Patent Publication Number: US-2016227248-A1

Title: Method and apparatus for encoding scalable video for encoding auxiliary picture, method and apparatus for decoding scalable video for decoding auxiliary picture

Description:
TECHNICAL FIELD 
     The present invention relates to methods and apparatus for encoding and decoding a multilayer video such as a scalable video and a multiview video, and more particularly, to a high-level syntax structure for signaling the multilayer video. 
     BACKGROUND ART 
     In general, video data is encoded by using a codec according to a predetermined data compression standard, e.g., the Moving Picture Experts Group (MPEG), and then the video data in the form of a bitstream is stored in a storage medium or is transmitted via a communication channel. 
     Scalable Video Coding (SVC) indicates a video compression method for appropriately adjusting and transmitting an amount of data while being adapted to various communication networks and terminals. The SVC provides a video coding method that can be adaptively serviced by one videostream, according to various transmitting networks and various receiving terminals. 
     In addition, due to the recent supply of three-dimensional (3D) multimedia apparatuses and 3D multimedia contents, a multiview video coding technique for 3D video coding has become increasingly widely used. 
     According to scalable video coding or multiview video coding of the related art, a video is encoded according to a limited encoding method based on a macroblock having a predetermined size. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     Provided are a video encoding technique or decoding technique based on various-size blocks with respect to scalable video coding or multiview video coding. 
     Technical Solution 
     A scalable video decoding method according to various embodiments include obtaining, from a bitstream, scalability mask information specifying whether scalable video decoding is performed according to each of scalability types in a current video; from among the scalability mask information, when scalability mask information for scalable video decoding an auxiliary picture indicates performance, obtaining a scalability index indicating a type of the auxiliary picture to be decoded in a current layer; and decoding the auxiliary picture of the current layer which is determined, by using the scalability index, as one of auxiliary picture types comprising an alpha plane and a depth picture of a primary picture of another layer. 
     Advantageous Effects of the Invention 
     A video encoding technique or decoding technique based on various-size blocks is performed with respect to scalable video coding or multiview video coding, thus, a video may be efficiently encoded or decoded by using a data block having an appropriate size for an image characteristic in each layer. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram illustrating a structure of a scalable video encoding apparatus, according to various embodiments. 
         FIG. 1B  illustrates a flowchart of a method of encoding a scalable video, according to various embodiments. 
         FIG. 2A  is a block diagram illustrating a structure of a scalable video decoding apparatus, according to various embodiments. 
         FIG. 2B  illustrates a flowchart of a method of decoding a scalable video, according to various embodiments. 
         FIG. 3A  illustrates a multilayer video. 
         FIG. 3B  illustrates a multilayer video encoding apparatus, according to an embodiment. 
         FIG. 4A  illustrates Network Abstraction Layer (NAL) units including encoded data of a multilayer video, according to various embodiments. 
         FIG. 4B  is a diagram for describing a layer set, according to various embodiments. 
         FIG. 4C  is a diagram for describing an output layer subset, according to an embodiment. 
         FIG. 5A  illustrates a mapping relationship between a scalability mask and a scalability type, according to an embodiment. 
         FIG. 5B  illustrates an auxiliary picture type, according to an embodiment. 
         FIG. 5C  illustrates syntax for mapping an auxiliary picture type to a scalability type, according to an embodiment. 
         FIG. 6  illustrates video parameter set (VPS) extension syntax, according to an embodiment. 
         FIG. 7  illustrates VPS extension syntax, according to another embodiment. 
         FIG. 8  illustrates a block diagram of a video encoding apparatus based on coding units of a tree structure, according to various embodiments. 
         FIG. 9  illustrates a block diagram of a video decoding apparatus based on coding units of a tree structure, according to various embodiments. 
         FIG. 10  illustrates a concept of coding units, according to various embodiments. 
         FIG. 11  illustrates a block diagram of an image encoder based on coding units, according to various embodiments. 
         FIG. 12  illustrates a block diagram of an image decoder based on coding units, according to various embodiments. 
         FIG. 13  illustrates deeper coding units according to depths, and partitions, according to various embodiments. 
         FIG. 14  illustrates a relationship between a coding unit and transformation units, according to various embodiments. 
         FIG. 15  illustrates a plurality of pieces of encoding information according to depths, according to various embodiments. 
         FIG. 16  illustrates deeper coding units according to depths, according to various embodiments. 
         FIGS. 17, 18, and 19  illustrate a relationship between coding units, prediction units, and transformation units, according to various embodiments. 
         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 in which a program is stored, according to various embodiments. 
         FIG. 22  illustrates a disc drive for recording and reading a program by using the disc. 
         FIG. 23  illustrates an overall structure of a content supply system for providing a content distribution service. 
         FIGS. 24 and 25  illustrate external and internal structures of a mobile phone to which a video encoding method and a video decoding method of the present invention are applied, according to various embodiments. 
         FIG. 26  illustrates a digital broadcasting system employing a communication system, according to various embodiments. 
         FIG. 27  illustrates a network structure of a cloud computing system using a video encoding apparatus and a video decoding apparatus, according to various embodiments. 
     
    
    
     BEST MODE 
     A scalable video decoding method according to various embodiments includes obtaining, from a bitstream, scalability mask information specifying whether scalable video decoding is performed according to each of scalability types in a current video; from among the scalability mask information, when scalability mask information for scalable video decoding an auxiliary picture indicates performance, obtaining a scalability index indicating a type of the auxiliary picture to be decoded in a current layer; and decoding the auxiliary picture of the current layer which is determined, by using the scalability index, as one of auxiliary picture types including an alpha plane and a depth picture of a primary picture of another layer. 
     The scalable video decoding method may further include obtaining target output layer information from the bitstream; when the target output layer information represents one layer, determining, as a target output layer, only a layer that is from among a default layer set including at least one layer and has an identifier of a highest layer including the primary picture associated with the auxiliary picture; and when the target output layer information represents two or more layers, determining, as a target output layer, all layers that are from among a default layer set including a plurality of layers and include the primary picture associated with the auxiliary picture, wherein each of the default layer sets is one of a group of default layer sets that each include at least one layer. 
     A scalable video encoding method according to various embodiments includes encoding an auxiliary picture of a current layer which corresponds to a primary picture of another layer; determining scalability mask information specifying whether scalable video encoding is performed according to each of scalability types of a current video; when a scalability type for the auxiliary picture is performed in the current layer, determining a scalability index indicating an auxiliary picture type including an alpha plane and a depth picture of the primary picture of the other layer; and generating a bitstream including the scalability mask information for the current video and the scalability index for the current layer. 
     A scalable video decoding apparatus according to various embodiments includes a scalability type information obtainer configured to obtain, from a bitstream, scalability mask information specifying whether scalable video decoding is performed according to each of scalability types in a current video, and, from among the scalability mask information, when scalability mask information for scalable video decoding an auxiliary picture indicates performance, to obtain a scalability index indicating a type of the auxiliary picture to be decoded in a current layer; and an auxiliary picture decoder configured to decode the auxiliary picture of the current layer, which is determined, by using the scalability index, as one of auxiliary picture types including an alpha plane and a depth picture of a primary picture of another layer. 
     A scalable video encoding apparatus according to various embodiments includes a scalability type determiner configured to determine scalability mask information specifying whether scalable video encoding is performed according to each of scalability types of a current video, and when a scalability type for an auxiliary picture is performed in a current layer, to determine a scalability index indicating an auxiliary picture type including an alpha plane and a depth picture of a primary picture of another layer; and a bitstream generator configured to generate a bitstream including encoded data of the auxiliary picture, the scalability mask information for the current video, and the scalability index for the current layer. 
     According to the scalable video encoding apparatus according to various embodiments, a layer that includes only auxiliary pictures and is from among a layer set including one or more layers may not be a target output layer. 
     According to various embodiments, the primary picture associated with the auxiliary picture may be present, the primary picture may be associated with one or more auxiliary pictures having different auxiliary picture types, and the auxiliary picture types may include the alpha plane, the depth picture, and chroma data. 
     According to various embodiments, a type of a Network Abstraction Layer (NAL) unit of the auxiliary picture may be equal to a type of a NAL unit of the associated primary picture, and a temporal layer identifier of the NAL unit of the auxiliary picture may be equal to a temporal layer identifier of the NAL unit of the primary picture. 
     According to various embodiments, a standard profile for decoding the auxiliary picture may not be limited to a profile that precedes a predetermined profile. 
     According to various embodiments, prediction decoding may not be performed between layers having different auxiliary picture type identifiers, and the prediction decoding may be performed between layers having a same auxiliary picture type identifier, wherein the prediction decoding includes inter-view prediction with respect to a plurality of depth view images. 
     According to various embodiments, the auxiliary picture and the primary picture associated with the auxiliary picture may have a same scalability identifier, and the auxiliary picture and the primary picture associated with the auxiliary picture may be allowed to have different scalability identifiers. 
     According to various embodiments, a chroma enhancement auxiliary picture from among the auxiliary picture may include a monochrome picture. 
     According to various embodiments, a Cr layer and a Cb layer of the chroma enhancement auxiliary picture may be paired. 
     According to various embodiments, there is provided a computer-readable recording medium having recorded thereon a program for executing the scalable video decoding method. 
     According to various embodiments, there is provided a computer-readable recording medium having recorded thereon a program for executing the scalable video encoding method. 
     MODE OF THE INVENTION 
     Hereinafter, a scalable video encoding apparatus, a scalable video decoding apparatus, a scalable video encoding method, and a scalable video decoding method according to various embodiments are provided with reference to  FIGS. 1A through 7 . Also, a video encoding apparatus and a video decoding apparatus, and a video encoding method and a video decoding method based on coding units of a tree structure according to various embodiments are provided with reference to  FIGS. 8 through 20 . Also, various embodiments to which the scalable video encoding method, a multilayer video decoding method, the video encoding method and the video decoding method according to the embodiments of  FIGS. 1A through 20  may be applied are provided with reference to  FIGS. 21 through 27 . Hereinafter, an ‘image’ may correspond to a still image of a video or a moving picture, i.e., the video itself. 
     First, with reference to  FIGS. 1A through 7 , the scalable video encoding apparatus and the scalable video encoding method, and the scalable video decoding apparatus and the scalable video decoding method according to various embodiments are provided. 
       FIG. 1A  is a block diagram illustrating a structure of a scalable video encoding apparatus, according to various embodiments. 
     Referring to  FIG. 1A , a scalable video encoding apparatus  10  according to various embodiments includes a scalability type determiner  11  and a bitstream generator  13 . 
     The scalable video encoding apparatus  10  according to various embodiments may divide a plurality of video streams according to layers and may encode each of them by using a scalable video coding technique. The video stream encoding apparatus  10  may encode base layer images and enhancement layer images to different layers. 
     For example, a multiview video may be encoded according to the scalable video coding technique. Left-view images may be encoded as the base layer images, and right-view images may be encoded as the enhancement layer images. Alternatively, each of center-view images, left-view images, and right-view images may be encoded, and among these images, the center-view images may be encoded as the base layer images, the left-view images may be encoded as first enhancement layer images, and the right-view images may be encoded as second enhancement layer images. A result of encoding the base layer images may be output as a base layer stream, and results of encoding the first enhancement layer images and the second enhancement layer images may be respectively output as a first enhancement layer stream and a second enhancement layer stream. 
     When the number of enhancement layers is at least three, the base layer images, the first enhancement layer images, the second enhancement layer images, . . . , K th  enhancement layer images may be encoded. Accordingly, the result of encoding the base layer images may be output as the base layer stream, and results of encoding the first enhancement layer images, the second enhancement layer images, . . . , and the K th  enhancement layer images may be respectively output as the first enhancement layer stream, the second enhancement layer stream, . . . , and a Kth enhancement layer stream. 
     The scalable video encoding apparatus  10  according to various embodiments may perform inter prediction by which a current image is predicted by referring to images of a same layer. Due to the inter prediction, a motion vector indicating motion information between the current image and a reference image, and a residual component between the current image and the reference image may be generated. 
     Also, the scalable video encoding apparatus  10  according to various embodiments may perform inter-layer prediction by which the enhancement layer images are predicted by referring to the base layer images. The scalable video encoding apparatus  10  may perform inter-layer prediction by which the second enhancement layer images are predicted by referring to the first enhancement layer images. Due to the inter-layer prediction, a position difference component (or a motion vector) between the current image and a reference image of another layer, and a residual component between the current image and the reference image of the other layer may be generated. 
     In a case where the scalable video encoding apparatus  10  according to the embodiment allows at least two enhancement layers, the inter-layer prediction may be performed between base layer images and at least two enhancement layers according to a multilayer prediction structure. 
     An inter-layer prediction structure will be described in detail with reference to  FIG. 3 . 
     The scalable video encoding apparatus  10  according to various embodiments encodes each of blocks of each of images of a video according to layers. A type of a block may be a square, a rectangle, or a random geometric shape. The block is not limited to a data unit of a constant size. The block according to the embodiment may be a largest coding unit, a coding unit, a prediction unit, a transformation unit, etc. from among coding units of a tree structure. For example, the scalable video encoding apparatus  10  may split, in each of layers, images based on the High Efficiency Video Coding (HEVC) standard into blocks in a quadtree structure, and may encode them. Video encoding and decoding methods using the coding units of the tree structure will be described with reference to  FIGS. 8 through 20 . The inter prediction and the inter-layer prediction may be performed by using a data unit in the form of the coding unit, the prediction unit, or the transformation unit. 
     The scalable video encoding apparatus  10  according to various embodiments may encode an image sequence according to each of layers. The scalable video encoding apparatus  10  may perform source coding operations including inter prediction or intra prediction on each of layers, and thus may generate symbol data. For example, the scalable video encoding apparatus  10  may generate the symbol data by performing transformation and quantization on an image block including result data obtained by performing the inter prediction or the intra prediction on image samples, and may perform entropy encoding on the symbol data. The bitstream generator  13  may generate a bitstream including the symbol data on which the entropy encoding has been performed. 
     The scalable video encoding apparatus  10  may encode an image sequence according to each of layers, and the bitstream generator  13  may generate each bitstream. As described above, the scalable video encoding apparatus  10  may encode an image sequence of a current layer by referring to symbol data of another layer due to the inter-layer prediction. Therefore, the scalable video encoding apparatus  10  according to various embodiments may encode an image sequence of each layer by referring to an image sequence of another layer or referring to an image sequence of a same layer. For example, during an intra mode, a current sample may be predicted by using neighboring samples in a current image, and during an inter mode, the current image may be predicted by using another image of same layer. During an inter-layer prediction mode, the current image may be predicted by using a reference image from among images of another layer, which has a same Picture Order Count (POC) as the current image. 
     The scalable video encoding apparatus  10  may encode a multiview video, and may encode an image sequence of a different view in each of layers. In an inter-layer prediction structure for the multiview video, a current view image is encoded by referring to a different-view image, thus, the structure may be referred to as an inter-view prediction structure. 
     The scalable video encoding apparatus  10  receives an input of image data including a multilayer video, encodes the image data, and generates a multilayer encoded image. The scalable video encoding apparatus  10  corresponds to a video coding layer that directly involves a process of encoding an input video. As described layer with reference to  FIGS. 8 through 20 , the scalable video encoding apparatus  10  may encode each of pictures included in the multilayer video, based on coding units of a tree structure. 
     The bitstream generator  13  corresponds to a Network Abstraction Layer (NAL) that adds the multilayer encoded image and auxiliary data which are generated by the scalable video encoding apparatus  10  to a transmission data unit according to a predetermined format, and outputs the transmission data unit. The transmission data unit may be an NAL unit. The bitstream generator  13  allows the NAL unit to include the multilayer encoded image and the auxiliary data, and outputs the NAL unit. The bitstream generator  13  may output a bitstream generated by using the NAL unit. 
     The scalable video encoding apparatus  10  according to various embodiments may encode the image data to the multilayer encoded image. The scalable video encoding apparatus  10  may generate the auxiliary data including an index indicating an output layer set, and may generate the bitstream including the generated index and the multilayer encoded image. Here, the output layer set represents a group including one or more layers that are transmitted from the scalable video encoding apparatus  10 , are received by a scalable video decoding apparatus  20 , and are decoded in and are output from the scalable video decoding apparatus  20 . Each of the layers is decoded by performing the intra prediction, the inter prediction, and the inter-layer prediction, and images of a decoded layer are reconstructed, however, not every decoded layer is output and displayed, but only some layers may be output and displayed. 
     For example, spatial scalable bitstreams include encoded layers having different resolutions. A low-resolution image may be encoded as a base layer, and inter-layer prediction may be performed on a high-resolution image by using the low-resolution image, so that the high-resolution image may be encoded as an enhancement layer. When the low-resolution image of the base layer is decoded and the high-resolution image of the enhancement layer is decoded, only the high-resolution image may be output. 
     However, the decoded low-resolution image of the base layer is not required to be output. That is, since the decoded high-resolution image includes overlapping information having a different resolution from that of the low-resolution image, the low-resolution image is not required to be output. 
     As another example, multiview scalable bitstream may include encoded layers having different views. For example, a base layer including a left-view image is encoded, and the inter-layer prediction is performed on a right-view image by using the left-view image, so that an enhancement layer including the right-view image may be encoded. When the left-view image of the base layer is decoded, and the right-view image of the enhancement layer is decoded, not only the right-view image of the enhancement layer is output but also the left-view image of the base layer has to be decoded and output. 
     The scalable video encoding apparatus  10  may encode the image data to the multilayer encoded image, and then may determine an output target layer from among one or more layers included in a layer set. Here, the multilayer video includes a plurality of layers, and the layer set denotes a group including one or more layers from among encoded layers. The scalable video encoding apparatus  10  may generate a bitstream including encoded data with respect to the layers in the layer set. 
     An auxiliary picture is a picture to be used when another picture corresponding to a primary picture is reconstructed. That is, by using the primary picture and the auxiliary picture, a primary picture of another layer which corresponds to the primary picture may be reconstructed. Representatively, the auxiliary picture of the primary picture may include an alpha plane and a depth picture. As another example, when a primary picture is a Y picture in an YCrCb format, a Cr picture and a Cb picture may be auxiliary pictures. Since the auxiliary picture does not need to be rendered, a layer that is not a layer of the auxiliary picture may be determined as an output layer. 
     The scalable video encoding apparatus  10  according to the embodiment may encode the primary picture as a first layer, and may encode the auxiliary picture of the primary picture as a second layer. 
     The scalable video encoding apparatus  10  according to the embodiment may perform scalable video coding according to one or more scalability types. A scalability type may be determined for each layer. 
     The scalability type determiner  11  may generate scalability type information representing a scalable video coding technique performed on a current layer. For example, a scalability type of a layer may include spatial scalability, quality scalability, multiview scalability, an auxiliary picture, etc. 
     The scalability type determiner  11  may determine scalability mask information specifying whether scalable video coding is performed according to each of scalability types of a current video. 
     For example, the scalability mask information may be determined according to each of the scalability types. The scalability mask information may be determined so as to represent a scalable video coding technique performed on each layer. 
     For example, when scalable video coding according to a spatial scalability type or a quality scalability type is performed on a first layer, scalability mask information may be generated to specify that a type of the scalable video coding for the first layer is the spatial scalability type or the quality scalability type. 
     For example, when scalable video coding according to a multiview scalability type is performed on a second layer, scalability mask information may be generated to specify that a type of the scalable video coding for the second layer is the multiview scalability type. 
     For example, when scalable video coding is performed on a third layer so as to encode an auxiliary picture as a separate layer, scalability mask information may be generated to specify that a type of the scalable video coding for the third layer is a scalability type by which the auxiliary picture is encoded as the separate layer. 
     When a scalability type for an auxiliary picture is performed in a current layer, the scalability type determiner  11  may determine a scalability index indicating an auxiliary picture type including an alpha plane and a depth picture of a primary picture of another layer. 
     For example, when the scalable video encoding apparatus  10  encodes the alpha plane as the auxiliary picture in the current layer, scalability mask information denoting that the auxiliary picture corresponds to a scalability type which is encoded as a separate layer, and an image scalability index indicating the alpha plane may be generated. 
     For example, when the scalable video encoding apparatus  10  encodes the depth picture as the auxiliary picture in the current layer, the scalability mask information denoting that the auxiliary picture corresponds to the scalability type which is encoded as the separate layer, and an image scalability index indicating the depth picture may be generated. 
     For example, when the scalable video encoding apparatus  10  encodes a Cr picture or a Cb picture as the auxiliary picture in the current layer, the scalability mask information denoting that the auxiliary picture corresponds to the scalability type which is encoded as the separate layer, and an image scalability index indicating the Cr picture or the Cb picture may be generated. 
     The bitstream generator  13  may generate a VPS NAL unit including a generated index, and may generate a bitstream including the VPS NAL unit. The bitstream generator  13  may generate the bitstream including the index and the multilayer encoded image which are generated in the scalable video encoding apparatus  10   
     In addition, the bitstream generator  13  may generate a bitstream including data about the auxiliary picture encoded in the current layer, the scalability mask information, and the scalability index. 
     For example, the scalability mask information may be included in a VPS extension NAL unit including additional information of the bitstream which is about a current video. For example, a scalability index that is set for each layer may be included in the VPS extension NAL. Thus, the bitstream generator  13  may generate the bitstream including the data about the auxiliary picture encoded in the current layer, scalability mask information of the current video, and the scalability index for each layer. 
     The scalable video encoding apparatus  10  may not include encoded data of all layers in the bitstream but may include only output target layers. 
     For example, when a mobile terminal decodes a multilayer video that was encoded according to a spatial scalable video coding method, even if the mobile terminal reconstructs and displays an image by decoding a low-resolution layer from among a high-resolution image and a low-resolution image, a viewer may watch the image without feeling inconvenience. A multilayer encoded image may include multiple layers including a low-resolution layer, a medium-resolution layer, and a high-resolution layer, and when it is reproduced by the mobile terminal, the mobile terminal may reconstruct the image by decoding only the low-resolution layer and the medium-resolution layer. 
     Therefore, there may be not only a layer set including all layers but may also be a layer set including the low-resolution layer and the medium-resolution layer. 
     The scalable video encoding apparatus  10  may determine a plurality of output layer sets with respect to the multilayer encoded image. The scalable video encoding apparatus  10  may generate additional information about the number of layer sets, based on the plurality of determined output layer sets. Detailed descriptions about the output layer sets are provided at a later time with reference to  FIGS. 4B and 4C . 
     The scalable video encoding apparatus  10  may determine a layer to be decoded from each of the output layer sets. That is, the scalable video encoding apparatus  10  may determine the layer to be decoded and output from each of the output layer sets. 
     A plurality of output layer sets, each including at least one layer decoded in the scalable video encoding apparatus  10 , may be determined. The output layer sets may include different combinations of output layers. Each of the output layer sets may include one or more layers, and output layer subsets including layers that are from among the layers included in the output layer set and are to be decoded and output from the scalable video decoding apparatus  20  may be variously determined. Each of the output layer subsets may include at least one output layer. The scalable video encoding apparatus  10  may generate an index to indicate one output layer subset from among at least three output layer subsets determined from among layers included in an output layer set. 
     The scalable video encoding apparatus  10  may determine, from among output layer sets, a target output layer set that indicates a layer set to be encoded and transmitted. The scalable video decoding apparatus  20  may determine an output layer set from among the target output layer set. 
     As the output layer set, a group of default output layer sets that each include at least one layer may be set. 
     According to an embodiment, from among the default output layer set including at least one layer, only a layer that has an identifier of a highest layer including a primary picture associated with an auxiliary picture may be determined as an output target layer. In this case, output target layer information indicating one layer may be generated. 
     According to another embodiment, from among a default output layer set including a plurality of layers, all layers including a primary picture associated with an auxiliary picture may be determined as a target output layer. In this case, target output layer information representing the plurality of layers may be generated. 
     The bitstream generator  13  may generate a bitstream including the target output layer information. According to an embodiment, the target output layer information may be included in the VPS extension NAL unit. 
       FIG. 1B  illustrates a flowchart of a method of encoding a scalable video, according to various embodiments. 
     In operation  12 , the scalable video encoding apparatus  10  may encode an auxiliary picture of a current layer which corresponds to a primary picture of another layer. 
     In operation  14 , the scalability type determiner  11  of the scalable video encoding apparatus  10  may determine scalability mask information denoting whether scalable video coding has been performed according to scalability types of a current video. 
     The method includes determining, when the scalability type for an auxiliary picture is performed in a current layer, a scalability index indicating an auxiliary picture type including an alpha plane and a depth picture of the primary picture of the other layer, and generating a bitstream including the scalability mask information for the current video and the scalability index for the current layer. In more detail, the scalable video encoding apparatus  10  may generate a VPS NAL unit including the generated index. The scalable video encoding apparatus  10  may generate a bitstream including the VPS NAL unit. 
       FIG. 2A  is a block diagram illustrating a structure of a scalable video decoding apparatus, according to various embodiments. 
     Referring to  FIG. 2A , the scalable video decoding apparatus  20  may include a bitstream obtainer  21  and an auxiliary picture decoder  23 . 
     The scalable video decoding apparatus  20  may receive a base layer stream and an enhancement layer stream. According to a scalable video coding technique, the scalable video decoding apparatus  20  may receive, as the base layer stream, a base layer stream including encoded data of base layer images, and may receive, as the enhancement layer stream, an enhancement layer stream including encoded data of enhancement layer images. 
     The scalable video decoding apparatus  20  may decode a plurality of layer streams according to a scalable video coding technique. The scalable video decoding apparatus  20  may reconstruct base layer images by reconstructing the base layer stream, and may reconstruct enhancement layer images by reconstructing the enhancement layer stream. 
     For example, a multiview video may be encoded according to the scalable video coding technique. For example, left-view images may be reconstructed by decoding the base layer stream, and right-view images may be reconstructed by decoding the enhancement layer stream. As another example, center-view images may be reconstructed by decoding the base layer stream. Left-view images may be reconstructed by further decoding a first enhancement layer stream in addition to the base layer stream. Right-view images may be reconstructed by further decoding a second enhancement layer stream in addition to the base layer stream. 
     In a case where there are at least three enhancement layers, first enhancement layer images with respect to a first enhancement layer may be reconstructed from a first enhancement layer stream, and second enhancement layer images may be further reconstructed by further reconstructing a second enhancement layer stream. K th  enhancement layer images may be further reconstructed by further decoding a K th  enhancement layer stream in addition to the first enhancement layer stream. 
     The scalable video decoding apparatus  20  may obtain encoded data of the base layer images and the enhancement layer images from the base layer stream and the enhancement layer stream, and may further obtain a motion vector generated by inter prediction, and disparity information generated by inter-layer prediction. 
     For example, the scalable video decoding apparatus  20  may decode inter-predicted data of each of layers, and may decode data that is inter-layer predicted between a plurality of layers. The reconstruction may be performed by motion compensation and inter-layer decoding, based on a coding unit or a prediction unit according to an embodiment. 
     Images of each layer stream may be reconstructed by performing motion compensation for a current image by referring to reconstructed images that are predicted via inter prediction using a same layer. The motion compensation means an operation of reconstructing a reconstructed image of the current image by synthesizing a reference image and a residual component of the current image, wherein the reference image is determined by using a motion vector of the current image. 
     Also, the scalable video decoding apparatus  20  according to the embodiment may perform the inter-layer decoding by referring to the base layer images, so as to reconstruct the enhancement layer image predicted via the inter-layer prediction. The inter-layer decoding means an operation of reconstructing a reconstructed image of the current image by synthesizing a reference image of another layer and the residual component of the current image, wherein the reference image is determined by using disparity information of the current image. 
     The scalable video decoding apparatus  20  according to the embodiment may perform inter-layer decoding so as to reconstruct the second enhancement layer images that are predicted by referring to the first enhancement layer images. 
     The scalable video decoding apparatus  20  decodes each of blocks of each of images of a video. The block according to the embodiment may be a largest coding unit, a coding unit, a prediction unit, a transformation unit, etc. from among coding units of a tree structure. For example, the scalable video decoding apparatus  20  may decode each layer stream, based on blocks in a quadtree structure determined according to the HEVC standard, and may reconstruct image sequences. 
     The scalable video decoding apparatus  20  may obtain symbol data reconstructed by performing entropy decoding on each layer. The scalable video decoding apparatus  20  may perform inverse-quantization and inverse-transformation by using the symbol data and thus may reconstruct quantized transform coefficients of the residual component. The scalable video decoding apparatus  20  according to another embodiment may receive a bitstream of the quantized transform coefficients. As a result of the inverse-quantization and the inverse-transformation that are performed on the quantized transform coefficients, the residual component of images may be reconstructed. 
     The scalable video decoding apparatus  20  according to various embodiments may reconstruct a received bitstream according to each of layers and may reconstruct an image sequence according to each of the layers. 
     The scalable video decoding apparatus  20  may generate reconstructed images of the image sequence according to each of the layers by performing motion compensation between images of a same layer or by performing inter-layer prediction between images of different layers. 
     Therefore, the scalable video decoding apparatus  20  according to various embodiments may decode an image sequence of each layer by referring to an image sequence of a same layer or an image sequence of different layer, according to a prediction mode. For example, during an intra prediction mode, a current block may be reconstructed by using neighboring samples in a same image, and during an inter prediction mode, the current block may be reconstructed by referring to another image of the same layer. During an inter-layer prediction mode, the current block may be reconstructed by using a reference image that has a same POC as the current image and is from among images of another layer. 
     The bitstream obtainer  21  may obtain a video stream of an image encoded according to a scalable video coding technique. 
     The scalable video decoding apparatus  20  may obtain, from a bitstream, an index indicating one of output layer sets included in a target output layer set. The output layer sets may each include at least one output layer. 
     The scalable video decoding apparatus  20  may determine, based on the obtained index, at least one layer included in a layer set, as an output layer to be decoded, and may decode an image included in the output layer. 
     The scalable video decoding apparatus  20  may obtain a VPS NAL unit including the index from the bitstream, and may obtain the index by using the obtained VPS NAL unit. 
     The scalable video decoding apparatus  20  may determine, based on the obtained index, layers that are from among layers included in the target output layer set and exclude a layer with respect to auxiliary picture data, as output layers. 
     The scalability type determiner  11  according to the embodiment may obtain, from the bitstream, scalability mask information denoting whether scalable video decoding has been performed according to scalability types in a current video. When scalability mask information for scalable video decoding an auxiliary picture indicates ‘performance’ in the scalability mask information, the scalability type determiner  11  may obtain a scalable index indicating an auxiliary picture type to be decoded in a current layer. 
     The bitstream obtainer  21  may obtain scalability type information indicating a scalable video coding technique performed on the current layer. For example, a scalability type of a layer may include spatial scalability, quality scalability, multiview scalability, an auxiliary picture, etc. 
     The scalability type determiner  11  may obtain scalability mask information denoting whether scalable video coding has been performed, according to scalability types of the current video. 
     For example, the scalability mask information may be obtained according to each of the scalability types. The scalability mask information obtained from each layer may indicate a scalable video decoding technique to be performed in each layer. 
     For example, scalability mask information obtained from a first layer indicates scalable video coding according to a spatial scalability type or a quality scalability type, the scalable video decoding apparatus  20  may perform scalable video decoding on the first layer according to the spatial scalability type or the quality scalability type. For example, a high-resolution image depending on a low resolution of a base layer may be decoded in the first layer according to the spatial scalability type. As another example, a high quality image of an additional layer which depends on a low quality image of a base layer may be decoded in the first layer according to the quality scalability type. 
     For example, when scalability mask information obtained from a second layer indicates scalable video coding according to a multiview scalability type, the scalable video decoding apparatus  20  may perform scalable video decoding on the second layer according to the multiview scalability type. Therefore, an additional view image corresponding to a base view image of the base layer may be decoded in the second layer. 
     For example, when scalability mask information indicates, in a third layer, scalable video coding for encoding an auxiliary picture as a separate layer, the scalable video decoding apparatus  20  may perform scalable video decoding on the third layer according to a scalability type by which the auxiliary picture is encoded as the separate layer. For example, an auxiliary view picture corresponding to a primary picture of the base layer may be decoded in the third layer. 
     When scalability mask information denotes that a scalability type for an auxiliary picture is performed in the current layer, the bitstream obtainer  21  may obtain a scalability index indicating a type of the auxiliary picture from a bitstream. The bitstream obtainer  21  may determine, by using the scalability index, that the auxiliary picture is one of an alpha plane and a depth picture of a primary picture of another layer. 
     For example, when the scalability index obtained by the bitstream obtainer  21  indicates the alpha plane, the auxiliary picture decoder  23  may reconstruct the alpha plane corresponding to the primary picture, by using symbols obtained from the bitstream. 
     For example, when the scalability index obtained by the bitstream obtainer  21  indicates the depth picture, the auxiliary picture decoder  23  may reconstruct the depth picture corresponding to the primary picture, by using symbols obtained from the bitstream. 
     For example, when the scalability index obtained by the bitstream obtainer  21  indicates a Cr picture or a Cb picture, the auxiliary picture decoder  23  may reconstruct the Cr picture or the Cb picture which corresponds to a Y picture, by using symbols obtained from the bitstream. 
     For example, the bitstream obtainer  21  may obtain additional information about the current video from a VPS extension NAL unit in the bitstream. For example, a scalability index set in each layer may be obtained from the VPS extension NAL unit. Therefore, the bitstream obtainer  21  may obtain, from the bitstream, data of the auxiliary picture encoded in the current layer, the scalability mask information about the current video, and the scalability index according to each of the layers. 
     The auxiliary picture decoder  23  may decode the auxiliary picture of the current layer which is determined, by using the scalability index, as one of auxiliary pictures each including an alpha plane and a depth image of a primary picture of another layer. 
     An auxiliary picture is a picture to be used when another picture corresponding to a primary picture is reconstructed. That is, by using the primary picture and the auxiliary picture, a primary picture of another layer which corresponds to the primary picture may be reconstructed. Representatively, the auxiliary picture of the primary picture may include an alpha plane and a depth picture. As another example, when a primary picture is a Y picture in an YCrCb format, a Cr picture and a Cb picture may also be auxiliary pictures. Since the auxiliary picture does not need to be rendered, a layer that is not a layer of the auxiliary picture may be determined as an output layer. 
     The scalable video decoding apparatus  20  according to the embodiment may decode the primary picture as a first layer, and may decode the auxiliary picture of the primary picture as a second layer. 
     The scalable video decoding apparatus  20  according to the embodiment may perform scalable video decoding according to at least one scalability type. A scalability type may be determined according to each of the layers. 
     The scalable video encoding apparatus  10  may not include encoded data of all layers in the bitstream but may include only output target layers. 
     For example, when a mobile terminal decodes a multilayer video that was encoded according to a spatial scalable video coding method, even if the mobile terminal reconstructs and displays an image by decoding a low-resolution layer from among a high-resolution image and a low-resolution image, a viewer may watch the image without feeling inconvenience. A multilayer encoded image may include multiple layers including a low-resolution layer, a medium-resolution layer, and a high-resolution layer, and when it is reproduced by the mobile terminal, the mobile terminal may reconstruct the image by decoding only the low-resolution layer and the medium-resolution layer. 
     Therefore, there may be not only a layer set including all layers but may also be a layer set including the low-resolution layer and the medium-resolution layer. 
     The scalable video encoding apparatus  10  may determine a plurality of output layer sets with respect to the multilayer encoded image. The scalable video encoding apparatus  10  may generate additional information about the number of layer sets, based on the plurality of determined output layer sets. Detailed descriptions about the output layer sets are provided at a later time with reference to  FIGS. 4B and 4C . 
     The scalable video encoding apparatus  10  may determine a layer to be decoded from each of the output layer sets. That is, the scalable video encoding apparatus  10  may determine the layer to be decoded and output from each of the output layer sets. 
     A plurality of output layer sets, each including at least one layer decoded in the scalable video encoding apparatus  10 , may be determined. The output layer sets may include different combinations of output layers. Each of the output layer sets may include one or more layers, and output layer subsets including layers that are from among the layers included in the output layer set and are to be decoded and output from the scalable video decoding apparatus  20  may be variously determined. Each of the output layer subsets may include at least one output layer. The scalable video encoding apparatus  10  may generate an index to indicate one output layer subset from among at least three output layer subsets determined from among layers included in an output layer set. 
     The scalable video encoding apparatus  10  may determine, from among output layer sets, an output target layer set that indicates a layer set to be encoded and transmitted. The scalable video decoding apparatus  20  may determine an output layer set from among the output target layer set. 
     As the output layer set, a group of default output layer sets that each include at least one layer may be set. 
     According to an embodiment, from among the default output layer set including at least one layer, only a layer that has an identifier of a highest layer including a primary picture associated with an auxiliary picture may be determined as an output target layer. In this case, output target layer information indicating one layer may be generated. 
     According to another embodiment, from among a default output layer set including a plurality of layers, all layers including a primary picture associated with an auxiliary picture may be determined as output target layers. In this case, output target layer information indicating the plurality of layers may be generated. 
     The bitstream generator  13  may generate a bitstream including the output target layer information. According to an embodiment, the output target layer information may be included in the VPS extension NAL unit. 
       FIG. 2B  illustrates a flowchart of a method of decoding a scalable video, according to various embodiments. 
     In operation  22 , the scalable video decoding apparatus  20  may obtain, from a bitstream, scalability mask information denoting whether scalable video decoding is performed on a current video according to scalability types. 
     In operation  24 , when scalability mask information for scalable video decoding an auxiliary picture indicates ‘performance’ in the scalability mask information, the scalable video decoding apparatus  20  may obtain a scalable index indicating an auxiliary picture type to be decoded in a current layer. 
     In operation  26 , the scalable video decoding apparatus  20  may decode the auxiliary picture of the current layer which is determined, by using the scalability index, as one of auxiliary picture types including an alpha plane and a depth image of a primary picture of another layer. 
     According to the embodiment, a layer that includes only auxiliary pictures from among a layer set including one or more layers may not be an output target layer. 
     According to the embodiment, there is always a primary picture associated with an auxiliary picture, but the primary picture may be related to one or more auxiliary pictures having different auxiliary picture types. 
     According to the embodiment, the auxiliary picture types may include an alpha plane, a depth picture, Cr chroma data, and Cb chroma data. 
     According to the embodiment, a NAL unit type of the auxiliary picture may be equal to a NAL unit type of the associated primary picture. A temporal layer identifier of the NAL unit type of the auxiliary picture may be equal to a temporal layer identifier of the NAL unit type of the primary picture. 
     According to the embodiment, a standard profile for decoding the auxiliary picture may not be limited to a profile preceding a predetermined profile. 
     According to the embodiment, prediction decoding may not be performed between layers having different auxiliary picture type identifiers. The prediction decoding may be performed between layers having a same auxiliary picture type identifier, and may include inter-view prediction with respect to a plurality of depth view images. 
     According to the embodiment, an auxiliary picture and a primary picture associated with the auxiliary picture may have a same scalability identifier, but the auxiliary picture and the primary picture associated with the auxiliary picture may have different auxiliary picture type identifiers. 
     According to the embodiment, a chroma enhancement auxiliary picture from among the auxiliary pictures may include a monochrome picture. Here, a Cr layer and a Cb layer of the chroma enhancement auxiliary picture may be paired. 
     The scalable video decoding apparatus  20  according to the embodiment may obtain output target layer information from a bitstream. 
     When the output target layer information indicates one layer, from among a default output layer set including at least one layer, only a layer that has an identifier of a highest layer including the primary picture associated with the auxiliary picture may be determined as an output target layer. 
     When the output target layer information indicates a plurality of layers, from among a default output layer set including a plurality of layers, all layers including the primary picture associated with the auxiliary picture may be determined as output target layers. 
       FIG. 3A  illustrates a multilayer video. 
     In order to provide an optimal service in various network environments and various terminals, the scalable video encoding apparatus  10  may output a scalable bitstream by encoding multilayer image sequences having various spatial resolutions, various qualities, various frame-rates, and different views. That is, the scalable video encoding apparatus  10  may generate a video bitstream by encoding an input image according to various scalability types and may output the video bitstream. Scalability includes temporal scalability, spatial scalability, quality scalability, multiview scalability, and combinations thereof. The scalabilities may be classified according to types. Also, the scalabilities may be identified as dimension identifiers in the types. 
     For example, scalability has scalability types including temporal scalability, spatial scalability, quality scalability, multiview scalability, or the like. According to the types, the scalabilities may be identified as dimension identifiers. For example, when they have different scalabilities, they may have different dimension identifiers. For example, when a scalability type corresponds to high-dimensional scalability, a higher scalability dimension may be assigned thereto. 
     When a bitstream is dividable into valid substreams, the bitstream is scalable. A spatially scalable bitstream includes substreams having various resolutions. In order to distinguish between different scalabilities in a same scalability type, a scalability dimension is used. The scalability dimension may be referred to as a scalability dimension identifier. 
     For example, the spatially-scalable bitstream may be divided into substreams having different resolutions such as a quarter video graphics array (QVGA), a video graphics array (VGA), a wide video graphics array (WVGA), or the like. For example, layers respectively having different resolutions may be distinguished therebetween by using dimension identifiers. For example, a QVGA substream may have 0 as a value of a spatial scalability dimension identifier, a VGA substream may have 1 as a value of the spatial scalability dimension identifier, and a WVGA sub stream may have 2 as a value of the spatial scalability dimension identifier. 
     A temporally-scalable bitstream includes substreams having various frame-rates. For example, the temporally-scalable bitstream may be divided into substreams that respectively have a frame-rate of 7.5 Hz, a frame-rate of 15 Hz, a frame-rate of 30 Hz, and a frame-rate of 60 Hz. A quality-scalable bitstream may be divided into substreams having different qualities according to a Coarse-Grained Scalability (CGS) technique, a Medium-Grained Scalability (MGS) technique, and a Fine-Grained Scalability (FGS) technique. The temporally-scalable bitstream may also be divided into different dimensions according to different frame-rates, and the quality-scalable bitstream may also be divided into different dimensions according to the different techniques. 
     A multiview scalable bitstream includes substreams having different views in one bitstream. For example, a bitstream of a stereoscopic video includes a left-view image and a right-view image. Also, a scalable bitstream may include substreams with respect to encoded data of a multiview image and a depth map. View-scalability may be divided into different dimensions according to views. 
     Different scalable extension types may be combined with each other. That is, a scalable video bitstream may include substreams obtained by encoding image sequences of multiple layers including images where one or more of temporal, spatial, quality, and multiview scalabilities are different therebetween. 
     Hereinafter, with reference to  FIG. 3B , a multilayer prediction system, as a scalable video encoding system according to various embodiments, which may be performed in a base layer and an enhancement layer is described in detail. 
       FIG. 3B  illustrates a multilayer video encoding apparatus  1600 , according to an embodiment. 
     A multilayer encoding system  1600  includes a base layer encoding terminal  1610 , an enhancement layer encoding terminal  1660 , and an inter-layer prediction terminal  1650  between the base layer encoding terminal  1610  and the enhancement layer encoding terminal  1660 . 
     The base layer encoding terminal  1610  receives an input of a base layer image sequence and encodes each image. The enhancement layer encoding terminal  1660  receives an input of an enhancement layer image sequence and encodes each image. Operations that overlap in operations of the base layer encoding terminal  1610  and operations of the enhancement layer encoding terminal  1660  are simultaneously described below. 
     A block splitter  1618  or  1668  splits an input image (a low-resolution image or a high-resolution image) to a largest coding unit, a coding unit, a prediction unit, a transformation unit, etc. In order to encode the coding unit that is output from the block splitter  1618  or  1668 , intra prediction or inter prediction may be performed with respect to each prediction unit of the coding unit. A prediction switch  1648  or  1698  may perform the inter prediction by referring to a reconstructed previous image output from a motion compensator  1640  or  1690  or may perform the intra prediction by using a neighbouring prediction unit of a current prediction unit in a current input image output from an intra predictor  1645  or  1695 , based on whether a prediction mode of the prediction unit is an intra prediction mode or an inter prediction mode. Residue information may be generated with respect to each prediction unit due to the inter prediction. 
     A residue component between the prediction unit and a neighbouring image is input to a transformer/quantizer  1620  or  1670 , according to each prediction unit of the coding unit. The transformer/quantizer  1620  or  1670  may perform transformation and quantization with respect to each transformation unit, based on the transformation unit of the coding unit, and may output a quantized transformation coefficient. 
     A scaling/inverse transformer  1625  or  1675  may perform scaling and inverse-transformation on the quantized transformation coefficient, according to each transformation unit of the coding unit, and may generate a residue component of a spatial domain. When it is controlled to an inter mode due to the prediction switch  1648  or  1698 , the residue component may be synthesized with the reconstructed previous image or the neighbouring prediction unit, so that a reconstructed image including the current prediction unit may be generated and a reconstructed current image may be stored in a storage  1630  or  1680 . The reconstructed current image may be transferred to the intra predictor  1645  or  1695 /the motion compensator  1640  or  1690 , according to a prediction mode of a prediction unit to be next encoded. 
     In particular, during the inter mode, an in-loop filter  1635  or  1685  may perform at least one of deblocking filtering and Sample Adaptive Offset (SAO) filtering on the reconstructed image stored in the storage  1630  or  1680 , according to each coding unit. At least one of the deblocking filtering and the SAO filtering to compensate for an encoding error between an original image and a reconstructed image may be performed on the coding unit. At least one of the deblocking filtering and the SAO filtering may be performed on the coding unit and at least one of a prediction unit and a transformation unit included in the coding unit. 
     The deblocking filtering is filtering for smoothing a blocking phenomenon of a data unit, and the SAO filtering is filtering for compensating for a pixel value that has been corrupted while data is encoded and decoded. Data that is filtered by the in-loop filter  1635  or  1685  may be transferred to the motion compensator  1640  or  1690 , according to each prediction unit. In order to encode a next coding unit output from the block splitter  1618  or  1668 , a residue component between the reconstructed current image and the next coding unit may be generated, wherein the reconstructed current image is output from the motion compensator  1640  or  1690  and the next coding unit is output from the block splitter  1618  or  1668 . 
     In this manner, the aforementioned encoding procedure may be repeated with respect to each coding unit of the input image. 
     Also, for inter-layer prediction, the enhancement layer encoding terminal  1660  may refer to the reconstructed image stored in the storage  1630  of the base layer encoding terminal  1610 . An encoding controller  1615  of the base layer encoding terminal  1610  may control the storage  1630  of the base layer encoding terminal  1610 , and may transfer the reconstructed image of the base layer encoding terminal  1610  to the enhancement layer encoding terminal  1660 . The transferred reconstructed base layer image may be used as a prediction image of the enhancement layer. 
     When the base layer and the enhancement layer have different resolutions, an in-loop filter  1655  of the inter-layer prediction terminal  1650  may upsample the reconstructed base layer image and may transfer an upsampled reconstructed base layer image to the enhancement layer encoding terminal  1660 . Therefore, the upsampled reconstructed base layer image may be used as the prediction image of the enhancement layer. 
     When the inter-layer prediction is performed in a manner that an encoding controller  1665  of the enhancement layer encoding terminal  1660  controls the switch  1698 , the enhancement layer image may be predicted by referring to the reconstructed base layer image that is transferred via the inter-layer prediction terminal  1650 . 
     In order to encode an image, various encoding modes for a coding unit, a prediction unit, and a transformation unit may be set. For example, as an encoding mode for the coding unit, a depth, split information (e.g., a split flag), or the like may be set. As an encoding mode for the prediction unit, a prediction mode, a partition type, intra direction information, reference list information, or the like may be set. As an encoding mode for the prediction unit, a transformation depth, split information or the like may be set. 
     The base layer encoding terminal  1610  may perform encoding by using each of various depths for the coding unit, each of various modes for the prediction unit, each of various partition types, each of various intra directions, each of various reference lists, and each of various transformation depths for the transformation unit, and according to results of the performances, the base layer encoding terminal  1610  may determine an encoding depth, a prediction mode, a partition type, intra direction/reference list, a transformation depth, etc. that have the highest encoding efficiency. However, an encoding mode determined by the base layer encoding terminal  1610  is not limited to the aforementioned encoding modes. 
     The encoding controller  1615  of the base layer encoding terminal  1610  may control various encoding modes to be appropriately applied to operations of each configuring element. Also, for inter-layer encoding in the enhancement layer encoding terminal  1660 , the encoding controller  1615  may control the enhancement layer encoding terminal  1660  to determine an encoding mode or a residue component by referring to the encoding results from the base layer encoding terminal  1610 . 
     For example, the enhancement layer encoding terminal  1660  may use an encoding mode of the base layer encoding terminal  1610  as an encoding mode for the enhancement layer image, or may determine the encoding mode for the enhancement layer image by referring to an encoding mode of the base layer encoding terminal  1610 . The encoding controller  1615  of the base layer encoding terminal  1610  may use a current encoding mode from the encoding mode of the base layer encoding terminal  1610  so as to determine a current encoding mode of the enhancement layer encoding terminal  1660  by controlling a control signal of the encoding controller  1665  of the enhancement layer encoding terminal  1660 . 
     In particular, the enhancement layer encoding terminal  1660  according to an embodiment may encode an inter-layer prediction error by using an SAO parameter. Thus, a prediction error between a predicted enhancement layer image determined from the reconstructed base layer image and a reconstructed enhancement layer image may be encoded as an offset of the SAO parameter. 
     Similar to the inter-layer encoding system  1600  shown in  FIG. 3B , an inter-layer decoding system based on the inter-layer prediction technique may be embodied. That is, the inter-layer decoding system may receive a base layer bitstream and an enhancement layer bitstream. A base layer decoding terminal of the inter-layer decoding system may reconstruct base layer images by decoding the base layer bitstream and. An enhancement layer decoding terminal of the inter-layer decoding system for a multilayer video may decode the enhancement layer bitstream by using a reconstructed base layer image and parsed encoding information and may reconstruct enhancement layer images. 
       FIG. 4A  illustrates NAL units including encoded data of a multilayer video, according to various embodiments. 
     As described above, the bitstream generator  13  outputs the NAL units including encoded multilayer video data and auxiliary data. 
     A video parameter set (VPS) includes information to be applied to multilayer image sequences  42 ,  43 , and  44  included in the multilayer video. A NAL unit including information about the VPS is referred to as a VPS NAL unit  41 . 
     The VPS NAL unit  41  includes a common syntax element shared by the multilayer image sequences  42 ,  43 , and  44 , information about an operation point to prevent transmission of unnecessary information, required information about an operation point which is required in a session negotiation step such as a profile or a level, or the like. In particular, the VPS NAL unit  41  according to an embodiment includes scalability information related to a scalability identifier to implement scalability in the multilayer video. The scalability information is information for determining scalability to be applied to the multilayer image sequences  42 ,  43 , and  44  included in the multilayer video. 
     The scalability information includes a type of the scalability to be applied to the multilayer image sequences  42 ,  43 , and  44  included in the multilayer video, and a dimension of the scalability. The scalability information in encoding and decoding methods according to a first embodiment of the present invention may be directly obtained from a value of a layer identifier included in an NAL unit header. Layer identifiers may be identifiers for identifying a plurality of layers included in the VPS. The VPS may signal the layer identifiers for the layers via VPS extension. The layer identifier for each layer of the VPS may be signaled in a manner that the layer identifier is included in the VPS NAL unit. For example, a layer identifier of NAL units included in a particular layer of the VPS may be included in the VPS NAL unit. For example, a layer identifier of a NAL unit included in the VPS may be signaled due to VPS extension. Therefore, the encoding and decoding methods according to an embodiment may obtain scalability information about a layer of NAL units included in a corresponding VPS, by using a layer identifier of the NAL units. 
       FIG. 4B  is a diagram for describing a layer set, according to various embodiments. 
     The scalable video encoding apparatus  10  may determine a layer set including one or more layers of a multilayer video. Here, determined layer sets may be plural in number. In the present embodiment, it is assumed that a layer having a minimum layer-identifier value is a base layer. Also, a layer having a great layer-identifier value may be an enhancement layer that is decoded by referring to other layers after the other layers are decoded. 
     It is assumed that an encoded multilayer video  500  according to an embodiment includes four layers. The layers have layer identifiers that are different from each other. Here, the multilayer video  500  may have three layer sets below. 
     A first layer set  510  may include all layers included in the multilayer video  500 . 
     A second layer set  520  may include three layers included in a multilayer video. 
     A layer set group  540  may include at least one layer set. For example, the layer set group  540  may include the first layer set  510  and the second layer set  520 . 
     A third layer set  530  may include two layers included in the multilayer video. 
     An additional layer set group  545  may include at least one additional layer set. For example, the additional layer set group  545  may include the third layer set  530  that is an additional layer set. 
     An output layer set group  550  may include at least one layer set group. For example, the output layer set group  550  may include the layer set group  540  and the additional layer set group  545 . 
     A layer set included in the output layer set group  550  is referred to as an output layer set. For example, the output layer set group  550  includes the first layer set  510 , the second layer set  520 , and the third layer set  530 . 
     The scalable video decoding apparatus  20  may determine one output layer set from among the output layer set group  550 . Such determined output layer set is a target output layer set. The scalable video decoding apparatus  20  may decode layers included in the target output layer set, by using the determined target output layer set as a decoding target layer set. 
     The scalable video decoding apparatus  20  is not limited to determining the three layer sets as shown in  FIG. 5A  but may determine layer sets by combining decodable layers. 
     After a multilayer video encoding apparatus  10  determines the output layer set group, the multilayer video encoding apparatus  10  may generate information indicating the number of the output layer sets included in the output layer set group, and may generate a bitstream including the generated information that indicates the number of the output layer sets. 
     The scalable video decoding apparatus  20  may obtain the generated bitstream, may obtain, from the obtained bitstream, information indicating the number of the output layer sets, and may determine the output layer set group by using the obtained information. 
     The multilayer video encoding apparatus  10  may determine a target output layer set from among the output layer set group, and may generate a bitstream including layers included in the determined target output layer set. 
     The scalable video decoding apparatus  20  may determine the output layer set group, based on the information of the bitstream which indicates the number of the output layer sets. 
     Here, the scalable video decoding apparatus  20  may determine in advance, from among the determined output layer set group, the target output layer set to be decoded. 
     For example, before the scalable video decoding apparatus  20  receives the bitstream, the scalable video decoding apparatus  20  may determine, from among the layer sets  510 ,  520 , and  530 , the first layer set  510  as the target output layer set to be decoded. 
     However, it is not limited thereto, and when the multilayer video encoding apparatus  10  determines the target output layer set from among the output layer set group, the scalable video encoding apparatus  10  generates an index indicating the target output layer set from among the output layer set group. The scalable video decoding apparatus  20  obtains the generated bitstream. The scalable video decoding apparatus  20  may obtain, from the obtained bitstream, the index indicating the target output layer set, may determine, by using the obtained index, the target output layer set from among the output layer set group, and may decode a layer included in the target output layer set. 
     The scalable video decoding apparatus  20  may determine the output layer set group, based on the information about the number of the output layer sets and the index, and when the scalable video decoding apparatus  20  determines the first layer set  510  that is the target output layer set from among the determined output layer set group, the scalable video decoding apparatus  20  may determine whether layers included in the bitstream have all of layers included in the first layer set  510 . When the scalable video decoding apparatus  20  determines that the layers included in the first layer set  510  are all included, the scalable video decoding apparatus  20  may decode the layers included in the first layer set  510  and thus may reconstruct a video. 
     In more detail, the scalable video decoding apparatus  20  may determine the number of layer sets (NumLayerSets), based on a syntax element vps_num_layer_sets_minus1 indicating the number of layer sets obtained from a bitstream −1 and a syntax element num_add_layer_set indicating the number of additional layer sets. The number of output layer sets (NumOutputLayerSets) may be determined based on the number of layer sets (NumLayerSets) and an additional output layer set (num_add_olss) obtained from the bitstream. 
       FIG. 4C  is a diagram for describing an output layer subset, according to an embodiment. 
     Referring to  FIG. 4C , it is assumed that the scalable video decoding apparatus  20  determined the layer set  510  of  FIG. 4B  as a target output layer set. 
     The scalable video decoding apparatus  20  decodes at least one of layers  511  included in the first layer set  510 . However, the scalable video decoding apparatus  20  may not display all decoded layers but may display at least one layer from among the decoded layers. 
     A multilayer decoding apparatus  20  may determine an output target layer from among the layers included in the first layer set  510 . In more detail, the multilayer decoding apparatus  20  may determine an output layer subset including the output target layer from among the layers included in the first layer set  510 . 
     For example, a first output layer subset  560  according to an embodiment may include only a layer  512  that has a maximum layer-identifier value and is from among the layers included in the first layer set  510 . In a case where a multilayer video has a spatial scalability type, a layer having a minimum layer-identifier value is a low-resolution layer, and a highest layer is a high-resolution layer. The high-resolution layer refers to the low-resolution layer. Therefore, when the high-resolution layer is decoded, since the low-resolution layer includes overlapping information, it is not required to display the low-resolution layer. Accordingly, the layer  512  having the maximum layer-identifier value may be included in the first output layer subset  560 . 
     For example, a second output layer subset  570  according to another embodiment may include all layers  511  included in the first layer set  510 . In a case where a multilayer video has a multiview scalability type, layers corresponding to a left view, a right view, and a central view may be included. The layers included in a layer set may be layers respectively corresponding to the left view, the right view, and the central view, and may be all displayed. 
     A third output layer subset  560  may include a layer  513  having a minimum layer-identifier value from among layers in the first layer set  510 . 
     The scalable video decoding apparatus  20  may obtain an index indicating an output layer subset, and may determine, by using the obtained index, one output layer subset from among output layer subsets  540 ,  550 , and  560 . 
     The scalable video decoding apparatus  20  decodes and then displays a layer included in a determined target output layer subset. 
     As described above with reference to  FIGS. 4B and 4C , it is assumed that a layer having the minimum layer-identifier value is the base layer, and a layer having the maximum layer-identifier value is a layer to be decoded and encoded last from among enhancement layers, but it is not limited thereto, and even if a layer-identifier value is great, the layer may be independently encoded without referring to another layer having a smaller layer-identifier value than the layer. 
     While it is described above by assuming that the layers shown in  FIGS. 4B and 4C  are primary pictures, but it is not limited thereto, and the layers may include layers corresponding to auxiliary pictures. For example, the auxiliary pictures may include an alpha plane picture and a depth picture. However, the pictures are only reference pictures used in decoding the primary pictures, and are not directly output and are displayed. 
     Therefore, the scalable video decoding apparatus  20  may determine, as an output layer subset, layers that exclude the layers corresponding to the auxiliary pictures. 
     As described above with reference to  FIG. 4C , it is assumed that the second layer set  510  is determined as the target output layer set, but it is not limited thereto, and thus, the scalable video decoding apparatus  20  may determine the output layer set group and may not determine in advance the target output layer set but may determine the target output layer set at a later time. In this case, the scalable video decoding apparatus  20  may determine output layer subsets for the layer sets  510 ,  520 , and  530 , respectively. 
     For example, the scalable video decoding apparatus  20  may determine an output layer set group including the layer sets  510 ,  520 , and  530 , may determine, as output layer subsets, layers having maximum layer-identifier values from the layer sets  510 ,  520 , and  530 , i.e., a layer  4  of the first layer set  510 , a layer  3  of the second layer set  520 , and a layer  2  of the third layer set  530 , and may determine an output layer subset group including the output layer subsets. In this regard, the scalable video decoding apparatus  20  may determine the output layer subset group, based on an index indicating the output layer subset group. 
     When the scalable video decoding apparatus  20  determines the target output layer set, the scalable video decoding apparatus  20  may determine one target output layer subset from among the output layer subset group. That is, the target output layer subset including at least one layer, which is included in the target output layer set, may be determined. 
     For example, when the scalable video decoding apparatus  20  determines the first layer set  510  as the target output layer set from among the output layer set group, the scalable video decoding apparatus  20  may determine the first output layer subset  560  from among the output layer subset group, and may determine, as an output layer, the layer  513  having the maximum layer-identifier value included in the first output layer subset  560 . 
       FIG. 5A  illustrates a mapping relationship between a scalability mask and a scalability type, according to an embodiment. 
     When the scalable video decoding apparatus  20  obtains scalability type information, a scalability video decoding technique to be performed on each layer may be determined according to an index of the scalability type information. 
     For example, when the index of the scalability type information is 1, the scalable video decoding apparatus  20  may perform, on a current layer, scalable video decoding according to a multiview scalability type. When the scalable video decoding apparatus  20  obtains the scalability type information having the index of 1, the scalable video decoding apparatus  20  may further obtain, from a bitstream, a scalability index ScalabilityID indicating a view-order index. 
     For example, when the index of the scalability type information is 2, the scalable video decoding apparatus  20  may perform, on the current layer, scalable video decoding according to a spatial scalability type or a quality scalability type. When the scalable video decoding apparatus  20  obtains the scalability type information having the index of 2, the scalable video decoding apparatus  20  may further obtain, from the bitstream, a scalability index ScalabilityID indicating an identifier DependencyID indicating a dependent relationship. 
     For example, when the index of the scalability type information is 3, the scalable video decoding apparatus  20  may perform scalable video decoding on the current layer so as to decode an auxiliary picture. When the scalable video decoding apparatus  20  obtains the scalability type information having the index of 3, the scalable video decoding apparatus  20  may further obtain, from the bitstream, a scalability index ScalabilityID indicating an identifier AuxID indicating a type of the auxiliary picture. 
     In a case where the index of the scalability type information is 0 or is between 4 through 15, the scalability type is not assigned thereto, thus, when the scalability type is added at a later time, a new scalability type may be assigned from among an index 0 or in the range of indexes 4 through 15. 
       FIG. 5B  illustrates an auxiliary picture type, according to an embodiment. 
     When the scalable video decoding apparatus  20  according to an embodiment performs scalable video decoding on a current layer so as to decode an auxiliary picture, a type of the auxiliary picture that is decoded in the current layer may be determined according to an auxiliary picture type identifier AuxId. 
     For example, when the auxiliary picture type identifier AuxId is an index 1, the auxiliary picture type corresponds to AUX_ALPHA, and the scalable video decoding apparatus  20  may interpret that the auxiliary picture decoded in the current layer is an alpha plane of a primary picture. 
     For example, when the auxiliary picture type identifier AuxId is an index 2, the auxiliary picture type corresponds to AUX_DEPTH, and the scalable video decoding apparatus  20  may interpret that the auxiliary picture decoded in the current layer is a depth picture of the primary picture. 
     In a case where the index of the scalability type information is between 3 through 255, the scalability type is not assigned thereto, thus, when the scalability type is added at a later time, a new scalability type may be assigned from among indexes 3 through 255. For example, when a Cr picture or a Cb picture which corresponds to a Y picture that is a primary picture is decoded as an auxiliary picture, the Cr picture or the Cb picture from among auxiliary picture type indexes that are preliminarily assigned with reference to  FIG. 5B  may be assigned thereto. 
     For example, when an auxiliary picture of only one auxiliary picture type is decoded in a layer, auxiliary pictures may have a scalability mask identifier value equal to that of the primary picture. 
     For example, when the auxiliary pictures have a same layer dependency as the primary picture, syntax for signaling a layer dependency may be skipped. 
       FIG. 5C  illustrates syntax for mapping an auxiliary picture type to a scalability type, according to an embodiment. 
     According to source syntax of  FIG. 5C , when scalability mask information scalability_mask_flag[smIdx] specifies that scalable video decoding is performed according to a predetermined scalability type indicated by a scalability mask index smIdx, a scalability index ScalabilityID is determined according to a corresponding scalability type. 
     According to last syntax of  FIG. 5C , when the scalability mask index is 3, an auxiliary picture type identifier may be determined according to a scalability index. That is, when the scalability mask index is 3, the scalability mask information represents a scalable video decoding technique for decoding an auxiliary picture of a primary picture. Therefore, such obtained scalability index ScalabilityID may be mapped to an auxiliary picture type index AuxID. 
     When the auxiliary picture type index AuxID[1ID] is 0, a layer having a layer identifier HD specifies that it does not include an auxiliary picture. When the auxiliary picture type index AuxID[1ID] has a value greater than 0, the layer having the layer identifier 1ID may include an auxiliary picture specified due to an auxiliary picture type according to  FIG. 5B . 
       FIGS. 5A, 5B, and 5C  are merely an example among syntaxes for determining a scalability type according to various embodiments, thus, it is required to note that syntax for the scalable video decoding apparatus  20  to interpret a scalability video decoding technique is not limited thereto. 
     Hereinafter, with reference to  FIG. 6 , syntax for defining a layer group including a layer of a primary picture and a layer of an auxiliary picture is proposed.  FIG. 6  illustrates VPS extension syntax, according to an embodiment. 
     First, when vps_aux_picture_use_flag is 1, it means that an auxiliary picture is included in a bitstream, and aux_mask_flag is obtained from a VPS. When vps_aux_picture_use_flag is 0, it may be determined that the auxiliary picture is not included in the bitstream and aux_mask_flag is 0. 
     When vps_aux_picture_use_flag is 1, aux_mask_flag may be obtained, and when aux_mask_flag[i] is 1, it means that an auxiliary picture of an i th  auxiliary picture type is obtained from a bitstream. When aux_mask_flag[i] is 0, it means that the auxiliary picture of the i th  auxiliary picture type is not obtained from the bitstream. 
     When vps_aux_picture_use_flag is 1, vps_aux_picture_grouped_flag may be further obtained from a VPS extension NAL unit. vps_aux_picture_grouped_flag specifies whether auxiliary pictures of a primary picture are grouped. 
     When vps_aux_picture_grouped_flag is 1, a layer identifier nuh_layer_id of an i th  auxiliary picture of a primary picture whose layer identifier nuh_layer_id is k may be k+i. Auxiliary pictures may have a layer dependency equal to that of a corresponding primary picture. 
     When vps_aux_picture_grouped_flag is not obtained from the bitstream, a value may be determined as 0. When vps_aux_picture_grouped_flag is 0, a limitation condition between a layer identifier of the primary picture and a layer identifier of the auxiliary picture is not necessary. 
     When a layer group for an auxiliary picture is set, the number of layers NumGroupLayers included in the layer group may be determined as a total sum of the number 1 of a primary picture and the number of auxiliary picture types NumAuxTypes (NumGroupLayers+=NumAuxTypes). 
     According to layers, by using direct_dependency_flag, whether a direct dependency is present between layers included in a same layer group (direct_dependency_flag[i][j]). That is, whether a j th  layer from among an i th  layer group has a direct dependency with respect to a base layer. 
     Also, a scalability index dimension_id according to each scalability type may be determined according to each layer group. 
     When the direct dependency is present between the layers included in a same layer group in each layer, according to direct_dependency_flag, information direct_dependency_type specifying that the j th  layer from among the i th  layer group is a direct dependency type with respect to the base layer may be obtained. 
       FIG. 7  illustrates VPS extension syntax, according to another embodiment. 
     When a limit is allowed so that a layer including only an auxiliary picture is to be a target output layer, a flag default_one_targe_t output_layer_flag specifying whether a default output layer is one target output layer may be obtained from a VPS extension NAL unit. 
     For example, when default_one_target_output_layer_flag is 1, only an uppermost layer including primary pictures from among each of default output layer sets may be a target output layer. 
     However, when default_one_target_output_layer_flag is 0, all layers including primary pictures from among each of the default output layer sets may be target output layers. 
     The scalable video encoding apparatus  10  according to  FIG. 1A  may generate samples by performing intra prediction, inter prediction, inter-layer prediction, transformation, and quantization on each of image blocks, may perform entropy encoding on the samples, and thus may output the samples in the form of a bitstream. In order to output a video encoding result of a videostream encoding apparatus  10 , i.e., in order to output a base layer videostream and an enhancement layer videostream, the videostream encoding apparatus  10  may operate in connection with an internal video encoding processor or an external video encoding processor so as to perform a video encoding operation including transformation and quantization. The internal video encoding processor of the videostream encoding apparatus  10  according to an embodiment may be a separate processor or may be implemented in a manner that a video encoding apparatus, a central processing unit (CPU) or a graphics processing unit (GPU) includes a video encoding processing module and thus performs a basic video encoding operation. 
     A videostream decoding apparatus  20  according to  FIG. 2A  performs decoding on each of the received base layer videostream and the received enhancement layer videostream. That is, inverse-quantization, inverse-transformation, intra prediction, and motion compensation (motion compensation with respect to images, inter-layer disparity compensation) may be performed on each of the image blocks of the base layer videostream and the enhancement layer videostream, so that samples of a base layer images may be reconstructed from the base layer videostream, and samples of enhancement layer images may be reconstructed from the enhancement layer videostream. 
     In order to output a reconstructed image generated by performing the decoding, the scalable video decoding apparatus  20  may operate in connection with an internal video decoding processor or an external video decoding processor so as to perform a video reconstructing operation including the inverse-quantization, the inverse-transformation, and the prediction/compensation. The internal video decoding processor of the scalable video decoding apparatus  20  according to an embodiment may be a separate processor or may be implemented in a manner that a video decoding apparatus, a CPU or a GPU includes a video decoding processing module and thus performs a basic video decoding operation. 
     As described above, the scalable video encoding apparatus  10  and the scalable video decoding apparatus  20  according to the embodiments split blocks of divided video data into coding units of a tree structure, and encoding units, prediction units, and transformation units are used for inter-layer prediction or inter-prediction with respect to the coding units. Hereinafter, with reference to  FIGS. 8 through 20 , a video encoding method and apparatus therefor, and a video decoding method and apparatus therefor based on coding units of a tree structure and transformation units according to embodiments are described. 
     Basically, in an encoding/decoding procedure for a multilayer video, an encoding/decoding procedure for base layer images, and an encoding/decoding procedure for enhancement layer images are separately performed. That is, when inter-layer prediction occurs in the multilayer video, encoding/decoding results with respect to a single layer video may be mutually referred to, but an encoding/decoding procedure is performed for each of single layer videos. 
     Therefore, for convenience of description, a video encoding procedure and a video decoding procedure based on coding units of a tree structure that are described later with reference to  FIGS. 8 through 20  are a video encoding procedure and a video decoding procedure for a single layer video, thus, inter-prediction and motion compensation are described in detail. However, as described above with reference to  FIGS. 1A through 7 , for encoding/decoding a video stream, inter-layer prediction and compensation between base layer images and enhancement layer images are performed. 
     Therefore, in order for an image encoder  12  of the scalable video encoding apparatus  10  according to the embodiment to encode a multilayer video, based on coding units of a tree structure, the image encoder  12  may include video encoding apparatuses  800  of  FIG. 8  corresponding to the number of layers of the multilayer video so as to perform video encoding on each of single layer videos, and may control the video encoding apparatuses  800  to encode the single layer videos, respectively. Also, the scalable video encoding apparatus  10  may perform inter-view prediction by using encoding results with respect to discrete single views obtained by the video encoding apparatuses  800 . Accordingly, the image encoder  12  of the scalable video encoding apparatus  10  may generate a base layer video stream and an enhancement layer video stream that include an encoding result of each layer. 
     Similarly, in order for the scalable video decoding apparatus  20  to decode a multilayer video, based on coding units of a tree structure, the scalable video decoding apparatus  20  may include video decoding apparatuses  900  of  FIG. 9  corresponding to the number of layers of a multilayer video so as to perform video decoding on each of layers of a received base layer videostream and a received enhancement layer videostream, and may control the video decoding apparatuses  900  to decode single layer videos, respectively. Then, the scalable video decoding apparatus  20  may perform inter-layer compensation by using decoding results with respect to discrete single layers obtained by the video decoding apparatuses  900 . Accordingly, the scalable video decoding apparatus  20  may generate base layer images and enhancement layer images that are reconstructed for each of the layers. 
       FIG. 8  illustrates a block diagram of a video encoding apparatus based on coding units of a tree structure  800 , according to various embodiments. 
     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 independently determined 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 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 various embodiments. 
     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 bit stream 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 various embodiments. 
     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 predictor  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 various embodiments. 
     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 . 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 encoder of  FIG. 1A  encodes a videostream of two or more layers, the video encoder  1100  may be provided for each layer. Similarly, if the decoder 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 various embodiments. 
     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 included 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 , 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 , 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 , i.e. a partition  1340  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 various embodiments. 
     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 various embodiments. 
     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 various embodiments. 
     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×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 coding 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 2N (d−1)×N (d−1), two partitions having a size of N (d−1)×2N (d−1), four partitions having a size of N (d−1)×N (d−1) from among the partition 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  having 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 various embodiments. 
     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  are 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 
                   
               
               
                 (Encoding on Coding Unit having Size of 
               
               
                 2N × 2N and Current Depth of d) 
               
            
           
           
               
               
               
            
               
                   
                 Size of Transformation Unit 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Partition Type 
                 Split 
                 Split 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Symmetrical 
                   
                 Information 0 of 
                 Information 1 of 
                   
               
               
                 Prediction 
                 Partition 
                 Asymmetrical 
                 Transformation 
                 Transformation 
                 Split 
               
               
                 Mode 
                 Type 
                 Partition Type 
                 Unit 
                 Unit 
                 Information 1 
               
               
                   
               
               
                 Intra 
                 2N × 2N 
                 2N × nU 
                 2N × 2N 
                 N × N 
                 Repeatedly 
               
               
                 Inter 
                 2N × N 
                 2N × nD 
                   
                 (Symmetrical 
                 Encode Coding 
               
               
                 Skip (Only 
                 N × 2N 
                 nL × 2N 
                   
                 Partition Type) 
                 Units having 
               
               
                 2N × 2N) 
                 N × N 
                 nR × 2N 
                   
                 N/2 × N/2 
                 Lower Depth 
               
               
                   
                   
                   
                   
                 (Asymmetrical 
                 of d + 1 
               
               
                   
                   
                   
                   
                 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 2N×2N, 2N×N, N×2N, and N×N, which are obtained by symmetrically splitting a height or a width of a prediction unit, and asymmetrical partition 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 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 one 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 one or more embodiments 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 various embodiments. 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 computer  12100 . 
     Video data captured by a camera  12600  may also be transmitted to the streaming server  11300  via the computer  12100 . The camera  12600  is an imaging device capable of capturing both still images and video images, similar to a digital camera. The video data captured by the camera  12600  may be encoded using the camera  12600  or the computer  12100 . Software that performs encoding and decoding 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 various embodiments. 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 various embodiments. 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  12840 . 
     In the set-top box  12870  connected to the antenna  12860  for a satellite/terrestrial broadcast or a cable antenna  12850  for receiving a cable television (TV) broadcast, 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 various embodiments. 
     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, various embodiments of methods of storing the video encoding method and the video decoding method in a storage medium or various 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 present invention can also be embodied as computer-readable codes on a computer readable recording medium. The computer-readable recording medium is any data storage device that can store programs or data which can be thereafter read by a computer system. Examples of the computer-readable recording medium include ROMs, RAMs, CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and the like. The computer-readable recording medium can also be distributed over network-coupled computer systems so that the computer-readable code is stored and executed in a distributed fashion. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the following claims. Therefore, the scope of the present invention is defined not by the detailed description of the present invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.