WO2019216736A1 - Inter prediction mode-based image processing method and apparatus therefor - Google Patents

Abstract

Disclosed are a method for performing decoding of a video signal and an apparatus therefor. Particularly, a method of decoding an image on the basis of an inter prediction mode may comprise the steps of: determining whether or not symmetric bi-prediction is applied to a current block when bi-prediction is applied to the current block; parsing at least one of a first reference index, a first motion vector predictor flag, and a first motion vector difference of a first reference picture list; and deriving at least one of a second reference index, a second motion vector predictor flag, and a second motion vector difference of a second reference picture list by using at least one of the first reference index, the first motion vector predictor flag, and the first motion vector difference when the symmetric bi-prediction is applied to the current block.

Description

Inter prediction mode based image processing method and apparatus therefor

The present invention relates to a still image or moving image processing method, and more particularly, to a method for encoding / decoding a still image or moving image based on an inter prediction mode and an apparatus supporting the same.

Compression coding refers to a series of signal processing techniques for transmitting digitized information through a communication line or for storing in a form suitable for a storage medium. Media such as an image, an image, an audio, and the like may be a target of compression encoding. In particular, a technique of performing compression encoding on an image is called video image compression.

Next-generation video content will be characterized by high spatial resolution, high frame rate and high dimensionality of scene representation. Processing such content would result in a tremendous increase in terms of memory storage, memory access rate, and processing power.

Accordingly, there is a need to design coding tools for more efficiently processing next generation video content.

In the conventional image compression technology, when bidirectional prediction is applied, separate data is transmitted for deriving motion information for each reference list direction. As such, if all motion data is signaled for each reference direction, the accuracy of prediction can be increased in performing bidirectional prediction. However, in many cases, since it may include an object having no motion or constant motion, transmitting all motion information for each reference list direction may be inefficient in terms of bit rate.

Accordingly, the present invention proposes a method of effectively encoding / decoding bidirectional motion information using symmetrical motion information to solve such a problem.

The technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

According to an aspect of the present invention, in a method of decoding an image based on an inter prediction mode, when bi-prediction is applied to a current block, symmetric bi-prediction is applied to the current block. Determining whether the image is symmetrical, wherein the symmetric bidirectional prediction indicates a mode of encoding motion information for bidirectional prediction using motion information symmetric with respect to both reference directions based on a current picture; Parsing at least one of a first reference index, a first motion vector predictor flag, or a first motion vector difference of the first reference picture list; And a second reference of a second reference picture list using at least one of the first reference index, the first motion vector predictor flag, or the first motion vector difference when the symmetric bidirectional prediction is applied to the current block. Deriving at least one of an index, a second motion vector predictor flag, or a second motion vector difference.

Preferably, deriving at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference may include: using the first reference index in the current reference list in the first reference picture list; And determining a first reference picture of the block, wherein the second reference index has a difference in picture order count (POC) between the current picture and the first reference picture in the second reference picture list. Can be derived to the same picture.

Preferably, when a picture having the same POC difference between the current picture and the first reference picture does not exist in the second reference picture list, the second reference index is a POC difference between the current picture and the second reference picture list. Can be derived to the smallest picture.

Preferably, the second motion vector difference may be derived from a value having the same magnitude as that of the first motion vector difference and having the opposite sign.

Preferably, a first POC difference indicating a picture order count (POC) difference between the current picture and the first reference picture is the same as a second POC difference indicating a POC difference between the current picture and the second reference picture. If not, the magnitude of the second motion vector difference may be scaled based on the first POC difference and the second POC difference.

Preferably, when the symmetric bidirectional prediction is not applied to the current block, the method may further include parsing at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference. have.

Preferably, the first reference index and the second reference index may be different from a picture order count (POC) between reference pictures of the first reference picture list and the current picture and reference of the first reference picture list, respectively. It may be derived based on the POC difference between pictures and the current picture.

According to another aspect of the present invention, in an apparatus for decoding an image based on an inter prediction mode, when bi-prediction is applied to a current block, symmetric bi-prediction is applied to the current block. A symmetric bidirectional prediction determiner determining whether or not to be applied, wherein the symmetric bidirectional prediction indicates a mode for encoding motion information for bidirectional prediction using motion information symmetrical with respect to both reference directions with respect to a current picture; A first reference picture list for parsing at least one of a first reference index, a first motion vector predictor flag, or a first motion vector difference of the first reference picture list Induction part; And a second reference of a second reference picture list using at least one of the first reference index, the first motion vector predictor flag, or the first motion vector difference when the symmetric bidirectional prediction is applied to the current block. It may include a second reference picture list derivation unit for deriving at least one of an index, a second motion vector predictor flag, or a second motion vector difference.

Preferably, the second reference picture list deriving unit determines the first reference picture of the current block in the first reference picture list using the first reference index, and the second reference index is the second reference picture. A picture order count (POC) difference between the current picture and the first reference picture in a list may be derived to the same picture.

Preferably, when a picture having the same POC difference between the current picture and the first reference picture does not exist in the second reference picture list, the second reference index is a POC difference between the current picture and the second reference picture list. Can be derived to the smallest picture.

Preferably, the second motion vector difference may be derived from a value having the same magnitude as that of the first motion vector difference and having the opposite sign.

Preferably, a first POC difference indicating a picture order count (POC) difference between the current picture and the first reference picture is the same as a second POC difference indicating a POC difference between the current picture and the second reference picture. If not, the magnitude of the second motion vector difference may be scaled based on the first POC difference and the second POC difference.

Preferably, when the symmetric bidirectional prediction is not applied to the current block, the second reference picture list deriving unit includes at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference. Can be parsed.

Preferably, the first reference index and the second reference index may be different from a picture order count (POC) between reference pictures of the first reference picture list and the current picture and reference of the first reference picture list, respectively. It may be derived based on the POC difference between pictures and the current picture.

According to an embodiment of the present invention, by effectively expressing the symmetrical movement of an object using less information, it is possible to reduce signaling bits and improve compression performance while maintaining prediction accuracy.

The effects obtainable in the present invention are not limited to the above-mentioned effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. .

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide embodiments of the present invention and together with the description, describe the technical features of the present invention.

1 is a schematic block diagram of an encoding apparatus in which an encoding of a video / image signal is performed, according to an embodiment to which the present invention is applied.

2 is a schematic block diagram of a decoding apparatus in which an embodiment of the present invention is applied and decoding of a video / image signal is performed.

3 is a diagram illustrating an example of a multi-type tree structure as an embodiment to which the present invention can be applied.

FIG. 4 is a diagram illustrating a signaling mechanism of partition partition information of a quadtree with nested multi-type tree structure according to an embodiment to which the present invention may be applied.

FIG. 5 is a diagram illustrating a method of dividing a CTU into multiple CUs based on a quadtree and accompanying multi-type tree structure as an embodiment to which the present invention may be applied.

FIG. 6 is a diagram illustrating a method of limiting ternary-tree splitting as an embodiment to which the present invention may be applied.

FIG. 7 is a diagram illustrating redundant division patterns that may occur in binary tree division and ternary tree division, as an embodiment to which the present invention may be applied.

8 and 9 illustrate an inter prediction based video / image encoding method and an inter prediction unit in an encoding apparatus according to an embodiment of the present invention.

10 and 11 illustrate an inter prediction based video / image decoding method and an inter prediction unit in a decoding apparatus according to an embodiment of the present invention.

FIG. 12 is a diagram for describing a neighboring block used in a merge mode or a skip mode as an embodiment to which the present invention is applied.

13 is a flowchart illustrating a merge candidate list construction method according to an embodiment to which the present invention is applied.

14 is a flowchart illustrating a merge candidate list construction method according to an embodiment to which the present invention is applied.

FIG. 15 is a diagram for describing a method of performing bidirectional prediction based on symmetrical motion information according to an embodiment to which the present invention is applied.

16 is a flowchart illustrating a method of generating an inter prediction block according to an embodiment to which the present invention is applied.

17 is a diagram illustrating an inter prediction apparatus according to an embodiment to which the present invention is applied.

18 shows a video coding system to which the present invention is applied.

19 is a diagram illustrating a structure of a content streaming system according to an embodiment to which the present invention is applied.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art appreciates that the present invention may be practiced without these specific details.

In some instances, well-known structures and devices may be omitted or shown in block diagram form centering on the core functions of the structures and devices in order to avoid obscuring the concepts of the present invention.

In addition, the terminology used in the present invention was selected as a general term widely used as possible now, in a specific case will be described using terms arbitrarily selected by the applicant. In such a case, since the meaning is clearly described in the detailed description of the part, it should not be interpreted simply by the name of the term used in the description of the present invention, and it should be understood that the meaning of the term should be understood and interpreted. .

Specific terms used in the following description are provided to help the understanding of the present invention, and the use of such specific terms may be changed to other forms without departing from the technical spirit of the present invention. For example, signals, data, samples, pictures, frames, blocks, etc. may be appropriately replaced and interpreted in each coding process.

Hereinafter, in the present specification, the 'processing unit' refers to a unit in which a process of encoding / decoding such as prediction, transformation, and / or quantization is performed. Hereinafter, for convenience of description, the processing unit may be referred to as a 'processing block' or 'block'.

The processing unit may be interpreted to include a unit for the luma component and a unit for the chroma component. For example, the processing unit may correspond to a Coding Tree Unit (CTU), a Coding Unit (CU), a Prediction Unit (PU), or a Transform Unit (TU).

In addition, the processing unit may be interpreted as a unit for a luma component or a unit for a chroma component. For example, the processing unit may be a coding tree block (CTB), a coding block (CB), a prediction block (PU), or a transform block (TB) for a luma component. May correspond to. Or, it may correspond to a coding tree block (CTB), a coding block (CB), a prediction block (PU), or a transform block (TB) for a chroma component. In addition, the present invention is not limited thereto, and the processing unit may be interpreted to include a unit for a luma component and a unit for a chroma component.

In addition, the processing unit is not necessarily limited to square blocks, but may also be configured in a polygonal form having three or more vertices.

In the following specification, a pixel, a pixel, and the like are referred to collectively as samples. In addition, using a sample may mean using a pixel value or a pixel value.

1 is a schematic block diagram of an encoding apparatus in which an encoding of a video / image signal is performed, according to an embodiment to which the present invention is applied.

Referring to FIG. 1, the encoding apparatus 100 may include an image splitter 110, a subtractor 115, a transformer 120, a quantizer 130, an inverse quantizer 140, an inverse transformer 150, The adder 155, the filter 160, the memory 170, the inter predictor 180, the intra predictor 185, and the entropy encoder 190 may be configured. The inter predictor 180 and the intra predictor 185 may be collectively referred to as a predictor. In other words, the predictor may include an inter predictor 180 and an intra predictor 185. The transform unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transform unit 150 may be included in the residual processing unit. The residual processing unit may further include a subtracting unit 115. As an example, the image divider 110, the subtractor 115, the transformer 120, the quantizer 130, the inverse quantizer 140, the inverse transformer 150, and the adder 155 may be described. The filtering unit 160, the inter prediction unit 180, the intra prediction unit 185, and the entropy encoding unit 190 may be configured by one hardware component (eg, an encoder or a processor). In addition, the memory 170 may include a decoded picture buffer (DPB) or may be configured by a digital storage medium.

The image divider 110 may divide the input image (or picture or frame) input to the encoding apparatus 100 into one or more processing units. For example, the processing unit may be called a coding unit (CU). In this case, the coding unit may be recursively divided according to a quad-tree binary-tree (QTBT) structure from a coding tree unit (CTU) or a largest coding unit (LCU). For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad tree structure and / or a binary tree structure. In this case, for example, the quad tree structure may be applied first and the binary tree structure may be applied later. Alternatively, the binary tree structure may be applied first. The coding procedure according to the present invention may be performed based on the final coding unit that is no longer split. In this case, the maximum coding unit may be used as the final coding unit immediately based on coding efficiency according to the image characteristic, or if necessary, the coding unit is recursively divided into coding units of lower depths and optimized. A coding unit of size may be used as the final coding unit. Here, the coding procedure may include a procedure of prediction, transform, and reconstruction, which will be described later. As another example, the processing unit may further include a prediction unit (PU) or a transform unit (TU). In this case, the prediction unit and the transform unit may be partitioned or partitioned from the aforementioned final coding unit, respectively. The prediction unit may be a unit of sample prediction, and the transformation unit may be a unit for deriving a transform coefficient and / or a unit for deriving a residual signal from the transform coefficient.

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

The encoding apparatus 100 subtracts the prediction signal (predicted block, prediction sample array) output from the inter prediction unit 180 or the intra prediction unit 185 from the input image signal (original block, original sample array). A signal may be generated (residual signal, residual block, residual sample array), and the generated residual signal is transmitted to the converter 120. In this case, as shown, 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 100 may be called a subtraction unit 115. The prediction unit may perform a prediction on a block to be processed (hereinafter, referred to as a current block) and generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied on a current block or CU basis. As described later in the description of each prediction mode, the prediction unit may generate various information related to prediction, such as prediction mode information, and transmit the generated information to the entropy encoding unit 190. The information about the prediction may be encoded in the entropy encoding unit 190 and output in the form of a bitstream.

The intra predictor 185 may predict the current block by referring to the samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located apart according to the prediction mode. In intra prediction, prediction modes may include a plurality of non-directional modes and a plurality of directional modes. Non-directional mode may include, for example, DC mode and planner mode (Planar mode). The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes according to the degree of detail of the prediction direction. However, as an example, more or less directional prediction modes may be used depending on the setting. The intra predictor 185 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

The inter predictor 180 may derive the predicted block with respect to the current block based on the reference block (reference sample array) specified by the motion vector on the 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, subblocks, or samples based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block existing 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 referred to as a collocated reference block, a collocated CU (colCU), and the like, and a reference picture including the temporal neighboring block is called a collocated picture (colPic). It may be. For example, the inter prediction unit 180 constructs a motion information candidate list based on neighboring blocks and provides information indicating which candidates are used to derive a motion vector and / or a reference picture index of the current block. Can be generated. Inter prediction may be performed based on various prediction modes. For example, in case of a skip mode and a merge mode, the inter prediction unit 180 may use motion information of a neighboring block as motion information of a current block. In the skip mode, unlike the merge mode, the residual signal may not be transmitted. In the motion vector prediction (MVP) mode, the motion vector of the neighboring block is used as a motion vector predictor and the motion vector difference is signaled by signaling a motion vector difference. Can be directed.

The prediction signal generated by the inter predictor 180 or the intra predictor 185 may be used to generate a reconstruction signal or to generate a residual signal.

The transformer 120 may apply transform techniques to the residual signal to generate transform coefficients. For example, the transformation technique may include at least one of a discrete cosine transform (DCT), a discrete sine transform (DST), a karhunen-loeve transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). It may include. Here, GBT means a conversion obtained from this graph when the relationship information between pixels is represented by a graph. CNT refers to a transform that is generated based on and generates a prediction signal using all previously reconstructed pixels. In addition, the conversion process may be applied to pixel blocks having the same size as the square, or may be applied to blocks of variable size rather than square.

The quantization unit 130 quantizes the transform coefficients and transmits them to the entropy encoding unit 190. The entropy encoding unit 190 encodes the quantized signal (information about the quantized transform coefficients) and outputs the bitstream. have. The information about the quantized transform coefficients may be referred to as residual information. The quantization unit 130 may rearrange block quantized transform coefficients into a one-dimensional vector form based on a coefficient scan order, and quantize the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector form. Information about transform coefficients may be generated. The entropy encoding unit 190 may perform various encoding methods such as, for example, exponential Golomb, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), and the like. The entropy encoding unit 190 may encode information necessary for video / image reconstruction other than quantized transform coefficients (for example, values of syntax elements) together or separately. Encoded information (eg, encoded video / image information) may be transmitted or stored in units of NALs (network abstraction layer) in the form of a bitstream. The bitstream may be transmitted over a network or may be stored in a digital storage medium. The network may include a broadcasting 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, and the like. The signal output from the entropy encoding unit 190 may include a transmitting unit (not shown) for transmitting and / or a storing unit (not shown) for storing as an internal / external element of the encoding apparatus 100, or the transmitting unit It may be a component of the entropy encoding unit 190.

The quantized transform coefficients output from the quantization unit 130 may be used to generate a prediction signal. For example, the quantized transform coefficients may be reconstructed in the residual signal by applying inverse quantization and inverse transform through inverse quantization unit 140 and inverse transform unit 150 in a loop. The adder 155 adds the reconstructed residual signal to the predicted signal output from the inter predictor 180 or the intra predictor 185 so that a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) is added. Can be generated. If there is no residual for the block to be processed, such as when the skip mode is applied, the predicted block may be used as the reconstructed block. The adder 155 may be called a restoration unit or a restoration block generation unit. The generated reconstruction signal may be used for intra prediction of a next processing target block in a current picture, and may be used for inter prediction of a next picture through filtering as described below.

The filtering unit 160 may improve subjective / objective image quality by applying filtering to the reconstruction signal. For example, the filtering unit 160 may generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and the modified reconstructed picture is stored in the memory 170, specifically, the DPB of the memory 170. Can be stored in The various filtering methods may include, for example, deblocking filtering, a sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. As described below in the description of each filtering method, the filtering unit 160 may generate various information about the filtering and transmit the generated information to the entropy encoding unit 190. The filtering information may be encoded in the entropy encoding unit 190 and output in the form of a bitstream.

The modified reconstructed picture transmitted to the memory 170 may be used as the reference picture in the inter predictor 180. When the inter prediction is applied through the encoding apparatus, the encoding apparatus may avoid prediction mismatch between the encoding apparatus 100 and the decoding apparatus, and may improve encoding efficiency.

The memory 170 DPB may store the modified reconstructed picture for use as a reference picture in the inter predictor 180. The memory 170 may store the motion information of the block from which the motion information in the current picture is derived (or encoded) and / or the motion information of the blocks in the picture that have already been reconstructed. The stored motion information may be transmitted to the inter predictor 180 to use the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture, and transfer the reconstructed samples to the intra predictor 185.

2 is a schematic block diagram of a decoding apparatus in which an embodiment of the present invention is applied and decoding of a video / image signal is performed.

Referring to FIG. 2, the decoding apparatus 200 includes an entropy decoding unit 210, an inverse quantizer 220, an inverse transform unit 230, an adder 235, a filter 240, a memory 250, and an inter The prediction unit 260 and the intra prediction unit 265 may be configured. The inter predictor 260 and the intra predictor 265 may be collectively called a predictor. That is, the predictor may include an inter predictor 180 and an intra predictor 185. The inverse quantization unit 220 and the inverse transform unit 230 may be collectively called a residual processing unit. That is, the residual processing unit may include an inverse quantization unit 220 and an inverse transformation unit 230. The entropy decoder 210, the inverse quantizer 220, the inverse transformer 230, the adder 235, the filter 240, the inter predictor 260, and the intra predictor 265 are described in the embodiment. Can be configured by one hardware component (eg, decoder or processor). In addition, the memory 170 may include a decoded picture buffer (DPB) or may be configured by a digital storage medium.

When a bitstream including video / image information is input, the decoding apparatus 200 may reconstruct an image corresponding to a process in which video / image information is processed in the encoding apparatus of FIG. 1. For example, the decoding apparatus 200 may perform decoding using a processing unit applied in the encoding apparatus. Thus the processing unit of decoding may be a coding unit, for example, which may be split along a quad tree structure and / or a binary tree structure from a coding tree unit or a maximum coding unit. The reconstructed video signal decoded and output through the decoding apparatus 200 may be reproduced through the reproducing apparatus.

The decoding apparatus 200 may receive a signal output from the encoding apparatus of FIG. 1 in the form of a bitstream, and the received signal may be decoded through the entropy decoding unit 210. For example, the entropy decoding unit 210 may parse the bitstream to derive information (eg, video / image information) necessary for image reconstruction (or picture reconstruction). For example, the entropy decoding unit 210 decodes information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, quantized values of syntax elements required for image reconstruction, and transform coefficients for residuals. Can be output. More specifically, the CABAC entropy decoding method receives a bin corresponding to each syntax element in a bitstream, and decodes syntax element information and decoding information of neighboring and decoding target blocks or information of symbols / bins decoded in a previous step. The context model may be determined using the context model, the probability of occurrence of a bin may be predicted according to the determined context model, and arithmetic decoding of the bin may be performed to generate a symbol corresponding to the value of each syntax element. have. In this case, the CABAC entropy decoding method may update the context model by using the information of the decoded symbol / bin for the context model of the next symbol / bean after determining the context model. The information related to the prediction among the information decoded by the entropy decoding unit 2110 is provided to the prediction unit (the inter prediction unit 260 and the intra prediction unit 265), and the entropy decoding performed by the entropy decoding unit 210 is performed. Dual values, that is, quantized transform coefficients and related parameter information, may be input to the inverse quantizer 220. In addition, information on filtering among information decoded by the entropy decoding unit 210 may be provided to the filtering unit 240. Meanwhile, a receiver (not shown) that receives a signal output from the encoding apparatus may be further configured as an internal / external element of the decoding apparatus 200, or the receiver may be a component of the entropy decoding unit 210.

The inverse quantization unit 220 may dequantize the quantized transform coefficients and output the transform coefficients. The inverse quantization unit 220 may rearrange the quantized transform coefficients in the form of a two-dimensional block. In this case, the reordering may be performed based on the coefficient scan order performed by the encoding apparatus. The inverse quantization unit 220 may perform inverse quantization on quantized transform coefficients using a quantization parameter (for example, quantization step size information), and may obtain transform coefficients.

The inverse transformer 230 inversely transforms the transform coefficients to obtain a residual signal (residual block, residual sample array).

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 information about the prediction output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode.

The intra predictor 265 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 apart according to the prediction mode. In intra prediction, prediction modes may include a plurality of non-directional modes and a plurality of directional modes. The intra predictor 265 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

The inter prediction unit 260 may derive the predicted block for the current block based on the reference block (reference sample array) specified by the motion vector on the 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, subblocks, or samples based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block existing in the current picture and a temporal neighboring block present in the reference picture. For example, the inter prediction unit 260 may 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 based on the received candidate selection information. Inter prediction may be performed based on various prediction modes, and the information about the prediction may include information indicating a mode of inter prediction for the current block.

The adder 235 adds the obtained residual signal to the predictive signal (predicted block, predictive sample array) output from the inter predictor 260 or the intra predictor 265 to restore the reconstructed signal (restored picture, reconstructed block). , Restore sample array). If there is no residual for the block to be processed, such as when the skip mode is applied, the predicted block may be used as the reconstructed block.

The adder 235 may be called a restoration unit or a restoration block generation unit. The generated reconstruction signal may be used for intra prediction of a next processing target block in a current picture, and may be used for inter prediction of a next picture through filtering as described below.

The filtering unit 240 may improve subjective / objective image quality by applying filtering to the reconstruction signal. For example, the filtering unit 240 may generate a modified reconstructed picture by applying various filtering methods to the reconstructed picture, and the modified reconstructed picture may be stored in the memory 250, specifically, the DPB of the memory 250. Can be sent to. The various filtering methods may include, for example, deblocking filtering, a sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 250 may be used as the reference picture in the inter predictor 260. The memory 250 may store the motion information of the block from which the motion information in the current picture is derived (or decoded) and / or the motion information of the blocks in the picture that have already been reconstructed. The stored motion information may be transmitted to the inter predictor 260 to use the motion information of the spatial neighboring block or the motion information of the temporal neighboring block. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture, and transfer the reconstructed samples to the intra predictor 265.

In the present specification, the embodiments described by the filtering unit 160, the inter prediction unit 180, and the intra prediction unit 185 of the encoding apparatus 100 are respectively the filtering unit 240 and the inter prediction of the decoding apparatus 200. The same may also apply to the unit 260 and the intra predictor 265.

Block Partitioning

The video / image coding method according to this document may be performed based on various detailed techniques, and each detailed technique will be described as follows. Techniques described below include prediction, residual processing ((inverse) transformation, (inverse) quantization, etc.), syntax element coding, filtering, partitioning / division, etc. in the video / image encoding / decoding procedures described above and / or described below. It will be apparent to those skilled in the art that they may be involved in related procedures.

The block partitioning procedure according to this document may be performed by the image splitter 110 of the encoding apparatus described above, and the partitioning related information may be processed (encoded) by the entropy encoding unit 190 and transmitted to the decoding apparatus in the form of a bitstream. . The entropy decoding unit 210 of the decoding apparatus derives a block partitioning structure of the current picture based on the partitioning related information obtained from the bitstream, and based on this, a series of procedures (eg, prediction and residual) for image decoding. Processing, block reconstruction, in-loop filtering, etc.).

Partitioning of picture into CTUs

Pictures can be divided into a sequence of coding tree units (CTUs). The CTU may correspond to a coding tree block (CTB). Alternatively, the CTU may include a coding tree block of luma samples and two coding tree blocks of corresponding chroma samples. In other words, for a picture that includes three sample arrays, the CTU may include an N × N block of luma samples and two corresponding blocks of chroma samples.

The maximum allowable size of the CTU for coding and prediction may be different from the maximum allowable size of the CTU for transform. For example, the maximum allowable size of the luma block in the CTU may be 128x128.

Partitionig of the CTUs using a tree structure

The CTU may be divided into CUs based on a quad-tree (QT) structure. The quadtree structure may be referred to as a quaternary tree structure. This is to reflect various local characteristics. Meanwhile, in this document, the CTU may be divided based on a multitype tree structure partition including a binary tree (BT) and a ternary tree (TT) as well as a quad tree. Hereinafter, the QTBT structure may include a quadtree and binary tree based partition structure, and the QTBTTT may include a quadtree, binary tree, and ternary tree based partition structure. Alternatively, the QTBT structure may include a quadtree, binary tree and ternary tree based partitioning structure. In a coding tree structure, a CU may have a square or rectangular shape. The CTU may first be divided into quadtree structures. After that, the leaf nodes of the quadtree structure may be further divided by the multitype tree structure.

3 is a diagram illustrating an example of a multi-type tree structure as an embodiment to which the present invention can be applied.

In one embodiment of the present invention, the multitype tree structure may include four partition types as shown in FIG. The four split types include vertical binary splitting (SPLIT_BT_VER), horizontal binary splitting (SPLIT_BT_HOR), vertical ternary splitting (SPLIT_TT_VER), and horizontal ternary splitting (SPLIT_TT_HOR). ) May be included. Leaf nodes of the multitype tree structure may be called CUs. These CUs can be used for prediction and transform procedures. In general, CU, PU, and TU may have the same block size in this document. However, when the maximum supported transform length is smaller than the width or height of the color component of the CU, the CU and the TU may have different block sizes.

FIG. 4 is a diagram illustrating a signaling mechanism of partition partition information of a quadtree with nested multi-type tree structure according to an embodiment to which the present invention may be applied.

Here, the CTU is treated as the root of the quadtree, and is partitioned for the first time into a quadtree structure. Each quadtree leaf node may then be further partitioned into a multitype tree structure. In the multitype tree structure, a first flag (ex. Mtt_split_cu_flag) is signaled to indicate whether the node is additionally partitioned. If the node is additionally partitioned, a second flag (ex. Mtt_split_cu_verticla_flag) may be signaled to indicate the splitting direction. Thereafter, a third flag (ex. Mtt_split_cu_binary_flag) may be signaled to indicate whether the partition type is binary partition or ternary partition. For example, based on the mtt_split_cu_vertical_flag and the mtt_split_cu_binary_flag, a multi-type tree splitting mode (MttSplitMode) of a CU may be derived as shown in Table 1 below.

FIG. 5 is a diagram illustrating a method of dividing a CTU into multiple CUs based on a quadtree and accompanying multi-type tree structure as an embodiment to which the present invention may be applied.

Here, bold block edges represent quadtree partitioning and the remaining edges represent multitype tree partitioning. Quadtree partitions involving a multitype tree can provide a content-adapted coding tree structure. The CU may correspond to a coding block (CB). Alternatively, the CU may include a coding block of luma samples and two coding blocks of corresponding chroma samples. The size of a CU may be as large as CTU, or may be cut by 4 × 4 in luma sample units. For example, in the 4: 2: 0 color format (or chroma format), the maximum chroma CB size may be 64x64 and the minimum chroma CB size may be 2x2.

For example, in this document, the maximum allowable luma TB size may be 64x64 and the maximum allowable chroma TB size may be 32x32. If the width or height of the CB divided according to the tree structure is larger than the maximum transform width or height, the CB may be automatically (or implicitly) split until the TB size limit in the horizontal and vertical directions is satisfied.

Meanwhile, for a quadtree coding tree scheme involving a multitype tree, the following parameters may be defined and identified as SPS syntax elements.

CTU size: the root node size of a quaternary tree

MinQTSize: the minimum allowed quaternary tree leaf node size

MaxBtSize: the maximum allowed binary tree root node size

MaxTtSize: the maximum allowed ternary tree root node size

MaxMttDepth: the maximum allowed hierarchy depth of multi-type tree splitting from a quadtree leaf

MinBtSize: the minimum allowed binary tree leaf node size

MinTtSize: the minimum allowed ternary tree leaf node size

As an example of a quadtree coding tree structure involving a multitype tree, the CTU size may be set to 64x64 blocks of 128x128 luma samples and two corresponding chroma samples (in 4: 2: 0 chroma format). In this case, MinOTSize can be set to 16x16, MaxBtSize to 128x128, MaxTtSzie to 64x64, MinBtSize and MinTtSize (for both width and height) to 4x4, and MaxMttDepth to 4. Quarttree partitioning may be applied to the CTU to generate quadtree leaf nodes. The quadtree leaf node may be called a leaf QT node. Quadtree leaf nodes may have a 128x128 size (i.e. the CTU size) from a 16x16 size (i.e. the MinOTSize). If the leaf QT node is 128x128, it may not be additionally divided into a binary tree / a ternary tree. This is because in this case, even if split, it exceeds MaxBtsize and MaxTtszie (i.e. 64x64). In other cases, leaf QT nodes may be further partitioned into a multitype tree. Therefore, the leaf QT node is the root node for the multitype tree, and the leaf QT node may have a multitype tree depth (mttDepth) 0 value. If the multitype tree depth reaches MaxMttdepth (ex. 4), further splitting may not be considered further. If the width of the multitype tree node is equal to MinBtSize and less than or equal to 2xMinTtSize, then no further horizontal split may be considered. If the height of the multitype tree node is equal to MinBtSize and less than or equal to 2xMinTtSize, no further vertical split may be considered.

FIG. 6 is a diagram illustrating a method of limiting ternary-tree splitting as an embodiment to which the present invention may be applied.

With reference to FIG. 6, to allow for 64x64 luma blocks and 32x32 chroma pipeline designs in a hardware decoder, TT partitioning may be limited in certain cases. For example, when the width or height of the luma coding block is greater than a predetermined specific value (eg, 32 and 64), TT partitioning may be limited as shown in FIG. 6.

In this document, the coding tree scheme may support that the luma and chroma blocks have separate block tree structures. For P and B slices, luma and chroma CTBs in one CTU may be limited to have the same coding tree structure. However, for I slices, luma and chroma blocks may have a separate block tree structure from each other. If an individual block tree mode is applied, the luma CTB may be split into CUs based on a particular coding tree structure, and the chroma CTB may be split into chroma CUs based on another coding tree structure. This may mean that a CU in an I slice may consist of a coding block of a luma component or coding blocks of two chroma components, and a CU of a P or B slice may be composed of blocks of three color components.

In the above-described "Partitionig of the CTUs using a tree structure", a quadtree coding tree structure involving a multitype tree has been described, but a structure in which a CU is divided is not limited thereto. For example, the BT structure and the TT structure may be interpreted as a concept included in a multiple partitioning tree (MPT) structure, and the CU may be interpreted to be divided through the QT structure and the MPT structure. In one example where a CU is split through a QT structure and an MPT structure, a syntax element (eg, MPT_split_type) that contains information about how many blocks the leaf node of the QT structure is divided into and the leaf node of the QT structure are vertical The partition structure may be determined by signaling a syntax element (eg, MPT_split_mode) including information about which direction is divided into and horizontally.

In another example, the CU may be partitioned in a different way than the QT structure, BT structure or TT structure. That is, according to the QT structure, the CU of the lower depth is divided into 1/4 size of the CU of the upper depth, or the CU of the lower depth is divided into 1/2 size of the CU of the upper depth according to the BT structure, or according to the TT structure. Unlike the CU of the lower depth is divided into 1/4 or 1/2 size of the CU of the upper depth, the CU of the lower depth is sometimes 1/5, 1/3, 3/8, 3 of the CU of the upper depth. It can be divided into / 5, 2/3 or 5/8 size, the way in which the CU is divided is not limited to this.

If a portion of a tree node block exceeds the bottom or right picture boundary, the tree node block is placed so that all samples of all coded CUs are located within the picture boundaries. May be limited. In this case, for example, the following division rule may be applied.

-If a portion of a tree node block exceeds both the bottom and the right picture boundaries,

-If the block is a QT node and the size of the block is larger than the minimum QT size, the block is forced to be split with QT split mode.

-Otherwise, the block is forced to be split with SPLIT_BT_HOR mode

-Otherwise if a portion of a tree node block exceeds the bottom picture boundaries,

-If the block is a QT node, and the size of the block is larger than the minimum QT size, and the size of the block is larger than the maximum BT size, the block is forced to be split with QT split mode.

-Otherwise, if the block is a QT node, and the size of the block is larger than the minimum QT size and the size of the block is smaller than or equal to the maximum BT size, the block is forced to be split with QT split mode or SPLIT_BT_HOR mode.

-Otherwise (the block is a BTT node or the size of the block is smaller than or equal to the minimum QT size), the block is forced to be split with SPLIT_BT_HOR mode.

-Otherwise if a portion of a tree node block exceeds the right picture boundaries,

-If the block is a QT node, and the size of the block is larger than the minimum QT size, and the size of the block is larger than the maximum BT size, the block is forced to be split with QT split mode.

-Otherwise, if the block is a QT node, and the size of the block is larger than the minimum QT size and the size of the block is smaller than or equal to the maximum BT size, the block is forced to be split with QT split mode or SPLIT_BT_VER mode.

-Otherwise (the block is a BTT node or the size of the block is smaller than or equal to the minimum QT size), the block is forced to be split with SPLIT_BT_VER mode.

On the other hand, the quadtree coded block structure with the multi-type tree described above can provide a very flexible block partitioning structure. Because of the partition types supported in a multitype tree, different partition patterns can potentially result in the same coding block structure in some cases. By limiting the occurrence of such redundant partition patterns, the data amount of partitioning information can be reduced. A description with reference to the drawings below.

FIG. 7 is a diagram illustrating redundant division patterns that may occur in binary tree division and ternary tree division, as an embodiment to which the present invention may be applied.

As shown in FIG. 7, two levels of consecutive binary splits in one direction have the same coding block structure as the binary split for the center partition after the ternary split. . In this case, the binary tree split in the given direction for the center partition of the ternary tree split may be limited. This restriction can be applied for CUs of all pictures. If this particular partitioning is restricted, the signaling of the corresponding syntax elements can be modified to reflect this limited case, thereby reducing the number of bits signaled for partitioning. For example, as shown in FIG. 7, when the binary tree split for the center partition of the CU is restricted, the mtt_split_cu_binary_flag syntax element indicating whether the split is a binary split or a tenary split is not signaled, and the value Can be inferred by the decoder to zero.

Prediction

The decoded portion of the current picture or other pictures in which the current processing unit is included may be used to reconstruct the current processing unit in which decoding is performed.

Intra picture or I picture (slice), which uses only the current picture for reconstruction, i.e. performs only intra picture prediction, predicts a picture (slice) using at most one motion vector and reference index to predict each unit A picture using a predictive picture or P picture (slice), up to two motion vectors, and a reference index (slice) may be referred to as a bi-predictive picture or a B picture (slice).

Intra prediction means a prediction method that derives the current processing block from data elements (eg, sample values, etc.) of the same decoded picture (or slice). That is, a method of predicting pixel values of the current processing block by referring to reconstructed regions in the current picture.

Hereinafter, the inter prediction will be described in more detail.

Inter prediction (or inter screen prediction)

Inter prediction means a prediction method of deriving a current processing block based on data elements (eg, sample values or motion vectors, etc.) of pictures other than the current picture. That is, a method of predicting pixel values of the current processing block by referring to reconstructed regions in other reconstructed pictures other than the current picture.

Inter prediction (or inter picture prediction) is a technique for removing redundancy existing between pictures, and is mostly performed through motion estimation and motion compensation.

The present invention describes the detailed description of the inter prediction method described above with reference to FIGS. 1 and 2, and the decoder may be represented by the inter prediction-based video / image decoding method of FIG. 10 described later and the inter prediction unit in the decoding apparatus of FIG. 11. . In addition, the encoder may be represented by the inter prediction based video / video encoding method of FIG. 8 and the inter prediction unit in the encoding apparatus of FIG. 9. In addition, the data encoded by FIGS. 8 and 9 may be stored in the form of a bitstream.

The prediction unit of the encoding apparatus / decoding apparatus may derive the prediction sample by performing inter prediction on a block basis. Inter prediction may represent prediction derived in a manner dependent on data elements (e.g. sample values, motion information, etc.) of the picture (s) other than the current picture. When inter prediction is applied to the current block, a predicted block (prediction sample array) for the current block is derived based on a reference block (reference sample array) specified by a motion vector on the reference picture indicated by the reference picture index. Can be.

In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information of the current block may be predicted in units of blocks, subblocks, or samples based on the correlation of the motion information between the neighboring block and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction type (L0 prediction, L1 prediction, Bi prediction, etc.) information.

When inter prediction is applied, the neighboring block may include a spatial neighboring block existing 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 referred to as a collocated reference block, a collocated CU (colCU), and the like, and a reference picture including the temporal neighboring block is called a collocated picture (colPic). It may be. For example, a motion information candidate list may be constructed based on neighboring blocks of the current block, and a flag indicating which candidate is selected (used) to derive a motion vector and / or a reference picture index of the current block. Or index information may be signaled.

Inter prediction may be performed based on various prediction modes. For example, in the case of the skip mode and the merge mode, the motion information of the current block may be the same as the motion information of the selected neighboring block. In the skip mode, unlike the merge mode, the residual signal may not be transmitted. In the case of a motion vector prediction (MVP) mode, a motion vector of a selected neighboring block is 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.

8 and 9 illustrate an inter prediction based video / image encoding method and an inter prediction unit in an encoding apparatus according to an embodiment of the present invention.

8 and 9, S801 may be performed by the inter prediction unit 180 of the encoding apparatus, and S802 may be performed by the residual processing unit of the encoding apparatus. In detail, S802 may be performed by the subtraction unit 115 of the encoding apparatus. In S803, the prediction information may be derived by the inter prediction unit 180 and encoded by the entropy encoding unit 190. In S803, the residual information may be derived by the residual processor and encoded by the entropy encoding unit 190. The residual information is information about the residual samples. The residual information may include information about quantized transform coefficients for the residual samples.

As described above, the residual samples may be derived as transform coefficients through the transform unit 120 of the encoding apparatus, and the transform coefficients may be derived as transform coefficients quantized through the quantization unit 130. Information about the quantized transform coefficients may be encoded by the entropy encoding unit 190 through a residual coding procedure.

The encoding apparatus performs inter prediction on the current block (S801). The encoding apparatus may derive inter prediction mode and motion information of the current block and generate prediction samples of the current block. In this case, the inter prediction mode determination, the motion information derivation, and the prediction samples generation procedure may be performed simultaneously, or one procedure may be performed before the other. For example, the inter prediction unit 180 of the encoding apparatus may include a prediction mode determination unit 181, a motion information derivation unit 182, and a prediction sample derivation unit 183, and the prediction mode determination unit 181. In FIG. 2, a prediction mode for the current block may be determined, motion information derivation unit 182 may derive motion information of the current block, and prediction sample derivation unit 183 may derive motion samples of the current block.

For example, the inter prediction unit 180 of the encoding apparatus searches for a block similar to the current block in a predetermined area (search area) of reference pictures through motion estimation, and a difference from the current block is determined. Reference blocks that are minimum or below a certain criterion may be derived. Based on this, a reference picture index indicating a reference picture in which the reference block is located may be derived, and a motion vector may be derived based on a position difference between the reference block and the current block. The encoding apparatus may determine a mode applied to the current block among various prediction modes. The encoding apparatus may compare RD costs for the various prediction modes and determine an optimal prediction mode for the current block.

For example, when a skip mode or a merge mode is applied to the current block, the encoding apparatus constructs a merge candidate list to be described later, and among the reference blocks indicated by merge candidates included in the merge candidate list. A reference block having a difference from the current block that is smaller than or equal to a predetermined criterion may be derived. In this case, a merge candidate associated with the derived reference block is selected, and merge index information indicating the selected merge candidate may be generated and signaled to the decoding apparatus. The motion information of the current block may be derived using the motion information of the selected merge candidate.

As another example, when the (A) MVP mode is applied to the current block, the encoding apparatus constructs a (A) MVP candidate list to be described later, and among the mvp (motion vector predictor) candidates included in the (A) MVP candidate list. The motion vector of the selected mvp candidate may be used as mvp of the current block. In this case, for example, a motion vector indicating a reference block derived by the above-described motion estimation may be used as the motion vector of the current block, and the difference with the motion vector of the current block is smallest among the mvp candidates. An mvp candidate with a motion vector 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, the information about the MVD may be signaled to the decoding device. In addition, when the (A) MVP mode is applied, the value of the reference picture index may be configured with reference picture index information and separately signaled to the decoding apparatus.

The encoding apparatus may derive residual samples based on the prediction samples (S802). The encoding apparatus may derive the residual samples by comparing the original samples of the current block with the prediction samples.

The encoding apparatus encodes image information including prediction information and residual information (S803). The encoding apparatus may output the encoded image information in the form of a bitstream. The prediction information may include prediction mode information (eg, skip flag, merge flag or mode index) and information on motion information as information related to the prediction procedure. The information about the motion information may include candidate selection information (eg, merge index, mvp flag or mvp index) that is information for deriving a motion vector. In addition, the information about the motion information may include the above-described information about the MVD and / or reference picture index information.

The information about the motion information may include information indicating whether L0 prediction, L1 prediction, or bi prediction is applied. The residual information is information about the residual samples. The residual information may include information about quantized transform coefficients for the residual samples.

The output bitstream may be stored in a (digital) storage medium and delivered to the decoding device, or may be delivered to the decoding device via a network.

Meanwhile, as described above, the encoding apparatus may generate a reconstructed picture (including the reconstructed samples and the reconstructed block) based on the reference samples and the residual samples. This is because the encoding apparatus derives the same prediction result as that performed in the decoding apparatus, and thus the coding efficiency can be increased. Accordingly, the encoding apparatus may store a reconstructed picture (or reconstructed samples, a reconstructed block) in a memory and use it as a reference picture for inter prediction. As described above, an in-loop filtering procedure may be further applied to the reconstructed picture.

10 and 11 illustrate an inter prediction based video / image decoding method and an inter prediction unit in a decoding apparatus according to an embodiment of the present invention.

10 and 11, the decoding apparatus may perform an operation corresponding to the operation performed by the encoding apparatus. The decoding apparatus may perform prediction on the current block and derive prediction samples based on the received prediction information.

S1001 to S1003 may be performed by the inter prediction unit 260 of the decoding apparatus, and the residual information of S1004 may be obtained from the bitstream by the entropy decoding unit 210 of the decoding apparatus. The residual processor of the decoding apparatus may derive residual samples for the current block based on the residual information. In detail, the inverse quantization unit 220 of the residual processing unit performs dequantization on the basis of the quantized transform coefficients derived based on the residual information to derive transform coefficients and inverse transform unit of the residual processing unit ( 230 may derive residual samples for the current block by performing an inverse transform on the transform coefficients. S1005 may be performed by the adder 235 or the reconstruction unit of the decoding apparatus.

In detail, the decoding apparatus may determine a prediction mode for the current block based on the received prediction information (S1001). The decoding apparatus may determine which inter prediction mode is applied to the current block based on the prediction mode information in the prediction information.

For example, it may be determined whether the merge mode is not applied to the current block or (A) MVP mode is determined based on the merge flag. Alternatively, one of various inter prediction mode candidates may be selected based on the mode index. The inter prediction mode candidates may include a skip mode, a merge mode, and / or (A) MVP mode, or may include various inter prediction modes described below.

The decoding apparatus derives motion information of the current block based on the determined inter prediction mode (S1002). For example, when a skip mode or a merge mode is applied to the current block, the decoding apparatus may construct a merge candidate list to be described later, and select one merge candidate among merge candidates included in the merge candidate list. The selection may be performed based on the above merge information. The motion information of the current block may be derived using the motion information of the selected merge candidate. The motion information of the selected merge candidate may be used as motion information of the current block.

As another example, when (A) MVP mode is applied to the current block, the decoding apparatus constructs (A) MVP candidate list to be described later, and among (m) mvp (motion vector predictor) candidates included in the (A) MVP candidate list. The motion vector of the selected mvp candidate may be used as mvp of the current block. The selection may be performed based on the above-described selection information (mvp flag or mvp index). In this case, the MVD of the current block may be derived based on the information on the MVD, and the motion vector of the current block may be derived based on mvp and the MVD of the current block. In addition, a reference picture index of the current block may be derived based on the reference picture index information. A picture indicated by the reference picture index in the reference picture list for the current block may be derived as a reference picture referred for inter prediction of the current block.

Meanwhile, as described below, motion information of the current block may be derived without constructing a candidate list, and in this case, motion information of the current block may be derived according to a procedure disclosed in a prediction mode to be described later. In this case, the candidate list structure as described above may be omitted.

The decoding apparatus may generate prediction samples for the current block based on the motion information of the current block (S1003). In this case, the reference picture may be derived based on the reference picture index of the current block, and the prediction samples of the current block may be derived using the samples of the reference block indicated by the motion vector of the current block on the reference picture. In this case, as described below, a prediction sample filtering procedure for all or some of the prediction samples of the current block may be further performed.

For example, the inter prediction unit 260 of the decoding apparatus may include a prediction mode determination unit 261, a motion information derivation unit 262, and a prediction sample derivation unit 263, and the prediction mode determination unit 261 may be used. Determining a prediction mode for the current block based on the prediction mode information received in the step, and based on the information on the motion information received from the motion information derivation unit 262, motion information (motion vector and / or A reference picture index, etc.), and the prediction sample derivation unit 263 may derive the prediction samples of the current block.

The decoding apparatus generates residual samples for the current block based on the received residual information (S1004). The decoding apparatus may generate reconstructed samples for the current block based on the prediction samples and the residual samples, and may generate a reconstructed picture based on the prediction samples (S1005). After that, the in-loop filtering procedure may be further applied to the reconstructed picture as described above.

As described above, the inter prediction procedure may include determining an inter prediction mode, deriving motion information according to the determined prediction mode, and performing prediction based on the derived motion information (prediction sample generation).

Determination of inter prediction mode

Various inter prediction modes may be used for prediction of the current block in the picture. For example, various modes, such as merge mode, skip mode, MVP mode, and affine mode, may be used. Decoder side motion vector refinement (DMVR) mode, adaptive motion vector resolution (AMVR) mode, and the like may be further used as a secondary mode. The affine mode may be called an affine motion prediction mode. MVP mode may be referred to as advanced motion vector prediction (AMVP) mode.

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 the bitstream and received by the decoding apparatus. The prediction mode information may include index information indicating one of a plurality of candidate modes. Alternatively, the inter prediction mode may be indicated through hierarchical signaling of flag information. In this case, the prediction mode information may include one or more flags.

For example, a skip flag is signaled to indicate whether a skip mode is applied, and if a skip mode is not applied, a merge flag is signaled to indicate whether a merge mode is applied, and if a merge mode is not applied, an MVP mode is applied. Or may further signal a flag for additional classification. The affine mode may be signaled in an independent mode, or may be signaled in a mode dependent on a merge mode or an MVP mode. For example, the affine mode may be configured with one candidate of a merge candidate list or an MVP candidate list as described below.

Derivation of motion information according to inter prediction mode

Inter prediction may be performed 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 a high correlation using the original block in the original picture for the current block in fractional pixel units within a predetermined search range in the reference picture, thereby deriving motion information. Can be. Similarity of blocks can be derived based on the difference of phase based sample values. For example, the similarity of the blocks may be calculated based on the SAD between the current block (or template of the current block) and the reference block (or template of the reference block). In this case, motion information may be derived based on a reference block having the smallest SAD in the search area. The derived motion information may be signaled to the decoding apparatus according to various methods based on the inter prediction mode.

Merge mode and skip mode

FIG. 12 is a diagram for describing a neighboring block used in a merge mode or a skip mode as an embodiment to which the present invention is applied.

When the merge mode is applied, the motion information of the current prediction block is not directly transmitted, and the motion information of the current prediction block is derived using the motion information of the neighboring prediction block. Accordingly, 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 neighboring prediction blocks are used.

The encoder may search for merge candidate blocks used to derive motion information of the current prediction block to perform the merge mode. For example, up to five merge candidate blocks may be used, but the present invention is not limited thereto. The maximum number of merge candidate blocks may be transmitted in a slice header (or tile group header), but the present invention is not limited thereto. After finding the merge candidate blocks, the encoder may generate a merge candidate list, and select the merge candidate block having the smallest cost among them as the final merge candidate block.

The present invention provides various embodiments of a merge candidate block constituting the merge candidate list.

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. As a specific example, in the case of the spatial merge candidate, the blocks shown in FIG. 12 may be used as the spatial merge candidate.

13 is a flowchart illustrating a merge candidate list construction method according to an embodiment to which the present invention is applied.

Referring to FIG. 13, the coding apparatus (encoder / decoder) inserts spatial merge candidates derived by searching for spatial neighboring blocks of the current block to the merge candidate list (S1301). For example, the spatial neighboring blocks may include a lower left corner peripheral block, a left peripheral block, a right upper corner peripheral block, an upper peripheral block, and an upper left corner peripheral block of the current block. However, as an example, in addition to the above-described spatial peripheral blocks, additional peripheral blocks such as a right peripheral block, a lower peripheral block, and a lower right peripheral block may be further used as the spatial peripheral blocks. The coding apparatus may search for the spatial neighboring blocks based on priority, detect available blocks, and derive motion information of the detected blocks as the spatial merge candidates. For example, the encoder and the decoder may search the five blocks shown in FIG. 12 in the order of A1, B1, B0, A0, and B2, and index the available candidates sequentially to form a merge candidate list.

The coding apparatus inserts the temporal merge candidate derived by searching the temporal neighboring block of the current block into the merge candidate list (S1302). The temporal neighboring block 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 in which the temporal neighboring block is located may be called a collocated picture or a col picture. The temporal neighboring block may be searched in the order of the lower right corner peripheral block 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 may be stored as representative motion information for each predetermined storage unit in the col picture. In this case, it is not necessary to store the motion information for all the blocks in the predetermined storage unit, thereby obtaining a motion data compression effect. In this case, the constant storage unit may be predetermined, for example, 16x16 sample units, 8x8 sample units, or the like, or size information about the constant storage unit may be signaled from the encoder to the decoder. When the motion data compression is applied, motion information of the temporal neighboring block may be replaced with representative motion information of the predetermined storage unit in which the temporal neighboring block is located.

That is, in this case, in terms of implementation, a position that is arithmetically shifted after arithmetic right shift by a predetermined value based on the coordinates (upper left sample position) of the temporal neighboring block, rather than the prediction block located at the coordinate of the temporal neighboring block The temporal merge candidate may be derived based on the motion information of the covering prediction block. For example, when the constant storage unit is 2nx2n sample units, assuming that the coordinates of the temporal neighboring block are (xTnb, yTnb), the modified positions are ((xTnb >> n) << n) and (yTnb >> The motion information of the prediction block located at n) << n)) may be used for the temporal merge candidate.

Specifically, for example, when the constant storage unit is a 16x16 sample unit, when the coordinates of the temporal neighboring block are (xTnb, yTnb), the modified position is ((xTnb >> 4) << 4), ( The motion information of the prediction block located at yTnb >> 4) << 4)) may be used for the temporal merge candidate. Or, for example, when the constant storage unit is an 8x8 sample unit, if the coordinates of the temporal neighboring block are (xTnb, yTnb), the modified position is ((xTnb >> 3) << 3), (yTnb> The motion information of the prediction block located at > 3) < 3) can be used for the temporal merge candidate.

The coding apparatus may check whether the number of current merge candidates is smaller than the number of maximum merge candidates (S1303). The maximum number of merge candidates may be predefined or signaled at the encoder to the decoder. For example, the encoder may generate information about the maximum number of merge candidates, encode the information, and transmit the encoded information to the decoder in the form of a bitstream. If the maximum number of merge candidates is filled up, the subsequent candidate addition process may not proceed.

As a result of the checking, when the number of the current merge candidates is smaller than the maximum merge candidates, the coding apparatus inserts an additional merge candidate into the merge candidate list (S1304). The additional merge candidate may include, for example, ATMVP, combined bi-predictive merge candidate (when the slice type of the current slice is B type) and / or zero vector merge candidate.

As a result of the checking, when the number of the current merge candidates is not smaller than the number of the maximum merge candidates, the coding apparatus may terminate the construction of the merge candidate list. In this case, the encoder may select an optimal merge candidate among merge candidates constituting the merge candidate list based on a rate-distortion (RD) cost, and signal selection information (ex. Merge index) indicating the selected merge candidate to the decoder. can do. The decoder may select the optimal merge candidate based on the merge candidate list and the selection information.

As described above, the motion information of the selected merge candidate may be used as the motion information of the current block, and the prediction samples of the current block may be derived based on the motion information of the current block. An encoder may derive residual samples of the current block based on the prediction samples, and may signal residual information about the residual samples to a decoder. The decoder may generate reconstructed samples based on the residual samples derived from the residual information and the prediction samples, and generate a reconstructed picture based on the same.

When the skip mode is applied, the motion information of the current block may be derived in the same manner as when the merge mode is applied. However, when the skip mode is applied, the residual signal for the corresponding block is omitted, and thus prediction samples may be used as reconstructed samples.

MVP mode

14 is a flowchart illustrating a merge candidate list construction method according to an embodiment to which the present invention is applied.

When the Motion Vector Prediction (MVP) mode is applied, the motion vector and / or the temporal neighboring block (or Col block) of the restored spatial neighboring block (for example, may be the neighboring block described above with reference to FIG. 12). Using the motion vector, a motion vector predictor candidate list may be generated. That is, the motion vector of the reconstructed spatial neighboring block and / or the motion vector corresponding to the temporal neighboring block may be used as a motion vector predictor candidate.

The information about the prediction may include selection information (eg, MVP flag or MVP index) indicating an optimal motion vector predictor candidate selected from the motion vector predictor candidates included in the list. At this time, the prediction unit may select the motion vector predictor of the current block from among the motion vector predictor candidates included in the motion vector candidate list using the selection information. The prediction unit of the encoding apparatus may obtain a motion vector difference (MVD) between the motion vector of the current block and the motion vector predictor, and may encode the output vector in a bitstream form. That is, MVD may be obtained by subtracting the motion vector predictor from the motion vector of the current block. In this case, the prediction unit of the decoding apparatus may obtain a motion vector difference included in the information about 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 apparatus may obtain or derive a reference picture index or the like indicating the reference picture from the information about the prediction. For example, the motion vector predictor candidate list may be configured as shown in FIG. 14.

Example 1

In a conventional image compression technique, in order to perform bi-prediction, it is necessary to encode / decode motion information for each of reference list 0 (L0, list 0) and reference list 1 (L1, list 1). Here, the motion information may include a reference index (or reference picture index), a motion vector predictor (or a motion vector predictor index), and / or a motion vector difference (MVD, motion). vector difference).

That is, the encoder / decoder may determine the reference picture and the motion vector of the current block (or the current coding block) by using the reference index, the motion vector predictor, and the motion vector difference of each of the reference list 0 and the reference list 1. In this case, the decoder may perform a motion information parsing process for the current block based on the syntax structure of Table 2 below.

Referring to Table 2, an example of a motion information decoding process for the current coding block will be described.

The decoder parses inter_pred_idc when the current slice (or tile group) is a B slice. Here, inter_pred_idc represents a syntax (or syntax element) indicating whether the current block uses reference list 0, reference list 1, or bidirectional prediction.

When the current block uses bidirectional prediction, the decoder derives motion information in the reference list 0 direction and derives motion information in the reference list 1 direction in the same manner.

In detail, the decoder parses ref_idx_l0. Here, ref_idx_l0 represents a reference index of reference list 0. The decoder then calls the mvd_coding_l0 syntax and parses the mvp_l0_flag. Here, the mvd_coding_l0 syntax indicates a syntax (or function) for deriving motion vector difference with respect to reference list 0. FIG. mvp_l0_flag represents a motion vector predictor index of reference list 0.

The decoder then parses ref_idx_l1. Here, ref_idx_l1 indicates a reference index of reference list 1. The decoder then calls the mvd_coding_l1 syntax and parses the mvp_l1_flag. Here, the mvd_coding_l1 syntax indicates a syntax (or function) for deriving motion vector difference with respect to reference list 1. mvp_l1_flag represents a motion vector predictor index of reference list 0.

In other words, in the conventional image compression technique, the encoder is used separately for each reference list as shown in the example of Table 2 to derive reference pictures, motion vectors (predictors and differentials) for L0 and L1 when bidirectional prediction is applied. Send the data. As such, if all motion data is signaled for each reference direction, the accuracy of prediction can be increased in performing bidirectional prediction. However, since most moving pictures include objects having no motion or constant motion, transmitting both motion information of L0 and L1 may be inefficient in terms of bit rate.

Accordingly, the present invention proposes a symmetrical bi-prediction method to solve such a problem.

FIG. 15 is a diagram for describing a method of performing bidirectional prediction based on symmetrical motion information according to an embodiment to which the present invention is applied.

Referring to FIG. 15, if it is assumed that an object has symmetrical motion in consecutive pictures (or frames), the motion information of reference list 0 and the motion information of reference list 1 may have symmetric values. Here, the motion information may include at least one of a reference index (or reference picture index), a motion vector predictor (prediction value) (or a motion vector predictor index (flag)), and / or a motion vector difference. In an embodiment, the motion vector of the reference list 0 and the motion vector of the reference list 1 may be expressed by Equation 1 below.

In Equation 1, MV ₀ represents a motion vector of reference list 0, and MV1 represents a motion vector of reference list 1. FIG. Similarly, assuming that the object has a symmetrical motion in consecutive pictures, the distance between the current picture and the reference pictures may be expressed by Equation 2 below. Referring to Equation 1, the motion vector of L1 may be derived as a motion vector that is opposite to the motion vector of L0, that is, symmetrical.

In Equation 2, td ₀ represents a distance between the current picture and a reference picture in the direction of the reference list 0 (for example, a difference in picture order count (POC)), and td ₁ is a reference in the current picture and the reference list 1 direction. The distance between pictures is shown. That is, since a continuous reference picture is assumed, the distance between the L0 reference picture and the L1 reference picture from the current picture may have the same value as in Equation 2.

Meanwhile, the distance between the current picture and the reference pictures may be different. In this case, the symmetric motion information may be scaled based on the distance between the current picture and the reference picture. For example, the motion vector of L1 may be expressed as Equation 3 below.

Referring to Equation 3, the L1 motion vector may be derived as a scaled value based on a reference picture distance with respect to a symmetric L0 motion vector value.

In one embodiment, when the L0 motion vector and the reference picture are determined using the L0 motion information, the encoder / decoder uses Equation 1 and 2 without reference to signaling / parsing the L1 motion information and uses the equations 1 and 2 to refer to the L1 motion vector and reference picture. The picture can be determined. Also, as an example, when there is no reference picture of L1 derived, the encoder / decoder may determine the picture closest to the current picture as the reference picture of L1, and at this time, the motion of L1 using Equation 3 described above. Vectors can be derived.

Table 3 below illustrates a syntax structure for symmetrical bi-prediction using symmetric motion information.

Referring to Table 3, an example of a motion information decoding process for the current coding block will be described. Descriptions overlapping with the above-described Table 2 will be omitted.

The decoder parses inter_pred_idc when the current slice (or tile group) is a B slice.

The decoder parses symmetrical_flag when bidirectional prediction is applied to the current block. Here, symmetrical_flag represents a flag indicating whether to apply symmetric bidirectional prediction that performs bidirectional prediction using symmetric motion information.

In an embodiment of the present invention, when symmetric bidirectional prediction is applied, the encoder / decoder may parse motion information in one direction from the bit stream and use it to derive motion information in the other direction. The decoder may parse the motion information in the reference list 0 direction and determine whether to parse the motion information in the reference list 1 direction according to whether symmetric bidirectional prediction is applied to the current block.

Specifically, the decoder parses ref_idx_l0, calls the mvd_coding_l0 syntax, and parses mvp_l0_flag. When the symmetrical_flag value is 0, that is, when symmetric bidirectional prediction is not applied to the current block, the decoder parses ref_idx_l1, calls mvd_coding_l1 syntax, and parses mvp_l1_flag.

In the present embodiment, when the symmetrical_flag value is 1, that is, when symmetric bidirectional prediction is applied to the current block, the decoder determines motion information (specifically, reference picture index, motion vector difference, and motion vector predictor flag of reference list 1). ) May not be parsed. The decoder may derive the motion information of the reference list 1 using the motion information of the reference list 0.

As shown in Table 3, when symmetric bidirectional prediction is applied, the decoder may determine the motion information of L1 using the method described with reference to FIG. 15 and Equations 1 to 3 above. If symmetric bidirectional prediction is not applied, the decoder may perform general bidirectional prediction.

In addition, in one embodiment, when symmetric bidirectional prediction is applied, the encoder / decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0, L1). In an embodiment, the encoder / decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the reference list in each direction and the current picture. As an example, when there is no reference picture having the same td ₀ and td ₁ , the encoder / decoder may determine the reference picture closest to the current picture as the reference picture.

According to an embodiment of the present invention, by effectively expressing the symmetrical movement of an object using less information, it is possible to reduce signaling bits and improve compression performance while maintaining prediction accuracy.

Example 2

According to an embodiment of the present invention, a method of improving the reliability of the motion information of the reference list while reducing signaling bits by encoding / decoding a part of the motion information of the reference list in a specific direction is proposed.

In detail, when symmetric bidirectional prediction is applied, the encoder may transmit only a reference picture index among the motion information of L1 to the decoder. The decoder may determine the reference picture of L1 according to the received reference picture index. That is, the decoder determines the reference picture of L1 using the reference picture index signaled from the encoder, and other motion information (eg, motion vector difference and / or motion vector predictor) is motion information symmetric to the motion information of L0. It can be derived using.

In an embodiment, if the determined reference picture of L1 does not exist in the reference list, the encoder / decoder may determine the closest picture as the reference picture based on the POC distance from the current picture. In this case, the motion vector of L1 may be calculated (or derived) using Equation 3 described above.

Table 4 below illustrates a syntax structure for symmetrical bi-prediction using symmetric motion information.

Referring to Table 4, an example of a motion information decoding process for the current coding block will be described. The overlapping description with Tables 2 and 3 described above will be omitted.

The decoder parses inter_pred_idc when the current slice (or tile group) is a B slice.

The decoder parses symmetrical_flag when bidirectional prediction is applied to the current block. Here, symmetrical_flag represents a flag indicating whether to apply symmetric bidirectional prediction.

In an embodiment of the present invention, when symmetric bidirectional prediction is applied, the encoder / decoder may parse motion information in one direction from the bit stream and use it to derive motion information in the other direction. That is, the decoder may parse the motion information in the reference list 0 direction and determine whether to parse the motion information in the reference list 1 direction according to whether symmetric bidirectional prediction is applied to the current block. In particular, in the present embodiment, the decoder may derive the motion vector of the reference list 1 using the motion information of the reference list 0, and may derive the reference picture of the reference list 1 with the reference picture index value transmitted from the encoder. .

Specifically, the decoder parses ref_idx_l0, calls the mvd_coding_l0 syntax, and parses mvp_l0_flag. The decoder then parses ref_idx_l1. The decoder may determine the reference picture of Reference List 1 using ref_idx_l1.

If the symmetrical_flag value is 0, that is, if symmetric bidirectional prediction is not applied to the current block, the decoder calls mvd_coding_l1 syntax and parses mvp_l1_flag.

In the present embodiment, when the symmetrical_flag value is 1, that is, when symmetric bidirectional prediction is applied to the current block, the decoder extracts the motion information of the reference list 1 (specifically, the motion vector difference and / or the motion vector predictor flag). Without parsing, the decoder may derive the motion vector difference and / or motion vector predictor flag of reference list 1 using the motion vector difference and / or motion vector predictor of reference list 0.

As shown in Table 4, when symmetric bidirectional prediction is applied, the decoder may determine the motion information of L1 using FIG. 15, Equations 1 to 3, and the aforementioned method. If symmetric bidirectional prediction is not applied, the decoder may perform general bidirectional prediction.

In addition, in one embodiment, when symmetric bidirectional prediction is applied, the encoder / decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0, L1). In an embodiment, the encoder / decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the reference list in each direction and the current picture. For example, when there is no reference picture having the same td ₀ and td ₁ , the encoder / decoder may determine the reference picture that is closest to the current picture as the reference picture.

According to an embodiment of the present invention, by effectively expressing the symmetrical movement of an object using less information, it is possible to reduce signaling bits and improve compression performance while maintaining prediction accuracy.

Example 3

According to an embodiment of the present invention, a method of improving the reliability of the motion information of the reference list while reducing signaling bits by encoding / decoding a part of the motion information of the reference list in a specific direction is proposed.

As an embodiment, when symmetric bidirectional prediction is applied, the encoder may transmit only a motion vector predictor index (or flag) among the motion information of L1 to the decoder. The decoder may determine the motion vector predictor of L1 according to the received motion vector predictor index. The decoder may derive the motion vector difference and the reference picture index of L1 using the remaining motion information of L0 (that is, the motion vector difference and the reference picture index).

In detail, the motion vector difference of L1 may be calculated (or derived) using Equation 4 below.

In Equation 4, td ₀ represents the distance between the current picture and the reference picture in the reference list 0 direction (eg, a POC difference), and td ₁ represents the distance between the current picture and the reference picture in the reference list 1 direction.

In one embodiment, the encoder / decoder may determine the reference picture such that td ₀ and td ₁ have the same value. If there is no L1 reference picture having the same td ₀ and td ₁ , the encoder / decoder may determine a picture closest to the current picture as the reference picture.

Table 5 below illustrates a syntax structure for symmetrical bi-prediction using symmetric motion information.

Referring to Table 5, an example of a motion information decoding process for the current coding block will be described. Descriptions overlapping with the aforementioned Tables 2 to 4 will be omitted.

The decoder parses inter_pred_idc when the current slice (or tile group) is a B slice.

The decoder parses symmetrical_flag when bidirectional prediction is applied to the current block. Here, symmetrical_flag represents a flag indicating whether to apply symmetric bidirectional prediction.

In an embodiment of the present invention, when symmetric bidirectional prediction is applied, the encoder / decoder may parse motion information in one direction from the bit stream and use it to derive motion information in the other direction. That is, the decoder may parse the motion information in the reference list 0 direction and determine whether to parse the motion information in the reference list 1 direction according to whether symmetric bidirectional prediction is applied to the current block. In particular, in the present embodiment, the decoder may derive the motion vector difference of the reference list 1 using the motion vector difference of the reference list 0, and decode the reference picture index of the reference list 1 using the reference picture index of the reference list 0. The motion vector predictor of reference list 1 may be determined using the motion vector predictor index value transmitted from the encoder.

Specifically, the decoder parses ref_idx_l0, calls the mvd_coding_l0 syntax, and parses mvp_l0_flag. The decoder then parses mvp_l1_flag. The decoder may determine the motion vector predictor of reference list 1 using mvp_l1_flag.

If the symmetrical_flag value is 0, that is, symmetric bidirectional prediction is not applied to the current block, the decoder calls mvd_coding_l1 syntax and parses ref_idx_l1.

In the present embodiment, when the symmetrical_flag value is 1, that is, when symmetric bidirectional prediction is applied to the current block, the decoder does not parse motion information (specifically, motion vector difference and / or reference picture index) of reference list 1. Instead, the decoder may derive the motion vector difference and / or the reference picture index of reference list 1 using the motion vector difference and / or the reference picture index of reference list 0.

As shown in Table 5, when symmetric bidirectional prediction is applied, the decoder may determine the motion information of L1 using FIG. 15, Equations 1 to 4, and the aforementioned method. If symmetric bidirectional prediction is not applied, the decoder may perform general bidirectional prediction.

In addition, in one embodiment, when symmetric bidirectional prediction is applied, the encoder / decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0, L1). In an embodiment, the encoder / decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the reference list in each direction and the current picture. For example, when there is no reference picture having the same td ₀ and td ₁ , the encoder / decoder may determine the reference picture that is closest to the current picture as the reference picture.

According to an embodiment of the present invention, by effectively expressing the symmetrical movement of an object using less information, it is possible to reduce signaling bits and improve compression performance while maintaining prediction accuracy.

Example 4

According to an embodiment of the present invention, a method of improving the reliability of the motion information of the reference list while reducing signaling bits by encoding / decoding a part of the motion information of the reference list in a specific direction is proposed.

As an embodiment, when symmetric bidirectional prediction is applied, the encoder may transmit only a motion vector predictor index (or a flag) and a reference picture index among the motion information of L1 to the decoder. The decoder may determine the motion vector predictor and the reference picture of L1 according to the received motion vector predictor index and the reference picture index. The decoder may derive the motion vector difference of L1 using the remaining motion information of L0 (that is, the motion vector difference). In detail, the motion vector difference of L1 may be calculated (or derived) using Equation 4 described above.

In one embodiment, the encoder / decoder may determine the reference picture such that td0 and td1 have the same value. If there is no L1 reference picture in which td0 and td1 are the same, the encoder / decoder may determine the picture closest to the current picture as the reference picture.

Table 6 below illustrates a syntax structure for symmetrical bi-prediction using symmetric motion information.

Referring to Table 6, an example of a motion information decoding process for the current coding block will be described. Descriptions overlapping with the aforementioned Tables 2 to 5 will be omitted.

The decoder parses inter_pred_idc when the current slice (or tile group) is a B slice.

The decoder parses symmetrical_flag when bidirectional prediction is applied to the current block. Here, symmetrical_flag represents a flag indicating whether to apply symmetric bidirectional prediction.

In an embodiment of the present invention, when symmetric bidirectional prediction is applied, the encoder / decoder may parse motion information in one direction from the bit stream and use it to derive motion information in the other direction. That is, the decoder may parse the motion information in the reference list 0 direction and determine whether to parse the motion information in the reference list 1 direction according to whether symmetric bidirectional prediction is applied to the current block. In particular, in the present embodiment, the decoder may derive the motion vector difference of the reference list 1 using the motion vector difference of the reference list 0. The decoder may determine the reference picture index of the reference list 1 using the reference picture index transmitted from the encoder, and determine the motion vector predictor of the reference list 1 using the motion vector predictor index value transmitted from the encoder.

Specifically, the decoder parses ref_idx_l0, calls the mvd_coding_l0 syntax, and parses mvp_l0_flag. The decoder then parses mvp_l1_flag and parses ref_idx_l1. The decoder may determine the motion vector predictor of reference list 1 using mvp_l1_flag and determine the reference picture index of reference list 1 using ref_idx_l1.

If the symmetrical_flag value is 0, that is, symmetric bidirectional prediction is not applied to the current block, the decoder calls the mvd_coding_l1 syntax.

In the present embodiment, when the symmetrical_flag value is 1, that is, when symmetric bidirectional prediction is applied to the current block, the decoder does not parse the motion information (specifically, the motion vector difference) of the reference list 1, and the decoder does not parse the reference list. A motion vector difference of reference list 1 may be derived using a motion vector difference of zero.

As shown in Table 6, when symmetric bidirectional prediction is applied, the decoder may determine the motion information of L1 using FIG. 15, Equations 1 to 4, and the aforementioned method. If symmetric bidirectional prediction is not applied, the decoder may perform general bidirectional prediction.

In addition, in one embodiment, when symmetric bidirectional prediction is applied, the encoder / decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0, L1). In an embodiment, the encoder / decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the reference list in each direction and the current picture. For example, when there is no reference picture having the same td ₀ and td ₁ , the encoder / decoder may determine the reference picture that is closest to the current picture as the reference picture.

According to an embodiment of the present invention, by effectively expressing the symmetrical movement of an object using less information, it is possible to reduce signaling bits and improve compression performance while maintaining prediction accuracy.

Example 5

According to an embodiment of the present invention, a method of improving the reliability of the motion information of the reference list while reducing signaling bits by encoding / decoding a part of the motion information of the reference list in a specific direction is proposed.

As an embodiment, when symmetric bidirectional prediction is applied, the encoder may transmit only the motion vector predictor index (or flag) and the motion vector difference among the motion information of L1 to the decoder. The decoder may determine the motion vector predictor and the motion vector difference of L1 according to the received motion vector predictor index and the motion vector difference. The decoder may derive the reference picture index of L1 using the remaining motion information of L0 (that is, the reference picture index).

In one embodiment, the encoder / decoder may determine the reference picture such that td0 and td1 have the same value. If there is no L1 reference picture in which td0 and td1 are the same, the encoder / decoder may determine the picture closest to the current picture as the reference picture.

Table 7 below illustrates a syntax structure for symmetrical bi-prediction using symmetric motion information.

An example of a motion information decoding process for the current coding block will be described with reference to Table 7. The description overlapping with the aforementioned Tables 2 to 6 will be omitted.

The decoder parses inter_pred_idc when the current slice (or tile group) is a B slice.

The decoder parses symmetrical_flag when bidirectional prediction is applied to the current block. Here, symmetrical_flag represents a flag indicating whether to apply symmetric bidirectional prediction.

In an embodiment of the present invention, when symmetric bidirectional prediction is applied, the encoder / decoder may parse motion information in one direction from the bit stream and use it to derive motion information in the other direction. That is, the decoder may parse the motion information in the reference list 0 direction and determine whether to parse the motion information in the reference list 1 direction according to whether symmetric bidirectional prediction is applied to the current block. In particular, in the present embodiment, the decoder may derive the reference picture index of reference list 1 using the reference picture index of reference list 0. FIG. The decoder may determine the motion vector predictor of the reference list 1 using the motion vector predictor index (or the flag) transmitted from the encoder, and determine the motion vector difference of the reference list 1 using the motion vector difference transmitted from the encoder. You can decide.

Specifically, the decoder parses ref_idx_l0, calls the mvd_coding_l0 syntax, and parses mvp_l0_flag. The decoder then parses mvp_l1_flag and calls the mvd_coding_l1 syntax. The decoder may determine the motion vector predictor of reference list 1 using mvp_l1_flag, and determine the motion vector difference of reference list 1 through mvd_coding_l1 syntax.

If the symmetrical_flag value is 0, that is, symmetric bidirectional prediction is not applied to the current block, the decoder parses the reference picture index.

In this embodiment, when the symmetrical_flag value is 1, that is, when symmetric bidirectional prediction is applied to the current block, the decoder does not parse the motion information of the reference list 1 (specifically, the reference picture index) and the decoder does not parse the reference list. A reference picture index of reference list 1 may be derived using a motion vector difference of zero.

As shown in Table 7, when symmetric bidirectional prediction is applied, the decoder may determine the motion information of L1 using FIG. 15, Equations 1 to 4, and the aforementioned method. If symmetric bidirectional prediction is not applied, the decoder may perform general bidirectional prediction.

In addition, in one embodiment, when symmetric bidirectional prediction is applied, the encoder / decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0, L1). In an embodiment, the encoder / decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the reference list in each direction and the current picture. For example, when there is no reference picture having the same td ₀ and td ₁ , the encoder / decoder may determine the reference picture that is closest to the current picture as the reference picture.

According to an embodiment of the present invention, by effectively expressing the symmetrical movement of an object using less information, it is possible to reduce signaling bits and improve compression performance while maintaining prediction accuracy.

Embodiments of the present invention described above have been described separately for the convenience of description, but the present invention is not limited thereto. That is, the embodiments described in Embodiments 1 to 5 described above may be performed independently, or one or more embodiments may be combined and performed.

16 is a flowchart illustrating a method of generating an inter prediction block according to an embodiment to which the present invention is applied.

Referring to FIG. 16, a decoder is mainly described for convenience of description, but the present invention is not limited thereto, and the method of generating an inter prediction block according to an embodiment of the present invention may be performed in the same manner in the encoder and the decoder.

When the bi-prediction is applied to the current block, the decoder determines whether symmetric bi-prediction is applied to the current block (S1601). In this case, the symmetric bidirectional prediction indicates a mode for encoding motion information for bidirectional prediction using motion information symmetrical with respect to both reference directions based on the current picture.

The decoder parses at least one of a first reference index, a first motion vector predictor flag, or a first motion vector difference of the first reference picture list (S1602). .

When the symmetric bidirectional prediction is applied to the current block, the decoder uses a second reference picture list using at least one of the first reference index, the first motion vector predictor flag, or the first motion vector difference. At least one of a reference index, a second motion vector predictor flag, or a second motion vector difference is derived (S1603).

As described above in Embodiments 1 to 5, the decoder may parse motion information of the first reference picture list L0 and determine the L0 motion vector and / or the L0 reference picture. The decoder may derive the motion information of the second reference picture list L1 using the parsed motion information or the determined motion information. The decoder may generate a prediction block by performing bidirectional prediction based on the derived motion information.

In an embodiment, the deriving of at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference may be performed by using the first reference index in the first reference picture list. Determining a first reference picture of a current block, wherein the second reference index is a difference in picture order count (POC) between the current picture and the first reference picture in the second reference picture list; Can be derived to the same picture.

In an embodiment, when a picture having the same POC difference between the current picture and the first reference picture does not exist in the second reference picture list, the second reference index is configured to include the current picture and the POC in the second reference picture list. The difference can be derived with the smallest picture.

In an embodiment, the second motion vector difference may be derived from a value having the same magnitude as that of the first motion vector difference and having the opposite sign.

In some embodiments, a first POC difference indicating a difference in picture order count (POC) between the current picture and the first reference picture and a second POC difference indicating a difference in POC between the current picture and the second reference picture may be determined. If not the same, the magnitude of the second motion vector difference may be scaled based on the first POC difference and the second POC difference.

In an embodiment, when the symmetric bidirectional prediction is not applied to the current block, the decoder may parse at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference.

In example embodiments, the first reference index and the second reference index may be different from each other in the picture order count (POC) between the reference pictures of the first reference picture list and the current picture and the first reference picture list. It may be derived based on the POC difference between the reference pictures and the current picture.

That is, as described above, the decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0 and L1). In one embodiment, the decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the direction reference list in each direction and the current picture. For example, the decoder may determine a picture closest to the current picture (ie, the smallest POC difference) in each direction reference picture list as the reference picture in each direction.

17 is a diagram illustrating an inter prediction apparatus according to an embodiment to which the present invention is applied.

In FIG. 17, the inter prediction unit is illustrated as one block for convenience of description, but the inter prediction unit may be implemented in a configuration included in the encoder and / or the decoder.

Referring to FIG. 17, the inter predictor implements the functions, processes, and / or methods proposed in FIGS. 8 to 16. In detail, the inter prediction unit may include a symmetric bidirectional prediction determination unit 1701, a first reference picture list deriving unit 1702, and a second reference picture list deriving unit 1703.

When bi-prediction is applied to the current block, the symmetric bidirectional prediction determiner 1701 determines whether symmetric bi-prediction is applied to the current block. In this case, the symmetric bidirectional prediction indicates a mode for encoding motion information for bidirectional prediction using motion information symmetrical with respect to both reference directions based on the current picture.

The first reference picture list deriving unit 1702 may include one of a first reference index, a first motion vector predictor flag, or a first motion vector difference of the first reference picture list. Parse at least one.

When the symmetric bidirectional prediction is applied to the current block, the second reference picture list derivator 1703 uses at least one of the first reference index, the first motion vector predictor flag, or the first motion vector difference. At least one of a second reference index, a second motion vector predictor flag, or a second motion vector difference of the second reference picture list may be derived.

As described above in Embodiments 1 to 5, the decoder may parse motion information of the first reference picture list L0 and determine the L0 motion vector and / or the L0 reference picture. The decoder may derive the motion information of the second reference picture list L1 using the parsed motion information or the determined motion information. The decoder may generate a prediction block by performing bidirectional prediction based on the derived motion information.

In an embodiment, the second reference picture list deriving unit 1703 determines the first reference picture of the current block in the first reference picture list using the first reference index, and the second reference index is determined by the second reference picture list derivation unit 1703. A difference in picture order count (POC) between the current picture and the first reference picture may be derived from the second reference picture list.

In an embodiment, when a picture having the same POC difference between the current picture and the first reference picture does not exist in the second reference picture list, the second reference index is configured to include the current picture and the POC in the second reference picture list. The difference can be derived with the smallest picture.

In an embodiment, the second motion vector difference may be derived from a value having the same magnitude as that of the first motion vector difference and having the opposite sign.

In some embodiments, a first POC difference indicating a difference in picture order count (POC) between the current picture and the first reference picture and a second POC difference indicating a difference in POC between the current picture and the second reference picture may be determined. If not the same, the magnitude of the second motion vector difference may be scaled based on the first POC difference and the second POC difference.

In an embodiment, when the symmetric bidirectional prediction is not applied to the current block, the second reference picture list derivator 1703 may include the second reference index, the second motion vector predictor flag, or the second motion vector. At least one of the differences may be parsed.

In example embodiments, the first reference index and the second reference index may be different from each other in the picture order count (POC) between the reference pictures of the first reference picture list and the current picture and the first reference picture list. It may be derived based on the POC difference between the reference pictures and the current picture.

That is, as described above, the decoder may derive according to a predefined criterion without parsing all of the reference picture indices for both directions (ie, L0 and L1). In one embodiment, the decoder may derive the reference picture index in each direction based on the POC difference between the reference pictures in the direction reference list in each direction and the current picture. For example, the decoder may determine a picture closest to the current picture (ie, the smallest POC difference) in each direction reference picture list as the reference picture in each direction.

18 shows a video coding system to which the present invention is applied.

The video coding system can include a source device and a receiving device. The source device may deliver the encoded video / image information or data to a receiving device through a digital storage medium or network in a file or streaming form.

The source device may include a video source, an encoding apparatus, and a transmitter. The receiving device may include a receiver, a decoding apparatus, and a renderer. The encoding device may be called a video / image encoding device, and the decoding device may be called a video / image decoding device. The transmitter may be included in the encoding device. The receiver may be included in the decoding device. The renderer may include a display unit, and the display unit may be configured as a separate device or an external component.

The video source may acquire the video / image through a process of capturing, synthesizing, or generating the video / image. The video source may comprise a video / image capture device and / or a video / image generation device. The video / image capture device may include, for example, one or more cameras, video / image archives including previously captured video / images, and the like. Video / image generation devices may include, for example, computers, tablets and smartphones, and may (electronically) generate video / images. For example, a virtual video / image may be generated through a computer or the like. In this case, the video / image capturing process may be replaced by a process of generating related data.

The encoding device may encode the input video / image. The encoding apparatus may perform a series of procedures such as prediction, transform, and quantization for compression and coding efficiency. The encoded data (encoded video / image information) may be output in the form of a bitstream.

The transmitter may transmit the encoded video / video information or data output in the form of a bitstream to the receiver of the receiving device through 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, and the like. The transmission unit may include an element for generating a media file through a predetermined file format, and may include an element for transmission through a broadcast / communication network. The receiver may extract the bitstream and transmit the extracted bitstream to the decoding apparatus.

The decoding apparatus may decode the video / image by performing a series of procedures such as inverse quantization, inverse transformation, and prediction corresponding to the operation of the encoding apparatus.

The renderer may render the decoded video / image. The rendered video / image may be displayed through the display unit.

19 is a diagram illustrating a structure of a content streaming system according to an embodiment to which the present invention is applied.

Referring to FIG. 19, a content streaming system to which the present invention is applied may largely include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device.

The encoding server compresses content input from multimedia input devices such as a smart phone, a camera, a camcorder, etc. into digital data to generate a bitstream and transmit the bitstream to the streaming server. As another example, when multimedia input devices such as smart phones, cameras, camcorders, etc. directly generate a bitstream, the encoding server may be omitted.

The bitstream may be generated by an encoding method or a bitstream generation method to which the present invention is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user device based on the user's request through the web server, and the web server serves as a medium for informing the user of what service. When a user requests a desired service from the web server, the web server delivers it to a streaming server, and the streaming server transmits multimedia data to the user. In this case, the content streaming system may include a separate control server. In this case, the control server plays a role of controlling a command / response between devices in the content streaming system.

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

Examples of the user device include a mobile phone, a smart phone, a laptop computer, a digital broadcasting terminal, a personal digital assistant (PDA), a portable multimedia player (PMP), navigation, a slate PC, Tablet PCs, ultrabooks, wearable devices, such as smartwatches, glass glasses, head mounted displays, digital TVs, desktops Computer, digital signage, and the like.

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

As described above, the embodiments described herein 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 addition, the decoder and encoder to which the present invention is applied include a multimedia broadcasting transmitting and receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video chat device, a real time communication device such as video communication, a mobile streaming device, Storage media, camcorders, video on demand (VoD) service providing devices, OTT video (Over the top video) devices, Internet streaming service providing devices, three-dimensional (3D) video devices, video telephony video devices, and medical video devices. It can be used to process video signals or data signals. For example, the OTT 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 digital video recorder (DVR), and the like.

In addition, the processing method to which the present invention 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 present invention 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 be, 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 disc. It may include a data storage device. The computer-readable recording medium also includes media embodied in the form of a carrier wave (eg, transmission over the Internet). In addition, the bitstream generated by the encoding method may be stored in a computer-readable recording medium or transmitted through a wired or wireless communication network.

In addition, an embodiment of the present invention may be implemented as a computer program product by program code, which may be performed on a computer by an embodiment of the present invention. The program code may be stored on a carrier readable by a computer.

The embodiments described above are the components and features of the present invention are combined in a predetermined form. Each component or feature is to be considered optional unless stated otherwise. Each component or feature may be embodied in a form that is not combined with other components or features. It is also possible to combine some of the components and / or features to form an embodiment of the invention. The order of the operations described in the embodiments of the present invention may be changed. Some components or features of one embodiment may be included in another embodiment or may be replaced with corresponding components or features of another embodiment. It is obvious that the claims may be combined to form an embodiment by combining claims that do not have an explicit citation relationship in the claims or as new claims by post-application correction.

Embodiments according to the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of a hardware implementation, an embodiment of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), FPGAs ( field programmable gate arrays), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that performs the functions or operations described above. The software code may be stored in memory and driven by the processor. The memory may be located inside or outside the processor, and may exchange data with the processor by various known means.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential features of the present invention. Accordingly, the above detailed description should not be construed as limiting in all aspects and should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention.

As mentioned above, preferred embodiments of the present invention are disclosed for purposes of illustration, and those skilled in the art can improve and change various other embodiments within the spirit and technical scope of the present invention disclosed in the appended claims below. , Replacement or addition would be possible.

Claims (14)

1. A method of decoding an image based on inter prediction mode,

   When bi-prediction is applied to the current block, determining whether symmetric bi-prediction is applied to the current block, wherein the symmetric bi-prediction is positive based on the current picture Indicates a mode of encoding motion information for bidirectional prediction using motion information symmetric with respect to a reference direction;

   Parsing at least one of a first reference index, a first motion vector predictor flag, or a first motion vector difference of the first reference picture list; And

   When the symmetric bidirectional prediction is applied to the current block, a second reference index of a second reference picture list using at least one of the first reference index, the first motion vector predictor flag, or the first motion vector difference. And deriving at least one of a second motion vector predictor flag or a second motion vector difference.
2. The method of claim 1,

   Deriving at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference,

   Determining a first reference picture of the current block in the first reference picture list by using the first reference index,

   And wherein the second reference index is derived from a picture having a same picture order count (POC) difference between the current picture and the first reference picture in the second reference picture list.
3. The method of claim 2,

   If a picture having the same POC difference between the current picture and the first reference picture does not exist in the second reference picture list, the second reference index has the smallest difference between the current picture and the POC in the second reference picture list. A method of decoding an image, which is derived from a picture.
4. The method of claim 1,

   The second motion vector difference is

   And a magnitude equal to the first motion vector difference and derived from an opposite sign.
5. The method of claim 4, wherein

   When the first POC difference indicating the difference in picture order count (POC) between the current picture and the first reference picture and the second POC difference indicating the difference in POC between the current picture and the second reference picture are not equal to each other, The magnitude of the second motion vector difference is scaled based on the first POC difference and the second POC difference.
6. The method of claim 1,

   Parsing at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference when the symmetric bidirectional prediction is not applied to the current block. Way.
7. The method of claim 1,

   The first reference index and the second reference index may be different from reference picture counts (POC) between the reference pictures of the first reference picture list and the current picture and reference pictures of the first reference picture list. And derived based on the POC difference between the current pictures.
8. An apparatus for decoding an image based on an inter prediction mode,

   When bi-prediction is applied to the current block, the symmetric bi-prediction determiner determines whether symmetric bi-prediction is applied to the current block, wherein the symmetric bi-prediction is the current picture Represents a mode of encoding motion information for bidirectional prediction using motion information symmetric with respect to both reference directions with respect to the reference;

   A first reference picture list for parsing at least one of a first reference index, a first motion vector predictor flag, or a first motion vector difference of the first reference picture list Induction part; And

   When the symmetric bidirectional prediction is applied to the current block, a second reference index of a second reference picture list using at least one of the first reference index, the first motion vector predictor flag, or the first motion vector difference. And a second reference picture list derivation unit for deriving at least one of a second motion vector predictor flag or a second motion vector difference.
9. The method of claim 8,

   The second reference picture list deriving unit,

   Determine a first reference picture of the current block from the first reference picture list by using the first reference index,

   And the second reference index is derived from a picture having a same picture order count (POC) difference between the current picture and the first reference picture in the second reference picture list.
10. The method of claim 9,

    If a picture having the same POC difference between the current picture and the first reference picture does not exist in the second reference picture list, the second reference index has the smallest difference between the current picture and the POC in the second reference picture list. An apparatus for decoding an image, which is guided to a picture.
11. The method of claim 8,

    The second motion vector difference is

    And a signal having the same magnitude as that of the first motion vector difference and having an opposite sign.
12. The method of claim 11,

    When the first POC difference indicating the difference in picture order count (POC) between the current picture and the first reference picture and the second POC difference indicating the difference in POC between the current picture and the second reference picture are not equal, The magnitude of the second motion vector difference is scaled based on the first POC difference and the second POC difference.
13. The method of claim 8,

    The second reference picture list deriving unit,

    And when the symmetric bidirectional prediction is not applied to the current block, at least one of the second reference index, the second motion vector predictor flag, or the second motion vector difference is parsed.
14. The method of claim 8,

    The first reference index and the second reference index may be different from reference picture counts (POC) between the reference pictures of the first reference picture list and the current picture and reference pictures of the first reference picture list. The apparatus for decoding an image derived based on the POC difference between the current pictures.

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