Image encoding / decoding method, bitstream transmission method, and recording medium storing bitstream

Abstract

An image encoding / decoding method, a bitstream transmission method, and a computer-readable recording medium storing a bitstream are provided. An image decoding method according to the present disclosure may be an image decoding method performed by an image decoding device, the image decoding method including: acquiring first information based on a prediction direction of a current block being bidirectional; deriving at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block based on the first information indicating that different prediction types are applied to each prediction direction of the current block; and bidirectionally predicting the current block with the different prediction types based on one of the at least one bidirectional motion information candidate, the different prediction types being a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type.

Description

[Technical field]

[0001] The present disclosure relates to an image encoding / decoding method, a bitstream transmission method, and a recording medium storing a bitstream, and more particularly, to bidirectional prediction based on different types of prediction units during inter prediction. [Background technology]

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

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

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

[0005] Another object of the present disclosure is to provide a method for performing bidirectional prediction based on prediction units of different types.

[0006] Another object of the present disclosure is to provide various methods for deriving prediction candidates for bidirectional prediction.

[0007] Another object of the present disclosure is to provide a method for encoding and obtaining motion vector differentials.

[0008] Another object of the present disclosure is to provide a non-transitory computer-readable recording medium for storing a bitstream generated by the image encoding method according to the present disclosure.

[0009] Another object of the present disclosure is to provide a non-transitory computer-readable recording medium for storing a bitstream that is received by an image decoding device according to the present disclosure, decoded, and used to reconstruct an image.

[0010] Another object of the present disclosure is to provide a method for transmitting a bitstream generated by the image encoding method according to the present disclosure.

[0011] The technical problems to be solved by the present disclosure are not limited to the above-mentioned technical problems, and other technical problems not described above will be clearly understood by a person having ordinary skill in the art to which the present disclosure pertains from the following description. [Means for solving the problem]

[0012] An image decoding method according to one aspect of the present disclosure may be an image decoding method performed by an image decoding device, the image decoding method including: acquiring first information based on the prediction direction of a current block being bidirectional; deriving at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block based on the first information indicating that different prediction types are applied to each prediction direction of the current block; and bidirectionally predicting the current block with the different prediction types based on one of the at least one bidirectional motion information candidate, wherein the different prediction types are a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type.

[0013] According to another aspect of the present invention, an image encoding method is an image encoding method performed by an image encoding device, the image encoding method including: determining whether different prediction types are applied to each prediction direction of the current block based on the prediction direction of the current block being bidirectional, wherein first information indicating whether different prediction types are applied to each prediction direction of the current block is encoded into a bitstream; deriving at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block based on the different prediction types being applied to each prediction direction of the current block; and bidirectionally predicting the current block with the different prediction types based on one of the at least one bidirectional motion information candidate, wherein the different prediction types are a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type.

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

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

[0016] The features described above in the brief summary of the present disclosure are merely exemplary embodiments of the detailed description of the present disclosure that follows and are not intended to limit the scope of the present disclosure. Effect of the Invention

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

[0018] Furthermore, according to the present disclosure, the motion information is adaptively derived for each prediction direction, thereby improving compression efficiency.

[0019] Furthermore, according to the present disclosure, candidates for inducing different motion information for each prediction direction can be efficiently configured.

[0020] Additionally, the present disclosure may provide improved bit efficiency for signaling motion vector differentials.

[0021] The effects obtained by the present disclosure are not limited to the effects described above, and other effects not described above will be clearly understood by those having ordinary skill in the art to which the present disclosure pertains from the following description. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram illustrating a video coding system to which an embodiment of the present disclosure can be applied.

[0023] [Diagram 2] 1 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied;

[0024] [Diagram 3] 1 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied;

[0025] [Figure 4] FIG. 2 is a diagram illustrating an inter prediction unit of an image encoding device.

[0026] [Diagram 5] 1 is a flowchart illustrating a method for encoding an image based on inter prediction.

[0027] [Figure 6] FIG. 2 is a diagram illustrating an inter prediction unit of an image decoding device.

[0028] [Figure 7] 1 is a flowchart illustrating a method for decoding an image based on inter prediction.

[0029] [Figure 8] 1 is a flowchart illustrating an inter prediction method.

[0030] [Figure 9] 1 is a flowchart illustrating an image encoding method according to an embodiment of the present disclosure.

[0031] [Figure 10] 1 is a flowchart illustrating an image decoding method according to an embodiment of the present disclosure.

[0032] [Figure 11] FIG. 2 is a diagram for explaining an example of a bidirectional prediction method according to an embodiment of the present disclosure.

[0033] [Figure 12-15] 11 is a diagram for explaining an example of deriving bidirectional motion information candidates. FIG.

[0034] [Figure 16] 13 is a flowchart illustrating an image encoding method and an image decoding method according to another embodiment of the present disclosure.

[0035] [Figure 17] 13 is a flowchart illustrating an image encoding method and an image decoding method according to another embodiment of the present disclosure.

[0036] [Figure 18] 11A and 11B are diagrams for explaining examples of neighboring blocks used to derive bidirectional motion information candidates.

[0037] [Figure 19] 10 is a flowchart illustrating an image encoding method according to another embodiment of the present disclosure.

[0038] [Figure 20] 13 is a flowchart illustrating an image decoding method according to another embodiment of the present disclosure.

[0039] [Figure 21] 10 is a flowchart illustrating an image encoding method according to another embodiment of the present disclosure.

[0040] [Figure 22] 13 is a flowchart illustrating an image decoding method according to another embodiment of the present disclosure.

[0041] [Diagram 23] FIG. 13 is a diagram illustrating an example of signaling of motion vector differentials according to the present disclosure.

[0042] [Figure 24] 11A and 11B are diagrams illustrating various positions of a reference block;

[0043] [Diagram 25] FIG. 1 is a diagram illustrating an example of a content streaming system to which an embodiment of the present disclosure can be applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present disclosure will be described in detail below with reference to the accompanying drawings so that those skilled in the art can easily implement the present disclosure. However, the present disclosure may be embodied in various different forms and is not limited to the embodiments described herein.

[0045] In describing the embodiments of the present disclosure, if it is determined that a specific description of a known configuration or function may make the gist of the present disclosure unclear, the detailed description thereof will be omitted. In addition, in the drawings, parts that are not related to the description of the present disclosure are omitted, and similar parts are denoted by similar reference numerals.

[0046] In the present disclosure, when a certain component is "connected," "coupled," or "connected" to another component, this includes not only a direct connection relationship, but also an indirect connection relationship in which another component exists between them. Furthermore, when a certain component is described as "including" or "having" another component, this does not mean that the other component is excluded, but that the other component can be further included, unless otherwise specified.

[0047] In this disclosure, terms such as "first" and "second" are used only for the purpose of distinguishing one component from another component, and do not limit the order or importance of the components unless otherwise specified. Therefore, within the scope of this disclosure, a first component in one embodiment may be called a second component in another embodiment, and similarly, a second component in one embodiment may be called a first component in another embodiment.

[0048] In this disclosure, components that are distinguished from one another are used to clearly describe the characteristics of each component, and do not necessarily mean that the components are separate. In other words, multiple components may be integrated and configured as a single hardware or software unit, or one component may be distributed and configured as multiple hardware or software units. Thus, even if not otherwise stated, such integrated or distributed embodiments are also included in the scope of the present disclosure.

[0049] In the present disclosure, the components described in the various embodiments are not necessarily essential components, and some may be optional components. Therefore, an embodiment consisting of a subset of the components described in one embodiment is also included in the scope of the present disclosure. In addition, an embodiment including other components in addition to the components described in the various embodiments is also included in the scope of the present disclosure.

[0050] The present disclosure relates to image encoding and decoding, and terms used in this disclosure may have ordinary meanings in the technical field to which the present disclosure belongs, unless they are newly defined in this disclosure.

[0051] In this disclosure, a "picture" generally means a unit indicating any one image in a particular time period, a slice / tile is a coding unit constituting a part of a picture, and one picture may be composed of one or more slices / tiles. Also, a slice / tile may include one or more coding tree units (CTUs).

[0052] In this disclosure, a "pixel" or a "pel" may refer to the smallest unit that constitutes one picture (or image). A "sample" may also be used as a term corresponding to a pixel. A sample may generally indicate a pixel or a pixel value, may indicate only a pixel / pixel value of a luma component, or may indicate only a pixel / pixel value of a chroma component.

[0053] In this disclosure, a "unit" may refer to a basic unit of image processing. A unit may include at least one of a specific region of a picture and information related to the region. A unit may be used interchangeably with terms such as "sample array," "block," or "area," depending on the case. In a general case, an M×N block may include a set (or array) of samples or transform coefficients consisting of M columns and N rows.

[0054] In the present disclosure, a "current block" may refer to any one of a "current coding block," a "current coding unit," a "block to be coded," a "block to be decoded," or a "block to be processed." If prediction is performed, a "current block" may refer to a "current predicted block" or a "block to be predicted." If transformation (inverse transformation) / quantization (inverse quantization) is performed, a "current block" may refer to a "current transformed block" or a "block to be transformed." If filtering is performed, a "current block" may refer to a "block to be filtered."

[0055] Additionally, in this disclosure, unless explicitly stated as a chroma block, a "current block" may refer to a block including both a luma component block and a chroma component block, or a "luma block of the current block." The luma component block of the current block may be explicitly expressed as including an explicit description of the luma component block, such as a "luma block" or a "current luma block." Additionally, the chroma component block of the current block may be explicitly expressed as including an explicit description of the chroma component block, such as a "chroma block" or a "current chroma block."

[0056] In the present disclosure, " / " and "," can be interpreted as "and / or." For example, "A / B" and "A, B" can be interpreted as "A and / or B." Also, "A / B / C" and "A, B, C" can mean "at least one of A, B, and / or C."

[0057] In this disclosure, "or" can be interpreted as "and / or." For example, "A or B" can mean 1) only "A," 2) only "B," or 3) "A and B." Alternatively, in this disclosure, "or" can mean "additionally or alternatively."

[0058] Video Coding System Overview

[0059] FIG. 1 is a schematic diagram of a video coding system to which embodiments of the present disclosure can be applied.

[0060] A video coding system according to an embodiment may include an encoding device 10 and a decoding device 20. The encoding device 10 may transmit encoded video and / or image information or data to the decoding device 20 in a file or streaming format via a digital storage medium or a network.

[0061] The encoding device 10 according to an embodiment may include a video source generating unit 11, an encoding unit 12, and a transmitting unit 13. The decoding device 20 according to an embodiment may include a receiving unit 21, a decoding unit 22, and a rendering unit 23. The encoding unit 12 may be referred to as a video / image encoding unit, and the decoding unit 22 may be referred to as a video / image decoding unit. The transmitting unit 13 may be included in the encoding unit 12. The receiving unit 21 may be included in the decoding unit 22. The rendering unit 23 may include a display unit, which may be configured as a separate device or an external component.

[0062] The video source generating unit 11 can obtain videos / images through a process of capturing, synthesizing, or generating videos / images. The video source generating unit 11 can include a video / image capture device and / or a video / image generation device. The video / image capture device can include, for example, one or more cameras, a video / image archive containing previously captured videos / images, etc. The video / image generation device can include, for example, a computer, a tablet, a smartphone, etc., and can (electronically) generate videos / images. For example, a virtual video / image can be generated through a computer, etc., in which case the video / image capture process can be replaced by a process in which related data is generated.

[0063] The encoder 12 may encode the input video / image. The encoder 12 may perform a series of steps such as prediction, transformation, quantization, etc. for compression and encoding efficiency. The encoder 12 may output the encoded data (encoded video / image information) in a bitstream format.

[0064] The transmitting unit 13 may obtain the encoded video / image information or data output in a bitstream format and may transmit the same to the receiving unit 21 of the decoding device 20 or other external objects in a file or streaming format via a digital storage medium or a network. The digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. The transmitting unit 13 may include elements for generating a media file via a predetermined file format and may include elements for transmitting the same via a broadcasting / communication network. The transmitting unit 13 may be provided as a transmitting device separate from the encoding device 12, in which case the transmitting device may include at least one processor for obtaining the encoded video / image information or data output in a bitstream format and a transmitting unit for transmitting the same in a file or stream format. The receiving unit 21 may extract / receive the bitstream from the storage medium or network and transmit it to the decoding unit 22.

[0065] The decoding unit 22 can decode the video / image by performing a series of steps such as inverse quantization, inverse transformation, and prediction corresponding to the operations of the encoding unit 12.

[0066] The rendering unit 23 can render the decoded video / images. The rendered video / images can be displayed via a display unit.

[0067] Overview of the image encoding device

[0068] FIG. 2 is a diagram illustrating an image encoding device to which an embodiment of the present disclosure can be applied.

[0069] As shown in Fig. 2, the image coding device 100 may include an image division unit 110, a subtraction unit 115, a transformation unit 120, a quantization unit 130, an inverse quantization unit 140, an inverse transformation unit 150, an addition unit 155, a filtering unit 160, a memory 170, an inter prediction unit 180, an intra prediction unit 185, and an entropy coding unit 190. The inter prediction unit 180 and the intra prediction unit 185 may be collectively referred to as a "prediction unit." The transformation unit 120, the quantization unit 130, the inverse quantization unit 140, and the inverse transformation unit 150 may be included in a residual processing unit. The residual processing unit may further include a subtraction unit 115.

[0070] All or at least some of the components constituting the image encoding device 100 may be realized by a single hardware component (e.g., an encoder or a processor) depending on the embodiment. Also, the memory 170 may include a decoded picture buffer (DPB) and may be realized by a digital storage medium.

[0071] The image division unit 110 may divide an input image (or picture, frame) input to the image encoding device 100 into one or more processing units. As an example, the processing units may be called coding units (CUs). The coding units may be obtained by recursively dividing a coding tree unit (CTU) or a largest coding unit (LCU) according to a QT / BT / TT (Quad-tree / Binary-tree / Ternal-tree) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad-tree structure, a binary-tree structure, and / or a ternary-tree structure. For dividing the coding units, a quad-tree structure may be applied first, and a binary-tree structure and / or a ternary-tree structure may be applied later. A coding procedure according to the present disclosure may be performed based on a final coding unit that is not further divided. The maximum coding unit may be used as the final coding unit, and a lower depth coding unit obtained by dividing the maximum coding unit may be used as the final coding unit. Here, the coding procedure may include procedures such as prediction, transformation, and / or restoration, which will be described later. As another example, a processing unit of the coding procedure may be a prediction unit (PU) or a transform unit (TU). The prediction unit and the transform unit may be divided or partitioned from the final coding unit, respectively. The prediction unit may be a unit of sample prediction, and the transform unit may be a unit for deriving transform coefficients and / or a unit for deriving a residual signal from the transform coefficients.

[0072] The prediction unit (inter prediction unit 180 or intra prediction unit 185) may perform prediction on a block to be processed (current block) and generate a predicted block including prediction samples for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied in units of a current block or a CU. The prediction unit may generate various information related to prediction of the current block and transmit it to the entropy encoding unit 190. The information related to prediction may be encoded by the entropy encoding unit 190 and output in a bitstream format.

[0073] The intra prediction unit 185 may predict the current block by referring to samples in the current picture. The referenced samples may be located in the neighborhood of the current block or may be located away from the current block according to an intra prediction mode and / or an intra prediction technique. The intra prediction mode may include a plurality of non-directional modes and a plurality of directional modes. The non-directional mode may include, for example, a DC mode and a Planar mode. The directional mode may include, for example, 33 directional prediction modes or 65 directional prediction modes depending on the fineness of the prediction direction. However, this is merely an example, and more or less directional prediction modes may be used depending on the setting. The intra prediction unit 185 may also determine a prediction mode to be applied to the current block using prediction modes applied to neighboring blocks.

[0074] The inter prediction unit 180 may derive a predicted block for a current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of a block, a sub-block, or a sample based on the correlation of motion information between a neighboring block and a current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring block may include a spatial neighboring block present in the current picture and a temporal neighboring block present in the reference picture. The reference picture including the reference block and the reference picture including the temporal neighboring block may be the same or different from each other. The temporal neighboring block may be called a collocated reference block, a collocated CU (colCU), etc. The reference picture including the temporal neighboring block may be called a collocated picture (colPic). For example, the inter prediction unit 180 may generate information indicating which candidate is used to derive a motion vector and / or a reference picture index of the current block by forming a motion information candidate list based on neighboring blocks. Inter prediction may be performed based on various prediction modes, and for example, in the case of a skip mode and a merge mode, the inter prediction unit 180 may use motion information of neighboring blocks as motion information of the current block. In the case of the skip mode, unlike the merge mode, a residual signal may not be transmitted.In the case of a motion vector prediction (MVP) mode, the motion vector of the current block can be signaled by using the motion vector of a neighboring block as a motion vector predictor and encoding a motion vector difference and an indicator for the motion vector predictor. The motion vector difference can mean the difference between the motion vector of the current block and the motion vector predictor.

[0075] The prediction unit may generate a prediction signal based on various prediction methods and / or prediction techniques, which will be described later. For example, the prediction unit may apply intra prediction or inter prediction for prediction of the current block, and may simultaneously apply intra prediction and inter prediction. A prediction method that simultaneously applies intra prediction and inter prediction for prediction of the current block may be called combined inter and intra prediction (CIIP). The prediction unit may also perform intra block copy (IBC) for prediction of the current block. Intra block copy can be used for content image / video coding such as games, for example, as in screen content coding (SCC). IBC is a method of predicting a current block using an already restored reference block in a current picture that is located a predetermined distance away from the current block. When IBC is applied, the position of the reference block in the current picture may be coded as a vector (block vector) corresponding to the predetermined distance. IBC is basically performed in the current picture, but may be performed similarly to inter prediction in that a reference block is derived in the current picture. That is, the IBC may use at least one of the inter prediction techniques described in this disclosure.

[0076] The prediction signal generated by the prediction unit may be used to generate a restored signal or a residual signal. The subtraction unit 115 may subtract the prediction signal (predicted block, prediction sample array) output from the prediction unit from the input image signal (original block, original sample array) to generate a residual signal (residual block, residual sample array). The generated residual signal may be transmitted to the conversion unit 120.

[0077] The transform unit 120 may generate transform coefficients by applying a transform technique to the residual signal. For example, the transform technique may include at least one of a discrete cosine transform (DCT), a discrete sine transform (DST), a Karhunen-Loeve transform (KLT), a graph-based transform (GBT), or a conditionally non-linear transform (CNT). Here, the GBT refers to a transform obtained from a graph when the relationship information between pixels is expressed as a graph. The CNT refers to a transform obtained based on a predicted signal generated using all previously reconstructed pixels. The transform process may be applied to pixel blocks having the same square size, or may be applied to non-square, variable-sized blocks.

[0078] The quantization unit 130 may quantize the transform coefficients and transmit the quantized transform coefficients to the entropy coding unit 190. The entropy coding unit 190 may code the quantized signal (information on the quantized transform coefficients) and output the coded signal in a bitstream format. The information on the quantized transform coefficients may be referred to as residual information. The quantization unit 130 may rearrange the quantized transform coefficients in a block format into a one-dimensional vector format based on a coefficient scan order, and may generate information on the quantized transform coefficients based on the quantized transform coefficients in the one-dimensional vector format.

[0079] The entropy coding unit 190 may perform various coding methods, such as exponential Golomb, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC). The entropy coding unit 190 may also code information required for video / image restoration (e.g., values ​​of syntax elements, etc.) together or separately in addition to the quantized transform coefficients. The coded information (e.g., coded video / image information) may be transmitted or stored in a network abstraction layer (NAL) unit unit in a bitstream format. The video / image information may further include information on various parameter sets, such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). The video / image information may further include general constraint information. The signaling information, transmitted information and / or syntax elements referred to in this disclosure may be encoded through the above-mentioned encoding procedures and included in the bitstream.

[0080] The bitstream may be transmitted via a network or may be stored in a digital storage medium. Here, the network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as a USB, SD, CD, DVD, Blu-ray, HDD, SSD, etc. A transmitting unit (not shown) for transmitting the signal output from the entropy encoding unit 190 and / or a storing unit (not shown) for storing the signal may be provided as an internal / external element of the image encoding device 100, or the transmitting unit may be provided as a component of the entropy encoding unit 190.

[0081] The quantized transform coefficients output from the quantization unit 130 can be used to generate a residual signal. For example, the quantized transform coefficients are subjected to inverse quantization and inverse transformation via the inverse quantization unit 140 and the inverse transformation unit 150, so that a residual signal (residual block or residual sample) can be restored.

[0082] The adder 155 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the reconstructed residual signal to a prediction signal output from the inter prediction unit 180 or the intra prediction unit 185. When there is no residual for the current block to be processed, such as when a skip mode is applied, a predicted block may be used as a reconstructed block. The adder 155 may be referred to as a reconstruction unit or a reconstructed block generation unit. The generated reconstructed signal may be used for intra prediction of the next current block to be processed in the current picture, and may also be used for inter prediction of the next picture after filtering as described below.

[0083] The filtering unit 160 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 160 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 170, specifically, in the DPB of the memory 170. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, and the like. The filtering unit 160 may generate various information related to filtering, as will be described later in the description of each filtering method, and transmit the information to the entropy coding unit 190. The information related to filtering may be coded by the entropy coding unit 190 and output in a bitstream format.

[0084] The modified reconstructed picture transmitted to the memory 170 may be used as a reference picture in the inter prediction unit 180. When inter prediction is applied through this, the image encoding device 100 may avoid a prediction mismatch between the image encoding device 100 and the image decoding device, and may also improve encoding efficiency.

[0085] The DPB in the memory 170 may store modified reconstructed pictures to be used as reference pictures in the inter prediction unit 180. The memory 170 may store motion information of blocks from which motion information in the current picture is derived (or coded) and / or motion information of already reconstructed intra-picture blocks. The stored motion information may be transmitted to the inter prediction unit 180 to be used as motion information of spatial surrounding blocks or motion information of temporal surrounding blocks. The memory 170 may store reconstructed samples of reconstructed blocks in the current picture and transmit them to the intra prediction unit 185.

[0086] Overview of the image decoding device

[0087] FIG. 3 is a diagram illustrating an image decoding device to which an embodiment of the present disclosure can be applied.

[0088] 3, the image decoding device 200 may include an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265. The inter prediction unit 260 and the intra prediction unit 265 may be collectively referred to as a "prediction unit." The inverse quantization unit 220 and the inverse transform unit 230 may be included in a residual processing unit.

[0089] All or at least some of the components constituting the image decoding device 200 may be realized by one hardware component (e.g., a decoder or a processor) depending on the embodiment. Also, the memory 170 may include a DPB and may be realized by a digital storage medium.

[0090] The image decoding device 200, which receives a bitstream including video / image information, can reconstruct an image by executing a process corresponding to the process performed by the image encoding device 100 of Fig. 2. For example, the image decoding device 200 can perform decoding using a processing unit applied in the image encoding device. Thus, the processing unit for decoding can be, for example, a coding unit. The coding unit can be obtained by dividing a coding tree unit or a maximum coding unit. Then, the reconstructed image signal decoded and output by the image decoding device 200 can be reproduced by a reproduction device (not shown).

[0091] The image decoding apparatus 200 may receive a signal output from the image encoding apparatus of FIG. 2 in the form of a bitstream. The received signal may be decoded via the entropy decoding unit 210. For example, the entropy decoding unit 210 may derive information (e.g., video / image information) required for image restoration (or picture restoration) by parsing the bitstream. The video / image information may further include information on various parameter sets such as an adaptation parameter set (APS), a picture parameter set (PPS), a sequence parameter set (SPS), or a video parameter set (VPS). In addition, the video / image information may further include general constraint information. The image decoding apparatus may further use information on the parameter set and / or the general constraint information to decode an image. The signaling information, received information, and / or syntax elements referred to in the present disclosure may be obtained from the bitstream by being decoded via the decoding procedure. For example, the entropy decoding unit 210 may decode information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, and output values ​​of syntax elements required for image restoration and quantized values ​​of transform coefficients related to the residual. More specifically, the CABAC entropy decoding method may receive bins corresponding to each syntax element from the bitstream, determine a context model using syntax element information to be decoded and decoded information of neighboring blocks and a block to be decoded, or information of a symbol / bin decoded in a previous step, predict the occurrence probability of the bin based on the determined context model, and perform arithmetic decoding of the bin to generate a symbol corresponding to the value of each syntax element. In this case, the CABAC entropy decoding method may update the context model using information of the decoded symbol / bin for the context model of the next symbol / bin after determining the context model.Among the information decoded by the entropy decoding unit 210, information related to prediction is provided to a prediction unit (inter prediction unit 260 and intra prediction unit 265), and residual values ​​entropy-decoded by the entropy decoding unit 210, i.e., quantized transform coefficients and related parameter information, may be input to the inverse quantization unit 220. Also, among the information decoded by the entropy decoding unit 210, information related to filtering may be provided to the filtering unit 240. Meanwhile, a receiving unit (not shown) for receiving a signal output from the image encoding device may be further provided as an internal / external element of the image decoding device 200, or the receiving unit may be provided as a component of the entropy decoding unit 210.

[0092] Meanwhile, the image decoding device according to the present disclosure may be called a video / image / picture decoding device. The image decoding device may include an information decoder (video / image / picture information decoder) and / or a sample decoder (video / image / picture sample decoder). The information decoder may include an entropy decoding unit 210, and the sample decoder may include at least one of an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, a memory 250, an inter prediction unit 260, and an intra prediction unit 265.

[0093] The inverse quantization unit 220 may inverse quantize the quantized transform coefficients to output transform coefficients. The inverse quantization unit 220 may rearrange the quantized transform coefficients in a two-dimensional block format. In this case, the rearrangement may be performed based on a coefficient scan order performed in the image encoding device. The inverse quantization unit 220 may perform inverse quantization on the quantized transform coefficients using a quantization parameter (e.g., quantization step size information) to obtain transform coefficients.

[0094] The inverse transform unit 230 can inversely transform the transform coefficients to obtain a residual signal (residual block, residual sample array).

[0095] The prediction unit may perform prediction on a current block and generate a predicted block including a prediction sample for the current block. The prediction unit may determine whether intra prediction or inter prediction is applied to the current block based on the prediction information output from the entropy decoding unit 210, and may determine a specific intra / inter prediction mode (prediction technique).

[0096] The prediction unit can generate a prediction signal based on various prediction methods (techniques) described below, as has been described in the explanation of the prediction unit of the image encoding device 100.

[0097] The intra prediction unit 265 may predict the current block by referring to samples in the current picture. The description of the intra prediction unit 185 may be similarly applied to the intra prediction unit 265.

[0098] The inter prediction unit 260 may derive a predicted block for the current block based on a reference block (reference sample array) identified by a motion vector on a reference picture. In this case, in order to reduce the amount of motion information transmitted in the inter prediction mode, the motion information may be predicted in units of blocks, sub-blocks, or samples based on correlation of motion information between neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction direction (L0 prediction, L1 prediction, Bi prediction, etc.) information. In the case of inter prediction, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. For example, the inter prediction unit 260 may configure a motion information candidate list based on the neighboring blocks, and derive a motion vector and / or a reference picture index for the current block based on the received candidate selection information. Inter prediction may be performed based on various prediction modes (techniques), and the prediction information may include information indicating a mode (technique) of inter prediction for the current block.

[0099] The adder 235 may generate a reconstructed signal (reconstructed picture, reconstructed block, reconstructed sample array) by adding the acquired residual signal to a prediction signal (predicted block, predicted sample array) output from a prediction unit (including the inter prediction unit 260 and / or the intra prediction unit 265). When there is no residual for the current block, such as when a skip mode is applied, the predicted block can be used as the reconstructed block. The description of the adder 155 may also be applied to the adder 235. The adder 235 may also be referred to as a reconstruction unit or a reconstructed block generator. The generated reconstructed signal may be used for intra prediction of the next current block in the current picture, and may also be used for inter prediction of the next picture via filtering, as described below.

[0100] The filtering unit 240 may apply filtering to the reconstructed signal to improve subjective / objective image quality. For example, the filtering unit 240 may apply various filtering methods to the reconstructed picture to generate a modified reconstructed picture, and may store the modified reconstructed picture in the memory 250, specifically, in the DPB of the memory 250. The various filtering methods may include, for example, deblocking filtering, sample adaptive offset, an adaptive loop filter, a bilateral filter, etc.

[0101] The (modified) reconstructed picture stored in the DPB of the memory 250 may be used as a reference picture in the inter prediction unit 260. The memory 250 may store motion information of a block from which motion information in the current picture is derived (or decoded) and / or motion information of a block in an already reconstructed picture. The stored motion information may be transmitted to the inter prediction unit 260 to be used as motion information of a spatial surrounding block or motion information of a temporal surrounding block. The memory 250 may store a reconstructed sample of a reconstructed block in the current picture and transmit it to the intra prediction unit 265.

[0102] In this specification, the embodiments described for the filtering unit 160, inter prediction unit 180 and intra prediction unit 185 of the image encoding device 100 can also be applied in a similar or corresponding manner to the filtering unit 240, inter prediction unit 260 and intra prediction unit 265 of the image decoding device 200, respectively.

[0103] Inter Prediction

[0104] The prediction unit of the image encoding device 100 and the image decoding device 200 may perform inter prediction on a block basis to derive a prediction sample. The inter prediction may be a prediction derived in a manner dependent on data elements (e.g., sample values ​​or motion information, etc.) of pictures other than the current picture. When inter prediction is applied to the current block, a predicted block (prediction sample array) for the current block may be derived based on a reference block (reference sample array) identified by a motion vector on a reference picture indicated by a reference picture index. 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 on a block, sub-block, or sample basis based on correlation between motion information of neighboring blocks and the current block. The motion information may include a motion vector and a reference picture index. The motion information may further include inter prediction type (L0 prediction, L1 prediction, Bi prediction, etc.) information. When inter prediction is applied, the neighboring blocks may include spatial neighboring blocks present in the current picture and temporal neighboring blocks present in the reference picture. The reference picture including the reference block and the reference picture including the temporal peripheral block may be the same or different from each other. The temporal peripheral block may be called a collocated reference block, a collocated CU (colCU), etc., and the reference picture including the temporal peripheral block may be called a collocated picture (colPic). For example, a motion information candidate list may be constructed based on the peripheral blocks of the current block, and a flag or index information indicating which candidate is selected (used) to derive the motion vector and / or reference picture index of the current block may be signaled.Inter prediction may be performed based on various prediction modes, and for example, in the case of skip mode and merge mode, the motion information of the current block may be the same as the motion information of the selected neighboring block. In the case of skip mode, unlike the merge mode, a residual signal may not be transmitted. In the case of motion vector prediction (MVP) mode, a motion vector of a selected neighboring block may be used as a motion vector predictor, and a motion vector difference may be signaled. In this case, the motion vector of the current block may be derived using the sum of the motion vector predictor and the motion vector difference.

[0105] The motion information may include L0 motion information and / or L1 motion information depending on an inter prediction type (such as L0 prediction, L1 prediction, Bi prediction, etc.). A motion vector in the L0 direction may be referred to as an L0 motion vector or MVL0, and a motion vector in the L1 direction may be referred to as an L1 motion vector or MVL1. A prediction based on an L0 motion vector may be referred to as an L0 prediction, a prediction based on an L1 motion vector may be referred to as an L1 prediction, and a prediction based on both the L0 motion vector and the L1 motion vector may be referred to as a bi-prediction (Bi) prediction. Here, the L0 motion vector may represent a motion vector associated with a reference picture list L0 (L0), and the L1 motion vector may represent a motion vector associated with a reference picture list L1 (L1). The reference picture list L0 may include a previous picture in output order from the current picture as a reference picture, and the reference picture list L1 may include a subsequent picture in output order from the current picture. The previous picture may be referred to as a forward (reference) picture, and the subsequent picture may be referred to as a backward (reference) picture. The reference picture list L0 may further include a subsequent picture in output order from the current picture as a reference picture. In this case, the previous picture may be indexed first in the reference picture list L0, and the subsequent picture may be indexed next. The reference picture list L1 may further include a previous picture in output order from the current picture as a reference picture. In this case, the subsequent picture may be indexed first in the reference picture list 1, and the previous picture may be indexed next. Here, the output order may correspond to a picture order count (POC) order.

[0106] FIG. 4 is a diagram illustrating an inter prediction unit (180) of the image decoding device 100, and FIG. 5 is a flowchart showing a method for encoding an image based on inter prediction.

[0107] The image encoding device 100 may perform inter prediction on a current block (S510). The image encoding device 100 may derive an inter prediction mode and motion information of the current block, and generate a prediction sample of the current block. Here, the steps of determining the inter prediction mode, deriving the motion information, and generating the prediction sample may be performed simultaneously, or one of the steps may be performed prior to the other steps. For example, the inter prediction unit 180 of the image encoding device 100 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 may determine a prediction mode for the current block, the motion information derivation unit 182 may derive motion information of the current block, and the prediction sample derivation unit 183 may derive a prediction sample of the current block. For example, the inter prediction unit 180 of the image encoding device 100 may search for a block similar to the current block within a certain region (search region) of a reference picture through motion estimation, and derive a reference block whose difference from the current block is minimum or equal to or less than a certain criterion. Based on this, a reference picture index indicating a reference picture in which the reference block is located may be derived, and a motion vector may be derived based on a position difference between the reference block and the current block. The image encoding device 100 may determine a mode to be applied to the current block among various prediction modes. The image encoding device 100 may compare RD costs for the various prediction modes and determine an optimal prediction mode for the current block.

[0108] For example, when a skip mode or a merge mode is applied to the current block, the image encoding device 100 may construct a merge candidate list (to be described later) and derive a reference block whose difference from the current block is minimum or equal to or less than a certain criterion among reference blocks indicated by merge candidates included in the merge candidate list. In this case, a merge candidate related to the derived reference block may be selected, and merge index information indicating the selected merge candidate may be generated and signaled to a decoding device. Motion information of the current block may be derived using motion information of the selected merge candidate.

[0109] As another example, when the (A)MVP mode is applied to the current block, the image encoding apparatus 100 may construct an (A)MVP candidate list (to be described later) and use a motion vector of a selected MVP candidate among the MVP (motion vector predictor) candidates included in the (A)MVP candidate list as the MVP of the current block. In this case, for example, a motion vector indicating a reference block derived by the above-mentioned motion estimation may be used as the motion vector of the current block, and an MVP candidate having a motion vector with the smallest difference from the motion vector of the current block among the MVP candidates may become the selected MVP candidate. A motion vector difference (MVD), which is a difference obtained by subtracting the MVP from the motion vector of the current block, may be derived. In this case, information on the MVD may be signaled to the image decoding apparatus 200. In addition, when the (A)MVP mode is applied, the value of the reference picture index may be configured as reference picture index information and separately signaled to the image decoding apparatus 200.

[0110] The image encoding device 100 may derive a residual sample based on the predicted sample (S520). The image encoding device 100 may derive the residual sample by comparing an original sample of the current block with the predicted sample.

[0111] The image encoding device 100 may encode image information including prediction information and residual information (S530). The image encoding device 100 may output the encoded image information in a bitstream format. The prediction information may include prediction mode information (e.g., skip flag, merge flag or mode index, etc.) and information on motion information as information related to the prediction procedure. The information on the motion information may include candidate selection information (e.g., merge index, mvp flag or mvp index) which is information for deriving a motion vector. In addition, the information on the motion information may include the above-mentioned information on MVD and / or reference picture index information. In addition, the information on the motion information may include information indicating whether L0 prediction, L1 prediction, or bi-prediction is applied. The residual information is information on the residual sample. The residual information may include information on a quantized transform coefficient for the residual sample.

[0112] The output bitstream may be stored in a (digital) storage medium and then transmitted to the image decoding device 200, or may be transmitted to the image decoding device 200 via a network.

[0113] Meanwhile, as described above, the image coding apparatus 100 may generate a reconstructed picture (including reconstructed samples and reconstructed blocks) based on the reference samples and the residual samples. This is because the same prediction result as that performed by the image decoding apparatus 200 is derived by the image coding apparatus 100, thereby improving coding efficiency. Therefore, the image coding apparatus 100 may store the reconstructed picture (or reconstructed sample, reconstructed block) in a memory and use it as a reference picture for inter prediction. As described above, an in-loop filtering procedure or the like may be further applied to the reconstructed picture.

[0114] FIG. 6 is a diagram illustrating the inter prediction unit 260 of the image decoding device 200, and FIG. 7 is a flowchart showing a method for decoding an image based on inter prediction.

[0115] The image decoding apparatus 200 may perform operations corresponding to those performed by the image encoding apparatus 100. The image decoding apparatus 200 may perform prediction on a current block based on received prediction information and derive a prediction sample.

[0116] Specifically, the image decoding apparatus 200 may determine a prediction mode for the current block based on the received prediction information (S710). The image decoding apparatus 200 may determine which inter prediction mode is applied to the current block based on prediction mode information in the prediction information.

[0117] For example, it may be determined whether the merge mode is applied to the current block or the (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 skip mode, merge mode, and / or (A)MVP mode, or may include various inter prediction modes described below.

[0118] The image decoding apparatus 200 may derive motion information of the current block based on the determined inter prediction mode (S720). For example, when a skip mode or a merge mode is applied to the current block, the image decoding apparatus 200 may form a merge candidate list (to be described later) and select one merge candidate from among the merge candidates included in the merge candidate list. The selection may be performed based on the above-mentioned selection information (merge index). Motion information of the selected merge candidate may be used to derive motion information of the current block. The motion information of the selected merge candidate may be used as motion information of the current block.

[0119] As another example, when the (A)MVP mode is applied to the current block, the image decoding apparatus 200 may construct an (A)MVP candidate list described below, and may use a motion vector of a selected MVP candidate among the MVP (motion vector predictor) candidates included in the (A)MVP candidate list as the MVP of the current block. The selection may be performed based on the above-mentioned selection information (mvp flag or mvp index). In this case, the MVD of the current block may be derived based on information on the MVD, and the motion vector of the current block may be derived based on the MVP of the current block and the MVD. Also, the reference picture index of the current block may be derived based on the reference picture index information. A picture pointed to by the reference picture index in a reference picture list for the current block may be derived as a reference picture referenced for inter prediction of the current block.

[0120] Meanwhile, the motion information of the current block may be derived without constructing a candidate list, as described below, in which case the motion information of the current block may be derived according to a procedure disclosed in a prediction mode, as described below. In this case, the candidate list construction as described above may be omitted.

[0121] The image decoding apparatus 200 may generate a prediction sample for the current block based on the motion information of the current block (S730). In this case, the reference picture may be derived based on a reference picture index of the current block, and the prediction sample of the current block may be derived using a sample of a reference block to which the motion vector of the current block points on the reference picture. In this case, as described below, a prediction sample filtering procedure may be further performed on all or a part of the prediction sample of the current block, if necessary.

[0122] For example, the inter prediction unit 260 of the image decoding device 200 may include a prediction mode determination unit 261, a motion information derivation unit 262, and a prediction sample derivation unit 263, and may determine a prediction mode for the current block based on prediction mode information received by the prediction mode determination unit 181, derive motion information (such as a motion vector and / or a reference picture index) of the current block based on information regarding the motion information received by the motion information derivation unit 182, and derive a prediction sample of the current block by the prediction sample derivation unit 183.

[0123] The image decoding apparatus 200 may generate a residual sample for the current block based on the received residual information (S740). The image decoding apparatus 200 may generate a reconstructed sample for the current block based on the predicted sample and the residual sample, and may generate a reconstructed picture based on the reconstructed sample (S750). Thereafter, an in-loop filtering procedure may be further applied to the reconstructed picture, as described above.

[0124] 8, as described above, the inter prediction procedure may include an inter prediction mode determination step (S810), a motion information derivation step (S820) according to the determined prediction mode, and a prediction execution step (a prediction sample generation step) based on the derived motion information (S830). The inter prediction procedure may be performed in the image encoding device 100 and the image decoding device 200 as described above.

[0125] Inter prediction mode decision

[0126] Various inter prediction modes can be used to predict a current block in a picture. For example, various modes such as merge mode, skip mode, motion vector prediction (MVP) mode, affine mode, sub-block merge mode, and merge with MVD (MVVD) mode can be used. Decoder side motion vector refinement (DMVR) mode, adaptive motion vector resolution (AMVR) mode, Bi-prediction with CU-level weight (BCW), Bi-directional optical flow (BDOF), etc. can be used in addition to or instead of the affine mode. The affine mode may also be referred to as affine motion prediction mode. The MVP mode may also be referred to as advanced motion vector prediction mode (AMVP). In this document, motion information candidates derived by some modes and / or some modes may be included as one of the motion information related candidates of other modes. For example, an HMVP candidate may be added as a merge candidate of the merge / skip mode, or may be added as an MVP candidate of the MVP mode.

[0127] Prediction mode information indicating an inter prediction mode of a current block may be signaled from the image encoding apparatus 100 to the image decoding apparatus 200. The prediction mode information may be included in a bitstream and received by the image decoding apparatus 200. The prediction mode information may include index information indicating one of a number of candidate modes. Alternatively, the inter prediction mode may be indicated through hierarchical signaling of flag information. In this case, the prediction mode information may include one or more flags. For example, a skip flag may be signaled to indicate whether or not a skip mode is applied, and if the skip mode is not applied, a merge flag may be signaled to indicate whether or not a merge mode is applied, and if the merge mode is not applied, an MVP mode may be indicated to be applied, or a flag for additional division may be further signaled. The affine mode may be signaled in an independent mode or in a mode dependent on the merge mode or MVP mode. For example, the affine mode may include an affine merge mode and an affine MVP mode.

[0128] Deriving motion information

[0129] Inter prediction may be performed using motion information of the current block. The image encoding device 100 may derive optimal motion information for the current block through a motion estimation procedure. For example, the image encoding device 100 may search for a similar reference block having high correlation with an original block in an original picture for the current block in a fractional pixel unit within a predetermined search range in the reference picture, thereby deriving motion information. The similarity of the block may be derived based on a difference in phase-based sample values. For example, the similarity of the block may be calculated based on the SAD between the current block (or a template of the current block) and the reference block (or a template of the reference block). In this case, the motion information may be derived based on the reference block having the smallest SAD in the search range. The derived motion information may be signaled to the image decoding device 200 according to various methods based on the inter prediction mode.

[0130] Generating prediction samples

[0131] A predicted block for the current block may be derived based on the motion information derived according to the prediction mode. The predicted block may include a prediction sample (prediction sample array) of the current block. If the motion vector of the current block points to a fractional sample unit, an interpolation procedure may be performed, whereby a prediction sample of the current block may be derived based on a reference sample of a fractional sample unit in a reference picture. If affine inter prediction is applied to the current block, a prediction sample may be generated based on a sample / sub-block unit MV. If bi-prediction is applied, a prediction sample derived through a weighted sum or weighted average (by phase) of a prediction sample derived based on L0 prediction (i.e., prediction using a reference picture in a reference picture list L0 and MVL0) and a prediction sample derived based on L1 prediction (i.e., prediction using a reference picture in a reference picture list L1 and MVL1) may be used as a prediction sample of the current block. When bi-prediction is applied, if the reference picture used for L0 prediction and the reference picture used for L1 prediction are located in different temporal directions from the current picture (i.e., bi-prediction but also bidirectional prediction), this can be called true bi-prediction.

[0132] As described above, reconstructed samples and reconstructed pictures can be generated based on the derived predicted samples, and then procedures such as in-loop filtering can be performed.

[0133] Affine prediction

[0134] Existing video coding systems use only one MV (motion vector) to represent the motion of a coding block (using a translation motion model). However, although the above method can represent the optimal motion in block units, if the optimal MV can be determined in pixel units, rather than the optimal motion of each actual pixel, the coding efficiency can be improved. For this reason, in this embodiment, an affine motion prediction method that uses an affine motion model for coding will be described. The fine motion prediction method can represent the MV in each pixel unit of a block using two, three, or four MVs.

[0135] An affine motion model can express four types of motion. Among the motions that an affine motion model can express, an affine motion model that expresses three types of motion (translation, scale, and rotation) is called a similarity (or simplified) affine motion model, and the proposed method will be described based on the similarity (or simplified) affine motion model. However, the method is not limited to this motion model.

[0136] Affine motion prediction can determine an MV for a pixel position included in a block using two or more control point motion vectors (CPMVs). In this case, a set of MVs is called an affine motion vector field (MVF), and in the encoding / decoding process, the affine MVF can be determined in pixel units or in predefined subblock units. When determined in pixel units, an MV is obtained based on each pixel value, and when determined in subblock units, an MV for the block can be obtained based on the pixel value at the center of the subblock (bottom right of the center, i.e., the bottom right sample of the four central samples).

[0137] When affine prediction is available, the motion models applicable to the current block may include the following three: a translational motion model, a 4-parameter affine motion model, and a 6-parameter affine motion model, where the translational motion model may indicate a model in which an existing block-based motion vector is used, the 4-parameter affine motion model may indicate a model in which two CPMVs are used, and the 6-parameter affine motion model may indicate a model in which three CPMVs are used.

[0138] The affine motion prediction may include an affine MVP (or affine inter) mode and an affine merge. In the affine motion prediction, a motion vector of a current block may be derived on a sample basis or a sub-block basis.

[0139] When the affine merge mode is applied, the CPMV of the current block may be derived using the CPMV of the neighboring blocks. In this case, the CPMV of the neighboring blocks may be used as the CPMV of the current block as is, or the CPMV of the neighboring blocks may be modified based on the size of the neighboring blocks and the size of the current block, and then used as the CPMV of the current block.

[0140] Meanwhile, in the case of affine merging in which MV is derived in subblock units, it may be called a subblock merging mode, which may be indicated based on merge_subblock_flag (value 1). In this case, an affine merging candidate list described later may be called a subblock merging candidate list. In this case, the subblock merging candidate list may further include a candidate derived by sbTMVP described later. In this case, the candidate derived by sbTMVP may be used as a candidate for the 0th index of the subblock merging candidate list. In other words, the candidate derived by sbTMVP may be positioned before inherited affine candidates and constructed affine candidates described later in the subblock merging candidate list.

[0141] In affine MVP mode, after determining two or more control point motion vector predictions (CPMVP) and CPMV for the current block, a control point motion vector difference (CPMVD) corresponding to the difference value can be transmitted from the image encoding device 100 to the image decoding device 200.

[0142] For example, when the value of affine merge flag or merge_subblock_flag is 0, the affine MVP mode can be applied. Or, for example, when the value of inter_affine_flag is 1, the affine MVP mode can be applied. The affine MVP mode may be called an affine CP MVP mode. Or, the affine MVP mode may be called an affine inter mode or an inter affine mode. The affine MVP candidate list may be called a control point motion vectors predictor candidate list.

[0143] Working Example

[0144] The present disclosure relates to bidirectional motion compensation in an inter prediction process, and more particularly, to a method for performing bidirectional prediction of a current block by applying different prediction types for each prediction direction (L0 or L1).

[0145] The different prediction types may be a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type. The sub PU prediction type may be called a sub PU prediction mode, and may include a sub-block merge mode and an affine mode in which an MV is derived in a sub-block unit. The non-sub PU prediction type is a prediction type that does not correspond to the sub PU prediction type, and may be called a non-sub PU prediction mode.

[0146] In a conventional bidirectional motion compensation process, when the current block is a sub PU prediction type, motion compensation is performed based on the sub PU prediction type in both directions (L0, L1), and when the current block is a non-sub PU prediction type, motion compensation is performed based on the non-sub PU prediction type in both directions to generate a predicted block.

[0147] For example, the determination of a prediction mode, the derivation of motion information, and the generation of a prediction sample through bidirectional prediction in a conventional merge mode may derive bidirectional motion information from prediction candidates. Specific examples of performing bidirectional prediction in a conventional merge mode can be classified as follows. That is, the conventional merge mode does not support a method of predicting one prediction direction with a sub PU prediction type and predicting the other prediction direction with a non-sub PU prediction type.

[0148] -If merge_subblock_flag is 1, prediction is performed based on the sub PU prediction type in both directions.

[0149] -If merge_subblock_flag is 0, prediction is performed based on the non-sub PU prediction type in both directions.

[0150] Furthermore, in the conventional merge mode, information used for prediction varies depending on each prediction type, that is, the conventional merge mode has a restriction that the same information must be used in both directions during the bidirectional prediction.

[0151] When applying the sub PU prediction type, if the neighboring block (reference block) is a block predicted in affine mode, the stored CPMV of the neighboring block is used.

[0152] -When applying non-sub PU prediction type, use the saved MV of the surrounding blocks.

[0153] As another example, the AMVP mode that signals MVD, MVP index, reference picture index, etc. to induce motion information can be classified as follows: That is, the conventional AMVP mode also does not support a method of predicting one prediction direction with a sub PU prediction type and predicting the other prediction direction with a non-sub PU prediction type.

[0154] - If inter_affine_flag is 1, prediction is performed based on the sub PU prediction type in both directions.

[0155] -If inter_affine_flag is 0, prediction is based on non-sub PU in both directions.

[0156] In addition, in the conventional AMVP mode, the information used for prediction varies depending on the prediction type, that is, the conventional AMVP mode has a restriction that the same information must be used in both directions during the bidirectional prediction process.

[0157] When applying the sub PU prediction type, if the neighboring block (reference block) is a block predicted in affine mode, the stored CPMV of the neighboring block is used.

[0158] -When applying non-sub PU prediction type, use the saved MV of the surrounding blocks.

[0159] Furthermore, conventional AMVP modes have differences in the amount of information signaled via the bitstream for each prediction type.

[0160] -When applying the sub PU prediction type, signal two or more MVDs (xoffset, yoffset) required to derive the MV of each CP

[0161] -When applying non-sub PU prediction type, signal the MVD required to generate the prediction block

[0162] The method proposed by the present disclosure may correspond to a method of simultaneously supporting different prediction types (i.e., sub PU prediction type and non-sub PU prediction type) in the process of generating a prediction block in each prediction direction unit when performing bidirectional prediction for one prediction block. Therefore, according to the method proposed by the present disclosure, different prediction types are adaptively induced for each prediction direction, thereby improving compression efficiency.

[0163] In addition, the method proposed by the present disclosure performs motion compensation via different prediction types for each prediction direction, and in this process, it is possible to adaptively refer to necessary information for each prediction direction. Therefore, according to the method proposed by the present disclosure, it is possible to improve efficiency of bits required for signaling necessary information.

[0164] Example 1

[0165] FIG. 9 is a flowchart showing an image coding method according to the first embodiment.

[0166] 9, the image encoding device 100 may determine whether the prediction direction of the current block is bidirectional (S900). For example, it may be determined whether the prediction direction of the current block is bidirectional based on whether the value of inter_pred_idc, which is a syntax element indicating the prediction direction of the current block, indicates bidirectional prediction (PRED_BI).

[0167] When the prediction direction of the current block is bidirectional, the image encoding device 100 may determine whether different prediction types are applied to each prediction direction (S910). Here, the different prediction types may be a sub PU prediction type and a non-sub PU prediction type. That is, the step S910 may be a step of determining whether a specific direction is predicted based on the sub PU prediction type and another direction is predicted based on the non-sub PU prediction type. First information (merge_mixed_pu_flag or inter_mixed_pu_flag), which is information indicating whether different prediction types are applied to each prediction direction of the current block, may be coded into a bitstream.

[0168] When different prediction types are applied to the prediction directions, the image encoding apparatus 100 may derive at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block of the current block (S920).

[0169] For example, when the L0 direction of the current block is a sub PU prediction type and the L1 direction is a non-sub PU prediction type, a bidirectional motion information candidate having sub PU-based motion information in the L0 direction and non-sub PU-based motion information in the L1 direction may be derived. As another example, when the L0 direction of the current block is a non-sub PU prediction type and the L1 direction is a sub PU prediction type, a bidirectional motion information candidate having non-sub PU-based motion information in the L0 direction and sub PU-based motion information in the L1 direction may be derived. Here, the sub PU-based motion information may be motion information used for the sub PU prediction type, and the non-sub PU-based motion information may be motion information used for the non-sub PU prediction type. At least one bidirectional motion information candidate may constitute a motion information candidate list.

[0170] The image encoding apparatus 100 may bidirectionally predict the current block with different prediction types based on any of the derived bidirectional motion information candidates, that is, based on the motion information candidate list (S930).

[0171] FIG. 10 is a flowchart of the image decoding method according to the first embodiment.

[0172] 10, the image decoding apparatus 200 may determine whether the prediction direction of the current block is bidirectional (S1000). For example, it may be determined whether the prediction direction of the current block is bidirectional based on whether the value of inter_pred_idc, which is a syntax element indicating the prediction direction of the current block, indicates bidirectional prediction (PRED_BI).

[0173] When the prediction direction of the current block is bidirectional, the image decoding device 200 can obtain first information (merge_mixed_pu_flag or inter_mixed_pu_flag), which is information indicating whether different prediction types are applied to each prediction direction of the current block, from the bitstream (S1010).

[0174] The image decoding apparatus 200 may determine whether different prediction types are applied to the prediction directions of the current block based on the first information (S1020). In addition, when the first information indicates that different prediction types are applied to the prediction directions of the current block, the image decoding apparatus 200 may derive at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block (S1030).

[0175] For example, when the L0 direction of the current block is a sub PU prediction type and the L1 direction is a non-sub PU prediction type, a bidirectional motion information candidate having sub PU-based motion information in the L0 direction and non-sub PU-based motion information in the L1 direction may be derived. As another example, when the L0 direction of the current block is a non-sub PU prediction type and the L1 direction is a sub PU prediction type, a bidirectional motion information candidate having non-sub PU-based motion information in the L0 direction and sub PU-based motion information in the L1 direction may be derived. Here, the sub PU-based motion information may be motion information used for the sub PU prediction type, and the non-sub PU-based motion information may be motion information used for the non-sub PU prediction type. At least one bidirectional motion information candidate may constitute a motion information candidate list.

[0176] The image decoding apparatus 200 may bidirectionally predict the current block with different prediction types based on one of the induced bidirectional motion information candidates, that is, based on the motion information candidate list (S1040).

[0177] Fig. 11 is a diagram for explaining an example of a bidirectional prediction method according to Example 1. In Fig. 11, it is assumed that the prediction type of the L0 direction (Reference picture list (L0)) of a current block (Current picture) is the sub PU prediction type, and the prediction type of the L1 direction (Reference picture list (L1)) is the non-sub PU prediction type.

[0178] Under this assumption, a bidirectional motion information candidate whose prediction type in the L0 direction is the sub PU prediction type and whose prediction type in the L1 direction is the non-sub PU prediction type can be used for bidirectional prediction of the current block.

[0179] Therefore, sub PU-based motion compensation (MC) is performed in one direction (e.g., L0), and non-sub PU-based motion compensation is performed in the opposite direction (e.g., L1) to generate a prediction block for the current block.

[0180] Example 2

[0181] As mentioned above, in merge mode, bidirectional motion information of the current block must be derived using only the motion information of the prediction candidate (motion information candidate) without any signaling for separate syntax (except for indexes), so the prediction candidate of the present disclosure must satisfy the following two conditions.

[0182] - Bidirectional movement information can be induced

[0183] - It is possible to induce the necessary motion information for each prediction direction (e.g., L0: CPMV, L1: MV)

[0184] For example, when a reference block is encoded / decoded by the method proposed in the present disclosure, or when the reference block is bidirectionally predicted by a sub PU prediction type, bidirectional motion information for applying the method proposed in the present disclosure is stored in the reference block. Therefore, in such cases, bidirectional predictions of different prediction types can be performed.

[0185] As another example, if the reference block is not encoded / decoded by the method proposed in the present disclosure, or if the reference block is bidirectionally predicted by a non-sub PU prediction type, bidirectional motion information for applying the method proposed in the present disclosure may not be stored in the reference block. Therefore, in such cases, the following solution may be performed.

[0186] - Reconstruct the motion information required for each prediction direction to derive bidirectional motion information candidates

[0187] - The motion information of the reference block is not used to derive bidirectional motion information candidates.

[0188] In this way, when the method proposed through the present disclosure is applied to merge mode, in the process of constructing a prediction candidate list (motion information candidate list) for a current block, if a reference candidate satisfies the following condition, the reference candidate can be configured as a prediction candidate, or prediction information regenerated based on the reference candidate can be configured as a prediction candidate for the current block.

[0189] Bidirectional prediction information (bidirectional motion information) can be derived from motion information of a decoded neighboring block / a block in a decoded previous picture (i.e., a reference block), and sub PU-based motion information can be derived from at least one of the bidirectional motion information.

[0190] The motion information of the reference block may be decomposed in units of prediction directions and recombined to derive bidirectional motion information, and sub PU-based motion information may be derived from at least one of the recombined bidirectional motion information.

[0191] For any prediction direction, the motion information of the current block can be combined with the motion information of the reference block to derive the motion information in sub PU units.

[0192] On the other hand, when the method proposed through the present disclosure is applied to AMVP mode, prediction blocks must be generated using different prediction types for each prediction direction, and therefore motion information candidates to be referenced for prediction of each prediction direction must be adaptively derived.

[0193] For example, as shown in the left side of Fig. 12(a), if the current block is coded in the proposed way (e.g., L0: non-sub PU, L1: sub PU),

[0194] - Motion information candidates can be compensated for with the signaled MVD to derive motion information of the current block.

[0195] In this process, information on a reference block that has already been decoded in the vicinity can be referred to and used as a motion information candidate, or motion information reconstructed using information on the reference block can be used as a motion information candidate.

[0196] That is, the proposed method performs prediction based on different prediction types for each prediction direction, and therefore, reference information differs for each prediction direction. Therefore, the proposed method can be applied only when necessary information is given.

[0197] For example, if a sub PU prediction type is applied to the L0 direction of the current block and a non-sub PU prediction type is applied to the L1 direction, two or more MVDs for CPMV must be signaled to induce motion information in the L0 direction, and less than two MVDs must be signaled to induce motion information in the L1 direction.

[0198] However, if the L0-direction CPMV is not derived from the prediction candidate (reference block), the L0-direction CPMV of the current block is not derived. For example, as shown on the right side of FIG. 12(a), when the reference block is bidirectionally predicted, but both directions are predicted by the non-sub PU prediction type, or the reference block is not bidirectionally predicted, appropriate motion information for the current block cannot be derived.

[0199] Next, we summarize the basic operations of the method proposed by this disclosure when applied to AMVP mode.

[0200] As illustrated in FIG. 12(b), when the L0 direction is predicted based on the sub PU prediction type and the L1 direction is predicted based on the non-sub PU prediction type, an MVP for each CP can be derived by referring to the CPMV of the L0 direction of the prediction candidate (reference block), and the L0 direction CPMV (motion information) of the current block can be derived by considering each induced MVP and the MVD of each signaled CP. That is, the L0 direction motion information candidate of the current block can be derived by applying an affine model to the L0 direction motion information of the prediction candidate. In addition, the L0 direction motion information candidate of the current block can be derived by considering the L1 direction motion information of the prediction candidate and the signaled MVD.

[0201] In the present disclosure, the induced bidirectional motion information candidates can form a single motion information candidate list. Therefore, compared with the conventional AMVP mode in which a separate motion information candidate list is formed for each prediction direction and separate index information is signaled, the present disclosure can easily form a motion information candidate list and reduce the amount of bits required for signaling index information.

[0202] Based on the reasons and contents described above, in the second embodiment, a method is proposed in which appropriate bidirectional motion information candidates are derived from reference blocks and these are configured into a prediction candidate list (motion information candidate list).

[0203] Example 2-1

[0204] Example 2-1 is an example in which, when bidirectional motion information can be derived from the motion information of a reference block and sub PU-based motion information can be derived from at least one direction of the bidirectional motion information, a bidirectional motion information candidate of a current block is derived from the bidirectional motion information. Here, "when sub PU-based motion information can be derived from at least one direction" may mean that at least one sub PU-based motion information is included in the bidirectional motion information of the reference block.

[0205] 13 and 14 are diagrams for explaining an example of deriving bidirectional motion information candidates when embodiment 2-1 is applied to the merge mode.

[0206] As illustrated in Figure 13(a), if the reference block of the current block is coded using the proposed method (L0: non-sub PU, L1: sub PU) or the bidirectional motion information of the reference block includes at least one sub-PU based motion information, it can be induced as a bidirectional motion information candidate.

[0207] Specifically, as illustrated in Fig. 13(b), the L0-direction motion information of the current block (Cur) can be derived by applying an affine model based on the L0-direction CPMV of the reference block, and the L1-direction motion information of the current block can be derived from the L1-direction motion information of the reference block, and thus the bidirectional motion information of the reference block can be derived as a bidirectional motion information candidate.

[0208] As another example, as illustrated in Figure 14(a), when a reference block is bidirectionally predicted and both directions are predicted with the sub PU prediction type, the bidirectional motion information of the reference block stores the CPMV of the surrounding blocks (left side of Figure 14(b)), and at the same time, the motion information (MV) of the reference block itself is also stored (right side of Figure 14(b)).

[0209] Therefore, as illustrated in FIG. 14(c), in this case, the L0 direction motion information of the current block can be derived by applying an affine model based on the CPMV of the reference block, and the MV of the reference block can be used as the L1 direction motion information of the current block. Thus, the bidirectional motion information of such a reference block can be derived as a bidirectional motion information candidate.

[0210] FIG. 15 is a diagram for explaining an example of deriving bidirectional motion information candidates when Example 2-1 is applied to the AMVP mode.

[0211] If the reference block stores CPMV, as in the case where the reference block is decoded using the proposed method or affine mode, bidirectional motion information candidates can be derived by combining the motion information (CPMV) with motion information (MV) stored at the position of the reference block that is not CPMV.

[0212] For example, as illustrated in FIG 15(a), different prediction types may be applied to each prediction direction of a current block (Current block) (L0: non-sub PU, L1: sub PU), and positions of reference blocks for constructing bidirectional motion information candidates may be defined as A to G. Also, motion information for each position of a reference block (Candidates Position) is as shown in Table 1.

[0213] [Table 1]

[0214] In this example, among the bidirectional motion information of the reference block, bidirectional motion information suitable for applying a prediction method (prediction type) of the current block may be induced as a bidirectional motion information candidate. That is, as shown in Table 2, bidirectional motion information in which the prediction type in the L0 direction is the non-sub PU prediction type and the prediction type in the L1 direction is the sub PU prediction type may be induced as a bidirectional motion information candidate.

[0215] [Table 2]

[0216] According to an embodiment, as illustrated in Fig. 15(b), the CPMV (CP) of the neighboring block is stored at position D, and at the same time, the motion information (D) of the reference block itself is also stored, so that the MV of the CP can be used for the sub PU prediction type, and the motion information of the reference block itself can be referenced for the non-sub PU prediction type. Therefore, the motion information coded by the sub PU at position D (i.e., sub PU-based motion information) can be processed like non-sub PU-based motion information, and through this process, the list in Table 2 can be modified as in Table 3 to configure a bidirectional motion information candidate list.

[0217] [Table 3]

[0218] As another example, in the example of FIG. 15(a), the motion information for each position (candidate position) of the reference block is as shown in Table 4.

[0219] [Table 4]

[0220] In this case, among the motion information of the reference blocks, motion information suitable for applying different prediction types of the current block can be derived as bidirectional motion information candidates as shown in Table 5.

[0221] [Table 5]

[0222] Example 2-2

[0223] Example 2-2 is an example in which bidirectional motion information of a reference block can be derived by decomposing the motion information of the reference block in units of prediction directions and recombining the decomposed bidirectional motion information, and when sub PU-based motion information can be derived from at least one direction among the recombined bidirectional motion information, a bidirectional motion information candidate of a current block is derived from the decomposed bidirectional motion information. Here, "when sub PU-based motion information can be derived from at least one direction" may mean that at least one or more sub PU-based motion information is included in the recombined bidirectional motion information.

[0224] FIG. 16 is a flowchart showing an image coding method and an image decoding method according to Example 2-2.

[0225] Referring to FIG. 16, the image encoding device 100 and the image decoding device 200 may determine a prediction direction of a reference block (S1600). If the prediction direction of the reference block is unidirectional, the image encoding device 100 and the image decoding device 200 may combine motion information of the reference block in units of prediction directions to derive combined bidirectional motion information (S1610). In addition, the image encoding device 100 and the image decoding device 200 may derive bidirectional motion information candidates based on motion information including one or more sub PU-based motion information among the combined bidirectional motion information (S1620).

[0226] According to an embodiment, steps S1610 and S1620 may be performed independently of step S1610. That is, combined bidirectional motion information and bidirectional motion information candidates may be derived without determining the prediction direction of the reference block.

[0227] Next, an example of deriving bidirectional motion information candidates when embodiment 2-2 is applied to the merge mode will be described.

[0228] As illustrated in FIG 15(a), different prediction types are applied to each prediction direction of the current block (L0: non-sub PU, L1: sub PU), and the positions of the reference block for constructing bidirectional motion information candidates are defined as A to G. In addition, the motion information for each position of the reference block (candidate position) may be unidirectional motion information as shown in Table 6.

[0229] [Table 6]

[0230] In this case, the unidirectional motion information of the reference block is recombined to derive recombined bidirectional motion information, and bidirectional motion information candidates as shown in Table 7 can be derived from the recombined bidirectional motion information.

[0231] [Table 7]

[0232] Next, an example of deriving bidirectional motion information candidates when embodiment 2-2 is applied to the AMVP mode will be described.

[0233] When the position of a reference block is as shown in Figure 15(a) and the motion information of the reference block is as shown in Table 1, the motion information of the reference block is separated for each prediction direction and recombined to derive recombined bidirectional motion information as shown in Table 8. The recombined bidirectional motion information can be used as a bidirectional motion information candidate for bidirectional prediction of the current block.

[0234] [Table 8]

[0235] Example 2-3

[0236] When the motion information of the reference block is all non-sub PU-based motion information, the motion information for deriving the sub PU-based motion information of the current block cannot be referenced from the reference block. For example, as shown in FIG. 15(a), when different prediction types are applied to each prediction direction of the current block and the motion information of the reference blocks (A to G) is all non-sub PU motion information, the CPMV for supporting the sub PU prediction type of the L1 direction of the current block cannot be derived.

[0237] In order to solve this problem, embodiment 2-3 proposes a method of deriving sub PU-based motion information of a current block from neighboring blocks located in the vicinity of the current block, rather than deriving it from a reference block.

[0238] For this purpose, the reference block is divided into adjacent blocks and peripheral blocks. The adjacent blocks may correspond to blocks adjacent to the current block. For example, in FIG. 15(a), blocks A to G adjacent to the current block may correspond to the adjacent blocks. In addition, the adjacent blocks may be predefined blocks used to derive motion information of the current block. The peripheral blocks are blocks located around the current block and may or may not be adjacent to the current block. The peripheral blocks may be blocks that do not correspond to the adjacent blocks. The reference blocks may include adjacent blocks and peripheral blocks.

[0239] FIG. 17 is a flowchart showing an image coding method and an image decoding method according to the embodiment 2-3.

[0240] Referring to FIG. 17, the image encoding device 100 and the image decoding device 200 may determine a prediction type of a reference block (S1700). If the prediction types of all the reference blocks are non-sub PU, the image encoding device 100 and the image decoding device 200 may combine the motion information of the reference blocks in units of prediction directions to derive combined bidirectional motion information (S1710). In addition, the image encoding device 100 and the image decoding device 200 may derive bidirectional motion information candidates based on the combined bidirectional motion information (S1720).

[0241] The neighboring blocks may include bidirectional motion information. Even if the motion information of the neighboring blocks is not bidirectional, the combined bidirectional motion information can be derived by adaptively referring to the motion information in each prediction direction unit.

[0242] An example of the neighboring blocks is shown in Figure 18. As illustrated in Figure 18, the neighboring blocks may be reconstruction blocks located around the CP position of the current block. That is, A0, A1, B0, and B1 may be neighboring blocks located around the lower left CP position of the current block, A2, A3, A4, B2, B3, and B4 may be neighboring blocks located around the upper left CP position of the current block, and A5, A6, B5, and B6 may be neighboring blocks located around the upper right CP position of the current block.

[0243] In the example of Figure 18, one motion information for a non-sub PU prediction type can be induced for the L0 direction, and two or more CPMVs for a sub PU prediction type can be induced for the L1 direction. As a result, bidirectional motion information candidates (prediction candidate list) can be configured as shown in Table 9.

[0244] [Table 9]

[0245] In Table 9, information in parentheses of MV() and CPMV() may indicate the positions of the neighboring blocks shown in Figure 18. That is, as shown in Table 9, non-sub PU-based motion information for the L0 direction may be derived using motion information of the neighboring blocks, and sub PU-based motion information for the L1 direction may be derived using CPMV combined with motion information of the neighboring blocks.

[0246] Example 3

[0247] In order to apply different prediction types to each prediction direction according to the method of the present disclosure, a syntax element for supporting the application needs to be signaled. Example 3 is an example of a method for signaling a syntax element.

[0248] FIG. 19 is a flowchart of an image coding method according to the third embodiment.

[0249] 19, the image encoding device 100 may determine whether a first condition (apply_proposed_method_condition) is satisfied (S1900). The first condition is a condition for whether to apply the method of the present disclosure, and may include whether a prediction direction of a current block is bidirectional. Specific content of the first condition will be described later.

[0250] If a first condition is satisfied, the image encoding device 100 may encode the first information into a bitstream (S1910). Here, the first information may be encoded into merge_mixed_pu_flag or inter_mixed_pu_flag. The image encoding device 100 may determine whether different prediction types are applied to each prediction direction of the current block (S1920).

[0251] Also, when it is determined that different prediction types are applied to the prediction directions of the current block, the image encoding device 100 may encode the second information (inter_mixed_pu_l0_flag) and the third information (cu_subpu_type_flag) (S1930). According to an embodiment, the step S1930 may be performed only when the prediction mode of the current block is the AMVP mode. That is, the second information and the third information may be encoded when the prediction mode of the current block is the AMVP mode.

[0252] FIG. 20 is a flowchart of an image decoding method according to the third embodiment.

[0253] Referring to FIG 20, the image decoding apparatus 200 may determine whether a first condition (apply_proposed_method_condition) is satisfied (S2000). The first condition is a condition for whether to apply the method of the present disclosure, and may include whether a prediction direction of a current block is bidirectional. Specific content of the first condition will be described later.

[0254] If the first condition is satisfied, the image decoding apparatus 200 may obtain first information (merge_mixed_pu_flag or inter_mixed_pu_flag) from a bitstream (S2010). The image decoding apparatus 200 may determine whether different prediction types are applied to the prediction directions of the current block based on the first information (S2020).

[0255] When the first information indicates that different prediction types are applied to the prediction directions of the current block, the image decoding apparatus 200 may obtain the second information (inter_mixed_pu_l0_flag) and the third information (cu_subpu_type_flag) from the bitstream (S2030). According to an embodiment, the step S2030 may be performed only when the prediction mode of the current block is the AMVP mode. That is, the second information and the third information may be obtained when the prediction mode of the current block is the AMVP mode.

[0256] Example 3 can be divided into 1) an example (example 3-1) in which no other information is signaled (except index information) and which is performed like the conventional merge mode, and 2) an example (example 3-2) in which other information is signaled and which is performed like the conventional AMVP mode.

[0257] Example 3-1

[0258] An example of the syntax structure according to Example 3-1 is shown in Table 10.

[0259] [Table 10-1] [Table 10-2]

[0260] merge_idx may indicate a merge candidate in the merge candidate list. merge_mixed_pu_flag is first information and may be encoded and acquired if a first condition (apply_proposed_method_condition) is satisfied. The first information may indicate whether different prediction types are applied to each prediction direction of the current block. merge_mixed_pu_flag=1 may indicate that different prediction types are applied to each prediction direction of the current block, and merge_mixed_pu_flag=0 may indicate that different prediction types are not applied to each prediction direction of the current block.

[0261] When merge_mixed_pu_flag=1, a prediction block is generated by applying the method proposed in the present disclosure, so that information such as merge_subblock_flag or regular_merge_flag, which is additionally signaled to support the conventional method, does not need to be signaled. That is, merge_subblock_flag may be signaled (merge_mixed_pu_flag=0) when the method proposed in the present disclosure is not applied.

[0262] The regular_merge_flag may indicate whether a merge mode (regular merge mode) is applied to the current block. The regular_merge_flag=1 may indicate that the regular merge mode is used to generate inter prediction parameters of the current block.

[0263] merge_subblock_flag may indicate whether a subblock merge mode (or an affine merge mode) is applied to the current block. That is, merge_subblock_flag may indicate whether a subblock merge mode is used to generate subblock-based inter prediction parameters for the current block. merge_subblock_idx may indicate a merge candidate index of a subblock-based merge candidate list.

[0264] The index information (merge_mixed_pu_idx) may indicate a bidirectional motion information candidate used for prediction of a current block among at least one bidirectional motion information candidate. The index information may be signaled when the maximum number (MaxNumCombinedSubblockMergeCand) of prediction candidates (bidirectional motion information candidates) for the method proposed in the present disclosure exceeds 1. That is, when the maximum number of prediction candidates is 1 or less, prediction may be performed using one bidirectional motion information candidate without signaling the index information.

[0265] The mmvd_merge_flag may indicate whether or not a merge mode with motion vector difference (MMVD) mode is applied to the current block. The mmvd_merge_flag=1 may indicate that the MMVD mode is used to generate inter prediction parameters of the current block. The mmvd_cand_flag may indicate whether or not the first candidate (0) or the second candidate (1) in a merging candidate list is used with the MVD derived from the mmvd_distance_idx and the mmvd_direction_idx. The mmvd_distance_idx may indicate an index used to derive the variable MmvdDistance[x0][y0], and the mmvd_direction_idx may indicate an index used to derive the variable MmvdSign[x0][y0].

[0266] ciip_flag may indicate whether a combined inter-picture merge and intra-picture prediction mode is applied to the current block. merge_gpm_partition_idx may indicate a partition shape of a geometric partitioning merge mode (GPM mode), and merge_gpm_idx0 and merge_gpm_idx1 may indicate a first candidate and a second candidate in a geometric partitioning based motion compensation candidate list, respectively.

[0267] Another example of the syntax structure according to Example 3-1 is shown in Table 11.

[0268] [Table 11-1] [Table 11-2]

[0269] The syntax structure in Table 11 is an example of syntactically combining the method proposed by the present disclosure with a conventional coding tool (e.g., subblock merge mode). That is, when the subblock merge mode is applied (merge_subblock_flag=1), it is determined whether the first condition (apply_proposed_method_condition) is satisfied, and when the first condition is satisfied, it is determined whether the method proposed by the present disclosure is applied (merge_mixed_pu_flag). Also, in the example of Table 11, the function of the index information (merge_mixed_pu_idx) can be performed by merge_subblock_idx.

[0270] Another example of the syntax structure according to Example 3-1 is shown in Table 12.

[0271] [Table 12]

[0272] The syntax structure of Table 12 is an example in which index information for selecting a bidirectional motion information candidate is not signaled. Specifically, according to Table 12, if(merge_mixed_pu_flag==1) and if(MaxNumCombinedSubblockMergeCand>1) in Table 11 are not performed, and merge_mixed_pu_idx in Table 11 does not need to be signaled. In this case, the bidirectional motion information candidate used for predicting the current block is determined based on a cost according to template matching, and may be determined to be the bidirectional motion information candidate having the minimum cost, for example.

[0273] Example 3-2

[0274] An example of the syntax structure according to Example 3-2 is shown in Table 13.

[0275] [Table 13-1] [Table 13-2]

[0276] inter_mixed_pu_flag is first information and may be encoded and acquired if a first condition (apply_proposed_method_condition) is satisfied. The first information may indicate whether different prediction types are applied to each prediction direction of the current block. inter_mixed_pu_flag=1 may indicate that different prediction types are applied to each prediction direction of the current block, and inter_mixed_pu_flag=0 may indicate that different prediction types are not applied to each prediction direction of the current block.

[0277] The second information (inter_mixed_pu_l0_flag) may be signaled when different prediction types are applied to each prediction direction of the current block (inter_mixed_pu_flag=1). inter_mixed_pu_l0_flag may indicate different prediction types applied to each prediction direction. For example, inter_mixed_pu_l0_flag=1 may indicate that the prediction type applied to the L0 direction is the sub PU prediction type, and inter_mixed_pu_l0_flag=0 may indicate that the prediction type applied to the L0 direction is the non-sub PU prediction type. As another example, inter_mixed_pu_l0_flag=1 may indicate that the prediction type applied to the L0 direction is the non-sub PU prediction type, and inter_mixed_pu_l0_flag=0 may indicate that the prediction type applied to the L0 direction is the sub PU prediction type.

[0278] The third information (cu_subpu_type_flag) may be signaled when different prediction types are applied to each prediction direction of the current block (inter_mixed_pu_flag=1). The cu_subpu_type_flag may be used to determine a parameter model to be applied to each prediction direction, and the determination of the parameter model will be described later.

[0279] The inter_affine_flag may indicate whether an affine mode is applied to the current block. The cu_affine_type_flag may indicate a parameter model applied to the current block. For example, cu_affine_type_flag=1 may indicate that a 6-parameter model is applied, and cu_affine_type_flag=0 may indicate that a 4-parameter model is applied.

[0280] sym_mvd_flag may indicate whether symmetric MVD is used in MVD coding. For example, sym_mvd_flag=1 may indicate that ref_idx_l0, ref_idx_l1, and mvd_coding are not present. ref_idx_l0 may indicate a reference picture in reference picture list 0 (L0 direction), and ref_idx_l1 may indicate a reference picture in reference picture list 1 (L1 direction).

[0281] mvp_l0_flag may indicate a candidate in MVP candidate list 0 that is selected to derive the MVP of the current block when MVP mode is applied, and mvp_l1_flag may indicate a candidate in MVP candidate list 1 that is selected to derive the MVP of the current block when MVP mode is applied.

[0282] Another example of the syntax structure according to Example 3-2 is shown in Table 14.

[0283] [Table 14-1] [Table 14-2]

[0284] The syntax structure of Table 14 is an example in which the method proposed in the present disclosure is syntactically integrated with a conventional coding tool (e.g., affine mode). If inter_affine_flag=1, a sub PU prediction mode may be applied to the current block. Thus, if the prediction direction of the current block is bidirectional (inter_pred_idc==PRED_BI), it may be signaled whether different prediction types are applied to each prediction direction (inter_mixed_pu_flag) and what prediction type is applied to each prediction direction. Also, in the example of Table 13, the function of the third information (cu_subpu_type_flag) may be performed by cu_affine_type_flag.

[0285] An example of a method for determining different prediction types for each prediction direction based on the second information (inter_mixed_pu_l0_flag) is shown in Table 15.

[0286] [Table 15]

[0287] Referring to Table 15, if inter_mixed_pu_flag=0, inter_mixed_pu_l0_flag is not signaled, and the bidirectional prediction of the current block may be determined as the sub PU prediction type. If inter_mixed_pu_flag=1 and inter_mixed_pu_l0_flag=0, the L0 direction of the current block may be determined as the sub PU prediction type, and the L1 direction of the current block may be determined as the non-sub PU prediction type. If inter_mixed_pu_flag=1 and inter_mixed_pu_l0_flag=1, the L0 direction of the current block may be determined as the non-sub PU prediction type, and the L1 direction of the current block may be determined as the sub PU prediction type.

[0288] According to an embodiment, the first information may indicate whether different prediction types are applied to each prediction direction, and may also indicate different prediction types applied to each prediction direction. That is, the first information may further indicate different prediction types applied to each prediction direction. To this end, the first information may be expressed in an index form (inter_mixed_pu_idc) as shown in Table 16, instead of the above-mentioned flag form.

[0289] [Table 16]

[0290] An example of a method for determining whether different prediction types are applied to each prediction direction and different prediction types for each prediction direction based on the first information in the form of an index is shown in Table 17.

[0291] [Table 17]

[0292] Referring to Table 17, when inter_mixed_PU_idc=0, the bidirectionality of the current block may be determined as the sub PU prediction type. When inter_mixed_PU_idc=10, the L0 direction of the current block may be determined as the sub PU prediction type, and the L1 direction of the current block may be determined as the non-sub PU prediction type. When inter_mixed_pu_idc=11, the L0 direction of the current block may be determined as the non-sub PU prediction type, and the L1 direction of the current block may be determined as the sub PU prediction type.

[0293] Example 4

[0294] Example 4 is an example for various examples of the first condition (apply_proposed_method_condition).

[0295] The first condition may include the following conditions: That is, if the following conditions are met, the first information (merge_mixed_pu_flag or inter_mixed_pu_flag) may be encoded and obtained.

[0296] Condition 1. Whether the current block generates a predicted block using Bi-prediction

[0297] Condition 1 is whether the prediction direction of the current block is bidirectional. For example, condition 1 is whether inter_pred_idc[x0][y0]==PRED_BI is satisfied.

[0298] Condition 2. Whether the size of the current block satisfies the predefined size

[0299] Condition 2 is whether the size of the current block (cbWidth, cbHeight) satisfies a predetermined size. For example, condition 2 may include cbWidth>=minimum width (MIN_COMB_PU_WIDTH), cbHeight>=minimum height (MIN_COMB_PU_HEIGHT), cbWidth<=maximum width (MAX_COMB_PU_WIDTH), and cbHeight<=maximum height (MAX_COMB_PU_HEIGHT).

[0300] MIN_COMB_PU_WIDTH, MIN_COMB_PU_HEIGHT, MAX_COMB_PU_WIDTH, and MAX_COMB_PU_HEIGHT may be predefined values ​​or may be values ​​signaled from a higher level of the bitstream. In addition, the values ​​of MIN_COMB_PU_WIDTH, MIN_COMB_PU_HEIGHT, MAX_COMB_PU_WIDTH, and MAX_COMB_PU_HEIGHT may have a range of values ​​within the minimum / maximum block size supported by the compression technique.

[0301] Condition 3. Whether or not the method proposed by this disclosure is supported at a higher level (SPS, PPS, PH, SH, etc.)

[0302] Whether the method proposed by the present disclosure is supported can be defined by a syntax element (comb_subpu_enable_flag) signaled from a higher level, and condition 3 can be satisfied when comb_subpu_enable_flag=1.

[0303] Condition 4. Whether or not MVD signaling of a specific prediction direction is supported at a higher level (SPS, PPS, PH, SH, etc.)

[0304] Whether MVD for a particular prediction direction is signaled can be defined by a syntax element (ph_mvd_l1_zero_flag) signaled from a higher level, and condition 4 is satisfied when ph_mvd_l1_zero_flag=0 (MVD is signaled).

[0305] Example 5

[0306] Example 5 is an example of a method for adaptively signaling MVD for each prediction direction.

[0307] In the conventional AMVP mode, the prediction candidate index (mvp_lx_flag), the reference picture index (ref_idx_lx), and the motion information offset (MvdLx, mvdCpLX) may be coded into a bitstream and signaled. When the prediction type of a specific direction of the current block is a sub PU prediction type (e.g., affine), at least two or more MVDs, i.e., mvdCpLXs, of each control point (CP) may be signaled. Therefore, the amount of bits of signaled information may be more disadvantageous than that of a non-sub PU prediction type (e.g., non-affine). Furthermore, in the conventional method, the same prediction type is applied to each prediction direction, so the amount of information signaled may be further increased in the case of bidirectional prediction.

[0308] The method proposed by the present disclosure applies different prediction types (sub PU prediction type and non-sub PU prediction type) for each prediction direction, thereby reducing the amount of bits signaled in the case of bidirectional prediction, thereby improving compression efficiency.

[0309] FIG. 21 is a flowchart of an image coding method according to the fifth embodiment.

[0310] Referring to FIG. 21, the image coding apparatus 100 may determine different prediction types to be applied to each prediction direction of a current block (S2100). For example, the image coding apparatus 100 may determine the prediction type of the L0 direction as the sub PU prediction type, and the prediction type of the L1 direction as the non-sub PU prediction type. As another example, the image coding apparatus 100 may determine the prediction type of the L0 direction as the non-sub PU prediction type, and the prediction type of the L1 direction as the sub PU prediction type. Information regarding the determination of the prediction type may be coded in second information (inter_mixed_pu_l0_flag) and signaled.

[0311] The image encoding device 100 may determine a parameter model to be applied to each prediction direction. The parameter model may include a translational motion model, a 4-parameter model (e.g., a 4-parameter affine motion model), and a 6-parameter model (e.g., a 6-parameter affine motion model). Information on the parameter model may be coded and signaled in the third information (cu_subpu_type_flag).

[0312] The image encoding device 100 may determine a prediction type applied to each prediction direction (S2120) and signal an MVD for each prediction direction based on the determination (S2130, S2140). For example, when a non-sub PU prediction type is applied to the L0 direction and a sub PU prediction type is applied to the L1 direction, one MVD may be encoded for the L0 direction and multiple MVDs may be encoded for the L1 direction. Here, the number of MVDs to be encoded for the prediction direction to which the sub PU prediction type is applied may be determined based on the parameter model determined through the process of S2110.

[0313] FIG. 22 is a flowchart of an image decoding method according to the fifth embodiment.

[0314] 22, the image decoding apparatus 200 may derive a parameter model (MotionModelIdc[lX][x][y]) of each prediction direction based on the signaled second information (inter_mixed_pu_l0_flag) and third information (cu_subpu_type_flag) (S2200). Here, MotionModelIdc[lx][x][y] is a variable indicating a motion model (parameter model) of a coding unit, and lx may indicate a reference picture list direction (prediction direction).

[0315] MotionModelIdc[lx][x][y] can be derived as follows:

[0316] -If general_merge_flag=1, MotionModelIdc[lx][x][y]=merge_subblock_flag[x0][y0] for lx=0, 1

[0317] - Otherwise (general_merge_flag=0),

[0318] If -inter_mixed_pu_flag=1,

[0319] MotionModelIdc[0][x][y]=inter_mixed_pu_l0_flag[x0][y0]? cu_subpu_type_flag[x0][y0]+1:0

[0320] MotionModelIdc[1][x][y]=inter_mixed_pu_l0_flag[x0][y0]?0: cu_subpu_type_flag[x0][y0]+1

[0321] If -inter_mixed_pu_flag=1,

[0322] MotionModelIdc[lx][x][y]=inter_affine_flag[x0][y0]+cu_affine_type_flag[x0][y0] for lx=0,1

[0323] The parameter model indicated by MotionModelIdc[lx][x][y] is as shown in Table 18.

[0324] [Table 18]

[0325] Since different prediction types are applied to each prediction direction, MotionModelIdc may indicate different values ​​for each prediction direction. In addition, since the proposed method is applied when inter_mixed_pu_flag=1, the first array of MotionModelIdc may be defined to indicate each prediction direction in order to define each prediction direction.

[0326] The image decoding apparatus 200 may perform mvd_coding for each prediction direction to obtain an MVD for each prediction direction (S2210 to S2250).

[0327] Specifically, the image decoding apparatus 200 may obtain a first MVD by performing mvd_coding (S2210) and may check the value of MotionModelIdc (S2220). If the value of MotionModelIdc indicates a 4-parameter model or a 6-parameter model (MotionModelIdc>0), the image decoding apparatus 200 may additionally perform mvd_coding to obtain a second MVD (S2230). Alternatively, if MotionModelIdc=0, additional mvd_coding may not be performed.

[0328] The image decoding apparatus 200 may additionally check the value of MotionModelIdc (S2240). As a result, if the value of MotionModelIdc indicates a 6-parameter model (MotionModelIdc>1), the image decoding apparatus 200 may additionally perform mvd_coding to obtain a third MVD (S2250). In contrast, if MotionModelIdc=1, additional mvd_coding may not be performed.

[0329] The above-described method for adaptively encoding and obtaining MVD can be expressed in a syntax structure as shown in Table 19.

[0330] [Table 19]

[0331] In mvd_coding(x0, y0, lx, n) in Table 19, lx indicates the prediction direction, and n may indicate the number of times mvd_coding is performed.

[0332] The image decoding apparatus 200 may derive bidirectional motion information of the current block based on at least one of the bidirectional motion information candidates and the MVD obtained for each prediction direction, and may perform bidirectional prediction based on the induced bidirectional motion information.

[0333] An example of bidirectionally predicting a current block with different prediction types using adaptively signaled MVDs is shown in Figure 23. In Figure 23, since the L0 direction is a sub PU prediction type, multiple MVDs (e.g., as many as the number of CPs) are signaled, and since the L1 direction is a non-sub prediction type, a single MVD can be signaled.

[0334] Example 6

[0335] Example 6 is an example for various positions of the reference block.

[0336] 24 is a diagram illustrating various positions of a reference block that can guide motion information in more detail. As illustrated in FIG. 24, the reference block may include a block adjacent to a current block and a block not adjacent to the current block.

[0337] Therefore, the image encoding device 100 and the image decoding device 200 can refer not only to motion information of blocks adjacent to the current block, but also to motion information of blocks that are not adjacent to the current block, thereby enabling more detailed deriving of the motion information of the current block.

[0338] FIG. 25 is a diagram illustrating an example of a content streaming system to which an embodiment of the present disclosure can be applied.

[0339] As shown in FIG. 25, a content streaming system to which an embodiment of the present disclosure is applied may broadly include an encoding server, a streaming server, a Web server, a media storage, a user device, and a multimedia input device.

[0340] The encoding server compresses content input from a multimedia input device such as a smartphone, camera, camcorder, etc. into digital data to generate a bitstream and transmits the bitstream to the streaming server. As another example, if a multimedia input device such as a smartphone, camera, video camera, etc. directly generates a bitstream, the encoding server can be omitted.

[0341] The bitstream may be generated by an image encoding method and / or image encoding device to which an embodiment of the present disclosure is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

[0342] The streaming server transmits multimedia data to a user device based on a user request via a web server, and the web server can act as a medium to inform the user of available services. When a user requests a desired service from the web server, the web server transmits the request to the streaming server, and the streaming server can transmit the multimedia data to the user. At this time, the content streaming system can include a separate control server, and in this case, the control server can control commands / responses between devices in the content streaming system.

[0343] The streaming server may receive the content from a media storage and / or an encoding server. For example, when receiving the content from the encoding server, the content may be received in real time. In this case, the streaming server may store the bitstream for a certain period of time in order to provide a smooth streaming service.

[0344] Examples of the user devices include mobile phones, smart phones, laptop computers, digital broadcasting terminals, personal digital assistants (PDAs), portable multimedia players (PMPs), navigation systems, slate PCs, tablet PCs, ultrabooks, wearable devices such as smartwatches, smart glass, head mounted displays (HMDs), digital TVs, desktop computers, and digital signage.

[0345] Each server in the content streaming system can be operated as a distributed server, in which case data received from each server can be processed in a distributed manner.

[0346] The scope of the present disclosure includes software or machine-executable commands (e.g., operating systems, applications, firmware, programs, etc.) that cause operations according to the methods of the various embodiments to be performed on a device or computer, and non-transitory computer-readable medium on which such software or commands can be stored and executed on a device or computer. [Industrial Applicability]

[0347] The embodiments of the present disclosure can be used to encode / decode images.

Claims

1. An image decoding method performed by an image decoding device, comprising: obtaining first information based on the prediction direction of the current block being bidirectional; deriving at least one bidirectional motion information candidate corresponding to each of the different prediction types from a reference block based on the first information indicating that different prediction types are applied to each prediction direction of the current block; bidirectionally predicting the current block with the different prediction types based on one of the at least one bidirectional motion information candidate; The different prediction types are a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type.

2. The bidirectional motion information candidates are derived based on motion information including at least one sub PU-based motion information among bidirectional motion information of the reference block, The image decoding method of claim 1 , wherein the sub PU-based motion information is motion information used for the sub PU prediction type.

3. The bidirectional motion information candidate is derived based on motion information including at least one sub PU-based motion information among motion information obtained by combining the motion information of the reference block in a prediction direction unit, The image decoding method of claim 1 , wherein the sub PU-based motion information is motion information used for the sub PU prediction type.

4. The image decoding method according to claim 3 , wherein the combined motion information is derived based on a prediction direction of the reference block being unidirectional.

5. The reference blocks include neighboring blocks located around the current block and predetermined neighboring blocks adjacent to the current block, 2. The image decoding method according to claim 1, wherein, based on the prediction mode of the predetermined neighboring block being the non-sub PU prediction type, the bidirectional motion information candidate is derived based on motion information in which motion information of the surrounding blocks is combined in a prediction direction unit.

6. The obtaining step includes: determining a first condition for determining whether the current block is to be bidirectionally predicted with the different prediction types; acquiring the first information based on the first condition being satisfied; The image decoding method according to claim 1 , wherein the first condition includes a condition on a prediction direction of the current block and a predetermined size condition on a size of the current block.

7. The image decoding method of claim 6 , wherein the first condition further includes a condition on whether a motion vector difference (mvd) in a specific direction of the two directions is signaled.

8. obtaining second information based on the first information indicating that different prediction types are applied to each prediction direction of the current block; The different prediction types to be applied to the respective prediction directions are determined based on the second information; The image decoding method according to claim 1 , wherein the bidirectional motion information candidates correspond to the determined different prediction types.

9. The image decoding method of claim 1 , wherein the first information further indicates different prediction types to be applied to the respective prediction directions.

10. obtaining third information based on the first information indicating that different prediction types are applied to the prediction directions of the current block; The image decoding method according to claim 8 , wherein the parameter model to be applied to each of the prediction directions is determined based on the third information.

11. Bidirectional motion information of the current block is derived based on one of the at least one bidirectional motion information candidate and an mvd obtained for each prediction direction of the current block; The image decoding method of claim 10 , wherein the number of mvds obtained for each prediction direction of the current block is determined based on the second information and the third information.

12. The image decoding method of claim 1 , wherein the mvd is obtained for a direction to which the non-sub PU-based prediction mode is applied among prediction directions of the current block.

13. The image decoding method of claim 1 , wherein the first information is obtained based on whether a sub-block merging mode is applied to the current block or an affine prediction mode is applied to the current block.

14. An image coding method performed by an image coding device, comprising: determining whether different prediction types are applied to the prediction directions of the current block based on whether the prediction directions of the current block are bidirectional, and first information indicating whether different prediction types are applied to the prediction directions of the current block is coded into a bitstream; deriving at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block based on the different prediction types being applied to each prediction direction of the current block; bidirectionally predicting the current block with the different prediction types based on one of the at least one bidirectional motion information candidate; The different prediction types are a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type.

15. A computer-readable recording medium storing a bitstream generated by the image encoding method according to claim 14.

16. A method for transmitting a bitstream generated by an image coding method, comprising the steps of: The image encoding method includes: determining whether different prediction types are applied to the prediction directions of the current block based on whether the prediction directions of the current block are bidirectional, and first information indicating whether different prediction types are applied to the prediction directions of the current block is coded into a bitstream; deriving at least one bidirectional motion information candidate corresponding to the different prediction types from a reference block based on the different prediction types being applied to each prediction direction of the current block; bidirectionally predicting the current block with the different prediction types based on one of the at least one bidirectional motion information candidate; The different prediction types are a sub-block unit (sub PU) prediction type and a non-sub-block unit (non-sub PU) prediction type.

Images