Patent Publication Number: US-2022239900-A1

Title: Video decoding method and device, and video encoding method and device

Description:
TECHNICAL FIELD 
     The disclosure relates to a video decoding method and a video decoding device, and more particularly, to a video encoding method and device and a video decoding method and device, in which, when an affine mode is to be applied to a current block, an affine parameter group is determined based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and prediction is performed on the current block based on the determined affine parameter group. 
     Also, the disclosure relates to a video encoding method and device and a video decoding method and device, in which: it is determined whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included; when the current block is the block to be reconstructed first, a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored is reset and prediction on the current block is performed based on motion information of an adjacent block to the current block; and when the current block is not the block to be reconstructed first, prediction is performed on the current block based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     BACKGROUND ART 
     Image data is encoded by a codec according to a preset data compression standard, for example, a moving picture experts group (MPEG) standard, and then stored in the form of a bitstream in a recording medium or transmitted through a communication channel. 
     With the development and propagation of hardware capable of reproducing and storing high-resolution or high-definition image content, the need for a codec for effectively encoding or decoding high-resolution or high-definition image content is increasing. Encoded image data may be reproduced by being decoded. Recently, methods for effectively compressing such high-resolution or high-definition image content have been implemented. For example, it has been proposed that image compression technology may be effectively implemented through a process of segmenting an image to be encoded by an arbitrary method or rendering data. 
     As one of the techniques for rendering data, a method of performing prediction on a current block by using surrounding information of the current block is generally used. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     Provided are a method and device, in which, during video encoding and decoding processes, when an affine mode is to be applied to a current block, an affine parameter group is determined based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and prediction is performed on the current block, based on the determined affine parameter group. 
     Also, provided are a method and a device, in which: during video encoding and decoding processes, it is determined whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included; when the current block is the block to be reconstructed first, a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored is reset and prediction is performed on the current block, based on motion information of an adjacent block to the current block; and when the current block is not the block to be reconstructed first, prediction is performed on the current block, based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     Solution to Problem 
     To overcome the above-described technical problems, a video decoding method proposed in the disclosure includes: receiving information about whether to apply an affine mode to a current block; when the information about whether to apply the affine mode indicates that the affine mode is to be applied, determining an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored; and performing prediction on the current block, based on the affine parameter group. 
     To overcome the above-described technical problems, a video decoding device proposed in the disclosure includes: a memory; and at least one processor connected to the memory, and the at least one processor is configured to: receive information about whether to apply an affine mode to a current block; when the information about whether to apply the affine mode indicates that the affine mode is to be applied, determine an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and perform prediction on the current block, based on the affine parameter group. 
     To overcome the above-described technical problems, a video encoding method proposed in the disclosure includes: determining whether to apply an affine mode to a current block; when it is determined that the affine mode is to be applied to the current block, determining an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks encoded in an affine mode before the current block are stored; performing prediction on the current block, based on the affine parameter group; and encoding information about whether to apply the affine mode to the current block. 
     To overcome the above-described technical problems, a video encoding device proposed in the disclosure includes a memory, and at least one processor connected to the memory, and the at least one processor is configured to: determine whether to apply an affine mode to a current block; when it is determined that the affine mode is to be applied to the current block, determine an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks encoded in an affine mode before the current block are stored; perform prediction on the current block, based on the affine parameter group; and encode information about whether to apply the affine mode to the current block. 
     To overcome the above-described technical problems, a video decoding method proposed in the disclosure includes: determining whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included; when the current block is the block to be reconstructed first, resetting a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored and performing prediction on the current block, based on motion information of an adjacent block to the current block; and when the current block is not the block to be reconstructed first, performing prediction on the current block, based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     To overcome the above-described technical problems, a video decoding device proposed in the disclosure includes: a memory; and at least one processor connected to the memory, and the at least one processor is configured to: determine whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included; when the current block is the block to be reconstructed first, reset a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored and perform prediction on the current block based on motion information of an adjacent block to the current block; and when the current block is not the block to be reconstructed first, perform prediction on the current block, based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     To overcome the above-described technical problems, a video encoding method proposed in the disclosure includes: determining whether a current block is a block to be encoded first in a predetermined area in which the current block is included; when the current block is the block to be encoded first, resetting a history-based motion vector candidate list in which motion vectors of blocks encoded before the current block are stored and performing prediction on the current block based on motion information of an adjacent block to the current block; and when the current block is not the block to be encoded first, performing prediction on the current block, based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     To overcome the above-described technical problems, a video encoding device proposed in the disclosure includes a memory, and at least one processor connected to the memory, and the at least one processor is configured to: determine whether a current block is a block to be encoded first in a predetermined area in which the current block is included; when the current block is the block to be encoded first, reset a history-based motion vector candidate list in which motion vectors of blocks encoded before the current block are stored and perform prediction on the current block based on motion information of an adjacent block to the current block; and when the current block is not the block to be encoded first, perform prediction on the current block, based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     Advantageous Effects of Disclosure 
     During video encoding and decoding processes, when an affine mode is to be applied to a current block, an affine parameter group is determined based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and prediction is performed on the current block based on the determined affine parameter group, and thus the amount of buffer may be reduced. 
     Also, when a current block is a block to be reconstructed first in a predetermined area in which the current block is included, a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored is reset, and the history-based motion vector candidate list is used for each predetermined area, and thus the amount of buffer may be reduced and efficient parallel processing may be performed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a schematic block diagram of an image decoding device according to an embodiment. 
         FIG. 2  illustrates a flowchart of an image decoding method according to an embodiment. 
         FIG. 3  illustrates a process, performed by an image decoding device, of determining at least one coding unit by splitting a current coding unit, according to an embodiment. 
         FIG. 4  illustrates a process, performed by an image decoding device, of determining at least one coding unit by splitting a non-square coding unit, according to an embodiment. 
         FIG. 5  illustrates a process, performed by an image decoding device, of splitting a coding unit based on at least one of block shape information and split shape mode information, according to an embodiment. 
         FIG. 6  illustrates a method, performed by an image decoding device, of determining a preset coding unit from among an odd number of coding units, according to an embodiment. 
         FIG. 7  illustrates an order of processing a plurality of coding units when an image decoding device determines the plurality of coding units by splitting a current coding unit, according to an embodiment. 
         FIG. 8  illustrates a process, performed by an image decoding device, of determining that a current coding unit is to be split into an odd number of coding units, when the coding units are not processable in a preset order, according to an embodiment. 
         FIG. 9  illustrates a process, performed by an image decoding device, of determining at least one coding unit by splitting a first coding unit, according to an embodiment. 
         FIG. 10  illustrates that a shape into which a second coding unit is splittable is restricted when the second coding unit having a non-square shape, which is determined when an image decoding device splits a first coding unit, satisfies a preset condition, according to an embodiment. 
         FIG. 11  illustrates a process, performed by an image decoding device, of splitting a square coding unit when split shape mode information indicates that the square coding unit is not to be split into four square coding units, according to an embodiment. 
         FIG. 12  illustrates that a processing order between a plurality of coding units may be changed depending on a process of splitting a coding unit, according to an embodiment. 
         FIG. 13  illustrates a process of determining a depth of a coding unit as a shape and size of the coding unit change, when the coding unit is recursively split such that a plurality of coding units are determined, according to an embodiment. 
         FIG. 14  illustrates depths that are determinable based on shapes and sizes of coding units, and part indexes (PIDs) that are for distinguishing the coding units, according to an embodiment. 
         FIG. 15  illustrates that a plurality of coding units are determined based on a plurality of preset data units included in a picture, according to an embodiment. 
         FIG. 16  illustrates a processing block serving as a unit for determining a determination order of reference coding units included in a picture, according to an embodiment. 
         FIG. 17  illustrates a block diagram of a video encoding device according to an embodiment. 
         FIG. 18  is a flowchart illustrating a video encoding method according to an embodiment. 
         FIG. 19  illustrates a block diagram of a video decoding device according to an embodiment. 
         FIG. 20  illustrates a flowchart of a video decoding method according to an embodiment. 
         FIG. 21A  illustrates a schematic diagram of history-based motion vector prediction (HMVP). 
         FIG. 21B  illustrates a process of processing a list used for HMVP. 
         FIG. 22A  illustrates in detail a method of deriving a motion vector applied to a sample of a current block in an affine mode.  FIG. 22B  illustrates a method of deriving an affine parameter group from surrounding information in an affine mode. 
         FIG. 23  illustrates a method of determining an affine parameter group candidate of a current block based on a history-based affine parameter group candidate list. 
         FIG. 24  illustrates a flowchart of a video encoding method according to another embodiment. 
         FIG. 25  illustrates a flowchart of a video decoding method according to another embodiment. 
         FIG. 26  illustrates a line buffer required to access motion information at a boundary of a largest coding unit. 
         FIG. 27A  illustrates an example of identifying each area having a history-based motion vector candidate list in one frame. 
         FIG. 27B  illustrates another example of identifying each area having a history-based motion vector candidate list in one frame. 
         FIG. 28  illustrates a method of determining a method of determining a motion vector of a current block in an affine mode. 
     
    
    
     BEST MODE 
     A video decoding method according to an embodiment proposed in the disclosure includes: receiving information about whether to apply an affine mode to a current block; when the information about whether to apply the affine mode indicates that the affine mode is to be applied, determining an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored; and performing prediction on the current block, based on the affine parameter group. 
     According to an embodiment, when an adjacent block reconstructed in an affine mode exists from among adjacent blocks to the current block, the affine parameter group may be determined based on affine parameter information of the adjacent block reconstructed in the affine mode and the history-based affine parameter group candidate list. 
     According to an embodiment, the video decoding method may further include adding the affine parameter group of the current block to the history-based affine parameter group candidate list. 
     According to an embodiment, the video decoding method may further include, when adding the affine parameter group of the current block to the history-based affine parameter group candidate list, in a case where a number of candidates included in the history-based affine parameter group candidate list is a predetermined maximum value, removing the earliest-stored affine parameter candidate from the history-based affine parameter group candidate list. 
     According to an embodiment, when adding the affine parameter group of the current block to the history-based affine parameter group candidate list, in a case where the affine parameter group of the current block is the same as an affine parameter group existing in the history-based affine parameter group candidate list, maintaining the history-based affine parameter group candidate list. 
     According to an embodiment, the affine mode may indicate an affine merge mode or an affine advanced motion vector prediction (AMVP) mode. 
     A video encoding method according to an embodiment proposed in the disclosure includes: determining whether to apply an affine mode to a current block; when it is determined that the affine mode is to be applied to the current block, determining an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks encoded in an affine mode before the current block are stored; performing prediction on the current block, based on the affine parameter group; and encoding information about whether to apply the affine mode to the current block. 
     According to an embodiment, when an adjacent block encoded in an affine mode exists from among adjacent blocks to the current block, the affine parameter group may be determined based on affine parameter information of the adjacent block encoded in the affine mode and the history-based affine parameter group candidate list. 
     A video decoding method according to another embodiment proposed in the disclosure includes: determining whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included; when the current block is the block to be reconstructed first, resetting a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored and performing prediction on the current block based on motion information of an adjacent block to the current block; and when the current block is not the block to be reconstructed first, performing prediction on the current block, based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     According to an embodiment, the video decoding method may further include storing a motion vector of the current block in the history-based motion vector candidate list. 
     According to an embodiment, the video decoding method may further include, when storing the motion vector of the current block in the history-based motion vector candidate list, in a case where a number of motion vector candidates stored in the history-based motion vector candidate list is a predetermined maximum value, removing a motion vector of the earliest-stored block from the history-based motion vector candidate list. 
     According to an embodiment, the video decoding method may further include, when storing the motion vector of the current block in the history-based motion vector candidate list, in a case where the motion vector of the current block is the same as a motion vector existing in the history-based motion vector candidate list, maintaining the history-based motion vector candidate list. 
     MODE OF DISCLOSURE 
     The advantages and features of the disclosure and methods of achieving the advantages and features will be described more fully with reference to the accompanying drawings, in which embodiments of the disclosure are shown. The disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather these embodiments are provided so that the disclosure will be thorough and complete, and will fully convey the concept of the disclosure to one of ordinary skill in the art. 
     The terms used herein will be briefly described, and disclosed embodiments will be described in detail. 
     The terms used herein are those general terms currently widely used in the art in consideration of functions in the disclosure, but the terms may vary according to the intention of one of ordinary skill in the art, precedents, or new technology in the art. Also, some of the terms used herein may be arbitrarily chosen by the present applicant, and in this case, these terms are defined in detail below. Accordingly, the specific terms used herein should be defined based on the unique meanings thereof and the whole context of the disclosure. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     It will be understood that when a certain part “includes” a certain component, the part does not exclude another component but may further include another component, unless the context clearly dictates otherwise. 
     Also, the term “unit” used herein refers to a software component or a hardware component, which performs certain tasks. However, the term “unit” is not limited to software or hardware. A “unit” may be configured to be in an addressable storage medium or configured to operate one or more processors. Thus, a “unit” may include, by way of example, components such as software components, object-oriented software components, class components, and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The functionality provided by the components and “units” may be combined into fewer components and “units” or further separated into additional components and “units”. 
     According to an embodiment of the disclosure, the “unit” may include a processor and a memory. The term “processor” should be interpreted broadly to include a general purpose processor, a central processing unit (CPU), a microprocessor, a digital signal processor (DSP), a controller, a microcontroller, a state machine, etc. In some circumstances, the “processor” may refer to an application specific semiconductor (ASIC), a programmable logic device (PLD), a field programmable gate array (FPGA), etc. The term “processor” may refer to a combination of processing devices such as, for example, a combination of a DSP and a microprocessor, a combination of a plurality of microprocessors, a combination of one or more microprocessors in conjunction with a DSP core, or a combination of any other such configuration. 
     The term “memory” should be interpreted broadly to include any electronic component capable of storing electronic information. The term “memory” may refer to various types of processor-readable media, such as a random access memory (RAM), a read-only memory (ROM), a non-volatile random access memory (NVRAM), a programmable read-only memory (PROM), an erase-programmable read-only memory (EPROM), an electrically erasable PROM (EEPROM), a flash memory, a magnetic or optical data storage device, a register, etc. When the processor can read information from a memory and/or write information to the memory, the memory is said to be in an electronic communication state with the processor. The memory integrated in the processor is in an electronic communication state with the processor. 
     Hereinafter, an “image” may indicate a still image of a video or may indicate a dynamic image such as a moving image, that is, the video itself. 
     Hereinafter, a “sample” denotes data assigned to a sampling location of an image, i.e., data to be processed. For example, pixel values of an image in a spatial domain and transform coefficients on a transform region may be samples. A unit including at least one such sample may be defined as a block. 
     Also, in the present specification, a “current block” may denote a block of a largest coding unit, a coding unit, a prediction unit, or a transform unit of a current image to be encoded or decoded. 
     The disclosure will now be described more fully with reference to the accompanying drawings for one of ordinary skill in the art to be able to perform the disclosure without any difficulty. Also, portions irrelevant to the descriptions of the disclosure will be omitted in the drawings for clear descriptions of the disclosure. 
     Hereinafter, an image encoding device, an image decoding device, an image encoding method, and an image decoding method, according to an embodiment, will be described with reference to  FIGS. 1 to 16 . A method of determining a data unit of an image according to an embodiment, will be described with reference to  FIGS. 3 through 16 . A video encoding/decoding method according to an embodiment, which receives information about whether to apply an affine mode to a current block, when the information about whether to apply the affine mode indicates that the affine mode is to be applied, determines an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and performs prediction on the current block based on the affine parameter group, will be described below with reference to  FIGS. 17 to 23 and 28 . A video encoding/decoding method, which determines whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included, when the current block is the block to be reconstructed first, resets a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored and performs prediction on the current block based on motion information of an adjacent block to the current block, and when the current block is not the block to be reconstructed first, performs prediction on the current block based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list, will be described with reference to  FIGS. 24 to 27 . 
     Hereinafter, a method and device for adaptively selecting a context model, based on various shapes of coding units, according to an embodiment of the disclosure, will be described with reference to  FIGS. 1 and 2 . 
       FIG. 1  illustrates a schematic block diagram of an image decoding device according to an embodiment. 
     The image decoding device  100  may include a receiver  110  and a decoder  120 . The receiver  110  and the decoder  120  may include at least one processor. Also, the receiver  110  and the decoder  120  may include a memory storing instructions to be performed by the at least one processor. 
     The receiver  110  may receive a bitstream. The bitstream includes information on an image encoded by an image encoding device  2200  described below. Also, the bitstream may be transmitted from the image encoding device  2200 . The image encoding device  2200  and the image decoding device  100  may be connected via wires or wirelessly, and the receiver  110  may receive the bitstream via wires or wirelessly. The receiver  110  may receive the bitstream from a storage medium, such as an optical medium or a hard disk. The decoder  120  may reconstruct an image based on information obtained from the received bitstream. The decoder  120  may obtain, from the bitstream, a syntax element for reconstructing the image. The decoder  120  may reconstruct the image based on the syntax element. 
     Operations of the image decoding device  100  will be described in detail with reference to  FIG. 2 . 
       FIG. 2  illustrates a flowchart of an image decoding method according to an embodiment. 
     According to an embodiment of the disclosure, the receiver  110  receives a bitstream. 
     The image decoding device  100  obtains, from a bitstream, a bin string corresponding to a split shape mode of a coding unit (operation  210 ). The image decoding device  100  determines a split rule of coding units (operation  220 ). Also, the image decoding device  100  splits the coding unit into a plurality of coding units, based on at least one of the bin string corresponding to the split shape mode and the split rule (operation  230 ). The image decoding device  100  may determine an allowable first range of a size of the coding unit, according to a ratio of the width and the height of the coding unit, so as to determine the split rule. The image decoding device  100  may determine an allowable second range of the size of the coding unit, according to the split shape mode of the coding unit, so as to determine the split rule. 
     Hereinafter, splitting of a coding unit will be described in detail according to an embodiment of the disclosure. 
     First, one picture may be split into one or more slices or one or more tiles. One slice or one tile may be a sequence of one or more largest coding units (coding tree units (CTUs)). There is a largest coding block (coding tree block (CTB)) conceptually compared to a largest coding unit (CTU). 
     The largest coding block (CTB) denotes an N×N block including N×N samples (where N is an integer). Each color component may be split into one or more largest coding blocks. 
     When a picture has three sample arrays (sample arrays for Y, Cr, and Cb components), a largest coding unit (CTU) includes a largest coding block of a luma sample, two corresponding largest coding blocks of chroma samples, and syntax structures used to encode the luma sample and the chroma samples. When a picture is a monochrome picture, a largest coding unit includes a largest coding block of a monochrome sample and syntax structures used to encode the monochrome samples. When a picture is a picture encoded in color planes separated according to color components, a largest coding unit includes syntax structures used to encode the picture and samples of the picture. 
     One largest coding block (CTB) may be split into M×N coding blocks including M×N samples (M and N are integers). 
     When a picture has sample arrays for Y, Cr, and Cb components, a coding unit (CU) includes a coding block of a luma sample, two corresponding coding blocks of chroma samples, and syntax structures used to encode the luma sample and the chroma samples. When a picture is a monochrome picture, a coding unit includes a coding block of a monochrome sample and syntax structures used to encode the monochrome samples. When a picture is a picture encoded in color planes separated according to color components, a coding unit includes syntax structures used to encode the picture and samples of the picture. 
     As described above, a largest coding block and a largest coding unit are conceptually distinguished from each other, and a coding block and a coding unit are conceptually distinguished from each other. That is, a (largest) coding unit refers to a data structure including a (largest) coding block including a corresponding sample and a syntax structure corresponding to the (largest) coding block. However, because it is understood by one of ordinary skill in the art that a (largest) coding unit or a (largest) coding block refers to a block of a preset size including a preset number of samples, a largest coding block and a largest coding unit, or a coding block and a coding unit are mentioned in the following specification without being distinguished unless otherwise described. 
     An image may be split into largest coding units (CTUs). A size of each largest coding unit may be determined based on information obtained from a bitstream. A shape of each largest coding unit may be a square shape of the same size. However, the embodiment is not limited thereto. 
     For example, information about a maximum size of a luma coding block may be obtained from a bitstream. For example, the maximum size of the luma coding block indicated by the information about the maximum size of the luma coding block may be one of 4×4, 8×8, 16×16, 32×32, 64×64, 128×128, and 256×256. 
     For example, information about a luma block size difference and a maximum size of a luma coding block that may be split into two may be obtained from a bitstream. The information about the luma block size difference may refer to a size difference between a luma largest coding unit and a largest luma coding block that may be split into two. Accordingly, when the information about the maximum size of the luma coding block that may be split into two and the information about the luma block size difference obtained from the bitstream are combined with each other, a size of the luma largest coding unit may be determined. A size of a chroma largest coding unit may be determined by using the size of the luma largest coding unit. For example, when a Y:Cb:Cr ratio is 4:2:0 according to a color format, a size of a chroma block may be half a size of a luma block, and a size of a chroma largest coding unit may be half a size of a luma largest coding unit. 
     According to an embodiment, because information about a maximum size of a luma coding block that is binary splittable is obtained from a bitstream, the maximum size of the luma coding block that is binary splittable may be variably determined. In contrast, a maximum size of a luma coding block that is ternary splittable may be fixed. For example, the maximum size of the luma coding block that is ternary splittable in an I-picture may be 32×32, and the maximum size of the luma coding block that is ternary splittable in a P-picture or a B-picture may be 64×64. 
     Also, a largest coding unit may be hierarchically split into coding units based on split shape mode information obtained from a bitstream. At least one of information indicating whether quad splitting is performed, information indicating whether multi-splitting is performed, split direction information, and split type information may be obtained as the split shape mode information from the bitstream. 
     For example, the information indicating whether quad splitting is performed may indicate whether a current coding unit is quad split (QUAD_SPLIT) or not. 
     When the current coding unit is not quad split, the information indicating whether multi-splitting is performed may indicate whether the current coding unit is no longer split (NO_SPLIT) or binary/ternary split. 
     When the current coding unit is binary split or ternary split, the split direction information indicates that the current coding unit is split in one of a horizontal direction and a vertical direction. 
     When the current coding unit is split in the horizontal direction or the vertical direction, the split type information indicates that the current coding unit is binary split or ternary split. 
     A split mode of the current coding unit may be determined according to the split direction information and the split type information. A split mode when the current coding unit is binary split in the horizontal direction may be determined to be a binary horizontal split mode (SPLIT_BT_HOR), a split mode when the current coding unit is ternary split in the horizontal direction may be determined to be a ternary horizontal split mode (SPLIT_TT_HOR), a split mode when the current coding unit is binary split in the vertical direction may be determined to be a binary vertical split mode (SPLIT_BT_VER), and a split mode when the current coding unit is ternary split in the vertical direction may be determined to be a ternary vertical split mode (SPLIT_TT_VER). 
     The image decoding device  100  may obtain, from the bitstream, the split shape mode information from one bin string. A form of the bitstream received by the image decoding device  100  may include fixed length binary code, unary code, truncated unary code, predetermined binary code, or the like. The bin string is information in a binary number. The bin string may include at least one bit. The image decoding device  100  may obtain the split shape mode information corresponding to the bin string, based on the split rule. The image decoding device  100  may determine whether to quad split a coding unit, whether not to split a coding unit, a split direction, and a split type, based on one bin string. 
     The coding unit may be smaller than or the same as the largest coding unit. For example, because a largest coding unit is a coding unit having a maximum size, the largest coding unit is one of coding units. When split shape mode information about a largest coding unit indicates that splitting is not performed, a coding unit determined in the largest coding unit has the same size as that of the largest coding unit. When split shape mode information about a largest coding unit indicates that splitting is performed, the largest coding unit may be split into coding units. Also, when split shape mode information about a coding unit indicates that splitting is performed, the coding unit may be split into smaller coding units. However, the splitting of the image is not limited thereto, and the largest coding unit and the coding unit may not be distinguished. The splitting of the coding unit will be described in detail with reference to  FIGS. 3 through 16 . 
     Also, one or more prediction blocks for prediction may be determined from a coding unit. The prediction block may be the same as or smaller than the coding unit. Also, one or more transform blocks for transformation may be determined from a coding unit. The transform block may be the same as or smaller than the coding unit. 
     The shapes and sizes of the transform block and prediction block may not be related to each other. 
     In another embodiment, prediction may be performed by using a coding unit as a prediction unit. Also, transformation may be performed by using a coding unit as a transform block. 
     The splitting of the coding unit will be described in detail with reference to  FIGS. 3 through 16 . A current block and an adjacent block of the disclosure may indicate one of the largest coding unit, the coding unit, the prediction block, and the transform block. Also, the current block of the current coding unit is a block that is currently being decoded or encoded or a block that is currently being split. The adjacent block may be a block reconstructed before the current block. The adjacent block may be adjacent to the current block spatially or temporally. The adjacent block may be located at one of the lower left, left, upper left, top, upper right, right, lower right of the current block. 
       FIG. 3  illustrates a process, performed by an image decoding device, of determining at least one coding unit by splitting a current coding unit, according to an embodiment. 
     A block shape may include 4N×4N, 4N×2N, 2N×4N, 4N×N, N×4N, 32N×N, N×32N, 16N×N, N×16N, 8N×N, or N×8N. Here, N may be a positive integer. Block shape information is information indicating at least one of a shape, a direction, a ratio of width and height, or size of a coding unit. 
     The shape of the coding unit may include a square and a non-square. When the lengths of the width and height of the coding unit are the same (i.e., when the block shape of the coding unit is 4N×4N), the image decoding device  100  may determine the block shape information on the coding unit as a square. The image decoding device  100  may determine the shape of the coding unit to be a non-square. 
     When the width and the height of the coding unit are different from each other (i.e., when the block shape of the coding unit is 4N×2N, 2N×4N, 4N×N, N×4N, 32N×N, N×32N, 16N×N, N×16N, 8N×N, or N×8N), the image decoding device  100  may determine the block shape information on the coding unit as a non-square shape. When the shape of the coding unit is non-square, the image decoding device  100  may determine the ratio of the width and height among the block shape information on the coding unit to be at least one of 1:2, 2:1, 1:4, 4:1, 1:8, 8:1, 1:16, 16:1, 1:32, and 32:1. Also, the image decoding device  100  may determine whether the coding unit is in a horizontal direction or a vertical direction, based on the length of the width and the length of the height of the coding unit. Also, the image decoding device  100  may determine the size of the coding unit, based on at least one of the length of the width, the length of the height, or the area of the coding unit. 
     According to an embodiment, the image decoding device  100  may determine the shape of the coding unit by using the block shape information, and may determine a splitting method of the coding unit by using the split shape mode information. That is, a coding unit splitting method indicated by the split shape mode information may be determined based on a block shape indicated by the block shape information used by the image decoding device  100 . 
     The image decoding device  100  may obtain the split shape mode information from a bitstream. However, an embodiment is not limited thereto, and the image decoding device  100  and the image encoding device  2200  may determine pre-agreed split shape mode information, based on the block shape information. The image decoding device  100  may determine the pre-agreed split shape mode information with respect to a largest coding unit or a smallest coding unit. For example, the image decoding device  100  may determine split shape mode information with respect to the largest coding unit to be a quad split. Also, the image decoding device  100  may determine split shape mode information regarding the smallest coding unit to be “not to perform splitting”. In particular, the image decoding device  100  may determine the size of the largest coding unit to be 256×256. The image decoding device  100  may determine the pre-agreed split shape mode information to be a quad split. The quad split is a split shape mode in which the width and the height of the coding unit are both bisected. The image decoding device  100  may obtain a coding unit of a 128×128 size from the largest coding unit of a 256×256 size, based on the split shape mode information. Also, the image decoding device  100  may determine the size of the smallest coding unit to be 4×4. The image decoding device  100  may obtain split shape mode information indicating “not to perform splitting” with respect to the smallest coding unit. 
     According to an embodiment, the image decoding device  100  may use the block shape information indicating that the current coding unit has a square shape. For example, the image decoding device  100  may determine whether not to split a square coding unit, whether to vertically split the square coding unit, whether to horizontally split the square coding unit, or whether to split the square coding unit into four coding units, based on the split shape mode information. Referring to  FIG. 3 , when the block shape information on a current coding unit  300  indicates a square shape, the decoder  120  may determine that a coding unit  310   a  having the same size as the current coding unit  300  is not split, based on the split shape mode information indicating not to perform splitting, or may determine coding units  310   b ,  310   c ,  310   d ,  310   e , or  310   f  split based on the split shape mode information indicating a preset splitting method. 
     Referring to  FIG. 3 , according to an embodiment, the image decoding device  100  may determine two coding units  310   b  obtained by splitting the current coding unit  300  in a vertical direction, based on the split shape mode information indicating to perform splitting in a vertical direction. The image decoding device  100  may determine two coding units  310   c  obtained by splitting the current coding unit  300  in a horizontal direction, based on the split shape mode information indicating to perform splitting in a horizontal direction. The image decoding device  100  may determine four coding units  310   d  obtained by splitting the current coding unit  300  in vertical and horizontal directions, based on the split shape mode information indicating to perform splitting in vertical and horizontal directions. According to an embodiment, the image decoding device  100  may determine three coding units  310   e  obtained by splitting the current coding unit  300  in a vertical direction, based on the split shape mode information indicating to perform ternary splitting in a vertical direction. The image decoding device  100  may determine three coding units  310   f  obtained by splitting the current coding unit  300  in a horizontal direction, based on the split shape mode information indicating to perform ternary splitting in a horizontal direction. However, splitting methods of the square coding unit are not limited to the above-described methods, and the split shape mode information may indicate various methods. Preset splitting methods of splitting the square coding unit will be described in detail below in relation to various embodiments. 
       FIG. 4  illustrates a process, performed by an image decoding device, of determining at least one coding unit by splitting a non-square coding unit, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may use block shape information indicating that a current coding unit has a non-square shape. The image decoding device  100  may determine whether not to split the non-square current coding unit or whether to split the non-square current coding unit by using a preset splitting method, based on split shape mode information. Referring to  FIG. 4 , when the block shape information on a current coding unit  400  or  450  indicates a non-square shape, the image decoding device  100  may determine a coding unit  410  or  460  having the same size as the current coding unit  400  or  450 , based on the split shape mode information indicating not to perform splitting, or determine coding units  420   a  and  420   b ,  430   a  to  430   c ,  470   a  and  470   b , or  480   a  to  480   c  split based on the split shape mode information indicating a preset splitting method. Preset splitting methods of splitting a non-square coding unit will be described in detail below in relation to various embodiments. 
     According to an embodiment, the image decoding device  100  may determine a splitting method of a coding unit by using the split shape mode information and, in this case, the split shape mode information may indicate the number of one or more coding units generated by splitting a coding unit. Referring to  FIG. 4 , when the split shape mode information indicates to split the current coding unit  400  or  450  into two coding units, the image decoding device  100  may determine two coding units  420   a  and  420   b , or  470   a  and  470   b  included in the current coding unit  400  or  450 , by splitting the current coding unit  400  or  450  based on the split shape mode information. 
     According to an embodiment, when the image decoding device  100  splits the non-square current coding unit  400  or  450  based on the split shape mode information, the image decoding device  100  may consider the location of a long side of the non-square current coding unit  400  or  450  to split a current coding unit. For example, the image decoding device  100  may determine a plurality of coding units by splitting the current coding unit  400  or  450  in a direction of splitting a long side of the current coding unit  400  or  450 , in consideration of the shape of the current coding unit  400  or  450 . 
     According to an embodiment, when the split shape mode information indicates to split (ternary split) a coding unit into an odd number of blocks, the image decoding device  100  may determine an odd number of coding units included in the current coding unit  400  or  450 . For example, when the split shape mode information indicates to split the current coding unit  400  or  450  into three coding units, the image decoding device  100  may split the current coding unit  400  or  450  into three coding units  430   a ,  430   b , and  430   c , or  480   a ,  480   b , and  480   c.    
     According to an embodiment, a ratio of the width and height of the current coding unit  400  or  450  may be 4:1 or 1:4. When the ratio of the width and height is 4:1, the block shape information may indicate a horizontal direction because the length of the width is longer than the length of the height. When the ratio of the width and height is 1:4, the block shape information may indicate a vertical direction because the length of the width is shorter than the length of the height. The image decoding device  100  may determine to split a current coding unit into the odd number of blocks, based on the split shape mode information. Also, the image decoding device  100  may determine a split direction of the current coding unit  400  or  450 , based on the block shape information on the current coding unit  400  or  450 . For example, when the current coding unit  400  is in the vertical direction, the image decoding device  100  may determine the coding units  430   a  to  430   c  by splitting the current coding unit  400  in the horizontal direction. Also, when the current coding unit  450  is in the horizontal direction, the image decoding device  100  may determine the coding units  480   a  to  480   c  by splitting the current coding unit  450  in the vertical direction. 
     According to an embodiment, the image decoding device  100  may determine the odd number of coding units included in the current coding unit  400  or  450 , and not all the determined coding units may have the same size. For example, a preset coding unit  430   b  or  480   b  from among the determined odd number of coding units  430   a ,  430   b , and  430   c , or  480   a ,  480   b , and  480   c  may have a size different from the size of the other coding units  430   a  and  430   c , or  480   a  and  480   c . That is, coding units which may be determined by splitting the current coding unit  400  or  450  may have multiple sizes and, in some cases, all of the odd number of coding units  430   a ,  430   b , and  430   c , or  480   a ,  480   b , and  480   c  may have different sizes. 
     According to an embodiment, when the split shape mode information indicates to split a coding unit into the odd number of blocks, the image decoding device  100  may determine the odd number of coding units included in the current coding unit  400  or  450 , and moreover, may put a preset restriction on at least one coding unit from among the odd number of coding units generated by splitting the current coding unit  400  or  450 . Referring to  FIG. 4 , the image decoding device  100  may set a decoding process regarding the coding unit  430   b  or  480   b  located at the center among the three coding units  430   a ,  430   b , and  430   c , or  480   a ,  480   b , and  480   c  generated as the current coding unit  400  or  450  is split to be different from that of the other coding units  430   a  and  430   c , or  480   a  and  480   c . For example, the image decoding device  100  may restrict the coding unit  430   b  or  480   b  at the center location to be no longer split or to be split only a preset number of times, unlike the other coding units  430   a  and  430   c , or  480   a  and  480   c.    
       FIG. 5  illustrates a process, performed by an image decoding device, of splitting a coding unit based on at least one of block shape information and split shape mode information, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine to split or not to split a square first coding unit  500  into coding units, based on at least one of the block shape information and the split shape mode information. According to an embodiment, when the split shape mode information indicates to split the first coding unit  500  in a horizontal direction, the image decoding device  100  may determine a second coding unit  510  by splitting the first coding unit  500  in a horizontal direction. A first coding unit, a second coding unit, and a third coding unit used according to an embodiment are terms used to understand a relation before and after splitting a coding unit. For example, a second coding unit may be determined by splitting a first coding unit, and a third coding unit may be determined by splitting the second coding unit. It will be understood that the relation of the first coding unit, the second coding unit, and the third coding unit follows the above descriptions. 
     According to an embodiment, the image decoding device  100  may determine to split or not to split the determined second coding unit  510  into coding units, based on the split shape mode information. Referring to  FIG. 5 , the image decoding device  100  may split the non-square second coding unit  510 , which is determined by splitting the first coding unit  500 , into one or more third coding units  520   a ,  520   b ,  520   c , and  520   d  based on at least one of the split shape mode information and the split shape mode information, or may not split the non-square second coding unit  510 . The image decoding device  100  may obtain the split shape mode information, and may obtain a plurality of various-shaped second coding units (e.g.,  510 ) by splitting the first coding unit  500 , based on the obtained split shape mode information, and the second coding unit  510  may be split by using a splitting method of the first coding unit  500  based on the split shape mode information. According to an embodiment, when the first coding unit  500  is split into the second coding units  510  based on the split shape mode information on the first coding unit  500 , the second coding unit  510  may also be split into the third coding units (e.g.,  520   a , or  520   b ,  520   c , and  520   d ) based on the split shape mode information on the second coding unit  510 . That is, a coding unit may be recursively split based on the split shape mode information on each coding unit. Therefore, a square coding unit may be determined by splitting a non-square coding unit, and a non-square coding unit may be determined by recursively splitting the square coding unit. 
     Referring to  FIG. 5 , a preset coding unit (e.g., a coding unit located at a center location, or a square coding unit) from among an odd number of third coding units  520   b ,  520   c , and  520   d  determined by splitting the non-square second coding unit  510  may be recursively split. According to an embodiment, the square third coding unit  520   c  from among the odd number of third coding units  520   b ,  520   c , and  520   d  may be split in a horizontal direction into a plurality of fourth coding units. A non-square fourth coding unit  530   b  or  530   d  from among the plurality of fourth coding units  530   a ,  530   b ,  530   c , and  530   d  may be re-split into a plurality of coding units. For example, the non-square fourth coding unit  530   b  or  530   d  may be re-split into an odd number of coding units. A method that may be used to recursively split a coding unit will be described below in relation to various embodiments. 
     According to an embodiment, the image decoding device  100  may split each of the third coding units  520   a , or  520   b ,  520   c , and  520   d  into coding units, based on the split shape mode information. Also, the image decoding device  100  may determine not to split the second coding unit  510  based on the split shape mode information. According to an embodiment, the image decoding device  100  may split the non-square second coding unit  510  into the odd number of third coding units  520   b ,  520   c , and  520   d . The image decoding device  100  may put a preset restriction on a preset third coding unit from among the odd number of third coding units  520   b ,  520   c , and  520   d . For example, the image decoding device  100  may restrict the third coding unit  520   c  at a center location from among the odd number of third coding units  520   b ,  520   c , and  520   d  to be no longer split or to be split a settable number of times. 
     Referring to  FIG. 5 , the image decoding device  100  may restrict the third coding unit  520   c , which is at the center location from among the odd number of third coding units  520   b ,  520   c , and  520   d  included in the non-square second coding unit  510 , to be no longer split, to be split by using a preset splitting method (e.g., split into only four coding units or split by using a splitting method of the second coding unit  510 ), or to be split only a preset number of times (e.g., split only n times (where n&gt;0)). However, the restrictions on the third coding unit  520   c  at the center location are not limited to the above-described examples, and may include various restrictions for decoding the third coding unit  520   c  at the center location differently from the other third coding units  520   b  and  520   d.    
     According to an embodiment, the image decoding device  100  may obtain the split shape mode information, which is used to split a current coding unit, from a preset location in the current coding unit. 
       FIG. 6  illustrates a method, performed by an image decoding device, of determining a preset coding unit from among an odd number of coding units, according to an embodiment. 
     Referring to  FIG. 6 , split shape mode information on a current coding unit  600  or  650  may be obtained from a sample of a preset location (e.g., a sample  640  or  690  of a center location) from among a plurality of samples included in the current coding unit  600  or  650 . However, the preset location in the current coding unit  600 , from which at least one piece of the split shape mode information may be obtained, is not limited to the center location in  FIG. 6 , and may include various locations included in the current coding unit  600  (e.g., top, bottom, left,-right, upper-left, lower-left, upper-right, lower-right locations, or the like). The image decoding device  100  may obtain the split shape mode information from the preset location and may determine to split or not to split the current coding unit into various-shaped and various-sized coding units. 
     According to an embodiment, when the current coding unit is split into a preset number of coding units, the image decoding device  100  may select one of the coding units. Various methods may be used to select one of a plurality of coding units, as will be described below in relation to various embodiments. 
     According to an embodiment, the image decoding device  100  may split the current coding unit into a plurality of coding units, and may determine a coding unit at a preset location. 
     According to an embodiment, image decoding device  100  may use information indicating locations of the odd number of coding units, to determine a coding unit at a center location from among the odd number of coding units. Referring to  FIG. 6 , the image decoding device  100  may determine the odd number of coding units  620   a ,  620   b , and  620   c  or the odd number of coding units  660   a ,  660   b , and  660   c  by splitting the current coding unit  600  or the current coding unit  650 . The image decoding device  100  may determine the middle coding unit  620   b  or the middle coding unit  660   b  by using information about the locations of the odd number of coding units  620   a ,  620   b , and  620   c  or the odd number of coding units  660   a ,  660   b , and  660   c . For example, the image decoding device  100  may determine the coding unit  620   b  of the center location by determining the locations of the coding units  620   a ,  620   b , and  620   c  based on information indicating locations of preset samples included in the coding units  620   a ,  620   b , and  620   c . In detail, the image decoding device  100  may determine the coding unit  620   b  at the center location by determining the locations of the coding units  620   a ,  620   b , and  620   c  based on information indicating locations of upper-left samples  630   a ,  630   b , and  630   c  of the coding units  620   a ,  620   b , and  620   c.    
     According to an embodiment, the information indicating the locations of the upper-left samples  630   a ,  630   b , and  630   c , which are included in the coding units  620   a ,  620   b , and  620   c , respectively, may include information about locations or coordinates of the coding units  620   a ,  620   b , and  620   c  in a picture. According to an embodiment, the information indicating the locations of the upper-left samples  630   a ,  630   b , and  630   c , which are included in the coding units  620   a ,  620   b , and  620   c , respectively, may include information indicating widths or heights of the coding units  620   a ,  620   b , and  620   c  included in the current coding unit  600 , and the widths or heights may correspond to information indicating differences between the coordinates of the coding units  620   a ,  620   b , and  620   c  in the picture. That is, the image decoding device  100  may determine the coding unit  620   b  at the center location by directly using the information about the locations or coordinates of the coding units  620   a ,  620   b , and  620   c  in the picture, or by using the information about the widths or heights of the coding units, which correspond to the difference values between the coordinates. 
     According to an embodiment, information indicating the location of the upper-left sample  630   a  of the upper coding unit  620   a  may include coordinates (xa, ya), information indicating the location of the upper-left sample  630   b  of the middle coding unit  620   b  may include coordinates (xb, yb), and information indicating the location of the upper-left sample  630   c  of the lower coding unit  620   c  may include coordinates (xc, yc). The image decoding device  100  may determine the middle coding unit  620   b  by using the coordinates of the upper-left samples  630   a ,  630   b , and  630   c  which are included in the coding units  620   a ,  620   b , and  620   c , respectively. For example, when the coordinates of the upper-left samples  630   a ,  630   b , and  630   c  are sorted in an ascending or descending order, the coding unit  620   b  including the coordinates (xb, yb) of the sample  630   b  at a center location may be determined as a coding unit at a center location from among the coding units  620   a ,  620   b , and  620   c  determined by splitting the current coding unit  600 . However, the coordinates indicating the locations of the upper-left samples  630   a ,  630   b , and  630   c  may include coordinates indicating absolute locations in the picture, or may use coordinates (dxb, dyb) indicating a relative location of the upper-left sample  630   b  of the middle coding unit  620   b  and coordinates (dxc, dyc) indicating a relative location of the upper-left sample  630   c  of the lower coding unit  620   c  with reference to the location of the upper-left sample  630   a  of the upper coding unit  620   a . A method of determining a coding unit at a preset location by using coordinates of a sample included in the coding unit, as information indicating a location of the sample, is not limited to the above-described method, and may include various arithmetic methods capable of using the coordinates of the sample. 
     According to an embodiment, the image decoding device  100  may split the current coding unit  600  into a plurality of coding units  620   a ,  620   b , and  620   c , and may select one of the coding units  620   a ,  620   b , and  620   c  based on a preset criterion. For example, the image decoding device  100  may select the coding unit  620   b , which has a size different from that of the others, from among the coding units  620   a ,  620   b , and  620   c.    
     According to an embodiment, the image decoding device  100  may determine the width or height of each of the coding units  620   a ,  620   b , and  620   c  by using the coordinates (xa, ya) that is the information indicating the location of the upper-left sample  630   a  of the upper coding unit  620   a , the coordinates (xb, yb) that is the information indicating the location of the upper-left sample  630   b  of the middle coding unit  620   b , and the coordinates (xc, yc) that is the information indicating the location of the upper-left sample  630   c  of the lower coding unit  620   c . The image decoding device  100  may determine the respective sizes of the coding units  620   a ,  620   b , and  620   c  by using the coordinates (xa, ya), (xb, yb), and (xc, yc) indicating the locations of the coding units  620   a ,  620   b , and  620   c . According to an embodiment, the image decoding device  100  may determine the width of the upper coding unit  620   a  to be the width of the current coding unit  600 . The image decoding device  100  may determine the height of the upper coding unit  620   a  to be yb-ya. According to an embodiment, the image decoding device  100  may determine the width of the middle coding unit  620   b  to be the width of the current coding unit  600 . The image decoding device  100  may determine the height of the middle coding unit  620   b  to be ya-yb. According to an embodiment, the image decoding device  100  may determine the width or height of the lower coding unit  620   c  by using the width or height of the current coding unit  600  or the widths or heights of the upper and middle coding units  620   a  and  620   b . The image decoding device  100  may determine a coding unit, which has a size different from that of the others, based on the determined widths and heights of the coding units  620   a  to  620   c . Referring to  FIG. 6 , the image decoding device  100  may determine the middle coding unit  620   b , which has a size different from the size of the upper and lower coding units  620   a  and  620   c , as the coding unit of the preset location. However, the above-described method, performed by the image decoding device  100 , of determining a coding unit having a size different from the size of the other coding units merely corresponds to an example of determining a coding unit at a preset location by using the sizes of coding units, which are determined based on coordinates of samples, and thus various methods of determining a coding unit at a preset location by comparing the sizes of coding units, which are determined based on coordinates of preset samples, may be used. 
     The image decoding device  100  may determine the width or height of each of the coding units  660   a ,  660   b , and  660   c  by using the coordinates (xd, yd) that is information indicating the location of an upper-left sample  670   a  of the left coding unit  660   a , the coordinates (xe, ye) that is information indicating the location of an upper-left sample  670   b  of the middle coding unit  660   b , and the coordinates (xf, yf) that is information indicating a location of the upper-left sample  670   c  of the right coding unit  660   c . The image decoding device  100  may determine the respective sizes of the coding units  660   a ,  660   b , and  660   c  by using the coordinates (xd, yd), (xe, ye), and (xf, yf) indicating the locations of the coding units  660   a ,  660   b , and  660   c.    
     According to an embodiment, the image decoding device  100  may determine the width of the left coding unit  660   a  to be xe-xd. The image decoding device  100  may determine the height of the left coding unit  660   a  to be the height of the current coding unit  650 . According to an embodiment, the image decoding device  100  may determine the width of the middle coding unit  660   b  to be xf-xe. The image decoding device  100  may determine the height of the middle coding unit  660   b  to be the height of the current coding unit  650 . According to an embodiment, the image decoding device  100  may determine the width or height of the right coding unit  660   c  by using the width or height of the current coding unit  650  or the widths or heights of the left and middle coding units  660   a  and  660   b . The image decoding device  100  may determine a coding unit, which has a size different from that of the others, based on the determined widths and heights of the coding units  660   a  to  660   c . Referring to  FIG. 6 , the image decoding device  100  may determine the middle coding unit  660   b , which has a size different from the sizes of the left and right coding units  660   a  and  660   c , as the coding unit of the preset location. However, the above-described method, performed by the image decoding device  100 , of determining a coding unit having a size different from the size of the other coding units merely corresponds to an example of determining a coding unit at a preset location by using the sizes of coding units, which are determined based on coordinates of samples, and thus various methods of determining a coding unit at a preset location by comparing the sizes of coding units, which are determined based on coordinates of preset samples, may be used. 
     However, locations of samples considered to determine locations of coding units are not limited to the above-described upper-left locations, and information about arbitrary locations of samples included in the coding units may be used. 
     According to an embodiment, the image decoding device  100  may select a coding unit at a preset location from among an odd number of coding units determined by splitting the current coding unit, considering the shape of the current coding unit. For example, when the current coding unit has a non-square shape, a width of which is longer than a height, the image decoding device  100  may determine the coding unit at the preset location in a horizontal direction. That is, the image decoding device  100  may determine one of coding units at different locations in a horizontal direction and put a restriction on the coding unit. When the current coding unit has a non-square shape, a height of which is longer than a width, the image decoding device  100  may determine the coding unit at the preset location in a vertical direction. That is, the image decoding device  100  may determine one of coding units at different locations in a vertical direction and may put a restriction on the coding unit. 
     According to an embodiment, the image decoding device  100  may use information indicating respective locations of an even number of coding units, to determine the coding unit at the preset location from among the even number of coding units. The image decoding device  100  may determine an even number of coding units by splitting (binary splitting) the current coding unit, and may determine the coding unit at the preset location by using the information about the locations of the even number of coding units. An operation related thereto may correspond to the operation of determining a coding unit at a preset location (e.g., a center location) from among an odd number of coding units, which has been described in detail above in relation to  FIG. 6 , and thus detailed descriptions thereof are not provided here. 
     According to an embodiment, when a non-square current coding unit is split into a plurality of coding units, preset information about a coding unit at a preset location may be used in a splitting operation to determine the coding unit at the preset location from among the plurality of coding units. For example, the image decoding device  100  may use at least one of block shape information and split shape mode information, which is stored in a sample included in a middle coding unit, in a splitting operation to determine a coding unit at a center location from among the plurality of coding units determined by splitting the current coding unit. 
     Referring to  FIG. 6 , the image decoding device  100  may split the current coding unit  600  into the plurality of coding units  620   a ,  620   b , and  620   c  based on the split shape mode information, and may determine the coding unit  620   b  at a center location from among the plurality of the coding units  620   a ,  620   b , and  620   c . Furthermore, the image decoding device  100  may determine the coding unit  620   b  at the center location, in consideration of a location from which the split shape mode information is obtained. That is, the split shape mode information on the current coding unit  600  may be obtained from the sample  640  at a center location of the current coding unit  600  and, when the current coding unit  600  is split into the plurality of coding units  620   a ,  620   b , and  620   c  based on the split shape mode information, the coding unit  620   b  including the sample  640  may be determined as the coding unit at the center location. However, information used to determine the coding unit at the center location is not limited to the split shape mode information, and various types of information may be used to determine the coding unit at the center location. 
     According to an embodiment, preset information for identifying the coding unit at the preset location may be obtained from a preset sample included in a coding unit to be determined. Referring to  FIG. 6 , the image decoding device  100  may use the split shape mode information, which is obtained from a sample at a preset location in the current coding unit  600  (e.g., a sample at a center location of the current coding unit  600 ) to determine a coding unit at a preset location from among the plurality of the coding units  620   a ,  620   b , and  620   c  determined by splitting the current coding unit  600  (e.g., a coding unit at a center location from among a plurality of split coding units). That is, the image decoding device  100  may determine the sample at the preset location by considering a block shape of the current coding unit  600 , determine the coding unit  620   b  including a sample, from which preset information (e.g., the split shape mode information) may be obtained, from among the plurality of coding units  620   a ,  620   b , and  620   c  determined by splitting the current coding unit  600 , and may put a preset restriction on the coding unit  620   b . Referring to  FIG. 6 , according to an embodiment, the image decoding device  100  may determine the sample  640  at the center location of the current coding unit  600  as the sample from which the preset information may be obtained, and may put a preset restriction on the coding unit  620   b  including the sample  640 , in a decoding operation. However, the location of the sample from which the preset information may be obtained is not limited to the above-described location, and may include arbitrary locations of samples included in the coding unit  620   b  to be determined for a restriction. 
     According to an embodiment, the location of the sample from which the preset information may be obtained may be determined based on the shape of the current coding unit  600 . According to an embodiment, the block shape information may indicate whether the current coding unit has a square or non-square shape, and the location of the sample from which the preset information may be obtained may be determined based on the shape. For example, the image decoding device  100  may determine a sample located on a boundary for splitting at least one of a width and height of the current coding unit in half, as the sample from which the preset information may be obtained, by using at least one of information about the width of the current coding unit and information about the height of the current coding unit. As another example, when the block shape information on the current coding unit indicates a non-square shape, the image decoding device  100  may determine one of samples adjacent to a boundary for splitting a long side of the current coding unit in half, as the sample from which the preset information may be obtained. 
     According to an embodiment, when the current coding unit is split into a plurality of coding units, the image decoding device  100  may use the split shape mode information to determine a coding unit at a preset location from among the plurality of coding units. According to an embodiment, the image decoding device  100  may obtain the split shape mode information from a sample at a preset location in a coding unit, and split the plurality of coding units, which are generated by splitting the current coding unit, by using the split shape mode information, which is obtained from the sample of the preset location in each of the plurality of coding units. That is, a coding unit may be recursively split based on the split shape mode information, which is obtained from the sample at the preset location in each coding unit. An operation of recursively splitting a coding unit has been described above in relation to  FIG. 5 , and thus detailed descriptions thereof will not be provided here. 
     According to an embodiment, the image decoding device  100  may determine one or more coding units by splitting the current coding unit, and may determine an order of decoding the one or more coding units, based on a preset block (e.g., the current coding unit). 
       FIG. 7  illustrates an order of processing a plurality of coding units when an image decoding device determines the plurality of coding units by splitting a current coding unit, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine second coding units  710   a  and  710   b  by splitting a first coding unit  700  in a vertical direction, determine second coding units  730   a  and  730   b  by splitting the first coding unit  700  in a horizontal direction, or determine second coding units  750   a ,  750   b ,  750   c , and  750   d  by splitting the first coding unit  700  in vertical and horizontal directions, based on split shape mode information. 
     Referring to  FIG. 7 , the image decoding device  100  may determine to process the second coding units  710   a  and  710   b , which are determined by splitting the first coding unit  700  in a vertical direction, in a horizontal direction order  710   c . The image decoding device  100  may determine to process the second coding units  730   a  and  730   b , which are determined by splitting the first coding unit  700  in a horizontal direction, in a vertical direction order  730   c . The image decoding device  100  may determine the second coding units  750   a ,  750   b ,  750   c , and  750   d , which are determined by splitting the first coding unit  700  in vertical and horizontal directions, according to a preset order (e.g., a raster scan order or Z-scan order  750   e ) by which coding units in a row are processed and then coding units in a next row are processed. 
     According to an embodiment, the image decoding device  100  may recursively split coding units. Referring to  FIG. 7 , the image decoding device  100  may determine the plurality of coding units  710   a  and  710   b ,  730   a  and  730   b , or  750   a ,  750   b ,  750   c , and  750   d  by splitting the first coding unit  700 , and recursively split each of the determined plurality of coding units  710   a ,  710   b ,  730   a ,  730   b ,  750   a ,  750   b ,  750   c , and  750   d . A splitting method of the plurality of coding units  710   a  and  710   b ,  730   a  and  730   b , or  750   a ,  750   b ,  750   c , and  750   d  may correspond to a splitting method of the first coding unit  700 . As such, each of the plurality of coding units  710   a  and  710   b ,  730   a  and  730   b , or  750   a ,  750   b ,  750   c , and  750   d  may be independently split into a plurality of coding units. Referring to  FIG. 7 , the image decoding device  100  may determine the second coding units  710   a  and  710   b  by splitting the first coding unit  700  in a vertical direction, and may determine to independently split or not to split each of the second coding units  710   a  and  710   b.    
     According to an embodiment, the image decoding device  100  may determine third coding units  720   a  and  720   b  by splitting the left second coding unit  710   a  in a horizontal direction, and may not split the right second coding unit  710   b.    
     According to an embodiment, a processing order of coding units may be determined based on an operation of splitting a coding unit. In other words, a processing order of split coding units may be determined based on a processing order of coding units immediately before being split. The image decoding device  100  may determine a processing order of the third coding units  720   a  and  720   b  determined by splitting the left second coding unit  710   a , independently of the right second coding unit  710   b . Because the third coding units  720   a  and  720   b  are determined by splitting the left second coding unit  710   a  in a horizontal direction, the third coding units  720   a  and  720   b  may be processed in a vertical direction order  720   c . Because the left and right second coding units  710   a  and  710   b  are processed in the horizontal direction order  710   c , the right second coding unit  710   b  may be processed after the third coding units  720   a  and  720   b  included in the left second coding unit  710   a  are processed in the vertical direction order  720   c . An operation of determining a processing order of coding units based on a coding unit before being split is not limited to the above-described example, and various methods may be used to independently process coding units, which are split and determined to various shapes, in a preset order. 
       FIG. 8  illustrates a process, performed by an image decoding device, of determining that a current coding unit is to be split into an odd number of coding units, when the coding units are not processable in a preset order, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine that the current coding unit is split into an odd number of coding units, based on obtained split shape mode information. Referring to  FIG. 8 , a square first coding unit  800  may be split into non-square second coding units  810   a  and  810   b , and the second coding units  810   a  and  810   b  may be independently split into third coding units  820   a  and  820   b , and  820   c  to  820   e . According to an embodiment, the image decoding device  100  may determine the plurality of third coding units  820   a  and  820   b  by splitting the left second coding unit  810   a  in a horizontal direction, and may split the right second coding unit  810   b  into the odd number of third coding units  820   c  to  820   e.    
     According to an embodiment, the video decoding device  100  may determine whether any coding unit is split into an odd number of coding units, by determining whether the third coding units  820   a  and  820   b , and  820   c  to  820   e  are processable in a preset order. Referring to  FIG. 8 , the image decoding device  100  may determine the third coding units  820   a  and  820   b , and  820   c  to  820   e  by recursively splitting the first coding unit  800 . The image decoding device  100  may determine whether any of the first coding unit  800 , the second coding units  810   a  and  810   b , or the third coding units  820   a  and  820   b , and  820   c  to  820   e  are split into an odd number of coding units, based on at least one of the block shape information and the split shape mode information. For example, a coding unit located in the right from among the second coding units  810   a  and  810   b  may be split into an odd number of third coding units  820   c ,  820   d , and  820   e . A processing order of a plurality of coding units included in the first coding unit  800  may be a preset order (e.g., a Z-scan order  830 ), and the image decoding device  100  may determine whether the third coding units  820   c ,  820   d , and  820   e , which are determined by splitting the right second coding unit  810   b  into an odd number of coding units, satisfy a condition for processing in the preset order. 
     According to an embodiment, the image decoding device  100  may determine whether the third coding units  820   a  and  820   b , and  820   c  to  820   e  included in the first coding unit  800  satisfy the condition for processing in the preset order, and the condition relates to whether at least one of a width and height of the second coding units  810   a  and  810   b  is to be split in half along a boundary of the third coding units  820   a  and  820   b , and  820   c  to  820   e . For example, the third coding units  820   a  and  820   b  determined when the height of the left second coding unit  810   a  of the non-square shape is split in half may satisfy the condition. It may be determined that the third coding units  820   c  to  820   e  do not satisfy the condition because the boundaries of the third coding units  820   c  to  820   e  determined when the right second coding unit  810   b  is split into three coding units are unable to split the width or height of the right second coding unit  810   b  in half. When the condition is not satisfied as described above, the image decoding device  100  may determine disconnection of a scan order, and may determine that the right second coding unit  810   b  is to be split into an odd number of coding units, based on a result of the determination. According to an embodiment, when a coding unit is split into an odd number of coding units, the image decoding device  100  may put a preset restriction on a coding unit at a preset location from among the split coding units. The restriction or the preset location has been described above in relation to various embodiments, and thus detailed descriptions thereof will not be provided herein. 
       FIG. 9  illustrates a process, performed by an image decoding device, of determining at least one coding unit by splitting a first coding unit, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may split the first coding unit  900 , based on split shape mode information, which is obtained through the receiver  110 . The square first coding unit  900  may be split into four square coding units, or may be split into a plurality of non-square coding units. For example, referring to  FIG. 9 , when the first coding unit  900  has a square shape and the split shape mode information indicates to split the first coding unit  900  into non-square coding units, the image decoding device  100  may split the first coding unit  900  into a plurality of non-square coding units. In detail, when the split shape mode information indicates to determine an odd number of coding units by splitting the first coding unit  900  in a horizontal direction or a vertical direction, the image decoding device  100  may split the square first coding unit  900  into an odd number of coding units, e.g., second coding units  910   a ,  910   b , and  910   c  determined by splitting the square first coding unit  900  in a vertical direction or second coding units  920   a ,  920   b , and  920   c  determined by splitting the square first coding unit  900  in a horizontal direction. 
     According to an embodiment, the image decoding device  100  may determine whether the second coding units  910   a ,  910   b ,  910   c ,  920   a ,  920   b , and  920   c  included in the first coding unit  900  satisfy a condition for processing in a preset order, and the condition relates to whether at least one of a width and height of the first coding unit  900  is to be split in half along a boundary of the second coding units  910   a ,  910   b ,  910   c ,  920   a ,  920   b , and  920   c . Referring to  FIG. 9 , because boundaries of the second coding units  910   a ,  910   b , and  910   c  determined by splitting the square first coding unit  900  in a vertical direction do not split the height of the first coding unit  900  in half, it may be determined that the first coding unit  900  does not satisfy the condition for processing in the preset order. Also, because boundaries of the second coding units  920   a ,  920   b , and  920   c  determined by splitting the square first coding unit  900  in a horizontal direction do not split the width of the first coding unit  900  in half, it may be determined that the first coding unit  900  does not satisfy the condition for processing in the preset order. When the condition is not satisfied as described above, the image decoding device  100  may decide disconnection of a scan order, and may determine that the first coding unit  900  is to be split into an odd number of coding units, based on a result of the decision. According to an embodiment, when a coding unit is split into an odd number of coding units, the image decoding device  100  may put a preset restriction on a coding unit at a preset location from among the split coding units. The restriction or the preset location has been described above in relation to various embodiments, and thus detailed descriptions thereof will not be provided herein. 
     According to an embodiment, the image decoding device  100  may determine various-shaped coding units by splitting a first coding unit. 
     Referring to  FIG. 9 , the image decoding device  100  may split the square first coding unit  900  or a non-square first coding unit  930  or  950  into various-shaped coding units. 
       FIG. 10  illustrates that a shape into which a second coding unit is splittable is restricted when the second coding unit having a non-square shape, which is determined when an image decoding device splits a first coding unit, satisfies a preset condition, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine to split the square first coding unit  1000  into non-square second coding units  1010   a , and  1010   b  or  1020   a  and  1020   b , based on split shape mode information, which is obtained by the receiver  110 . The second coding units  1010   a  and  1010   b , or  1020   a  and  1020   b  may be independently split. As such, the image decoding device  100  may determine to split or not to split each of the second coding units  1010   a  and  1010   b , or  1020   a  and  1020   b  into a plurality of coding units, based on the split shape mode information on each of the second coding units  1010   a  and  1010   b , or  1020   a  and  1020   b . According to an embodiment, the image decoding device  100  may determine third coding units  1012   a  and  1012   b  by splitting the non-square left second coding unit  1010   a , which is determined by splitting the first coding unit  1000  in a vertical direction, in a horizontal direction. However, when the left second coding unit  1010   a  is split in a horizontal direction, the image decoding device  100  may restrict the right second coding unit  1010   b  to not be split in a horizontal direction in which the left second coding unit  1010   a  is split. When third coding units  1014   a  and  1014   b  are determined by splitting the right second coding unit  1010   b  in the same direction, because the left and right second coding units  1010   a  and  1010   b  are independently split in a horizontal direction, the third coding units  1012   a  and  1012   b , or  1014   a  and  1014   b  may be determined. However, this case serves equally as a case in which the image decoding device  100  splits the first coding unit  1000  into four square second coding units  1030   a ,  1030   b ,  1030   c , and  1030   d , based on the split shape mode information, and may be inefficient in terms of image decoding. 
     According to an embodiment, the image decoding device  100  may determine third coding units  1022   a  and  1022   b , or  1024   a  and  1024   b  by splitting the non-square second coding unit  1020   a  or  1020   b , which is determined by splitting the first coding unit  1000  in a horizontal direction, in a vertical direction. However, when a second coding unit (e.g., the upper second coding unit  1020   a ) is split in a vertical direction, for the above-described reason, the image decoding device  100  may restrict the other second coding unit (e.g., the lower second coding unit  1020   b ) to not be split in a vertical direction in which the upper second coding unit  1020   a  is split. 
       FIG. 11  illustrates a process, performed by an image decoding device, of splitting a square coding unit when split shape mode information indicates that the square coding unit is not to be split into four square coding units, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine second coding units  1110   a  and  1110   b , or  1120   a  and  1120   b , etc. by splitting a first coding unit  1100 , based on split shape mode information. The split shape mode information may include information about various methods of splitting a coding unit but, the information about various splitting methods may not include information for splitting a coding unit into four square coding units. According to such split shape mode information, the image decoding device  100  may not split the square first coding unit  1100  into four square second coding units  1130   a ,  1130   b ,  1130   c , and  1130   d . The image decoding device  100  may determine the non-square second coding units  1110   a  and  1110   b , or  1120   a  and  1120   b , etc., based on the split shape mode information. 
     According to an embodiment, the image decoding device  100  may independently split the non-square second coding units  1110   a  and  1110   b , or  1120   a  and  1120   b , etc. Each of the second coding units  1110   a  and  1110   b , or  1120   a  and  1120   b , etc. may be recursively split in a preset order, and this splitting method may correspond to a method of splitting the first coding unit  1100 , based on the split shape mode information. 
     For example, the image decoding device  100  may determine square third coding units  1112   a  and  1112   b  by splitting the left second coding unit  1110   a  in a horizontal direction, and may determine square third coding units  1114   a  and  1114   b  by splitting the right second coding unit  1110   b  in a horizontal direction. Furthermore, the image decoding device  100  may determine square third coding units  1116   a ,  1116   b ,  1116   c , and  1116   d  by splitting both of the left and right second coding units  1110   a  and  1110   b  in a horizontal direction. In this case, coding units having the same shape as the four square second coding units  1130   a ,  1130   b ,  1130   c , and  1130   d  split from the first coding unit  1100  may be determined. 
     As another example, the image decoding device  100  may determine square third coding units  1122   a  and  1122   b  by splitting the upper second coding unit  1120   a  in a vertical direction, and may determine square third coding units  1124   a  and  1124   b  by splitting the lower second coding unit  1120   b  in a vertical direction. Furthermore, the image decoding device  100  may determine square third coding units  1126   a ,  1126   b ,  1126   c , and  1126   d  by splitting both of the upper and lower second coding units  1120   a  and  1120   b  in a vertical direction. In this case, coding units having the same shape as the four square second coding units  1130   a ,  1130   b ,  1130   c , and  1130   d  split from the first coding unit  1100  may be determined. 
       FIG. 12  illustrates that a processing order between a plurality of coding units may be changed depending on a process of splitting a coding unit, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may split a first coding unit  1200 , based on split shape mode information. When a block shape indicates a square shape and the split shape mode information indicates to split the first coding unit  1200  in at least one of horizontal and vertical directions, the image decoding device  100  may determine second coding units  1210   a  and  1210   b , or  1220   a  and  1220   b , etc. by splitting the first coding unit  1200 . Referring to  FIG. 12 , the non-square second coding units  1210   a  and  1210   b , or  1220   a  and  1220   b  determined by splitting the first coding unit  1200  in only a horizontal direction or vertical direction may be independently split based on the split shape mode information on each coding unit. For example, the image decoding device  100  may determine third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d  by splitting the second coding units  1210   a  and  1210   b , which are generated by splitting the first coding unit  1200  in a vertical direction, in a horizontal direction, and may determine third coding units  1226   a ,  1226   b ,  1226   c , and  1226   d  by splitting the second coding units  1220   a  and  1220   b , which are generated by splitting the first coding unit  1200  in a horizontal direction, in a vertical direction. An operation of splitting the second coding units  1210   a  and  1210   b , or  1220   a  and  1220   b  has been described above in relation to  FIG. 11 , and thus detailed descriptions thereof will not be provided herein. 
     According to an embodiment, the image decoding device  100  may process coding units in a preset order. An operation of processing coding units in a preset order has been described above in relation to  FIG. 7 , and thus detailed descriptions thereof will not be provided herein. Referring to  FIG. 12 , the image decoding device  100  may determine foursquare third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d , and  1226   a ,  1226   b ,  1226   c , and  1226   d  by splitting the square first coding unit  1200 . According to an embodiment, the image decoding device  100  may determine processing orders of the third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d , and  1226   a ,  1226   b ,  1226   c , and  1226   d  based on a split shape by which the first coding unit  1200  is split. 
     According to an embodiment, the image decoding device  100  may determine the third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d  by splitting the second coding units  1210   a  and  1210   b  generated by splitting the first coding unit  1200  in a vertical direction, in a horizontal direction, and may process the third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d  in a processing order  1217  for initially processing the third coding units  1216   a  and  1216   c , which are included in the left second coding unit  1210   a , in a vertical direction and then processing the third coding unit  1216   b  and  1216   d , which are included in the right second coding unit  1210   b , in a vertical direction. 
     According to an embodiment, the image decoding device  100  may determine the third coding units  1226   a ,  1226   b ,  1226   c , and  1226   d  by splitting the second coding units  1220   a  and  1220   b  generated by splitting the first coding unit  1200  in a horizontal direction, in a vertical direction, and may process the third coding units  1226   a ,  1226   b ,  1226   c , and  1226   d  in a processing order  1227  for initially processing the third coding units  1226   a  and  1226   b , which are included in the upper second coding unit  1220   a , in a horizontal direction and then processing the third coding unit  1226   c  and  1226   d , which are included in the lower second coding unit  1220   b , in a horizontal direction. 
     Referring to  FIG. 12 , the square third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d , and  1226   a ,  1226   b ,  1226   c , and  1226   d  may be determined by splitting the second coding units  1210   a  and  1210   b , and  1220   a  and  1220   b , respectively. Although the second coding units  1210   a  and  1210   b  are determined by splitting the first coding unit  1200  in a vertical direction differently from the second coding units  1220   a  and  1220   b  which are determined by splitting the first coding unit  1200  in a horizontal direction, the third coding units  1216   a ,  1216   b ,  1216   c , and  1216   d , and  1226   a ,  1226   b ,  1226   c , and  1226   d  split therefrom eventually show same-shaped coding units split from the first coding unit  1200 . As such, by recursively splitting a coding unit in different manners based on the split shape mode information, the image decoding device  100  may process a plurality of coding units in different orders even when the coding units are eventually determined to be the same shape. 
       FIG. 13  illustrates a process of determining a depth of a coding unit as a shape and size of the coding unit change, when the coding unit is recursively split such that a plurality of coding units are determined, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine the depth of the coding unit, based on a preset criterion. For example, the preset criterion may be the length of a long side of the coding unit. When the length of a long side of a coding unit before being split is 2n times (n&gt;0) the length of a long side of a split current coding unit, the image decoding device  100  may determine that a depth of the current coding unit is increased from a depth of the coding unit before being split, by n. In the following description, a coding unit having an increased depth is expressed as a coding unit of a deeper depth. 
     Referring to  FIG. 13 , according to an embodiment, the image decoding device  100  may determine a second coding unit  1302  and a third coding unit  1304  of deeper depths by splitting a square first coding unit  1300  based on block shape information indicating a square shape (e.g., the block shape information may be expressed as ‘0: SQUARE’). Assuming that the size of the square first coding unit  1300  is 2N×2N, the second coding unit  1302  determined by splitting a width and height of the first coding unit  1300  in ½ may have a size of N×N. Furthermore, the third coding unit  1304  determined by splitting a width and height of the second coding unit  1302  in ½ may have a size of N/2×N/2. In this case, a width and height of the third coding unit  1304  are ¼ times those of the first coding unit  1300 . When a depth of the first coding unit  1300  is D, a depth of the second coding unit  1302 , the width and height of which are ½ times those of the first coding unit  1300 , may be D+1, and a depth of the third coding unit  1304 , the width and height of which are ¼ times those of the first coding unit  1300 , may be D+2. 
     According to an embodiment, the image decoding device  100  may determine a second coding unit  1312  or  1322  and a third coding unit  1314  or  1324  of deeper depths by splitting a non-square first coding unit  1310  or  1320  based on block shape information indicating a non-square shape (e.g., the block shape information may be expressed as ‘1: NS_VER’ indicating a non-square shape, a height of which is longer than a width, or as ‘2: NS_HOR’ indicating a non-square shape, a width of which is longer than a height). 
     The image decoding device  100  may determine a second coding unit  1302 ,  1312 , or  1322  by splitting at least one of a width and height of the first coding unit  1310  having a size of N×2N. That is, the image decoding device  100  may determine the second coding unit  1302  having a size of N×N or the second coding unit  1322  having a size of N×N/2 by splitting the first coding unit  1310  in a horizontal direction, or may determine the second coding unit  1312  having a size of N/2×N by splitting the first coding unit  1310  in horizontal and vertical directions. 
     According to an embodiment, the image decoding device  100  may determine the second coding unit  1302 ,  1312 , or  1322  by splitting at least one of a width and height of the first coding unit  1320  having a size of 2N×N. That is, the image decoding device  100  may determine the second coding unit  1302  having a size of N×N or the second coding unit  1312  having a size of N/2×N by splitting the first coding unit  1320  in a vertical direction, or may determine the second coding unit  1322  having a size of N×N/2 by splitting the first coding unit  1320  in horizontal and vertical directions. 
     According to an embodiment, the image decoding device  100  may determine a third coding unit  1304 ,  1314 , or  1324  by splitting at least one of a width and height of the second coding unit  1302  having a size of N×N. That is, the image decoding device  100  may determine the third coding unit  1304  having a size of N/2×N/2, the third coding unit  1314  having a size of N/4×N/2, or the third coding unit  1324  having a size of N/2×N/4 by splitting the second coding unit  1302  in vertical and horizontal directions. 
     According to an embodiment, the image decoding device  100  may determine the third coding unit  1304 ,  1314 , or  1324  by splitting at least one of a width and height of the second coding unit  1312  having a size of N/2×N. That is, the image decoding device  100  may determine the third coding unit  1304  having a size of N/2×N/2 or the third coding unit  1324  having a size of N/2×N/4 by splitting the second coding unit  1312  in a horizontal direction, or may determine the third coding unit  1314  having a size of N/4×N/2 by splitting the second coding unit  1312  in vertical and horizontal directions. 
     According to an embodiment, the image decoding device  100  may determine the third coding unit  1304 ,  1314 , or  1324  by splitting at least one of a width and height of the second coding unit  1322  having a size of N×N/2. That is, the image decoding device  100  may determine the third coding unit  1304  having a size of N/2×N/2 or the third coding unit  1314  having a size of N/4×N/2 by splitting the second coding unit  1322  in a vertical direction, or may determine the third coding unit  1324  having a size of N/2×N/4 by splitting the second coding unit  1322  in vertical and horizontal directions. 
     According to an embodiment, the image decoding device  100  may split the square coding unit  1300 ,  1302 , or  1304  in a horizontal or vertical direction. For example, the image decoding device  100  may determine the first coding unit  1310  having a size of N×2N by splitting the first coding unit  1300  having a size of 2N×2N in a vertical direction, or may determine the first coding unit  1320  having a size of 2N×N by splitting the first coding unit  1300  in a horizontal direction. According to an embodiment, when a depth is determined based on the length of the longest side of a coding unit, a depth of a coding unit determined by splitting the first coding unit  1300  having a size of 2N×2N in a horizontal or vertical direction may be the same as the depth of the first coding unit  1300 . 
     According to an embodiment, a width and height of the third coding unit  1314  or  1324  may be ¼ times those of the first coding unit  1310  or  1320 . When a depth of the first coding unit  1310  or  1320  is D, a depth of the second coding unit  1312  or  1322 , the width and height of which are ½ times those of the first coding unit  1310  or  1320 , may be D+1, and a depth of the third coding unit  1314  or  1324 , the width and height of which are ¼ times those of the first coding unit  1310  or  1320 , may be D+2. 
       FIG. 14  illustrates depths that are determinable based on shapes and sizes of coding units, and part indexes (PIDs) that are for distinguishing the coding units, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine various-shape second coding units by splitting a square first coding unit  1400 . Referring to  FIG. 14 , the image decoding device  100  may determine second coding units  1402   a  and  1402   b ,  1404   a  and  1404   b , and  1406   a ,  1406   b ,  1406   c , and  1406   d  by splitting the first coding unit  1400  in at least one of vertical and horizontal directions based on split shape mode information. That is, the image decoding device  100  may determine the second coding units  1402   a  and  1402   b ,  1404   a  and  1404   b , and  1406   a ,  1406   b ,  1406   c , and  1406   d , based on the split shape mode information on the first coding unit  1400 . 
     According to an embodiment, depths of the second coding units  1402   a  and  1402   b ,  1404   a  and  1404   b , and  1406   a ,  1406   b ,  1406   c , and  1406   d  that are determined based on the split shape mode information on the square first coding unit  1400  may be determined based on the length of a long side thereof. For example, because the length of a side of the square first coding unit  1400  equals the length of a long side of the non-square second coding units  1402   a  and  1402   b , and  1404   a  and  1404   b , the first coding unit  1400  and the non-square second coding units  1402   a  and  1402   b , and  1404   a  and  1404   b  may have the same depth, e.g., D. However, when the image decoding device  100  splits the first coding unit  1400  into the four square second coding units  1406   a ,  1406   b ,  1406   c , and  1406   d  based on the split shape mode information, because the length of a side of the square second coding units  1406   a ,  1406   b ,  1406   c , and  1406   d  is ½ times the length of a side of the first coding unit  1400 , a depth of the second coding units  1406   a ,  1406   b ,  1406   c , and  1406   d  may be D+1 which is deeper than the depth D of the first coding unit  1400  by  1 . 
     According to an embodiment, the image decoding device  100  may determine a plurality of second coding units  1412   a  and  1412   b , and  1414   a ,  1414   b , and  1414   c  by splitting a first coding unit  1410 , a height of which is longer than a width, in a horizontal direction based on the split shape mode information. According to an embodiment, the image decoding device  100  may determine a plurality of second coding units  1422   a  and  1422   b , and  1424   a ,  1424   b , and  1424   c  by splitting a first coding unit  1420 , a width of which is longer than a height, in a vertical direction based on the split shape mode information. 
     According to an embodiment, a depth of the second coding units  1412   a  and  1412   b , and  1414   a ,  1414   b , and  1414   c , or  1422   a  and  1422   b , and  1424   a ,  1424   b , and  1424   c , which are determined based on the split shape mode information on the non-square first coding unit  1410  or  1420 , may be determined based on the length of a long side thereof. For example, because the length of a side of the square second coding units  1412   a  and  1412   b  is ½ times the length of a long side of the first coding unit  1410  having a non-square shape, a height of which is longer than a width, a depth of the square second coding units  1412   a  and  1412   b  is D+1 which is deeper than the depth D of the non-square first coding unit  1410  by  1 . 
     Furthermore, the image decoding device  100  may split the non-square first coding unit  1410  into an odd number of second coding units  1414   a ,  1414   b , and  1414   c  based on the split shape mode information. The odd number of second coding units  1414   a ,  1414   b , and  1414   c  may include the non-square second coding units  1414   a  and  1414   c  and the square second coding unit  1414   b . In this case, because the length of a long side of the non-square second coding units  1414   a  and  1414   c  and the length of a side of the square second coding unit  1414   b  are ½ times the length of a long side of the first coding unit  1410 , a depth of the second coding units  1414   a ,  1414   b , and  1414   c  may be D+1 which is deeper than the depth D of the non-square first coding unit  1410  by  1 . The image decoding device  100  may determine depths of coding units split from the first coding unit  1420  having a non-square shape, a width of which is longer than a height, by using the above-described method of determining depths of coding units split from the first coding unit  1410 . 
     According to an embodiment, the image decoding device  100  may determine PIDs for identifying split coding units, based on a size ratio between the coding units when an odd number of split coding units do not have equal sizes. Referring to  FIG. 14 , a coding unit  1414   b  of a center location among an odd number of split coding units  1414   a ,  1414   b , and  1414   c  may have a width equal to that of the other coding units  1414   a  and  1414   c  and a height which is two times that of the other coding units  1414   a  and  1414   c . That is, in this case, the coding unit  1414   b  at the center location may include two of the other coding unit  1414   a  or  1414   c . Therefore, when a PID of the coding unit  1414   b  at the center location is 1 based on a scan order, a PID of the coding unit  1414   c  located next to the coding unit  1414   b  may be increased by 2 and thus may be 3. That is, discontinuity in PID values may be present. According to an embodiment, the image decoding device  100  may determine whether an odd number of split coding units do not have equal sizes, based on whether discontinuity is present in PIDs for identifying the split coding units. 
     According to an embodiment, the image decoding device  100  may determine whether to use a specific splitting method, based on PID values for identifying a plurality of coding units determined by splitting a current coding unit. Referring to  FIG. 14 , the image decoding device  100  may determine an even number of coding units  1412   a  and  1412   b  or an odd number of coding units  1414   a ,  1414   b , and  1414   c  by splitting the first coding unit  1410  having a rectangular shape, a height of which is longer than a width. The image decoding device  100  may use PIDs indicating respective coding units so as to identify the respective coding units. According to an embodiment, the PID may be obtained from a sample at a preset location of each coding unit (e.g., an upper-left sample). 
     According to an embodiment, the image decoding device  100  may determine a coding unit at a preset location from among the split coding units, by using the PIDs for distinguishing the coding units. According to an embodiment, when the split shape mode information on the first coding unit  1410  having a rectangular shape, a height of which is longer than a width, indicates to split a coding unit into three coding units, the image decoding device  100  may split the first coding unit  1410  into three coding units  1414   a ,  1414   b , and  1414   c . The image decoding device  100  may assign a PID to each of the three coding units  1414   a ,  1414   b , and  1414   c . The image decoding device  100  may compare PIDs of an odd number of split coding units to determine a coding unit at a center location from among the coding units. The image decoding device  100  may determine the coding unit  1414   b  having a PID corresponding to a middle value among the PIDs of the coding units, as the coding unit at the center location from among the coding units determined by splitting the first coding unit  1410 . According to an embodiment, the image decoding device  100  may determine PIDs for distinguishing split coding units, based on a size ratio between the coding units when the split coding units do not have equal sizes. Referring to  FIG. 14 , the coding unit  1414   b  generated by splitting the first coding unit  1410  may have a width equal to that of the other coding units  1414   a  and  1414   c  and a height which is two times that of the other coding units  1414   a  and  1414   c . In this case, when the PID of the coding unit  1414   b  at the center location is 1, the PID of the coding unit  1414   c  located next to the coding unit  1414   b  may be increased by 2 and thus may be 3. When the PID is not uniformly increased as described above, the image decoding device  100  may determine that a coding unit is split into a plurality of coding units including a coding unit having a size different from that of the other coding units. According to an embodiment, when the split shape mode information indicates to split a coding unit into an odd number of coding units, the image decoding device  100  may split a current coding unit in such a manner that a coding unit of a preset location among an odd number of coding units (e.g., a coding unit of a center location) has a size different from that of the other coding units. In this case, the image decoding device  100  may determine the coding unit of the center location, which has a different size, by using PIDs of the coding units. However, the PIDs and the size or location of the coding unit of the preset location are not limited to the above-described examples, and various PIDs and various locations and sizes of coding units may be used. 
     According to an embodiment, the image decoding device  100  may use a preset data unit where a coding unit starts to be recursively split. 
       FIG. 15  illustrates that a plurality of coding units are determined based on a plurality of preset data units included in a picture, according to an embodiment. 
     According to an embodiment, a preset data unit may be defined as a data unit where a coding unit starts to be recursively split by using split shape mode information. That is, the preset data unit may correspond to a coding unit of an uppermost depth, which is used to determine a plurality of coding units split from a current picture. In the following descriptions, for convenience of explanation, the preset data unit is referred to as a reference data unit. 
     According to an embodiment, the reference data unit may have a preset size and a preset shape. According to an embodiment, a reference coding unit may include M×N samples. Herein, M and N may be equal to each other, and may be integers expressed as powers of 2. That is, the reference data unit may have a square or non-square shape, and may be split into an integer number of coding units. 
     According to an embodiment, the image decoding device  100  may split the current picture into a plurality of reference data units. According to an embodiment, the image decoding device  100  may split the plurality of reference data units, which are split from the current picture, by using the split shape mode information on each reference data unit. The operation of splitting the reference data unit may correspond to a splitting operation using a quadtree structure. 
     According to an embodiment, the image decoding device  100  may previously determine the minimum size allowed for the reference data units included in the current picture. Accordingly, the image decoding device  100  may determine various reference data units having sizes equal to or greater than the minimum size, and may determine one or more coding units by using the split shape mode information with reference to the determined reference data unit. 
     Referring to  FIG. 15 , the image decoding device  100  may use a square reference coding unit  1500  or a non-square reference coding unit  1502 . According to an embodiment, the shape and size of reference coding units may be determined based on various data units capable of including one or more reference coding units (e.g., sequences, pictures, slices, slice segments, tiles, tile groups, largest coding units, or the like). 
     According to an embodiment, the receiver  110  of the image decoding device  100  may obtain, from a bitstream, at least one of reference coding unit shape information and reference coding unit size information with respect to each of the various data units. An operation of splitting the square reference coding unit  1500  into one or more coding units has been described above in relation to the operation of splitting the current coding unit  300  of  FIG. 3 , and an operation of splitting the non-square reference coding unit  1502  into one or more coding units has been described above in relation to the operation of splitting the current coding unit  400  or  450  of  FIG. 4 . Thus, detailed descriptions thereof will not be provided herein. 
     According to an embodiment, the image decoding device  100  may use a PID for identifying the size and shape of reference coding units, to determine the size and shape of reference coding units according to some data units previously determined based on a preset condition. That is, the receiver  110  may obtain, from the bitstream, only the PID for identifying the size and shape of reference coding units with respect to each slice, slice segment, tile, tile group, or largest coding unit which is a data unit satisfying a preset condition (e.g., a data unit having a size equal to or smaller than a slice) among the various data units (e.g., sequences, pictures, slices, slice segments, tiles, tile groups, largest coding units, or the like). The image decoding device  100  may determine the size and shape of reference data units with respect to each data unit, which satisfies the preset condition, by using the PID. When the reference coding unit shape information and the reference coding unit size information are obtained and used from the bitstream according to each data unit having a relatively small size, efficiency of using the bitstream may not be high, and therefore, only the PID may be obtained and used instead of directly obtaining the reference coding unit shape information and the reference coding unit size information. In this case, at least one of the size and shape of reference coding units corresponding to the PID for identifying the size and shape of reference coding units may be previously determined. That is, the image decoding device  100  may determine at least one of the size and shape of reference coding units included in a data unit serving as a unit for obtaining the PID, by selecting the previously determined at least one of the size and shape of reference coding units based on the PID. 
     According to an embodiment, the image decoding device  100  may use one or more reference coding units included in a largest coding unit. That is, a largest coding unit split from a picture may include one or more reference coding units, and coding units may be determined by recursively splitting each reference coding unit. According to an embodiment, at least one of a width and height of the largest coding unit may be integer times at least one of the width and height of the reference coding units. According to an embodiment, the size of reference coding units may be obtained by splitting the largest coding unit n times based on a quadtree structure. That is, the image decoding device  100  may determine the reference coding units by splitting the largest coding unit n times based on a quadtree structure, and may split the reference coding unit based on at least one of the block shape information and the split shape mode information according to various embodiments. 
       FIG. 16  illustrates a processing block serving as a unit for determining a determination order of reference coding units included in a picture, according to an embodiment. 
     According to an embodiment, the image decoding device  100  may determine one or more processing blocks split from a picture. The processing block is a data unit including one or more reference coding units split from a picture, and the one or more reference coding units included in the processing block may be determined according to a specific order. That is, a determination order of one or more reference coding units determined in each processing block may correspond to one of various types of orders for determining reference coding units, and may vary depending on the processing block. The determination order of reference coding units, which is determined with respect to each processing block, may be one of various orders, e.g., raster scan order, Z-scan, N-scan, up-right diagonal scan, horizontal scan, and vertical scan, but is not limited to the above-mentioned scan orders. 
     According to an embodiment, the image decoding device  100  may obtain processing block size information and may determine the size of one or more processing blocks included in the picture. The image decoding device  100  may obtain the processing block size information from a bitstream and may determine the size of one or more processing blocks included in the picture. The size of processing blocks may be a preset size of data units, which is indicated by the processing block size information. 
     According to an embodiment, the receiver  110  of the image decoding device  100  may obtain the processing block size information from the bitstream according to each specific data unit. For example, the processing block size information may be obtained from the bitstream in a data unit such as an image, sequence, picture, slice, slice segment, tile, or tile group. That is, the receiver  110  may obtain the processing block size information from the bitstream according to each of the various data units, and the image decoding device  100  may determine the size of one or more processing blocks, which are split from the picture, by using the obtained processing block size information. The size of the processing blocks may be integer times that of the reference coding units. 
     According to an embodiment, the image decoding device  100  may determine the size of processing blocks  1602  and  1612  included in the picture  1600 . For example, the image decoding device  100  may determine the size of processing blocks based on the processing block size information obtained from the bitstream. Referring to  FIG. 16 , according to an embodiment, the image decoding device  100  may determine a width of the processing blocks  1602  and  1612  to be four times the width of the reference coding units, and may determine a height of the processing blocks  1602  and  1612  to be four times the height of the reference coding units. The image decoding device  100  may determine a determination order of one or more reference coding units in one or more processing blocks. 
     According to an embodiment, the image decoding device  100  may determine the processing blocks  1602  and  1612 , which are included in the picture  1600 , based on the size of processing blocks, and may determine a determination order of one or more reference coding units in the processing blocks  1602  and  1612 . According to an embodiment, determination of reference coding units may include determination of the size of the reference coding units. 
     According to an embodiment, the image decoding device  100  may obtain, from the bitstream, determination order information on one or more reference coding units included in one or more processing blocks, and may determine a determination order with respect to one or more reference coding units based on the obtained determination order information. The determination order information may be defined as an order or direction for determining the reference coding units in the processing block. That is, the determination order of reference coding units may be independently determined with respect to each processing block. 
     According to an embodiment, the image decoding device  100  may obtain, from the bitstream, the determination order information on reference coding units according to each specific data unit. For example, the receiver  110  may obtain the determination order information on reference coding units from the bitstream according to each data unit such as an image, sequence, picture, slice, slice segment, tile, tile group, or processing block. Because the determination order information on reference coding units indicates an order for determining reference coding units in a processing block, the determination order information may be obtained with respect to each specific data unit including an integer number of processing blocks. 
     According to an embodiment, the image decoding device  100  may determine one or more reference coding units based on the determined determination order. 
     According to an embodiment, the receiver  110  may obtain the determination order information on reference coding units from the bitstream as information related to the processing blocks  1602  and  1612 , and the image decoding device  100  may determine a determination order of one or more reference coding units included in the processing blocks  1602  and  1612  and determine one or more reference coding units, which are included in the picture  1600 , based on the determination order. Referring to  FIG. 16 , the image decoding device  100  may determine determination orders  1604  and  1614  of one or more reference coding units in the processing blocks  1602  and  1612 , respectively. For example, when the determination order information on reference coding units is obtained with respect to each processing block, different types of the determination order information on reference coding units may be obtained for the processing blocks  1602  and  1612 . When the determination order  1604  of reference coding units in the processing block  1602  is a raster scan order, reference coding units included in the processing block  1602  may be determined according to a raster scan order. On the contrary, when the determination order  1614  of reference coding units in the other processing block  1612  is a backward raster scan order, reference coding units included in the processing block  1612  may be determined according to the backward raster scan order. 
     According to an embodiment, the image decoding device  100  may decode the determined one or more reference coding units. The image decoding device  100  may decode an image, based on the reference coding units determined as described above. A method of decoding the reference coding units may include various image decoding methods. 
     According to an embodiment, the image decoding device  100  may obtain block shape information indicating the shape of a current coding unit or split shape mode information indicating a splitting method of the current coding unit, from the bitstream, and may use the obtained information. The split shape mode information may be included in the bitstream related to various data units. For example, the image decoding device  100  may use the split shape mode information included in a sequence parameter set, a picture parameter set, a video parameter set, a slice header, a slice segment header, a tile header, or a tile group header. Furthermore, the image decoding device  100  may obtain, from the bitstream, a syntax element corresponding to the block shape information or the split shape mode information according to each largest coding unit, each reference coding unit, or each processing block, and may use the obtained syntax element. 
     Hereinafter, a method of determining a split rule, according to an embodiment of the disclosure will be described in detail. 
     The image decoding device  100  may determine a split rule of an image. The split rule may be predetermined between the image decoding device  100  and the image encoding device  2200 . The image decoding device  100  may determine the split rule of the image, based on information obtained from a bitstream. The image decoding device  100  may determine the split rule based on the information obtained from at least one of a sequence parameter set, a picture parameter set, a video parameter set, a slice header, a slice segment header, a tile header, and a tile group header. The image decoding device  100  may determine the split rule differently according to frames, slices, tiles, temporal layers, largest coding units, or coding units. 
     The image decoding device  100  may determine the split rule based on a block shape of a coding unit. The block shape may include a size, shape, a ratio of width and height, and a direction of the coding unit. The image encoding device  2200  and the image decoding device  100  may pre-determine to determine the split rule based on the block shape of the coding unit. However, the embodiment is not limited thereto. The image decoding device  100  may determine the split rule based on the information obtained from the bitstream received from the image encoding device  2200 . 
     The shape of the coding unit may include a square and a non-square. When the lengths of the width and height of the coding unit are the same, the image decoding device  100  may determine the shape of the coding unit to be a square. Also, when the lengths of the width and height of the coding unit are not the same, the image decoding device  100  may determine the shape of the coding unit to be a non-square. 
     The size of the coding unit may include various sizes, such as 4×4, 8×4, 4×8, 8×8, 16×4, 16×8, and to 256×256. The size of the coding unit may be classified based on the length of a long side of the coding unit, the length of a short side, or the area. The image decoding device  100  may apply the same split rule to coding units classified as the same group. For example, the image decoding device  100  may classify coding units having the same lengths of the long sides as having the same size. Also, the image decoding device  100  may apply the same split rule to coding units having the same lengths of long sides. 
     The ratio of the width and height of the coding unit may include 1:2, 2:1, 1:4, 4:1, 1:8, 8:1, 1:16, 16:1, 32:1, 1:32, or the like. Also, a direction of the coding unit may include a horizontal direction and a vertical direction. The horizontal direction may indicate a case in which the length of the width of the coding unit is longer than the length of the height thereof. The vertical direction may indicate a case in which the length of the width of the coding unit is shorter than the length of the height thereof. 
     The image decoding device  100  may adaptively determine the split rule based on the size of the coding unit. The image decoding device  100  may differently determine an allowable split shape mode based on the size of the coding unit. For example, the image decoding device  100  may determine whether splitting is allowed based on the size of the coding unit. The image decoding device  100  may determine a split direction according to the size of the coding unit. The image decoding device  100  may determine an allowable split type according to the size of the coding unit. 
     The split rule determined based on the size of the coding unit may be a split rule predetermined between the image encoding device  2200  and the image decoding device  100 . Also, the image decoding device  100  may determine the split rule based on the information obtained from the bitstream. 
     The image decoding device  100  may adaptively determine the split rule based on a location of the coding unit. The image decoding device  100  may adaptively determine the split rule based on the location of the coding unit in the image. 
     Also, the image decoding device  100  may determine the split rule such that coding units generated via different splitting paths do not have the same block shape. However, an embodiment is not limited thereto, and the coding units generated via different splitting paths have the same block shape. The coding units generated via the different splitting paths may have different decoding processing orders. Because the decoding processing orders have been described above with reference to  FIG. 12 , details thereof are not provided again. 
     Hereinafter, a video encoding or decoding method according to an embodiment of the disclosure will be described in detail with reference to  FIGS. 17 to 20 , in which information about whether to apply an affine mode to a current block is received, when the information about whether to apply the affine mode indicates that the affine mode is to be applied, an affine parameter group is determined based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and prediction is performed on the current block based on the affine parameter group. 
       FIG. 17  is a block diagram of a video encoding device according to an embodiment. 
     A video encoding device  1700  according to an embodiment may include a memory  1710 , and at least one processor  1720  connected to the memory  1710 . Operations of the video encoding device  1700  according to an embodiment may be performed by individual processors or by a control of a central processor. Also, the memory  1710  of the video encoding device  1700  may store data received from external devices and data generated by processors, for example, information about whether to apply an affine mode to a current block, a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, etc. 
     The processor  1720  of the video encoding device  1700  may determine whether to apply an affine mode to a current block, when it is determined that the affine mode is to be applied to the current block, determine an affine parameter group based on history-based affine parameter group candidate list in which affine parameter groups of one or more blocks encoded in an affine mode before the current block are stored, perform prediction on the current block based on the affine parameter group, and encode information about whether to apply the affine mode to the current block. 
     Hereinafter, specific operations of a video encoding method will be described in detail with reference to  FIG. 18 , in which the video encoding device  1700  according to an embodiment determines whether to apply an affine mode to a current block, when it is determined that the affine mode is to be applied to the current block, determines an affine parameter group based on history-based affine parameter group candidate list in which affine parameter groups of one or more blocks encoded in an affine mode before the current block are stored, performs prediction on the current block based on the affine parameter group, and encodes information about whether to apply the affine mode to the current block. 
     The term “affine parameter group” used herein refers to a set of affine parameters required to apply the affine mode, and the affine parameter will be described below in  FIG. 22A . 
       FIG. 18  is a flowchart illustrating a video encoding method according to an embodiment. 
     Referring to  FIG. 18 , in operation S 1810 , the video encoding device  1700  may determine whether to apply an affine mode to a current block. 
     According to an embodiment, the affine mode may be indicate affine merge mode or an affine advanced motion vector prediction (AMVP) mode. 
     In operation S 1820 , when it is determined that the affine mode is to be applied to the current block, the video encoding device  1700  may determine an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks encoded in an affine mode before the current block are stored. 
     According to an embodiment, when an adjacent block encoded in an affine mode exists from among adjacent blocks of the current block, the affine parameter group may be determined based on affine parameter information of the adjacent block encoded in the affine mode and the history-based affine parameter group candidate list. The affine parameter information of the adjacent block encoded in the affine mode may be information of an affine merge mode candidate or an affine AMVP mode candidate of the current block. 
     In operation S 1850 , the video encoding device  1700  may perform prediction on the current block based on the affine parameter group. 
     According to an embodiment, the video encoding device  1700  may add the affine parameter group of the current block to the history-based affine parameter group candidate list. 
     According to an embodiment, when adding the affine parameter group of the current block to the history-based affine parameter group candidate list, in a case where the number of candidates included in the history-based affine parameter group candidate list is a predetermined maximum value, the video encoding device  1700  may remove the earliest-stored affine parameter candidate from the history-based affine parameter group candidate list. 
     According to an embodiment, when adding the affine parameter group of the current block to the history-based affine parameter group candidate list, in a case where the affine parameter group of the current block is the same as an affine parameter group existing in the history-based affine parameter group candidate list, the video encoding device  1700  may maintain the history-based affine parameter group candidate list. 
     According to an embodiment, in a case where the current block is a block to be encoded first in a predetermined area in which the current block is included, the video encoding device  1700  may reset the history-based affine parameter group candidate list, determine the affine parameter group based on the affine parameter information of the adjacent block encoded in the affine mode, which is located around the current block, and perform prediction on the current block based on the determined affine parameter group. The predetermined area may be a largest coding unit, a row of a largest coding unit, or a tile. Also, the video encoding device  1700  may add the affine parameter group of the current block to the history-based affine parameter group candidate list. 
     According to an embodiment, in a case where the current block is not the block to be encoded first in the predetermined area in which the current block is included, the video encoding device  1700  may determine the affine parameter group based on the affine parameter information of the adjacent block encoded in the affine mode, which is located around the current block, and the history-based affine parameter group candidate list, and perform prediction on the current block based on the determined affine parameter group. Also, the video encoding device  1700  may add the affine parameter group of the current block to the history-based affine parameter group candidate list. 
     In operation S 1870 , the video encoding device  1700  may encode information about whether to apply the affine mode to the current block. 
     According to an embodiment, whether to apply the affine mode to the current block may be determined by calculating a sum of transform difference (SATD) or a rate of distortion optimization (RDO), and information about whether to apply the affine mode to the current block may be encoded and signaled. 
       FIGS. 19 and 20  illustrate a block diagram of a video decoding device according to an embodiment and a flowchart of a video decoding method according to an embodiment, respectively corresponding to the video encoding device and the video encoding method described above. 
       FIG. 19  illustrates a block diagram of a video decoding device according to an embodiment. 
     A video decoding device  1900  according to an embodiment may include a memory  1910 , and at least one processor  1920  connected to the memory  1910 . Operations of the video decoding device  1900  according to an embodiment may be performed by individual processors or by a control of a central processor. Also, the memory  1910  of the video decoding device  1900  may store data received from external devices and data generated by processors, for example, information about whether to apply an affine mode to a current block, a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, etc. 
     The processor  1920  of the video decoding device  1900  may receive information about whether to apply an affine mode to a current block, when the information about whether to apply the affine mode indicates that the affine mode is to be applied, determine an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and perform prediction on the current block based on the affine parameter group. 
     Hereinafter, specific operations of a video decoding method will be described in detail with reference to  FIG. 20 , in which the video decoding device  1900  according to an embodiment receives information about whether to apply an affine mode to a current block, when the information about whether to apply the affine mode indicates that the affine mode is to be applied, determines an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored, and performs prediction on the current block based on the affine parameter group. 
       FIG. 20  illustrates a flowchart of a video decoding method according to an embodiment. 
     Referring to  FIG. 20 , in operation S 2010 , the video decoding device  1900  may receive information about whether to apply an affine mode to a current block. 
     According to an embodiment, information about whether to apply the affine mode to the current block may have been determined and signaled during an encoding process. 
     According to an embodiment, the affine mode may indicate an affine merge mode or an affine AMVP mode. 
     In operation S 2030 , the video decoding device  1900  may determine an affine parameter group based on a history-based affine parameter group candidate list in which affine parameter groups of one or more blocks reconstructed in an affine mode before the current block are stored. 
     According to an embodiment, when an adjacent block reconstructed in an affine mode exists from among adjacent blocks of the current block, the affine parameter group may be determined based on affine parameter information of the adjacent block reconstructed in the affine mode and the history-based affine parameter group candidate list. The affine parameter information of the adjacent block reconstructed in the affine mode may be information of an affine merge mode candidate or an affine AMVP mode candidate of the current block. 
     According to an embodiment, the video decoding device  1900  may add the affine parameter group of the current block to the history-based affine parameter group candidate list. 
     According to an embodiment, when adding the affine parameter group of the current block to the history-based affine parameter group candidate list, in a case where the number of candidates included in the history-based affine parameter group candidate list is a predetermined maximum value, the video decoding device  1900  may remove the earliest-stored affine parameter candidate from the history-based affine parameter group candidate list. 
     According to an embodiment, when adding the affine parameter group of the current block to the history-based affine parameter group candidate list, in a case where the affine parameter group of the current block is the same as an affine parameter group existing in the history-based affine parameter group candidate list, the video decoding device  1900  may maintain the history-based affine parameter group candidate list. 
     In operation S 2050 , the video decoding device  1900  may perform prediction on the current block based on the affine parameter group. 
     According to an embodiment, in a case where the current block is a block to be reconstructed first in a predetermined area in which the current block is included, the video decoding device  1900  may reset the history-based affine parameter group candidate list, determine the affine parameter group based on the affine parameter information of the adjacent block reconstructed in the affine mode, which is located around the current block, and perform prediction on the current block based on the determined affine parameter group. The predetermined area may be a largest coding unit, a row of a largest coding unit, or a tile. Also, the video decoding device  1900  may add the affine parameter group of the current block to the history-based affine parameter group candidate list. 
     According to an embodiment, in a case where the current block is not the block to be reconstructed first in the predetermined area in which the current block is included, the video decoding device  1900  may determine the affine parameter group based on the affine parameter information of the adjacent block reconstructed in the affine mode, which is located around the current block, and the history-based affine parameter group candidate list, and perform prediction on the current block based on the determined affine parameter group. Also, the video decoding device  1900  may add the affine parameter group of the current block to the history-based affine parameter group candidate list. 
       FIG. 21A  illustrates a schematic diagram of history-based motion vector prediction (HMVP), and  FIG. 21B  illustrates a process of processing a list used for HMVP. 
     Referring to  FIG. 21A , HMVP candidates in which pieces of motion information of a previously encoded block or a previously reconstructed block are stored are used for HMVP. In detail, an HMVP list is maintained through a process of loading a table in which HMVP candidates are stored, that is, an HMVP list, decoding the block based on the HMVP candidates, and updating motion information of the decoded block in an HMVP candidate table. 
     Referring to  FIG. 21B , the HMVP list refers to a list of N motion vectors which are most recently decoded. When the number of motion vectors that are storable in the HMVP list is L, HMVP candidates from HMVP 0  to HMVP- 1  from indexes 0 to L-1 may be stored in the HMVP list before the HMVP list is updated. Thereafter, when the HMVP list is updated and then a new HMVP candidate is added, motion information of the earliest-stored HMVP 0  among candidates stored in the HMVP list may be removed. That is, the HMVP list is updated according to first-in first-out (FIFO) logic. In addition, before adding the new HMVP candidate to the HMVP list, whether the same candidate as the new HMVP candidate exists among the existing HMVP candidates may be checked. As a result of the checking, when the same candidate exists, the new HMVP candidate may not be added. 
       FIG. 22A  illustrates in detail a method of deriving a motion vector applied to a sample of a current block  2200  in an affine mode. 
     In the affine mode, at least three affine parameters are required to derive a motion vector of a sample of the current block  2200 . In detail, the affine mode may include a 6-parameter affine mode, a 4-parameter affine mode, and a 3-parameter affine mode. Hereinafter, a method of deriving a motion vector of a sample of the current block  2200  according to each affine mode will be described. 
     In the 6-parameter affine mode, the processor  1720  may obtain three motion vectors  2202 ,  2204 , and  2206  from adjacent samples  2201 ,  2203 , and  2205  of the current block  2200 . A first motion vector  2202  may be obtained from adjacent samples of an upper-left coordinate  2201  of the current block  2200 . Also, a second motion vector  2204  may be obtained from adjacent samples of an upper-right coordinate  2203  of the current block  2200 . A third motion vector  2206  may be obtained from adjacent samples of a lower-left coordinate  2205  of the current block  2200 . In  FIG. 22 , the third motion vector  2206  is obtained based on the lower-left coordinate  2205  of the current block  2200 , but may also be obtained based on a lower-right coordinate  2207  of the current block  2200  according to embodiments. Also, the processor  1720  may determine an x component and a y component of the first motion vector  2202 , an x component and a y component of the second motion vector  2204 , and an x component and a y component of the third motion vector  2206  as affine parameters. 
     According to an embodiment, the first motion vector  2202  may be determined as an average of motion vectors of a plurality of adjacent blocks adjacent to the upper-left coordinate  2201  of the current block  2200 . Similarly, the second motion vector  2204  may be determined as an average of motion vectors of a plurality of adjacent blocks adjacent to the upper-right coordinate  2203  of the current block  2200 . Also, the third motion vector  2206  may be determined as an average of motion vectors of a plurality of adjacent blocks adjacent to the lower-left coordinate  2205  or the lower-right coordinate  2207  of the current block  2200 . 
     A motion vector  2210  of a sample  2208  of the current block  2200  may be determined based on the first motion vector  2202 , the second motion vector  2204 , and the third motion vector  2206  according to Equations 1 to 3. 
     In Equations 1 to 3, x denotes a horizontal distance difference between the upper-left coordinate  2201  of the current block  2200  and the sample  2208  of the current block  2200 , and y denotes a vertical distance difference between the upper-left coordinate  2201  of the current block  2200  and the sample  2208  of the current block  2200 . MV0 denotes the first motion vector  2202 , MV1 denotes the second motion vector  2204 , and MV2 denotes the third motion vector  2206 . MV denotes the motion vector  2210  of the sample  2208  of the current block  2200 . w denotes a width of the current block  2200 , and h denotes a height of the current block  2200 . dMVx denotes a horizontal rate of change of the motion vector  2210 , and dMVy denotes a vertical rate of change of the motion vector  2210 . 
       dMVx=( MV 1− MV 0)/ w   [Equation 1]
 
       dMVy=( MV 2− MV 0)/ h   [Equation 2]
 
         MV=MV 0+ x ·dMVx+ y ·dMVy  [Equation 3]
 
     Equation 1 indicates a method of obtaining the horizontal rate of change (dMVx) of the motion vector  2210 . According to Equation 1, a value obtained by subtracting the first motion vector  2202  from the second motion vector  2204  and dividing the resultant value by the width of the current block  2200  is determined as the horizontal rate of change of the motion vector  2210 . 
     Equation 2 indicates a method of obtaining the vertical rate of change (dMVy) of the motion vector  2210 . According to Equation 2, a value obtained by subtracting the first motion vector  2202  from the third motion vector  2206  and dividing the resultant value by the height of the current block  2200  is determined as the vertical rate of change of the motion vector  2210 . 
     Equation 3 indicates a method of obtaining the motion vector  2210 . According to Equation 3, the motion vector  2210  is determined as a value obtained by adding the first motion vector (MV0)  2202  and inner products of the coordinates (x, y) of the sample  2208  of the current block  2200  with respect to the upper-left coordinate  2201  of the current block  2200  and the vertical rate of change and the horizontal rate of change (dMVx, dMVy). 
     Motion vectors of all samples or sub-blocks included in the current block  2200  may be determined according to Equations 1 to 3. According to Equations 1 to 3, the motion vector of the sample may be determined different according to a location of the sample. Equations 1 and 2 may be applied to a case where vertical components of coordinates from which the first motion vector  2202  and the second motion vector  2204  are extracted are the same and horizontal components of coordinates from which the first motion vector  2202  and the third motion vector  2206  are extracted are the same. Accordingly, a generalized equation of determining a motion vector of the current block  2200  will be described below with reference to  FIG. 28 . 
     In the 6-parameter affine mode, the motion vector  2210  is determined by three motion vectors, and thus a reference block of the current block  2200  may be zoomed, rotated, and sheared from the current block  2200 . 
     In the 4-parameter affine mode, the processor  1720  may obtain two motion vectors  2202  and  2204  from adjacent samples of the current block  2200 . As in the 6-parameter affine mode, the first motion vector  2202  may be obtained from adjacent samples of the upper-left coordinate of the current block  2200 . Similarly, the second motion vector  2204  may be obtained from adjacent samples of the upper-right coordinate of the current block  2200 . Also, the processor  1720  may determine the x component and the y component of the first motion vector  2202  and the x component and the y component of the second motion vector  2204  as affine parameters. 
     In the 4-parameter affine mode, the third motion vector  2206  is not determined from the lower-left coordinate or the lower-right coordinate of the current block  2200 , but instead determined by combining the first motion vector  2202  and the second motion vector  2204 . 
     Equations 4 and 5 indicate methods of determining the third motion vector  2206  by combining the first motion vector  2202  and the second motion vector  2204 . In Equations 4 and 5, x denotes a horizontal component of a motion vector and y denotes a vertical component of a motion vector. MV0 denotes the first motion vector  2202 , MV1 denotes the second motion vector  2204 , and MV2 denotes the third motion vector  2206 . w denotes a width of the current block  2200 , and h denotes a height of the current block  2200 . 
         MV 2[ x ]=( MV 1[ y ]− MV 0[ y ])* w/h+MV 0[ x ]  [Equation 4]
 
         MV 2[ y ]=( MV 0[ x ]− MV 1[ x ])* w/h+MV 0[ y ]  [Equation 5]
 
     According to Equation 4, a horizontal coordinate value (MV2[x]) of the third motion vector  2206  is determined as a value ((MV1[y]−MV0[y])*w/h+MV0[x]), which is obtained by adding a horizontal coordinate value (MV0[x]) of the first motion vector  2202  to a product of a value (MV1[y]−MV0[y]) obtained by subtracting a vertical coordinate value of the first motion vector  2202  from a vertical coordinate value of the second motion vector  2204 , and a value (w/h) obtained by dividing the width of the current block by the height of the current block. 
     According to Equation 5, a vertical coordinate value (MV2[y]) of the third motion vector  2206  is determined as a value ((MV0[x]−MV1[x])*w/h+MV0[y]), which is obtained by adding a vertical coordinate value (MV0[y]) of the first motion vector  2202  to a value (MV0[x]−MV1[x]), obtained by subtracting a horizontal coordinate value of the second motion vector  2204  from a horizontal coordinate value of the first motion vector  2202 , and a value (w/h) obtained by dividing the width of the current block by the height of the current block. 
     In the 4-parameter affine mode, the x component and the y component of the third motion vector  2206  are derived from the first motion vector  2202  and the second motion vector  2204 . Accordingly, unlike the 6-parameter affine mode, in the 4-parameter affine mode, a reference block of the current block  2200  may only be zoomed and rotated from the current block  2200  based on first motion vector  2202  and the second motion vector  2204 . That is, in the 4-parameter affine mode, the current block  2200  is not sheared. 
     In the 3-parameter affine mode, the processor  1720  may obtain two motion vectors  2202  and  2204  from adjacent samples of the current block  2200 . The first motion vector  2202  may be obtained from adjacent samples of the upper-left coordinate of the current block  2200 . Similarly, the second motion vector  2204  may be obtained from adjacent samples of the upper-right coordinate of the current block  2200 . However, unlike the 4-parameter affine mode, in the 3-parameter affine mode, only the x component or the y component is obtained from the second motion vector  2204 . Therefore, the processor  1720  may determine the x component or the y component of the second motion vector  2204  as well as the x component and the y component of the first motion vector  2202  as affine parameters. 
     When the x component of the second motion vector  2204  is obtainable, the y component of the second motion vector  2204  is obtained from the y component of the first motion vector  2202 . On the contrary, when the y component of the second motion vector  2204  is obtainable, the x component of the second motion vector  2204  is obtained from the x component and the y component of the first motion vector  2202  and the y component of the second motion vector  2204 . Equations 6 and 7 below indicate methods of determining the y component and the x component of the second motion vector  2204 , respectively. 
     In Equations 6 and 7, x denotes a horizontal component of a motion vector and y denotes a vertical component of a motion vector. MV0 denotes the first motion vector  2202 , MV1 denotes the second motion vector  2204 , and MV2 denotes the third motion vector. w denotes a width of the current block  2200 , and h denotes a height of the current block  2200 . 
         MV 1[ y ]= MV 0[ y ]  [Equation 6]
 
         MV 1[ x ]=sqrt( w 2−( MV 1[ y ]− MV 0[ y ])2)+ MV 0[ x ]− w   [Equation 7]
 
     According to Equation 6, when only the x component of the second motion vector  2204  is obtainable, the processor  1720  determines the y component of the second motion vector  2204  to be the same as the y component of the first motion vector  2202 . 
     According to Equation 7, when only the y component of the second motion vector  2204  is obtainable, the processor  1720  determines the x component of the second motion vector  2204  based on the x component and the y component (MV0[x], MV0[y]) of the first motion vector  2202  and the y component (MV1[y]) of the second motion vector  2204 . 
     Also, the x component and the y component of the third motion vector  2206  are determined according to Equations 4 and 5, as in the 4-parameter affine mode. In the 3-parameter affine mode, together with the third motion vector  2206 , a component that is not obtainable among the x component and the y component of the second motion vector  2204  is derived from a component that is obtainable among the x component and the y component of the first motion vector  2202  and the second motion vector  2204 . Accordingly, in the 3-parameter affine mode, the reference block of the current block  2200  may only be zoomed or rotated from the current block  2200  based on the first motion vector  2202  and the second motion vector  2204 . When the x component of the second motion vector  2204  is obtainable, the reference block of the current block  2200  may only be zoomed from the current block  2200 . On the contrary, the y component of the second motion vector  2204  is obtainable, the reference block of the current block  2200  may only be rotated from the current block  2200 . 
       FIG. 22B  illustrates a method of deriving an affine parameter group from surrounding information in an affine mode. 
     The affine mode refers to an inter prediction method of obtaining motion vectors for samples included in a current block by affine-transforming motion vectors obtained from adjacent samples of the current block. The affine mode may include an affine merge mode, an affine AMVP mode (or affine inter mode), etc. 
     A method of deriving an affine parameter group candidate from a block to which an affine mode is applied around a current block will be described in detail with reference to  FIG. 22B . 
     In the affine merge mode or the affine AMVP mode, an affine parameter group for applying an affine mode to a current block may be obtained from an affine merge candidate list or an affine AMVP candidate list. However, when an appropriate affine parameter group is not obtainable from the affine merge candidate list or the affine AMVP candidate list, the affine parameter group of the current block may be derived from information of an adjacent block. 
     In  FIG. 22B , when an upper-left coordinate of a current picture is (0, 0), by using a motion vector {right arrow over (v E0 )} in the upper left (x E0 , y E0 ), a motion vector {right arrow over (v E1 )} in the upper right (x E1 , y E1 ), and a motion vector {right arrow over (v E2 )} in the lower left (x E2 , y E2 ) of an upper-left adjacent block to which an affine mode is applied, among adjacent blocks of a current prediction unit included in the current picture, a motion vector {right arrow over (v 0 )} in the upper left (x 0 , y 0 ) and a motion vector v in the upper right (x 1 , y 1 ) of the current prediction unit may be derived in the same manner as in the equations of  FIG. 22B . In Equations of  FIG. 22B , v E0x , v E1x , and v E2x  denote x component vectors of {right arrow over (v E0 )}, {right arrow over (v E1 )}, and {right arrow over (v E2 )}, respectively, and v E0y , v E1y , and v E2y  denote y component vectors of {right arrow over (v E0 )}, {right arrow over (v E1 )}, and {right arrow over (v E2 )}, respectively. 
     When the affine parameter group is derived in the method described above, there is a problem that occurs when an upper boundary line of the current prediction unit coincides with an upper boundary line of a largest coding unit. In detail, because a motion vector line buffer is used to store motion information of blocks in contact with the upper side of the largest coding unit, when the upper side of the current prediction unit is the upper boundary line of the largest coding unit, there is a problem that an additional line buffer is required instead of a general line buffer, in order to derive a motion vector v 0  in the upper left (x 0 , y 0 ) and a motion vector v 1  in the upper right (x 1 , y 1 ) of the current prediction unit in  FIG. 22B . This is because in the general line buffer, only the motion vector v E  is accessible and the motion vectors v E0  and v E1  are not accessible. 
     To solve this problem, a history-based affine parameter group candidate list may be used. The history-based affine parameter candidate group list will be described in detail in  FIG. 23 . 
       FIG. 23  illustrates a method of determining an affine parameter group candidate of a current block based on a history-based affine parameter group candidate list. 
     With reference to  FIG. 23 , when a current frame  2300  is split into a plurality of blocks  2301  to  2323  and an affine mode is to be applied to a current block  2317 , blocks  2301 ,  2303 ,  2305 ,  2307 ,  2311 ,  2314 , and  2316  reconstructed in the affine mode before the current block exist from among the blocks. Information of the blocks  2301 ,  2303 ,  2305 ,  2307 ,  2311 ,  2314 , and  2316  reconstructed in the affine mode before the current block is separately stored in the history-based affine parameter group candidate list. 
     First, an affine parameter group required for the current block to which the affine mode is to be applied may be obtained from an affine merge candidate list or an affine AMVP candidate list of the current block according to an affine merge mode or an affine AMVP mode. However, when the affine parameter group for the current block is not obtainable from the affine merge candidate list or the affine AMVP candidate list of the current block, the affine parameter group of the current block may be determined by obtaining affine information from the history-based affine parameter group candidate list in which affine parameter groups of the blocks reconstructed in the affine mode before the current block are stored. 
     For example, in  FIG. 23 , the affine parameter group of the current block may be determined from the history-based affine parameter group candidate list in which affine parameter groups of the blocks  2301 ,  2303 ,  2305 ,  2307 ,  2311 ,  2314 , and  2316  reconstructed by applying the affine mode before the current block  2317  are stored. Accordingly, the affine parameter group of the current block may be determined based on affine information on a block other than an adjacent block which is adjacent to the current block  2317 . 
     Also, when selecting an available affine parameter group candidate from the history-based affine parameter group candidate list to determine the affine parameter group for the current block, a pruning process may be applied to select a more appropriate model. 
     According to an embodiment, only when a boundary of the current block is shared, an affine parameter group candidate in the history-based affine parameter group candidate list may be considered. 
     According to an embodiment, affine parameter group candidates may be arranged in the history-based affine parameter group candidate list according to a distance from a location of the current block, for example, a center location or a location of an upper-left corner of the current block. 
     According to an embodiment, affine parameter group candidates in the history-based affine parameter group candidate list within a specific threshold distance from the current block may be considered. 
     According to an embodiment, affine parameter group candidates in the history-based affine parameter group candidate list may be considered, when an x-coordinate at a right edge of a block reconstructed in the affine mode before the current block and an x-coordinate at an upper-left corner of the current block are the same as each other, or when a y-coordinate at a lower edge of the block reconstructed in the affine mode before the current block and a y-coordinate at the upper-left corner of the current block are the same as each other. 
     In this case, unlike deriving the affine parameter group from the motion information of the adjacent block in  FIG. 22B , the affine parameter group may be determined from the history-based affine parameter group candidate list in which N predetermined affine parameter group candidates are stored. Therefore, the amount of buffer may be reduced by using a buffer having an appropriate capacity in which N predetermined affine parameter group candidates are stored, instead of an unnecessary additional line buffer. 
     The affine parameter group may include a width and a height of a block, an upper-left location, an upper-left motion vector, an upper-right motion vector, and a lower-left motion vector, may include an upper-left location, an upper-right location, a lower-left location, an upper-left motion vector, an upper-right motion vector, and a lower-left motion vector, may include a width, a height, an upper-left location, an upper-left motion vector, and an upper-right motion vector, or may include a width, a height, an upper-left location, an upper-left motion vector, a vertical amount of change of a motion vector, and a horizontal amount of change of a motion vector. 
       FIG. 24  illustrates a flowchart of a video encoding method according to another embodiment. 
     The video encoding device  1700  of  FIG. 17  may perform an operation according to the video encoding method of  FIG. 24 . 
     The video encoding device  1700  may include the memory  1710  and the at least one processor  1720  connected to the memory  1710 . Operations of the video encoding device  1700  according to an embodiment may be performed by individual processors or by a control of a central processor. Also, the memory  1710  of the video encoding device  1700  may store data received from external devices and data generated by processors, for example, information about a history-based motion vector candidate list in which motion vectors of blocks encoded before a current block are stored, etc. 
     The processor  1720  of the video encoding device  1700  may determine whether a current block is a block to be encoded first in a predetermined area in which the current block is included, when the current block is the block to be encoded first, reset a history-based motion vector candidate list in which motion vectors of blocks encoded before the current block are stored and perform prediction on the current block based on motion information of an adjacent block to the current block, and when the current block is not the block to be encoded first, perform prediction on the current block based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     Referring to  FIG. 24 , in operation S 2410 , the video encoding device  1700  may determine whether a current block is a block to be encoded first in a predetermined area in which the current block is included. 
     According to an embodiment, the predetermined area may be a largest coding unit (CTU), a row of a largest coding unit (CTU ROW), or a tile. In operation S 2430 , when the current block is the block to be encoded first, the video encoding device  1700  may reset a history-based motion vector candidate list in which motion vectors of blocks encoded before the current block are stored and perform prediction on the current block based on motion information of an adjacent block to the current block. 
     In operation S 2450 , when the current block is not the block to be encoded first, the video encoding device  1700  may perform prediction on the current block based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     According to an embodiment, the video encoding device  1700  may store a motion vector of the current block in the history-based motion vector candidate list. 
     According to an embodiment, when storing the motion vector of the current block in the history-based motion vector candidate list, in a case where the number of motion vector candidates stored in the history-based motion vector candidate list is a predetermined maximum value, the video encoding device  1700  may remove a motion vector of the earliest-stored block from the history-based motion vector candidate list. 
     According to an embodiment, when storing the motion vector of the current block in the history-based motion vector candidate list, in a case where the motion vector of the current block is the same as a motion vector existing in the history-based motion vector candidate list, the video encoding device  1700  may maintain the history-based motion vector candidate list. 
       FIG. 25  illustrates a flowchart of a video decoding method according to another embodiment. 
     The video decoding device  1900  may perform an operation according to the video decoding method. 
     The video decoding device  1900  may include the memory  1910  and the at least one processor  1920  connected to the memory  1910 . Operations of the video decoding device  1900  according to an embodiment may be performed by individual processors or by a control of a central processor. Also, the memory  1910  of the video decoding device  1900  may store data received from external devices and data generated by processors, for example, information about a history-based motion vector candidate list in which motion vectors of blocks reconstructed before a current block are stored, etc. 
     The processor  1920  of the video decoding device  1900  may determine whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included, when the current block is the block to be reconstructed first, reset a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored and perform prediction on the current block based on motion information of an adjacent block to the current block, and when the current block is not the block to be reconstructed first, perform prediction on the current block based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     Referring to  FIG. 25 , in operation S 2510 , the video decoding device  1900  may determine whether a current block is a block to be reconstructed first in a predetermined area in which the current block is included. 
     According to an embodiment, the predetermined area may be a largest coding unit (CTU), a row of a largest coding unit (CTU ROW), or a tile. 
     In operation S 2530 , when the current block is the block to be reconstructed first, the video decoding device  1900  may reset a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored and perform prediction on the current block based on motion information of an adjacent block to the current block. 
     In operation S 2550 , when the current block is not the block to be reconstructed first, the video decoding device  1900  may perform prediction on the current block based on the motion information of the adjacent block to the current block and the history-based motion vector candidate list. 
     According to an embodiment, the video decoding device  1900  may store a motion vector of the current block in the history-based motion vector candidate list. 
     According to an embodiment, when storing the motion vector of the current block in the history-based motion vector candidate list, in a case where the number of motion vector candidates stored in the history-based motion vector candidate list is a predetermined maximum value, the video decoding device  1900  may remove a motion vector of the earliest-stored block from the history-based motion vector candidate list. 
     According to an embodiment, when storing the motion vector of the current block in the history-based motion vector candidate list, in a case where the motion vector of the current block is the same as a motion vector existing in the history-based motion vector candidate list, the video decoding device  1900  may maintain the history-based motion vector candidate list. 
       FIG. 26  illustrates a line buffer required to access motion information at a boundary of a largest coding unit. 
     Referring to  FIG. 26 , when the upper side of a current block coincides with an upper boundary line of a largest coding unit, blocks A, B, C, and D that the current block can refer to are located above the largest coding unit including the current block, and thus access to a motion vector line buffer is required to use motion information of the blocks A, B, C, and D. Access to the line buffer may be completely eliminated by using a history-based motion vector candidate list in which motion vectors of blocks reconstructed before the current block are stored. Even though the access to the line buffer is completely eliminated, derivation of an AMVP candidate or a merge candidate only affects blocks immediately below the upper boundary line of the largest coding unit, and thus the impact on performance is minimized. 
       FIG. 27A  illustrates an example of identifying each area having a history-based motion vector candidate list in one frame, and  FIG. 27B  illustrates another example of identifying each area having a history-based motion vector candidate list in one frame. 
     When the history-based motion vector candidate list is used, all blocks processed before the current block may be requested according to a coding order. In this case, because adjacent blocks must be completely processed, efficient parallel processing is impossible. 
     A method of using a plurality of history-based motion vector candidate lists based on a certain area, that is, a method of using a history-based motion vector candidate list for each certain area, will be described in detail with reference to  FIGS. 27A and 27B . In detail, when accessing a history-based motion vector candidate list of a current block, a history-based motion vector candidate list belonging to a certain area corresponding to the current block is selected, and when the history-based motion vector candidate list is updated, the history-based motion vector candidate list in the certain area corresponding to the current block is updated. Accordingly, because each area has a history-based motion vector candidate list, dependency on other areas may be removed and efficient parallel processing may be allowed. 
     Referring to  FIG. 27A , a current frame (or current picture) may be divided into areas  1  to  6 , and each history-based motion vector candidate list may be used. In detail, a history-based motion vector candidate list in which motion vector candidates of the area  1  are stored is used for a block belonging to the area  1  having a size of M×N, and when prediction is performed on a block of the area  2 , the history-based motion vector candidate list may be reset. Accordingly, when prediction is performed on the area  2 , samples of the area  2  may be reconstructed without being dependent on data of samples of the area  1 . Thereafter, motion vector candidates of reconstructed blocks belonging to the area  2  may be stored in the history-based motion vector candidate list. 
     The areas  1  to  6  may be a tile unit, a slice unit, a largest coding unit, etc. 
     Referring to  FIG. 27B , a current frame (or current picture) may be divided into areas  1  to  4 , and each history-based motion vector candidate list may be used. In detail, a history-based motion vector candidate list in which motion vector candidates of the area  1  are stored is used for a block belonging to the area  1  having M rows, and when prediction is performed on a block of the area  2 , the history-based motion vector candidate list may be reset. Accordingly, when prediction is performed on the area  2 , samples of the area  2  may be reconstructed without being dependent on data of samples of the area  1 . Thereafter, motion vector candidates of reconstructed blocks belonging to the area  2  may be stored in the history-based motion vector candidate list. 
     The areas  1  to  4  may be a tile unit, a slice unit, a largest coding unit, etc. Also, the areas  1  to  4  may be a row of a largest coding unit (CTU row). 
       FIG. 28  illustrates a method of determining a method of determining a motion vector of a current block in an affine mode. Equations 8 to 10 below indicate a method of determining a motion vector of a current block according to motion vector extraction locations  2800 ,  2810 , and  2820  of  FIG. 28 . 
     In Equations 8 and 9, w denotes a horizontal distance between a first motion vector extraction location  2800  and a second motion vector extraction location  2810 . h denotes a vertical distance between the first motion vector extraction location  2800  and a third motion vector extraction location  2820 . x denotes a horizontal distance between the first motion vector extraction location  2800  and the third motion vector extraction location  2820 . y denotes a vertical distance between the first motion vector extraction location  2800  and the second motion vector extraction location  2810 . 
     P 0  denotes a first motion vector, P 1  denotes a second motion vector, and P 2  denotes a third motion vector. dx and dy denote a horizontal amount of change and a vertical amount of change, respectively. 
     
       
         
           
             
               
                 
                   dx 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             hP 
                             1 
                           
                           - 
                           
                             yP 
                             2 
                           
                         
                         ) 
                       
                       - 
                       
                         ( 
                         
                           
                             hP 
                             0 
                           
                           - 
                           
                             yP 
                             0 
                           
                         
                         ) 
                       
                     
                     
                       wh 
                       - 
                       xy 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
             
               
                 
                   dy 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             xP 
                             1 
                           
                           - 
                           
                             wP 
                             2 
                           
                         
                         ) 
                       
                       - 
                       
                         ( 
                         
                           
                             xP 
                             0 
                           
                           - 
                           
                             wP 
                             0 
                           
                         
                         ) 
                       
                     
                     
                       xy 
                       - 
                       wh 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
     The horizontal amount of change is determined according to Equation 8, and the vertical amount of change is determined according to Equation 9. Further, according to Equation 10, a motion vector of a sample  2830  of the current block is determined based on the horizontal amount of change and the vertical amount of change. In Equation 10, Pa denotes the motion vector of the sample  2830  of the current block. Also, i denotes a horizontal distance between the first motion vector extraction location  2800  and the sample  2830  of the current block, and j denotes a vertical distance between the first motion vector extraction location  2800  and the sample  2830  of the current block. 
         Pa=P 0+ idx+jdy   [Equation 10]
 
     When three motion vectors and an extraction location of each motion vector are given according to Equations 8 to 10, motion vectors of samples included in the current block may be determined. 
     So far, various embodiments have been described. It will be understood by one of ordinary skill in the art to which the disclosure belongs that modifications can be made within a range not deviating from the intrinsic properties of the disclosure. Therefore, the disclosed embodiments should be considered from a descriptive standpoint rather than a restrictive standpoint. The scope of the disclosure is defined in the accompanying claims rather than the above detailed description, and it should be noted that all differences falling within the claims and equivalents thereof are included in the scope of the disclosure. 
     Meanwhile, the embodiments of the disclosure may be written as a program that is executable on a computer, and implemented on a general-purpose digital computer that operates a program using a computer-readable recording medium. The computer-readable recording medium may include a storage medium, such as a magnetic storage medium (for example, read only memory (ROM), a floppy disk, a hard disk, etc.) and an optical reading medium (for example, compact disc ROM (CD-ROM), digital versatile disc (DVD), etc.).