Patent Publication Number: US-11665356-B2

Title: Method and apparatus for video decoding, and method and apparatus for video encoding

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of U.S. application Ser. No. 16/651,126 filed Mar. 26, 2020, which is a National Stage of International Application No. PCT/KR2018/012311 filed Oct. 18, 2018, which claims priority to U.S. Provisional Application No. 62/573,915 filed Oct. 18, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a video decoding method and a video decoding apparatus, and provides a method and apparatus for configuring an additional mode set based on most probable mode (MPM) modes of a current block, determining an intra prediction mode of the current block based on the MPM modes and the additional mode set, and thus performing intra prediction on the current block. 
     BACKGROUND ART 
     Image data is encoded by a preset codec conforming to a data compression standard, e.g., the Moving Picture Expert Group (MPEG) standard, and then is stored in a recording medium or transmitted through a communication channel in the form of a bitstream. 
     As hardware capable of reproducing and storing high-resolution or high-quality image content has been developed and become widely popular, a codec capable of efficiently encoding or decoding the high-resolution or high-quality image content is in high demand. The encoded image content may be reproduced by decoding it. Recently, methods of effectively compressing high-resolution or high-quality image content are used. For example, a method of randomly splitting an image to be encoded or a procedure of manipulating data is proposed to allow an image compression technique to be effectively implemented. 
     As one of data manipulation techniques, it is general, in intra prediction, to use at least two most probable mode (MPM) modes and to code data and perform signaling under the same conditions in other modes. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     Provided are a method and apparatus for configuring an additional mode set based on most probable mode (MPM) modes of a current block, determining an intra prediction mode based on the MPM modes and the configured additional mode set, and performing prediction on the current block according to the determined intra prediction mode. 
     Solution to Problem 
     To solve the technical problems, a video decoding method provided in the present disclosure includes configuring an additional mode set based on most probable mode (MPM) modes of a current block; determining an intra prediction mode of the current block, based on the MPM modes and the additional mode set; and performing intra prediction on the current block, based on the intra prediction mode. 
     To solve the technical problems, a video decoding apparatus provided in the present disclosure includes a memory; and at least one processor connected with the memory, wherein the at least one processor is configured to configure an additional mode set based on MPM modes of a current block, determine an intra prediction mode of the current block, based on the MPM modes and the additional mode set, and perform intra prediction on the current block, based on the intra prediction mode. 
     To solve the technical problems, a video encoding method provided in the present disclosure includes configuring an additional mode set based on MPM modes of a current block; determining an intra prediction mode of the current block, based on the MPM modes and the additional mode set; and performing intra prediction on the current block, based on the intra prediction mode. 
     To solve the technical problems, a video encoding apparatus provided in the present disclosure includes a memory; and at least one processor connected with the memory, wherein the at least one processor is configured to configure an additional mode set based on MPM modes of a current block, determine an intra prediction mode of the current block, based on the MPM modes and the additional mode set, and perform intra prediction on the current block, based on the intra prediction mode. 
     Advantageous Effects of Disclosure 
     In a video encoding and decoding procedure, an additional mode set may be configured based on most probable mode (MPM) modes of a current block, and then prediction may be performed on the current block by using an intra prediction mode determined based on the MPM modes and the additional mode set, such that performance may be enhanced due to accuracy in intra prediction, a bit amount occurring in the coding based on the intra prediction mode may be decreased, and thus, efficiency in mode signaling may be increased and reliability may be increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic block diagram of an image decoding apparatus, according to an embodiment. 
         FIG.  2    is a flowchart of an image decoding method, according to an embodiment. 
         FIG.  3    illustrates a process, performed by an image decoding apparatus, of determining at least one coding unit by splitting a current coding unit, according to an embodiment. 
         FIG.  4    illustrates a process, performed by the image decoding apparatus, 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 the image decoding apparatus, 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 the image decoding apparatus, 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 the image decoding apparatus determines the plurality of coding units by splitting a current coding unit, according to an embodiment. 
         FIG.  8    illustrates a process, performed by the image decoding apparatus, 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 the image decoding apparatus, 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 by the image decoding apparatus is restricted when a second coding unit having a non-square shape, which is determined by splitting a first coding unit, satisfies a preset condition, according to an embodiment. 
         FIG.  11    illustrates a process, performed by the image decoding apparatus, 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 a 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 decoding apparatus, according to an embodiment. 
         FIG.  18    illustrates a flowchart of a video decoding method, according to an embodiment. 
         FIG.  19    illustrates a block diagram of a video encoding apparatus, according to an embodiment. 
         FIG.  20    illustrates a flowchart of a video encoding method, according to an embodiment. 
         FIG.  21    illustrates an embodiment of modes of left and top neighboring blocks and a mode set of a current block. 
         FIG.  22    illustrates an embodiment of intra prediction mode directions. 
         FIG.  23    illustrates an embodiment of an additional mode set. 
         FIG.  24    illustrates an embodiment of intra prediction mode signaling syntax. 
     
    
    
     BEST MODE 
     According to an embodiment of the present disclosure, a video decoding method includes configuring an additional mode set based on most probable mode (MPM) modes of a current block; determining an intra prediction mode of the current block, based on the MPM modes and the additional mode set; and performing intra prediction on the current block, based on the intra prediction mode. 
     According to an embodiment, the MPM modes of the current block may include a prediction mode of a neighboring block adjacent to a left side of the current block and a prediction mode of a neighboring block adjacent to a top side of the current block. 
     According to an embodiment, a neighboring block adjacent to a right side of the current block and a neighboring block adjacent to a top side of the current block may be reconstructed before the current block, and the MPM modes of the current block may include a prediction mode of the neighboring block adjacent to the right side of the current block and a prediction mode of the neighboring block adjacent to the top side of the current block. 
     According to an embodiment, the additional mode set may be determined based on types of the MPM modes. 
     According to an embodiment, the additional mode set may include at least one of a DC mode, a planar mode, a vertical mode, a horizontal mode, and a diagonal mode. 
     According to an embodiment, when the MPM modes are all angular modes, the additional mode set may include at least one of the DC mode, the planar mode, and intra prediction modes having index increased by n from index of each of the MPM modes, wherein n is a non-zero integer. 
     According to an embodiment, when the MPM modes are all angular modes, the additional mode set may include at least one of a DC mode, a planar mode, intra prediction modes having an index increased by 1 from index of each of the MPM modes, intra prediction modes having an index decreased by 1 from the index of each of the MPM modes, and intra prediction modes having an index decreased by 2 from the index of each of the MPM modes. 
     According to an embodiment, when an intra prediction mode of the neighboring block adjacent to the left side of the current block and an intra prediction mode of the neighboring block adjacent to the top side of the current block are equal and angular modes, the MPM modes and the additional mode set may include the intra prediction mode of the neighboring block adjacent to the left side, a DC mode, a planar mode, a mode of increasing, by 1, an index of the intra prediction mode of the neighboring block adjacent to the left side, an intra prediction mode having an index decreased by 1 from the index of the intra prediction mode of the neighboring block adjacent to the left side, and an intra prediction mode having an index decreased by 2 from the index of the intra prediction mode of the neighboring block adjacent to the left side. 
     According to an embodiment, when a first MPM mode of the MPM modes is a non-angular mode, and a second MPM mode of the MPM modes is an angular mode, the additional mode set may include at least one of a mode different from the first MPM mode from among a DC mode and a planar mode, an intra prediction mode having an index increased by n from an index of the second MPM mode, a vertical mode, and a horizontal mode, wherein n is a non-zero integer. 
     According to an embodiment of the present disclosure, a video encoding method includes configuring an additional mode set based on MPM modes of a current block; determining an intra prediction mode of the current block, based on the MPM modes and the additional mode set; and performing intra prediction on the current block, based on the intra prediction mode. 
     According to an embodiment of the present disclosure, a video decoding apparatus includes a memory; and at least one processor connected with the memory, wherein the at least one processor is configured to configure an additional mode set based on MPM modes of a current block, determine an intra prediction mode of the current block, based on the MPM modes and the additional mode set, and perform intra prediction on the current block, based on the intra prediction mode. 
     According to an embodiment of the present disclosure, a video encoding apparatus includes a memory; and at least one processor connected with the memory, wherein the at least one processor is configured to configure an additional mode set based on MPM modes of a current block, determine an intra prediction mode of the current block, based on the MPM modes and the additional mode set, and perform intra prediction on the current block, based on the intra prediction mode. 
     MODE OF DISCLOSURE 
     Advantages and features of embodiments and methods of accomplishing the same may be understood more readily by reference to the embodiments and the accompanying drawings. In this regard, the disclosure may have different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this 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 in the specification will be briefly defined, and the embodiments will be described in detail. 
     All terms including descriptive or technical terms which are used in the specification should be construed as having meanings that are obvious to one of ordinary skill in the art. However, the terms may have different meanings according to the intention of one of ordinary skill in the art, precedent cases, or the appearance of new technologies. Also, some terms may be arbitrarily selected by the applicant, and in this case, the meaning of the selected terms will be described in detail in the detailed description of the disclosure. Therefore, the terms used in the disclosure should not be interpreted based on only their names but have to be defined based on the meaning of the terms together with the descriptions throughout the specification. 
     In the following specification, the singular forms include plural forms unless the context clearly indicates otherwise. 
     When a part “includes” or “comprises” an element, unless there is a particular description contrary thereto, the part may further include other elements, not excluding the other elements. 
     In the following description, terms such as “unit” indicate a software or hardware component and the “unit” performs certain functions. However, the “unit” is not limited to software or hardware. The “unit” may be formed so as to be in an addressable storage medium, or may be formed so as to operate one or more processors. Thus, for example, the term “unit” may refer to components such as software components, object-oriented software components, class components, and task components, and may include processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, micro codes, circuits, data, a database, data structures, tables, arrays, or variables. A function provided by the components and “units” may be associated with the smaller number of components and “units”, or may be divided 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, and the like. In some environments, the “processor” may refer to an application specific semiconductor (ASIC), a programmable logic device (PLD), a field programmable gate array (FPGA), or the like. 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 configurations. 
     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, registers, and the like. 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 be a static image such as a still image of a video or may be a dynamic image such as a moving image, that is, the video itself. 
     Hereinafter, a “sample” denotes data assigned to a sampling position 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 domain may be samples A unit including at least one such sample may be defined as a block. 
     Hereinafter, 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 embodiments without any difficulty. In addition, portions irrelevant to the description will be omitted in the drawings for a clear description of the disclosure. 
     Hereinafter, an image encoding apparatus and an image decoding apparatus, and an image encoding method and an image decoding method according to embodiments will be described with reference to  FIGS.  1  through  16   . With reference to  FIGS.  3  to  16   , a method of determining a data unit of an image according to an embodiment will be described, and with reference to  FIGS.  17  to  24   , a method of configuring an additional mode set based on most probable mode (MPM) modes of a current block, determining an intra prediction mode of the current block based on the MPM modes and the additional mode set, and performing intra prediction on the current block, based on the determined intra prediction mode according to an embodiment will be described. 
     Hereinafter, the method and apparatus for adaptively selecting a context model, based on various shapes of coding unit according to an embodiment of the disclosure will now be described with reference to  FIGS.  1  and  2   . 
       FIG.  1    is a schematic block diagram of an image decoding apparatus  100 , according to an embodiment. 
     The image decoding apparatus  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 of an image encoded by an image encoding apparatus  2200  described later. Also, the bitstream may be transmitted from the image encoding apparatus  2200 . The image encoding apparatus  2200  and the image decoding apparatus  100  may be connected via wire or wirelessly, and the receiver  110  may receive the bitstream via wire 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 apparatus  100  will be described in detail with reference to  FIG.  2   . 
       FIG.  2    is 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 apparatus  100  obtains, from a bitstream, a bin string corresponding to a split shape mode of a coding unit (operation  210 ). The image decoding apparatus  100  determines a split rule of the coding unit (operation  220 ). Also, the image decoding apparatus  100  splits the coding unit into a plurality of coding units, based on at least one of the split rule and the bin string corresponding to the split shape mode (operation  230 ). The image decoding apparatus  100  may determine an allowable first range of a size of the coding unit, according to a ratio of the width to the height of the coding unit, so as to determine the split rule. The image decoding apparatus  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. One slice may be a sequence of 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). 
     A largest coding block (CTB) refers to an N×N block including N×N samples (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 lama 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 having 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 having 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 disclosure 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 information about a maximum size of a luma coding block that is bi-splittable 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 is bi-splittable. Accordingly, when the information about the maximum size of the luma coding block that is bi-splittable 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 the size of the luma largest coding unit. 
     According to an embodiment, because the information about the maximum size of the luma coding block that is bi-splittable is obtained from the bitstream, the maximum size of the luma coding block that is bi-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-slice may be 32×32, and the maximum size of the luma coding block that is ternary splittable in a P-slice or a B-slice 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_BTVER), 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 apparatus  100  may obtain split shape mode information from a bitstream from one bin string. The bitstream received by the image decoding apparatus  100  may include a fixed length binary code, a unary code, a truncated unary code, a pre-determined binary code, etc. The bin string is a binary sequence of information. The bin string may include at least one bit. The image decoding apparatus  100  may obtain the split shape mode information corresponding to the bin string based on a split rule. The image decoding apparatus  100  may determine whether or not to quad split a coding unit, a split direction, and a split type, based on one bin string. 
     A coding unit may be equal to or smaller than a 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 code information about a largest coding unit indicates that splitting is to be 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 to be performed, the coding unit may be split into smaller coding units. However, splitting of an image is not limited thereto, and a largest coding unit and a coding unit may not be distinguished from each other. Splitting of a coding unit will be described in more detail with reference to  FIGS.  3  through  16   . 
     Also, one or more prediction blocks for prediction may be determined from a coding unit. A prediction block may be equal to or smaller than a coding unit. Also, one or more transform blocks for transform may be determined from a coding unit. A transform block may be equal to or smaller than a coding unit. 
     Shapes and sizes of a transform block and a 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. 
     Splitting of a coding unit will be described in detail with reference to  FIGS.  3  to  16   . Each of a current block and a neighboring block of the present disclosure may indicate one of a largest coding unit, a coding unit, a prediction block, and a transform block. Also, a current block or a current coding unit is a block on which decoding or encoding is currently performed or a block on which splitting is currently performed. A neighboring block may be a block that is reconstructed before a current block. The neighboring block may be spatially or temporally adjacent to the current block. The neighboring block may be located at one of a lower-left side, a left side, an upper-left side, an upper side, an upper-right side, a right side, and a lower-right side of the current block. 
       FIG.  3    illustrates a process, performed by the image decoding apparatus  100 , 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. In this regard, N may be a positive integer. Block shape information is information indicating at least one of a shape, a direction, a ratio of a width to a height, or a size of a coding unit. 
     The shape of the coding unit may include a square shape and a non-square shape. When the width and the height of the coding unit are the same (i.e., when the block shape of the coding unit is 4N×4N), the image decoding apparatus  100  may determine the block shape information of the coding unit as a square shape. The image decoding apparatus  100  may determine the shape of the coding unit as a non-square shape. 
     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 apparatus  100  may determine the block shape information of the coding unit as a non-square shape. When the shape of the coding unit is a non-square shape, the image decoding apparatus  100  may determine the ratio of the width to the height in the block shape information of the coding unit as 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 apparatus  100  may determine whether the coding unit is in a horizontal direction or a vertical direction, based on a length of the width and a length of the height of the coding unit. Also, the image decoding apparatus  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 an area, which is of the coding unit. 
     According to an embodiment, the image decoding apparatus  100  may determine a shape of the coding unit by using the block shape information, and may determine which shape the coding unit is split into by using split shape mode information. That is, a coding unit splitting method indicated by the split shape mode information may be determined according to which block shape is indicated by the block shape information used by the image decoding apparatus  100 . 
     The image decoding apparatus  100  may determine the split shape mode information from a bitstream. However, the present disclosure is not limited thereto, and the image decoding apparatus  100  and the image encoding apparatus  200  may determine the split shape mode information that is predetermined based on the block shape information. The image decoding apparatus  100  may determine the split shape mode information that is predetermined for a largest coding unit or a smallest coding unit. For example, the image decoding apparatus  100  may determine that split shape mode information of a largest coding unit indicates quad-split. Also, the image decoding apparatus  100  may determine that split shape mode information of a smallest coding unit indicates “not to perform splitting”. For example, the image decoding apparatus  100  may determine a size of the largest coding unit to be 256×256. The image decoding apparatus  100  may determine that the predetermined split shape mode information indicates quad-split. The quad-split is a split shape mode in which a width and a height of a coding unit are halved. The image decoding apparatus  100  may obtain a coding unit having a size of 128×128 from the largest coding unit having a size of 256×256, based on the split shape mode information. Also, the image decoding apparatus  100  may determine a size of the smallest coding unit to be 4×4. The image decoding apparatus  100  may obtain the split shape mode information indicating “not to perform splitting” for the smallest coding unit. 
     According to an embodiment, the image decoding apparatus  100  may use the block shape information indicating that the current coding unit has a square shape. For example, the image decoding apparatus  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 of 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 to be 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 , and  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 apparatus  100  may determine two coding units  310   b  obtained by vertically splitting the current coding unit  300 , based on the split shape mode information indicating to vertically perform splitting. The image decoding apparatus  100  may determine two coding units  310   c  obtained by horizontally splitting the current coding unit  300 , based on the split shape mode information indicating to horizontally perform splitting. The image decoding apparatus  100  may determine four coding units  310   d  obtained by vertically and horizontally splitting the current coding unit  300 , based on the split shape mode information indicating to vertically and horizontally perform splitting. The image decoding apparatus  100  may determine three coding units  310   e  obtained by vertically splitting the current coding unit  300 , based on the split shape mode information indicating to vertically perform ternary splitting according to an embodiment. The image decoding apparatus  100  may determine three coding units  310   f  obtained by horizontally splitting the current coding unit  300 , based on the split shape mode information indicating to horizontally perform ternary splitting. However, split shapes by which the square coding unit is splittable are not limited to the aforementioned split shapes, and the split shape mode information may include various shapes. Preset split shapes by which the square coding unit is split will now be described in detail through various embodiments. 
       FIG.  4    illustrates a process, performed by the image decoding apparatus  100 , 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 apparatus  100  may use block shape information indicating that a current coding unit has a non-square shape. The image decoding apparatus  100  may determine, according to split shape mode information, whether not to split the non-square current coding unit or whether to split the non-square current coding unit by using a preset method. Referring to  FIG.  4   , when the block shape information of a current coding unit  400  or  450  indicates a non-square shape, the image decoding apparatus  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 now be described in detail through various embodiments. 
     According to an embodiment, the image decoding apparatus  100  may determine, by using the split shape mode information, a split shape by which a coding unit is to be split, and in this case, the split shape mode information may indicate the number of one or more coding units to be generated by splitting the 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 apparatus  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 apparatus  100  splits the non-square current coding unit  400  or  450  based on the split shape mode information, the image decoding apparatus  100  may split the current coding unit  400  or  450 , in consideration of the location of a long side of the non-square current coding unit  400  or  450 . For example, the image decoding apparatus  100  may determine a plurality of coding units by splitting the 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 apparatus  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 apparatus  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 a width to a height of the current coding unit  400  or  450  may be 4:1 or 1:4. When the ratio of the width to the height is 4:1, a length of the width is greater than a length of the height, and thus the block shape information may indicate a horizontal direction. When the ratio of the width to the height is 1:4, a length of the width is smaller than a length of the height, and thus the block shape information may indicate a vertical direction. The image decoding apparatus  100  may determine to split the current coding unit into an odd number of blocks, based on the split shape mode information. Also, the image decoding apparatus  100  may determine a split direction of the current coding unit  400  or  450 , based on the block shape information of the current coding unit  400  or  450 . For example, when the current coding unit  400  is in a vertical direction, the image decoding apparatus  100  may horizontally split the current coding unit  400  and may determine the coding units  430   a ,  430   b , and  430   c . Also, when the current coding unit  450  is in a horizontal direction, the image decoding apparatus  100  may vertically split the current coding unit  450  and may determine the coding units  480   a ,  480   b , and  480   c.    
     According to an embodiment, the image decoding apparatus  100  may determine an odd number of coding units included in the current coding unit  400  or  450 , and sizes of all of the determined coding units may not be the same. 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 sizes 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 an odd number of blocks, the image decoding apparatus  100  may determine an odd number of coding units included in the current coding unit  400  or  450 , and 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 apparatus  100  may allow a decoding method of the coding unit  430   b  or  480   b  to be different from that of the other coding units  430   a  and  430   c , or  480   a  and  480   c , wherein the coding unit  430   b  or  480   b  is at a center location from among the three coding units  430   a ,  430   b , and  430   c , or  480   a ,  480   b , and  480   c  generated by splitting the current coding unit  400  or  450 . For example, the image decoding apparatus  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 the image decoding apparatus  100 , 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 apparatus  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 apparatus  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 a coding unit is split. For example, the second coding unit may be determined by splitting the first coding unit, and the third coding unit may be determined by splitting the second coding unit. Hereinafter, it will be understood that a relationship among the first coding unit, the second coding unit, and the third coding unit applies to the following descriptions. 
     According to an embodiment, the image decoding apparatus  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 apparatus  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 , or  520   b ,  520   c , and  520   d  based on the split shape mode information, or may not split the non-square second coding unit  510 . The image decoding apparatus  100  may obtain the split shape mode information, and may split 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 of the first coding unit  500 , the second coding unit  510  may also be split into the third coding units  520   a , or  520   b ,  520   c , and  520   d , based on the split shape mode information of the second coding unit  510 . That is, a coding unit may be recursively split based on the split shape mode information of 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   b  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 split again into a plurality of coding units. For example, the non-square fourth coding unit  530   b  or  530   d  may be split again into an odd number of coding units. A method that may be used to recursively split a coding unit will be described below through various embodiments. 
     According to an embodiment, the image decoding apparatus  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 apparatus  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 apparatus  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 apparatus  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 apparatus  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 apparatus  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 aforementioned examples, and it should be interpreted that the restrictions 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 apparatus  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 the image decoding apparatus  100 , 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 of 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 of the split shape mode information may be obtained, is not limited to the center location in  FIG.  6   , and it should be interpreted that the preset location may include various locations (e.g., top, bottom, left, right, upper-left, lower-left, upper-right, lower-right locations, or the like) included in the current coding unit  600 . The image decoding apparatus  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 apparatus  100  may select one of the coding units. Various methods that may be used to select one of a plurality of coding units will be described below through various embodiments. 
     According to an embodiment, the image decoding apparatus  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, the image decoding apparatus  100  may use information indicating locations of an odd number of coding units so as to determine a coding unit at a center location from among the odd number of coding units. Referring to  FIG.  6   , the image decoding apparatus  100  may determine an odd number of coding units  620   a ,  620   b , and  620   c  or an 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 apparatus  100  may determine the coding unit  620   b  at a center location or the coding unit  660   b  at a center location by using information about 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 apparatus  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 apparatus  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 top-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 top-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 top-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 apparatus  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 top-left sample  630   a  of the upper coding unit  620   a  may include coordinates (xa, ya), information indicating the location of the top-left sample  530   b  of the middle coding unit  620   b  may include coordinates (xb, yb), and information indicating the location of the top-left sample  630   c  of the lower coding unit  620   c  may include coordinates (xc, yc). The image decoding apparatus  100  may determine the middle coding unit  620   b  by using the coordinates of the top-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 top-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 top-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 top-left sample  630   b  of the middle coding unit  620   b  and coordinates (dxc, dyc) indicating a relative location of the top-left sample  630   c  of the lower coding unit  620   c , with reference to the location of the top-left sample  630   a  of the upper coding unit  620   a . Also, 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 apparatus  100  may split the current coding unit  600  into the 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 apparatus  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 apparatus  100  may determine the widths or heights of the coding units  620   a ,  620   b , and  620   c  by using the coordinates (xa, ya) indicating the location of the top-left sample  630   a  of the upper coding unit  620   a , the coordinates (xb, yb) indicating the location of the top-left sample  630   b  of the middle coding unit  620   b , and the coordinates (xc, yc) indicating the location of the top-left sample  630   c  of the lower coding unit  620   c . The image decoding apparatus  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 apparatus  100  may determine the width of the upper coding unit  620   a  to be a width of the current coding unit  600 . The image decoding apparatus  100  may determine the height of the upper coding unit  620   a  to be yb-ya. According to an embodiment, the image decoding apparatus  100  may determine the width of the middle coding unit  620   b  to be a width of the current coding unit  600 . The image decoding apparatus  100  may determine the height of the middle coding unit  620   b  to be yc-yb. According to an embodiment, the image decoding apparatus  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  and the widths or heights of the upper and middle coding units  620   a  and  620   b . The image decoding apparatus  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 apparatus  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 apparatus  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 apparatus  100  may determine a width or a height of each of the coding units  660   a ,  660   b , and  660   c  by using coordinates (xd, yd) that are information indicating a location of a top-left sample  670   a  of the left coding unit  660   a , coordinates (xe, ye) that are information indicating a location of a top-left sample  670   b  of the middle coding unit  660   b , and coordinates (xf, yf) that are information indicating a location of a top-left sample  670   c  of the right coding unit  660   c . The image decoding apparatus  100  may determine 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 apparatus  100  may determine the width of the left coding unit  660   a  to be xe-xd. The image decoding apparatus  100  may determine the height of the left coding unit  660   a  as the height of the current coding unit  650 . According to an embodiment, the image decoding apparatus  100  may determine the width of the middle coding unit  660   b  to be xf-xe. The image decoding apparatus  100  may determine the height of the middle coding unit  660   b  to be the height of the current coding unit  600 . According to an embodiment, the image decoding apparatus  100  may determine the width or the height of the right coding unit  660   c  by using the width or the height of the current coding unit  650  and the width and the height of the left coding unit  660   a  and the middle coding unit  660   b . The image decoding apparatus  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 ,  660   b , and  660   c . Referring to  FIG.  6   , the image decoding apparatus  100  may determine the middle coding unit  660   b , which has a size different from the size of the left coding unit  660   a  and the right coding unit  660   c , as the coding unit of the preset location. However, the above-described method, performed by the image decoding apparatus  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 top-left locations, and information about arbitrary locations of samples included in the coding units may be used. 
     According to an embodiment, the image decoding apparatus  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 where a width of which is longer than a height, the image decoding apparatus  100  may determine the coding unit at the preset location in a horizontal direction. That is, the image decoding apparatus  100  may determine one of coding units at different locations in a horizontal direction and may put a restriction on the coding unit. When the current coding unit has a non-square shape where a height of which is longer than a width, the image decoding apparatus  100  may determine the coding unit at the preset location in a vertical direction. That is, the image decoding apparatus  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 apparatus  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 apparatus  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 with reference 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 apparatus  100  may use at least one of block shape information and split shape mode information, which is stored in a sample included in a coding unit at a center location, in a splitting operation to determine the coding unit at the center location from among the plurality of coding units determined by splitting the current coding unit. 
     Referring to  FIG.  6   , the image decoding apparatus  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 apparatus  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 of 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 kinds 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 apparatus  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 apparatus  100  may determine the sample at the preset location by considering a block shape of the current coding unit  600 , may 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 apparatus  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 is obtainable 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 is obtainable may be determined based on the shape. For example, the image decoding apparatus  100  may determine a sample located on a boundary for dividing at least one of a width and height of the current coding unit in half, as the sample from which the preset information is obtainable, 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 of the current coding unit indicates a non-square shape, the image decoding apparatus  100  may determine one of samples adjacent to a boundary for dividing a long side of the current coding unit in half, as the sample from which the preset information is obtainable. 
     According to an embodiment, when the current coding unit is split into a plurality of coding units, the image decoding apparatus  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 apparatus  100  may obtain the split shape mode information from a sample at a preset location in a coding unit, and may 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 with reference to  FIG.  5   , and thus detailed descriptions thereof will not be provided here. 
     According to an embodiment, the image decoding apparatus  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 the image decoding apparatus  100  determines the plurality of coding units by splitting a current coding unit, according to an embodiment. 
     According to an embodiment, the image decoding apparatus  100  may determine second coding units  710   a  and  710   b  by splitting a first coding unit  700  in a vertical direction, may determine second coding units  730   a  and  730   b  by splitting the first coding unit  700  in a horizontal direction, or may 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 apparatus  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 apparatus  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 apparatus  100  may determine to process 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 apparatus  100  may recursively split coding units. Referring to  FIG.  7   , the image decoding apparatus  100  may determine the plurality of second coding units  710   a ,  710   b ,  730   a ,  730   b ,  750   a ,  750   b ,  750   c , and  750   d  by splitting the first coding unit  700 , and may recursively split each of the determined plurality of second 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 second coding units  710   a ,  710   b ,  730   a ,  730   b ,  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 second coding units  710   a ,  710   b ,  730   a ,  730   b ,  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 apparatus  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 apparatus  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 apparatus  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 the image decoding apparatus  100 , 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 apparatus  100  may determine whether the current coding unit is to be 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 apparatus  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 an odd number of third coding units  820   c  to  820   e.    
     According to an embodiment, the image decoding apparatus  100  may determine whether any coding unit is to be 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 apparatus  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 apparatus  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 ,  820   b ,  820   c ,  820   d  and  820   e  is to be split into an odd number of coding units, based on at least one of block shape information and split shape mode information. For example, a second 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 apparatus  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 apparatus  100  may determine whether the third coding units  820   a  and  820   b , and  820   c ,  820   d , and  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 divided in half along a boundary of the third coding units  820   a  and  820   b , and  820   c ,  820   d , and  820   e . For example, the third coding units  820   a  and  820   b  determined by dividing the height of the non-square left second coding unit  810   a  in half may satisfy the condition. However, because boundaries of the third coding units  820   c ,  820   d , and  820   e  determined by splitting the right second coding unit  810   b  into three coding units do not divide the width or height of the right second coding unit  810   b  in half, it may be determined that the third coding units  820   c ,  820   d , and  820   e  do not satisfy the condition. When the condition is not satisfied as described above, the image decoding apparatus  100  may decide disconnection of a scan order, and 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 decision. According to an embodiment, when a coding unit is split into an odd number of coding units, the image decoding apparatus  100  may put a preset restriction on a coding unit at a preset location among the split coding units, and the restriction or the preset location has been described above through various embodiments and thus detailed descriptions thereof will not be provided here. 
       FIG.  9    illustrates a process, performed by the image decoding apparatus  100 , of determining at least one coding unit by splitting a first coding unit  900 , according to an embodiment. 
     According to an embodiment, the image decoding apparatus  100  may split the first coding unit  900 , based on split shape mode information, which is obtained by 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 apparatus  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 apparatus  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 apparatus  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 divided 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 divide 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. In addition, 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 divide 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 apparatus  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 apparatus  100  may put a preset restriction on a coding unit at a preset location from among the split coding units, and the restriction or the preset location has been described above through various embodiments and thus detailed descriptions thereof will not be provided here. 
     According to an embodiment, the image decoding apparatus  100  may determine various-shaped coding units by splitting a first coding unit. 
     Referring to  FIG.  9   , the image decoding apparatus  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 by the image decoding apparatus  100  is restricted when a second coding unit having a non-square shape, which is determined by splitting a first coding unit  1000 , satisfies a preset condition, according to an embodiment. 
     According to an embodiment, the image decoding apparatus  100  may determine to split the square first coding unit  1000  into non-square second coding units  1010   a ,  1010   b ,  1020   a , and  1020   b , based on split shape mode information, which is obtained by the receiver  110 . The second coding units  1010   a ,  1010   b ,  1020   a , and  1020   b  may be independently split. As such, the image decoding apparatus  100  may determine to split or not to split the first coding unit  1000  into a plurality of coding units, based on the split shape mode information of each of the second coding units  1010   a ,  1010   b ,  1020   a , and  1020   b . According to an embodiment, the image decoding apparatus  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 apparatus  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 ,  1012   b ,  1014   a , and  1014   b  may be determined. However, this case serves equally as a case in which the image decoding apparatus  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 apparatus  100  may determine third coding units  1022   a ,  1022   b ,  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 apparatus  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 the image decoding apparatus  100 , 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 apparatus  100  may determine second coding units  1110   a ,  1110   b ,  1120   a ,  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 shapes by which a coding unit is splittable but, the information about various shapes may not include information for splitting a coding unit into four square coding units. According to such split shape mode information, the image decoding apparatus  100  cannot split the first square coding unit  1100  into four square second coding units  1130   a ,  1130   b ,  1130   c , and  1130   d . The image decoding apparatus  100  may determine the non-square second coding units  1110   a ,  1110   b ,  1120   a ,  1120   b , etc., based on the split shape mode information. 
     According to an embodiment, the image decoding apparatus  100  may independently split the non-square second coding units  1110   a ,  1110   b ,  1120   a ,  1120   b , etc. Each of the second coding units  1110   a ,  1110   b ,  1120   a ,  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 apparatus  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 apparatus  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 apparatus  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 apparatus  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 apparatus  100  may split a first coding unit  1200 , based on split shape mode information. When a block shape is 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 apparatus  100  may determine second coding units  1210   a ,  1210   b ,  1220   a ,  1220   b , etc. by splitting the first coding unit  1200 . Referring to  FIG.  12   , the non-square second coding units  1210   a ,  1210   b ,  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 of each second coding unit. For example, the image decoding apparatus  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 horizontal direction. An operation of splitting the second coding units  1210   a ,  1210   b ,  1220   a , and  1220   b  has been described above with reference to  FIG.  11   , and thus detailed descriptions thereof will not be provided here. 
     According to an embodiment, the image decoding apparatus  100  may process coding units in a preset order. An operation of processing coding units in a preset order has been described above with reference to  FIG.  7   , and thus detailed descriptions thereof will not be provided here. Referring to  FIG.  12   , the image decoding apparatus  100  may determine four square 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 apparatus  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 splitting method of the first coding unit  1200 . 
     According to an embodiment, the image decoding apparatus  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 apparatus  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 ,  1210   b ,  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 apparatus  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 a 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 apparatus  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 apparatus  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 apparatus  100  may determine a second coding unit  1302 , a third coding unit  1304 , etc. 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 dividing 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 dividing 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 apparatus  100  may determine a second coding unit  1312  or  1322 , a third coding unit  1314  or  1324 , etc. 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 apparatus  100  may determine the second coding unit  1302 ,  1312 , or  1322  by dividing 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 apparatus  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 apparatus  100  may determine the second coding unit  1302 ,  1312 , or  1322  by dividing 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 apparatus  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 apparatus  100  may determine the third coding unit  1304 ,  1314 , or  1324  by dividing 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 apparatus  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 apparatus  100  may determine the third coding unit  1304 ,  1314 , or  1324  by dividing 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 apparatus  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 apparatus  100  may determine the third coding unit  1304 ,  1314 , or  1324  by dividing 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 apparatus  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 apparatus  100  may split the square coding unit  1300 ,  1302 , or  1304  in a horizontal or vertical direction. For example, the image decoding apparatus  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 apparatus  100  may determine various-shaped second coding units by splitting a square first coding unit  1400 . Referring to  FIG.  14   , the image decoding apparatus  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 apparatus  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 of the first coding unit  1400 . 
     According to an embodiment, a depth 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 , which are determined based on the split shape mode information of 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 apparatus  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 apparatus  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 apparatus  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 ,  1414   a ,  1414   b , and  1414   c ,  1422   a  and  1422   b , and  1424   a ,  1424   b , and  1424   c , which are determined based on the split shape mode information of 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 apparatus  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 apparatus  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 apparatus  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   , the coding unit  1414   b  of a center location among the 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 apparatus  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 apparatus  100  may determine whether to use a specific split shape, 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 apparatus  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 apparatus  100  may use PIDs to identify respective coding units. According to an embodiment, the PID may be obtained from a sample of a preset location (e.g., an upper-left sample) of each coding unit. 
     According to an embodiment, the image decoding apparatus  100  may determine a coding unit of 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 of 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 apparatus  100  may split the first coding unit  1410  into three coding units  1414   a ,  1414   b , and  1414   c . The image decoding apparatus  100  may assign a PID to each of the three coding units  1414   a ,  1414   b , and  1414   c . The image decoding apparatus  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 apparatus  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 of the center location from among the coding units determined by splitting the first coding unit  1410 . According to an embodiment, the image decoding apparatus  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 apparatus  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 apparatus  100  may split a current coding unit in such a manner that a coding unit of a preset location (e.g., a coding unit of a center location) among an odd number of coding units has a size different from that of the other coding units. In this case, the image decoding apparatus  100  may determine the coding unit of the center location, which has a different size, by using PIDs of the coding units. However, the PID and the size or location of the coding unit of the preset location to be determined 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 apparatus  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 apparatus  100  may split the current picture into a plurality of reference data units. According to an embodiment, the image decoding apparatus  100  may split the plurality of reference data units, which are split from the current picture, by using split shape mode information for 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 apparatus  100  may previously determine the minimum size allowed for the reference data units included in the current picture. Accordingly, the image decoding apparatus  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 apparatus  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, largest coding units, or the like). 
     According to an embodiment, the receiver  110  of the image decoding apparatus  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 determining one or more coding units included in the square reference coding unit  1500  has been described above in relation to the operation of splitting the current coding unit  300  of  FIG.  3   , and an operation of determining one or more coding units included in the non-square reference coding unit  1502  has been described above in relation to the operation of splitting the current coding unit  400  or  450  of  FIG.  4   , and thus, detailed descriptions thereof will not be provided here. 
     According to an embodiment, the image decoding apparatus  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, 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, largest coding units, or the like). The image decoding apparatus  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 predetermined. That is, the image decoding apparatus  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 predetermined at least one of the size and shape of reference coding units based on the PID. 
     According to an embodiment, the image decoding apparatus  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 apparatus  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  1600 , according to an embodiment. 
     According to an embodiment, the image decoding apparatus  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 an image, 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, 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 apparatus  100  may obtain processing block size information and may determine the size of one or more processing blocks included in the image. The image decoding apparatus  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 image. 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 apparatus  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, or slice segment. That is, the receiver  110  may obtain the processing block size information from the bitstream according to each of the various data units, the image decoding apparatus  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, and the size of the processing blocks may be integer times that of the reference coding units. 
     According to an embodiment, the image decoding apparatus  100  may determine the size of processing blocks  1602  and  1612  included in the picture  1600 . For example, the image decoding apparatus  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 apparatus  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 apparatus  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 apparatus  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 included 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 apparatus  100  may obtain, from the bitstream, determination order information of 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 apparatus  100  may obtain, from the bitstream, the determination order information of reference coding units according to each specific data unit. For example, the receiver  110  may obtain the determination order information of reference coding units from the bitstream according to each data unit such as an image, sequence, picture, slice, slice segment, or processing block. Because the determination order information of 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 apparatus  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 of reference coding units from the bitstream as information related to the processing blocks  1602  and  1612 , and the image decoding apparatus  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 apparatus  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 of reference coding units is obtained with respect to each processing block, different kinds of the determination order information of 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 the 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 apparatus  100  may decode the determined one or more reference coding units. The image decoding apparatus  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 apparatus  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 apparatus  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, or a slice segment header. Furthermore, the image decoding apparatus  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 present disclosure will be described in detail. 
     The image decoding apparatus  100  may determine a split rule of an image. The split rule may be pre-determined between the image decoding apparatus  100  and the image encoding apparatus  200 . The image decoding apparatus  100  may determine the split rule of the image, based on information obtained from a bitstream, The image decoding apparatus  100  may determine the split rule, based on information obtained from at least one of a sequence parameter set, a picture parameter set, a video parameter set, a slice header, and a slice segment header. The image decoding apparatus  100  may differently determine the split rule according to a frame, a slice, a temporal layer, a largest coding unit, or a coding unit. 
     The image decoding apparatus  100  may determine the split rule, based on a block shape of a coding unit. The block shape may include a size, a shape, a ratio of a width to a height, and a direction of the coding unit. The image encoding apparatus  200  and the image decoding apparatus  100  may previously determine to determine the split rule, based on the block shape of the coding unit. However, the present disclosure is not limited thereto. The image decoding apparatus  100  may determine the split rule, based on the information obtained from the bitstream received from the image encoding apparatus  200 . 
     The shape of the coding unit may include a square shape and a non-square shape. When the width and the height of the coding unit are the same, the image decoding apparatus  100  may determine that the shape of the coding unit is a square shape. Also, when the width and the height of the coding unit are not the same, the image decoding apparatus  100  may determine that the shape of the coding unit is a non-square shape. 
     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 256×256. The size of the coding unit may be classified according to the length of a long side, the length of a short side, or the area of the coding unit. The image decoding apparatus  100  may apply the same split rule to coding units classified in the same group. For example, the image decoding apparatus  100  may classify coding units whose long sides have the same length as coding units having the same size. Also, the image decoding apparatus  100  may apply the same split rule to coding units whose long sides have the same length. 
     The ratio of the width to the 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 or 1:32. Also, the direction of the coding unit may include a horizontal direction and a vertical direction. The horizontal direction may indicate a case where the length of the width of the coding unit is greater than the length of the height of the coding unit. The vertical direction may indicate a case where the length of the width of the coding unit is smaller than the length of the height of the coding unit. 
     The image decoding apparatus  100  may adaptively determine the split rule, based on the size of the coding unit. The image decoding apparatus  100  may differently determine an allowable split shape mode based on the size of the coding unit. For example, the image decoding apparatus  100  may determine whether splitting is allowed based on the size of the coding unit. The image decoding apparatus  100  may determine a split direction according to the size of the coding unit. The image decoding apparatus  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 pre-determined between the image encoding apparatus  2200  and the image decoding apparatus  100 . Also, the image decoding apparatus  100  may determine the split rule, based on the information obtained from the bitstream. 
     The image decoding apparatus  100  may adaptively determine the split rule based on a location of the coding unit. The image decoding apparatus  100  may adaptively determine the split rule based on the location of the coding unit in the image. 
     Also, the image decoding apparatus  100  may determine the split rule such that coding units generated via different split paths do not have the same block shape. However, the present disclosure is not limited thereto, and the coding units generated via different split paths may have the same block shape. The coding units generated via different split paths may have different decoding processing orders. A decoding processing order has been described with reference to  FIG.  12   , and thus detailed descriptions thereof are not provided here. 
     Hereinafter, with reference to  FIGS.  17  to  20   , a method and apparatus for encoding or decoding a video by configuring an additional mode set based on MPM modes of a current block, determining an intra prediction mode of the current block based on the MPM modes and the additional mode set, and performing intra prediction on the current block, based on the determined intra prediction mode according to an embodiment disclosed in the present disclosure application will now be described. 
       FIG.  17    illustrates a block diagram of a video decoding apparatus  1700 , according to an embodiment. 
     The video decoding apparatus  1700  according to an embodiment may include a memory  1710  and at least one processor  1720  connected with the memory  1710 . Operations of the video decoding apparatus  1700  according to an embodiment may be performed by separate processors or may be performed by the control of a central processor. Also, the memory  1710  of the video decoding apparatus  1700  may store data received from an external source, and data generated by a processor, e.g., information about the MPM modes of the current block and information about the configured additional mode set. 
     The processor  1720  of the video decoding apparatus  1700  may configure the additional mode set including at least one candidate mode, based on the MPM modes of the current block, may determine the intra prediction mode of the current block based on the MPM modes and the additional mode set, and may perform intra prediction, based on the intra prediction mode. 
     Hereinafter, with reference to  FIG.  18   , detailed operations of a video decoding method, performed by the video decoding apparatus  1700  according to an embodiment, of configuring an additional mode set based on MPM modes of a current block, determining an intra prediction mode of the current block based on the MPM modes and the additional mode set, and performing intra prediction will now be described. 
       FIG.  18    illustrates a flowchart of a video decoding method, according to an embodiment. 
     Referring to  FIG.  18   , in operation S 1810 , the video decoding apparatus  1700  may configure an additional mode set based on MPM modes of a current block. 
     According to an embodiment, the MPM modes of the current block may include a prediction mode of a neighboring block adjacent to the left side of the current block and a prediction mode of a neighboring block adjacent to the top side of the current block. 
     According to another embodiment, a neighboring block adjacent to the right side of the current block and a neighboring block adjacent to the top side of the current block may be reconstructed before the current block, and the MPM modes of the current block may include a prediction mode of the neighboring block adjacent to the right side of the current block and the prediction mode of the neighboring block adjacent to the top side of the current block. In a case where the neighboring block adjacent to the right side of the current block and the neighboring block adjacent to the top side of the current block exist, when the neighboring block adjacent to the left side of the current block simultaneously exists, an MPM mode may be configured by using information about the prediction mode of the left block and the prediction mode of the top block, and then an additional mode set may be configured by using information about the MPM mode and mode information of the right block. When mode information of the neighboring blocks adjacent in the left side, the top side, and the right side all exist, and some of the left block, the top block, and the right block are based on a same mode, a mode set may be configured by using only information about two different modes or may include more angular modes in directions close to index of duplicated mode from among modes of the neighboring blocks. 
     In operation S 1830 , the video decoding apparatus  1700  may determine an intra prediction mode of the current block, based on the MPM modes and the additional mode set. 
     According to an embodiment, as the intra prediction mode of the current block, an optimal intra prediction mode of the current block from among the MPM modes and modes of the additional mode set may be determined by using information obtained from a bitstream. 
     In operation S 1850 , the video decoding apparatus  1700  may perform intra prediction on the current block, based on the intra prediction mode. 
     According to an embodiment, the intra prediction mode of the current block is highly probable to have a relation to an intra prediction mode of a neighboring block, and thus, N intra prediction modes that are highly probable to become the intra prediction mode of the current block may constitute the additional mode set, by using mode information of the neighboring block or MPM information of the current block. The additional mode set may be configured of an intra prediction mode having an index that is increased by n from an intra prediction mode index of a left block mode, and an intra prediction mode having an index that is increased by m from an intra prediction mode index of a top block mode (where, n and m are non-zero integers). For example, when the left block and the top block are based on an angular mode, the additional mode set may be configured of an intra prediction mode having an index increased by 1 from the intra prediction mode index of the left block mode, an intra prediction mode having an index increased by 1 from the intra prediction mode index of the top block mode, an intra prediction mode having an index decreased by 1 from the intra prediction mode index of the left block mode, and an intra prediction mode having an index decreased by 1 from the intra prediction mode index of the top block mode. Alternatively, the additional mode set may be configured of the intra prediction mode having the index increased by 1 from the intra prediction mode index of the top block mode, an intra prediction mode having an index increased by 2 from the intra prediction mode index of the top block mode, an intra prediction mode having an index decreased by 2 from the intra prediction mode index of the left block mode, and an intra prediction mode having an index obtained by averaging intra prediction mode indices of the left block mode and the top block mode and then rounding off a result thereof. 
       FIG.  21    illustrates an embodiment of modes of left and top neighboring blocks and a mode set of the current block. 
     Referring to  FIG.  21   , when intra prediction modes of a top neighboring block  2120  adjacent to the top side of a current block  2110  and a left neighboring block  2130  adjacent to the left side of the current block  2110  are a vertical mode and a horizontal mode, respectively, an MPM mode of the current block  2110  may be determined as the vertical mode and the horizontal mode, and an additional mode set may include modes that have a high relation to the MPM mode and are close to the MPM mode, the modes being from among modes between the vertical mode and the horizontal mode. For example, the additional mode set may be configured of an intra prediction mode having an index increased by 1 from an intra prediction mode index of the vertical mode, an intra prediction mode having an index increased by 2 from the intra prediction mode index of the vertical mode, an intra prediction mode having an index decreased by 2 from an intra prediction mode index of the horizontal mode, an intra prediction mode having an index decreased by 1 from the intra prediction mode index of the horizontal mode, and an intra prediction mode having an index obtained by averaging and then rounding off intra prediction mode indices of the vertical mode and the horizontal mode. 
     According to an embodiment, the additional mode set may include N modes (where, N is a predetermined integer) according to the number of intra prediction modes or the number of MPMs. 
     According to an embodiment, the additional mode set may differently include N or M (where, N and M are positive integers) modes, according to a type of an intra prediction mode of a neighboring block adjacent to a current block. In detail, the additional mode set may vary according to a case where an intra prediction mode of the neighboring block is an angular mode and a case where an intra prediction mode of the neighboring block is a non-angular mode such as a DC mode, a planar mode, or the like. 
       FIG.  22    illustrates an embodiment of intra prediction mode directions. 
     In detail, a vertical direction  2240  that is a y-axis positive direction and a horizontal direction  2220  that is an x-axis negative direction, from among intra prediction directions of an intra prediction mode of  FIG.  22   , indicate a vertical mode and a horizontal mode of the intra prediction mode, respectively.  FIG.  22    also illustrates, from among the intra prediction directions, a diagonal angular mode  2250  in an upper right quadrant which is a last direction of the intra prediction directions, a diagonal angular mode  2210  in a lower left quadrant which is a start direction of an angular mode, and a diagonal mode  2230  that is perpendicular to a direction of the angular mode  2250  which is a 45-degree direction. The direction of the angular mode  2250  of the intra prediction mode of  FIG.  22    may be a direction other than the 45-degree direction. 
     According to an embodiment, the intra prediction mode may include 67 modes. In detail, the intra prediction mode may include a DC mode, a planar mode, and 65 angular modes. The intra prediction mode may be distinguished therein by indicating indices of the intra prediction mode to be 0 to 66 (0 indicates the planar mode, 1 indicates the DC mode, and 2 to 66 indicate angular modes). 
     Referring to  FIG.  22   , a mode of the intra prediction mode, the mode having an index of 2, may be the angular mode  2210  in a direction opposite to the angular mode  2250  in the 45-degree direction, a mode of the intra prediction mode, the mode having an index of 34, may be the diagonal mode  2230  that is perpendicular to the direction of the angular mode  2250  which is the 45-degree direction, a mode of the intra prediction mode, the mode having an index of 66, may be the angular mode  2250  in the 45-degree direction, an index of the horizontal direction  2220  that is the x-axis negative direction may be 18, and an index of the vertical direction  2240  that is the y-axis positive direction may be 50. 
     According to an embodiment, when the number of MPM modes is 2 and the number of other intra prediction modes is 65, an additional mode set may include 4 or 8 modes according to types of modes selected to be the MPM modes. 
     Table 1 below illustrates an example of the additional mode set configured according to types of two MPM modes when MPM[0] and MPM[1] respectively indicate intra prediction modes designated as MPM modes. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 When both MPMs are 
                 When both MPMs 
               
               
                   
                 non-angular modes. 
                 are angular modes. 
               
               
                   
                   
               
             
            
               
                   
                 DC or planar mode 
                 Planar mode 
               
               
                   
                 Vertical mode 
                 DC mode 
               
               
                   
                 Horizontal mode 
                 MPM[0] − 2 
               
               
                   
                 Diagonal mode 
                 MPM[0] − 1 
               
               
                   
                   
                 MPM[0] + 1 
               
               
                   
                   
                 MPM[1] − 2 
               
               
                   
                   
                 MPM[1] − 1 
               
               
                   
                   
                 MPM[1] + 1 
               
               
                   
                   
               
            
           
         
       
     
     In detail, when two MPM modes are all non-angular modes, an additional mode set may include 4 modes that are DC or planar mode, a vertical mode, a horizontal mode, and a diagonal mode. Alternatively, the additional mode set may include at least one of the DC or planar mode, the vertical mode, the horizontal mode, and the diagonal mode. 
     In another embodiment, when two MPM modes are all non-angular modes, the additional mode set may include at least one of the DC or planar mode, the vertical mode, the horizontal mode, an intra prediction mode of an index increased by 4 from an intra prediction mode index of the horizontal mode, and, intra prediction mode of an index decreased by 4 from the intra prediction mode index of the horizontal mode. 
     When two MPM modes are all angular modes, the additional mode set may include 8 modes that are a planar mode, a DC mode, an intra prediction mode (MPM[0]-2) having an index decreased by 2 from an index of MPM[0] mode, an intra prediction mode (MPM[0]-1) having an index decreased by 1 from the index of MPM[0] mode, an intra prediction mode (MPM[0]+1) having an index increased by 1 from the index of MPM[0] mode, an intra prediction mode (MPM[1]-2) having an index decreased by 2 from an index of MPM[1] mode, an intra prediction mode (MPM[1]-1) having an index decreased by 1 from the index of MPM[1] mode, and an intra prediction mode (MPM[1]+1) having an index increased by 1 from the index of MPM[1] mode. Alternatively, the additional mode set may include at least one of the planar mode, the DC mode, the intra prediction mode (MPM[0]-2) having the index decreased by 2 from the index of MPM[0] mode, the intra prediction mode (MPM[0]-1) having the index decreased by 1 from the index of MPM[0] mode, the intra prediction mode (MPM[0]+1) having the index increased by 1 from the index of MPM[0] mode, the intra prediction mode (MPM[1]-2) having the index decreased by 2 from the index of MPM[1] mode, the intra prediction mode (MPM[1]-1) having the index decreased by 1 from the index of MPM[1] mode, and the intra prediction mode (MPM[1]+1) having the index increased by 1 from the index of MPM[1] mode. 
     Alternatively, when an intra prediction mode (a candidate of MPM[0]) of a neighboring block adjacent to the left side of a current block and an intra prediction mode (a candidate of MPM[1]) of a neighboring block adjacent to the top side of the current block are same angular mode, MPM[0] may be determined to be the intra prediction mode of the neighboring block adjacent to the left side of the current block, and the rest of MPM modes and an additional mode set may include 5 modes that are a planar mode, a DC mode, an intra prediction mode (MPM[0]-2) having an index decreased by 2 from an index of MPM[0] mode, an intra prediction mode (MPM[0]-1) having an index decreased by 1 from the index of MPM[0] mode, and an intra prediction mode (MPM[0]+1) having an index increased by 1 from the index of MPM[0] mode. Alternatively, MPM[0] may be determined to be the intra prediction mode of the neighboring block adjacent to the left side of the current block, and the rest of MPM modes and the additional mode set may include at least one of the planar mode, the DC mode, the intra prediction mode (MPM[0]-2) having the index decreased by 2 from the index of MPM[0] mode, the intra prediction mode (MPM[0]-1) having the index decreased by 1 from the index of MPM[0] mode, and the intra prediction mode (MPM[0]+1) having the index increased by 1 from the index of MPM[0] mode. 
     In another embodiment, when two MPM modes are all angular modes, the additional mode set may include at least one of the DC mode, the planar mode, an intra prediction mode having an index increased by 1 from an intra prediction mode index of an MPM mode which have greater intra prediction mode index from among the MPM modes and an intra prediction mode having an index decreased by 1 from the intra prediction mode index of the MPM mode which have greater intra prediction mode index from among the MPM modes, or an intra prediction mode having an index increased by 2 from the intra prediction mode index of the MPM mode which have greater intra prediction mode index from among the MPM modes and an intra prediction mode having an index decreased by 2 from the intra prediction mode index of the MPM mode which have greater intra prediction mode index from among the MPM modes. 
     In another embodiment, when two MPM modes are all angular modes, the additional mode set may include at least one of the DC mode, the planar mode, and intra prediction modes having index increased by n from index of each of the MPM modes, wherein n is a non-zero integer. 
     Table 2 below shows an example of an additional mode set configured when one of the MPM modes is a non-angular mode and the other one is an angular mode, i.e., when MPM[0] is a non-angular mode and MPM[1] is an angular mode. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Case where angular + non- 
                 Case where angular + non- 
               
               
                 angular modes (4 modes) 
                 angular modes (8 modes) 
               
               
                 DC or planar mode 
                 DC or planar mode 
               
               
                 (Mode different from MPM[0]) 
                 (Mode different from MPM[0]) 
               
               
                   
               
             
            
               
                 MPM[1] − 2 
                 MPM[1] + 2 
               
               
                 MPM[1] + 2 
                 MPM[1] + 1 
               
               
                 Default mode 
                 MPM[1] − 1 
               
               
                   
                 MPM[1] − 2 
               
               
                   
                 Default mode 
               
               
                   
                 Default mode 
               
               
                   
                 Default mode 
               
               
                   
               
            
           
         
       
     
     In detail, when one of two MPM modes is a non-angular mode and the other one is an angular mode, the additional mode set may include 4 modes that are a mode is different from MPM[0] that is a non-angular mode from among “DC or planar mode”, an intra prediction mode having an index decreased by 2 from a mode index of MPM[1] that is an angular mode, an intra prediction mode having an index increased by 2 from the mode index of MPM[1], and a default mode. Alternatively, the additional mode set may include at least one of 4 modes that are the mode that is from among “DC or planar mode” and is different from MPM[0] that is a non-angular mode, the intra prediction mode having the index decreased by 2 from the mode index of MPM[1] that is an angular mode, the intra prediction mode having the index increased by 2 from the mode index of MPM[1], and the default mode. 
     Also, the additional mode set may include 8 modes that are 3 default modes, the mode that is from among “DC or planar mode” and is different from MPM[0] that is a non-angular mode, the intra prediction mode having the index decreased by 2 from the mode index of MPM[1] that is an angular mode, an intra prediction mode having an index decreased by 1 from the mode index of MPM[1], an intra prediction mode having an index increased by 1 from the mode index of MPM[1], and the intra prediction mode having the index increased by 2 from the mode index of MPM[1]. Alternatively, the additional mode set may include 3 default modes, the mode that is from among “DC or planar mode” and is different from MPM[0] that is a non-angular mode, a mode of decreasing, by 2, the mode index of MPM[1] that is an angular mode, a mode of decreasing, by 1, the mode index of MPM[1], a mode of increasing, by 1, the mode index of MPM[1], and a mode of increasing, by 2, the mode index of MPM[1]. 
     According to an embodiment, the additional mode set may include at least one of the mode that is from among “DC or planar mode” and is different from MPM[0] that is a non-angular mode, the intra prediction mode having the index decreased by 2 from the mode index of MPM[1] that is an angular mode, the intra prediction mode having the index decreased by 1 from the mode index of MPM[1], and the intra prediction mode having the index increased by 1 from the mode index of MPM[1]. 
     According to an embodiment, a first MPM mode from among the MPM modes may be a non-angular mode, a second MPM mode may be an angular mode, and the additional mode set may include at least one of a mode different from the first MPM mode from among the DC mode and the planar mode, an intra prediction mode having an index increased by n from an index of the second MPM mode, a vertical mode, and a horizontal mode, wherein n is a non-zero integer. 
     According to an embodiment, the default mode may be selected from a default mode candidate list pre-configured to prepare a case in which candidates to be included in the additional mode set are decreased, the case including a case where at least two candidate modes are overlapped or a case where at least one of a mode or an MPM mode of a neighboring block is a non-angular mode. In detail, the default mode candidate list may include intra prediction modes that are frequently selected in the statistics, and an order of candidates included in the list may be listed from a mode having a higher probability. 
     According to an embodiment, for the default mode, an additional mode set may be configured in a manner that modes that have been used up to a current block in a current frame are counted and then intra prediction modes that are frequently selected are adaptively selected. Alternatively, the default mode may be selected by using a history about an intra prediction mode selected for a previous intra block of the current block in the current frame, or may be selected by applying priority numbers to modes of blocks adjacent to the current block. 
       FIG.  23    illustrates an embodiment of an additional mode set. 
     Referring to  FIG.  23   , the additional mode set may include a vertical direction  2340  that is a y-axis positive direction, a horizontal direction  2320  that is an x-axis negative direction, an angular mode  2350  in a 45-degree direction, an angular mode  2310  that is a direction opposite to the angular mode  2350  in a 45-degree direction, a diagonal mode  2330  that is perpendicular to a direction of the angular mode  2350  which is a 45-degree direction, an angular mode  2370  in a direction by which an angle between the vertical direction  2340  and the angular mode  2350  in a 45-degree direction is halved, and an angular mode  2360  in a direction by which an angle between the horizontal direction  2320  and the angular mode  2310  that is a direction opposite to the angular mode  2350  in a 45-degree direction is halved. In a case where an angle between two angular modes is halved, when there is no direction exactly corresponding thereto, an angular mode in the most adjacent direction may be used or an intra prediction mode having an index obtained by averaging respective intra prediction mode indices and then rounding off a result thereof may be used. 
       FIGS.  19  and  20    illustrate a block diagram of a video encoding apparatus  1900  according to an embodiment and a flowchart of a video encoding method according to an embodiment that respectively correspond to a video decoding apparatus and a video decoding method described above. 
     The video encoding apparatus  1900  according to an embodiment may include a memory  1910  and at least one processor  1920  connected with the memory  1910 . Operations of the video encoding apparatus  1900  according to an embodiment may be performed by separate processors or may be performed by the control of a central processor. Also, the memory  1910  of the video encoding apparatus  1900  may store data received from an external source, and data generated by a processor, e.g., information about the MPM modes of the current block and information about the configured additional mode set. 
     The processor  1920  of the video encoding apparatus  1900  may configure the additional mode set, based on the MPM modes of the current block, may determine the intra prediction mode of the current block based on the MPM modes and the additional mode set, and may perform intra prediction, based on the intra prediction mode. 
     Hereinafter, with reference to  FIG.  20   , detailed operations of the video encoding method, performed by the video encoding apparatus  1900  according to an embodiment, of configuring an additional mode set based on MPM modes of a current block, determining an intra prediction mode of the current block based on the MPM modes and the additional mode set, and performing intra prediction will now be described. 
       FIG.  20    illustrates a flowchart of a video encoding method, according to an embodiment. 
     Referring to  FIG.  20   , in operation S 2010 , the video encoding apparatus  1900  may configure an additional mode set based on MPM modes of a current block. 
     According to an embodiment, the MPM modes of the current block may be determined by using prediction modes of a neighboring block. 
     In operation S 2030 , the video encoding apparatus  1900  may determine an intra prediction mode of the current block, based on the MPM modes and the additional mode set. 
     According to an embodiment, the video encoding apparatus  1900  may encode intra prediction mode information by comparing the MPM modes and modes of the additional mode set with an optimal intra prediction mode determined in the current block. The optimal intra prediction mode of the current block may be determined by calculating rate-distortion cost of the current block. 
     In operation S 2050 , the video encoding apparatus  1900  may perform intra prediction on the current block, based on the intra prediction mode. 
       FIG.  24    illustrates an embodiment of intra prediction mode signaling syntax. 
     Referring to  FIG.  24   , when an MPM flag indicates 1, intra prediction is performed by using an MPM mode. When an MPM flag indicates 0, a flag of an additional mode set is checked and thus, when the flag of the additional mode set indicates 1, intra prediction is performed by using N modes of the additional mode set, and when the flag of the additional mode set indicates 0, intra prediction may be performed by using other remaining intra prediction mode. When the number of modes of the additional mode set is N, the flag of the additional mode set may be coded with allocated log 2 (N) bits and then may be signaled. 
     According to an embodiment, a bit amount may be decreased by using unary coding or truncated unary coding according to the number of the modes of the additional mode set and probability that each mode is to be selected. 
     According to an embodiment, a same bit may be allocated to the additional mode set, and each bit may be efficiently coded using context modeling and then may be signaled. 
     According to an embodiment, when an intra prediction mode that is not a mode of the MPM mode and the additional mode set is selected, a bit is allocated except for the MPM mode and the modes of the additional mode set, such that the bit may be efficiently coded and then signaled. 
     According to an embodiment, a block unit flag may be used for the additional mode set, equally to the MPM flag. 
     According to an embodiment, whether to use only an MPM or also use an additional mode set may be determined based on an image. 
     According to an embodiment, a flag for determining whether to use an additional mode set may be transmitted in a unit of a frame. 
     According to an embodiment, whether to use an additional mode set and the number of modes of the additional mode set may be differently applied, according to sizes of blocks. 
     The disclosure has been particularly shown and described with reference to embodiments thereof. In this regard, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the disclosure. Therefore, the embodiments should be considered in a descriptive sense only and not for purposes of limitation. The scope of the disclosure is defined not by the detailed descriptions of the disclosure but by the following claims, and all differences within the scope will be construed as being included in the disclosure. 
     Meanwhile, the aforedescribed embodiments of the disclosure can be written as a program executable on a computer, and can be implemented in general-use digital computers that execute the program by using a computer-readable recording medium. Examples of the computer-readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, or DVDs), or the like.