Patent Publication Number: US-2016227250-A1

Title: Method and apparatus for depth inter coding, and method and apparatus for depth inter decoding

Description:
TECHNICAL FIELD 
     The present invention relates to a method of encoding and decoding a video and, more particularly, to an inter prediction method for a method and apparatus for decoding/encoding a depth image of a video. 
     BACKGROUND ART 
     A three-dimensional (3D) video provides depth and spatial shape information together with video information. While a stereoscopic video provides videos of different views to the left and right eyes, a 3D video provides a video shown from a different direction whenever a user changes views. Thus, videos captured in multiple views are required to generate the 3D video. 
     The videos captured in multiple views to generate the 3D video have an enormous amount of data. Accordingly, in consideration of network infrastructures, terrestrial bandwidths, etc., even when the 3D video is encoded by a coding apparatus optimized for single-view video coding, e.g., MPEG-2, H.264/AVC, or HEVC, implementation thereof is almost impossible. 
     Therefore, a multi-view (multilayer) video coding apparatus optimized to generate a 3D video is required. Particularly, development of a technology for efficiently reducing temporal and inter-view redundancy is necessary. 
     For example, a multi-view video codec may increase a compression ratio by encoding base-view pictures by using single-view video coding, and encoding extended-view pictures with reference to the base-view pictures. Furthermore, by additionally encoding auxiliary data such as a depth image, pictures of a larger number of views compared to the number of views of input pictures may be generated by a decoder. Herein, since the depth image is not directly shown to a user but is used to generate intermediate-view composite pictures, deterioration of the depth image reduces the quality of the composite pictures. Accordingly, the multi-view video codec should efficiently encode the depth image as well as the multi-view pictures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     The present invention provides a video encoding apparatus and method for encoding a depth image by applying an inter simplified depth coding (SDC) mode to a coding unit of the depth image. The present invention also provides a video decoding apparatus and method for reconstructing a depth image encoded by applying an inter SDC mode to a coding unit thereof. The present invention also provides a computer-readable recording medium having recorded thereon a computer program for executing the video encoding method and the video decoding method. However, the present invention is not limited thereto. 
     Technical Solution 
     According to an aspect of the present invention, a video decoding method performed by a video decoding apparatus includes acquiring simplified depth coding (SDC) mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, acquiring the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied based on the SDC mode information, and reconstructing a current block of the coding unit by using the residual DC component and a reference block of the prediction unit. 
     The video decoding method may further include determining whether the SDC mode is enabled for the depth image, based on SDC mode enable information indicating whether the SDC mode is enabled for the depth image. 
     The acquiring of the SDC mode information may include acquiring the SDC mode information if the SDC mode enable information indicates the SDC mode is enabled for the depth image. 
     The acquiring of the SDC mode information may include acquiring the SDC mode information determined based on partition mode information of the prediction unit. 
     The acquiring of the SDC mode information may include acquiring the SDC mode information indicating to apply the SDC mode, if the partition mode information indicates a 2N×2N mode. 
     The acquiring of the residual DC component may include acquiring the residual DC component determined as an average value of one or more residual pixel values of the residual block. 
     The acquiring of the residual DC component may include acquiring the residual DC component determined as an average value of a top left residual pixel value, a top right residual pixel value, a bottom left residual pixel value, and a bottom right residual pixel value of the residual block. 
     The acquiring of the residual DC component may include determining the residual pixel values to be used to calculate the average value, based on a size of at least one of the coding unit and the prediction unit, and acquiring the residual DC component determined as an average value of the residual pixel values. 
     The acquiring of the residual DC component may include acquiring an average value of the residual pixel values, acquiring a plurality of residual DC component candidates by adding integer multiples of an offset value to the average value, and acquiring an optimal residual DC component among the residual DC component candidates based on rate-distortion optimization. 
     The acquiring of the residual DC component may include acquiring an absolute value of the residual DC component and then acquiring a sign of the residual DC component if the absolute value is not 0. 
     According to another aspect of the present invention, a video decoding apparatus includes a simplified depth coding (SDC) mode information acquirer for acquiring SDC mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, a residual DC component acquirer for acquiring the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied based on the SDC mode information, and a decoder for reconstructing a current block of the coding unit by using the residual DC component and a reference block of the prediction unit. 
     According to another aspect of the present invention, a video encoding method performed by a video encoding apparatus includes generating simplified depth coding (SDC) mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, determining the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied, and generating a bitstream including the SDC mode information and the residual DC component. 
     According to another aspect of the present invention, a video encoding apparatus includes a simplified depth coding (SDC) mode information generator for generating SDC mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, a residual DC component determiner for determining the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied, and an encoder for generating a bitstream including the SDC mode information and the residual DC component. 
     According to another aspect of the present invention, a computer-readable recording medium has recorded thereon a computer program for executing the above video decoding method. 
     According to another aspect of the present invention, a computer-readable recording medium has recorded thereon a computer program for executing the above video encoding method. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of a video encoding apparatus  10  according to an embodiment.  FIG. 1B  is a flowchart of a video encoding method  11  according to an embodiment. 
         FIG. 2A  is a block diagram of a video decoding apparatus  20  according to an embodiment.  FIG. 2B  is a flowchart of a video decoding method  21  according to an embodiment. 
         FIG. 3A  is a diagram for describing a flag indicating whether to enable an inter SDC mode.  FIG. 3B  is a diagram for describing a flag indicating whether an inter SDC mode is applied to a coding unit.  FIG. 3C  is a diagram for describing a procedure for acquiring a residual DC component by the video decoding apparatus  20 . 
         FIG. 4  shows an interlayer prediction structure according to an embodiment. 
         FIG. 5  is a flowchart of a method of encoding a residual block based on a prediction mode by an interlayer video encoding apparatus according to an embodiment. 
         FIG. 6  is a flowchart of a method of encoding a residual block based on a prediction mode by an interlayer video encoding apparatus according to an embodiment. 
         FIGS. 7A and 7B  are diagrams for describing examples of generating residual data of a coding unit in a case when a prediction mode is an SDC mode, according to embodiments. 
         FIG. 8  illustrates a block diagram of a video encoding apparatus based on coding units of a tree structure  800 , according to an embodiment of the present invention. 
         FIG. 9  illustrates a block diagram of a video decoding apparatus based on coding units of a tree structure  900 , according to an embodiment. 
         FIG. 10  illustrates a concept of coding units, according to an embodiment. 
         FIG. 11  illustrates a block diagram of a video encoder  1100  based on coding units, according to an embodiment. 
         FIG. 12  illustrates a block diagram of a video decoder  1200  based on coding units, according to an embodiment. 
         FIG. 13  illustrates deeper coding units according to depths, and partitions, according to an embodiment. 
         FIG. 14  illustrates a relationship between a coding unit and transformation units, according to an embodiment. 
         FIG. 15  illustrates a plurality of pieces of encoding information according to depths, according to an embodiment. 
         FIG. 16  illustrates deeper coding units according to depths, according to an embodiment. 
         FIGS. 17, 18, and 19  illustrate a relationship between coding units, prediction units, and transformation units, according to an embodiment. 
         FIG. 20  illustrates a relationship between a coding unit, a prediction unit, and a transformation unit, according to encoding mode information of Table 1. 
         FIG. 21  illustrates a physical structure of a disc  26000  in which a program is stored, according to an embodiment. 
         FIG. 22  illustrates a disc drive  26800  for recording and reading a program by using the disc  26000 . 
         FIG. 23  illustrates an overall structure of a content supply system  11000  for providing a content distribution service. 
         FIG. 24  illustrates an external structure of a mobile phone  12500  to which a video encoding method and a video decoding method of the present invention are applied, according to an embodiment. 
         FIG. 25  illustrates an internal structure of the mobile phone  12500 . 
         FIG. 26  illustrates a digital broadcasting system employing a communication system, according to an embodiment. 
         FIG. 27  illustrates a network structure of a cloud computing system using a video encoding apparatus and a video decoding apparatus, according to an embodiment. 
     
    
    
     BEST MODE 
     According to an aspect of the present invention, a video decoding method performed by a video decoding apparatus includes acquiring simplified depth coding (SDC) mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, acquiring the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied based on the SDC mode information, and reconstructing a current block of the coding unit by using the residual DC component and a reference block of the prediction unit. 
     According to another aspect of the present invention, a video decoding apparatus includes a simplified depth coding (SDC) mode information acquirer for acquiring SDC mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, a residual DC component acquirer for acquiring the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied based on the SDC mode information, and a decoder for reconstructing a current block of the coding unit by using the residual DC component and a reference block of the prediction unit. 
     According to another aspect of the present invention, a video encoding method performed by a video encoding apparatus includes generating simplified depth coding (SDC) mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, determining the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied, and generating a bitstream including the SDC mode information and the residual DC component. 
     According to another aspect of the present invention, a video encoding apparatus includes a simplified depth coding (SDC) mode information generator for generating SDC mode information indicating whether an SDC mode for encoding a residual block of a prediction unit included in a coding unit of a depth image by using one residual DC component is applied to the coding unit, a residual DC component determiner for determining the residual DC component corresponding to the prediction unit of the coding unit to which the SDC mode is applied, and an encoder for generating a bitstream including the SDC mode information and the residual DC component. 
     According to another aspect of the present invention, a computer-readable recording medium has recorded thereon a computer program for executing the above video decoding method. 
     According to another aspect of the present invention, a computer-readable recording medium has recorded thereon a computer program for executing the above video encoding method. 
     MODE OF THE INVENTION 
     Hereinafter, an inter prediction method of a depth image for a video decoding and encoding apparatus and a method thereof according to embodiments will be described with reference to  FIGS. 1A to 7B . 
     In addition, a video encoding method and a video decoding method based on coding units having a tree structure which are applicable to the above-mentioned video encoding and decoding method according to embodiments will be described with reference to  FIGS. 8 to 20 . Furthermore, examples to which the above-mentioned video encoding and decoding method is applicable according to embodiments will be described with reference to  FIGS. 21 to 27 . 
     In the following description, the term ‘image’ may refer to a still image of a video, or a moving image, i.e., the video itself. 
     The term ‘sample’ refers to data assigned to an image sampling location and data to be processed. For example, pixels of an image of the spatial domain may be samples. 
     The term ‘current block’ may refer to a block of a coding unit or a prediction unit of a depth image to be encoded or decoded. 
     An inter prediction method of a depth image based on an inter simplified depth coding (SDC) mode for an interlayer video decoding and encoding apparatus and a method thereof is now described with reference to  FIGS. 1A to 7B . 
       FIG. 1A  is a block diagram of a video encoding apparatus  10  according to an embodiment.  FIG. 1B  is a flowchart of a video encoding method  11  according to an embodiment. 
     The video encoding apparatus  10  according to an embodiment may include a simplified depth coding (SDC) mode information generator  12 , a residual DC component determiner  14 , and an encoder  16 . In addition, the video encoding apparatus  10  according to an embodiment may include a central processor (not shown) for controlling all of the SDC mode information generator  12 , the residual DC component determiner  14 , and the encoder  16 . Alternatively, the SDC mode information generator  12 , the residual DC component determiner  14 , and the encoder  16  may be controlled by individual processors (not shown) and the processors may operate in association with each other to control the video encoding apparatus  10 . Otherwise, the SDC mode information generator  12 , the residual DC component determiner  14 , and the encoder  16  may be controlled by an external processor (not shown) of the video encoding apparatus  10 . 
     In addition, the video encoding apparatus  10  may include one or more memories (not shown) for storing input and output data of the SDC mode information generator  12 , the residual DC component determiner  14 , and the encoder  16 . The video encoding apparatus  10  may include a memory controller (not shown) for controlling data input and output to and from the memories. 
     To output a video encoding result, the video encoding apparatus  10  may perform video encoding operations including transformation in association with an internal or external video encoding processor. The internal video encoding processor of the video encoding apparatus  10  may implement the video encoding operations as a separate processor. Alternatively, the video encoding apparatus  10 , a central processing unit, or a graphic processing unit may include a video encoding module to implement basic video encoding operations. 
     The video encoding apparatus  10  according to an embodiment may classify a plurality of video sequences based on layers by using a scalable video coding scheme, may encode each video sequence, and may output separate streams each including encoded data per layer. The video encoding apparatus  10  may encode a first layer video sequence and a second layer video sequence as different layers. 
     For example, according to a scalable video coding scheme based on spatial scalability, low-resolution pictures may be encoded as first layer pictures and high-resolution pictures may be encoded as second layer pictures. A result of encoding the first layer pictures may be output as a first layer stream, and a result of encoding the second layer pictures may be output as a second layer stream. 
     As another example, a multi-view video may be encoded by using a scalable video coding scheme. In this case, center-view pictures may be encoded as first layer pictures, and left-view pictures and right-view pictures may be encoded as second layer pictures which refer to the first layer pictures. Alternatively, when the video encoding apparatus  10  allows three or more layers, e.g., a first layer, a second layer, and a third layer, the center-view pictures may be encoded as first layer pictures, the left-view pictures may be encoded as second layer pictures, and the right-view pictures may be encoded as third layer pictures. However, the layers are not limited to the above-described configuration and the layers assigned to and referred by the center-view, left-view, and right-view pictures may vary. 
     As another example, a scalable video coding scheme may be performed by using temporal hierarchical prediction based on temporal scalability. A first layer stream including encoding information generated by encoding pictures of a basic frame rate may be output. Temporal layers (temporal levels) may be classified based on frame rates and each temporal layer may be encoded as each layer. A second layer stream including encoding information of a high frame rate may be output by further encoding pictures of the high frame rate with reference to the pictures of the basic frame rate. 
     Alternatively, scalable video coding may be performed on a first layer and a plurality of second layers. When the number of second layers is three or more, first layer pictures, 1 st  second layer pictures, 2 nd  second layer pictures, . . . , and K th  second layer pictures may be encoded. As such, a result of encoding the first layer pictures may be output as a first layer stream, and a results of encoding the 1 st , 2 nd , . . . , and K th  second layer pictures may be output as 1 st , 2 nd , K th  second layer streams, respectively. 
     The video encoding apparatus  10  according to an embodiment may perform inter prediction to predict a current picture with reference to pictures of a single layer. Due to inter prediction, a motion vector indicating motion information between a current picture and a reference picture, and a residual component between the current picture and the reference picture may be generated. 
     In addition, the video encoding apparatus  10  may perform interlayer prediction to predict second layer pictures with reference to first layer pictures. 
     Alternatively, when the video encoding apparatus  10  allows three or more layers, e.g., a first layer, a second layer, and a third layer, the video encoding apparatus  10  may perform interlayer prediction between one first layer picture and third layer pictures and perform interlayer prediction between second layer pictures and third layer pictures by using a multilayer prediction structure. 
     Due to interlayer prediction, a location difference component between a current picture and a reference picture of another layer and a residual component between the current picture and the reference picture of the other layer may be generated. 
     A detailed description of the interlayer prediction structure will be given below with reference to  FIG. 4 . 
     The video encoding apparatus  10  according to an embodiment encodes each picture of a video per block, in each layer. The block may have a square shape, a rectangular shape, or a geometric shape, and is not limited to a certain-sized data unit. The block may be the largest coding unit, a coding unit, a prediction unit, a transformation unit, or the like among coding units having a tree structure. The largest coding unit including the coding units having a tree structure may be variously named as a coding tree unit, a coding block tree, a block tree, a root block tree, a coding tree, a coding root, or a tree trunk. A video encoding/decoding scheme based on coding units having a tree structure will be described below with reference to  FIGS. 8 to 20 . 
     When the video encoding apparatus  10  according to an embodiment encodes a multi-view video, by additionally encoding auxiliary data such as a depth image, pictures of a larger number of views compared to the number of views of input pictures may be generated by a decoder. Herein, since the depth image is not directly viewed to a user but is used to generate intermediate-view composite pictures, deterioration of the depth image may influence the quality of the composite pictures. 
     A variation in a depth value of the depth image is large near the edge of an object and is relatively small in the object. Accordingly, errors of the composite pictures may be minimized by minimizing errors generated at the edge of the object where the variation in the depth value is large. In addition, encoding efficiency of the depth image may be increased by reducing the amount of data of the inside of the object and a background region where the variation in the depth value is small. 
     Accordingly, the video encoding apparatus  10  may increase encoding efficiency of the depth image by encoding a current block by using an inter simplified depth coding (SDC) mode among inter frame prediction modes. In a conventional inter frame prediction mode, a residual block having pixel values corresponding to errors between a current block and a reference block is compressed by using an encoding procedure including transformation and quantization. However, in an inter SDC mode, a residual block is not compressed or is compressed to a residual DC component. The residual DC component is a value representative of the pixel values of the residual block and is determined as an average value of all or a part of the pixel values of the residual block. 
     Therefore, in the inter SDC mode, the video encoding apparatus  10  transmits a bitstream including a reference picture index and a motion vector indicating a reference block of a prediction unit, and a residual DC component corresponding to the residual block. 
     The SDC mode information generator  12  generates SDC mode information of a coding unit of the depth image. The SDC mode information is information indicating whether the inter SDC mode is applied to the coding unit. The SDC mode information may be implemented in the form of a flag. For example, the flag serving as the SDC mode information may be expressed as an sdc_flag. If the sdc_flag indicates the value 0, the inter SDC mode is not applied to the coding unit corresponding to the sdc_flag. Otherwise, if the sdc_flag indicates the value 1, the inter SDC mode is applied to the coding unit corresponding to the sdc_flag. 
     The SDC mode information generator  12  may determine whether to apply the SDC mode information, based on a partition mode of a prediction unit included in the coding unit. The SDC mode information generator  12  generates the SDC mode information based on the determination of whether to apply the SDC mode information. For example, when the partition mode of the prediction unit is a 2N×2N mode, if the inter SDC mode is applied, the SDC mode information generator  12  generates SDC mode information indicating that the inter SDC mode is enabled, for a coding unit including a prediction unit having a partition mode of a 2N×2N mode. That is, when the partition mode of the prediction unit is a 2N×2N mode, if the sdc_flag indicates the value 0, the inter SDC mode is not applied to the coding unit corresponding to the sdc_flag. Otherwise, when the partition mode of the prediction unit is a 2N×2N mode, if the sdc_flag indicates the value 1, the inter SDC mode is applied to the coding unit corresponding to the sdc_flag. If the partition mode of the prediction unit is a 2N×N, N×2N, or N×N mode other than a 2N×2N mode, the SDC mode information is not generated. 
     The SDC mode information may be determined based on whether the inter SDC mode is enabled for the depth image. If the inter SDC mode is not enabled for the depth image, the SDC mode information generator  12  determines that the inter SDC mode is not applied to the coding unit. Otherwise, if the inter SDC mode is enabled for the depth image, the SDC mode information generator  12  determines whether the inter SDC mode is applied to the coding unit, based on a condition such as the partition mode of the prediction unit. 
     Whether the inter SDC mode is enabled for the depth image may be determined based on SDC mode enable information. According to embodiments, the SDC mode enable information may be or may not be predefined. When the SDC mode enable information is predefined, whether the inter SDC mode is enabled for the depth image is determined based on the SDC mode enable information. Therefore, when the SDC mode enable information is predefined, the SDC mode information generator  12  may determine whether to apply the inter SDC mode based on the SDC mode enable information and the partition mode of the prediction unit, and then may generate the SDC mode information. 
     If the SDC mode enable information is not predefined, it may be determined that the inter SDC mode is not enabled for the depth image. Accordingly, when the SDC mode enable information is not predefined, the SDC mode information generator  12  does not apply the inter SDC mode to all coding units. However, according to another embodiment, when the SDC mode enable information is not predefined, it may be determined that the inter SDC mode is enabled for the depth image. 
     The SDC mode enable information may be implemented in the form of a flag. For example, the flag serving as the SDC mode enable information may be expressed as an inter_sdc_flag. If the inter_sdc_flag indicates the value 0, the inter SDC mode is not enabled for all coding units of the depth image corresponding to the inter_sdc_flag. Otherwise, if the inter_sdc_flag indicates the value 1, the inter SDC mode may be enabled for all coding units of the depth image corresponding to the inter_sdc_flag. 
     The residual DC component determiner  14  may determine a residual DC component based on residual pixel values included in a residual block corresponding to the prediction unit of the coding unit to which the SDC mode is applied. Specifically, the residual DC component determiner  14  may determine an average value of one or more residual pixel values included in the residual block, as the residual DC component. 
     Accordingly, the residual DC component determiner  14  may determine an average value of all residual pixel values as the residual DC component Likewise, the residual DC component determiner  14  may select only some of the residual pixel values and may determine an average value of the selected residual pixel values as the residual DC component. 
     If an average value of some of the residual pixel values is calculated, the residual DC component determiner  14  may select residual pixel values based on a partition size of the coding unit or the prediction unit. 
     Alternatively, the residual DC component determiner  14  may determine an average value of corner residual pixel values of the residual block as the residual DC component. Specifically, an average value of a top left residual pixel value, a top right residual pixel value, a bottom left residual pixel value, and a bottom right residual pixel value the residual block may be determined as the residual DC component. 
     As another example, the residual DC component determiner  14  may determine an average value of corner residual pixel values and center residual pixel values of the residual block as the residual DC component. 
     The residual DC component determiner  14  may determine an optimal residual DC component among a plurality of residual DC component candidates. The residual DC component determiner  14  may acquire an average value of one or more residual pixel values as described above, and then acquire a plurality of residual DC component candidates by adding multiple offset values to the average value. For example, when the average value is 3 and the offset values are −2, −1, 0, 1, and 2, the residual DC component determiner  14  may acquire five residual DC component candidates having the values 1, 2, 3, 4, and 5. 
     Thereafter, the residual DC component determiner  14  may determine an optimal residual DC component among the residual DC component candidates based on rate-distortion optimization. Rate-distortion optimization is a procedure for selecting an optimal compression method among compression methods selectable for an encoding target picture in consideration of a compression ratio and deterioration of an encoded picture. Therefore, based on rate-distortion optimization, the residual DC component determiner  14  may determine a residual DC component optimized for an encoding target picture among the residual DC component candidates. 
     For example, when the residual DC component candidates have the values 1, 2, and 3, the residual DC component determiner  14  encodes an encoding target picture by using the residual DC component candidate having the value 1, and then calculates a bitrate of the encoded picture and an error between the encoding target picture and the encoded picture. The residual DC component determiner  14  also performs the above procedure by using the other residual DC component candidates having the values 2 and 3. Thereafter, the residual DC component determiner  14  may determine an optimal residual DC component by comparing the bitrates of the encoded picture and the errors between the encoding target picture and the encoded picture. 
     The residual DC component determiner  14  may not determine the residual DC component. If a small encoding error is predicted when the residual block is encoded by determining the residual DC component, the residual block may not be encoded. In this case, the inter SDC mode may operate similarly to a skip mode. 
     The encoder  16  generates a bitstream including the SDC mode information and the residual DC component. When the SDC mode enable information is present, the encoder  16  may generate the bitstream further including the SDC mode enable information. 
     A detailed description is now given of the video encoding method  11  of the video encoding apparatus  10  according to an embodiment with reference to  FIG. 1B . 
     In operation  13 , SDC mode information indicating whether an SDC mode is applied to a coding unit of a depth image may be generated. As mentioned above, the SDC mode information may be implemented in the form of a flag. 
     Whether to apply the SDC mode information may be determined based on a partition mode of a prediction unit included in the coding unit. The SDC mode information is generated based on the determination of whether to apply the SDC mode information. 
     The SDC mode information may be determined based on whether an inter SDC mode is enabled for the depth image. Whether the inter SDC mode is enabled for the depth image may be determined based on SDC mode enable information. 
     In operation  15 , a residual DC component of the coding unit, to which the SDC mode is applied, is determined based on residual pixel values included in a residual block corresponding to the prediction unit of the coding unit. Specifically, an average value of one or more residual pixel values included in the residual block may be determined as the residual DC component. For example, an average value of all residual pixel values may be determined as the residual DC component. 
     Likewise, an average value of some selected among the residual pixel values may be determined as the residual DC component. If an average value of some of the residual pixel values is calculated, residual pixel values may be selected based on a partition size of the coding unit or the prediction unit. 
     Alternatively, an average value of corner residual pixel values of the residual block may be determined as the residual DC component. As another example, an average value of corner residual pixel values and center residual pixel values of the residual block may be determined as the residual DC component. 
     An optimal residual DC component may be determined among a plurality of residual DC component candidates. An average value of one or more residual pixel values may be acquired as described above, and then a plurality of residual DC component candidates may be acquired by adding multiple offset values to the average value. An optimal residual DC component may be determined among the residual DC component candidates based on rate-distortion optimization. 
     In operation  17 , a bitstream including the SDC mode information and the residual DC component is generated. 
     According to the above description, the video encoding apparatus  10  may efficiently encode a depth image by reducing the amount of data of a residual block having pixel values corresponding to errors between a current block and a reference block. 
       FIG. 2A  is a block diagram of a video decoding apparatus  20  according to an embodiment. 
     The video decoding apparatus  20  according to an embodiment may include an SDC mode information acquirer  22 , a residual DC component acquirer  24 , and a decoder  26 . In addition, the video decoding apparatus  20  according to an embodiment may include a central processor (not shown) for controlling all of the SDC mode information acquirer  22 , the residual DC component acquirer  24 , and the decoder  26 . Alternatively, the SDC mode information acquirer  22 , the residual DC component acquirer  24 , and the decoder  26  may be controlled by individual processors (not shown) and the processors may operate in association with each other to control the video decoding apparatus  20 . Otherwise, the SDC mode information acquirer  22 , the residual DC component acquirer  24 , and the decoder  26  may be controlled by an external processor (not shown) of the video decoding apparatus  20  according to an embodiment. 
     In addition, the video decoding apparatus  20  according to an embodiment may include one or more memories (not shown) for storing input and output data of the SDC mode information acquirer  22 , the residual DC component acquirer  24 , and the decoder  26 . The video decoding apparatus  20  may include a memory controller (not shown) for controlling data input and output to and from the memories. 
     To reconstruct a video by decoding the video, the video decoding apparatus 20 according to an embodiment may perform video decoding operations including inverse transformation in association with an internal or external video decoding processor. The internal video decoding processor of the video decoding apparatus  20  according to an embodiment may implement the video decoding operations as a separate processor, or the video decoding apparatus  20 , a central processing unit, or a graphic processing unit may include a video decoding module to implement basic video decoding operations. 
     The video decoding apparatus  20  according to an embodiment may receive bitstreams based on layers by using a scalable coding scheme. The number of layers of the bitstreams received by the video decoding apparatus  20  is not limited. 
     For example, the video decoding apparatus  20  based on spatial scalability may receive streams in which video sequences having different resolutions are encoded in different layers. A low-resolution video sequence may be reconstructed by decoding a first layer stream, and a high-resolution video sequence may be reconstructed by decoding a second layer stream. 
     As another example, a multi-view video may be decoded by using a scalable video coding scheme. When stereoscopic video streams of multiple layers are received, left-view pictures may be reconstructed by decoding a first layer stream. Right-view pictures may be reconstructed by further decoding a second layer stream in addition to the first layer stream. 
     Alternatively, when multi-view video streams of multiple layers are received, center-view pictures may be reconstructed by decoding a first layer stream. Left-view pictures may be reconstructed by further decoding a second layer stream in addition to the first layer stream. Right-view pictures may be reconstructed by further decoding a third layer stream in addition to the first layer stream. 
     As another example, a scalable video coding scheme may be performed based on temporal scalability. Pictures of a basic frame rate may be reconstructed by decoding a first layer stream. Pictures of a high frame rate may be reconstructed by further decoding a second layer stream in addition to the first layer stream. 
     Alternatively, when the number of second layers is three or more, first layer pictures may be reconstructed from a first layer stream, and second layer pictures may be further reconstructed by further decoding a second layer stream with reference to the reconstructed first layer pictures. K th  pictures may be further reconstructed by further decoding a K th  layer stream with reference to the reconstructed second layer pictures. 
     The video decoding apparatus  20  may acquire encoded data of first layer pictures and second layer pictures from a first layer stream and a second layer stream, and may further acquire a motion vector generated due to inter prediction and prediction information generated due to interlayer prediction. 
     For example, the video decoding apparatus  20  may decode data inter-predicted per layer, and may decode data interlayer-predicted among multiple layers. Reconstruction may be implemented by performing motion compensation and interlayer decoding based on a coding unit or a prediction unit. 
     Pictures of each layer stream may be reconstructed by performing motion compensation on a current picture with reference to reconstructed pictures which are predicted by performing inter prediction within the same layer. Motion compensation refers to an operation for reconfiguring a reconstructed image of a current picture by combining a reference picture determined by using a motion vector of the current picture, and a residual component of the current picture. 
     In addition, the video decoding apparatus  20  may perform interlayer decoding with reference to prediction information of the first layer pictures to decode the second layer pictures predicted by performing interlayer prediction. Interlayer decoding refers to an operation for reconfiguring prediction information of a current picture by using prediction information of a reference block of another layer to determine the prediction information of the current picture. 
     The video decoding apparatus  20  according to an embodiment may perform interlayer decoding to reconstruct third layer pictures predicted with reference to the second layer pictures. A detailed description of the interlayer prediction structure will be given below with reference to  FIG. 3 . 
     The video decoding apparatus  20  decodes each image of a video per block. The block may be the largest coding unit, a coding unit, a prediction unit, a transformation unit, or the like among coding units having a tree structure. A video encoding/decoding scheme based on coding units having a tree structure will be described below with reference to  FIGS. 8 to 20 . 
     In an inter SDC mode, the video decoding apparatus  20  acquires a reference block based on a reference picture index and a motion vector, and decodes a current block of the coding unit by using the reference block and the residual DC component. Specifically, the current block may be decoded by adding the residual DC component to all pixel values of the reference block. 
     The SDC mode information acquirer  22  acquires SDC mode information indicating whether the SDC mode is applied to the coding unit of the depth image. The SDC mode information is information indicating whether the inter SDC mode is applied to the coding unit. The SDC mode information may be implemented in the form of a flag and may be expressed as an sdc_flag. If the sdc_flag indicates the value 0, the inter SDC mode is not applied to the coding unit corresponding to the sdc_flag. Otherwise, if the sdc_flag indicates the value 1, the inter SDC mode is applied to the coding unit corresponding to the sdc_flag. 
     The SDC mode information acquirer  22  may acquire the SDC mode information determined based on a partition mode of a prediction unit included in the coding unit. For example, when the partition mode of the prediction unit is a 2N×2N mode, if the inter SDC mode is applied, the SDC mode information acquirer  22  acquires SDC mode information indicating that the inter SDC mode is enabled, for a coding unit including a prediction unit having a partition mode of a 2N×2N mode. That is, when the partition mode of the prediction unit is a 2N×2N mode, if the sdc_flag indicates the value 0, the inter SDC mode is not applied to the coding unit corresponding to the sdc_flag. Otherwise, when the partition mode of the prediction unit is a 2N×2N mode, if the sdc_flag indicates the value 1, the inter SDC mode is applied to the coding unit corresponding to the sdc_flag. If the partition mode of the prediction unit is a 2N×N, N×2N, or N×N mode other than a 2N×2N mode, the SDC mode information is not acquired. 
     The SDC mode information may be determined based on whether the inter SDC mode is enabled for the depth image. If the inter SDC mode is not enabled for the depth image, the SDC mode information acquirer  22  determines that the inter SDC mode is not applied to the coding unit. Otherwise, if the inter SDC mode is enabled for the depth image, the SDC mode information acquirer  22  acquires the SDC mode information indicating a different value based on a condition such as partition mode information. 
     Whether the inter SDC mode is enabled for the depth image may be determined based on SDC mode enable information. The SDC mode enable information may be or may not be predefined. The SDC mode enable information may be implemented in the form of a flag. 
     The residual DC component acquirer  24  may acquire a residual DC component determined based on residual pixel values included in a residual block corresponding to the prediction unit of the coding unit to which the SDC mode is applied. Specifically, the residual DC component acquirer  24  may acquire the residual DC component determined as an average value of one or more residual pixel values included in the residual block. 
     Accordingly, the residual DC component acquirer  24  may acquire the residual DC component determined as an average value of all residual pixel values Likewise, the residual DC component acquirer  24  may acquire the residual DC component determined as an average value of some selected among the residual pixel values. 
     The residual DC component acquirer  24  may acquire the residual DC component determined as an average value of residual pixel values selected based on a partition size of the coding unit or the prediction unit. Alternatively, the residual DC component acquirer  24  may acquire the residual DC component determined as an average value of corner residual pixel values of the residual block. As another example, the residual DC component acquirer  24  may acquire the residual DC component determined as an average value of corner residual pixel values and center residual pixel values of the residual block. 
     The residual DC component acquired by the residual DC component acquirer  24  may be an optimal residual DC component determined among a plurality of residual DC component candidates. 
     If a small encoding error is predicted when the residual block is encoded by determining the residual DC component, the residual block may not be generated. Therefore, the residual DC component acquirer  24  may not acquire the residual DC component. Accordingly, in this case, the inter SDC mode may operate similarly to a skip mode. 
     The decoder  26  reconstructs a current block of the coding unit by using the residual DC component. 
     A detailed description is now given of a video decoding method  21  of the video decoding apparatus  20  according to an embodiment with reference to  FIG. 2B . 
     In operation  23 , SDC mode information indicating whether an SDC mode is applied to a coding unit of a depth image is acquired. The SDC mode information is information indicating whether an inter SDC mode is applied to the coding unit. The SDC mode information may be implemented in the form of a flag. 
     The acquired SDC mode information may be information determined based on a partition mode of a prediction unit included in the coding unit. 
     The acquired SDC mode information may be information determined based on whether the inter SDC mode is enabled for the depth image. If the inter SDC mode is not enabled for the depth image, the acquired SDC mode information indicates that the inter SDC mode is not applied to all coding units. Otherwise, if the inter SDC mode is enabled for the depth image, the acquired SDC mode information indicates a different value based on a condition such as partition mode information. 
     Whether the inter SDC mode is enabled for the depth image may be determined based on SDC mode enable information. The SDC mode enable information may be or may not be predefined. The SDC mode enable information may be implemented in the form of a flag. 
     In operation  25 , a residual DC component determined based on residual pixel values included in a residual block corresponding to the prediction unit of the coding unit to which the SDC mode is applied is determined. 
     The residual DC component determined as an average value of one or more residual pixel values included in the residual block may be acquired. For example, the residual DC component determined as an average value of residual pixel values selected based on a partition size of the coding unit or the prediction unit may be acquired. Alternatively, the residual DC component determined as an average value of corner residual pixel values of the residual block may be acquired. 
     The residual DC component may be an optimal residual DC component determined among a plurality of residual DC component candidates. The residual DC component candidates may be acquired by adding integer multiples of an offset value to an average value of one or more residual pixel values. The residual DC component may be determined among the residual DC component candidates based on rate-distortion optimization. 
     If a small encoding error is predicted when the residual block is encoded by determining the residual DC component, the residual block may not be generated. Therefore, the residual DC component may not be acquired. Accordingly, in this case, the inter SDC mode may operate similarly to a skip mode. 
     In operation  27 , a current block of the coding unit is reconstructed by using the residual DC component. 
     According to the above description, the video decoding apparatus  20  may decode a current block which is encoded in an inter SDC mode by the video encoding apparatus  10 . 
       FIG. 3A  is a diagram for describing a flag indicating whether to enable an inter SDC mode.  FIG. 3A  shows vps_extension2 syntax. A shaded part of  FIG. 3A  shows an inter_sdc_flag[layerID] serving as the flag indicating whether to enable the inter SDC mode. The layerID indicates a unique ID of an interlayer picture. The inter_sdc_flag[layerID] is predefined in a case when a picture indicated by the layerID is a depth image. If the inter_sdc_flag[layerID] has the value 0, the inter SDC mode is not enabled for the depth image. Otherwise, if the inter_sdc_flag[layerID] has the value 1, the inter SDC mode is enabled for the depth image. 
     When the inter_sdc_flag[layerID] is not predefined, the inter_sdc_flag[layerID] is assumed to have the value 0. 
       FIG. 3B  is a diagram for describing a flag indicating whether an inter SDC mode is applied to a coding unit.  FIG. 3B  shows coding_unit syntax. A shaded part of  FIG. 3B  shows an sdc_flag[x0][y0] serving as the flag indicating whether the inter SDC mode is applied to the coding unit. The sdc_flag[x0][y0] indicates whether the inter SDC mode is applied to a coding unit provided at a location x0 from the left of and a location y0 from the top of a depth image. If the sdc_flag[x0][y0] has the value 1, the inter SDC mode is applied to the coding unit corresponding to the sdc_flag[x0][y0]. Otherwise, if the sdc_flag[x0][y0] has the value 0, the inter SDC mode is not applied to the coding unit corresponding to the sdc_flag[x0][y0]. 
     An sdcEnableFlag corresponds to a condition for the value 1 of the sdc_flag[x0][y0]. The sdcEnableFlag has the value 1 if the inter_sdc_flag[layerID] described above in relation to  FIG. 3A  has the value 1, a current mode is an inter mode, and a partition mode of the coding unit is 2N×2N. Otherwise, if the above condition is not satisfied, the sdcEnableFlag has the value 0. Otherwise, if the above condition is not satisfied, the sdcEnableFlag has the value 0. 
     When the sdc_flag[x0][y0] is not predefined, the sdc_flag[x0][y0] is assumed to have the value 0. 
       FIG. 3C  shows a procedure for acquiring a residual DC component by the video decoding apparatus  20 . 
     Based on cu_extension syntax, in an intra SDC mode corresponding to a partition mode of N×N, the video decoding apparatus  20  determines whether a residual DC component is present in each of four prediction modes. However, since the inter SDC mode is configured to be applied only in a partition mode of 2N×2N, the video decoding apparatus  20  determines whether a residual DC component is present, only for one prediction unit having the same size as a coding unit. 
     Since a dcNumSeg is always set to the value 1 for decoding in the inter SDC mode, the value of i of the syntax is always 0. Accordingly, i is not considered in the inter SDC mode. 
     Initially, the video decoding apparatus  20  acquires a depth_dc_flag[x0][y0] of a prediction unit. The depth_dc_flag[x0][y0] indicates whether a residual DC component is present in a prediction unit of a coding unit provided at a location x0 from the left of and a location y0 from the top of a depth image. 
     If the depth_dc_flag[x0][y0] is acquired, the video decoding apparatus  20  acquires a depth_dc_abs[x0][y0]. The depth_dc_abs[x0][y0] indicates an absolute value of the residual DC component corresponding to the prediction unit of the coding unit provided at the location x0 from the left of and the location y0 from the top of the depth image. 
     When the depth_dc_abs[x0][y0] is acquired, if the depth_dc_abs[x0][y0] does not have the value 0, the video decoding apparatus  20  acquires a depth_dc_sign_flag[x0][y0]. The depth_dc_sign_flag[x0][y0] indicates a sign of the residual DC component corresponding to the prediction unit of the coding unit provided at the location x0 from the left of and the location y0 from the top of the depth image. 
     If the absolute value and the sign of the residual DC component are separately transmitted as the residual DC component, the amount of data transmission may be reduced compared to a case when the residual DC component is directly transmitted. 
       FIG. 4  illustrates an interlayer prediction structure according to an embodiment. 
     The video encoding apparatus  10  according to an embodiment may prediction-encode base-view pictures, left-view pictures, and right-view pictures based on a reproduction order  400  of the multi-view video prediction structure illustrated in  FIG. 4 . 
     Based on the reproduction order  400  of the multi-view video prediction structure according to a related art, pictures of the same view are arranged in a horizontal direction. Accordingly, the left-view pictures marked as ‘Left’ are arranged in a row in a horizontal direction, the base-view pictures marked as ‘Center’ are arranged in a row in a horizontal direction, and the right-view pictures marked as ‘Right’ are arranged in a row in a horizontal direction. The base-view pictures may be center-view pictures compared to the left-view/right-view pictures. 
     In addition, pictures of the same picture order count (POC) order are arranged in a vertical direction. The POC order of the pictures indicates a reproduction order of pictures included in a video. ‘POC X’ marked in the multi-view video prediction structure indicates a relative reproduction order of pictures located in each column. A small value of X indicates an early reproduction order, and a large value thereof indicates a late reproduction order. 
     Therefore, based on the reproduction order  400  of the multi-view video prediction structure according to a related art, the left-view pictures marked as ‘Left’ are arranged based on the POC order (reproduction order) in a horizontal direction, the base-view pictures marked as ‘Center’ are arranged based on the POC order (reproduction order) in a horizontal direction, and the right-view pictures marked as ‘Right’ are arranged based on the POC order (reproduction order) in a horizontal direction. A left-view picture and a right-view picture located at the same column as a base-view picture have different views but have the same POC order (reproduction order). 
     Per view, four sequential pictures configure one group of pictures (GOP). Each GOP includes pictures located between two sequential anchor pictures, and one anchor picture (key picture). 
     An anchor picture is a random access point (RAP) picture. When a video is reproduced, at a certain reproduction order, that is, if a reproduction location is arbitrarily selected among the pictures arranged based on the POC order, an anchor picture which is the closest to the reproduction location in POC order is reproduced. The base-view pictures include base-view anchor pictures  411 ,  412 ,  413 ,  414 , and  415 , the left-view pictures include left-view anchor pictures  421 ,  422 ,  423 ,  424 , and  425 , and the right-view pictures include right-view anchor pictures  431 ,  432 ,  433 ,  434 , and  435 . 
     The multi-view pictures may be reproduced and predicted (reconstructed) in the order of the GOPs. Initially, according to the reproduction order  400  of the multi-view video prediction structure, per view, the pictures included in GOP  0  may be reproduced and then the pictures included in GOP  1  may be reproduced. That is, the pictures included in every GOP may be reproduced in the order of GOP  0 , GOP  1 , GOP  2 , and GOP  3 . In addition, based on a coding order of the multi-view video prediction structure, per view, the pictures included in GOP  0  may be predicted (reconstructed) and then the pictures included in GOP  1  may be predicted (reconstructed). That is, the pictures included in every GOP may be predicted (reconstructed) in the order of GOP  0 , GOP  1 , GOP  2 , and GOP  3 . 
     Based on the reproduction order  400  of the multi-view video prediction structure, both inter-view prediction (interlayer prediction) and inter prediction are performed on the pictures. In the multi-view video prediction structure, a picture from which an arrow starts is a reference picture, and a picture to which the arrow is directed is a picture to be predicted by using the reference picture. 
     A result of predicting the base-view pictures may be encoded and then output in the form of a base-view video stream, and a result of predicting the additional-view pictures may be encoded and then output in the form of a layer bitstream. In addition, a result of prediction-encoding the left-view pictures may be output in the form of a first layer bitstream, and a result of prediction-encoding the right-view pictures may be output in the form of a second layer bitstream 
     Only inter prediction is performed on the base-view pictures. That is, although the I-type anchor pictures  411 ,  412 ,  413 ,  414 , and  415  do not refer to other pictures, the other B-type and b-type pictures are predicted with reference to other base-view pictures. The B-type pictures are predicted with reference to I-type anchor pictures preceding the same in POC order and I-type anchor pictures following the same in POC order. The b-type pictures are predicted with reference to I-type anchor pictures preceding the same in POC order and B-type pictures following the same in POC order, or with reference to B-type anchor pictures preceding the same in POC order and I-type anchor pictures following the same in POC order. 
     On the left-view pictures and the right-view pictures, inter-view prediction (interlayer prediction) is performed with reference to pictures of another view and inter prediction is performed with reference to pictures of the same view. 
     Inter-view prediction (interlayer prediction) may be performed on the left-view anchor pictures  421 ,  422 ,  423 ,  424 , and  425  with reference to the base-view anchor pictures  411 ,  412 ,  413 ,  414 , and  415  corresponding thereto in POC order. Inter-view prediction may be performed on the right-view anchor pictures  431 ,  432 ,  433 ,  434 , and  435  with reference to the base-view anchor pictures  411 ,  412 ,  413 ,  414 , and  415  or the left-view anchor pictures  421 ,  422 ,  423 ,  424 , and  425  corresponding thereto in POC order. In addition, inter-view prediction (interlayer prediction) may be performed on left-view non-anchor pictures and right-view non-anchor pictures with reference to other-view pictures corresponding thereto in POC order. 
     The left-view non-anchor pictures and the right-view non-anchor pictures are predicted with reference to pictures of the same view. 
     However, the left-view pictures and the right-view pictures may not be predicted with reference to anchor pictures preceding the same in reproduction order among the additional-view pictures of the same view. That is, for inter prediction of a current left-view picture, left-view non-anchor pictures preceding the current left-view picture in reproduction order may be referred to. Likewise, for inter prediction of a current right-view picture, right-view non-anchor pictures preceding the current right-view picture in reproduction order may be referred to. 
     Alternatively, for inter prediction of a current left-view picture, a left-view picture belonging to a previous GOP preceding a current GOP including the current left-view picture may not be referred to, and a left-view picture belonging to the current GOP and preceding the current left-view picture in reconstruction order may be referred to. The above principle is equally applied to a right-view picture. 
     The video decoding apparatus  20  according to an embodiment may reconstruct the base-view pictures, the left-view pictures, and the right-view pictures based on the reproduction order  400  of the multi-view video prediction structure illustrated in  FIG. 4 . 
     The left-view pictures may be reconstructed by performing inter-view disparity compensation with reference to the base-view pictures and performing inter motion compensation with reference to the left-view pictures. The right-view pictures may be reconstructed by performing inter-view disparity compensation with reference to the base-view pictures and the left-view pictures and performing inter motion compensation with reference to the right-view pictures. Reference pictures should be reconstructed first for disparity compensation and motion compensation of the left-view pictures and the right-view pictures. 
     For inter motion compensation of the left-view picture, the left-view pictures may be reconstructed by performing inter motion compensation with reference to reconstructed left-view reference pictures. For inter motion compensation of the right-view picture, the right-view pictures may be reconstructed by performing inter motion compensation with reference to reconstructed right-view reference pictures. 
     Alternatively, for inter motion compensation of a current left-view picture, a left-view picture belonging to a previous GOP preceding a current GOP including the current left-view picture may not be referred to, and only a left-view picture belonging to the current GOP and preceding the current left-view picture in reconstruction order may be referred to. The above principle is equally applied to a right-view picture. 
       FIG. 5  is a flowchart of a method of encoding a residual block based on a prediction mode by an interlayer video encoding apparatus according to an embodiment. 
     In operation  52 , the video encoding apparatus  10  may define a part of predetermined partition modes as an inter SDC mode. For example, the video encoding apparatus  10  may configure a 2N×2N partition mode as the inter SDC mode. If the 2N×2N partition mode is defined as the inter SDC mode, the residual block may not be encoded or an average value of one or more of residual pixel values of the residual block may be encoded, and thus encoding efficiency may be achieved. 
     In operation  54 , the video encoding apparatus  10  determines whether the inter SDC mode is applied to a coding unit, based on a partition mode of the coding unit. If the partitions are encoded in the inter SDC mode, the method proceeds to operation  56 . Otherwise, if the partition mode is not configured as the inter SDC mode or if the partitions are not encoded in the inter SDC mode, the method proceeds to operation  58 . 
     After determining whether the inter SDC mode is applied to the coding unit, based on the partition mode of the coding unit, the video encoding apparatus  10  may generate SDC mode information indicating whether the inter SDC mode is applied to the coding unit. 
     In operation  56 , the video encoding apparatus  10  may not encode a residual block or may encode an average value of one or more of residual pixel values of the residual block. When the residual signal is not encoded, the video encoding apparatus  10  may operate in the inter SDC mode similarly to a skip mode. 
     In operation  58 , the video encoding apparatus  10  encodes the residual block by using a general encoding method. For example, discrete cosine transformation (DCT) and quantization may be performed on the residual block. 
       FIG. 6  is a flowchart of a method of encoding a residual block based on a prediction mode by an interlayer video encoding apparatus according to an embodiment. 
     In operation  62 , the video encoding apparatus  10  may define a part of predetermined partition modes as an inter SDC mode. 
     In operation  64 , the video encoding apparatus  10  determines whether the inter SDC mode is applied to a coding unit, based on a partition mode of the coding unit. If the partitions are encoded in the inter SDC mode, the method proceeds to operation  66 . Otherwise, if the partition mode is not configured as the inter SDC mode or if the partitions are not encoded in the inter SDC mode, the method proceeds to operation  69 . 
     In operation  66 , the video encoding apparatus  10  may acquire an average value of one or more of residual pixel values of a residual block. The video encoding apparatus  10  may acquire a plurality of residual DC component candidates by adding integer multiples of an offset value to the average value. 
     In operation  68 , the video encoding apparatus  10  may determine an optimal residual DC component among the residual DC component candidates acquired in operation  66 , based on rate-distortion optimization. 
     In operation  69 , the video encoding apparatus  10  encodes the residual block by using a general encoding method. 
       FIGS. 7A and 7B  are diagrams for describing examples of generating residual data of a coding unit in a case when a prediction mode is an SDC mode, according to embodiments. 
       FIG. 7A  shows a case when a residual block is not compressed. That is, if a small encoding error is predicted, the residual block having pixel values corresponding to errors between a current block and a reference block may not be compressed. In this case, the SDC mode may operate similarly to a skip mode. 
       FIG. 7B  shows a case when an average value of four corner residual pixel values of a residual block is determined as a residual DC component. Specifically, an average value of four pixels  715 ,  720 ,  725 , and  730  of  FIG. 7B  is determined as the residual DC component. 
     Alternatively, an average value of four corner residual pixel values and center residual pixel values of the residual block may be determined as the residual DC component. 
       FIG. 8  illustrates a block diagram of a video encoding apparatus based on coding units of a tree structure  800 , according to an embodiment of the present invention. 
     The video encoding apparatus involving video prediction based on coding units of the tree structure  800  includes a coding unit determiner  820  and an output unit  830 . Hereinafter, for convenience of description, the video encoding apparatus involving video prediction based on coding units of the tree structure  800  is referred to as the ‘video encoding apparatus  800 ’. 
     The coding unit determiner  820  may split a current picture based on a largest coding unit that is a coding unit having a maximum size for a current picture of an image. If the current picture is larger than the largest coding unit, image data of the current picture may be split into the at least one largest coding unit. The largest coding unit according to an 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 embodiment may be characterized by a maximum size and a depth. The depth denotes the number of times the coding unit is spatially split from the largest coding unit, and as the depth deepens, deeper coding units according to depths may be split from the largest coding unit to a smallest coding unit. A depth of the largest coding unit may be defined as an uppermost depth and a depth of the smallest coding unit may be defined as a lowermost depth. Since a size of a coding unit corresponding to each depth decreases as the depth of the largest 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 largest coding units according to a maximum size of the coding unit, and each of the largest coding units may include deeper coding units that are split according to depths. Since the largest coding unit according to an embodiment is split according to depths, the image data of a spatial domain included in the largest 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 largest coding unit are hierarchically split, may be predetermined. 
     The coding unit determiner  820  encodes at least one split region obtained by splitting a region of the largest coding unit according to depths, and determines a depth to output a finally encoded image data according to the at least one split region. That is, the coding unit determiner  820  determines a final depth by encoding the image data in the deeper coding units according to depths, according to the largest coding unit of the current picture, and selecting a depth having the least encoding error. The determined final depth and image data according to largest coding units are output to the output unit  830 . 
     The image data in the largest 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 based on each of the deeper coding units are compared. A depth having the least encoding error may be selected after comparing encoding errors of the deeper coding units. At least one final depth may be selected for each largest coding unit. 
     The size of the largest 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 largest 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 largest coding unit, the encoding errors may differ according to regions in the one largest coding unit, and thus the final depths may differ according to regions in the image data. Thus, one or more final depths may be determined in one largest coding unit, and the image data of the largest coding unit may be divided according to coding units of at least one final depth. 
     Accordingly, the coding unit determiner  820  according to the embodiment may determine coding units having a tree structure included in the largest coding unit. The ‘coding units having a tree structure’ according to an embodiment include coding units corresponding to a depth determined to be the final depth, from among all deeper coding units included in the largest coding unit. A coding unit of a final depth may be hierarchically determined according to depths in the same region of the largest coding unit, and may be independently determined in different regions. Equally, a final depth in a current region may be determined independently from a final depth in another region. 
     A maximum depth according to an embodiment is an index related to the number of splitting times from a largest coding unit to a smallest coding unit. A first maximum depth according to an embodiment may denote the total number of splitting times from the largest coding unit to the smallest coding unit. A second maximum depth according to an embodiment may denote the total number of depth levels from the largest coding unit to the smallest coding unit. For example, when a depth of the largest coding unit is 0, a depth of a coding unit, in which the largest coding unit is split once, may be set to 1, and a depth of a coding unit, in which the largest coding unit is split twice, may be set to 2. Here, if the smallest coding unit is a coding unit in which the largest coding unit is split four times, 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 largest 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 largest coding unit. 
     Since the number of deeper coding units increases whenever the largest 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. Hereinafter, for convenience of description, the prediction encoding and the transformation will be described based on a coding unit of a current depth in at least one largest coding unit. 
     The video encoding apparatus  800  according to the 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  800  may select not only a coding unit for encoding the image data, but may also select 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 in the largest coding unit, the prediction encoding may be performed based on a coding unit of a final depth, i.e., based on the coding unit that is no longer split. 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 and a data unit obtained by splitting at least one selected from a height and a width of the prediction unit. A partition is a data unit where a prediction unit of a coding unit is split, and a prediction unit may be a partition having the same size as a 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, and a size of a partition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partition mode may selectively include symmetrical partitions obtained by symmetrically splitting a height or width of the prediction unit, and may selectively include partitions obtained by asymmetrically splitting the height or width of the prediction unit, such as 1:n or n:1, partitions 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 may be independently performed on one prediction unit in a coding unit, thereby selecting a prediction mode having a least encoding error. 
     The video encoding apparatus  800  according to the 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 data 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 may include a data unit for an intra mode and a transformation unit for an inter mode. 
     The transformation unit in the coding unit may be recursively split into smaller sized regions in the similar manner as the coding unit according to the tree structure, thus, residual data of the coding unit may be divided according to the transformation unit having the tree structure according to a transformation depth. 
     A transformation depth indicating the number of splitting times to reach the transformation unit by splitting the height and width of the coding unit may also be set in the transformation unit. For example, in a current coding unit of 2N×2N, a transformation depth may be 0 when the size of a transformation unit is 2N×2N, may be 1 when the size of the transformation unit is N×N, and may be 2 when the size of the transformation unit is N/2×N/2. That is, with respect to the transformation unit, the transformation unit having the tree structure may be set according to the transformation depths. 
     Split information according to depths requires not only information about a depth but also requires information related to prediction and transformation. Accordingly, the coding unit determiner  820  may determine not only a depth generating a least encoding error but may also determine a partition mode in which a prediction unit is split to partitions, a prediction mode according to prediction units, and a size of a transformation unit for transformation. 
     Coding units according to a tree structure in a largest coding unit and methods of determining a prediction unit/partition, and a transformation unit, according to embodiments, will be described in detail later with reference to  FIGS. 9 through 19 . 
     The coding unit determiner  820  may measure an encoding error of deeper coding units according to depths by using Rate-Distortion Optimization based on Lagrangian multipliers. 
     The output unit  830  outputs, in bitstreams, the image data of the largest coding unit, which is encoded based on the at least one depth determined by the coding unit determiner  820 , and information according to depths. 
     The encoded image data may correspond to a result obtained by encoding residual data of an image. 
     The split information according to depths may include depth information, partition mode information of the prediction unit, prediction mode information, and the split information of the transformation unit. 
     Final depth information may be defined by using split information according to depths, which specifies 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 a depth, the current coding unit is encoded by using the coding unit of the current depth, and thus split information of the current depth may be defined not to split the current coding unit to a lower depth. On the contrary, if the current depth of the current coding unit is not the depth, the encoding has to be performed on the coding unit of the lower depth, and thus the split information of the current depth may be defined to split the current coding unit to the coding units of the lower depth. 
     If the current depth is not the 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 largest coding unit, and at least one piece of split information has to be determined for a coding unit of a depth, at least one piece of split information may be determined for one largest coding unit. Also, a depth of data of the largest coding unit may vary according to locations since the data is hierarchically split according to depths, and thus a depth and split information may be set for the data. 
     Accordingly, the output unit  830  according to the embodiment may assign encoding information about a corresponding depth and an encoding mode to at least one of the coding unit, the prediction unit, and a minimum unit included in the largest coding unit. 
     The minimum unit according to an embodiment is a square data unit obtained by splitting the smallest coding unit constituting the lowermost depth by 4. Alternatively, the minimum unit according to an embodiment may be a maximum square data unit that may be included in all of the coding units, prediction units, partition units, and transformation units included in the largest coding unit. 
     For example, the encoding information output by the output unit  830  may be classified into encoding information according to deeper coding units, and encoding information according to prediction units. The encoding information according to the deeper 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 during 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 during the intra mode. 
     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, or a picture parameter set. 
     Information about a maximum size of the transformation unit allowed with respect to a current video, and information about a minimum size of the transformation unit may also be output through a header of a bitstream, a sequence parameter set, or a picture parameter set. The output unit  830  may encode and output reference information, prediction information, and slice type information, which are related to prediction. 
     According to the simplest embodiment for the video encoding apparatus  800 , 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 above, by two. That is, 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, a current coding unit having a size of 2N×2N may maximally include four lower-depth coding units having a size of N×N. 
     Accordingly, the video encoding apparatus  800  may form the coding units having the tree structure by determining coding units having an optimum shape and an optimum size for each largest coding unit, based on the size of the largest coding unit and the maximum depth determined considering characteristics of the current picture. Also, since encoding may be performed on each largest coding unit by using any one of various prediction modes and transformations, an optimal encoding mode may be determined by taking into account characteristics of the coding unit of various image sizes. 
     Thus, if an image having a high resolution or a large data amount is encoded in a conventional macroblock, the number of macroblocks per picture excessively increases. Accordingly, the 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 according to the embodiment, 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 inter-layer video encoding apparatus including configuration described above with reference to  FIG. 1A  may include the video encoding apparatuses  800  corresponding to the number of layers so as to encode single layer images in each of the layers of a multilayer video. For example, a first layer encoder may include one video encoding apparatus  800 , and a second layer encoder may include the video encoding apparatuses  800  corresponding to the number of second layers. 
     When the video encoding apparatuses  800  encode first layer images, the coding unit determiner  820  may determine a prediction unit for inter-image prediction according to each of coding units of a tree structure in each largest coding unit, and may perform the inter-image prediction on each prediction unit. 
     When the video encoding apparatuses  800  encode the second layer images, the coding unit determiner  820  may determine prediction units and coding units of a tree structure in each largest coding unit, and may perform inter-prediction on each of the prediction units. 
     The video encoding apparatuses  800  may encode a luminance difference so as to compensate for the luminance difference between the first layer image and the second layer image. However, whether to perform luminance compensation may be determined according to an encoding mode of a coding unit. For example, the luminance compensation may be performed only on a prediction unit having a size of 2N×2N. 
       FIG. 9  illustrates a block diagram of a video decoding apparatus based on coding units of a tree structure  900 , according to an embodiment. 
     The video decoding apparatus involving video prediction based on coding units of the tree structure  900  according to the embodiment includes a receiver  910 , an image data and encoding information extractor  920 , and an image data decoder  930 . Hereinafter, for convenience of description, the video decoding apparatus involving video prediction based on coding units of the tree structure  900  according to the embodiment is referred to as the ‘video decoding apparatus  900 ’. 
     Definitions of various terms, such as a coding unit, a depth, a prediction unit, a transformation unit, and various types of split information for decoding operations of the video decoding apparatus  900  according to the embodiment are identical to those described with reference to  FIG. 8  and the video encoding apparatus  800 . 
     The receiver  910  receives and parses a bitstream of an encoded video. The image data and encoding information extractor  920  extracts encoded image data for each coding unit from the parsed bitstream, wherein the coding units have a tree structure according to each largest coding unit, and outputs the extracted image data to the image data decoder  930 . The image data and encoding information extractor  920  may extract information about a maximum size of a coding unit of a current picture, from a header about the current picture, a sequence parameter set, or a picture parameter set. 
     Also, the image data and encoding information extractor  920  extracts, from the parsed bitstream, a final depth and split information about the coding units having a tree structure according to each largest coding unit. The extracted final depth and the extracted split information are output to the image data decoder  930 . That is, the image data in a bitstream is split into the largest coding unit so that the image data decoder  930  may decode the image data for each largest coding unit. 
     A depth and split information according to each of the largest coding units may be set for one or more pieces of depth information, and split information according to depths may include partition mode information of a corresponding coding unit, prediction mode information, and split information of a transformation unit. Also, as the depth information, the split information according to depths may be extracted. 
     The depth and the split information according to each of the largest coding units extracted by the image data and encoding information extractor  920  are a depth and split information determined to generate a minimum encoding error when an encoder, such as the video encoding apparatus  800 , repeatedly performs encoding for each deeper coding unit according to depths according to each largest coding unit. Accordingly, the video decoding apparatus  900  may reconstruct an image by decoding data according to an encoding method that generates the minimum encoding error. 
     Since encoding information about the 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  920  may extract the depth and the split information according to the predetermined data units. If a depth and split information of a corresponding largest coding unit are recorded according to each of the predetermined data units, predetermined data units having the same depth and the split information may be inferred to be the data units included in the same largest coding unit. 
     The image data decoder  930  reconstructs the current picture by decoding the image data in each largest coding unit based on the depth and the split information according to each of the largest coding units. That is, the image data decoder  930  may decode the encoded image data, based on a read partition mode, a prediction mode, and a transformation unit for each coding unit from among the coding units having the tree structure included in each largest coding unit. A decoding process may include a prediction process including intra prediction and motion compensation, and an inverse transformation process. 
     The image data decoder  930  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 depths. 
     In addition, for inverse transformation for each largest coding unit, the image data decoder  930  may read information about a transformation unit according to a tree structure for each coding unit so as to perform inverse transformation based on transformation units for each coding unit. Due to the inverse transformation, a pixel value of a spatial domain of the coding unit may be reconstructed. 
     The image data decoder  930  may determine a depth of a current largest 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 depth. Accordingly, the image data decoder  930  may decode the image data of the current largest coding unit by using the information about the partition mode of the prediction unit, the prediction mode, and the size of the transformation unit for each coding unit corresponding to the current depth. 
     That is, 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  930  in the same encoding mode. As such, the current coding unit may be decoded by obtaining the information about the encoding mode for each coding unit. 
     The inter-layer video decoding apparatus including configuration described above with reference to  FIG. 2A  may include the video decoding apparatuses  900  corresponding to the number of views, so as to reconstruct first layer images and second layer images by decoding a received first layer imagestream and a received second layer imagestream. 
     When the first layer imagestream is received, the image data decoder  930  of the video decoding apparatus  900  may split samples of the first layer images, which are extracted from the first layer imagestream by an extractor  920 , into coding units according to a tree structure of a largest coding unit. The image data decoder  930  may perform motion compensation, based on prediction units for the inter-image prediction, on each of the coding units according to the tree structure of the samples of the first layer images, and may reconstruct the first layer images. 
     When the second layer imagestream is received, the image data decoder  930  of the video decoding apparatus  900  may split samples of the second layer images, which are extracted from the second layer imagestream by the extractor  920 , into coding units according to a tree structure of a largest coding unit. The image data decoder  930  may perform motion compensation, based on prediction units for the inter-image prediction, on each of the coding units of the samples of the second layer images, and may reconstruct the second layer images. 
     The extractor  920  may obtain, from a bitstream, information related to a luminance error so as to compensate for a luminance difference between the first layer image and the second layer image. However, whether to perform luminance compensation may be determined according to an encoding mode of a coding unit. For example, the luminance compensation may be performed only on a prediction unit having a size of 2N×2N. 
     Thus, the video decoding apparatus  900  may obtain information about at least one coding unit that generates the minimum encoding error when encoding is recursively performed for each largest coding unit, and may use the information to decode the current picture. That is, the coding units having the tree structure determined to be the optimum coding units in each largest coding unit may be decoded. 
     Accordingly, even if an image has high resolution or has an excessively large data amount, the image may be efficiently decoded and reconstructed by using a size of a coding unit and an encoding mode, which are adaptively determined according to characteristics of the image, by using optimal split information received from an encoding terminal. 
       FIG. 10  illustrates a concept of coding units, according to an embodiment. 
     A size of a coding unit may be expressed by 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, and 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  1010 , a resolution is 1920×1080, a maximum size of a coding unit is 64, and a maximum depth is 2. In video data  1020 , a resolution is 1920×1080, a maximum size of a coding unit is 64, and a maximum depth is 3. In video data  1030 , 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. 10  denotes the total number of splits from a largest coding unit to a smallest coding unit. 
     If a resolution is high or a data amount is large, it is preferable that a maximum size of a coding unit is 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  1010  and  1020  having a higher resolution than the video data  1030  may be selected to  64 . 
     Since the maximum depth of the video data  1010  is 2, coding units  1015  of the vide data  1010  may include a largest 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 largest coding unit twice. On the other hand, since the maximum depth of the video data  1030  is 1, coding units  1035  of the video data  1030  may include a largest 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 largest coding unit once. 
     Since the maximum depth of the video data  1020  is 3, coding units  1025  of the video data  1020  may include a largest 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 largest coding unit three times. As a depth deepens, an expression capability with respect to detailed information may be improved. 
       FIG. 11  illustrates a block diagram of a video encoder 1100 based on coding units, according to various embodiments. 
     The video encoder  1100  according to an embodiment performs operations of a picture encoder  1520  of the video encoding apparatus  800  so as to encode image data. That is, an intra predictor  1120  performs intra prediction on coding units in an intra mode, from among a current image  1105 , and an inter predictor  1115  performs inter prediction on coding units in an inter mode by using the current image  1105  and a reference image obtained from a reconstructed picture buffer  1110  according to prediction units. The current image  1105  may be split into largest coding units and then the largest coding units may be sequentially encoded. In this regard, the largest coding units that are to be split into coding units having a tree structure may be encoded. 
     Residue data is generated by removing prediction data regarding a coding unit of each mode which is output from the intra predictor  1120  or the inter predictor  1115  from data regarding an encoded coding unit of the current image  1105 , and the residue data is output as a quantized transformation coefficient according to transformation units through a transformer  1125  and a quantizer  1130 . The quantized transformation coefficient is reconstructed as the residue data in a spatial domain through an inverse-quantizer  1145  and an inverse-transformer  1150 . The reconstructed residual image data in the spatial domain is added to prediction data for the coding unit of each mode which is output from the intra pictor  1120  or the inter predictor  1115  and thus is reconstructed as data in a spatial domain for a coding unit of the current image  1105 . The reconstructed data in the spatial domain is generated as a reconstructed image through a deblocking unit  1155  and an SAO performer  1160  and the reconstructed image is stored in the reconstructed picture buffer  1110 . The reconstructed images stored in the reconstructed picture buffer  1110  may be used as reference images for inter predicting another image. The transformation coefficient quantized by the transformer  1125  and the quantizer  1130  may be output as a bitstream  1140  through an entropy encoder  1135 . 
     In order for the video encoder  1100  to be applied in the video encoding apparatus  800 , all elements of the video encoder  1100 , i.e., the inter predictor  1115 , the intra predictor  1120 , the transformer  1125 , the quantizer  1130 , the entropy encoder  1135 , the inverse-quantizer  1145 , the inverse-transformer  1150 , the deblocking unit  1155 , and the SAO performer  1160 , may perform operations based on each coding unit among coding units having a tree structure according to each largest coding unit. 
     In particular, the intra predictor  1120  and the inter predictor  1115  may determine a partition mode and a prediction mode of each coding unit from among the coding units having a tree structure, by taking into account the maximum size and the maximum depth of a current largest coding unit, and the transformer  1125  may determine whether to split a transformation unit according to a quadtree in each coding unit from among the coding units having a tree structure. 
       FIG. 12  illustrates a block diagram of a video decoder  1200  based on coding units, according to an embodiment. 
     An entropy decoder  1215  parses, from a bitstream  1205 , encoded image data to be decoded and encoding information required for decoding. The encoded image data corresponds to a quantized transformation coefficient, and an inverse-quantizer  1220  and an inverse-transformer  1225  reconstruct residue data from the quantized transformation coefficient. 
     An intra predictor  1240  performs intra prediction on a coding unit in an intra mode according to prediction units. An inter predictor  1235  performs inter prediction by using a reference image with respect to a coding unit in an inter mode from among a current image, wherein the reference image is obtained by a reconstructed picture buffer  1230  according to prediction units. 
     Prediction data and residue data regarding coding units of each mode, which passed through the intra predictor  1240  and the inter predictor  1235 , are summed, so that data in a spatial domain regarding coding units of the current image  1205  may be reconstructed, and the reconstructed data in the spatial domain may be output as a reconstructed image  1260  through a deblocking unit  1245  and an SAO performer  1250 . Also, reconstructed images stored in the reconstructed picture buffer  30  may be output as reference images. 
     In order for a picture decoder  930  of the video decoding apparatus  900  to decode the image data, operations after the entropy decoder  1215  of the video decoder  1200  according to an embodiment may be performed. 
     In order for the video decoder  1200  to be applied in the video decoding apparatus  900  according to an embodiment, all elements of the video decoder  1200 , i.e., the entropy decoder  1215 , the inverse-quantizer  1220 , the inverse-transformer  1225 , the intra predictor  1240 , the inter predictor  1235 , the deblocking unit  1245 , and the SAO performer  1250  may perform operations based on coding units having a tree structure for each largest coding unit. 
     In particular, the intra predictor  1240  and the inter predictor  1235  may determine a partition mode and a prediction mode of each coding unit from among the coding units according to a tree structure, and the inverse-transformer  1225  may determine whether or not to split a transformation unit according to a quadtree in each coding unit. 
     The encoding operation of  FIG. 10  and the decoding operation of  FIG. 11  are described as a videostream encoding operation and a videostream decoding operation, respectively, in a single layer. Thus, if the video encoding apparatus  10  of  FIG. 1A  encodes a videostream of two or more layers, the video encoder  1100  may be provided for each layer. Similarly, if the inter-layer decoding apparatus  20  of  FIG. 2A  decodes a videostream of two or more layers, the video decoder  1200  may be provided for each layer. 
       FIG. 13  illustrates deeper coding units according to depths, and partitions, according to an embodiment. 
     The video encoding apparatus  800  according to an embodiment and the video decoding apparatus  900  according to an 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 variously set according to user requirements. 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 of coding units  1300  according to an 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 represents a total number of times the coding unit is split from the largest coding unit to the smallest coding unit. Since a depth deepens along a vertical axis of the hierarchical structure of coding units  1300 , 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 of coding units  1300 . 
     That is, a coding unit  1310  is a largest coding unit in the hierarchical structure of coding units  1300 , wherein a depth is 0 and a size, i.e., a height by width, is 64×64. The depth deepens along the vertical axis, and a coding unit  1320  having a size of 32×32 and a depth of 1, a coding unit  1330  having a size of 16×16 and a depth of 2, and a coding unit  1340  having a size of 8×8 and a depth of 3. The coding unit  1340  having the size of 8×8 and the depth of 3 is a smallest coding unit. 
     The prediction unit and the partitions of a coding unit are arranged along the horizontal axis according to each depth. That is, if the coding unit  1310  having a size of 64×64 and a depth of 0 is a prediction unit, the prediction unit may be split into partitions include in the coding unit  1310  having the size of 64×64, i.e. a partition  1310  having a size of 64×64, partitions  1312  having the size of 64×32, partitions  1314  having the size of 32×64, or partitions  1316  having the size of 32×32. 
     Equally, a prediction unit of the coding unit  1320  having the size of 32×32 and the depth of 1 may be split into partitions included in the coding unit  1320  having the size of 32×32, i.e. a partition  1320  having a size of 32×32, partitions  1322  having a size of 32×16, partitions  1324  having a size of 16×32, and partitions  1326  having a size of 16×16. 
     Equally, a prediction unit of the coding unit  1330  having the size of 16×16 and the depth of 2 may be split into partitions included in the coding unit  1330  having the size of 16×16, i.e. a partition having a size of 16×16 included in the coding unit  1330 , partitions  1332  having a size of 16×8, partitions  1334  having a size of 8×16, and partitions  1336  having a size of 8×8. 
     Equally, a prediction unit of the coding unit  1340  having the size of 8×8 and the depth of 3 may be split into partitions included in the coding unit  1340  having the size of 8×8, i.e. a partition having a size of 8×8 included in the coding unit  1340 , partitions  1342  having a size of 8×4, partitions  1344  having a size of 4×8, and partitions  1346  having a size of 4×4. 
     In order to determine a depth of the largest coding unit  1310 , the coding unit determiner  820  of the video encoding apparatus  800  has to perform encoding on coding units respectively corresponding to depths included in the largest coding unit  1310 . 
     The 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 results of encoding the same data according to depths, the data has to be encoded by using each of the coding unit corresponding to the depth of 1 and four coding units corresponding to the depth of 2. 
     In order to perform encoding according to each of the depths, a least encoding error that is a representative encoding error of a corresponding depth may be selected by performing encoding on each of prediction units of the coding units according to depths, along the horizontal axis of the hierarchical structure of coding units  1300 . Also, the minimum encoding error may be searched for by comparing representative encoding errors according to depths, by performing encoding for each depth as the depth deepens along the vertical axis of the hierarchical structure of coding units  1300 . A depth and a partition generating the minimum encoding error in the largest coding unit  1310  may be selected as a depth and a partition mode of the largest coding unit  1310 . 
       FIG. 14  illustrates a relationship between a coding unit and transformation units, according to an embodiment. 
     The video encoding apparatus  800  according to an embodiment or the video decoding apparatus  900  according to an embodiment encodes or decodes an image according to coding units having sizes smaller than or equal to a largest coding unit for each largest coding unit. Sizes of transformation units for transformation during an encoding process may be selected based on data units that are not larger than a corresponding coding unit. 
     For example, in the video encoding apparatus  800  or the video decoding apparatus  900 , when a size of the coding unit  1410  is 64×64, transformation may be performed by using the transformation units  1420  having a size of 32×32. 
     Also, data of the coding unit  1410  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 with respect to an original image may be selected. 
       FIG. 15  illustrates a plurality of pieces of encoding information, according to an embodiment. 
     The output unit  830  of the video encoding apparatus  800  according to an embodiment may encode and transmit, as split information, partition mode information  1500 , prediction mode information  1510 , and transformation unit size information  1520  for each coding unit corresponding to a depth. 
     The partition mode information  1500  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  1502  having a size of 2N×2N, a partition  1504  having a size of 2N×N, a partition  1506  having a size of N×2N, and a partition  1508  having a size of N×N. In this case, the partition mode information  1500  about a current coding unit is set to indicate one of the partition  1502  having a size of 2N×2N, the partition  1504  having a size of 2N×N, the partition  1506  having a size of N×2N, and the partition  1508  having a size of N×N. 
     The prediction mode information  1510  indicates a prediction mode of each partition. For example, the prediction mode information  1510  may indicate a mode of prediction encoding performed on a partition indicated by the partition mode information  1500 , i.e., an intra mode  1512 , an inter mode  1514 , or a skip mode  1516 . 
     The transformation unit size information  1520  represents a transformation unit to be based on when transformation is performed on a current coding unit. For example, the transformation unit may be one of a first intra transformation unit  1522 , a second intra transformation unit  1524 , a first inter transformation unit  1526 , and a second inter transformation unit  1528 . 
     The image data and encoding information extractor  1610  of the video decoding apparatus  900  may extract and use the partition mode information  1500 , the prediction mode information  1510 , and the transformation unit size information  1520  for decoding, according to each deeper coding unit. 
       FIG. 16  illustrates deeper coding units according to depths, according to an embodiment. 
     Split information may be used to represent a change in a depth. The spilt information specifies whether a coding unit of a current depth is split into coding units of a lower depth. 
     A prediction unit  1610  for prediction encoding a coding unit  1600  having a depth of 0 and a size of 2N_0×2N_0 may include partitions of a partition mode  1612  having a size of 2N_0×2N_0, a partition mode  1614  having a size of 2N_0×N_0, a partition mode  1616  having a size of N_0×2N_0, and a partition mode  1618  having a size of N_0×N_0. Only the partition modes  1612 ,  1614 ,  1616 , and  1618  which are obtained by symmetrically splitting the prediction unit are illustrated, but as described above, a partition mode is not limited thereto and may include asymmetrical partitions, partitions having a predetermined shape, and partitions having a geometrical shape. 
     According to each partition mode, prediction encoding has to be 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. 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 may be performed only on the partition having the size of 2N_0×2N_0. 
     If an encoding error is smallest in one of the partition modes  1612 ,  1614 , and  1616  having the sizes of 2N_0×2N_0, 2N_0×N_0 and N_0×2N_0, the prediction unit  1610  may not be split into a lower depth. 
     If the encoding error is the smallest in the partition mode  1618  having the size of N_0×N_0, a depth is changed from 0 to 1 and split is performed (operation  1620 ), and encoding may be repeatedly performed on coding units  1630  of a partition mode having a depth of 2 and a size of N_0 so as to search for a minimum encoding error. 
     A prediction unit  1630  for prediction encoding the coding unit  1630  having a depth of 1 and a size of 2N_1×2N_1 (=N_0×N_0) may include a partition mode  1642  having a size of 2N_1×2N_1, a partition mode  1644  having a size of 2N_1×N_1, a partition mode  1646  having a size of N_1×2N_1, and a partition mode  1648  having a size of N_1×N_1. 
     If an encoding error is the smallest in the partition mode  1648  having the size of N_1×N_1, a depth is changed from 1 to 2 and split is performed (in operation  1650 ), and encoding is repeatedly performed on coding units  1660  having a depth of 2 and a size of N_2×N_2 so as to search for a minimum encoding error. 
     When a maximum depth is d, deeper conding units according to depths may be set until when a depth corresponds to d-1, and split information may be set until when a depth corresponds to d-2. That is, when encoding is performed up to when the depth is d-1 after a coding unit corresponding to a depth of d-2 is split (in operation  1670 ), a prediction unit  1690  for prediction encoding a coding unit  1680  having a depth of d-1 and a size of 2N_(d-1)×2N_(d-1) may include partitions of a partition mode  1692  having a size of 2N_(d-1)×2N_(d-1), a partition mode  1694  having a size of 2N_(d-1)×N_(d-1), a partition mode  1696  having a size of N_(d-1)×2N_(d-1), and a partition mode  1698  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 N_(d-1)×2N_(d-1), four partitions having a size of N_(d-1)×N_(d-1) from among the partition modes so as to search for a partition mode generating a minimum encoding error. 
     Even when the partition type  1698  having the size of N_(d-1)×N_(d-1) 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 into a lower depth, and a depth for the coding units constituting a current largest coding unit  1600  is determined to be d-1 and a partition mode of the current largest coding unit  1600  may be determined to be N_(d-1)×N_(d-1). Also, since the maximum depth is d, split information for a coding unit  1652  corresponding to a depth of d-1 is not set. 
     A data unit  1699  may be a ‘minimum unit’ for the current largest coding unit. A minimum unit according to the embodiment may be a square data unit obtained by splitting a smallest coding unit having a lowermost depth by 4. By performing the encoding repeatedly, the video encoding apparatus  800  according to the embodiment may select a depth having the least encoding error by comparing encoding errors according to depths of the coding unit  1600  to determine a depth, and set a corresponding partition type and a prediction mode as an encoding mode of the depth. 
     As such, the minimum encoding errors according to depths are compared in all of the depths of 0, 1, . . . , d-1, d, and a depth having the least encoding error may be determined as a depth. The depth, the partition mode of the prediction unit, and the prediction mode may be encoded and transmitted as split information. Also, since a coding unit has to be split from a depth of 0 to a depth, only split information of the depth is set to ‘0’, and split information of depths excluding the depth is set to ‘1’. 
     The image data and encoding information extractor  920  of the video decoding apparatus  900  according to the embodiment may extract and use a depth and prediction unit information about the coding unit  1600  so as to decode the coding unit  1612 . The video decoding apparatus  900  according to the embodiment may determine a depth, in which split information is ‘0’, as a depth by using split information according to depths, and may use, for decoding, split information about the corresponding depth. 
       FIGS. 17, 18, and 19  illustrate a relationship between coding units, prediction units, and transformation units, according to an embodiment. 
     Coding units  1710  are deeper coding units according to depths determined by the video encoding apparatus  800 , in a largest coding unit. Prediction units  1760  ar partitions of prediction units of each of the coding units  1710  according to depths, and transformation units  1770  are transformation units of each of the coding units according to depths. 
     When a depth of a largest coding unit is 0 in the deeper coding units  1710 , depths of coding units  1712  and  1054  are 1, depths of coding units  1714 ,  1716 ,  1718 ,  1728 ,  1750 , and  1752  are 2, depths of coding units  1720 ,  1722 ,  1724 ,  1726 ,  1730 ,  1732 , and  1748  are 3, and depths of coding units  1740 ,  1742 ,  1744 , and  1746  are 4. 
     Some partitions  1714 ,  1716 ,  1722 ,  1732 ,  1748 ,  1750 ,  1752 , and  1754  from among the prediction units  1760  are obtained by splitting the coding unit. That is, partitions  1714 ,  1722 ,  1750 , and  1754  are a partition mode having a size of 2N×N, partitions  1716 ,  1748 , and  1752  are a partition mode having a size of N×2N, and a partition  1732  is a partition mode having a size of N×N. Prediction units and partitions of the deeper coding units  1710  are smaller than or equal to each coding unit. 
     Transformation or inverse transformation is performed on image data of the coding unit  1752  in the transformation units  1770  in a data unit that is smaller than the coding unit  1752 . Also, the coding units  1714 ,  1716 ,  1722 ,  1732 ,  1748 ,  1750 ,  1752 , and  1754  in the transformation units  1760  are data units different from those in the prediction units  1760  in terms of sizes and shapes. That is, the video encoding apparatus  800  and the video decoding apparatus  900  according to the embodiments may perform intra prediction/motion estimation/motion compensation/and transformation/inverse transformation on an individual 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 largest coding unit so as to determine an optimum coding unit, and thus coding units according to a recursive tree structure may be obtained. Encoding information may include split information about a coding unit, partition mode information, prediction mode information, and transformation unit size information. Table 1 below shows the encoding information that may be set by the video encoding apparatus  800  and the video decoding apparatus  900  according to the embodiments. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Split Information 0 
                 Split 
               
               
                 (Encoding on Coding Unit having Size of 2N × 2N and Current Depth of d) 
                 Information 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Prediction 
                 Partition Type 
                 Size of Transformation Unit 
                 Repeatedly 
               
               
                 Mode 
                   
                   
                 Encode Coding 
               
            
           
           
               
               
               
               
               
               
            
               
                 Intra 
                 Symmetrical 
                 Asymmetrical 
                 Split 
                 Split 
                 Units having 
               
               
                 Inter 
                 Partition 
                 Partition 
                 Information 0 of 
                 Information 1 of 
                 Lower Depth 
               
               
                 Skip (Only 
                 Type 
                 Type 
                 Transformation 
                 Transformation 
                 of d + 1 
               
               
                 2N × 2N) 
                   
                   
                 Unit 
                 Unit 
               
               
                   
                 2N × 2N 
                 2N × nU 
                 2N × 2N 
                 N × N 
               
               
                   
                 2N × N 
                 2N × nD 
                   
                 (Symmetrical 
               
               
                   
                 N × 2N 
                 nL × 2N 
                   
                 Partition Type) 
               
               
                   
                 N × N 
                 nR × 2N 
                   
                 N/2 × N/2 
               
               
                   
                   
                   
                   
                 (Asymmetrical 
               
               
                   
                   
                   
                   
                 Partition Type) 
               
               
                   
               
            
           
         
       
     
     The output unit  830  of the video encoding apparatus  800  according to the embodiment may output the encoding information about the coding units having a tree structure, and the image data and encoding information extractor  920  of the video decoding apparatus  900  according to the embodiment may extract the encoding information about the coding units having a tree structure from a received bitstream. 
     Split information specifies 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 depth, and thus partition mode information, prediction mode information, and transformation unit size information may be defined for the depth. If the current coding unit has to be further split according to the split information, encoding has to be independently performed on each of 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 modes, and the skip mode is defined only in a partition mode having a size of 2N×2N. 
     The partition mode information may indicate symmetrical partition modes having sizes of 2×2N, 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 modes 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 modes 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 modes 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. That is, 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 mode of the current coding unit having the size of 2N×2N is a symmetrical partition mode, a size of a transformation unit may be N×N, and if the partition mode of the current coding unit is an asymmetrical partition mode, the size of the transformation unit may be N/2×N/2. 
     The encoding information about coding units having a tree structure according to the embodiment may be assigned to at least one of a coding unit corresponding to a depth, a 01  prediction unit, and a minimum unit. The coding unit corresponding to the 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 depth by comparing encoding information of the adjacent data units. Also, a corresponding coding unit corresponding to a depth is determined by using encoding information of a data unit, and thus a distribution of depths in a largest coding unit may be inferred. 
     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. 
     In another embodiment, if a current coding unit is predicted based on encoding information of adjacent data units, data units adjacent to the current coding unit may be searched by using encoded information of the data units, and the searched adjacent coding units may be referred for predicting the current coding unit. 
       FIG. 20  illustrates a relationship between a coding unit, a prediction unit, and a transformation unit, according to encoding mode information of Table 1. 
     A largest coding unit  2000  includes coding units  2002 ,  2004 ,  2006 ,  2012 ,  2014 ,  2016 , and  2018  of depths. Here, since the coding unit  2018  is a coding unit of a depth, split information may be set to 0. Partition mode information of the coding unit  2018  having a size of 2N×2N may be set to be one of partition modes including 2N×2N  2022 , 2N×N  2024 , N×2N  2026 , N×N  2028 , 2N×nU  2032 , 2N×nD  2034 , nL×2N  2036 , and nR×2N  2038 . 
     Transformation unit split information (TU size flag) is a type of a transformation index, and a size of a transformation unit corresponding to the transformation index may be changed according to a prediction unit type or partition mode of the coding unit. 
     For example, when the partition mode information is set to be on e of symmetrical partition modes 2N×2N  2022 , 2N×N  2024 , N×2N  2026 , and N×N  2028 , if the transformation unit split information is 0, a transformation unit  2042  having a size of 2N×2N is set, and if the transformation unit split information is 1, a transformation unit  2044  having a size of N×N may be set. 
     When the partition mode information is set to be one of asymmetrical partition modes 2N×nU  2032 , 2N×nD  2034 , nL×2N  2036 , and nR×2N  2038 , if the transformation unit split information (TU size flag) is 0, a transformation unit  2052  having a size of 2N×2N may be set, and if the transformation unit split information is 1, a transformation unit  2054  having a size of N/2×N/2 may be set. 
     The transformation unit split information (TU size flag) described above with reference to  FIG. 19  is a flag having a value or 0 or 1, but the transformation unit split information according to an embodiment is not limited to a flag having 1 bit, and the transformation unit may be hierarchically split while the transformation unit split information increases in a manner of 0, 1, 2, 3 . . . etc., according to setting. The transformation unit split information may be an example of the transformation index. 
     In this case, the size of a transformation unit that has been actually used may be expressed by using the transformation unit split information according to the embodiment, together with a maximum size of the transformation unit and a minimum size of the transformation unit. The video encoding apparatus  800  according to the embodiment may encode maximum transformation unit size information, minimum transformation unit size information, and maximum transformation unit split information. The result of encoding the maximum transformation unit size information, the minimum transformation unit size information, and the maximum transformation unit split information may be inserted into an SPS. The video decoding apparatus  900  according to the embodiment may decode video by using the maximum transformation unit size information, the minimum transformation unit size information, and the maximum transformation unit split information. 
     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 smaller 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))   (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. That is, 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 by the 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 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)   (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)   (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 embodiment, and a factor for determining the current maximum transformation unit size is not limited thereto. 
     According to the video encoding method based on coding units of a tree structure described above with reference to  FIGS. 8 through 20 , image data of a spatial domain is encoded in each of the coding units of the tree structure, and the image data of the spatial domain is reconstructed in a manner that decoding is performed on each largest coding unit according to the video decoding method based on the coding units of the tree structure, so that a video that is formed of pictures and picture sequences may be reconstructed. The reconstructed video may be reproduced by a reproducing apparatus, may be stored in a storage medium, or may be transmitted via a network. 
     The embodiments of the present invention may be written as computer programs and may be implemented in general-use digital computers that execute the programs by using a computer-readable recording medium. Examples of the computer-readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, or DVDs), etc. 
     For convenience of description, the video encoding methods and/or the video encoding method, which are described with reference to  FIGS. 1A through 20 , will be collectively referred to as ‘the video encoding method of the present invention’. Also, the video decoding methods and/or the video decoding method, which are described with reference to  FIGS. 1A through 20 , will be collectively referred to as ‘the video decoding method of the present invention’. 
     Also, a video encoding apparatus including the video encoding apparatus, the video encoding apparatus  800  or the video encoder  1100  which are described with reference to  FIGS. 1A through 20  will be collectively referred to as a ‘video encoding apparatus of the present invention’. Also, a video decoding apparatus including the inter-layer video decoding apparatus, the video decoding apparatus  900 , or the video decoder  1200  which are described with reference to  FIGS. 1A through 20  will be collectively referred to as a ‘video decoding apparatus of the present invention’. 
     A computer-readable recording medium storing a program, e.g., a disc  26000 , according to an embodiment will now be described in detail. 
       FIG. 21  illustrates a physical structure of the disc  26000  in which a program is stored, according to an embodiment. The disc  26000 , as 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  26000  includes a plurality of concentric tracks Tr that are each divided into a specific number of sectors Se in a circumferential direction of the disc  26000 . In a specific region of the disc  26000 , a program that executes the quantized parameter determining method, the video encoding method, and the video decoding method described above may be assigned and stored. 
     A computer system embodied using a storage medium that stores a program for executing the video encoding method and the video decoding method as described above will now be described with reference to  FIG. 22 . 
       FIG. 22  illustrates a disc drive  26800  for recording and reading a program by using the disc  26000 . A computer system  26700  may store a program that executes at least one of the video encoding method and the video decoding method of the present invention, in the disc  26000  via the disc drive  26800 . In order to run the program stored in the disc  26000  in the computer system  26700 , the program may be read from the disc  26000  and may be transmitted to the computer system  26700  by using the disc drive  26800 . 
     The program that executes at least one of the video encoding method and the video decoding method of the present invention may be stored not only in the disc  26000  illustrated in  FIGS. 21 and 22  but may also be stored in a memory card, a ROM cassette, or a solid state drive (SSD). 
     A system to which the video encoding method and the video decoding method according to the embodiments described above are applied will be described below. 
       FIG. 23  illustrates an overall structure of a content supply system  11000  for providing 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 as illustrated in  FIG. 23 , and devices may be selectively connected thereto. The plurality of independent devices may be directly connected to the communication network  11400 , not 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  allows 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 by 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 puter  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 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 accessed by the computer  12100 . 
     If video data is captured by a camera built in the mobile phone  12500 , the video data may be received from the mobile phone  12500 . 
     The video data may be encoded by a large scale integrated circuit (LSI) system installed in the video camera  12300 , the mobile phone  12500 , or the camera  12600 . 
     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 may 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  allows the clients to receive and reproduce the encoded content data. Also, the content supply system  11000  allows 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 the video encoding apparatus and the video decoding apparatus of the present invention. 
     With reference to  FIGS. 24 and 25 , the mobile phone  12500  included in the content supply system  11000  according to an embodiment will now be described in detail. 
       FIG. 24  illustrates an external structure of the mobile phone  12500  to which a video encoding method and a video decoding method are applied, according to an embodiment. The mobile phone  12500  may be a smart phone, the functions of which are not limited and a large number 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  12000 , 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 diode (OLED) screen. The mobile phone  12500  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 mobile phone  12500  includes a speaker  12580  for outputting voice and sound or another type of a sound output unit, and a microphone  12550  for inputting voice and sound or another type of a sound input unit. The mobile phone  12500  further includes the camera  12530 , such as a charge-coupled device (CCD) camera, to capture video and still images. The mobile phone  12500  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. 25  illustrates an internal structure of the mobile phone  12500 . In order 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 encoder  12720 , a camera interface  12630 , an LCD controller  12620 , an image decoder  12690 , a multiplexer/demultiplexer  12680 , a recording/reading unit  12670 , a modulation/demodulation unit  12660 , 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 sets 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  to an operation mode. 
     The central controller  12710  includes a CPU, a read-only memory (ROM), and a random access memory (RAM). 
     While the mobile phone  12500  transmits communication data to the outside, a digital signal is generated by the mobile phone  12500  under control of the central controller  12710 . For example, the sound processor  12650  may generate a digital sound signal, the video encoder  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 transmitted to the modulation/demodulation unit  12660  by 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  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 , by 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 text message, e.g., email, is transmitted during 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 . By 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 . 
     In order to transmit image data during the data communication mode, image data captured by the camera  12530  is provided to the image encoder  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 encoder  12720  may correspond to that of the video encoding apparatus  100  described above. The image encoder  12720  may transform the image data received from the camera  12530  into compressed and encoded image data according to the aforementioned video encoding method, 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 transmitted to the multiplexer/demultiplexer  12680 . 
     The multiplexer/demultiplexer  12680  multiplexes the encoded image data received from the image encoder  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 analog-to-digital conversion (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 decoder  12690 , the sound processor  12650 , or the LCD controller  12620 , according to the type of the digital signal. 
     During 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 , by control of the central controller  12710 . 
     When during the data communication mode, data of a video file accessed at an Internet website is received, a signal received from the wireless base station  12000  via the antenna  12510  is output as multiplexed data via the modulation/demodulation unit  12660 , and the multiplexed data is transmitted to the multiplexer/demultiplexer  12680 . 
     In order 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 video decoder  12690  and the sound processor  12650 , respectively. 
     A structure of the image decoder  12690  may correspond to that of the video decoding apparatus described above. The image decoder  12690  may decode the encoded video data to obtain reconstructed video data and provide the reconstructed video data to the display screen  12520  via the LCD controller  12620 , by using the aforementioned video decoding method of the present invention. 
     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 transmitting terminal including only the video encoding apparatus, or may be a receiving terminal including only the video decoding apparatus. 
     A communication system according to an embodiment is not limited to the communication system described above with reference to  FIG. 24 . For example,  FIG. 26  illustrates a digital broadcasting system employing a communication system, according to an embodiment. 
     The digital broadcasting system of  FIG. 26  may receive a digital broadcast transmitted via a satellite or a terrestrial network by using the video encoding apparatus and the video decoding apparatus according to the embodiments. 
     In more detail, 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 the video decoding apparatus of the present invention 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 reconstruct digital signals. Thus, the reconstructed video signal may be reproduced, for example, on a monitor  2840 . 
     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, the video decoding apparatus of the present invention may be installed. Data output from the set-top box  12870  may also be reproduced on a TV monitor  12880 . 
     As another example, the video decoding apparatus of the present invention may be installed in the TV receiver  12810  instead of the set-top box  12870 . 
     An automobile  12920  that has 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  installed in the automobile  12920 . 
     A video signal may be encoded by the video encoding apparatus of the present invention and may then be stored in a storage medium. In more detail, 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 the video decoding apparatus according to the 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 video encoder  12720  of  FIG. 26 . For example, the computer  12100  and the TV receiver  12810  may not include the camera  12530 , the camera interface  12630 , and the video encoder  12720  of  FIG. 26 . 
       FIG. 27  illustrates a network structure of a cloud computing system using a video encoding apparatus and a video decoding apparatus, according to an embodiment. 
     The cloud computing system may include a cloud computing server  14100 , 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, a storage, an operating system (OS), and security software, into his/her own terminal in order to use them, but may select and use desired services from among services in a virtual space generated through the virtualization technology, at a desired point in 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 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 desktop 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 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 database distributed in different regions according to the virtualization technology. 
     User information about users who have subscribed for a cloud computing service is stored in the user DB  14100 . The user information may include logging 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 the 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. 24 . 
     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 this video, performed by the cloud computing server  14000 , may vary according to the request from the user terminal, i.e., 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. On the other hand, 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. 
     Here, the user terminal may include the video decoding apparatus as described above with reference to  FIGS. 1A through 20 . As another example, the user terminal may include the video encoding apparatus as described above with reference to  FIGS. 1A through 20 . Alternatively, the user terminal may include both the video encoding apparatus and the video decoding apparatus as described above with reference to  FIGS. 1A through 20 . 
     Various applications of the video encoding method, the video decoding method, the video encoding apparatus, and the video decoding apparatus described above with reference to  FIGS. 1A through 20  are described above with reference to  FIGS. 21 through 27 . However, embodiments of methods of storing the video encoding method and the video decoding method in a storage medium or embodiments of methods of implementing the video encoding apparatus and the video decoding apparatus in a device described above with reference to  FIGS. 1A through 20  are not limited to the embodiments of  FIGS. 21 through 27 . 
     The method, process, apparatus, product, and/or system according to the present invention are simple, cost-effective, various, and accurate. Furthermore, efficient and economical production, application, and utilization may be implemented by applying known components to the process, apparatus, product, and system according to the present invention. In addition, the present invention complies with current trends requiring cost reduction, system simplification, and performance enhancement. As such, the level of current technology may be enhanced. 
     While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims. The exemplary embodiments should be considered in a descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the following claims, and all differences within the scope will be construed as being included in the present invention.