / video decoding method and associated components according to an embodiment of the present document. [Figure 23] FIG. 1 illustrates a schematic diagram of an example of an image / video decoding method and associated components according to an embodiment of the present document. [Figure 24] FIG. 1 illustrates an example of a content streaming system in which the embodiments disclosed herein can be applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Although the disclosure of this document may be modified in various ways and may have various embodiments, a specific embodiment will be illustrated in the drawings and described in detail. However, this is not intended to limit the disclosure to a specific embodiment. The terms used in this document are merely used to describe a specific embodiment, and are not intended to limit the technical ideas of the embodiments in this document. A singular expression includes a plural expression unless the context clearly indicates otherwise. In this document, the terms "include" or "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the document, and should be understood not to preclude the possibility of the presence or addition of one or more different features, numbers, steps, operations, components, parts, or combinations thereof.

[0028] Meanwhile, each configuration in the drawings described in this document is shown independently for the convenience of describing different characteristic functions, and does not mean that each configuration is realized by separate hardware or software. For example, two or more of each configuration may be combined to form one configuration, and one configuration may be divided into multiple configurations. Embodiments in which each configuration is integrated and / or separated are also included in the disclosure scope of this document.

[0029] Hereinafter, the embodiments of the present document will be described with reference to the accompanying drawings. Hereinafter, the same reference numerals may be used for the same components in the drawings, and duplicated descriptions of the same components may be omitted.

[0030] FIG. 1 illustrates a schematic diagram of an example of a video / image coding system in which embodiments of the present document can be applied.

[0031] As shown in Figure 1, a video / image coding system may include a first device (source device) and a second device (receiving device). The source device may transmit encoded video / image information or data to the receiving device via a digital storage medium or a network in a file or streaming form.

[0032] The source device may comprise a video source, an encoding device, and a sending unit. The receiving device may comprise a receiving unit, a decoding device, and a renderer. The encoding device may be referred to as a video / image encoding device, and the decoding device may be referred to as a video / image decoding device. The transmitter may be comprised in the encoding device. The receiver may be comprised in the decoding device. The renderer may comprise a display unit, which may also be configured as a separate device or an external component.

[0033] A video source may acquire video / images through a video / image capture, synthesis, or generation process, etc. A video source may include a video / image capture device and / or a video / image generation device. A video / image capture device may include, for example, one or more cameras, a video / image archive containing previously captured video / images, etc. A video / image generation device may include, for example, a computer, a tablet, a smartphone, etc., and may (electronically) generate video / images. For example, a virtual video / image may be generated through a computer, etc., in which case the video / image capture process may be replaced by a process in which the associated data is generated.

[0034] An encoding device can encode an input video / image. The encoding device can perform a series of steps such as prediction, transformation, quantization, etc. for compression and coding efficiency. The encoded data (encoded video / image information) can be output in the form of a bitstream.

[0035] The transmitting unit may transmit the encoded video / image information or data output in the form of a bitstream to a receiving unit of a receiving device via a digital storage medium or a network in the form of a file or streaming. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmitting unit may include elements for generating a media file through a predetermined file format and may include elements for transmission through a broadcasting / communication network. The receiving unit may receive / extract the bitstream and transmit it to a decoding device.

[0036] The decoding device can decode the video / image by performing a series of steps such as inverse quantization, inverse transformation, prediction, etc., which correspond to the operations of the encoding device.

[0037] The renderer can render the decoded video / images. The rendered video / images can be displayed via a display unit.

[0038] This document relates to video / image coding. For example, the methods / embodiments disclosed in this document may be applied to methods disclosed in the Versatile Video Coding (VVC) standard. Also, the methods / embodiments disclosed in this document may be applied to methods disclosed in the Essential Video Coding (EVC) standard, the AOMedia Video 1 (AV1) standard, the 2nd generation of Audio Video coding Standard (AVS2) or next generation video / image coding standards (e.g., H.267 or H.268).

[0039] In this document, various embodiments relating to video / image coding are presented, which may be combined with each other unless otherwise stated.

[0040] In this document, video may mean a collection of a series of images over time. A picture generally means a unit showing one image at a particular time, and a slice / tile is a unit constituting a part of a picture in coding. A slice / tile may include one or more coding tree units (CTUs). One picture may be composed of one or more slices / tiles. A tile is a rectangular region of CTUs within a particular tile column and a particular tile row in a picture. The tile column is a rectangular region of CTUs having a height equal to the height of the picture and a width specified by syntax elements in the picture parameter set. The tile row is a rectangular region of CTUs having a height specified by syntax elements in the picture parameter set and a width equal to the width of the picture.A tile scan may indicate a specific sequential ordering of CTUs partitioning a picture in which the CTUs are ordered consecutively in CTU raster scan in a tile whereas tiles in a picture are ordered consecutively in a raster scan of the tiles of the picture. A slice includes an integer number of complete tiles or an integer number of consecutive complete CTU rows within a tile of a picture that may be exclusively contained in a single NAL unit.

[0041] On the other hand, a picture can be partitioned into two or more sub-pictures, where a sub-picture can be a rectangular region of one or more slices within a picture.

[0042] A pixel or a pel may refer to the smallest unit that constitutes one picture (or image). A term corresponding to a pixel may be "sample." 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.

[0043] A unit may indicate a basic unit of image processing. A unit may include at least one of a specific region of a picture and information related to the region. A unit may include one luma block and two chroma (e.g., cb, cr) blocks. A unit may be mixed with terms such as block or area in some cases. In a general case, an M×N block may include a set (or array) of samples or transform coefficients consisting of M columns and N rows.

[0044] In this document, "A or B" may mean "only A," "only B," or "both A and B." In other words, in this document, "A or B" may be interpreted as "A and / or B." For example, in this document, "A, B or C" may mean "only A," "only B," "only C," or "any combination of A, B and C."

[0045] As used in this document, a slash ( / ) or a comma can mean "and / or." For example, "A / B" can mean "A and / or B." Thus, "A / B" can mean "only A," "only B," or "both A and B." For example, "A, B, C" can mean "A, B, or C."

[0046] In this document, "at least one of A and B" may mean "only A," "only B," or "both A and B." Also, in this document, the expressions "at least one of A or B" and "at least one of A and / or B" may be interpreted as "at least one of A and B."

[0047] Also, in this document, "at least one of A, B and C" can mean "only A," "only B," "only C," or "any combination of A, B and C." Also, "at least one of A, B or C" and "at least one of A, B and / or C" can mean "at least one of A, B and C."

[0048] Furthermore, parentheses used in this document may mean "for example." Specifically, when "prediction (intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction." In other words, "prediction" in this document is not limited to "intra prediction," and "intra prediction" may be proposed as an example of "prediction." Furthermore, even when "prediction (i.e., intra prediction)" is displayed, "intra prediction" may be proposed as an example of "prediction."

[0049] Technical features described separately in one drawing in this document may be embodied separately or simultaneously.

[0050] 2 is a diagram for illustrating a schematic configuration of a video / image encoding device to which the embodiments of this document can be applied. Hereinafter, the encoding device may include an image encoding device and / or a video encoding device.

[0051] As shown in FIG. 2, the encoding device 200 may be configured to include an image partitioner 210, a predictor 220, a residual processor 230, an entropy encoder 240, an adder 250, a filter 260, and a memory 270. The predictor 220 may include an inter predictor 221 and an intra predictor 222. The residual processor 230 may include a transformer 232, a quantizer 233, a dequantizer 234, and an inverse transformer 235. The residual processor 230 may further include a subtractor 231. The adder 250 may be referred to as a reconstructor or a reconstructed block generator. The above-mentioned image division unit 210, prediction unit 220, residual processing unit 230, entropy encoding unit 240, addition unit 250, and filtering unit 260 may be configured by one or more hardware components (e.g., an encoder chip set or a processor) depending on the embodiment. In addition, the memory 270 may include a DPB (Decoded Picture Buffer) and may be configured by a digital storage medium. The above hardware components may further include the memory 270 as an internal / external component.

[0052] The image division unit 210 may divide an input image (or picture, frame) input to the encoding device 200 into one or more processing units. As an example, the processing unit may be called a coding unit (CU). In this case, the coding unit may be recursively divided from a coding tree unit (CTU) or a largest coding unit (LCU) according to a quad-tree binary-tree ternary-tree (QTBTTT) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad-tree structure, a binary tree structure, and / or a ternary tree structure. In this case, for example, the quad-tree structure may be applied first, and then the binary tree structure and / or the ternary tree structure may be applied. Alternatively, the binary tree structure may be applied first. A coding procedure according to the present disclosure may be performed based on a final coding unit that is not further divided. In this case, the largest coding unit may be used as the final coding unit based on coding efficiency according to image characteristics, or the coding unit may be recursively divided into coding units of lower depths as necessary, and a coding unit of an optimal size may be used as the final coding unit. Here, the coding procedure may include procedures such as prediction, transformation, and restoration, which will be described later. As another example, the processing unit may further include a prediction unit (PU) or a transform unit (TU). In this case, the prediction unit and the transform unit may each be divided or partitioned from the final coding unit described above.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.

[0053] The unit may be mixed with terms such as block or area in some cases. In the general case, an M×N block may refer to a collection of samples or transform coefficients consisting of M columns and N rows. A sample may generally refer to a pixel or a pixel value, may refer to only a pixel / pixel value of a luma component, or may refer to only a pixel / pixel value of a chroma component. A sample may be used as a term corresponding to a pixel or pel of one picture (or image).

[0054] The encoding device 200 may generate a residual signal (residual block, residual sample array) by subtracting a prediction signal (predicted block, prediction sample array) output from the inter prediction unit 221 or the intra prediction unit 222 from an input image signal (original block, original sample array), and the generated residual signal is transmitted to the conversion unit 232. In this case, as shown in the figure, a unit that subtracts a prediction signal (prediction block, prediction sample array) from an input image signal (original block, original sample array) in the encoder 200 may be called a subtraction unit 231. The prediction unit may perform prediction on a block to be processed (hereinafter, referred to as a current block) and generate a predicted block including a prediction sample for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied on a current block or CU basis. The prediction unit may generate various information related to prediction, such as prediction mode information, as will be described later in relation to each prediction mode, and transmit the generated information to the entropy encoding unit 240. The prediction information may be encoded by the entropy encoding unit 240 and output in the form of a bitstream.

[0055] The intra prediction unit 222 may predict the current block by referring to samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located away from the current block depending on the prediction mode. In intra prediction, the prediction mode may include a plurality of non-directional modes and a plurality of directional modes. The non-directional mode may include, for example, a DC mode and a planar mode. The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes depending on the fineness of the prediction direction. However, this is an example, and more or less directional prediction modes may be used depending on the setting. The intra prediction unit 222 may also determine the prediction mode to be applied to the current block using the prediction mode applied to the neighboring block.

[0056] The inter prediction unit 221 may derive a predicted block for the current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on the correlation of the motion information between the neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, BI prediction, etc.) information. In the case of inter prediction, the neighboring blocks may include a spatial neighboring block present in the current picture and a temporal neighboring block present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different. The temporal neighboring block may be called a collocated reference block, a collocated CU (col CU), etc., and the reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, the inter prediction unit 221 may generate information indicating which candidate is used to construct a motion information candidate list based on neighboring blocks and derive a motion vector and / or a reference picture index of the current block. Inter prediction may be performed based on various prediction modes, and for example, in the case of skip mode and merge mode, the inter prediction unit 221 may use the motion information of the neighboring blocks as the motion information of the current block. In the case of skip mode, unlike the merge mode, a residual signal may not be transmitted.In the case of a Motion Vector Prediction (MVP) mode, the motion vector of a neighboring block can be used as a motion vector predictor, and the motion vector of the current block can be indicated by signaling the motion vector difference.

[0057] The prediction unit 220 may generate a prediction signal based on various prediction methods described later. For example, the prediction unit may apply intra prediction or inter prediction for prediction of one block, as well as intra prediction and inter prediction simultaneously. This may be called Combined Inter and Intra Prediction (CIIP). The prediction unit may also be based on an Intra Block Copy (IBC) prediction mode or a palette mode for prediction of a block. The IBC prediction mode or palette mode may be used for content image / video coding such as games, for example, such as Screen Content Coding (SCC). IBC basically performs prediction within a current picture, but may be performed similarly to inter prediction in deriving a reference block within the current picture. That is, IBC may use at least one of the inter prediction techniques described in this document. The palette mode may be considered as an example of intra coding or intra prediction. When the palette mode is applied, a sample value within a picture may be signaled based on information regarding a palette table and a palette index.

[0058] The prediction signal generated through the prediction unit (including the inter prediction unit 221 and / or the intra prediction unit 222) may be used to generate a restored signal or may be used to generate a residual signal. The transform unit 232 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 graph-based transform (GBT), and a conditionally non-linear transform (CNT). Here, the GBT refers to a transform obtained from a graph when the relationship information between pixels is expressed as a graph. The CNT refers to a transform obtained based on a prediction signal generated using all previously reconstructed pixels. In addition, the transform process may be applied to pixel blocks having the same square size, or may be applied to blocks of variable sizes that are not square.

[0059] The quantizer 233 quantizes the transform coefficients and transmits the quantized transform coefficients to the entropy encoder 240, which may encode the quantized signal (information on the quantized transform coefficients) and output the quantized signal to a bitstream. The information on the quantized transform coefficients may be referred to as residual information. The quantizer 233 may rearrange the quantized transform coefficients in a block form into a one-dimensional vector form based on a coefficient scan order, and may generate information on the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector form. The entropy encoder 240 may perform various encoding methods, such as exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC). The entropy encoder 240 may also encode information required for video / image restoration (e.g., values ​​of syntax elements, etc.) together with or separately from the quantized transform coefficients. The encoded information (e.g., encoded video / image information) may be transmitted or stored in the form of a bitstream in Network Abstraction Layer (NAL) units. The video / image information may further include information on various parameter sets, such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). The video / image information may further include general constraint information. In this document, information and / or syntax elements transmitted / signaled from an encoding device to a decoding device may be included in the video / image information. The video / image information may be encoded through the above-mentioned encoding procedure and included in the bitstream.The bitstream may be transmitted over a network or stored in a digital storage medium. Here, the network may include a broadcast network and / or a communication network, and the digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. A transmitting unit (not shown) for transmitting the signal output from the entropy encoding unit 240 and / or a storage unit (not shown) for storing the signal may be configured as an internal / external element of the encoding device 200, or the transmitting unit may be included in the entropy encoding unit 240.

[0060] The quantized transform coefficients output from the quantizer 233 may be used to generate a prediction signal. For example, a residual signal (residual block or residual sample) may be restored by applying inverse quantization and inverse transform to the quantized transform coefficients via the inverse quantizer 234 and the inverse transformer 235. 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 predictor 221 or the intra predictor 222. When there is no residual for the current block, such as when a skip mode is applied, the predicted block may be used as the reconstructed block. The adder 250 may be referred to as a reconstruction unit or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of the next current block in the current picture, and may also be used for inter prediction of the next picture after filtering, as described below.

[0061] Meanwhile, Luma Mapping with Chroma Scaling (LMCS) may be applied during picture encoding and / or restoration.

[0062] The filtering unit 260 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 260 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 270, specifically, in the DPB of the memory 270. The various filtering methods may include, for example, deblock filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, etc. The filtering unit 260 may generate various information related to filtering and transmit the information to the entropy encoding unit 240, as will be described later in relation to each filtering method. The information related to filtering may be encoded by the entropy encoding unit 240 and output in the form of a bitstream.

[0063] The modified reconstructed picture transmitted to the memory 270 may be used as a reference picture in the inter prediction unit 221. When inter prediction is applied through this, the encoding apparatus may avoid a prediction mismatch between the encoding apparatus 100 and the decoding apparatus, and may also improve encoding efficiency.

[0064] The DPB of the memory 270 may store the modified reconstructed picture to be used as a reference picture in the inter prediction unit 221. The memory 270 may store motion information of a block from which motion information in the current picture is derived (or encoded) and / or motion information of a block in an already reconstructed picture. The stored motion information may be transmitted to the inter prediction unit 221 to be utilized as motion information of a spatial neighboring block or motion information of a temporal neighboring block. The memory 270 may store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra prediction unit 222.

[0065] 3 is a diagram for illustrating a schematic configuration of a video / image decoding device to which the embodiments of the present document can be applied. Hereinafter, the decoding device may include an image decoding device and / or a video decoding device.

[0066] As shown in FIG. 3, the decoding device 300 may be configured to include an entropy decoder 310, a residual processor 320, a predictor 330, an adder 340, a filter 350, and a memory 360. The predictor 330 may include an intra predictor 331 and an inter predictor 332. The residual processor 320 may include a dequantizer 321 and an inverse transformer 321. The entropy decoder 310, the residual processor 320, the predictor 330, the adder 340, and the filter 350 may be configured as one hardware component (e.g., a decoder chipset or processor) according to an embodiment. In addition, the memory 360 may include a decoded picture buffer (DPB) and may be configured as a digital storage medium. The above hardware components may further include a memory 360 as an internal / external component.

[0067] When a bitstream including video / image information is input, the decoding device 300 can restore an image corresponding to the process in which the video / image information is processed by the encoding device of FIG. 2. For example, the decoding device 300 can derive a unit / block based on block division related information obtained from the bitstream. The decoding device 300 can perform decoding using a processing unit applied in the encoding device. Thus, the processing unit of the decoding can be, for example, a coding unit, and the coding unit can be divided from a coding tree unit or a maximum coding unit according to a quadtree structure, a binary tree structure, and / or a ternary tree structure. One or more transform units can be derived from the coding unit. Then, the restored image signal decoded and output via the decoding device 300 can be reproduced via a reproduction device.

[0068] The decoding device 300 may receive a signal output from the encoding device of FIG. 2 in the form of a bitstream, and the received signal may be decoded via the entropy decoding unit 310. For example, the entropy decoding unit 310 may derive information (e.g., video / image information) required for image restoration (or picture restoration) by parsing the bitstream. The video / image information may further include information on various parameter sets, such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video / image information may further include general constraint information. The decoding device may further decode pictures based on information on the parameter set and / or the general constraint information. Signaling / received information and / or syntax elements described later in this document may be decoded via the decoding procedure and obtained from the bitstream. For example, the entropy decoding unit 310 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 may receive bins corresponding to each syntax element in the bitstream, determine a context model using syntax element information to be decoded and decode information of adjacent and to-be-decoded blocks or information of symbols / bins decoded in a previous step, predict the occurrence probability of bins according to the determined context model, and perform arithmetic decoding of the bins to generate symbols corresponding to the values ​​of each syntax element. In this case, the CABAC entropy decoding method may update the context model using information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model.Among the information decoded by the entropy decoding unit 310, information related to prediction is provided to a prediction unit (inter prediction unit 332 and intra prediction unit 331), and residual values ​​entropy-decoded by the entropy decoding unit 310, i.e., quantized transform coefficients and related parameter information, may be input to a residual processing unit 320. The residual processing unit 320 may derive a residual signal (residual block, residual sample, residual sample array). Also, among the information decoded by the entropy decoding unit 310, information related to filtering may be provided to a filtering unit 350. Meanwhile, a receiving unit (not shown) for receiving a signal output from the encoding device may be further configured as an internal / external element of the decoding device 300, or the receiving unit may be a component of the entropy decoding unit 310. Meanwhile, the decoding device according to this document may be called a video / image / picture decoding device, and the decoding device may be divided into an information decoder (video / image / picture information decoder) and a sample decoder (video / image / picture sample decoder). The information decoder may include the entropy decoding unit 310, and the sample decoder may include at least one of the inverse quantization unit 321, the inverse transform unit 322, the addition unit 340, the filtering unit 350, the memory 360, the inter prediction unit 332, and the intra prediction unit 331.

[0069] The inverse quantization unit 321 may inverse quantize the quantized transform coefficients and output the transform coefficients. The inverse quantization unit 321 may rearrange the quantized transform coefficients in a two-dimensional block form. In this case, the rearrangement may be performed based on a coefficient scan order performed in the encoding device. The inverse quantization unit 321 may perform inverse quantization on the quantized transform coefficients using a quantization parameter (e.g., quantization step size information) to obtain transform coefficients.

[0070] The inverse transform unit 322 inversely transforms the transform coefficients to obtain a residual signal (residual block, residual sample array).

[0071] The prediction unit may perform prediction on the 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 the prediction information output from the entropy decoding unit 310, and may determine a specific intra / inter prediction mode.

[0072] The prediction unit 330 may generate a prediction signal based on various prediction methods described later. For example, the prediction unit may apply intra prediction or inter prediction for prediction of one block, as well as apply intra prediction and inter prediction simultaneously. This may be called Combined Inter and Intra Prediction (CIIP). The prediction unit may also be based on an Intra Block Copy (IBC) prediction mode or a palette mode for prediction of a block. The IBC prediction mode or palette mode may be used for content image / video coding such as games, for example, Screen Content Coding (SCC). IBC basically performs prediction within a current picture, but may be performed similarly to inter prediction in deriving a reference block within the current picture. That is, IBC may utilize at least one of the inter prediction techniques described in this document. The palette mode may be considered as an example of intra coding or intra prediction. When the palette mode is applied, information regarding a palette table and a palette index may be included in the video / image information and signaled.

[0073] The intra prediction unit 331 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 depending on the prediction mode. In the intra prediction, the prediction mode may include a plurality of non-directional modes and a plurality of directional modes. The intra prediction unit 331 may also determine the prediction mode applied to the current block using the prediction mode applied to the neighboring block.

[0074] The inter prediction unit 332 may derive a predicted block for the current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on the correlation of the motion information between the neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, BI prediction, etc.) information. In the case of inter prediction, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. For example, the inter prediction unit 332 may configure a motion information candidate list based on the neighboring blocks and derive a motion vector and / or a reference picture index for the current block based on the received candidate selection information. Inter prediction may be performed based on various prediction modes, and the information on the prediction may include information indicating a mode of inter prediction for the current block.

[0075] The adder 340 can generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the acquired residual signal to a prediction signal (predicted block, predicted sample array) output from a prediction unit (including an inter prediction unit 332 and / or an intra prediction unit 331). When there is no residual for a current block, such as when a skip mode is applied, the predicted block can be used as a reconstructed block.

[0076] The adder 340 may be referred to as a reconstruction unit or a reconstruction block generator. The generated reconstruction signal may be used for intra prediction of a next block to be processed in the current picture, and may be output after filtering as described below, or may be used for inter prediction of a next picture.

[0077] Meanwhile, Luma Mapping with Chroma Scaling (LMCS) can be applied during the picture decoding process.

[0078] The filtering unit 350 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 350 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may transmit the modified reconstructed picture to the memory 360, specifically, to the DPB of the memory 360. The various filtering methods may include, for example, deblock filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, etc.

[0079] The (modified) reconstructed picture stored in the DPB of the memory 360 can be used as a reference picture in the inter prediction unit 332. The memory 360 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 prediction unit 260 to be utilized as motion information of a spatial neighboring block or motion information of a temporal neighboring block. The memory 360 can store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra prediction unit 331.

[0080] In this specification, the embodiments described for the filtering unit 260, inter prediction unit 221, and intra prediction unit 222 of the encoding device 200 may also be applied identically or correspondingly to the filtering unit 350, inter prediction unit 332, and intra prediction unit 331 of the decoding device 300, respectively.

[0081] As described above, in performing video coding, prediction is performed to improve compression efficiency. Through this, a predicted block including a predicted sample for a current block, which is a block to be coded, can be generated. Here, the predicted block includes a predicted sample in a spatial domain (or a pixel domain). The predicted block is derived in the same manner by an encoding device and a decoding device, and the encoding device can improve image coding efficiency by signaling information (residual information) on a residual between the original block and the predicted block, which is not an original sample value of the original block, to a decoding device. The decoding device can derive a residual block including residual samples based on the residual information, combine the residual block and the predicted block to generate a reconstructed block including reconstructed samples, and generate a reconstructed picture including the reconstructed block.

[0082] The residual information may be generated through a transform and quantization procedure. For example, an encoding apparatus may derive a residual block between the original block and the predicted block, perform a transform procedure on residual samples (residual sample array) included in the residual block to derive transform coefficients, and perform a quantization procedure on the transform coefficients to derive quantized transform coefficients, and then signal the related residual information to a decoding apparatus (through a bitstream). Here, the residual information may include information such as value information, position information, transform technique, transform kernel, and quantization parameter of the quantized transform coefficients. The decoding apparatus may derive a residual sample (or a residual block) by performing an inverse quantization / inverse transform procedure based on the residual information. The decoding apparatus may generate a reconstructed picture based on the predicted block and the residual block. In addition, the encoding apparatus may derive a residual block by inverse quantizing / inverse transforming the quantized transform coefficients for reference for inter-prediction of a future picture, and generate a reconstructed picture based on the residual block.

[0083] In this document, at least one of quantization / dequantization and / or transform / inverse transform can be omitted. When the quantization / dequantization is omitted, the quantized transform coefficients can be called transform coefficients. When the transform / inverse transform is omitted, the transform coefficients can also be called coefficients or residual coefficients, or still be called transform coefficients for uniformity of expression.

[0084] In this document, the quantized transform coefficients and the transform coefficients may be referred to as transform coefficients and scaled transform coefficients, respectively. In this case, the residual information may include information about the transform coefficient(s), and the information about the transform coefficient(s) may be signaled via a residual coding syntax. Transform coefficients may be derived based on the residual information (or information about the transform coefficient(s)), and scaled transform coefficients may be derived via an inverse transform (scaling) on ​​the transform coefficients. Residual samples may be derived based on an inverse transform (transform) on the scaled transform coefficients. This may be similarly applied / expressed in other parts of this document.

[0085] Intra prediction may refer to a prediction that generates a prediction sample for a current block based on a reference sample in a picture to which the current block belongs (hereinafter referred to as a current picture). When intra prediction is applied to a current block, neighboring reference samples used for intra prediction of the current block may be derived. The neighboring reference samples of the current block may include a sample adjacent to the left boundary of the current block having a size of nW×nH and a total of 2×nH samples adjacent to the bottom-left side, a sample adjacent to the top boundary of the current block and a total of 2×nW samples adjacent to the top-right side, and one sample adjacent to the top-left side of the current block. Alternatively, the neighboring reference samples of the current block may include upper neighboring samples of multiple columns and left neighboring samples of multiple rows. In addition, the neighboring reference samples of the current block may also include a total of nH samples adjacent to the right boundary of the current block of size nW×nH, a total of nW samples adjacent to the bottom boundary of the current block, and one sample adjacent to the bottom-right of the current block.

[0086] However, some of the neighboring reference samples of the current block may not yet be decoded or available. In this case, the decoder may substitute the unavailable samples as available samples to construct the neighboring reference samples used for prediction. Alternatively, the decoder may construct the neighboring reference samples used for prediction through interpolation of available samples.

[0087] When a neighboring reference sample is derived, (i) a predicted sample can be derived (induced) based on an average or an interpolation of neighboring reference samples of the current block, or (ii) the predicted sample can be derived based on a reference sample that exists in a specific (prediction) direction with respect to the predicted sample among the neighboring reference samples of the current block. The case (i) can be called a non-directional mode or a non-angular mode, and the case (ii) can be called a directional mode or an angular mode.

[0088] In addition, the prediction sample may be generated by interpolating a first neighboring sample located in a prediction direction of an intra prediction mode of the current block and a second neighboring sample located in an opposite direction to the prediction direction based on the prediction sample of the current block among the neighboring reference samples. The above case may be called Linear Interpolation Intra Prediction (LIP). In addition, a chroma prediction sample may be generated based on a luma sample using a linear model. In this case, it may be called LM mode.

[0089] Also, a temporal prediction sample of the current block may be derived based on the filtered neighboring reference samples, and a prediction sample of the current block may be derived by weighting the temporal prediction sample and at least one reference sample derived according to the intra prediction mode among the existing neighboring reference samples, i.e., non-filtered neighboring reference samples. The above case may be called Position Dependent Intra Prediction (PDPC).

[0090] Also, the reference sample line with the highest prediction accuracy is selected from the adjacent multiple reference sample lines of the current block, a prediction sample is derived using a reference sample located in the prediction direction of the corresponding line, and the reference sample line used at this time is signaled to a decoding device, thereby performing intra prediction encoding. The above case can be called multi-reference line intra prediction or MRL-based intra prediction.

[0091] Also, the current block may be divided into vertical or horizontal sub-partitions, and intra prediction may be performed based on the same intra prediction mode, and adjacent reference samples may be derived and used in units of the sub-partitions. That is, in this case, the intra prediction mode for the current block is applied to the sub-partitions in the same manner, and adjacent reference samples may be derived and used in units of the sub-partitions, thereby improving intra prediction performance in some cases. Such a prediction method may be called ISP (Intra Sub-Partitions) based intra prediction.

[0092] The above-mentioned intra prediction method may be called an intra prediction type, distinguished from the intra prediction mode. The intra prediction type may be called various terms such as an intra prediction technique or an additional intra prediction mode. For example, the intra prediction type (or the additional intra prediction mode, etc.) may include at least one of the above-mentioned LIP, PDPC, MRL, and ISP. A general intra prediction method excluding a specific intra prediction type such as the LIP, PDPC, MRL, and ISP may be called a normal intra prediction type. The normal intra prediction type may be generally applied when the above-mentioned specific intra prediction type is not applied, and prediction may be performed based on the above-mentioned intra prediction mode. Meanwhile, post-processing filtering may be performed on the derived prediction sample, if necessary.

[0093] Specifically, the intra prediction procedure may include an intra prediction mode / type determination step, an adjacent reference sample derivation step, and an intra prediction mode / type-based prediction sample derivation step. If necessary, a post-processing filtering step may be performed on the derived prediction sample.

[0094] When intra prediction is applied, the intra prediction mode applied to the current block may be determined using the intra prediction mode of the neighboring block. For example, the decoding device may select one of the MPM candidates in the MPM (Most Probable Mode) list derived based on the intra prediction mode of the neighboring block (e.g., the left and / or upper neighboring block) of the current block and an additional candidate mode based on the received MPM index, or may select one of the remaining intra prediction modes not included in the MPM candidates (and the planar mode) based on the remaining (remaining) intra prediction mode information. The MPM list may be configured with or without including the planar mode as a candidate. For example, if the MPM list includes the planar mode as a candidate, the MPM list may have six candidates, and if the MPM list does not include the planar mode as a candidate, the MPM list may have five candidates. If the MPM list does not include the planar mode as a candidate, a not planar flag (e.g., intra_luma_not_planar_flag) indicating that the intra prediction mode of the current block is not a planar mode may be signaled. For example, the MPM flag may be signaled first, and the MPM index and the not planar flag may be signaled when the value of the MPM flag is 1. Also, the MPM index may be signaled when the value of the not planar flag is 1. Here, the reason why the MPM list is configured not to include the planar mode as a candidate is to first signal a flag (not planar flag) to first check whether the planar mode is a planar mode, since the planar mode is always considered as an MPM, rather than meaning that the planar mode is not an MPM.

[0095] For example, whether the intra prediction mode applied to the current block is among the MPM candidates (and planar modes) or among the remaining modes may be indicated based on an MPM flag (e.g., intra_luma_mpm_flag). A value of 1 of the MPM flag may indicate that the intra prediction mode for the current block is among the MPM candidates (and planar modes), and a value of 0 of the MPM flag may indicate that the intra prediction mode for the current block is not among the MPM candidates (and planar modes). A value of 0 of the not planar flag (e.g., intra_luma_not_planar_flag) may indicate that the intra prediction mode for the current block is a planar mode, and a value of 1 of the not planar flag may indicate that the intra prediction mode for the current block is not a planar mode. The MPM index may be signaled in the form of an mpm_idx or intra_luma_mpm_idx syntax element, and the remaining intra prediction mode information may be signaled in the form of a rem_intra_luma_pred_mode or intra_luma_mpm_remainder syntax element. For example, the remaining intra prediction mode information may index the remaining intra prediction modes not included in the MPM candidates (and plane modes) among all intra prediction modes in order of prediction mode number and point to one of them. The intra prediction mode is an intra prediction mode for a luma component (sample). Hereinafter, the intra prediction mode information may include at least one of the MPM flag (e.g., intra_luma_mpm_flag), the not planar flag (e.g., intra_luma_not_planar_flag), the MPM index (e.g., mpm_idx or intra_luma_mpm_idx), and the remaining intra prediction mode information (rem_intra_luma_pred_mode or intra_luma_mpm_remainder). In this document, the MPM list may be referred to by various terms such as an MPM candidate list, candModeList, etc.If MIP is applied to the current block, a separate mpm flag (e.g., intra_mip_mpm_flag), mpm index (e.g., intra_mip_mpm_idx), and remaining intra prediction mode information (e.g., intra_mip_mpm_remainder) for MIP may be signaled, and the not planar flag is not signaled.

[0096] That is, in general, when an image is divided into blocks, a current block to be coded and a neighboring block have similar image characteristics. Therefore, the current block and the neighboring blocks are highly likely to have the same or similar intra prediction modes. Therefore, an encoder can use the intra prediction modes of the neighboring blocks to encode the intra prediction mode of the current block.

[0097] For example, the encoder / decoder may configure a Most Probable Modes (MPM) list for the current block. The MPM list may also be referred to as an MPM candidate list. Here, MPM may refer to a mode used to improve coding efficiency by considering similarity between the current block and neighboring blocks during intra prediction mode coding. As described above, the MPM list may be configured to include the planar mode or to exclude the planar mode. For example, if the MPM list includes the planar mode, the number of candidates in the MPM list is six. And, if the MPM list does not include the planar mode, the number of candidates in the MPM list is five.

[0098] The encoder / decoder can construct an MPM list containing 5 or 6 MPMs.

[0099] To construct the MPM list, three types of modes can be considered: default intra modes, neighbor intra modes, and derived intra modes.

[0100] For the neighboring intra mode, two neighboring blocks can be considered: the left neighboring block and the top neighboring block.

[0101] As described above, if the MPM list is configured not to include planar modes, planar modes are excluded from the list and the number of MPM list candidates may be set to five.

[0102] In addition, among the intra prediction modes, the non-directional mode (or non-angular mode) may include a DC mode based on the average of neighboring reference samples of the current block or a planar mode based on interpolation.

[0103] When inter prediction is applied, a prediction unit of an encoding device / decoding device may perform inter prediction on a block-by-block basis to derive a prediction sample. Inter prediction may indicate a 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 a current block, a predicted block (prediction sample array) for the current block may be derived based on a reference block (reference sample array) identified by a motion vector on a reference picture pointed to by an index of the reference picture. In this case, in order 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 of motion information between a neighboring block and the current block. The motion information may include a motion vector and an index of a reference picture. The motion information may further include information on an inter prediction type (L0 prediction, L1 prediction, BI prediction, etc.). When inter prediction is applied, the neighboring block may include a spatial neighboring block present in the current picture and a temporal neighboring block present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different. The temporal neighboring block may be called a collocated reference block, a collocated CU (colCU), etc., and the reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, a candidate list of motion information may be constructed based on the neighboring blocks of the current block, and a flag or index information indicating which candidate is selected (used) to derive the motion vector and / or the index of the reference picture of the current block may be signaled.Inter prediction is performed based on various prediction modes, for example, in the case of skip mode and merge mode, the motion information of the current block may be the same as that of the selected neighboring block. In the case of skip mode, unlike the merge mode, a residual signal may not be transmitted. In the case of a Motion Vector Prediction (MVP) mode, the motion vector of the 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.

[0104] The motion information may include L0 motion information and / or L1 motion information depending on an inter prediction type (such as L0 prediction, L1 prediction, BI prediction, etc.). 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. A prediction based on an L0 motion vector may be referred to as an L0 prediction, a prediction based on an L1 motion vector may be referred to as an L1 prediction, and a prediction based on both the L0 motion vector and the L1 motion vector may be referred to as a bi-prediction. Here, the L0 motion vector may indicate a motion vector associated with a reference picture list L0 (L0), and the L1 motion vector may indicate a motion vector associated with a reference picture list L1 (L1). The reference picture list L0 may include 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 called a forward (reference) picture, and the subsequent picture may be called a backward (reference) picture. The reference picture list L0 may further include a picture subsequent to the current picture in output order as a reference picture. In this case, the previous picture may be indexed first in the reference picture list L0, and the subsequent picture may be indexed thereafter. The reference picture list L1 may further include a picture subsequent to the current picture in output order as a reference picture. In this case, the subsequent picture may be indexed first in the reference picture list 1, and the previous picture may be indexed thereafter. Here, the output order may correspond to a POC (Picture Order Count) order.

[0105] A video / image encoding procedure based on inter prediction generally includes, for example:

[0106] FIG. 4 illustrates an example of an inter-prediction based video / image encoding method.

[0107] The encoding device performs inter prediction for a current block (S400). The encoding device derives an inter prediction mode and motion information for the current block, and generates a prediction sample for the block. Here, the procedures of determining an inter prediction mode, deriving motion information, and generating a prediction sample may be performed simultaneously, or one procedure may be performed before the other procedures. For example, the inter prediction unit of the encoding device includes a prediction mode determination unit, a motion information derivation unit, and a prediction sample derivation unit, and the prediction mode determination unit determines a prediction mode for the current block, the motion information derivation unit derives motion information for the current block, and the prediction sample derivation unit derives a prediction sample for the current block. For example, the inter prediction unit of the encoding device searches for a block similar to the current block within a certain region (search region) of a reference picture by motion estimation, and derives a reference block whose difference with 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 encoding apparatus determines a mode to be applied to the current block from among various prediction modes. The encoding apparatus can compare RD costs for the various prediction modes and determine an optimal prediction mode for the current block.

[0108] For example, when a skip mode or a merge mode is applied to the current block, the encoding apparatus may construct a merge candidate list (to be described later) and 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 related to the derived reference block is selected, and merge index information indicating the selected merge candidate is generated and signaled to a decoding apparatus. Motion information of the current block may be derived using motion information of the selected merge candidate.

[0109] As another example, when the (A)MVP mode is applied to the current block, the encoding apparatus may construct an (A)MVP candidate list, which will be described later, and may use a motion vector of a selected MVP candidate among the MVP (motion vector predictor) candidates included in the (A)MVP candidate list as the MVP of the current block. In this case, for example, a motion vector indicating a reference block derived by the above-mentioned motion estimation may be used as the motion vector of the current block, and an MVP candidate having a motion vector with the smallest difference from the motion vector of the current block among the MVP candidates may be the selected MVP candidate. A motion vector difference (MVD), which is a difference obtained by subtracting the MVP from the motion vector of the current block, may be derived. In this case, information regarding the MVD may be signaled to the decoding apparatus. In addition, when the (A)MVP mode is applied, the value of the reference picture index is configured as reference picture index information and is separately signaled to the decoding apparatus.

[0110] The encoding apparatus derives residual samples based on the predicted samples (S410). The encoding apparatus can derive the residual samples by comparing original samples of the current block with the predicted samples.

[0111] The encoding apparatus encodes image information including prediction information and residual information (S420). The encoding apparatus outputs the encoded image information in the form of a bitstream. The prediction information is information related to the prediction procedure and includes prediction mode information (e.g., skip flag, merge flag, or mode index) and information on motion information. The information on the motion information includes candidate selection information (e.g., merge index, mvp flag, or mvp index) which is information for deriving a motion vector. The information on the motion information also includes information on the above-mentioned MVD and / or reference picture index information. The information on the motion information also includes information indicating whether L0 prediction, L1 prediction, or bi prediction is applied (The information on the motion information may include information indicating whether L0 prediction, L1 prediction, or bi prediction is applied). The residual information is information on the residual sample. The residual information includes information on quantized transform coefficients for the residual sample.

[0112] The output bitstream may be stored on a (digital) storage medium and transmitted to the decoding device, or may be transmitted to the decoding device via a network.

[0113] Meanwhile, as described above, the encoding apparatus generates a reconstructed picture (including reconstructed samples and reconstructed blocks) based on the reference samples and the residual samples. This is because the same prediction result as that performed by the decoding apparatus is derived from the encoding apparatus, thereby improving coding efficiency. Therefore, the encoding 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, etc. can be further applied to the reconstructed picture.

[0114] A video / image decoding procedure based on inter prediction generally includes, for example:

[0115] FIG. 5 shows an example of an inter-prediction based video / image decoding method.

[0116] As shown in Figure 5, the decoding device performs operations corresponding to those performed by the encoding device. The decoding device can perform prediction on the current block based on the received prediction information and derive a prediction sample.

[0117] Specifically, the decoding device determines a prediction mode for the current block based on received prediction information (S500). The decoding device may determine which inter prediction mode is applied to the current block based on prediction mode information in the prediction information.

[0118] For example, it may determine whether the merge mode is applied to the current block or whether the (A)MVP mode is determined based on the merge flag. Alternatively, it may select one of various inter prediction mode candidates based on the mode index. The inter prediction mode candidates may include a skip mode, a merge mode, and / or an (A)MVP mode, or may include various inter prediction modes described below.

[0119] The decoding apparatus derives motion information of the current block based on the determined inter prediction mode (S510). For example, when a skip mode or a merge mode is applied to the current block, the decoding apparatus forms a merge candidate list (described later) and selects one of the merge candidates included in the merge candidate list. The selection is performed based on the above-mentioned selection information (merge index). The motion information of the selected merge candidate may be used to derive motion information of the current block. The motion information of the selected merge candidate may be used as motion information of the current block.

[0120] As another example, when the (A)MVP mode is applied to the current block, the decoding device configures an (A)MVP candidate list described later, and uses a motion vector of a selected MVP candidate among the MVP (motion vector predictor) candidates included in the (A)MVP candidate list as the MVP of the current block. The selection is performed based on the above-mentioned selection information (mvp flag or mvp index). In this case, the MVD of the current block can be derived based on information on the MVD, and the motion vector of the current block can be derived based on the mvp of the current block and the MVD. Also, the reference picture index of the current block can be derived based on the reference picture index information. A picture pointed to by the reference picture index in the reference picture list for the current block can be derived as a reference picture referenced for inter prediction of the current block.

[0121] On the other hand, the motion information of the current block may be derived without constructing a candidate list as described below, in which case the motion information of the current block may be derived according to a procedure disclosed in a prediction mode as described below. In this case, the candidate list construction as described above may be omitted.

[0122] The decoding apparatus generates a prediction sample for the current block based on the motion information of the current block (S520). In this case, the reference picture may be derived based on the reference picture index of the current block, and the prediction sample of the current block may be derived using a sample of a reference block pointed to on the reference picture by the motion vector of the current block. In this case, as described below, a prediction sample filtering procedure may be further performed on all or some of the prediction samples of the current block, depending on the case.

[0123] For example, the inter prediction unit of the decoding device may include a prediction mode determination unit, a motion information derivation unit, and a prediction sample derivation unit, and may determine a prediction mode for the current block based on prediction mode information received by the prediction mode determination unit, derive motion information (such as a motion vector and / or a reference picture index) of the current block based on information regarding the motion information received by the motion information derivation unit, and derive a prediction sample for the current block from the prediction sample derivation unit.

[0124] The decoding apparatus generates a residual sample for the current block based on the received residual information (S530). The decoding apparatus generates a reconstructed sample for the current block based on the prediction sample and the residual sample, and generates a reconstructed picture based on the reconstructed sample (S540). Thereafter, as described above, an in-loop filtering procedure can be further applied to the reconstructed picture.

[0125] FIG. 6 exemplarily illustrates an inter prediction procedure.

[0126] Referring to Figure 6, as described above, the inter prediction procedure includes an inter prediction mode determination step, a motion information derivation step according to the determined prediction mode, and a prediction execution (prediction sample generation) step based on the derived motion information. As described above, the inter prediction procedure is performed in an encoding device and a decoding device. In this document, a coding device includes an encoding device and / or a decoding device.

[0127] As shown in FIG. 6, the coding apparatus determines an inter prediction mode for a current block (S600). Various inter prediction modes may be used for prediction of a current block in a picture. For example, various modes such as merge mode, skip mode, Motion Vector Prediction (MVP) mode, Affine mode, sub-block merge mode, MMVD (Merge with MVD) mode, etc. may be used. Decoder side Motion Vector Refinement (DMVR) mode, Adaptive Motion Vector Resolution (AMVR) mode, Bi-prediction with CU-level Weight (BCW), Bi-Directional Optical Flow (BDOF), etc. may be used in addition or instead as an additional mode. The affine mode may be referred to as an affine motion prediction mode. The MVP mode may also be referred to as "Advanced Motion Vector Prediction (AMVP) mode." In this document, some modes and / or motion information candidates derived by some modes may be included as one of the motion information related candidates of other modes. For example, an HMVP candidate may be added as a merge candidate of the merge / skip mode, or may be added as an MVP candidate of the MVP mode. When the HMVP candidate is used as a motion information candidate of the merge mode or skip mode, the HMVP candidate may be referred to as an HMVP merge candidate.

[0128] Prediction mode information indicating the inter prediction mode of the current block may be signaled from the encoding device to the decoding device. The prediction mode information may be included in a bitstream and received by the decoding device. The prediction mode information includes index information indicating one of a number of candidate modes. Alternatively, the inter prediction mode may be indicated through hierarchical signaling of flag information. In this case, the prediction mode information includes one or more flags. For example, a skip flag may be signaled to indicate whether a skip mode is applied, a merge flag may be signaled to indicate whether a merge mode is applied if the skip mode is not applied, and an MVP mode may be indicated to be applied if the merge mode is not applied, or a flag for additional division may be further signaled. The affine mode may be signaled in an independent mode, or may be signaled in a mode dependent on the merge mode or MVP mode, etc. For example, the affine mode includes an affine merge mode and an affine MVP mode.

[0129] Meanwhile, information indicating whether the above-mentioned List0 (L0) prediction, List1 (L1) prediction, or bi-prediction is used for the current block (current coding unit) may be signaled. The information may be called motion prediction direction information, inter prediction direction information, or inter prediction indication information, and may be configured / encoded / signaled in the form of, for example, an inter_pred_idc syntax element. That is, the inter_pred_idc syntax element may indicate whether the above-mentioned List0 (L0) prediction, List1 (L1) prediction, or bi-prediction is used for the current block (current coding unit). In this document, for convenience of explanation, the inter prediction type (L0 prediction, L1 prediction, or BI prediction) indicated by the inter_pred_idc syntax element may be represented as a motion prediction direction. L0 prediction may be represented as pred_L0, L1 prediction as pred_L1, and bi-prediction as pred_BI. For example, the value of the inter_pred_idc syntax element can indicate the prediction type as follows:

[0130] [Table 1]

[0131] As described above, one picture includes one or more slices. A slice may have one of slice types including an I (Intra) slice, a P (Predictive) slice, and a B (Bi-Predictive) slice. The slice type is indicated based on slice type information. For blocks in an I slice, inter prediction is not used for prediction, and only intra prediction is used. Of course, in this case, original sample values ​​can also be coded and signaled without prediction. For blocks in a P slice, intra prediction or inter prediction is used, and when inter prediction is used, only uni prediction can be used. Meanwhile, for blocks in a B slice, intra prediction or inter prediction is used, and when inter prediction is used, up to bi prediction can be used.

[0132] L0 and L1 include reference pictures encoded / decoded before the current picture. For example, L0 includes reference pictures before and / or after the current picture in POC order, and L1 includes reference pictures after and / or before the current picture in POC order. In this case, L0 is assigned a lower reference picture index relative to a reference picture before the current picture in POC order, and L1 is assigned a lower reference picture index relative to a reference picture after the current picture in POC order. For B slices, bi-prediction is applied, and unidirectional bi-prediction may also be applied in this case, or bi-directional bi-prediction may also be applied. Bi-directional bi-prediction is also called true bi-prediction.

[0133] The following table shows the syntax for a coding unit according to one embodiment of this document.

[0134] [Table 2-1]

[0135] [Table 2-2]

[0136] [Table 2-3]

[0137] [Table 2-4]

[0138] [Table 2-5]

[0139] The coding apparatus derives motion information for the current block (S610). The motion information may be derived based on the inter prediction mode.

[0140] The coding apparatus may perform inter prediction using motion information of the current block. The encoding apparatus may derive optimal motion information for the current block through a motion estimation procedure. For example, the encoding apparatus may search for a similar reference block having high correlation using an original block in an original picture for the current block in a fractional pixel unit within a determined search range in the reference picture, thereby deriving motion information. The similarity of the blocks may be derived based on a difference in phase-based sample values. For example, the similarity of the blocks may be calculated based on the Sum of Absolute Differences (SAD) between the current block (or a template of the current block) and the reference block (or a template of the reference block). In this case, the motion information may be derived based on the reference block having the smallest SAD in the search area. The derived motion information may be signaled to the decoding apparatus by various methods based on the inter prediction mode.

[0141] The coding device performs inter prediction based on motion information for the current block (S620). The coding device may derive predictive sample(s) for the current block based on the motion information. The current block including the predictive samples may be referred to as a predicted block.

[0142] When a merge mode is applied, the motion information of the current prediction block is derived using the motion information of the surrounding prediction blocks without being directly transmitted. Therefore, the motion information of the current prediction block can be indicated by transmitting flag information indicating that the merge mode is used and a merge index indicating which surrounding prediction block is used. The merge mode may be called a regular merge mode.

[0143] To perform the merge mode, the encoder must search for merge candidate blocks that are used to derive motion information of the current prediction block. For example, up to five merge candidate blocks can be used, but the embodiment of this document is not limited thereto. The maximum number of merge candidate blocks is transmitted in the slice header or tile group header. After finding the merge candidate blocks, the encoder can generate a merge candidate list and select the merge candidate block with the smallest cost among them as the final merge candidate block.

[0144] The merge candidate list may use, for example, five merge candidate blocks. For example, four spatial merge candidates and one temporal merge candidate may be used. Hereinafter, the spatial merge candidates or spatial MVP candidates described later may be referred to as SMVPs, and the temporal merge candidates or temporal MVP candidates described later may be referred to as TMVPs.

[0145] FIG. 7 illustrates a schematic of a method for constructing a merge candidate list according to the present document.

[0146] A coding apparatus (encoder / decoder) searches for spatial peripheral blocks of a current block and inserts derived spatial merge candidates into a merge candidate list (S700). For example, the spatial peripheral blocks include a lower left corner peripheral block, a left peripheral block, a upper right corner peripheral block, an upper peripheral block, and an upper left corner peripheral block of the current block. However, this is an example, and additional peripheral blocks such as a right peripheral block, a lower peripheral block, and a lower right peripheral block can be used as the spatial peripheral blocks in addition to the spatial peripheral blocks. The coding apparatus searches the spatial peripheral blocks based on a priority order to detect usable blocks, and derives motion information of the detected blocks as the spatial merge candidates.

[0147] The coding device inserts the time merge candidates derived by searching for the time neighboring blocks of the current block into the merge candidate list (S710). The time neighboring blocks may be located on a reference picture that is a picture different from the current picture in which the current block is located. The reference picture on which the time neighboring blocks are located may be referred to as a collocated picture or a col picture. The time neighboring blocks can be searched in the order of the peripheral blocks of the lower right corner and the lower right center block of the co-located block with respect to the current block on the col picture. On the other hand, when motion data compression is applied, specific motion information is stored as representative motion information for each fixed storage unit in the col picture. In this case, it is not necessary to store the motion information for all blocks within the fixed storage unit, and thus the effect of motion data compression can be obtained. In this case, the fixed storage unit may be predetermined, for example, in units of 16×16 samples or 8×8 samples, or size information regarding the fixed storage unit may be signaled from the encoder to the decoder. When the motion data compression is applied, the motion information of the time neighboring blocks can be replaced with the representative motion information of the fixed storage unit in which the time neighboring blocks are located. That is, in this case, from the perspective of implementation, instead of the prediction block located at the coordinates of the time neighboring blocks, based on the coordinates (the upper left sample position (position)) of the time neighboring blocks, after arithmetic right shift by a certain value, the time merge candidate is derived based on the motion information of the prediction block covering the position after arithmetic left shift. For example, when the fixed storage unit is a 2n×2n sample unit, if the coordinates of the time neighboring blocks are (xTnb, yTnb), the motion information of the prediction block located at the corrected position ((xTnb>>n)<<n), (yTnb>>n)<<n)) is used for the time merge candidate.Specifically, for example, when the fixed storage unit is a 16×16 sample unit, if the coordinates of the temporal peripheral block are (xTnb, yTnb), the motion information of the prediction block located at the modified position ((xTnb>>4)<<4), (yTnb>>4)<<4)) is used for the temporal merge candidate. Alternatively, for example, when the fixed storage unit is an 8×8 sample unit, if the coordinates of the temporal peripheral block are (xTnb, yTnb), the motion information of the prediction block located at the modified position ((xTnb>>3)<<3), (yTnb>>3)<<3)) is used for the temporal merge candidate.

[0148] The coding device may check whether the current number of merging candidates is smaller than the maximum number of merging candidates (S720). The maximum number of merging candidates may be predefined or signaled from the encoder to the decoder. For example, the encoder generates information regarding the maximum number of merging candidates, encodes the information, and transmits the information to the decoder in the form of a bitstream. When the maximum number of merging candidates is filled, no further candidate addition process may be performed.

[0149] If the current number of merging candidates is less than the maximum number of merging candidates, the coding apparatus inserts an additional merging candidate into the merging candidate list (S730).

[0150] If it is determined that the current number of merge candidates is not less than the maximum number of merge candidates, the coding apparatus terminates constructing the merge candidate list (S740). In this case, the encoder may select an optimal merge candidate from among the merge candidates constituting the merge candidate list based on a rate-distortion (RD) cost, and may signal selection information (e.g., a merge index) indicating the selected merge candidate to a decoder. The decoder selects the optimal merge candidate based on the merge candidate list and the selection information.

[0151] As mentioned above, the motion information of the selected merging candidate may be used as the motion information of the current block, and a predictive sample of the current block may be derived based on the motion information of the current block. An encoder may derive a residual sample of the current block based on the predictive sample, and signal residual information regarding the residual sample to a decoder. As mentioned above, a decoder may generate a reconstructed sample based on the residual sample derived based on the residual information and the predictive sample, and generate a reconstructed picture based thereon.

[0152] When a skip mode is applied, the motion information of the current block can be derived in the same manner as when the merge mode is applied, except that when the skip mode is applied, the residual signal for the corresponding block is omitted, and therefore the predicted samples can be used directly as reconstructed samples.

[0153] When the MVP mode is applied, a motion vector predictor (mvp) candidate list is generated using the motion vector of the restored spatial surrounding block and / or the motion vector corresponding to the temporal surrounding block (or Col block). That is, the motion vector of the restored spatial surrounding block and / or the motion vector corresponding to the temporal surrounding block may be used as a motion vector predictor candidate. When bi-prediction is applied, an mvp candidate list for L0 motion information derivation and an mvp candidate list for L1 motion information derivation may be generated and used separately. The prediction information (or prediction information) includes selection information (e.g., an MVP flag or an MVP index) indicating a selected optimal motion vector predictor candidate among the motion vector predictor candidates included in the list. Here, the prediction unit may select a motion vector predictor of the current block from the motion vector predictor candidates included in the motion vector candidate list using the selection information. The prediction unit of the encoding device may obtain a motion vector difference (MVD) between the motion vector of the current block and the motion vector predictor, encode the MVD, and output it in the form of a bitstream. That is, the MVD is calculated as a value obtained by subtracting the motion vector predictor from the motion vector of the current block. Here, the prediction unit of the decoding device can obtain a motion vector difference included in the information on the prediction, and derive the motion vector of the current block by adding the motion vector difference and the motion vector predictor. The prediction unit of the decoding device can obtain or derive a reference picture index indicating a reference picture from the information on the prediction.

[0154] FIG. 8 is a flow chart illustrating a method for constructing a motion vector predictor candidate list.

[0155] As shown in FIG. 8, in one embodiment, first, a spatial candidate block for motion vector prediction is searched and inserted into a prediction candidate list (S800). Then, in one embodiment, it is determined whether the number of spatial candidate blocks is less than 2 (S810). For example, in one embodiment, if the number of spatial candidate blocks is less than 2, a temporal candidate block is searched and added to the prediction candidate list (S820), and if the temporal candidate block is unavailable, a zero motion vector is used. That is, a zero motion vector can be added to the prediction candidate list (S830). Then, in one embodiment, the configuration of a reserve candidate list is terminated (S840). Alternatively, in one embodiment, if the number of spatial candidate blocks is not less than 2, the configuration of a reserve candidate list is terminated (S840). Here, the reserve candidate list indicates an MVP candidate list.

[0156] On the other hand, when the MVP mode is applied, the reference picture index is explicitly signaled. In this case, the reference picture index for L0 prediction (refidxL0) and the reference picture index for L1 prediction (refidxL1) may be signaled separately. For example, when the MVP mode is applied and bi-prediction (BI prediction) is applied, both the information on refidxL0 and the information on refidxL1 may be signaled.

[0157] When the MVP mode is applied, as described above, information on the MVD derived from the encoding device is signaled to the decoding device. The information on the MVD may include, for example, information indicating x, y components for the absolute value and sign of the MVD. In this case, information indicating whether the absolute value of the MVD is greater than 0, whether it is greater than 1, and the remainder of the MVD may be signaled stepwise. For example, information indicating whether the absolute value of the MVD is greater than 1 may be signaled only if the value of flag information indicating whether the absolute value of the MVD is greater than 0 is 1.

[0158] For example, information regarding the MVD is configured in the syntax shown in the table below, encoded in the encoding device, and signaled to the decoding device.

[0159] [Table 3]

[0160] For example, in Table 3, the abs_mvd_greater0_flag syntax element indicates information regarding whether the difference (MVD) is greater than 0, and the abs_mvd_greater1_flag syntax element indicates information regarding whether the difference (MVD) is greater than 1. Furthermore, the abs_mvd_minus2 syntax element indicates information regarding the value obtained by subtracting 2 from the difference (MVD), and the mvd_sign_flag syntax element indicates information regarding the sign of the difference (MVD). Furthermore, in Table 3, [0] in each syntax element indicates information regarding L0, and [1] indicates information regarding L1.

[0161] For example, MVD[compIdx] is derived based on abs_mvd_greater0_flag[compIdx]*(abs_mvd_minus2[compIdx]+2)*(1-2*mvd_sign_flag[compIdx]). Here, compIdx (or cpIdx) indicates an index of each component and can have a value of 0 or 1. compIdx0 indicates the x component and compIdx1 indicates the y component. However, this is merely an example, and values ​​for each component can be expressed using other coordinate systems instead of the x, y coordinate system.

[0162] Meanwhile, an MVD for L0 prediction (MVD L0) and an MVD for L1 prediction (MVD L1) may be signaled separately, and the information on the MVD may include information on the MVD L0 and / or information on the MVD L1. For example, when the MVP mode and BI prediction are applied to the current block, both the information on the MVD L0 and the information on the MVD L1 are signaled.

[0163] FIG. 9 is a diagram for explaining SMVD (Symmetric Motion Vector Differences).

[0164] When BI prediction is applied, SMVD (Symmetric MVD) may be used in consideration of coding efficiency. In this case, signaling of some of the motion information may be omitted. For example, when SMVD is applied to the current block, information on refidxL0, information on refidxL1, and information on MVD L1 can be internally derived without being signaled from the encoding device to the decoding device. For example, when MVP mode and BI prediction are applied to the current block, flag information (e.g., SMVD flag information or sym_mvd_flag syntax element) indicating whether SMVD can be applied is signaled, and if the value of the flag information is 1, the decoding device determines that SMVD is applied to the current block.

[0165] When the SMVD mode is applied (i.e., the value of the SMVD flag information is 1), information on mvp_l0_flag, mvp_l1_flag, and MVD L0 (motion vector difference L0) are explicitly signaled, and as described above, information on refidxL0, information on refidx1, and information on MVD L1 (motion vector difference L1) are omitted and can be derived internally. For example, refidxL0 can be derived as an index pointing to a previous reference picture that is closest to the current picture in the POC procedure in a reference picture list 0 (which may be called List0 or L0). refidxL1 can be derived as an index pointing to a subsequent reference picture that is closest to the current picture in the POC procedure in a reference picture list 1 (which may be called List1 or L1). Alternatively, for example, refidxL0 and refidxL1 can both be derived as 0, respectively. Alternatively, for example, the refidxL0 and refidxL1 can be derived as the smallest indexes having the same POC difference in relation to the current picture. Specifically, for example, when "POC of the current picture - POC of the first reference picture indicated by refidxL0" is referred to as a first POC difference and "POC of the current picture - POC of the second reference picture indicated by refidxL1" is referred to as a second POC difference, only when the first POC difference and the second POC difference are the same, the value of refidxL0 indicating the first reference picture may be derived as the refidxL0 of the current block, and the value of refidxL1 indicating the second reference picture may be derived as the refidxL1 of the current block. Furthermore, for example, when there are a plurality of sets in which the first POC difference and the second POC difference are the same, refidxL0 and refidxL1 of the set with the smallest difference among them can be derived as refidxL0 and refidxL1 of the current block.

[0166] 9, reference picture list 0, reference picture list 1, MVD L0, and MVD L1 are shown, where MVD L1 is symmetrical to MVD L0.

[0167] MVD L1 may be derived as minus (−)MVD L0. For example, the final (refined or revised) motion information (motion vector: MV) for the current block is derived based on the following equation:

[0168] <Formula 1>

number

[0169] In Equation 1, mvx0 and mvy0 indicate the x and y components of the motion vector for L0 motion information or L0 prediction, and mvx1 and mvy1 indicate the x and y components of the motion vector for L1 motion information or L1 prediction. Also, mvpx0 and mvpy0 indicate the x and y components of the motion vector predictor for L0 prediction, and mvpx1 and mvpy1 indicate the x and y components of the motion vector predictor for L1 prediction. Also, mvdx0 and mvdy0 indicate the x and y components of the motion vector difference for L0 prediction.

[0170] Meanwhile, in the MMVD mode, as a method of applying MVD (Motion Vector Difference) to the merge mode, motion information directly used to generate a predicted sample of the current block (i.e., the current CU) may be implicitly derived. For example, an MMVD flag (e.g., mmvd_flag) indicating whether or not to use MMVD for the current block (i.e., the current CU) may be signaled, and MMVD may be performed based on the MMVD flag. If MMVD is applied to the current block (e.g., mmvd_flag is 1), additional information regarding MMVD may be signaled.

[0171] Here, the additional information about the MMVD includes a merge candidate flag (e.g., mmvd_cand_flag) indicating whether the first or second candidate in the merge candidate list is used along with the MVD, a distance index (e.g., mmvd_distance_idx) to indicate the motion magnitude, and a direction index (mmvd_direction_idx) to indicate the motion direction.

[0172] In the MMVD mode, two candidates (i.e., the first candidate or the second candidate) located at the first and second entries of the candidates in the merge candidate list can be used, and any one of the two candidates (i.e., the first candidate or the second candidate) can be used as the base MV. For example, a merge candidate flag (e.g., mmvd_cand_flag) can be signaled to indicate any one of the two candidates (i.e., the first candidate or the second candidate) in the merge candidate list.

[0173] Also, the distance index (e.g., mmvd_distance_idx) indicates information of the magnitude of motion and may indicate a predetermined offset from the starting point. The offset may be added to the horizontal or vertical component of the starting motion vector. The relationship between the distance index and the predetermined offset may be shown as in the following table.

[0174] [Table 4]

[0175] Referring to Table 4 above, the MVD distance (e.g., MmvdDistance) is determined by the value of the distance index (e.g., mmvd_distance_idx), and the MVD distance (e.g., MmvdDistance) can be derived using integer sample precision or fractional sample precision based on the value of tile_group_fpel_mmvd_enabled_flag. For example, when tile_group_fpel_mmvd_enabled_flag is 1, it indicates that the MVD distance is derived using integer sample units in the current tile group (or picture header), and when tile_group_fpel_mmvd_enabled_flag is 0, it indicates that the MVD distance is derived using fractional sample units in the tile group (or picture header). In Table 1, information (flag) for the tile group can be replaced with information for the picture header, for example, tile_group_fpel_mmvd_enabled_flag can be replaced with ph_fpel_mmvd_enabled_flag (or ph_mmvd_fullpel_only_flag).

[0176] Also, a direction index (e.g., mmvd_direction_idx) indicates the direction of the MVD based on the start point, and indicates four directions as shown in Table 5 below. Here, the direction of the MVD can indicate the code of the MVD. The relationship between the direction index and the MVD code is shown in the table below.

[0177] [Table 5]

[0178] Referring to Table 5 above, the sign of the MVD (eg, MmvdSign) is determined by the value of the direction index (eg, mmvd_direction_idx), and the sign of the MVD (eg, MmvdSign) is derived for the L0 reference picture and the L1 reference picture.

[0179] Based on the distance index (eg, mmvd_distance_idx) and direction index (eg, mmvd_direction_idx) as described above, the MVD offset can be calculated as follows:

[0180] <Formula 2>

number

[0181] <Formula 3>

number

[0182] In Equation 2 and Equation 3, the MMVD distance (MmvdDistance[x0][y0]) and the MMVD sign (MmvdSign[x0][y0][0], MmvdSign[x0][y0][1]) are derived based on Table 4 and / or Table 5. In summary, in the MMVD mode, a merge candidate indicated by a merge candidate flag (e.g., mmvd_cand_flag) among the merge candidate children in the merge candidate list derived based on the neighboring blocks can be selected, and the selected merge candidate can be used as a base candidate (e.g., MVP). Then, the motion information (i.e., a motion vector) of the current block can be derived by adding the MVD derived using the distance index (e.g., mmvd_distance_idx) and the direction index (e.g., mmvd_direction_idx) based on the base candidate.

[0183] A predicted block for the current block may be derived based on the motion information derived by the prediction mode. The predicted block includes a prediction sample (prediction sample array) of the current block. If the motion vector of the current block points to a fractional sample unit, an interpolation procedure may be performed, whereby a prediction sample of the current block may be derived based on a reference sample of the fractional sample unit in a reference picture. If bi-prediction is applied, a prediction sample derived by a weighted sum or weighted average (according to phase) of a prediction sample derived based on an L0 prediction (i.e., prediction using a reference picture in a reference picture list L0 and MVL0) and a prediction sample derived based on an L1 prediction (i.e., prediction using a reference picture in a reference picture list L1 and MVL1) may be used as a prediction sample of the current block. If bi-prediction is applied, when a reference picture used for L0 prediction and a reference picture used for L1 prediction are located in different time directions with respect to the current picture (i.e., when bi-prediction is applied but corresponds to bidirectional prediction), this may be referred to as true bi-prediction.

[0184] As mentioned above, reconstructed samples and reconstructed pictures are generated based on the derived predicted samples, after which procedures such as in-loop filtering can be performed.

[0185] As described above, according to this document, when bi-prediction is applied to the current block, a prediction sample can be derived based on a weighted average. Conventionally, a bi-prediction signal (i.e., a bi-prediction sample) is derived by a simple average of an L0 prediction signal (L0 prediction sample) and an L1 prediction signal (L1 prediction sample). That is, a bi-prediction sample is derived as an average of an L0 prediction sample based on an L0 reference picture and an MVL0 and an L1 prediction sample based on an L1 reference picture and an MVL1. However, according to this document, when bi-prediction is applied, a bi-prediction signal (bi-prediction sample) can be derived by a weighted average of an L0 prediction signal and an L1 prediction signal as follows:

[0186] In the above-mentioned embodiment related to MMVD, a method that considers long-term reference pictures in the MVD derivation process of MMVD can be proposed, thereby making it possible to maintain and increase compression efficiency in various applications. In addition, the method proposed in the embodiment of this document can be similarly applied to SMVD, which is a symmetric MVD technique used in inter mode (MVP mode), in addition to the MMVD technique used in MERGE.

[0187] FIG. 10 is a diagram illustrating a method of deriving a motion vector in inter prediction.

[0188] In one embodiment of this document, a MV derivation method is used that considers long-term reference pictures during the motion vector scaling (MV scaling) of a temporal motion candidate (temporal motion candidate, temporal merge candidate, or temporal mvp candidate). A temporal motion candidate may correspond to mvCol (mvLXCol). A temporal motion candidate may be referred to as "TMVP".

[0189] The following table describes the definition of a long-term reference picture.

[0190] [Table 6]

[0191] Referring to Table 6 above, if LongTermRefPic (aPic, aPb, refIdx, LX) is 1 (true), the corresponding reference picture is marked as used for long-term reference. For example, a reference picture that is not marked as used for long-term reference may be a reference picture marked as used for short-term reference. In another example, a reference picture that is not marked as used for long-term reference and is not marked as unused may be a reference picture marked as used for short-term reference. Hereinafter, a reference picture marked as used for long-term reference may be referred to as a long-term reference picture, and a reference picture marked as used for short-term reference may be referred to as a short-term reference picture.

[0192] The following table describes the derivation of TMVP(mvLXCol).

[0193] [Table 7]

[0194] Referring to FIG. 10 and Table 7, if the reference picture type (e.g., long-term reference picture (LTRP) or short-term reference picture (STRP)) pointed to by the current picture is not the same as the type of the collocated reference picture pointed to by the collocated picture, the temporal motion vector (mvLXCol) is not used. That is, if all are long-term reference pictures or all are short-term reference pictures, colMV is derived, and if they have other types, colMV is not derived. Also, if all are long-term reference pictures and the POC difference between the current picture and the reference picture of the current picture is the same as the POC difference between the collocated picture and the reference picture of the collocated picture, the motion vector of the collocated block can be used as is without scaling. If it is a short-term reference picture and the POC difference is different, the motion vector of the scaled collocated block is used.

[0195] In the embodiment of this document, the MMVD used in the MERGE / SKIP mode signals a base motion vector index, a distance index, and a direction index for one coding block as information for deriving MVD information. When performing unidirectional prediction, MVD is derived from the motion information, and when performing bidirectional prediction, mirroring and scaling methods are used to generate symmetric MVD information.

[0196] When performing bi-prediction, the MVD information for L0 or L1 is scaled to generate the MVD for L1 or L0, but when referring to long-term reference pictures, a change in the MVD derivation process is required.

[0197] 11 illustrates a process of deriving an MVD for an MMVD according to an embodiment of the present document. The method illustrated in FIG. 11 may be for a block to which bi-prediction is applied.

[0198] Referring to Figure 11, when the distance to the L0 reference picture and the distance to the L1 reference picture are the same, the derived MmvdOffset can be used as the MVD as is, and when the POC difference (the POC difference between the L0 reference picture and the current picture and the POC difference between the L1 reference picture and the current picture) is different, the MVD can be derived by scaling or simple mirroring (i.e., -1*MmvdOffset) according to the POC difference and whether it is a long-term or short-term reference picture.

[0199] As an example, the method of deriving a symmetric MVD using MMVD for a block to which bidirectional prediction is applied is not suitable for a block using a long-term reference picture, and in particular, when the reference picture types of each direction are different, it is difficult to expect performance improvement when using MMVD. Therefore, in the following figures and embodiments, when the reference picture types of L0 and L1 are different, an example is introduced in which MMVD is not applied.

[0200] 12 illustrates a process of deriving an MVD for an MMVD according to another embodiment of the present document. The method illustrated in FIG. 12 may be for a block to which bi-prediction is applied.

[0201] Referring to Figure 12, different MVD derivation methods are applied depending on whether the reference picture referred to by the current picture (or current slice, current block) is a Long-Term Reference Picture (LTRP) or a Short-Term Reference Picture (STRP). In one example, when the method of the embodiment of Figure 12 is applied, a part of the standard document according to this embodiment is described as in the following table.

[0202] [Table 8-1]

[0203] [Table 8-2]

[0204] 13 illustrates a process of deriving an MVD for an MMVD according to another embodiment of the present document. The method illustrated in FIG. 13 may be for a block to which bi-prediction is applied.

[0205] Referring to Fig. 13 above, different MVD derivation methods are applied depending on whether the reference picture referred to by the current picture (or current slice, current block) is a Long-Term Reference Picture (LTRP) or a Short-Term Reference Picture (STRP). In one example, when the method of the embodiment of Fig. 13 is applied, a part of the standard document according to this embodiment is described as in the following table.

[0206] [Table 9-1]

[0207] [Table 9-2]

[0208] In summary, an MVD derivation process for MMVD that does not derive MVD when the reference picture types for each direction are different is described.

[0209] In one embodiment of this document, MVD is not derived in all cases where a long-term reference picture is referenced. That is, when at least one L0 or L1 reference picture is a long-term reference picture, MVD is set to 0, and MVD can be derived only when there is a short-term reference picture. The following figures and tables will be specifically described.

[0210] 14 illustrates a process of deriving an MVD for an MMVD according to an embodiment of the present document. The method illustrated in FIG. 14 may be for a block to which bi-prediction is applied.

[0211] Referring to FIG. 14, when the current picture (or the current slice, the current block) refers only to short-term reference pictures based on the highest priority condition (RefPicL0!=LTRP&&RefPicL1!=STRP), the MVD for the MMVD can be derived. In one example, when the method of the embodiment of FIG. 14 is applied, a part of the standard document according to the embodiment is described as follows:

[0212] [Table 10-1]

[0213] [Table 10-2]

[0214] In one embodiment of this document, when the reference picture types in each direction are different, if there is a short-term reference picture, the MVD is derived, and if there is a long-term reference picture, the MVD is derived to 0. This will be specifically described in the following drawings and tables.

[0215] 15 illustrates a process of deriving an MVD for an MMVD according to an embodiment of the present document. The method illustrated in FIG. 15 may be for a block to which bi-prediction is applied.

[0216] Referring to Fig. 15 above, when the reference picture types in each direction are different, MmvdOffset is applied when referring to a reference picture (short-term reference picture) close to the current picture, and MVD has a value of 0 when referring to a reference picture (long-term reference picture) far from the current picture. Here, a picture close to the current picture can be considered to have a short-term reference picture, but if the close picture is a long-term reference picture, mmvdOffset can be applied to the motion vector of the list pointing to the short-term reference picture.

[0217] [Table 11]

[0218] For example, the four paragraphs included in Table 11 above can sequentially replace the bottom block (contents) of the flowchart included in FIG.

[0219] In one example, when the method of the embodiment according to FIG. 15 is applied, a portion of the standard document according to this embodiment is described as in the following table.

[0220] [Table 12-1]

[0221] [Table 12-2]

[0222] The following table provides a comparison table between the embodiments contained in this document.

[0223] [Table 13]

[0224] With reference to Table 13, a comparison is shown between methods of applying offsets taking into account reference picture type for MVD derivation of MMVD described in the embodiments according to Figures 11 to 15. In Table 13, embodiment A relates to the existing MMVD, embodiment B shows an embodiment according to Figures 11 to 13, embodiment C shows an embodiment according to Figure 14, and embodiment D shows an embodiment according to Figure 15.

[0225] That is, in the embodiments according to Figures 11, 12, and 13, a method of deriving an MVD only when the reference picture types in both directions are the same is described, and in the embodiment according to Figure 14, a method of deriving an MVD only when both directions are short-term reference pictures is described. In the embodiment according to Figure 14, if the reference picture is a long-term reference picture for unidirectional prediction, the MVD is set to 0. Also, in the embodiment according to Figure 15, a method of deriving an MVD in only one direction when the reference picture types in both directions are different is described. The differences between the embodiments show various characteristics of the technology described in this document, and it can be understood by a person skilled in the art to which this specification pertains that the effects to be achieved by the embodiments according to this document can be achieved based on the above characteristics.

[0226] In the embodiment of this document, when the reference picture type is a long-term reference picture, there is a separate process. When a long-term reference picture is included, since POCDiff-based scaling or mirroring does not affect performance improvement, the MVD in the direction having a short-term reference picture is assigned the MmvdOffset value, and the MVD in the direction having a long-term reference picture is assigned the 0 value. In one example, when this embodiment is applied, a part of the standard document according to this embodiment is described as follows:

[0227] [Table 14-1]

[0228] [Table 14-2]

[0229] In another example, a portion of Table 14 above can be replaced with the following table: Referring to Table 15, Offset is applied based on the reference picture type, not POCDiff.

[0230] [Table 15]

[0231] In yet another example, a portion of Table 14 above can be replaced with the following table: Referring to Table 16, it is possible to always set MmvdOffset for L0 and −MmvdOffset for L1 without considering the reference picture type.

[0232] [Table 16]

[0233] According to an embodiment of this document, SMVD in inter mode can be performed similarly to the MMVD used in the above-mentioned MERGE mode. When performing bidirectional prediction, the encoding device signals to the decoding device whether symmetric MVD derivation is possible, and when a related flag (e.g., sym_mvd_flag) is true (or its value is 1), the second direction MVD (e.g., MVD L1) is derived by mirroring the first direction MVD (e.g., MVD L0). In this case, scaling for the first direction MVD may not be performed.

[0234] The following table shows the syntax for a coding unit according to one embodiment of this document.

[0235] [Table 17]

[0236] [Table 18]

[0237] Referring to Tables 17 and 18 above, if inter_pred_idc==PRED_BI and L0 and L1 reference pictures are available (eg, RefIdxSymL0>-1&&RefIdxSymL1>-1), then sym_mvd_flag is signaled.

[0238] The following table shows the decoding procedure for an MMVD reference index according to one example.

[0239] [Table 19]

[0240] With reference to Table 19, the procedure for deriving the availability of L0 and L1 reference pictures is described. That is, if there is a reference picture in the forward direction among the L0 reference pictures, the reference picture index closest to the current picture is set to RefIdxSymL0, and the corresponding value is set to the reference index of L0. Also, if there is a reference picture in the backward direction among the L1 reference pictures, the reference picture index closest to the current picture is set to RefIdxSymL1, and the corresponding value is set to the reference index of L1.

[0241] Table 20 below shows a decoding procedure for an MMVD reference index according to another example.

[0242] [Table 20]

[0243] Referring to Table 20, when the L0 or L1 reference picture types are different, i.e., when long-term and short-term reference pictures are used, as in the embodiments described in conjunction with Figures 11, 12, and 13, in order to prevent SMVD, after reference index derivation for SMVD, if the reference picture types of L0 and L1 are different, SMVD is not used (see the last paragraph of Table 20).

[0244] In one embodiment of this document, similar to the MMVD used in the merge mode, the SMVD can be applied in the inter mode. When a long-term reference picture is used, as in the embodiment described with reference to FIG. 14, in order to prevent the SMVD, the long-term reference picture can be excluded in the process of deriving a reference index for the SMVD as shown in the following table.

[0245] [Table 21]

[0246] The following table according to another example of this embodiment shows an example of processing in which SMVD is not applied when a long-term reference picture is used after derivation of a reference picture index for SMVD.

[0247] [Table 22]

[0248] In one embodiment of this document, if the reference picture type of the current picture and the reference picture type of the co-located picture are different during the TMVP colMV derivation process, the motion vector MV is set to 0. However, since this differs from the derivation method for MMVD and SMVD, this is unified.

[0249] Even when the reference picture type of the current picture is a long-term reference picture and the reference picture type of the co-located picture is also a long-term reference picture, the motion vector uses the co-located motion vector value as is, but in this case, in MMVD and SMVD, MV is set to 0. Here, TMVP also sets MV to 0 without additional derivation.

[0250] Also, even if the reference picture type is different, there may be a long-term reference picture that is close to the current picture. Taking this into consideration, instead of setting MV to 0, colMV can be used as MV without scaling.

[0251] FIG. 16 is a diagram illustrating SMVD according to one embodiment of the present document.

[0252] For the derivation of SMVD, a method as shown in Fig. 16 can be used. That is, SMVD can be derived based on STRP (short-term reference picture) and / or LTRP (long-term reference picture). When using the mirrored L0 MVD for L1 MVD, if the types of reference pictures are different, an inaccurate MVD may be derived. This is because the ratio of distances (distance between reference picture 0 and the current picture and distance between reference picture 1 and the current picture) becomes large, and the correlation of the motion vectors in each direction decreases.

[0253] According to one embodiment of this document, the availability of reference pictures is checked, and if the condition is met, sym_mvd_flag can be parsed. If sym_mvd_flag is true, the MVD of L1 (MVDL1) can be derived as the mirrored MVDL0 (MVD of L0).

[0254] The following table shows a portion of the coding unit syntax according to this embodiment.

[0255] [Table 23]

[0256] Based on Table 23, the procedure for deriving sym_mvd_flag according to this embodiment can be explained.

[0257] In this embodiment, reference picture indexes (RefIdxSymLX with X=0,1) for SMVD can be derived. RefIdxSymL0 can indicate (the index of) the closest reference picture with a POC smaller than that of the current picture. RefIdxSymL1 can indicate (the index of) the closest reference picture with a POC larger than that of the current picture.

[0258] The following table describes, in the form of a standard document, how to derive reference picture indexes for SMVD according to the present embodiment.

[0259] [Table 24]

[0260] The following table shows the comparison results between the embodiments. The embodiment in Table 25 considers the reference picture type, so that the accuracy of MVD in SMVD can be improved. In Table 25, MVD can indicate MVD 0 (MVD of L0).

[0261] [Table 25]

[0262] Referring to Table 25, Example P shows how to derive the existing SMVD. Example Q may restrict SMVD when mixed reference picture types (e.g., STRP / LTRP or LTRP / STRP) are used in L0 and L1. Example R may restrict SMVD when referring to long-term reference pictures (LTRP).

[0263] The following table describes, in standard document format, how to derive reference picture indexes for SMVD according to embodiment Q of Table 25.

[0264] [Table 26]

[0265] The following table describes, in standard document format, how to derive reference picture indexes for SMVD according to embodiment Q of Table 25.

[0266] [Table 27]

[0267] [Table 28]

[0268] With reference to Table 27 and / or Table 28, SMVD may be restricted when referring to a long-term reference picture (LTRP). For example, with reference to Table 27, long-term reference pictures may be excluded in a reference picture checking process. This allows other reference pictures (e.g., not long-term reference pictures) to be considered for SMVD. With reference to Table 28, SMVD may not be performed when the reference picture closest to the current picture is a long-term reference picture. For example, even if the reference picture list includes a short-term reference picture, SMVD may not be performed when the reference picture closest to the current picture is a long-term reference picture.

[0269] In one example according to one embodiment of this document, if the POC distance of L0 is greater than or equal to the POC of L1 in the MMVD procedure, the L1 MVD can be derived as a scaled or mirrored L0 MVD. If the POC distance of L0 is less than the POC of L1 in the MMVD procedure, the L0 MVD can be derived as a scaled or mirrored L1 MVD in the MMVD procedure.

[0270] FIG. 17 is a flow chart illustrating a method for deriving the MMVD according to one embodiment of the present document.

[0271] In one embodiment of this document, MVD can be derived in MMVD considering POC difference and / or reference picture type. With reference to FIG. 17, currPocDiffLX can mean the difference between the POC of the current picture and the POC of the reference picture LX. CurrPocDiffL0 and currPocDiffL1 can be compared with each other, and the type of the reference picture can be checked ("refPicList0 != LTRP" or "refPicList1 != LTRP"). Considering the condition, MmvdOffset (derived using mmvd_cand_flag, mmvd_distance_idx, and / or mmvd_direction_idx) can be assigned as the same value as mMvdLX, a mirrored value, or a scaled value.

[0272] The following table shows a portion of a standard document according to this embodiment.

[0273] [Table 29-1]

[0274] [Table 29-2]

[0275] In the case where the current picture refers to one or more long-term reference pictures (LTRPs), the mirroring procedure considering the POC distance may not be necessary. This is because the mirroring MVD obtained from the reference picture that is much farther away than the other MVDs is not effective in terms of accuracy. A proposal to solve this is described below.

[0276] The following table shows the comparison results between the examples.

[0277] [Table 30]

[0278] Referring to Table 30, Example X shows an existing MMVD derivation method. Example Y may restrict the MMVD procedure when one or more long-term reference pictures are referenced to the current block. That is, in Example Y, the POC distance comparison procedure for the long-term reference picture may be omitted. Example Z may restrict the MMVD derivation procedure for all cases. That is, in Example Z, the POC distance comparison procedure may be omitted for all cases. In Table 30, offset may refer to MmvdOffset.

[0279] 18 is a flowchart showing a method for deriving an MMVD according to an embodiment of the present document. The flowchart in FIG. 18 can show a method for deriving an MMVD according to Example Y described above.

[0280] Referring to FIG. 18, the condition of comparing the POC difference when the reference picture type is a long-term reference picture may be removed, and the anchor MVD used for the mirroring procedure can be fixed to the L0 MVD.

[0281] The following table describes, in standard document format, how to derive the MMVD according to Example Y of Table 30.

[0282] [Table 31-1]

[0283] [Table 31-2]

[0284] 19 is a flowchart showing a method for deriving the MMVD according to one embodiment of the present document. The flowchart in FIG. 19 can show a method for deriving the MMVD according to Example Z described above.

[0285] 19, in embodiment Z, for all cases, the derivation procedure of the MMVD may be restricted. For all cases, the condition of comparing the POC difference may be removed, and the anchor MVD used for the mirroring or scaling procedure may be fixed to the L0 MVD.

[0286] The following table describes, in standard document format, how to derive the MMVD according to Example Z of Table 30.

[0287] [Table 32-1]

[0288] [Table 32-2]

[0289] Also, in one embodiment of the present invention, the requirement to compare POC differences for all cases may be removed and only the mirroring scheme may be used. The following table describes, in the form of a standard document, how to derive the MMVD in this example.

[0290] [Table 33]

[0291] The following drawings are created to explain a specific example of the present specification. The names of specific devices and names of specific signals / messages / fields described in the drawings are presented for illustrative purposes only, and the technical features of the present specification are not limited to the specific names used in the following drawings.

[0292] 20 and 21 illustrate an example of a video / image encoding method and related components according to an embodiment of the present document. The method disclosed in FIG. 20 may be performed by the encoding device disclosed in FIG. 2. Specifically, for example, S2000 to S2040 in FIG. 20 may be performed by the prediction unit 220 of the encoding device, and S2050 may be performed by the residual processing unit 230 of the encoding device. S2060 may be performed by the entropy encoding unit 240 of the encoding device. The method disclosed in FIG. 20 may include the embodiments described above in this document.

[0293] 20, the encoding apparatus derives an inter prediction mode for a current block in a current picture (S2000). Here, the inter prediction mode may include the above-mentioned merge mode, AMVP mode (a mode using a motion vector predictor candidate), MMVD, and SMVD.

[0294] The encoding device derives reference pictures for the inter prediction modes (S2010). In one example, the reference pictures may be included in reference picture list 0 (or L0, reference picture list L0) or reference picture list 1 (or L1, reference picture list L1). For example, the encoding device may construct a reference picture list for each slice included in the current picture.

[0295] The encoding apparatus derives motion information for prediction of the current block based on the inter prediction mode (S2020). The motion information may include a reference picture index and a motion vector. For example, the encoding apparatus may derive a reference index for SMVD (symmetric motion vector reference index). The reference index for SMVD may point to a reference picture for application of SMVD. The reference index for SMVD may include a reference index L0 (RefIdxSumL0) and a reference index L1 (RefIdxSumL1).

[0296] The encoding device may construct a motion vector predictor candidate list and derive a motion vector predictor based on the list. The encoding device may derive a motion vector based on the symmetric MVD and the motion vector predictor.

[0297] The encoding apparatus generates a prediction sample based on the motion information (S2030). The encoding apparatus can generate the prediction sample based on a motion vector and a reference picture index included in the motion information. For example, the prediction sample can be generated based on a block (or sample) indicated by the motion vector among blocks (or samples) in the reference picture pointed to by the reference picture index.

[0298] The encoding apparatus generates prediction related information including the inter prediction mode (S2040). The prediction related information may include information about the MMVD, information about the SMVD, and the like.

[0299] The encoding apparatus derives residual information based on the predicted samples (S2050). Specifically, the encoding apparatus may derive residual samples based on the predicted samples and the original samples. The encoding apparatus may derive residual information based on the residual samples. In order to derive the residual information, the above-mentioned transform and quantization processes may be performed.

[0300] The encoding device encodes the image / video information including the prediction related information and the residual information (S2060). The encoded image / video information may be output in the form of a bitstream. The bitstream may be transmitted to a decoding device via a network or a (digital) storage medium.

[0301] The image / video information may include various information according to embodiments of this document, for example, the image / video information may include at least one of the information disclosed in Tables 1 to 33 above.

[0302] In one embodiment, the prediction related information may include inter prediction type information indicating whether bi-prediction is applied to the current block in the current picture. For example, based on the inter prediction type information, the prediction related information may include symmetric motion vector differential reference flag information indicating whether symmetric motion vector differential reference is applicable. Also, the reference picture may include a short-term reference picture. Based on the symmetric motion vector differential reference flag information, a symmetric motion vector differential reference index may be derived from a reference index pointing to a short-term reference picture. The motion information may include a motion vector for the current block and the symmetric motion vector differential reference index. Based on the motion vector and the symmetric motion vector differential reference index, the prediction sample may be generated.

[0303] In one embodiment, the symmetric motion vector differential reference index may be derived based on a POC difference between each of the short-term reference pictures and the current picture, where, according to one example, the POC difference between the current picture and a previous reference picture from the current picture may be greater than 0. In another example, the POC difference between the current picture and a future reference picture from the current picture may be less than 0, although this is merely an example.

[0304] In one embodiment, the encoding apparatus may configure a reference picture list L0 (or reference picture list 0) for L0 prediction and a reference picture list L1 (or reference picture list 0) for L1 prediction. As an example, the short-term reference picture may include a short-term reference picture L0 included in the reference picture list L0 and a short-term reference picture L1 included in the reference picture list L1. The POC difference may include a first POC difference between the short-term reference picture L0 and the current picture, and a second POC difference between the short-term reference picture L1 and the current picture. For example, the symmetric motion vector differential reference index may include a symmetric motion vector differential reference index L0 and a symmetric motion vector differential reference index L1. Based on the first POC difference, the symmetric motion vector differential reference index L0 may be derived. Based on the second POC difference, the symmetric motion vector differential reference index L1 may be derived.

[0305] In one embodiment, the first POC difference may be the same as the second POC difference.

[0306] In one embodiment, the encoding apparatus may configure a reference picture list L0 for L0 prediction. The short-term reference picture may include a first short-term reference picture L0 and a second short-term reference picture L0 included in the reference picture list L0. As an example, the POC difference may include a third POC difference between the first short-term reference picture L0 and the current picture, and a fourth POC difference between the second short-term reference picture L0 and the current picture. For example, the symmetric motion vector difference reference index may include a symmetric motion vector difference reference index L0. Based on a comparison between the third and fourth POC differences, a reference picture index pointing to the first short-term reference picture L0 may be used as the symmetric motion vector difference reference index L0.

[0307] In one embodiment, if the third POC differential is smaller than the fourth POC differential, the reference picture index pointing to the first short-term reference picture L0 may be used as the symmetric motion vector differential reference index L0.

[0308] In one embodiment, the image information may include information about MVD (Motion Vector Differences). The motion information may include MV (Motion Vector). Based on the information about the MVD, an MVDL0 for L0 prediction may be derived. The MV may be derived based on the MVDL0 and the MVDL1.

[0309] In one embodiment, the magnitude of the MVDL1 may be the same as the magnitude of the MVDL0. The sign of the MVDL1 may be opposite to the sign of the MVDL0.

[0310] 22 and 23 illustrate an example of an image / video decoding method and related components according to an embodiment of the present document. The method disclosed in FIG. 22 may be performed by the decoding device disclosed in FIG. 3. Specifically, for example, S2200 in FIG. 22 may be performed by the entropy decoding unit 310 of the decoding device, S2210 to S2230 may be performed by the prediction unit 330 of the decoding device, S2240 may be performed by the residual processing unit 320 of the decoding device, and S2250 may be performed by the addition unit 340 of the decoding device. The method disclosed in FIG. 22 may include the embodiments described above in this document.

[0311] Referring to FIG. 22, a decoding device receives / acquires image / video information (S2200). The decoding device may receive / acquire the image / video information via a bitstream. The image / video information may include prediction-related information (including prediction mode information) and residual information. The prediction-related information may include information related to MMVD, information related to SMVD, etc. Also, the image / video information may include various information according to embodiments of the present document. For example, the image / video information may include information described in conjunction with FIGS. 1 to 19 and / or information disclosed in at least one of Tables 1 to 33 above.

[0312] The decoding apparatus derives an inter prediction mode for the current block based on the prediction related information (S2210). Here, the inter prediction mode may include the above-mentioned merge mode, AMVP mode (a mode using a motion vector predictor candidate), MMVD, and SMVD.

[0313] The decoding apparatus derives motion information for prediction of the current block based on the inter prediction mode (S2220). The motion information may include a reference picture index and a motion vector. For example, the decoding apparatus may derive a reference index for SMVD. The reference index for SMVD may point to a reference picture for application of SMVD. The reference index for SMVD may include a reference index L0 (RefIdxSumL0) and a reference index L1 (RefIdxSumL1).

[0314] The decoding device can construct a motion vector predictor candidate list and derive a motion vector predictor based on the list. The decoding device can derive a motion vector based on the symmetric MVD and the motion vector predictor.

[0315] The decoding device generates a prediction sample based on the motion information (S2230). The decoding device can generate the prediction sample based on a motion vector and a reference picture index included in the motion information. For example, the prediction sample can be generated based on a block (or sample) indicated by the motion vector among blocks (or samples) in the reference picture pointed to by the reference picture index.

[0316] The decoding apparatus generates residual samples based on the residual information (S2240). Specifically, the decoding apparatus may derive quantized transform coefficients based on the residual information. The quantized transform coefficients may have a one-dimensional vector form based on a coefficient scan order. The decoding apparatus may derive transform coefficients based on an inverse quantization procedure on the quantized transform coefficients. The decoding apparatus may derive residual samples based on an inverse transform procedure on the transform coefficients.

[0317] The decoding device generates reconstructed samples of the current picture based on the prediction samples and the residual samples (S2250). The decoding device may further perform a filtering procedure to generate (modified) reconstructed samples.

[0318] In one embodiment, the prediction related information may include inter prediction type information indicating whether bi-prediction is applied to the current block in the current picture. For example, based on the inter prediction type information, the prediction related information may include symmetric motion vector differential reference flag information indicating whether symmetric motion vector differential reference is applicable. Also, the reference picture may include a short-term reference picture. Based on the symmetric motion vector differential reference flag information, a symmetric motion vector differential reference index may be derived from a reference index pointing to a short-term reference picture. The motion information may include a motion vector for the current block and the symmetric motion vector differential reference index. Based on the motion vector and the symmetric motion vector differential reference index, the prediction sample may be generated.

[0319] In one embodiment, the symmetric motion vector differential reference index may be derived based on a POC difference between each of the short-term reference pictures and the current picture, where, according to one example, the POC difference between the current picture and a previous reference picture from the current picture may be greater than 0. In another example, the POC difference between the current picture and a future reference picture from the current picture may be less than 0, although this is merely an example.

[0320] In one embodiment, the encoding apparatus may configure a reference picture list L0 (or reference picture list 0) for L0 prediction and a reference picture list L1 (or reference picture list 0) for L1 prediction. As an example, the short-term reference picture may include a short-term reference picture L0 included in the reference picture list L0 and a short-term reference picture L1 included in the reference picture list L1. The POC difference may include a first POC difference between the short-term reference picture L0 and the current picture, and a second POC difference between the short-term reference picture L1 and the current picture. For example, the symmetric motion vector differential reference index may include a symmetric motion vector differential reference index L0 and a symmetric motion vector differential reference index L1. Based on the first POC difference, the symmetric motion vector differential reference index L0 may be derived. Based on the second POC difference, the symmetric motion vector differential reference index L1 may be derived.

[0321] In one embodiment, the first POC difference may be the same as the second POC difference.

[0322] In one embodiment, the encoding apparatus may configure a reference picture list L0 for L0 prediction. The short-term reference picture may include a first short-term reference picture L0 and a second short-term reference picture L0 included in the reference picture list L0. As an example, the POC difference may include a third POC difference between the first short-term reference picture L0 and the current picture, and a fourth POC difference between the second short-term reference picture L0 and the current picture. For example, the symmetric motion vector difference reference index may include a symmetric motion vector difference reference index L0. Based on a comparison between the third and fourth POC differences, a reference picture index pointing to the first short-term reference picture L0 may be used as the symmetric motion vector difference reference index L0.

[0323] In one embodiment, if the third POC differential is smaller than the fourth POC differential, the reference picture index pointing to the first short-term reference picture L0 may be used as the symmetric motion vector differential reference index L0.

[0324] In one embodiment, the image information may include information about MVD (Motion Vector Differences). The motion information may include MV (Motion Vector). Based on the information about the MVD, an MVDL0 for L0 prediction may be derived. The MV may be derived based on the MVDL0 and the MVDL1.

[0325] In one embodiment, the magnitude of the MVDL1 may be the same as the magnitude of the MVDL0. The sign of the MVDL1 may be opposite to the sign of the MVDL0.

[0326] In the above-described embodiments, the method is described based on the flowchart as a series of steps or blocks, but the embodiment is not limited to the order of steps, and certain steps may occur in different steps and different orders than those described above, or simultaneously. Also, those skilled in the art can understand that the steps shown in the flowchart are not exclusive, and different steps may be included, or one or more steps of the flowchart may be deleted without affecting the scope of the embodiments of this document.

[0327] The methods according to the embodiments of the present document described above may be embodied in the form of software, and the encoding device and / or decoding device according to the present document may be included in an image processing device such as a TV, a computer, a smartphone, a set-top box, or a display device.

[0328] In this document, when the embodiments are implemented in software, the methods described above may be implemented in modules (processes, functions, etc.) performing the functions described above. The modules may be stored in a memory and executed by a processor. The memory may be internal or external to the processor and may be coupled to the processor in various well-known ways. The processor may include an application-specific integrated circuit (ASIC), other chipsets, logic circuits, and / or data processing devices. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and / or other storage devices. That is, the embodiments described in this document may be implemented and performed on a processor, microprocessor, controller, or chip. For example, the functional units shown in each drawing may be implemented and performed on a computer, processor, microprocessor, controller, or chip. In this case, information (e.g., information on instructions) or algorithms for implementation may be stored in a digital storage medium.

[0329] In addition, the decoding device and the encoding device to which the embodiments of this document are applied may be included in a multimedia broadcast transmitting / receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video interactive device, a real-time communication device such as a video communication, a mobile streaming device, a storage medium, a camcorder, a video on demand (customized video) (VoD) service providing device, an OTT video (Over The Top video) device, an Internet streaming service providing device, a three-dimensional (3D) video device, a VR (Virtual Reality) device, an AR (Augmented (Argumente) Reality) device, an image telephone video device, a means of transportation terminal (e.g., a vehicle (including an autonomous vehicle) terminal, an airplane terminal, a ship terminal, etc.), and a medical video device, and may be used to process a video signal or a data signal. For example, the OTT video (Over The Top video) device may include a game console, a Blu-ray player, an Internet access TV, a home theater system, a smartphone, a tablet PC, a DVR (Digital Video Recorder), etc.

[0330] In addition, the processing method to which the embodiment of this document is applied can be produced in the form of a program executed by a computer and can be stored in a computer-readable recording medium. Multimedia data having a data structure according to the embodiment of this document can also be stored in a computer-readable recording medium. The computer-readable recording medium includes all kinds of storage devices and distributed storage devices in which computer-readable data is stored. The computer-readable recording medium may include, for example, a Blu-ray disc (BD), a universal serial bus (USB), a ROM, a PROM, an EPROM, an EEPROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage device. The computer-readable recording medium also includes a medium embodied in the form of a carrier wave (e.g., transmission via the Internet). Also, a bit stream generated by the encoding method can be stored in a computer-readable recording medium or transmitted via a wired or wireless communication network.

[0331] Furthermore, the embodiments of the present document may be embodied in a computer program product with program code, which may be executed by a computer in accordance with the embodiments of the present document. The program code may be stored on a carrier readable by a computer.

[0332] FIG. 24 illustrates an example of a content streaming system in which the embodiments disclosed in this document can be applied.

[0333] Referring to FIG. 24, a content streaming system to which the embodiments of this document are applied may roughly include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device.

[0334] The encoding server compresses content input from a multimedia input device such as a smartphone, camera, camcorder, etc. into digital data to generate a bitstream and transmits the bitstream to the streaming server. As another example, if a multimedia input device such as a smartphone, camera, camcorder, etc. directly generates a bitstream, the encoding server may be omitted.

[0335] The bitstream may be generated by an encoding method or a bitstream generation method to which an embodiment of this document is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

[0336] The streaming server transmits multimedia data to a user device based on a user request via a web server, and the web server acts as a medium to inform the user of available services. When a user requests a desired service from the web server, the web server transfers the request to the streaming server, and the streaming server transmits multimedia data to the user. At this time, the content streaming system may include a separate control server, and in this case, the control server controls commands / responses between devices in the content streaming system.

[0337] The streaming server may receive content from a media storage device 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 in order to provide a smooth streaming service.

[0338] Examples of the user devices include mobile phones, smart phones, laptop computers, digital broadcasting terminals, Personal Digital Assistants (PDAs), Portable Multimedia Players (PMPs), navigation systems, slate PCs, tablet PCs, ultrabooks (ULTRABOOK (registered trademark)), wearable devices (e.g., smart watches (watch-type terminals), smart glasses (glass-type terminals), and Head Mounted Displays (HMDs)), digital TVs, desktop computers, and digital signatures.

[0339] Each server in the content streaming system can be operated as a distributed server, in which case data received by each server can be processed in a distributed manner.

[0340] The claims described herein may be combined in various ways. For example, the technical features of the method claims herein may be combined and embodied as an apparatus, and the technical features of the apparatus claims herein may be combined and embodied as a method. In addition, the technical features of the method claims herein and the technical features of the apparatus claims herein may be combined and embodied as an apparatus, and the technical features of the method claims herein and the technical features of the apparatus claims herein may be combined and embodied as a method.

Claims

1. An image decoding method performed by a decoding device, comprising: receiving image information from a bitstream, the image information including residual information and prediction related information including information about motion vector difference (MVD); deriving an inter prediction mode for a current block in a current picture based on the prediction related information; deriving motion information for the current block based on the inter prediction mode; generating a prediction sample based on the motion information; generating residual samples based on the residual information; generating reconstructed samples of the current picture based on the prediction samples and the residual samples; The prediction related information includes inter-prediction type information indicating whether bi-prediction is applied to the current block, Based on the inter prediction type information, the prediction related information further includes symmetric motion vector difference flag information indicating whether a symmetric motion vector difference is applied; deriving a symmetric motion vector difference reference index from a reference index indicating a short-term reference picture based on the symmetric motion vector difference flag information; The image decoding method, wherein the symmetric motion vector differential reference indexes are derived based on a Picture Order Count (POC) difference between each of the short-term reference pictures and the current picture.

2. 1. An image encoding method performed by an encoding device, comprising: deriving an inter prediction mode for a current block in a current picture; deriving a reference picture for the inter prediction mode; deriving motion information for prediction of a current block based on the inter prediction mode; generating a prediction sample based on the motion information; generating prediction-related information including information about the inter prediction mode and information about a motion vector difference (MVD); generating residual information based on the prediction samples; encoding image information including the residual information and the prediction related information including the information on the inter prediction mode and the information on the MVD; The prediction related information includes inter-prediction type information indicating whether bi-prediction is applied to the current block, Based on the inter prediction type information, the prediction related information further includes symmetric motion vector difference flag information indicating whether a symmetric motion vector difference is applied; the reference pictures include short-term reference pictures; deriving a symmetric motion vector difference reference index from a reference index indicating a short-term reference picture based on the symmetric motion vector difference flag information; The image encoding method, wherein the symmetric motion vector differential reference indexes are derived based on a Picture Order Count (POC) difference between each of the short-term reference pictures and the current picture.

3. A method for transmitting data relating to an image, comprising the steps of: obtaining a bitstream of the data, the bitstream comprising: deriving an inter prediction mode for a current block in a current picture; deriving a reference picture for the inter prediction mode; deriving motion information for prediction of a current block based on the inter prediction mode; generating a prediction sample based on the motion information; generating prediction-related information including information about the inter prediction mode and information about a motion vector difference (MVD); generating residual information based on the prediction samples; encoding image information including the residual information and the prediction related information including the information on the inter prediction mode and the information on the MVD; transmitting the data including the bitstream; The prediction related information includes inter-prediction type information indicating whether bi-prediction is applied to the current block, Based on the inter prediction type information, the prediction related information further includes symmetric motion vector difference flag information indicating whether a symmetric motion vector difference is applied; the reference pictures include short-term reference pictures; deriving a symmetric motion vector difference reference index from a reference index indicating a short-term reference picture based on the symmetric motion vector difference flag information; The transmission method, wherein the symmetric motion vector differential reference indexes are derived based on a Picture Order Count (POC) difference between each of the short-term reference pictures and the current picture.

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