Patent Publication Number: US-2015071351-A1

Title: Inter prediction method in which reference picture lists can be changed and apparatus for the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage Entry of PCT/KR2013/003083, filed on Apr. 12, 2013, which claims priority to U.S. provisional patent application No. 61/624,355, filed on Apr. 15, 2012 in the U.S. Patent and Trademark Office, the entire disclosures of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The exemplary embodiments relate to video prediction and video encoding and decoding that are accompanied by video prediction. 
     BACKGROUND OF THE RELATED ART 
     As hardware for reproducing and storing high resolution or high quality video content is being developed and supplied, a need for a video codec for effectively encoding or decoding the high resolution or high quality video content is increasing. According to a related art video codec, a video is encoded according to a limited encoding method based on a macroblock having a predetermined size. 
     A video codec reduces an amount of encoded data by using a prediction method based on a characteristic whereby video images are temporally or spatially correlated to each other. According to the prediction method, to predict a current image by using peripheral images, temporal distances or spatial distances and prediction errors between images are used to record image information. 
     SUMMARY 
     The exemplary embodiments provide a method and apparatus for inter prediction using modification of a reference picture list. 
     The exemplary embodiments also provide a method of efficiently encoding, transmitting, receiving, and reading information related to modification of a reference picture list. 
     According to an aspect of an exemplary embodiment, there is provided an inter prediction method including: determining whether a restriction condition is provided which restricts an identity of reference picture lists to be used for slices of a same slice type among slices included in a same picture; determining a modification possibility of reference picture lists determined for a same slice among the slices of the same slice type; and determining reference picture lists for blocks in an inter mode that belong to the same slice based on the determining of whether the restriction condition is provided and the determining of the modification possibility of the reference picture lists. 
     The determining of the modification possibility of the reference picture lists may include determining the modification possibility of the reference picture lists independently from determining whether the restriction condition is provided. 
     The determining of whether the restriction condition is provided may include determining whether the restriction condition is provided for reference picture lists determined for blocks that belong to a same sequence, and the determining of the modification possibility of the reference picture lists may include determining the modification possibility of the reference picture lists determined for the blocks that belong to the same sequence. 
     The inter prediction method may further include determining a combination possibility of a first reference picture list and a second reference picture list that are determined for the blocks that belong to the same sequence, independently from the modification possibility of the reference picture lists. 
     The determining of the combination possibility may include determining whether it is possible to combine reference picture lists determined for a current slice, and if the determining of whether it is possible to combine the reference picture lists indicates that it is possible to combine the reference picture lists, determining whether combined reference picture lists, which are combined by using reference pictures of the first reference picture list and the second reference picture list for each slice among the slices of the same slice type, are provided; and if the determining of whether the combined reference picture lists are provided indicates that the combined reference picture lists are provided in the current slice, determining a modification possibility of the combined reference picture lists for each slice. 
     The determining of whether the restriction condition is provided may include parsing, from a sequence parameter set (SPS) of a received bitstream, information indicating whether the restriction condition is provided; and the determining of the modification possibility of the reference picture lists may include parsing information indicating the modification possibility of the reference picture lists from the SPS independently from the determining of whether the restriction condition is provided. 
     The determining of the combination possibility may include parsing information indicating the combination possibility of the first reference picture list and the second reference picture list from an SPS of a received bitstream, and the determining of whether the combined reference picture lists are provided may include parsing, from a slice header of the received bitstream, information indicating whether the combined reference picture lists are provided, and the determining of the modification possibility of the combined reference picture lists may include parsing, from the slice header, information indicating the modification possibility of the combined reference picture lists. 
     The determining of the reference picture lists may include parsing a reference index, a motion vector, and a residue for each of the blocks in the inter mode that belong to the picture from the received bitstream; determining at least one of a first reference picture list and a second reference picture list by using the parsed reference index; determining at least one reference picture for each of the blocks by using the determination of whether the restriction condition is provided, the determined modification possibility, and the determined at least one of the first reference picture list and the second reference picture list; and performing motion compensation by using the determined reference index, the determined motion vector, and the determined residue for each of the blocks and generating a reconstruction image based on the motion compensation. 
     The inter prediction method may further include encoding first information indicating whether the restriction condition is provided and inserting the encoded first information into an SPS of a bitstream; and encoding second information indicating the modification possibility of the reference picture lists and inserting the encoded second information into the SPS, independently from the determining of whether the restriction condition is provided. 
     The inter prediction method may further include encoding third information indicating a combination possibility of a first reference picture list and a second reference picture list and inserting the encoded third information into the SPS; encoding fourth information indicating whether combined reference picture lists are provided and inserting the encoded fourth information into a slice header of the bitstream; and encoding fifth information indicating a modification possibility of the combined reference picture lists and inserting the fifth information into the slice header. 
     The inter prediction method may further include determining at least one reference picture for each of the blocks by using the determined reference picture lists; performing motion prediction by using the determined at least one reference picture for each of the blocks and determining a residue; and encoding a reference index indicating the determined at least one reference picture for each of the blocks and the residue and inserting the encoded reference index and the encoded residue into a block of the bitstream. 
     According to another aspect of an exemplary embodiment, there is provided an inter prediction apparatus including: a reference picture list determiner configured to determine whether a restriction condition is provided which restricts an identity of reference picture lists to be used for slices of a same slice type among slices included in a same picture, determine a modification possibility of reference picture lists determined for a same slice among the slices of the same slice type, and determine reference picture lists for blocks in an inter mode that belong to the same slice based on the determination of whether the restriction condition is provided and the determined modification possibility of the reference picture lists; and an inter predictor configured to perform inter prediction for each of the blocks by using the determined reference picture lists. 
     The reference picture list determiner may be configured to determine at least one of a first reference picture list and a second reference picture list based on a reference index determined for each of the blocks in the inter mode that belong to the same picture, and to determine at least one reference picture for each of the blocks based on at least one of the determination of whether the restriction condition is provided, the determined modification possibility, and the determined at least one of the first reference picture list and the second reference picture list, and the inter predictor may be configured to perform motion compensation by using the determined at least one reference picture for each of the blocks, a motion vector, and a residue, and generate a reconstruction image based on the motion compensation. 
     The reference picture list determiner may be configured to determine at least one of a first reference picture list and a second reference picture list based on a reference index determined for each of the blocks in the inter mode that belong to the same picture, and determine at least one reference picture for each of the blocks based on at least one of the determination of whether the restriction condition is provided, the determined modification possibility, and the determined at least one of the first reference picture list and the second reference picture list, and the inter predictor may be configured to perform motion prediction by using the determined at least one reference picture for each of the blocks, and generate a reconstruction image based on the motion prediction. 
     According to another aspect of an exemplary embodiment, there is provided a non-transitory computer readable recording medium having recorded thereon a program for executing the inter prediction method according to an aspect of an exemplary embodiment. 
     Inter prediction according to the exemplary embodiments determines a modification possibility of a reference picture list, independently from whether there is a restriction condition that limits the reference picture list according to slice types, and thus a process of transmitting information related to an unnecessary reference picture list is omitted, thereby reducing a transmission bit rate. Also, a process of parsing the information related to the unnecessary reference picture list is omitted, thereby reducing a data parsing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of an inter prediction apparatus according to an exemplary embodiment; 
         FIG. 1B  is a flowchart of an inter prediction method according to an exemplary embodiment; 
         FIG. 2A  is a block diagram of a motion prediction apparatus corresponding to an inter prediction apparatus according to an exemplary embodiment; 
         FIG. 2B  is a flowchart of a motion prediction method of performing an inter prediction method according to an exemplary embodiment; 
         FIG. 3A  is a block diagram of a motion compensation apparatus corresponding to an inter prediction apparatus according to an exemplary embodiment; 
         FIG. 3B  is a flowchart of a motion compensation method of performing an inter prediction method according to an exemplary embodiment; 
         FIG. 4  is a diagram of a syntax of a sequence parameter set (SPS) according to an exemplary embodiment; 
         FIG. 5  is a diagram of a syntax of an SPS according to another exemplary embodiment; 
         FIG. 6  is a diagram of a syntax of an SPS according to another exemplary embodiment; 
         FIG. 7  is a diagram of a syntax of a parameter set related to a combination of reference picture lists according to another exemplary embodiment; 
         FIG. 8  is a diagram of a syntax of a parameter set related to a modification of reference picture lists according to another exemplary embodiment; 
         FIG. 9  is a block diagram of a video encoding apparatus configured to perform video prediction based on coding units having a tree structure according to an exemplary embodiment; 
         FIG. 10  is a block diagram of a video decoding apparatus configured to perform video prediction based on coding units having a tree structure according to an exemplary embodiment; 
         FIG. 11  is a diagram for describing a concept of coding units according to an exemplary embodiment; 
         FIG. 12  is a block diagram of an image encoder based on coding units according to an exemplary embodiment; 
         FIG. 13  is a block diagram of an image decoder based on coding units according to an exemplary embodiment; 
         FIG. 14  is a diagram illustrating deeper coding units according to depths, and partitions according to an exemplary embodiment; 
         FIG. 15  is a diagram for describing a relationship between a coding unit and transformation units, according to an exemplary embodiment; 
         FIG. 16  is a diagram for describing encoding information of coding units corresponding to a coded depth, according to an exemplary embodiment; 
         FIG. 17  is a diagram of deeper coding units according to depths, according to an exemplary embodiment; 
         FIGS. 18 ,  19  and  20  are diagrams for describing a relationship between coding units, prediction units, and transformation units, according to an exemplary embodiment; 
         FIG. 21  is a diagram for describing a relationship between a coding unit, a prediction unit or a partition, and a transformation unit, according to encoding mode information of Table 1; 
         FIG. 22  illustrates a physical structure of a disc that stores a program, according to an exemplary embodiment; 
         FIG. 23  illustrates a disc drive that records and reads a program by using a disc; 
         FIG. 24  illustrates an entire structure of a content supply system that provides a content distribution service; 
         FIGS. 25 and 26  illustrate external and internal structures of a mobile phone to which a video encoding method and a video decoding method are applied, according to an exemplary embodiment; 
         FIG. 27  illustrates a digital broadcasting system employing a communication system, according to an exemplary embodiment; and 
         FIG. 28  illustrates a network structure of a cloud computing system using a video encoding apparatus and a video decoding apparatus, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Hereinafter, an inter prediction method and apparatus capable of bi-prediction by using a bi-directional reference picture list, a motion prediction method and apparatus corresponding to the inter prediction method and apparatus, and a motion compensation method and apparatus will be described with reference to  FIGS. 1A through 8 . Video encoding and decoding apparatuses and video encoding and decoding methods according to an exemplary embodiment based on coding units having a tree structure will be described with reference to  FIGS. 9 through 21 . In addition, various exemplary embodiments to which video encoding and decoding methods according to an exemplary embodiment are applicable will be described with reference to  FIGS. 22 through 28 . Hereinafter, the term ‘image’ may refer to a still image or a moving picture, that is, a video. 
     First, with reference to  FIGS. 1A through 8 , an inter prediction method and apparatus capable of bi-prediction by using a bi-directional reference picture list, a motion prediction method and apparatus corresponding to the inter prediction method and apparatus, and a motion compensation method and apparatus according to exemplary embodiments will be described. 
       FIG. 1A  is a block diagram of an inter prediction apparatus  10  according to an exemplary embodiment.  FIG. 1B  is a flowchart of an inter prediction method according to an exemplary embodiment. 
     The inter prediction apparatus  10  according to an exemplary embodiment may include a reference picture list determiner  12  and an inter predictor  14 . 
     The inter prediction apparatus  10  according to an exemplary embodiment may include a central processor (not shown) for generally controlling the reference picture list determiner  12  and the inter predictor  14 . Alternatively, the reference picture list determiner  12  and the inter predictor  14  may be controlled by respective processors (not shown) and the processors may cooperatively interact with each other so as to control an overall operation of the inter prediction apparatus  10 . Alternatively, the reference picture list determiner  12  and the inter predictor  14  may be controlled according to control of an external processor (not shown) of the inter prediction apparatus  10 . 
     The inter prediction apparatus  10  according to an exemplary embodiment may include at least one data storage unit (not shown) for storing data that is input to and output from the reference picture list determiner  12  and the inter predictor  14 . The inter prediction apparatus  10  may include a memory controller (not shown) for controlling input/output of data of a data storage unit (not shown). 
     The inter prediction apparatus  10  according to an exemplary embodiment performs temporal prediction on video images. The inter prediction apparatus  10  determines prediction information indicating temporal distances between a current image and peripheral images, residues, etc. Thus, the prediction information, instead of entire data of images, may be used to record image information. 
     According to temporal prediction encoding, the current image may be predicted by temporally referring to previous and subsequent images. The current image and the reference images may be image data units including pictures, frames, fields, slices, etc. 
     The inter prediction apparatus  10  according to an exemplary embodiment may split the current image into a plurality of blocks for rapid calculation of inter prediction and perform inter prediction on the blocks. That is, for performing inter prediction on a current block among the plurality of blocks split from the current image, one of the plurality of blocks split from the current image may be referred to. 
     Reference picture lists may be divided into L0 lists and L1 lists according to reference directions. For example, a reference picture list for forward prediction of a P slice type image may include an L0 list for list 0 prediction. A reference picture list for a B slice type image capable of bi-prediction including forward prediction, backward prediction, bi-directional prediction, etc. may include an L1 list for list 1 prediction as well as the L0 list. 
     The L0 list and the L1 list may each include an index indicating one or more reference images and reference order information. A basic valid number of reference images allocated to the reference picture list may be previously limited. However, the number or the reference order of the reference images may be modified for each image according to necessity. Thus, the inter prediction apparatus  10  may set information regarding the basic valid number of the reference images of the reference picture list, information regarding a modification of the number of the reference images, information regarding a modification of the reference images, information regarding a modification of the reference order, etc. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine if at least one reference picture list among the L0 list and the L1 list is used in the current image. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine a modification possibility of the reference images included in the reference picture list in the current image. If the modification of the reference images is possible, the reference picture list determiner  12  may determine how the reference picture list is modified. 
     The reference picture list may include information regarding the reference images and the reference order of the reference images. For example, the L0 list may preferentially include an index indicating reference images for forward prediction, and the L1 list may preferentially include an index indicating reference images for backward prediction. However, the L0 list and the L1 list are not necessarily limited to including reference information only for forward prediction and backward prediction, respectively. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine the reference order of the reference images allocated to each reference picture list. For example, the reference order may be determined to preferentially refer to the reference image closer to the current image among the reference images allocated to the reference picture list according to a display order. 
     The reference picture list determiner  12  according to an exemplary embodiment may set attributes related to the reference picture lists, such as the basic valid number of the reference images allocated to the reference picture lists and modification of the reference picture lists. 
     The attributes related to the reference picture lists according to an exemplary embodiment may include a modification of the number of the reference images, a modification of the reference images allocated to the reference picture lists, or methods of modifying the reference order. 
     Hereinafter, a process in which the reference picture list determiner  12  according to an exemplary embodiment determines a reference picture list for inter prediction will be described with reference to  FIG. 1B . 
     In operation  111 , the reference picture list determiner  12  according to an exemplary embodiment may determine whether slices of the same slice type, among slices included in the same picture, have a restriction condition for identity of reference picture lists. For example, according to the restriction condition of the reference picture lists, reference picture lists of the slices of the same slice type may need to be identical in a current picture. In operation  112 , the reference picture list determiner  12  according to an exemplary embodiment may determine a modification possibility of the reference picture lists determined in the same slices. In operation  113 , the reference picture list determiner  12  according to an exemplary embodiment may determine a reference picture list for a block in an inter mode, among blocks of a current slice, based on whether the restriction condition exists for the reference picture lists and the modification possibility of the reference picture lists. 
     In this regard, the reference picture list determiner  12  according to an exemplary embodiment may determine the modification possibility of the reference picture lists, independently from whether there is the restriction condition for the reference picture lists. That is, irrespective of whether there is the restriction condition for the reference picture lists, the modification possibility of the reference picture lists may be determined. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine whether blocks of pictures that belong to the same sequence in the inter mode have a restriction condition that the same slice type needs to use the same reference picture list. 
     The reference picture list determiner  12  may also determine the modification possibility of the reference picture lists determined in the blocks of pictures that belong to the same sequence in the inter mode. 
     The reference picture list determiner  12  according to another exemplary embodiment may further include an LC (List Combination) list as a B slice type reference image list. The LC list may include one or more reference images among reference images of the L0 list and the L1 list. That is, the LC list may include a combination of at least one reference image of the L0 list and at least one reference image of the L1 list. The LC list including at least one reference image of the L0 list or the LC list including at least one reference image of the L1 list may be generated. Thus, the LC list may include at least one of the index indicating reference images for forward prediction and the index indicating reference images for backward prediction. 
     The reference picture list determiner  12  may also determine a use possibility of an LC reference picture list that is a combination of the L0 list and the L1 list. If the LC reference picture list is used, the reference picture list determiner  12  may determine which images of the reference images of the L0 list and the L1 list are included in the LC reference picture list. 
     If the LC list is used, the inter predictor  14  may determine a current reference image from among previous images included in the L0 list and next images included in the L1 list. 
     The reference picture list determiner  12  according to another exemplary embodiment may determine a combination possibility of the L0 list and the L1 list that is determined for the blocks of pictures that belong to the same sequence in the inter mode, independently from the modification possibility of the reference picture lists. That is, whether to user the LC list for the blocks of pictures that belong to the same sequence in the inter mode may be determined, independently from the modification possibility of the reference picture lists. 
     The reference picture list determiner  12  according to another exemplary embodiment may determine whether there is a reference picture list combined by using reference pictures of the L0 list and the L1 list for each slice, if combinations of reference picture lists determined in a current slice are possible. If a reference picture list combined in the current slice is used, a modification possibility of combined reference picture lists may be determined. 
     The inter predictor  14  may determine a reference image for inter prediction of a B slice type current image. The inter predictor  14  according to an exemplary embodiment may determine an image that is to be referred to by the current image from among the reference images allocated to the reference picture lists determined by the reference picture list determiner  12 . The inter predictor  14  may determine reference information indicating at least one reference image for prediction of an image. 
     The inter predictor  14  may determine the reference image from among a previous image preceding the current image and a next image following the current image according to a display order, for bi-prediction of the B slice type current image. The inter predictor  14  may determine the reference image from among images that are processed and reconstructed prior to a coding order of the current image. The inter predictor  14  may determine a current reference image from among previous images included in the L0 list. The inter predictor  14  may determine the current reference image from among next images included in the L1 list. 
     The inter predictor  14  according to another exemplary embodiment may determine the current reference image from among the previous images included in the L0 list and the next images included in the L1 list. 
     A reference picture list may be used to determine a reference block for motion prediction of an inter mode block. The reference picture list may be used to determine a reference block for motion compensation of the inter mode block. 
     Hereinafter, a process of performing motion prediction by using a reference picture list determined by the reference picture list determiner  12  according to an exemplary embodiment will now be described in detail with reference to  FIGS. 2A and 2B . A process of performing motion compensation by using the reference picture list determined by the reference picture list determiner  12  according to an exemplary embodiment will be described in detail with reference to  FIGS. 3A and 3B . 
       FIG. 2A  is a block diagram of a motion prediction apparatus  20  corresponding to the inter prediction apparatus  10  according to an exemplary embodiment.  FIG. 2B  is a flowchart of a motion prediction method of performing an inter prediction method according to an exemplary embodiment. 
     The motion prediction apparatus  20  according to an exemplary embodiment includes the reference picture list determiner  12  and a motion predictor  24 . 
     In operation  214 , the motion predictor  24  according to an exemplary embodiment may perform motion prediction by using a reference picture for each block of a current image. 
     The motion predictor  24  according to an exemplary embodiment may determine a reference image for a current block from among reference images allocated to a reference picture list determined by the reference picture list determiner  12 . The motion predictor  24  may determine a similarity between blocks of the determined reference image and the current block of the reference image and detect a block having a least error with the current block. The detected block may be determined as a reference block. The motion predictor  24  according to an exemplary embodiment may perform motion prediction by using the reference block determined for each block. 
     The motion predictor  24  may output information indicated by the determined reference image, for example, an image number, an index, etc. A motion vector indicating the reference block from among the blocks of the reference image may be determined. The motion predictor  24  may determine a residue between the reference block and the current block. 
       FIG. 3A  is a block diagram of a motion compensation apparatus  30  corresponding to the inter prediction apparatus  10  according to an exemplary embodiment.  FIG. 3B  is a flowchart of a motion compensation method of performing an inter prediction method according to an exemplary embodiment. 
     The motion compensation apparatus  30  according to an exemplary embodiment includes the reference picture list determiner  12  and a motion compensator  34 . 
     In operation  314 , the motion compensator  34  according to an exemplary embodiment may determine a reference picture for each block of a current image and perform motion compensation. 
     The motion compensator  34  may determine a reference image indicated by a reference index from among images allocated to a reference picture list by the reference picture list determiner  12  and determine a reference block indicated by a motion vector from among blocks of the reference image. The motion compensator  34  may reconstruct a current block by compensating for the reference block according to a residue. 
     In general, during a video decoding process, motion prediction and motion compensation may be performed. During a video decoding process, motion compensation may be performed. After motion prediction is performed on an original image, to generate the same reconstruction image as the original image through motion compensation, a reference image generated through motion prediction and a residue need to be used to perform motion compensation. Thus, during the video encoding and decoding processes, to encode and decode an inter mode block, the reference information (the reference index and the motion vector) and information regarding the residue should be transmitted and received. 
     Likewise, a reference picture list should be determined by using the same method during the video encoding and decoding processes. Thus, information regarding a method of determining the reference picture list used by the motion predictor  20  according to an exemplary embodiment may be encoded and output. The motion compensation apparatus  30  according to an exemplary embodiment may be used to receive the information regarding the method of determining the reference picture list and determine the reference picture list. 
     First, the video encoding process will be described. 
     The motion prediction apparatus  20  according to an exemplary embodiment may encode information indicating whether there is a restriction condition that the same slice type should use the same reference picture lists and transmit the information to inter mode blocks of the same pictures. In the case where the same slice type has the same reference picture list according to the restriction condition of the reference picture lists according to an exemplary embodiment, if there is information indicating whether to override a valid number of reference images, information regarding a valid number of reference images of an L0 list, and information regarding a valid number of reference images of an L1 list, the information may be applied to reference picture lists of the same slice type in the same way. When reference picture lists are combined and used, if the restriction condition is activated, and there is information regarding if the reference picture lists are combined and used and information regarding a valid number of reference images of an LC list, the information may be applied to an LC lists of the same slice type, e.g., the combined reference picture lists, in the same way. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine whether there is the restriction condition of the reference picture lists for each of pictures that belong to the same sequences, and thus information indicating whether there is the restriction condition of the reference picture lists may be inserted into a sequence parameter set (SPS) of a bitstream. 
     The motion prediction apparatus  20  according to an exemplary embodiment may encode information indicating a modification possibility of reference picture lists for inter mode blocks that belong to the same slices. The reference picture list determiner  12  according to an exemplary embodiment may determine a modification possibility of reference picture lists for each of slices that belong to the same sequences, and thus the motion prediction apparatus  20  according to an exemplary embodiment may insert information indicating the modification possibility of reference picture lists into the SPS of the bitstream. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine the modification possibility of reference picture lists irrespective of whether there is the restriction condition of the reference picture lists, and thus the motion prediction apparatus  20  according to an exemplary embodiment may independently insert the information indicating whether there is the restriction condition of the reference picture lists and the information indicating the modification possibility of the reference picture lists into the SPS. 
     The motion prediction apparatus  20  according to another exemplary embodiment may encode information indicating a combination possibility of the L0 list and the L1 list for the inter mode blocks that belong to the same slices. The reference picture list determiner  12  according to another exemplary embodiment may determine a combination possibility of the reference picture lists for each of the slices that belong to the same sequences, and thus the motion prediction apparatus  20  according to another exemplary embodiment may insert information indicating the combination possibility of the reference picture lists into the SPS of the bitstream. 
     The reference picture list determiner  12  according to another exemplary embodiment may determine the combination possibility of the reference picture lists irrespective of whether there is the restriction condition of the reference picture lists, and thus the motion prediction apparatus  20  according to another exemplary embodiment may independently insert the information indicating whether there is the restriction condition of the reference picture lists and the information indicating the combination possibility of the reference picture lists into the SPS. 
     The motion prediction apparatus  20  according to another exemplary embodiment may encode information indicating whether there is a combined reference picture list, from among the reference picture lists for inter mode blocks that belong to a current slice. The motion prediction apparatus  20  according to another exemplary embodiment may insert information indicating whether there is the combined reference picture list into a slice header of the bitstream. 
     The motion prediction apparatus  20  according to another exemplary embodiment may encode information indicating a modification possibility of the combined reference picture list if there are combined reference picture lists determined for the inter mode blocks that belong to the current slice. The motion prediction apparatus  20  according to another exemplary embodiment may insert the information indicating the modification possibility of the combined reference picture list into the slice header of the bitstream. 
     The motion prediction apparatus  20  may encode and insert a reference index indicating the determined reference picture for each block and a residue, along with the information regarding the method of determining the reference picture list into a block of the bitstream. 
     The motion prediction apparatus  20  may express an image by using prediction information instead of overall data of the image, and thus the motion prediction apparatus  20  may perform video compression encoding requiring a reduction of an amount of video data. 
     The motion prediction apparatus  20  according to an exemplary embodiment may be included in or may interact with a video encoder that encodes video based on coding units that are split from a video image according to spatial domains to perform inter prediction for video encoding. For inter prediction on a coding unit, the coding unit may be split into prediction units and partitions and then inter prediction may be performed based on the prediction units and the partitions. 
     A coding unit according to an exemplary embodiment may include coding units according to a tree structure according to an exemplary embodiment and a block in fixed shapes as well. According to an exemplary embodiment, the coding units according to the tree structure, prediction units thereof, and partitions thereof will be described in more detail with reference to  FIGS. 9 through 21 . 
     The motion prediction apparatus  20  according to an exemplary embodiment may perform inter prediction on an image block and image data in the coding units to output a prediction error of a reference image, e.g., a residue. The motion prediction apparatus  20  may generate the residue as a quantized transformation coefficient through transformation and quantization, perform entropy encoding on symbols such as a transformation coefficient, reference information, coding information, etc., and output a bitstream. The motion prediction apparatus  20  according to an exemplary embodiment may encode and output symbols including L0 list related information and L1 list related information that include information regarding a reference order of images that belong to each reference picture list or the number of the images, and reference picture list related information such as information regarding a modification of reference picture lists. 
     The motion prediction apparatus  20  may generate a reconstruction image by performing inverse quantization, inverse transformation, and prediction compensation on the transformation coefficient, reconstructing an image within a spatial domain, and performing in-loop filtering on the image. The reconstruction image may be used as a reference image for prediction of a next image. That is, the motion prediction apparatus  20  according to an exemplary embodiment may refer to the reconstructed image generated by the video encoder by using at least one of the L0 list and the L1 list, for bi-prediction of a B slice type current image. In doing so, the motion prediction apparatus  20  may determine the reference image and the residue through inter prediction. 
     Therefore, video compression encoding may be implemented through motion prediction performed by the motion prediction apparatus  20 . 
     The motion prediction apparatus  20  according to an exemplary embodiment may cooperatively interact with an internal video encoding processor installed therein or an external video encoding processor in order to output a video encoding result, and thus, a video encoding operation including motion prediction may be performed. The internal video encoding processor of the motion prediction apparatus  20  according to an exemplary embodiment may be embodied by adding a video encoding processing module to a central operating device or a graphic operating device as well as to a separate processor, which performs a basic video encoding operation. 
     Next, a video decoding process will now be described. 
     The motion compensation apparatus  30  according to an exemplary embodiment may receive, for inter mode blocks of the same picture, information indicating whether there is a restriction condition that the same slice types should use the same reference picture lists. The motion compensation apparatus  30  according to an exemplary embodiment may also parse the information regarding the restriction condition of reference picture lists from an SPS of a received bitstream. Based on the information regarding the restriction condition of reference picture lists extracted from the SPS, it may be determined whether there is the restriction condition of reference picture lists for each of pictures that belong to a current sequence. 
     The motion compensation apparatus  30  according to an exemplary embodiment may receive information indicating a modification possibility of reference picture lists for inter mode blocks that belong to the same slice. The motion compensation apparatus  30  according to an exemplary embodiment may parse the information indicating a modification possibility of reference picture lists from the SPS of the received bitstream. Based on the information indicating the modification possibility of reference picture lists extracted from the SPS, the modification possibility of reference picture lists for each of slices that belong to the current sequence may be determined. 
     The motion compensation apparatus  30  according to an exemplary embodiment may independently parse the information indicating whether there is the restriction condition of reference picture lists and the information indicating the modification possibility of reference picture lists from the SPS. Thus, the motion compensation apparatus  30  may determine the modification possibility of reference picture lists irrespective of whether there is the restriction condition of reference picture lists. 
     The motion compensation apparatus  30  according to another exemplary embodiment may receive information indicating a combination possibility of the L0 list and the L1 list for inter mode blocks that belong to a current picture. The motion compensation apparatus  30  according to another exemplary embodiment may parse information indicating a combination possibility of reference picture lists from the SPS. Thus, the combination possibility of reference picture lists for each of the slices that belong to the same sequences may be determined. 
     The motion compensation apparatus  30  according to another exemplary embodiment may independently parse the information indicating whether there is the restriction condition of reference picture lists and the information indicating the combination possibility of reference picture lists from the SPS. Thus, the motion compensation apparatus  30  according to another exemplary embodiment may determine the combination possibility of reference picture lists irrespective of whether there is the restriction condition of reference picture lists. 
     The motion compensation apparatus  30  according to another exemplary embodiment may receive information indicating whether there is a combined reference picture list from among the reference picture lists for the inter mode blocks that belong to the current slice. The motion compensation apparatus  30  according to another exemplary embodiment may parse the information indicating whether there is the combined reference picture list from a slice header of the bitstream. 
     The motion compensation apparatus  30  according to another exemplary embodiment may receive information indicating a modification possibility of the combined reference picture list if there are combined reference picture lists determined for the inter mode blocks that belong to the current slice. The motion compensation apparatus  30  according to another exemplary embodiment may parse the information indicating the modification possibility of the combined reference picture list from the slice header of the bitstream. 
     The motion compensation apparatus  30  may parse a reference index indicating a reference picture for a current block and a residue from a block of the bitstream, along with information regarding a method of determining a reference picture list. 
     The motion compensation apparatus  30  according to an exemplary embodiment may be included in or may interact with a video decoder that decodes video based on coding units that are split from a video image according to spatial domains to perform motion compensation for video decoding. A coding unit for motion compensation may include prediction units and partitions, and motion compensation may be performed based on the prediction units and partitions. A coding unit according to an exemplary embodiment may include coding units according to a tree structure according to an exemplary embodiment and a block in fixed shapes as well, as described above. 
     The motion compensation apparatus  30  may perform entropy decoding on the received bitstream to parse symbols such as a transformation coefficient, reference information, coding information, etc. The motion compensation apparatus  30  according to an exemplary embodiment may also parse symbols including reference picture list related information. 
     The motion compensation apparatus  30  according to an exemplary embodiment may perform inverse quantization and inverse transformation on the parsed transformation coefficient for each transformation unit to reconstruct a residue of the spatial domain. The motion compensation apparatus  30  according to an exemplary embodiment may determine a reference image by referring to a previously reconstructed image that is recorded on at least one of the L0 list and the L1 list, for bi-prediction of a B slice type current partition, and determine a reference block of the reference image indicated by a motion vector. 
     The motion compensation apparatus  30  according to an exemplary embodiment may perform deblocking filtering and sample adaptive offset (SAO) filtering on an image of the spatial domain, thereby generating a reconstruction image. The reconstruction image may be used as a reference image for prediction of a next image. 
     Therefore, video compression decoding may be implemented through motion compensation performed by the motion compensation apparatus  30 . 
     The motion compensation apparatus  30  according to an exemplary embodiment may cooperatively interact with an internal video decoding processor installed therein or an external video decoding processor in order to output a video decoding result, and thus, a video decoding operation including motion compensation may be performed. The internal video decoding processor of the motion compensation apparatus  30  according to an exemplary embodiment may be embodied by adding a video decoding processing module to a central operating device or a graphic operating device as well as to a separate processor, which performs a basic video decoding operation. 
     A syntax of reference picture list related information that is transmitted by the motion prediction apparatus  20  and is parsed by the motion compensation apparatus  30  according to an exemplary embodiment will now be described in detail with reference to  FIGS. 4 through 8  below. 
       FIG. 4  is a diagram of a syntax of an SPS  40  according to an exemplary embodiment. 
     A ‘seq_parameter_set_rbsp( )’  40  indicates the SPS. A ‘restricted_ref_pic_lists_flag’  42  indicates information regarding whether there is a restriction condition of a reference picture list. A ‘lists_modification_present_flag’  46  indicates information regarding a modification possibility of the reference picture list. 
     The motion prediction apparatus  20  may record various types of information necessary for decoding a current sequence in the ‘seq_parameter_set_rbsp( )’  40  to encode parameters related to the sequence. The motion compensation apparatus  30  may parse the parameters related to the sequence from the ‘seq_parameter_set_rbsp( )’  40  to read the various types of information necessary for decoding the current sequence. 
     The reference picture list determiner  12  according to an exemplary embodiment may determine an attribute of a reference picture list determined in each of sequences. 
     The motion prediction apparatus  20  according to an exemplary embodiment may record the ‘restricted_ref_pic_lists_flag’  42  indicating whether the same reference picture lists should be determined for slices of the same slice type that belong to the current sequence in the ‘seq_parameter_set_rbsp( )’  40  including various types of information regarding the current sequence. 
     The motion prediction apparatus  20  according to an exemplary embodiment may also record a ‘lists_modification_present_flag’  46  indicating whether a list modification needs to be considered with respect to reference picture lists determined in the current sequence, e.g., indicating the modification possibility of the reference picture list, in the ‘seq_parameter_set_rbsp( )’  40 . In particular, without having to consider a value of the ‘restricted_ref_pic_lists_flag’  42   44 , the ‘lists_modification_present_flag’  46  indicating the modification possibility of the reference picture list may be defined. 
     The motion compensation apparatus  30  according to an exemplary embodiment may parse the various types of information regarding the current sequence from the ‘seq_parameter_set_rbsp( )’  40 . The motion compensation apparatus  30  according to an exemplary embodiment may parse the ‘restricted_ref_pic_lists_flag’  42  from the ‘seq_parameter_set_rbsp( )’  40  to read whether the same reference picture lists should be determined for pictures of the same slice type that belong to the current sequence. 
     The motion compensation apparatus  30  according to an exemplary embodiment may parse the ‘lists_modification_present_flag’  46  from the ‘seq_parameter_set_rbsp( )’  40  to check whether the list modification needs to be considered with respect to the reference picture lists determined in the current sequence, e.g., the modification possibility of the reference picture list. In particular, without having to consider a reading result of the ‘restricted_ref_pic_lists_flag’  42 , the ‘lists_modification_present_flag’  46  indicating the modification possibility of the reference picture list may be parsed. 
     Therefore, irrespective of whether there is a restriction condition that the reference picture lists determined in the slices of the same slice type should be the same, the modification possibility of the reference picture list may be determined. 
       FIG. 5  is a diagram of a syntax of an SPS  50  according to another exemplary embodiment. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record a ‘lists_combination_present_flag’  54  indicating a combination possibility of reference picture lists and a ‘restricted_ref_pic_lists_flag’  52  indicating whether there is a restriction condition of reference picture lists determined in a current sequence and a ‘lists_modification_present_flag’  56  indicating a modification possibility of reference picture lists as well in a ‘seq_parameter_set_rbsp( )’  50 . 
     That is, the motion prediction apparatus  20  according to another exemplary embodiment may record the ‘lists_combination_present_flag’  54  in the ‘seq_parameter_set_rbsp( )’  50  with respect to the reference picture lists determined in the current sequence so as to indicate whether to use an LC list configured by combining reference images that belong to an L0 list and an L1 list. 
     The ‘lists_combination_present_flag’  54  may also be defined without having to consider a value of the ‘restricted_ref piclists_flag’  52 . 
     The motion compensation apparatus  30  according to an exemplary embodiment may parse the ‘lists_combination_present_flag’  54  and the ‘restricted_ref piclists_flag’  52  and the ‘lists_modification_present_flag’  56  as well from the ‘seq_parameter_set_rbsp( )’  50  to check whether a list combination of the reference picture lists determined in the current sequence should be considered, e.g., the combination possibility of reference picture lists. 
     In particular, without having to consider a reading result of the ‘restricted_ref_pic_lists_flag’  52 , the ‘lists_combination_present_flag’  54  indicating the combination possibility of reference picture lists may be parsed. 
       FIG. 6  is a diagram of a syntax of an SPS  60  according to another exemplary embodiment. 
     A ‘slice_header( )’  60  indicates a slice header. A ‘ref piclist_combination( )’  64  indicates a parameter set related to a combination of reference picture lists. A ‘ref_pic_list_modification( )’  68  indicates a parameter set related to a modification of reference picture lists. 
     The motion prediction apparatus  20  may record various types of information which may be used for decoding a current slice the ‘slice_header( )’  60  to encode parameters related to the slice. The motion compensation apparatus  30  may parse the parameters related to the slice from the ‘slice_header( )’  60  to read the various types of information which may be used for decoding the current slice. 
     The reference picture list determiner  12  according to another exemplary embodiment may determine a reference picture list according to a slice type for each slice. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record the ‘ref piclist_combination( )’  64  including information regarding how to combine reference picture lists in the current slice in the ‘slice_header( )’  60  including various types of information regarding the current slice. In particular, only in the case where the ‘lists_combination_present_flag’  54  included in the SPS ‘seq_parameter_set_rbsp( )’  50  regarding a sequence including the current slice indicates a combination of reference picture lists, may the ‘ref piclist_combination( )’  64  including information regarding a specific method of combining the reference picture lists be recorded in the ‘slice_header( )’  60 . 
     The motion prediction apparatus  20  according to an exemplary embodiment may also record the ‘ref_pic_list_modification( )’  68  including information regarding a specific method of modifying the reference picture lists in the ‘slice_header( )’  60  only in the case where the ‘lists_modification_present_flag’  56  included in the SPS ‘seq_parameter_set_rbsp( )’  50  indicates that a list modification of the reference picture lists determined in a current sequence is possible. 
     In particular, independently from a modification possibility of reference picture lists indicated by the ‘lists_modification_present_flag’  56  included in the SPS ‘seq_parameter_set_rbsp( )’  50 , the ‘ref_pic_list_combination( )’  64  indicating the information regarding the specific method of combining the reference picture lists may be included in the ‘slice_header( )’  60 . 
     The motion compensation apparatus  30  according to another exemplary embodiment may parse the ‘ref piclist_combination( )’  64  included in the ‘slice_header( )’  60  to read the information regarding how to combine the reference picture lists in the current slice. In particular, only in the case where the parsed ‘lists_combination_present_flag’  54  included in the SPS ‘seq_parameter_set_rbsp( )’  50  regarding the sequence including the current slice indicates that the reference picture lists can be combined, may the ‘ref piclist_combination( )’  64  indicating the information regarding the specific method of combining the reference picture lists be parsed from the ‘slice_header( )’  60 . 
     The motion compensation apparatus  30  according to an exemplary embodiment may also parse the ‘ref_pic_list_modification( )’  68  including the information regarding the specific method of modifying the reference picture lists from the ‘slice_header( )’  60  only in the case where the ‘lists_modification_present_flag’  56  included in the SPS ‘seq_parameter_set_rbsp( )’  50  indicates that the list modification of the reference picture lists determined in the current sequence is possible. 
     In particular, independently from a modification possibility of reference picture lists determined according to the ‘lists_modification_present_flag’  56 , the motion compensation apparatus  30  according to an exemplary embodiment may parse the ‘ref piclist_combination( )’  64  indicating the information regarding the specific method of combining the reference picture lists from the ‘slice_header( )’  60 . 
       FIG. 7  is a diagram of a syntax of a parameter set related to a combination of reference picture lists according to another exemplary embodiment. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record information regarding a specific method of combining reference picture lists in the ‘ref_pic_list_combination( )’  64  that is the parameter set related to the combination of reference picture lists. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record a ‘ref_pic_list_combination_flag’  72  indicating whether reference picture lists are combined and used for inter prediction of a current list in a B slice type in the ‘ref piclist_combination( )’  64 . If the reference picture lists are combined and used, a ‘num_ref_idx_lc_active_minus1’  74  indicating a number of valid reference images that belong to a combined reference picture list may be recorded in the ‘ref piclist_combination( )’  64 . 
     When an L0 list and an L1 list are not the same with respect to the slice of the B slice type, it may be necessary to determine the number of the valid reference images that belong to the combined reference picture list in order to prevent the combined reference picture list from being parsed. Thus, if the reference picture lists are combined and used in the ‘ref piclist_combination( )’  64 , the ‘num_ref_idx_lc_active_minus1’  74  may be further transmitted. 
     However, the motion prediction apparatus  20  according to another exemplary embodiment may not record information  76  related to a method of modifying combined reference picture lists in the ‘ref piclist_combination( )’  64  for recording the information regarding the method of combining reference picture lists. 
     The motion compensation apparatus  30  according to another exemplary embodiment may read the information regarding the specific method of combining reference picture lists from the ‘ref_pic_list_combination( )’  64  that is the parameter set related to the combination of reference picture lists. 
     The motion compensation apparatus  30  according to another exemplary embodiment may also parse the ‘ref_pic_list_combination_flag’  72  from the ‘ref_pic_list_combination( )’  64  to read whether the reference picture lists are combined and used for inter prediction of the current list in the B slice type. If the motion compensation apparatus  30  reads that the reference picture lists are combined and used for the current list, the motion compensation apparatus  30  may further parse the ‘num_ref_idx_lc_active_minus1’  74  from the ‘ref_pic_list_combination( )’  64  to read the number of the valid reference images that belong to the combined reference picture list. 
     When the L0 list and the L1 list are not the same with respect to the slice of the B slice type, during a parsing process, the number of the valid reference images that belong to the combined reference picture list may be read from the ‘num_ref_idx_lc_active_minus1’  74 , and an unnecessarily combined reference picture list may be prevented from being parsed. 
     However, the motion compensation apparatus  30  according to another exemplary embodiment may read only the information regarding the method of combining reference picture lists from the ‘ref piclist_combination( )’  64  and may not necessarily read the information  76  related to the method of modifying the combined reference picture lists. 
       FIG. 8  is a diagram of a syntax of a parameter set related to a modification of reference picture lists according to another exemplary embodiment. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record information regarding a specific method of modifying reference picture lists in the ‘ref_pic_list_modification( )’  68  that is the parameter set related to the modification of reference picture lists. In particular, information regarding a method of modifying reference picture lists may be individually recorded in the ‘ref_pic_list_modification( )’  68  for each direction of a reference picture. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record at least one of information  81  regarding a method of modifying an L0 list when a current slice is a P or B slice type, information  84  regarding a method of modifying an L1 list when the current slice is the B slice type, and information  76  regarding a method of modifying a combined reference picture list in the ‘ref_pic_list_modification( )’  68 . 
     The information  81  regarding the method of modifying the L0 list may include a ‘ref_pic_list_modification_flag_l0’  82  indicating whether the L0 list has been modified and a ‘list_entry_l0’  83  indicating an index of reference images included in the L0 list. Likewise, the information  84  regarding the method of modifying the L1 list may include a ‘ref piclist_modification_flag_l1’  85  indicating whether the L1 list has been modified and a ‘list_entry_l1’  86  indicating an index of reference images included in the L1 list. 
     If the ‘ref piclist_combination_flag’  72  previously inserted into a slice header  60  indicates that reference picture lists are combined and used in a current slice, the motion prediction apparatus  20  according to another exemplary embodiment may also record the information  76  related to the method of modifying the combined reference picture lists in the ‘ref piclist_modification( )’  68  that is the parameter set related to the modification of reference picture lists. 
     The motion prediction apparatus  20  according to another exemplary embodiment may record a ‘ref piclist_modification_flag_lc’  87  indicating whether the combined reference picture list has been modified, a ‘pic_from_list — 0_flag’  88  indicating whether the reference images of the L0 list are combined, and a ‘ref_idx_list_curr’  89  indicating an index of a currently added reference image, as the information  76  related to the method of modifying the combined reference picture lists. 
     The motion compensation apparatus  30  according to another exemplary embodiment may read the information regarding a specific method of modifying reference picture lists from the ‘ref_pic_list_modification( )’  68  that is the parameter set related to the modification of reference picture lists. In particular, the information regarding the method of modifying reference picture lists individually defined for each direction of the reference picture may be parsed from the ‘ref piclist_modification( )’  68 . 
     The motion compensation apparatus  30  according to another exemplary embodiment may read at least one of the information  81  regarding the method of modifying the L0 list when the current slice is the P or B slice type, the information  84  regarding the method of modifying the L1 list when the current slice is the B slice type, and the information  76  regarding the method of modifying the combined reference picture list from the ‘ref_pic_list_modification( )’  68 . 
     The motion compensation apparatus  30  according to another exemplary embodiment may parse the ‘ref_pic_list_modification_flag_l0’  82  as the information  81  regarding the method of modifying the L0 list to read whether the L0 list has been modified. The motion compensation apparatus  30  according to another exemplary embodiment may also parse the ‘list_entry_l0’  83  as the information  81  regarding the method of modifying the L0 list to read the index of the reference images included in the L0 list. 
     Likewise, whether the L1 list has been modified may be read from the parsed ‘ref_piclist_modification_flag_l1’  85  as the information  84  regarding the method of modifying the L1 list. The index of the reference images included in the L0 list may be read from the parsed ‘list_entry_l1’  86  as the information  84  regarding the method of modifying the L1 list. 
     If the motion compensation apparatus  30  according to another exemplary embodiment reads that the reference picture lists are combined and used in the current slice from the ‘ref piclist_combination_flag’  72  previously parsed in the slice header  60 , the motion compensation apparatus  30  according to another exemplary embodiment may further read the information  76  regarding the method of modifying the combined reference picture list from the ‘ref_pic_list_modification( )’  68  that is the parameter set related to the modification of reference picture lists. 
     The motion prediction apparatus  20  according to another exemplary embodiment may parse the ‘ref_pic_list_modification_flag_lc’  87  from the information  76  regarding the method of modifying the combined reference picture list to read whether the combined reference picture list has been modified. The ‘pic_from_list — 0_flag’  88  may be parsed from the information  76  regarding the method of modifying the combined reference picture list to read whether the reference images of the L0 list have been combined. The ‘ref_idx_list_curr’  89  may be parsed from the information  76  regarding the method of modifying the combined reference picture list to read an index of the index of the currently added reference image. 
     Therefore, at least one of the inter prediction apparatus  10 , the motion prediction apparatus  20 , and the motion compensation apparatus  30  according to an exemplary embodiment may determine a modification possibility of reference picture lists, independently from whether there is a restriction condition of limiting reference picture lists for each slice type. When it is possible to combine reference picture lists according to another exemplary embodiment, information regarding a combination of reference picture lists and information regarding a modification of reference picture lists may be independently transmitted and received. Thus, a process of transmitting unnecessary information regarding reference picture lists is omitted, thereby reducing a transmission bit rate. Likewise, a process of parsing unnecessary information regarding reference picture lists is omitted, thereby reducing a process of parsing data. 
     As described above, in the inter prediction apparatus  10 , the motion prediction apparatus  20 , and the motion compensation apparatus  30  according to the above-described various exemplary embodiments described with reference to  FIGS. 1A through 8  above, motion prediction and motion compensation are performed for each partition determined according to coding units having a tree structure. Hereinafter, with reference to  FIGS. 9 through 21 , a video encoding method and a video decoding method based on a coding unit according to a tree structure will be described. 
       FIG. 9  is a block diagram of a video encoding apparatus  100  configured to perform an encoding operation based on a coding unit according to a tree structure, according to an exemplary embodiment. 
     The video encoding apparatus  100  configured to perform video prediction based on a coding unit according to a tree structure according to an exemplary embodiment includes a coding determiner  120  and an output unit  130 . Hereinafter, for convenience of description, the video encoding apparatus  100  configured to perform video prediction based on a coding unit according to a tree structure according to an exemplary embodiment is referred to as ‘the video encoding apparatus  100 ’. 
     The coding determiner  120  may split a current picture based on a maximum coding unit for the current picture of an image. If the current picture is larger than the maximum coding unit, image data of the current picture may be split into the at least one maximum coding unit. The maximum coding unit according to an exemplary embodiment may be a data unit having a size of 32×32, 64×64, 128×128, 256×256, etc., wherein a shape of the data unit is a square having a width and length in squares of 2. 
     A coding unit according to an exemplary embodiment may be characterized by a maximum size and a depth. The depth denotes a number of times the coding unit is spatially split from the maximum coding unit, and as the depth deepens, deeper encoding units according to depths may be split from the maximum coding unit to a minimum coding unit. A depth of the maximum coding unit is an uppermost depth and a depth of the minimum coding unit is a lowermost depth. Since a size of a coding unit corresponding to each depth decreases as the depth of the maximum coding unit deepens, a coding unit corresponding to an upper depth may include a plurality of coding units corresponding to lower depths. 
     As described above, the image data of the current picture is split into the maximum coding units according to a maximum size of the coding unit, and each of the maximum coding units may include deeper coding units that are split according to depths. Since the maximum coding unit according to an exemplary embodiment is split according to depths, the image data of a spatial domain included in the maximum coding unit may be hierarchically classified according to depths. 
     A maximum depth and a maximum size of a coding unit, which limit the total number of times a height and a width of the maximum coding unit are hierarchically split, may be predetermined. 
     The coding unit determiner  120  encodes at least one split region obtained by splitting a region of the maximum coding unit according to depths, and determines a depth to output finally encoded image data according to the at least one split region. In other words, the coding unit determiner  120  determines a coded depth by encoding the image data in the deeper coding units according to depths, according to the maximum coding unit of the current picture, and selecting a depth having the least encoding error. Thus, the encoded image data of the coding unit corresponding to the determined coded depth is finally output. Also, the coding units corresponding to the coded depth may be regarded as encoded coding units. The determined coded depth and the encoded image data according to the determined coded depth are output to the output unit  130 . 
     The image data in the maximum coding unit is encoded based on the deeper coding units corresponding to at least one depth equal to or below the maximum depth, and results of encoding the image data are compared based on each of the deeper coding units. A depth having the least encoding error may be selected after comparing encoding errors of the deeper coding units. At least one coded depth may be selected for each maximum coding unit. 
     The size of the maximum coding unit is split as a coding unit is hierarchically split according to depths, and as the number of coding units increases. Also, even if coding units correspond to the same depth in one maximum coding unit, it is determined whether to split each of the coding units corresponding to the same depth to a lower depth by measuring an encoding error of the image data of the each coding unit, separately. Accordingly, even when image data is included in one maximum coding unit, the image data is split into regions according to the depths and the encoding errors may differ according to regions in the one maximum coding unit, and thus the coded depths may differ according to regions in the image data. Thus, one or more coded depths may be determined in one maximum coding unit, and the image data of the maximum coding unit may be divided according to coding units of at least one coded depth. 
     Accordingly, the coding unit determiner  120  according to an exemplary embodiment may determine coding units having a tree structure included in the maximum coding unit. The ‘coding units having a tree structure’ according to an exemplary embodiment may include coding units corresponding to a depth determined to be the coded depth, from among all deeper coding units included in the maximum coding unit. A coding unit of a coded depth may be hierarchically determined according to depths in the same region of the maximum coding unit, and may be independently determined in different regions. Similarly, a coded depth in a current region may be independently determined from a coded depth in another region. 
     A maximum depth according to an exemplary embodiment is an index related to the number of times splitting is performed from a maximum coding unit to a minimum coding unit. A first maximum depth according to an exemplary embodiment may denote the total number of times splitting is performed from the maximum coding unit to the minimum coding unit. A second maximum depth according to an exemplary embodiment may denote the total number of depth levels from the maximum coding unit to the minimum coding unit. For example, when a depth of the maximum coding unit is 0, a depth of a coding unit, in which the maximum coding unit is split once, may be set to 1, and a depth of a coding unit, in which the maximum coding unit is split twice, may be set to 2. Here, if the minimum coding unit is a coding unit in which the maximum coding unit is split four times, 5 depth levels of depths 0, 1, 2, 3 and 4 exist, and thus the first maximum depth may be set to 4, and the second maximum depth may be set to 5. 
     Prediction encoding and transformation may be performed according to the maximum coding unit. The prediction encoding and the transformation are also performed based on the deeper coding units according to a depth equal to or depths less than the maximum depth, according to the maximum coding unit. Transformation may be performed according to a method of orthogonal transformation or integer transformation. 
     Since the number of deeper coding units increases whenever the maximum coding unit is split according to depths, encoding including the prediction encoding and the transformation is performed on all of the deeper coding units generated as the depth deepens. For convenience of description, the prediction encoding and the transformation will now be described based on a coding unit of a current depth, in a maximum coding unit. 
     The video encoding apparatus  100  according to an exemplary embodiment may variously select a size or shape of a data unit for encoding the image data. In order to encode the image data, operations, such as prediction encoding, transformation, and entropy encoding, are performed, and at this time, the same data unit may be used for all operations or different data units may be used for each operation. 
     For example, the video encoding apparatus  100  may select not only a coding unit for encoding the image data, but also a data unit different from the coding unit so as to perform the prediction encoding on the image data in the coding unit. 
     In order to perform prediction encoding on the maximum coding unit, the prediction encoding may be performed based on a coding unit corresponding to a coded depth according to an exemplary embodiment, e.g., based on a coding unit that is no longer split into coding units corresponding to a lower depth. Hereinafter, the coding unit that is no longer split and becomes a basis unit for prediction encoding will now be referred to as a ‘prediction unit’. A partition obtained by splitting the prediction unit may include a prediction unit or a data unit obtained by splitting at least one of a height and a width of the prediction unit. The partition is a data unit obtained by dividing the prediction unit of the coding unit and the prediction unit may be a partition having the same size as the coding unit. 
     For example, when a coding unit of 2N×2N (where N is a positive integer) is no longer split and becomes a prediction unit of 2N×2N, a size of a partition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partition type according to an exemplary embodiment include symmetrical partitions that are obtained by symmetrically splitting a height or width of the prediction unit, partitions obtained by asymmetrically splitting the height or width of the prediction unit, such as 1:n or n:1, partitions that are obtained by geometrically splitting the prediction unit, and partitions having arbitrary shapes. 
     A prediction mode of the prediction unit may be at least one of an intra mode, an inter mode, and a skip mode. For example, the intra mode or the inter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, or N×N. Also, the skip mode may be performed only on the partition of 2N×2N. The encoding is independently performed on one prediction unit in a coding unit, thereby selecting a prediction mode having a least encoding error. 
     The video encoding apparatus  100  according to an exemplary embodiment may also perform the transformation on the image data in a coding unit based not only on the coding unit for encoding the image data, but also based on a transformation unit that is different from the coding unit. In order to perform the transformation in the coding unit, the transformation may be performed based on a data unit having a size smaller than or equal to the coding unit. For example, the transformation unit for the transformation may include a transformation unit for an intra mode and a data unit for an inter mode. 
     Similarly to the coding unit according to the tree structure according to an exemplary embodiment, the transformation unit in the coding unit may be recursively split into smaller sized regions and residual data in the coding unit may be divided according to the transformation having the tree structure according to transformation depths. 
     According to an exemplary embodiment, a transformation depth indicating the number of times splitting is performed to obtain the transformation unit by splitting the height and width of the coding unit may also be set in the transformation unit. For example, when the size of a transformation unit of a current coding unit is 2N×2N, a transformation depth may be set to 0. When the size of a transformation unit is N×N, the transformation depth may be set to 1. In addition, when the size of the transformation unit is N/2×N/2, the transformation depth may be set to 2. That is, the transformation unit according to the tree structure may also be set according to the transformation depth. 
     Encoding information according to coding units corresponding to a coded depth requires not only information about the coded depth, but also about information related to prediction encoding and transformation. Accordingly, the coding unit determiner  120  not only determines a coded depth having a least encoding error, but also determines a partition type in a prediction unit, a prediction mode according to prediction units, and a size of a transformation unit for transformation. 
     Coding units and a prediction unit/partition according to a tree structure in a maximum coding unit, and a method of determining a transformation unit, according to exemplary embodiments, will be described in detail later with reference to  FIGS. 10 through 20 . 
     The coding unit determiner  120  may measure an encoding error of deeper coding units according to depths by using Rate-Distortion Optimization based on Lagrangian multipliers. 
     The output unit  130  outputs the image data of the maximum coding unit, which is encoded based on the at least one coded depth determined by the coding unit determiner  120 , and information about the encoding mode according to the coded depth, in bitstreams. 
     The encoded image data may be obtained by encoding residual data of an image. 
     The information about the encoding mode according to the coded depth may include information about the coded depth, the partition type in the prediction unit, the prediction mode, and the size of the transformation unit. 
     The information about the coded depth may be defined by using split information according to depths, which indicates whether encoding is performed on coding units of a lower depth instead of a current depth. If the current depth of the current coding unit is the coded depth, image data in the current coding unit is encoded and output, and thus the split information may be defined not to split the current coding unit to a lower depth. Alternatively, if the current depth of the current coding unit is not the coded depth, the encoding is performed on the coding unit of the lower depth, and thus the split information may be defined to split the current coding unit to obtain the coding units of the lower depth. 
     If the current depth is not the coded depth, encoding is performed on the coding unit that is split into the coding unit of the lower depth. Since at least one coding unit of the lower depth exists in one coding unit of the current depth, the encoding is repeatedly performed on each coding unit of the lower depth, and thus the encoding may be recursively performed for the coding units having the same depth. 
     Since the coding units having a tree structure are determined for one maximum coding unit, and information about at least one encoding mode is determined for a coding unit of a coded depth, information about at least one encoding mode may be determined for one maximum coding unit. Also, a coded depth of the image data of the maximum coding unit may be different according to locations since the image data is hierarchically split according to depths, and thus information about the coded depth and the encoding mode may be set for the image data. 
     Accordingly, the output unit  130  according to an exemplary embodiment may assign encoding information about a corresponding coded depth and an encoding mode to at least one of the coding unit, the prediction unit, and a minimum unit included in the maximum coding unit. 
     The minimum unit according to an exemplary embodiment is a rectangular data unit obtained by splitting the minimum coding unit constituting the lowermost depth by 4. Alternatively, the minimum unit may be a maximum rectangular data unit having a maximum size, which is included in all of the coding units, prediction units, partition units, and transformation units included in the maximum coding unit. 
     For example, the encoding information output through the output unit  130  may be classified into encoding information according to coding units, and encoding information according to prediction units. The encoding information according to the coding units may include the information about the prediction mode and about the size of the partitions. The encoding information according to the prediction units may include information about an estimated direction of an inter mode, about a reference image index of the inter mode, about a motion vector, about a chroma component of an intra mode, and about an interpolation method of the intra mode. 
     Also, information about a maximum size of the coding unit defined according to pictures, slices, or GOPs, and information about a maximum depth may be inserted into a header of a bitstream, a sequence parameter set (SPS) or a picture parameter set (PPS). 
     In addition, information about a maximum size of a transformation unit and information about a minimum size of a transformation, which are acceptable for a current video, may also be output via a header of a bitstream, a SPS or a PPS. The output unit  130  may encode and output reference information, prediction information, and information about a slice type, which are related to prediction. 
     In the video encoding apparatus  100 , the deeper coding unit may be a coding unit obtained by dividing a height or width of a coding unit of an upper depth, which is one layer higher than the current depth, by two. In other words, when the size of the coding unit of the current depth is 2N×2N, the size of the coding unit of the lower depth is N×N. Also, the coding unit of the current depth having the size of 2N×2N may include a maximum value 4 of the coding unit of the lower depth. 
     Accordingly, the video encoding apparatus  100  may form the coding units having the tree structure by determining coding units having an optimum shape and an optimum size for each maximum coding unit, based on the size of the maximum coding unit and the maximum depth determined considering characteristics of the current picture. Also, since encoding may be performed on each maximum coding unit by using any one of various prediction modes and transformations, an optimum encoding mode may be determined considering characteristics of the coding unit of various image sizes. 
     Thus, if an image having high resolution or a large data amount is encoded in a related art macroblock, a number of macroblocks per picture excessively increases. Accordingly, a number of pieces of compressed information generated for each macroblock increases, and thus it is difficult to transmit the compressed information and data compression efficiency decreases. However, by using the video encoding apparatus  100 , image compression efficiency may be increased since a coding unit is adjusted while considering characteristics of an image while increasing a maximum size of a coding unit while considering a size of the image. 
     The video encoding apparatus  100  may determine a reference picture list and perform inter prediction according to the motion prediction method described with reference to  FIGS. 2A and 2B  above. 
     The coding unit determiner  120  may determine a prediction unit for inter prediction for each of the respective coding units according to a tree structure for each maximum coding unit, and may perform inter prediction for each respective prediction unit and one or more partitions thereof. 
     The coding unit determiner  120  according to an exemplary embodiment may determine whether there is a restriction condition for identity of reference picture lists of slices of the same slice type among slices included in the same picture for each of pictures that belong to a current sequence. Independently from the restriction condition of reference picture lists, the coding unit determiner  120  may determine a modification possibility of reference picture lists determined in the current sequence. The coding unit determiner  120  may determine a reference picture list for blocks in an inter mode that belongs to a picture based on whether there is the restriction condition of reference picture lists and the modification possibility of reference picture lists that are previously determined. 
     The coding unit determiner  120  according to another exemplary embodiment may determine a combination possibility of an L0 list and an L1 list that are determined for blocks that belong to the same picture, independently from the modification possibility of reference picture lists that belong to the current sequence. 
     If it is possible to combine the reference picture lists determined in the current slice for each of slices that belong to the same picture, the coding unit determiner  120  according to another exemplary embodiment may determine whether there are reference picture lists combined by using reference pictures of the L0 list and the L1 list. If it is determined that there are combined reference picture lists in the current slice for each of the slices that belong to the same picture, a modification possibility of the combined reference picture lists may be determined. 
     The output unit  130  according to an exemplary embodiment may encode information indicating whether there is the restriction condition of reference picture lists in the current sequence to insert the information into an SPS of a bitstream. Irrespective of whether there is the restriction condition of the reference picture list, the output unit  130  may encode information indicating the modification possibility of reference picture lists determined in the current sequence to insert the information into the SPS. 
     The output unit  130  according to another exemplary embodiment may encode information indicating the combination possibility of the L0 list and the L1 list in the current sequence to insert the information into the SPS. The output unit  130  according to another exemplary embodiment may encode information indicating whether there is a combined reference picture list in the current slice to insert the information into a slice header of the bitstream. The output unit  130  according to another exemplary embodiment may encode information indicating a modification possibility of combined reference picture lists in the current slice to insert the information into the slice header. 
       FIG. 10  is a block diagram of a video decoding apparatus  200  configured to perform a decoding operation based on a coding unit according to a tree structure, according to an exemplary embodiment. 
     The video decoding apparatus  200  configured to perform video prediction based on the coding unit according to the tree structure according to an exemplary embodiment includes a receiver  210 , an image data and encoding information extractor  220 , and an image data decoder  230 . Hereinafter, for convenience of description, the video decoding apparatus  200  configured to perform video prediction based on a coding unit according to a tree structure will be referred to as the ‘video decoding apparatus  200 ’. 
     Definitions of various terms, such as a coding unit, a depth, a prediction unit, a transformation unit, and information about various encoding modes, for decoding operations of the video decoding apparatus  200 , may be identical to those described with reference to  FIG. 9  and the video encoding apparatus  100 . 
     The receiver  210  receives and parses a bitstream of an encoded video. The image data and encoding information extractor  220  extracts encoded image data for each coding unit from the parsed bitstream, wherein the coding units have a tree structure according to each maximum coding unit, and outputs the extracted image data to the image data decoder  230 . The image data and encoding information extractor  220  may extract information about a maximum size of a coding unit of a current picture, from a header about the current picture, an SPS, or a PPS. 
     Also, the image data and encoding information extractor  220  extracts information about a coded depth and an encoding mode for the coding units having a tree structure according to each maximum coding unit, from the parsed bitstream. The extracted information about the coded depth and the encoding mode is output to the image data decoder  230 . In other words, the image data in a bitstream is split into the maximum coding unit so that the image data decoder  230  decodes the image data for each maximum coding unit. 
     The information about the coded depth and the encoding mode according to the maximum coding unit may be set for information about at least one coding unit corresponding to the coded depth, and information about an encoding mode may include information about a partition type of a corresponding coding unit corresponding to the coded depth, about a prediction mode, and a size of a transformation unit. Also, splitting information according to depths may be extracted as the information about the coded depth. 
     The information about the coded depth and the encoding mode according to each maximum coding unit extracted by the image data and encoding information extractor  220  is information about a coded depth and an encoding mode determined to generate a minimum encoding error when an encoder, such as the video encoding apparatus  100 , repeatedly performs encoding for each deeper coding unit according to depths according to each maximum coding unit. Accordingly, the video decoding apparatus  200  may restore an image by decoding the image data according to a coded depth and an encoding mode that generates the minimum encoding error. 
     Since encoding information about the coded depth and the encoding mode may be assigned to a predetermined data unit from among a corresponding coding unit, a prediction unit, and a minimum unit, the image data and encoding information extractor  220  may extract the information about the coded depth and the encoding mode according to the predetermined data units. The predetermined data units to which the same information about the coded depth and the encoding mode is assigned may be inferred to be the data units included in the same maximum coding unit. 
     The image data decoder  230  restores the current picture by decoding the image data in each maximum coding unit based on the information about the coded depth and the encoding mode according to the maximum coding units. In other words, the image data decoder  230  may decode the encoded image data based on the extracted information about the partition type, the prediction mode, and the transformation unit for each coding unit from among the coding units having the tree structure included in each maximum coding unit. A decoding process may include prediction including intra prediction and motion compensation, and inverse transformation. Inverse transformation may be performed according to a method of inverse orthogonal transformation or inverse integer transformation. 
     The image data decoder  230  may perform intra prediction or motion compensation according to a partition and a prediction mode of each coding unit, based on the information about the partition type and the prediction mode of the prediction unit of the coding unit according to coded depths. 
     In addition, the image data decoder  230  may read transformation unit information according to a tree structure for each coding unit for inverse transformation for each maximum coding unit and perform inverse transformation based on transformation units for each coding unit. Via the inverse transformation, a pixel value of a spatial region of the coding unit may be restored. 
     The image data decoder  230  may determine at least one coded depth of a current maximum coding unit by using split information according to depths. If the split information indicates that image data is no longer split in the current depth, the current depth is a coded depth. Accordingly, the image data decoder  230  may decode encoded data of at least one coding unit corresponding to each coded depth in the current maximum coding unit by using the information about the partition type of the prediction unit, the prediction mode, and the size of the transformation unit for each coding unit corresponding to the coded depth, and output the image data of the current maximum coding unit. 
     In other words, data units containing the encoding information including the same split information may be gathered by observing the encoding information set assigned for the predetermined data unit from among the coding unit, the prediction unit, and the minimum unit, and the gathered data units may be considered to be one data unit to be decoded by the image data decoder  230  in the same encoding mode. For each coding unit determined as described above, information about an encoding mode may be obtained so as to decode the current coding unit. 
     The video decoding apparatus  200  may determine a reference picture list according to the motion compensation method described with reference to  FIGS. 3A and 3B  to perform motion compensation. 
     The image data decoder  230  may determine a prediction unit for motion prediction for each respective coding unit according to a tree structure for each maximum coding unit and may perform motion prediction for each respective prediction unit and one or more partitions thereof. 
     The image data and encoding information extractor  220  according to another exemplary embodiment may parse information indicating whether there is a restriction condition of reference picture lists in a current sequence from an SPS of a bitstream. Irrespective of whether there is the restriction condition of reference picture lists, the image data and encoding information extractor  220  may parse information indicating a modification possibility of reference picture lists in the current sequence from the SPS. 
     The image data and encoding information extractor  220  according to another exemplary embodiment may parse information indicating a combination possibility of an L0 list and an L1 list in the current sequence from the SPS. The image data and encoding information extractor  220  according to another exemplary embodiment may parse information indicating whether there is a combined reference picture list in the current slice from a slice header of the bitstream. The image data and encoding information extractor  220  according to another exemplary embodiment may parse information indicating a modification possibility of combined reference picture lists in the current slice from the slice header. 
     The image data decoder  230  according to an exemplary embodiment may determine whether there is a restriction condition for identity of reference picture lists of slices of the same slice type from among slices included in the same picture, for each of pictures that belong to the current sequence. Independently from the restriction condition of reference picture lists, the image data decoder  230  may determine the modification possibility of reference picture lists determined in the current sequence. The image data decoder  230  may determine a reference picture list for blocks in an inter mode that belong to a picture, based on whether there is the restriction condition of reference picture lists and the modification possibility of reference picture lists that are previously determined. 
     If it is possible to combine the reference picture lists determined in the current slice for each of slices that belong to the same picture, the image data decoder  230  according to another exemplary embodiment may determine whether there are reference picture lists combined by using reference pictures of the L0 list and the L1 list. If it is determined that there are combined reference picture lists in the current slice for each of slices that belong to the same picture, a modification possibility of the combined reference picture lists may be determined. 
       FIG. 11  is a diagram for describing a concept of coding units according to an exemplary embodiment. 
     A size of a coding unit may be expressed in width×height, and may be 64×64, 32×32, 16×16, and 8×8. A coding unit of 64×64 may be split into partitions of 64×64, 64×32, 32×64, or 32×32, a coding unit of 32×32 may be split into partitions of 32×32, 32×16, 16×32, or 16×16, a coding unit of 16×16 may be split into partitions of 16×16, 16×8, 8×16, or 8×8, and a coding unit of 8×8 may be split into partitions of 8×8, 8×4, 4×8, or 4×4. 
     In video data  310 , a resolution is 1920×1080, a maximum size of a coding unit is 64, and a maximum depth is 2. In video data  320 , a resolution is 1920×1080, a maximum size of a coding unit is 64, and a maximum depth is 3. In video data  330 , a resolution is 352×288, a maximum size of a coding unit is 16, and a maximum depth is 1. The maximum depth shown in  FIG. 11  denotes a total number of splits from a maximum coding unit to a minimum decoding unit. 
     If a resolution is high or a data amount is large, a maximum size of a coding unit may be large so as to not only increase encoding efficiency but also to accurately reflect characteristics of an image. Accordingly, the maximum size of the coding unit of the video data  310  and  320  having a higher resolution than the video data  330  may be 64. 
     Since the maximum depth of the video data  310  is 2, coding units  315  of the video data  310  may include a maximum coding unit having a long axis size of 64, and coding units having long axis sizes of 32 and 16 since depths are deepened to two layers by splitting the maximum coding unit twice. Meanwhile, since the maximum depth of the video data  330  is 1, coding units  335  of the video data  330  may include a maximum coding unit having a long axis size of 16, and coding units having a long axis size of 8 since depths are deepened to one layer by splitting the maximum coding unit once. 
     Since the maximum depth of the video data  320  is 3, coding units  325  of the video data  320  may include a maximum coding unit having a long axis size of 64, and coding units having long axis sizes of 32, 16, and 8 since the depths are deepened to 3 layers by splitting the maximum coding unit three times. As a depth deepens, detailed information may be precisely expressed. 
       FIG. 12  is a block diagram of an image encoder  400  configured to perform an encoding operation based on coding units, according to an exemplary embodiment. 
     The image encoder  400  according to an exemplary embodiment performs operations of the coding unit determiner  120  of the video encoding apparatus  100  to encode image data. In other words, an intra predictor  410  performs intra prediction on coding units in an intra mode, from among a current frame  405 , and a motion estimator  420  and a motion compensator  425  perform inter estimation and motion compensation on coding units in an inter mode from among the current frame  405  by using the current frame  405  and a reference frame  495 . 
     Data output from the intra predictor  410 , the motion estimator  420 , and the motion compensator  425  is output as quantized transformation coefficients through a transformer  430  and a quantizer  440 . The quantized transformation coefficients are restored as data in a spatial domain through an inverse quantizer  460  and an inverse transformer  470 , and the restored data in the spatial domain is output as the reference frame  495  after being post-processed through a deblocking filter  480  and an SAO filter  490  (e.g., loop filter). The quantized transformation coefficients may be output as a bitstream  455  through an entropy encoder  450 . 
     In order for the image encoder  400  to be implemented in the video encoding apparatus  100  according to an exemplary embodiment, all elements of the image encoder  400 , e.g., the intra predictor  410 , the motion estimator  420 , the motion compensator  425 , the transformer  430 , the quantizer  440 , the entropy encoder  450 , the inverse quantizer  460 , the inverse transformer  470 , the deblocking filter  480 , and the SAO filter  490  perform operations based on each coding unit from among coding units having a tree structure while considering the maximum depth of each maximum coding unit. 
     Specifically, the intra predictor  410 , the motion estimator  420 , and the motion compensator  425  determine partitions and a prediction mode of each coding unit from among the coding units having a tree structure while considering the maximum size and the maximum depth of a current maximum coding unit, and the transformer  430  determines the size of the transformation unit in each coding unit from among the coding units having a tree structure. 
     The motion estimator  420  and the motion compensator  425  may determine reference picture lists based on the inter prediction method described with reference to  FIGS. 1A and 1B  above, determine reference pictures among the reference picture lists, and perform inter prediction. 
       FIG. 13  is a block diagram of an image decoder  500  configured to perform a decoding operation based on coding units, according to an exemplary embodiment. 
     A parser  510  parses encoded image data to be decoded and information about encoding required for decoding from a bitstream  505 . The encoded image data is output as inverse quantized data through an entropy decoder  520  and an inverse quantizer  530 , and the inverse quantized data is restored to image data in a spatial domain through an inverse transformer  540 . 
     An intra predictor  550  performs intra prediction on coding units in an intra mode with respect to the image data in the spatial domain, and a motion compensator  560  performs motion compensation on coding units in an inter mode by using a reference frame  585 . 
     The image data in the spatial domain, which passed through the intra predictor  550  and the motion compensator  560 , may be output as a restored frame  595  after being post-processed through a deblocking filter  570  and an SAO filter  580  (e.g., loop filter). Also, the image data that is post-processed through the deblocking filter  570  and the SAO filter  580  may be output as the reference frame  585 . 
     In order to decode the image data in the image data decoder  230  of the video decoding apparatus  200 , the image decoder  500  according to an exemplary embodiment may perform operations that are performed after the parser  510  performs an operation. 
     In order for the image decoder  500  to be implemented in the video decoding apparatus  200  according to an exemplary embodiment, all elements of the image decoder  500 , e.g., the parser  510 , the entropy decoder  520 , the inverse quantizer  530 , the inverse transformer  540 , the intra predictor  550 , the motion compensator  560 , the deblocking filter  570 , and the SAO filter  580  perform operations based on coding units having a tree structure for each maximum coding unit. 
     Specifically, the intra predictor  550  and the motion compensator  560  perform operations based on partitions and a prediction mode for each of the coding units having a tree structure, and the inverse transformer  540  performs operations based on a size of a transformation unit for each coding unit. 
     The motion compensator  560  may determine reference picture lists based on the inter prediction method described with reference to  FIGS. 1A and 1B  above, determine reference pictures among the reference picture lists, and perform motion compensation. 
       FIG. 14  is a diagram illustrating deeper coding units according to depths, and partitions, according to an exemplary embodiment. 
     The video encoding apparatus  100  according to an exemplary embodiment and the video decoding apparatus  200  according to an exemplary embodiment use hierarchical coding units so as to consider characteristics of an image. A maximum height, a maximum width, and a maximum depth of coding units may be adaptively determined according to the characteristics of the image, or may be differently set by a user. Sizes of deeper coding units according to depths may be determined according to the predetermined maximum size of the coding unit. 
     In a hierarchical structure  600  of coding units according to an exemplary embodiment, according to an exemplary embodiment, the maximum height and the maximum width of the coding units are each 64, and the maximum depth is 3. In this case, the maximum depth refers to a total number of times the coding unit is split from the maximum coding unit to the minimum coding unit. Since a depth deepens along a vertical axis of the hierarchical structure  600  according to an exemplary embodiment, a height and a width of the deeper coding unit are each split. Also, a prediction unit and partitions, which are bases for prediction encoding of each deeper coding unit, are shown along a horizontal axis of the hierarchical structure  600 . 
     In other words, a coding unit  610  is a maximum coding unit in the hierarchical structure  600 , wherein a depth is 0 and a size, e.g., a height by width, is 64×64. The depth deepens along the vertical axis, and a coding unit  620  having a size of 32×32 and a depth of 1, a coding unit  630  having a size of 16×16 and a depth of 2, and a coding unit  640  having a size of 8×8 and a depth of 3 exist. The coding unit  640  having the size of 8×8 and the depth of 3 is a minimum coding unit. 
     The prediction unit and the partitions of a coding unit are arranged along the horizontal axis according to each depth. In other words, if the coding unit  610  having the size of 64×64 and the depth of 0 is a prediction unit, the prediction unit may be split into partitions included in the encoding unit  610 , e.g., a partition  610  having a size of 64×64, partitions  612  having the size of 64×32, partitions  614  having the size of 32×64, or partitions  616  having the size of 32×32. 
     Similarly, a prediction unit of the coding unit  620  having the size of 32×32 and the depth of 1 may be split into partitions included in the coding unit  620 , e.g., a partition  620  having a size of 32×32, partitions  622  having a size of 32×16, partitions  624  having a size of 16×32, and partitions  626  having a size of 16×16. 
     Similarly, a prediction unit of the coding unit  630  having the size of 16×16 and the depth of 2 may be split into partitions included in the coding unit  630 , e.g., a partition having a size of 16×16 included in the coding unit  630 , partitions  632  having a size of 16×8, partitions  634  having a size of 8×16, and partitions  636  having a size of 8×8. 
     Similarly, a prediction unit of the coding unit  640  having the size of 8×8 and the depth of 3 may be split into partitions included in the coding unit  640 , e.g., a partition having a size of 8×8 included in the coding unit  640 , partitions  642  having a size of 8×4, partitions  644  having a size of 4×8, and partitions  646  having a size of 4×4. 
     In order to determine the at least one coded depth of the coding units constituting the maximum coding unit  610 , the coding unit determiner  120  of the video encoding apparatus  100  according to an exemplary embodiment performs encoding for coding units corresponding to each depth included in the maximum coding unit  610 . 
     A number of deeper coding units according to depths including data in the same range and the same size increases as the depth deepens. For example, four coding units corresponding to a depth of 2 are required to cover data that is included in one coding unit corresponding to a depth of 1. Accordingly, in order to compare encoding results of the same data according to depths, the coding unit corresponding to the depth of 1 and four coding units corresponding to the depth of 2 are each encoded. 
     In order to perform encoding for a current depth from among the depths, a least encoding error may be selected for the current depth by performing encoding for each prediction unit in the coding units corresponding to the current depth, along the horizontal axis of the hierarchical structure  600 . Alternatively, the minimum encoding error may be searched for by comparing the least encoding errors according to depths, by performing encoding for each depth as the depth deepens along the vertical axis of the hierarchical structure  600 . A depth and a partition having the minimum encoding error in the coding unit  610  may be selected as the coded depth and a partition type of the coding unit  610 . 
       FIG. 15  is a diagram for describing a relationship between a coding unit  710  and transformation units  720 , according to an exemplary embodiment. 
     The video encoding apparatus  100  according to an exemplary embodiment or the video decoding apparatus  200  according to an exemplary embodiment encodes or decodes an image according to coding units having sizes smaller than or equal to a maximum coding unit for each maximum coding unit. Sizes of transformation units for transformation during encoding may be selected based on data units that are not larger than a corresponding coding unit. 
     For example, in the video encoding apparatus  100  according to an exemplary embodiment or the video decoding apparatus  200  according to an exemplary embodiment, if a size of the coding unit  710  is 64×64, transformation may be performed by using the transformation units  720  having a size of 32×32. 
     Also, data of the coding unit  710  having the size of 64×64 may be encoded by performing the transformation on each of the transformation units having the size of 32×32, 16×16, 8×8, and 4×4, which are smaller than 64×64, and then a transformation unit having the least coding error may be selected. 
       FIG. 16  is a diagram for describing encoding information of coding units corresponding to a coded depth, according to an exemplary embodiment. 
     The output unit  130  of the video encoding apparatus  100  according to an exemplary embodiment may encode and transmit information  800  about a partition type, information  810  about a prediction mode, and information  820  about a size of a transformation unit for each coding unit corresponding to a coded depth, as information about an encoding mode. 
     The information  800  indicates information about a shape of a partition obtained by splitting a prediction unit of a current coding unit, wherein the partition is a data unit for prediction encoding the current coding unit. For example, a current coding unit CU — 0 having a size of 2N×2N may be split into any one of a partition  802  having a size of 2N×2N, a partition  804  having a size of 2N×N, a partition  806  having a size of N×2N, and a partition  808  having a size of N×N. Here, the information  800  about a partition type is set to indicate one of the partition  804  having a size of 2N×N, the partition  806  having a size of N×2N, and the partition  808  having a size of N×N 
     The information  810  indicates a prediction mode of each partition. For example, the information  810  may indicate a mode of prediction encoding performed on a partition indicated by the information  800 , e.g., an intra mode  812 , an inter mode  814 , or a skip mode  816 . 
     The information  820  indicates a transformation unit to be based on when transformation is performed on a current coding unit. For example, the transformation unit may be a first intra transformation unit  822 , a second intra transformation unit  824 , a first inter transformation unit  826 , or a second inter transformation unit  828 . 
     The image data and encoding information extractor  220  of the video decoding apparatus  200  according to an exemplary embodiment may extract and use the information  800 ,  810 , and  820  for decoding, according to each deeper coding unit. 
       FIG. 17  is a diagram of deeper coding units according to depths, according to an exemplary embodiment. 
     Split information may be used to indicate a change of a depth. The spilt information indicates whether a coding unit of a current depth is split into coding units of a lower depth. 
     A prediction unit  910  for prediction encoding a coding unit  900  having a depth of 0 and a size of 2N — 0×2N — 0 may include partitions of a partition type  912  having a size of 2N — 0×2N — 0, a partition type  914  having a size of 2N — 0×N — 0, a partition type  916  having a size of N — 0×2N — 0, and a partition type  918  having a size of N — 0×N — 0.  FIG. 17  only illustrates the partition types  912  through  918  which are obtained by symmetrically splitting the prediction unit  910 , but a partition type is not limited thereto, and the partitions of the prediction unit  910  may include asymmetrical partitions, partitions having a predetermined shape, and partitions having a geometrical shape. 
     Prediction encoding is repeatedly performed on one partition having a size of 2N — 0×2N — 0, two partitions having a size of 2N — 0×N — 0, two partitions having a size of N — 0×2N — 0, and four partitions having a size of N — 0×N — 0, according to each partition type. The prediction encoding in an intra mode and an inter mode may be performed on the partitions having the sizes of 2N — 0×2N — 0, N —  0×2N — 0, 2N — 0×N — 0, and N — 0×N — 0. The prediction encoding in a skip mode is performed only on the partition having the size of 2N — 0×2N — 0. 
     Errors of encoding including the prediction encoding in the partition types  912  through  918  are compared, and the least encoding error is determined among the partition types. If an encoding error is smallest in one of the partition types  912  through  916 , the prediction unit  910  may not be split into a lower depth. 
     If the encoding error is the smallest in the partition type  918 , a depth is changed from 0 to 1 to split the partition type  918  in operation  920 , and encoding is repeatedly performed on coding units  930  having a depth of 2 and a size of N — 0×N — 0 to search for a minimum encoding error. 
     A prediction unit  940  for prediction encoding the coding unit  930  having a depth of 1 and a size of 2N — 1×2N — 1 (=N — 0×N — 0) may include partitions of a partition type  942  having a size of 2N — 1×2N — 1, a partition type  944  having a size of 2N — 1×N — 1, a partition type  946  having a size of N — 1×2N — 1, and a partition type  948  having a size of N — 1×N — 1. 
     If an encoding error is the smallest in the partition type  948 , a depth is changed from 1 to 2 to split the partition type  948  in operation  950 , and encoding is repeatedly performed on coding units  960 , which have a depth of 2 and a size of N — 2×N — 2 to search for a minimum encoding error. 
     When a maximum depth is d, a split operation according to each depth may be performed until a depth becomes d−1, and split information may be encoded for up to when a depth is one of 0 to d−2. In other words, when encoding is performed until the depth is d−1 after a coding unit corresponding to a depth of d−2 is split in operation  970 , a prediction unit  990  for prediction encoding a coding unit  980  having a depth of d−1 and a size of 2N_(d−1)×2N_(d−1) may include partitions of a partition type  992  having a size of 2N_(d−1)×2N_(d−1), a partition type  994  having a size of 2N_(d−1)×N_(d−1), a partition type  996  having a size of N_(d−1)×2N_(d−1), and a partition type  998  having a size of N_(d−1)×N_(d−1). 
     Prediction encoding may be repeatedly performed on one partition having a size of 2N_(d−1)×2N_(d−1), two partitions having a size of 2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), four partitions having a size of N_(d−1)×N_(d−1) from among the partition types  992  through  998  to search for a partition type having a minimum encoding error. 
     Even when the partition type  998  has the minimum encoding error, since a maximum depth is d, a coding unit CU_(d−1) having a depth of d−1 is no longer split to a lower depth, and a coded depth for the coding units constituting a current maximum coding unit  900  is determined to be d−1 and a partition type of the current maximum coding unit  900  may be determined to be N_(d−1)×N_(d−1). Also, since the maximum depth is d and a minimum coding unit  980  having a lowermost depth of d−1 is no longer split to a lower depth, split information for the minimum coding unit  980  is not set. 
     A data unit  999  may be a ‘minimum unit’ for the current maximum coding unit. A minimum unit according to an exemplary embodiment may be a rectangular data unit obtained by splitting a minimum coding unit  980  by  4 . By performing the encoding repeatedly, the video encoding apparatus  100  may select a depth having the least encoding error by comparing encoding errors according to depths of the coding unit  900  to determine a coded depth, and set a corresponding partition type and a prediction mode as an encoding mode of the coded depth. 
     As such, the minimum encoding errors according to depths are compared in all of the depths of 1 through d, and a depth having the least encoding error may be determined as a coded depth. The coded depth, the partition type of the prediction unit, and the prediction mode may be encoded and transmitted as information about an encoding mode. Also, since a coding unit is split from a depth of 0 to a coded depth, only split information of the coded depth is set to 0, and split information of depths excluding the coded depth is set to 1. 
     The image data and encoding information extractor  220  of the video decoding apparatus  200  according to an exemplary embodiment may extract and use the information about the coded depth and the prediction unit of the coding unit  900  to decode the partition  912 . The video decoding apparatus  200  may determine a depth, in which split information is 0, as a coded depth by using split information according to depths, and use information about an encoding mode of the corresponding depth for decoding. 
       FIGS. 18 through 20  are diagrams for describing a relationship between coding units  1010 , prediction units  1060 , and transformation units  1070 , according to an exemplary embodiment. 
     The coding units  1010  are coding units having a tree structure, corresponding to coded depths determined by the video encoding apparatus  100  according to an exemplary embodiment, in a maximum coding unit. The prediction units  1060  are partitions of prediction units of each of the coding units  1010 , and the transformation units  1070  are transformation units of each of the coding units  1010 . 
     When a depth of a maximum coding unit is 0 in the coding units  1010 , depths of coding units  1012  and  1054  are 1, depths of coding units  1014 ,  1016 ,  1018 ,  1028 ,  1050 , and  1052  are 2, depths of coding units  1020 ,  1022 ,  1024 ,  1026 ,  1030 ,  1032 , and  1048  are 3, and depths of coding units  1040 ,  1042 ,  1044 , and  1046  are 4. 
     In the prediction units  1060 , some encoding units  1014 ,  1016 ,  1022 ,  1032 ,  1048 ,  1050 ,  1052 , and  1054  are obtained by splitting the coding units in the encoding units  1010 . In other words, partition types in the coding units  1014 ,  1022 ,  1050 , and  1054  have a size of 2N×N, partition types in the coding units  1016 ,  1048 , and  1052  have a size of N×2N, and a partition type of the coding unit  1032  has a size of N×N. Prediction units and partitions of the coding units  1010  are smaller than or equal to each coding unit. 
     Transformation or inverse transformation is performed on image data of the coding unit  1052  in the transformation units  1070  in a data unit that is smaller than the coding unit  1052 . Also, the coding units  1014 ,  1016 ,  1022 ,  1032 ,  1048 ,  1050 , and  1052  in the transformation units  1070  are different from those in the prediction units  1060  in terms of sizes and shapes. In other words, the video encoding and decoding apparatuses  100  and  200  according to an exemplary embodiment may perform intra prediction, motion estimation, motion compensation, transformation, and inverse transformation individually on a data unit in the same coding unit. 
     Accordingly, encoding is recursively performed on each of coding units having a hierarchical structure in each region of a maximum coding unit to determine an optimum coding unit, and thus coding units having a recursive tree structure may be obtained. Encoding information may include split information about a coding unit, information about a partition type, information about a prediction mode, and information about a size of a transformation unit. Table 1 shows the encoding information that may be set by the video encoding and decoding apparatuses  100  and  200  according to an exemplary embodiment. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Split Information 0 
                   
               
               
                 (Encoding on Coding Unit having Size of 2N × 2N and Current Depth of d) 
               
            
           
           
               
               
               
            
               
                   
                 Size of 
                   
               
               
                   
                 Transformation Unit 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Split 
                 Split 
                   
               
               
                   
                 Partition Type 
                 Information 0 
                 Information 1 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Symmetrical 
                 Asymmetrical 
                 of 
                 of 
                   
               
               
                 Prediction 
                 Partition 
                 Partition 
                 Transformation 
                 Transformation 
                 Split 
               
               
                 Mode 
                 Type 
                 Type 
                 Unit 
                 Unit 
                 Information 1 
               
               
                   
               
               
                 Intra 
                 2N × 2N 
                 2N × nU 
                 2N × 2N 
                 N × N 
                 Repeatedly 
               
               
                 Inter 
                 2N × N 
                 2N × nD 
                   
                 (Symmetrical 
                 Encode 
               
               
                 Skip 
                 N × 2N 
                 nL × 2N 
                   
                 Type) 
                 Coding Units 
               
               
                 (Only 
                 N × N 
                 nR × 2N 
                   
                 N/2 × N/2 
                 having 
               
               
                 2N × 2N) 
                   
                   
                   
                 (Asymmetrical 
                 Lower Depth 
               
               
                   
                   
                   
                   
                 Type) 
                 of d + 1 
               
               
                   
               
            
           
         
       
     
     The output unit  130  of the video encoding apparatus  100  according to an exemplary embodiment may output the encoding information about the coding units having a tree structure, and the image data and encoding information extractor  220  of the video decoding apparatus  200  according to an exemplary embodiment may extract the encoding information about the coding units having a tree structure from a received bitstream. 
     Split information indicates whether a current coding unit is split into coding units of a lower depth. If split information of a current depth d is 0, a depth, in which a current coding unit is no longer split into a lower depth, is a coded depth, and thus information about a partition type, prediction mode, and a size of a transformation unit may be defined for the coded depth. If the current coding unit is further split according to the split information, encoding is independently performed on four split coding units of a lower depth. 
     A prediction mode may be one of an intra mode, an inter mode, and a skip mode. The intra mode and the inter mode may be defined in all partition types, and the skip mode is defined only in a partition type having a size of 2N×2N. 
     The information about the partition type may indicate symmetrical partition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which are obtained by symmetrically splitting a height or a width of a prediction unit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N, which are obtained by asymmetrically splitting the height or width of the prediction unit. The asymmetrical partition types having the sizes of 2N×nU and 2N×nD may be respectively obtained by splitting the height of the prediction unit in 1:3 and 3:1, and the asymmetrical partition types having the sizes of nL×2N and nR×2N may be respectively obtained by splitting the width of the prediction unit in 1:3 and 3:1 
     The size of the transformation unit may be set to be two types in the intra mode and two types in the inter mode. In other words, if split information of the transformation unit is 0, the size of the transformation unit may be 2N×2N, which is the size of the current coding unit. If split information of the transformation unit is 1, the transformation units may be obtained by splitting the current coding unit. Also, if a partition type of the current coding unit having the size of 2N×2N is a symmetrical partition type, a size of a transformation unit may be N×N, and if the partition type of the current coding unit is an asymmetrical partition type, the size of the transformation unit may be N/2×N/2. 
     The encoding information about coding units having a tree structure may include at least one of a coding unit corresponding to a coded depth, a prediction unit, and a minimum unit. The coding unit corresponding to the coded depth may include at least one of a prediction unit and a minimum unit containing the same encoding information. 
     Accordingly, it is determined whether adjacent data units are included in the same coding unit corresponding to the coded depth by comparing encoding information of the adjacent data units. Also, a corresponding coding unit corresponding to a coded depth is determined by using encoding information of a data unit, and thus a distribution of coded depths in a maximum coding unit may be determined. 
     Accordingly, if a current coding unit is predicted based on encoding information of adjacent data units, encoding information of data units in deeper coding units adjacent to the current coding unit may be directly referred to and used. 
     Alternatively, if a current coding unit is predicted based on encoding information of adjacent data units, data units adjacent to the current coding unit are searched using encoded information of the data units, and the searched adjacent coding units may be referred to for predicting the current coding unit. 
       FIG. 21  is a diagram for describing a relationship between a coding unit, a prediction unit or a partition, and a transformation unit, according to encoding mode information of Table 1. 
     A maximum coding unit  1300  includes coding units  1302 ,  1304 ,  1306 ,  1312 ,  1314 ,  1316 , and  1318  of coded depths. Here, since the coding unit  1318  is a coding unit of a coded depth, split information may be set to 0. Information about a partition type of the coding unit  1318  having a size of 2N×2N may be set to be one of a partition type  1322  having a size of 2N×2N, a partition type  1324  having a size of 2N×N, a partition type  1326  having a size of N×2N, a partition type  1328  having a size of N×N, a partition type  1332  having a size of 2N×nU, a partition type  1334  having a size of 2N×nD, a partition type  1336  having a size of nL×2N, and a partition type  1338  having a size of nR×2N. 
     Split information (TU (Transformation Unit)size flag) of a transformation unit is a type of a transformation index. The size of the transformation unit corresponding to the transformation index may be changed according to a prediction unit type or partition type of the coding unit. 
     For example, when the partition type is set to be symmetrical, e.g., the partition type  1322 ,  1324 ,  1326 , or  1328 , a transformation unit  1342  having a size of 2N×2N is set if split information (TU size flag) of a transformation unit is 0, and a transformation unit  1344  having a size of N×N is set if a TU size flag is 1. 
     When the partition type is set to be asymmetrical, e.g., the partition type  1332 ,  1334 ,  1336 , or  1338 , a transformation unit  1352  having a size of 2N×2N is set if a TU size flag is 0, and a transformation unit  1354  having a size of N/2×N/2 is set if a TU size flag is 1. 
     Referring to  FIG. 21 , the TU size flag is a flag having a value or 0 or 1, but the TU size flag according to an exemplary embodiment is not limited to 1 bit, and a transformation unit may be hierarchically split having a tree structure while the TU size flag increases from 0. Split information (TU size flag) of a transformation unit may be an example of a transformation index. 
     In this case, the size of a transformation unit that has been actually used may be expressed by using a TU size flag of a transformation unit, according to an exemplary embodiment, together with a maximum size and minimum size of the transformation unit. According to an exemplary embodiment, the video encoding apparatus  100  is capable of encoding maximum transformation unit size information, minimum transformation unit size information, and a maximum TU size flag. A result of encoding the maximum transformation unit size information, the minimum transformation unit size information, and the maximum TU size flag may be inserted into an SPS. According to an exemplary embodiment, the video decoding apparatus  200  may decode video by using the maximum transformation unit size information, the minimum transformation unit size information, and the maximum TU size flag. 
     For example, (a) if the size of a current coding unit is 64×64 and a maximum transformation unit size is 32×32, (a−1) then the size of a transformation unit may be 32×32 when a TU size flag is 0, (a−2) may be 16×16 when the TU size flag is 1, and (a−3) may be 8×8 when the TU size flag is 2. 
     As another example, (b) if the size of the current coding unit is 32×32 and a minimum transformation unit size is 32×32, (b−1) then the size of the transformation unit may be 32×32 when the TU size flag is 0. Here, the TU size flag cannot be set to a value other than 0, since the size of the transformation unit cannot be less than 32×32. 
     As another example, (c) if the size of the current coding unit is 64×64 and a maximum TU size flag is 1, then the TU size flag may be 0 or 1. Here, the TU size flag cannot be set to a value other than 0 or 1. 
     Thus, if it is defined that the maximum TU size flag is ‘MaxTransformSizeIndex’, a minimum transformation unit size is ‘MinTransformSize’, and a transformation unit size is ‘RootTuSize’ when the TU size flag is 0, then a current minimum transformation unit size ‘CurrMinTuSize’ that can be determined in a current coding unit, may be defined by Equation (1): 
       CurrMinTuSize=max(MinTransformSize,RootTuSize/(2̂MaxTransformSizeIndex))  Equation (1)
 
     Compared to the current minimum transformation unit size ‘CurrMinTuSize’ that can be determined in the current coding unit, a transformation unit size ‘RootTuSize’ when the TU size flag is 0 may denote a maximum transformation unit size that can be selected in the system. In Equation (1), ‘RootTuSize/(2̂MaxTransformSizeIndex)’ denotes a transformation unit size when the transformation unit size ‘RootTuSize’, when the TU size flag is 0, is split a number of times corresponding to the maximum TU size flag, and ‘MinTransformSize’ denotes a minimum transformation size. Thus, a smaller value from among ‘RootTuSize/(2̂MaxTransformSizeIndex)’ and ‘MinTransformSize’ may be the current minimum transformation unit size ‘CurrMinTuSize’ that can be determined in the current coding unit. 
     According to an exemplary embodiment, the maximum transformation unit size RootTuSize may vary according to the type of a prediction mode. 
     For example, if a current prediction mode is an inter mode, then ‘RootTuSize’ may be determined by using Equation (2) below. In Equation (2), ‘MaxTransformSize’ denotes a maximum transformation unit size, and ‘PUSize’ denotes a current prediction unit size. 
       RootTuSize=min(MaxTransformSize,PUSize)  Equation (2)
 
     That is, if the current prediction mode is the inter mode, the transformation unit size ‘RootTuSize’, when the TU size flag is 0, may be a smaller value from among the maximum transformation unit size and the current prediction unit size. 
     If a prediction mode of a current partition unit is an intra mode, ‘RootTuSize’ may be determined by using Equation (3) below. In Equation (3), ‘PartitionSize’ denotes the size of the current partition unit. 
       RootTuSize=min(MaxTransformSize,PartitionSize)  Equation (3)
 
     That is, if the current prediction mode is the intra mode, the transformation unit size ‘RootTuSize’ when the TU size flag is 0 may be a smaller value from among the maximum transformation unit size and the size of the current partition unit. 
     However, the current maximum transformation unit size ‘RootTuSize’ that varies according to the type of a prediction mode in a partition unit is just an example and the exemplary embodiments are not limited thereto. 
     According to the video encoding method based on coding units having a tree structure as described with reference to  FIGS. 9 through 21 , image data of a spatial region is encoded for each coding unit of a tree structure. According to the video decoding method based on coding units having a tree structure, decoding is performed for each maximum coding unit to restore image data of a spatial region. Thus, a picture and a video that is a picture sequence may be restored. The restored video may be reproduced by a reproducing apparatus, stored in a storage medium, or transmitted through a network. 
     The exemplary embodiments may be written as computer programs and may be implemented in general-use digital computers that execute the programs 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.) and optical recording media (e.g., CD-ROMs, or DVDs). 
     For convenience of description, a video encoding method according to the inter prediction method, the motion prediction method, or the motion compensation method, which has been described with reference to  FIGS. 1A through 21 , may be collectively referred to as a ‘video encoding method according to an exemplary embodiment’. In addition, the video decoding method according to the inter prediction method or the motion compensation method, which has been described with reference to  FIGS. 1A through 20 , may be referred to as a ‘video decoding method according to an exemplary embodiment’. 
     A video encoding apparatus including the inter prediction apparatus  10 , the motion prediction apparatus  20 , the motion compensation apparatus  30 , the video encoding apparatus  100 , or the image encoder  400 , which has been described with reference to  FIGS. 1A through 21 , may be referred to as a ‘video encoding apparatus according to an exemplary embodiment’. In addition, a video decoding apparatus including the inter prediction apparatus  10 , the motion compensation apparatus  30 , the video decoding apparatus  200 , or the image decoder  500 , which has been described with reference to  FIGS. 1A through 20 , may be referred to as a ‘video decoding apparatus according to an exemplary embodiment’. 
     A computer readable recording medium storing a program, e.g., a disc  26000 , according to an exemplary embodiment will now be described in detail. 
       FIG. 22  illustrates a physical structure of a disc  26000  that stores a program, according to an exemplary embodiment. The disc  26000  which is a storage medium may be a hard drive, a compact disc-read only memory (CD-ROM) disc, a Blu-ray disc, or a digital versatile disc (DVD). The disc  260000  includes a plurality of concentric tracks Tf each being divided into a specific number of sectors Se in a circumferential direction of the disc  26000 . In a specific region of the disc  260000  according to an exemplary embodiment, a program that executes a method of determining a quantization parameter, a video encoding method, and a video decoding method as described above may be assigned and stored. 
     A computer system embodied using a storage medium that stores a program for executing a video encoding method and a video decoding method as described above will now be described with reference to  FIG. 23 . 
       FIG. 23  illustrates a disc drive  26800  that records and reads a program by using a disc  26000 . A computer system  26700  may store a program that executes at least one of a video encoding method and a video decoding method according to an exemplary embodiment, in the disc  26000  via the disc drive  26800 . To run the program stored in the disc  26000  in the computer system  26700 , the program may be read from the disc  26000  and be transmitted to the computer system  26700  by using the disc drive  26800 . 
     The program that executes at least one of a video encoding method and a video decoding method according to an exemplary embodiment may be stored not only in the disc  26000  illustrated in  FIGS. 22 and 23  but also in a memory card, a ROM cassette, or a solid state drive (SSD). 
     A system to which the video encoding method and a video decoding method described above are applied will be described below. 
       FIG. 24  illustrates an entire structure of a content supply system  11000  that provides a content distribution service. A service area of a communication system is divided into predetermined-sized cells, and wireless base stations  11700 ,  11800 ,  11900 , and  12000  are installed in these cells, respectively. 
     The content supply system  11000  includes a plurality of independent devices. For example, the plurality of independent devices, such as a computer  12100 , a personal digital assistant (PDA)  12200 , a video camera  12300 , and a mobile phone  12500 , are connected to the Internet  11100  via an internet service provider  11200 , a communication network  11400 , and the wireless base stations  11700 ,  11800 ,  11900 , and  12000 . 
     However, the content supply system  11000  is not limited to the configuration illustrated in  FIG. 24 , and devices may be selectively connected thereto. The plurality of independent devices may be directly connected to the communication network  11400 , rather than via the wireless base stations  11700 ,  11800 ,  11900 , and  12000 . 
     The video camera  12300  is an imaging device, e.g., a digital video camera, which is capable of capturing video images. The mobile phone  12500  may employ at least one communication method from among various protocols, e.g., Personal Digital Communications (PDC), Code Division Multiple Access (CDMA), Wideband-Code Division Multiple Access (W-CDMA), Global System for Mobile Communications (GSM), and Personal Handyphone System (PHS). 
     The video camera  12300  may be connected to a streaming server  11300  via the wireless base station  11900  and the communication network  11400 . The streaming server  11300  enables content received from a user via the video camera  12300  to be streamed via a real-time broadcast. The content received from the video camera  12300  may be encoded using the video camera  12300  or the streaming server  11300 . Video data captured by the video camera  12300  may be transmitted to the streaming server  11300  via the computer  12100 . 
     Video data captured by a camera  12600  may also be transmitted to the streaming server  11300  via the computer  12100 . The camera  12600  is an imaging device capable of capturing both still images and video images, similar to a digital camera. The video data captured by the camera  12600  may be encoded using the camera  12600  or the computer  12100 . Software that performs encoding and decoding of video may be stored in a computer readable recording medium, e.g., a CD-ROM disc, a floppy disc, a hard disc drive, an SSD, or a memory card, which may be accessible by the computer  12100 . 
     If video data is captured by a camera built into the mobile phone  12500 , the video data may be received from the mobile phone  12500 . 
     The video data may also be encoded by a large scale integrated circuit (LSI) system installed in the video camera  12300 , the mobile phone  12500 , or the camera  12600 . 
     According to an exemplary embodiment, the content supply system  11000  may encode content data recorded by a user using the video camera  12300 , the camera  12600 , the mobile phone  12500 , or another imaging device, e.g., content recorded during a concert, and transmit the encoded content data to the streaming server  11300 . The streaming server  11300  may transmit the encoded content data in a type of a streaming content to other clients that request the content data. 
     The clients are devices capable of decoding the encoded content data, e.g., the computer  12100 , the PDA  12200 , the video camera  12300 , or the mobile phone  12500 . Thus, the content supply system  11000  enables the clients to receive and reproduce the encoded content data. Also, the content supply system  11000  enables the clients to receive the encoded content data and decode and reproduce the encoded content data in real time, thereby enabling personal broadcasting. 
     Encoding and decoding operations of the plurality of independent devices included in the content supply system  11000  may be similar to those of a video encoding apparatus and a video decoding apparatus according to an exemplary embodiment. 
     The mobile phone  12500  included in the content supply system  11000  according to an exemplary embodiment will now be described in greater detail with reference to  FIGS. 25 and 26 . 
       FIG. 25  illustrates an external structure of a mobile phone  12500  to which a video encoding method and a video decoding method are applied, according to an exemplary embodiment. The mobile phone  12500  may be a smart phone, the functions of which are not limited and a large part of the functions of which may be changed or expanded. 
     The mobile phone  12500  includes an internal antenna  12510  via which a radio-frequency (RF) signal may be exchanged with the wireless base station (e.g.,  12000 ) of  FIG. 24 , and includes a display screen  12520  for displaying images captured by a camera  12530  or images that are received via the antenna  12510  and decoded, e.g., a liquid crystal display (LCD) or an organic light-emitting diodes (OLED) screen. The smart phone  12510  includes an operation panel  12540  including a control button and a touch panel. If the display screen  12520  is a touch screen, the operation panel  12540  further includes a touch sensing panel of the display screen  12520 . The smart phone  12500  includes a speaker  12580  for outputting voice and sound or may include another type of sound output unit, and a microphone  12550  for inputting voice and sound or may include another type of sound input unit. The smart phone  12510  further includes the camera  12530 , such as a charge-coupled device (CCD) camera, to capture video and still images. The smart phone  12510  may further include a storage medium  12570  for storing encoded/decoded data, e.g., video or still images captured by the camera  12530 , received via email, or obtained according to various ways; and a slot  12560  via which the storage medium  12570  is loaded into the mobile phone  12500 . The storage medium  12570  may be a flash memory, e.g., a secure digital (SD) card or an electrically erasable and programmable read only memory (EEPROM) included in a plastic case. 
       FIG. 26  illustrates an internal structure of the mobile phone  12500 , according to an exemplary embodiment. To systemically control parts of the mobile phone  12500  including the display screen  12520  and the operation panel  12540 , a power supply circuit  12700 , an operation input controller  12640 , an image encoding unit  12720  (e.g., image encoder), a camera interface  12630 , an LCD controller  12620 , an image decoding unit  12690  (e.g., image decoder), a multiplexer/demultiplexer  12680 , a recording/reading unit  12670  (e.g., recorder/reader), a modulation/demodulation unit  12660  (e.g., modulator/demodulator), and a sound processor  12650  are connected to a central controller  12710  via a synchronization bus  12730 . 
     If a user operates a power button and thereby turns the mobile phone  12500  from a ‘power off’ state to a ‘power on’ state, the power supply circuit  12700  supplies power to all the parts of the mobile phone  12500  from a battery pack, thereby setting the mobile phone  12500  in an operation mode. 
     The central controller  12710  includes a central processing unit (CPU), a ROM, and a random access memory (RAM). 
     While the mobile phone  12500  transmits communication data to the outside, a digital signal is generated in the mobile phone  12500  under control of the central controller. For example, the sound processor  12650  may generate a digital sound signal, the image encoding unit  12720  may generate a digital image signal, and text data of a message may be generated via the operation panel  12540  and the operation input controller  12640 . When a digital signal is delivered to the modulation/demodulation unit  12660  under control of the central controller  12710 , the modulation/demodulation unit  12660  modulates a frequency band of the digital signal, and a communication circuit  12610  performs digital-to-analog conversion (DAC) and frequency conversion on the frequency band-modulated digital sound signal. A transmission signal output from the communication circuit  12610  may be transmitted to a voice communication base station or the wireless base station (e.g.,  12000 ) via the antenna  12510 . 
     For example, when the mobile phone  12500  is in a conversation mode, a sound signal obtained via the microphone  12550  is transformed into a digital sound signal by the sound processor  12650 , under control of the central controller  12710 . The digital sound signal may be transformed into a transformation signal via the modulation/demodulation unit  12660  and the communication circuit  12610 , and may be transmitted via the antenna  12510 . 
     When a message, e.g., text message or email, is transmitted in a data communication mode, text data of the text message is input via the operation panel  12540  and is transmitted to the central controller  12610  via the operation input controller  12640 . Under control of the central controller  12610 , the text data is transformed into a transmission signal via the modulation/demodulation unit  12660  and the communication circuit  12610  and is transmitted to the wireless base station  12000  via the antenna  12510 . 
     To transmit image data in the data communication mode, image data captured by the camera  12530  is provided to the image encoding unit  12720  via the camera interface  12630 . The captured image data may be directly displayed on the display screen  12520  via the camera interface  12630  and the LCD controller  12620 . 
     A structure of the image encoding unit  12720  may correspond to a structure of the video encoding apparatus  100  described above. The image encoding unit  12720  may transform the image data received from the camera  12530  into compressed and encoded image data according to the video encoding method described above, and then output the encoded image data to the multiplexer/demultiplexer  12680 . During a recording operation of the camera  12530 , a sound signal obtained by the microphone  12550  of the mobile phone  12500  may be transformed into digital sound data via the sound processor  12650 , and the digital sound data may be delivered to the multiplexer/demultiplexer  12680 . 
     The multiplexer/demultiplexer  12680  multiplexes the encoded image data received from the image encoding unit  12720 , together with the sound data received from the sound processor  12650 . A result of multiplexing the data may be transformed into a transmission signal via the modulation/demodulation unit  12660  and the communication circuit  12610 , and may then be transmitted via the antenna  12510 . 
     While the mobile phone  12500  receives communication data from the outside, frequency recovery and ADC are performed on a signal received via the antenna  12510  to transform the signal into a digital signal. The modulation/demodulation unit  12660  modulates a frequency band of the digital signal. The frequency-band modulated digital signal is transmitted to the video decoding unit  12690 , the sound processor  12650 , or the LCD controller  12620 , according to the type of the digital signal. 
     In the conversation mode, the mobile phone  12500  amplifies a signal received via the antenna  12510 , and obtains a digital sound signal by performing frequency conversion and ADC on the amplified signal. A received digital sound signal is transformed into an analog sound signal via the modulation/demodulation unit  12660  and the sound processor  12650 , and the analog sound signal is output via the speaker  12580 , under control of the central controller  12710 . 
     When in the data communication mode, data of a video file accessed at an Internet website is received, a signal received from wireless base station (e.g.,  12000 ) via the antenna  12510  is output as multiplexed data via the modulation/demodulation unit  1266 , and the multiplexed data is transmitted to the multiplexer/demultiplexer  12680 . 
     To decode the multiplexed data received via the antenna  12510 , the multiplexer/demultiplexer  12680  demultiplexes the multiplexed data into an encoded video data stream and an encoded audio data stream. Via the synchronization bus  12730 , the encoded video data stream and the encoded audio data stream are provided to the image decoding unit  12690  and the sound processor  12650 , respectively. 
     A structure of the image decoding unit  12690  may correspond to a structure of the video decoding apparatus  200  described above. The image decoding unit  1269  may decode the encoded video data to obtain restored video data and provide the restored video data to the display screen  12520  via the LCD controller  12602 , according to the video decoding method described above. 
     Thus, the data of the video file accessed at the Internet website may be displayed on the display screen  12520 . At the same time, the sound processor  12650  may transform audio data into an analog sound signal, and provide the analog sound signal to the speaker  12580 . Thus, audio data contained in the video file accessed at the Internet website may also be reproduced via the speaker  12580 . 
     The mobile phone  12500  or another type of communication terminal may be a transceiving terminal including both a video encoding apparatus and a video decoding apparatus according to an exemplary embodiment, may be a transceiving terminal including only the video encoding apparatus, or may be a transceiving terminal including only the video decoding apparatus. 
     A communication system according to the exemplary embodiments is not limited to the communication system described above with reference to  FIG. 24 . For example,  FIG. 27  illustrates a digital broadcasting system employing a communication system, according to an exemplary embodiment. The digital broadcasting system of  FIG. 27  may receive a digital broadcast transmitted via a satellite or a terrestrial network by using a video encoding apparatus and a video decoding apparatus according to an exemplary embodiment. 
     Specifically, a broadcasting station  12890  transmits a video data stream to a communication satellite or a broadcasting satellite  12900  by using radio waves. The broadcasting satellite  12900  transmits a broadcast signal, and the broadcast signal is transmitted to a satellite broadcast receiver via a household antenna  12860 . In every house, an encoded video stream may be decoded and reproduced by a TV receiver  12810 , a set-top box  12870 , or another device. 
     When a video decoding apparatus according to an exemplary embodiment is implemented in a reproducing apparatus  12830 , the reproducing apparatus  12830  may parse and decode an encoded video stream recorded on a storage medium  12820 , such as a disc or a memory card, to restore digital signals. Thus, the restored video signal may be reproduced, for example, on a monitor  12840 . 
     In the set-top box  12870  connected to the antenna  12860  for a satellite/terrestrial broadcast or a cable antenna  12850  for receiving a cable television (TV) broadcast, a video decoding apparatus according to an exemplary embodiment may be installed. Data output from the set-top box  12870  may also be reproduced on a TV monitor  12880 . 
     As another example, a video decoding apparatus according to an exemplary embodiment may be installed in the TV receiver  12810  instead of the set-top box  12870 . 
     An automobile  12920  including an appropriate antenna  12910  may receive a signal transmitted from the satellite  12900  or the wireless base station  11700 . A decoded video may be reproduced on a display screen of an automobile navigation system  12930  built into the automobile  12920 . 
     A video signal may be encoded by a video encoding apparatus according to an exemplary embodiment and may then be stored in a storage medium. Specifically, an image signal may be stored in a DVD disc  12960  by a DVD recorder or may be stored in a hard disc by a hard disc recorder  12950 . As another example, the video signal may be stored in an SD card  12970 . If the hard disc recorder  12950  includes a video decoding apparatus according to an exemplary embodiment, a video signal recorded on the DVD disc  12960 , the SD card  12970 , or another storage medium may be reproduced on the TV monitor  12880 . 
     The automobile navigation system  12930  may not include the camera  12530 , the camera interface  12630 , and the image encoding unit  12720  of  FIG. 26 . For example, the computer  12100  and the TV receiver  12810  may not be included in the camera  12530 , the camera interface  12630 , or the image encoding unit  12720  of  FIG. 26 . 
       FIG. 28  illustrates a network structure of a cloud computing system using a video encoding apparatus and a video decoding apparatus, according to an exemplary embodiment. 
     The cloud computing system may include a cloud computing server  14000 , a user database (DB)  14100 , a plurality of computing resources  14200 , and a user terminal. 
     The cloud computing system provides an on-demand outsourcing service of the plurality of computing resources  14200  via a data communication network, e.g., the Internet, in response to a request from the user terminal. Under a cloud computing environment, a service provider provides users with desired services by combining computing resources at data centers located at physically different locations by using virtualization technology. A service user does not have to install computing resources, e.g., an application, storage, an operating system (OS), and security, in his or her own terminal in order to use the computing resources, but may select and use desired services from among services in a virtual space generated through the virtualization technology, at a desired point of time. 
     A user terminal of a specified service user is connected to the cloud computing server  14000  via a data communication network including the Internet and a mobile telecommunication network. User terminals may be provided with cloud computing services, and particularly video reproduction services, from the cloud computing server  14000 . The user terminals may be various types of electronic devices capable of being connected to the Internet, e.g., a desk-top PC  14300 , a smart TV  14400 , a smart phone  14500 , a notebook computer  14600 , a portable multimedia player (PMP)  14700 , a tablet PC  14800 , and the like. 
     The cloud computing server  14100  may combine the plurality of computing resources  14200  distributed in a cloud network and provide user terminals with a result of the combining. The plurality of computing resources  14200  may include various data services, and may include data uploaded from user terminals. As described above, the cloud computing server  14100  may provide user terminals with desired services by combining video databases distributed in different regions according to the virtualization technology. 
     User information about users who have subscribed to a cloud computing service is stored in the user DB  14100 . The user information may include login information, addresses, names, and personal credit information of the users. The user information may further include indexes of videos. Here, the indexes may include a list of videos that have already been reproduced, a list of videos that are being reproduced, a pausing point of a video that was being reproduced, and the like. 
     Information about a video stored in the user DB  14100  may be shared between user devices. For example, when a video service is provided to the notebook computer  14600  in response to a request from the notebook computer  14600 , a reproduction history of the video service is stored in the user DB  14100 . When a request to reproduce this video service is received from the smart phone  14500 , the cloud computing server  14000  searches for and reproduces the video service, based on the user DB  14100 . When the smart phone  14500  receives a video data stream from the cloud computing server  14000 , a process of reproducing video by decoding the video data stream is similar to an operation of the mobile phone  12500  described above with reference to  FIG. 25 . 
     The cloud computing server  14000  may refer to a reproduction history of a desired video service, stored in the user DB  14100 . For example, the cloud computing server  14000  receives a request to reproduce a video stored in the user DB  14100 , from a user terminal. If this video was being reproduced, then a method of streaming the video, performed by the cloud computing server  14000 , may vary according to the request from the user terminal, e.g., according to whether the video will be reproduced, starting from a start thereof or a pausing point thereof. For example, if the user terminal requests to reproduce the video, starting from the start thereof, the cloud computing server  14000  transmits streaming data of the video starting from a first frame thereof to the user terminal. If the user terminal requests to reproduce the video, starting from the pausing point thereof, the cloud computing server  14000  transmits streaming data of the video starting from a frame corresponding to the pausing point, to the user terminal. 
     In this case, the user terminal may include a video decoding apparatus as described above with reference to  FIGS. 1A through 21 . As another example, the user terminal may include a video encoding apparatus as described above with reference to  FIGS. 1A through 21 . Alternatively, the user terminal may include both the video decoding apparatus and the video encoding apparatus as described above with reference to  FIGS. 1A through 21 . 
     Various applications of a video encoding method, a video decoding method, a video encoding apparatus, and a video decoding apparatus according to exemplary embodiments described above with reference to  FIGS. 1A through 21  have been described above with reference to  FIGS. 22 through 28 . However, methods of storing the video encoding method and the video decoding method in a storage medium or methods of implementing the video encoding apparatus and the video decoding apparatus in a device according to various exemplary embodiments described above with reference to  FIGS. 1A through 21  are not limited to the exemplary embodiments described above with reference to  FIGS. 22 through 28 . 
     While the exemplary embodiments have been particularly shown and described with reference to certain exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope according to the exemplary embodiments as defined by the following claims.