CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. Ser. No. 11/405,420 filed in the United States on Apr. 18, 2006, which claims the benefit of U.S. Provisional Patent Application No. 60/672,047, filed on Apr. 18, 2005, in the U.S. Trademark and Patent Office, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a moving picture coding/decoding method and apparatus, and more particularly, to a moving picture coding/decoding method and apparatus using the H.264/MPEG-4 AVC FRExt (Advanced Video Coding Fidelity Range Extensions) standard. 
     2. Description of Related Art 
     A new RGB coding technology referred to as “residual color transform” was developed during the development of the H.264/MPEG-4 AVC FRExt standard. This technology is for preventing image quality deterioration that occurs when a transform from a RGB color space to a YCbCr color space is performed. However, RGB coding technologies according to H.264/MPEG-4 AVC FRExt still do not ensure a sufficiently high coding efficiency to be applied to moving picture reproducing apparatuses. 
     Thus, there is a need for coding technologies according to H.264/MPEG-4 AVC FRExt that ensure a sufficiently high coding efficiency to be applied to moving picture reproducing apparatuses. 
     BRIEF SUMMARY 
     An aspect of the present invention provides a moving picture coding/decoding method and apparatus that can increase the coding efficiency of a moving picture using a RGB coding technology according to H.264/MPEG-4 AVC FRExt. 
     An aspect of the present invention also provides a computer readable recording medium having embodied thereon a computer program for the moving picture coding/decoding method. 
     According to an aspect of the present invention, there is provided a moving picture coding method comprising: (a) selecting a prediction mode to be commonly applied to all the color components constituting a color space; (b) generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components according to the prediction mode selected in operation (a); (c) generating second residual data corresponding to differences between the first residual data for each of the color components; and (d) coding the generated second residual data. 
     According another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture coding method. 
     According to another aspect of the present invention, there is provided a moving picture coding apparatus comprising: a selection unit selecting a prediction mode to be commonly applied to all the color components constituting a color space; a subtractor generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components according to the prediction mode selected by the selection unit; a transform unit generating second residual data corresponding to differences between the first residual data generated by the subtractor; and a coding unit coding the second residual data generated by the transform unit. 
     According to another aspect of the present invention, there is provided a moving picture coding method comprising: (a) selecting a color space from among a plurality of color spaces; (b) generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components constituting the selected color space; (c) generating second residual data corresponding to differences between the generated first residual data; and (d) coding the generated second residual data. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture coding method. 
     According to another aspect of the present invention, there is provided a moving picture coding apparatus comprising: a selection unit selecting a color space from among a plurality of color spaces; a subtractor generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components constituting the color space selected by the selection unit; a transform unit generating second residual data corresponding to differences between the first residual data generated by the subtractor; and a coding unit coding the second residual data generated by the transform unit. 
     According to another aspect of the present invention, there is provided a moving picture coding method comprising: (a) selecting a color space from among a plurality of color spaces; (b) selecting a prediction mode to be commonly applied to all the color components constituting the selected color space; (b) generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components constituting the selected color space according to the selected prediction mode; (c) generating second residual data corresponding to differences between the generated first residual data; and (d) coding the generated second residual data. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture coding method. 
     According to another aspect of the present invention, there is provided a moving picture coding apparatus comprising: a first selection unit selecting a color space from among a plurality of color spaces; a second selection unit selecting a prediction mode to be commonly applied to all the color components constituting the color space selected by the first selection unit; a subtractor generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components according to the prediction mode selected by the second selection unit; a transform unit generating second residual data corresponding to differences between the first residual data generated by the subtractor; and a coding unit coding the second residual data generated by the transform unit. 
     According to another aspect of the present invention, there is provided a moving picture coding method comprising: (a) selecting first prediction modes to be independently applied to color components constituting a color space or a second prediction mode to be commonly applied to all the color components of the color space; and (b) generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components according to the first prediction modes or the second prediction mode selected in operation (a). 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture coding method. 
     According to another aspect of the present invention, there is provided a moving picture coding apparatus comprising: a selection unit selecting first prediction modes to be independently applied to color components constituting a color space or a second prediction mode to be commonly applied to all the color components of the color space; and a subtractor generating first residual data corresponding to differences between a current picture and a predicted picture for each of the color components according to the first prediction modes or the second prediction mode selected by the selection unit. 
     According to another aspect of the present invention, there is provided a moving picture decoding method comprising: (a) generating second residual data corresponding to differences between first residual data by decoding a bitstream; (b) generating the first residual data corresponding to the sum of the generated second residual data in a color space; (c) generating a prediction picture for each color component constituting the color space according to a prediction mode that is commonly applied to all the color components; and (d) generating a reconstructed picture corresponding to the sum of the generated first residual data and the generated predicted pictures. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture decoding method. 
     According to another aspect of the present invention, there is provided a moving picture decoding apparatus comprising: a decoding unit generating second residual data corresponding to differences between first residual data by decoding a bitstream; an inverse transform unit generating first residual data corresponding to the sum of the generated second residual data in a color space; a prediction unit generating a prediction picture for each color component constituting the color space according to a prediction mode that is commonly applied to all the color components; and an adder generating a reconstructed picture corresponding to the sum of the first residual data generated by the inverse transform unit and the predicted pictures generated by the prediction unit. 
     According to another aspect of the present invention, there is provided a moving picture decoding method comprising: (a) generating second residual data corresponding to differences between first residual data by decoding a bitstream; (b) generating the first residual data corresponding to the sum of the generated second residual data in a color space selected from among a plurality of color spaces; (c) generating a reconstructed picture corresponding to the sum of the generated first residual data and predicted pictures. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture decoding method. 
     According to another aspect of the present invention, there is provided a moving picture decoding apparatus comprising: a decoding unit generating second residual data corresponding to differences between first residual data by decoding a bitstream; an inverse transform unit generating the first residual data corresponding to the sum of the generated second residual data in a color space selected from among a plurality of color spaces; and an adder generating a reconstructed picture corresponding to the sum of the first residual data generated by the inverse transform unit and predicted pictures. 
     According to another aspect of the present invention, there is provided a moving picture decoding method comprising: (a) generating second residual data corresponding to differences between first residual data by decoding a bitstream; (b) generating the first residual data corresponding to the sum of the generated second residual data in a color space selected from among a plurality of color spaces; (c) generating a predicted picture for each color component of the color space according to a prediction mode that is commonly applied to all the color components; and (d) generating a reconstructed picture corresponding to the sum of the generated first residual data and the generated predicted pictures. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture decoding method. 
     According to another aspect of the present invention, there is provided a moving picture decoding apparatus comprising: a decoding unit generating second residual data corresponding to differences between first residual data by decoding a bitstream; an inverse transform unit generating the first residual data corresponding to the sum of the generated second residual data in a color space selected from among a plurality of color spaces; a prediction unit generating a predicted picture for each of the color components constituting the color space according to a prediction mode that is commonly applied to all the color components; and an adder generating a reconstructed picture corresponding to the sum of the first residual data generated by the inverse transform unit and the predicted pictures generated by the prediction unit. 
     According to another aspect of the present invention, there is provided a moving picture decoding method comprising: (a) generating a prediction picture for each color component constituting the color space according to first prediction modes that are independently applied to the color components constituting the color space or a second prediction mode that is commonly applied to all the color components; and (b) generating a reconstructed picture based on the generated predicted pictures. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for the above-described moving picture decoding method. 
     According to another aspect of the present invention, there is provided a moving picture decoding apparatus comprising: a prediction unit generating a prediction picture for each color component constituting the color space according to first prediction modes that are independently applied to the color components constituting the color space or a second prediction mode that is commonly applied to all the color components constituting the color space; and an adder generating a reconstructed picture based on the predicted pictures generated by the prediction unit. 
     According to another aspect of the present invention, there is provided an image decoding method comprising: obtaining information about a color representation from a bitstream, entropy decoding a residue, which corresponds to a difference between a current image and a predicted image of the current image, and reconstructing the current image by using the entropy decoded residue and the predicted image, based on the information about the color representation, wherein the reconstructing of the current image comprises: obtaining a prediction direction of a chroma component from among a plurality of prediction directions associated with a prediction direction of a luma component, wherein the plurality of prediction modes include a first mode in which the prediction direction of the chroma component is identical to the prediction direction of the luma component and a second mode in which the prediction direction of the chroma component is different from the prediction direction of the luma component; and obtaining the predicted image based on the prediction direction of the luma component and the prediction direction of the chroma component. 
     According to another aspect of the present invention, there is provided a method, wherein the information about the color representation indicates a color space into which color components are represented. 
     Additional and/or other aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following detailed description, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram of a moving picture coding apparatus according to an embodiment of the present invention; 
         FIG. 2  illustrates the selection of a size of blocks in the moving picture coding apparatus of  FIG. 1 ; 
         FIG. 3  illustrates the selection of a motion vector in the moving picture coding apparatus of  FIG. 1 ; 
         FIG. 4  illustrates the selection of a prediction direction in the moving picture coding apparatus in  FIG. 1 ; 
         FIG. 5  illustrates changes in correlation between residual data when a single prediction mode is applied to different color components in an embodiment according to the present invention; 
         FIG. 6  illustrates the correlation between color components when a single prediction mode is applied to all the color components in an embodiment of the present invention. 
         FIG. 7  is a block diagram of a moving picture decoding apparatus according to an embodiment of the present invention; 
         FIGS. 8A and 8B  are flowcharts of a moving picture coding method according to an embodiment of the present invention; 
         FIG. 9  is a flowchart of a moving picture decoding method according to an embodiment of the present invention; 
         FIG. 10  is the results of a simulation test according to embodiments of the present invention. 
         FIG. 11  is a graph comparatively showing the coding efficiencies in a lossless mode in embodiments according to the present invention; and 
         FIGS. 12A and 12B  are rate distortion (RD) curves obtained through a simulation of embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
       FIG. 1  is a block diagram of a moving picture coding apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 1 , a moving picture coding apparatus according to an embodiment of the present invention includes a color space selection unit  101 , an inter prediction mode selection unit  102 , a motion estimation unit  103 , a motion compensation unit  104 , an intra prediction mode selection unit  105 , an intra prediction unit  106 , a subtractor  107 , a residue transform unit  108 , a frequency domain transform unit  109 , a quantization unit  110 , an entropy coding unit  111 , an dequantization unit  112 , a frequency domain inverse transform unit  113 , a residue inverse transform unit  114 , an adder  115 , and a filter  116 . 
     The color space selection unit  101  adaptively selects a color space from among a plurality of color spaces based on the characteristics of a current picture. Non-limiting examples of color spaces include a YCgCo color space and a RGB color space. In the YCgCo color space, Y indicates a luma component, Co indicates a chroma orange component, and Cg indicates a chroma green component. In the RGB color space, R indicates red color component, G indicates a green color component, and B indicates a blue color component. 
     A scheme of coding a current picture will be referred to hereafter as “residual color transform” or (RCT) in the YCgCo color space and as “inter-plane prediction” or (IPP) in the RGB color space. 
     The RGB color space includes an R component, a G component, and a B component which can be perceived by humans. Accordingly, when the color space selection unit  101  selects another color space, for example, the YCgCo color space, not the RGB color space, after coding has been performed in the YCgCo color space, a transform from the YCgCo color space to the RGB color space should be performed. 
     According to the results of numerous experiments performed using sets of high quality images, such as high definition (HD) moving pictures, film scan images, Thompson Viper sequences, etc., there is no panacea for images having various characteristics and bit rates. Some images contain serious film grain noise in a color component, other images contain thermal noise, and other images have saturated color characteristics. RCT is efficient in a certain case, and IPP is efficient in another case. Therefore, in the present embodiment, one of RCT and IPP is adaptively used according to image characteristics. A moving picture decoding apparatus may be informed as to which of RCT and IPP is used in the moving picture coding apparatus by a single bit flag according to H.264/MPEG-4 AVC FRExt. 
     In a high bit rate environment, the coding efficiency of moving pictures is higher in RCT than in IPP. However, in RCT, due to the transform between different color spaces, the image quality deteriorates. Especially, when many noise components exist in a current picture or when the configuration of a current picture is complex, such image quality deterioration is serious. The color space selection unit  101  selects the RGB color space when many noise components exist in a current picture or when the configuration of a current picture is complex, or the YCgCo color space when almost no noise components exist in a current picture or when the configuration of a current picture is simple. In other words, the color space selection unit  101  adaptively selects a color space based on the characteristics of the moving picture, such as bit rate, coded sequences, etc. 
     The inter prediction mode selection unit  102  selects inter prediction modes to be independently applied to the color components constituting the color space selected by the color selection unit  101  or a single inter prediction mode to be commonly applied to all the color components constituting the color space selected by the color space selection unit  101 . In general, when all color components have different motion vectors in an inter prediction mode, residual data of the all color components have different characteristics, and thus there is almost no correlation between the residual data of the different color components. Thus, in the present embodiment, the use of a single prediction mode, i.e., a single block size, a single motion vector, etc., for all the color components is suggested to increase the correlation between the residual data of the different color components. The use of a single prediction mode provides more natural images than when RGB components have similar texture characteristics. In addition, a moving picture decoding apparatus may be informed as to which of a single mode and a conventional independent mode is used in the moving picture coding apparatus by a single bit flag according to H.264/MPEG-4 AVC FRExt. 
     A case where the inter prediction mode selection unit  102  selects a single mode will be described in detail. The inter prediction mode selection unit  102  selects a size of blocks to be commonly applied to all the color components constituting the color space selected by the color space selection unit  101 . The sizes of blocks constituting the current picture can be 16×16, 16×8, 8×1, 8×8, 8×4, 4×8, 4×4, etc. In general, a 16×16 block is referred to as “macroblock”, and blocks obtained by dividing a macroblock to various sizes are referred as to “subblocks”. Moving picture coding and decoding are performed in such block units. A portion of a moving picture that requires more accurate coding and decoding is coded and decoded in smaller block units. 
     In particular, the inter prediction mode selection unit  102  selects a size of blocks to be commonly applied to all color components. For example, the inter prediction mode selection unit  102  selects a size of blocks to be commonly applied to the Y component, the Co component, and the Cg component from among the sizes of blocks of the Y component, the sizes of blocks of the Cg component, and the sizes of blocks of the Cg component. Alternatively, the inter prediction mode selection unit  102  selects a size of blocks to be commonly applied to the R component, the G component, and the B component from among the sizes of blocks of the R component, the sizes of blocks of the G component, and the sizes of the blocks of the B component. 
       FIG. 2  illustrates the selection of a size of blocks in the moving picture coding apparatus of  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , the inter prediction mode selection unit  102  selects a size of blocks that is optimal to the macroblocks of all the color components and are commonly applied to all the color components from among the sizes of blocks constituting a macroblock of the R component, the sizes of blocks constituting a macroblock of the G component, and the sizes of blocks constituting a macroblock of the B component. This selection can apply to the YCgCo color space. 
     Next, the inter prediction mode selection unit  102  selects a single motion vector to be commonly applied to all the color components constituting the color space from among motion vectors of all the color components in units of blocks corresponding to the selected size of blocks. In particular, the inter prediction mode selection unit  102  selects a single motion vector to be commonly applied to all the color components from among the motion vectors of the color components calculated by the motion estimation unit  103 . For example, the inter prediction mode selection unit  102  selects a single motion vector to be commonly applied to all the Y component, Co component, and Cg component from among the motion vector of the Y component, the motion vector of the Co component, and the motion vector of the Cg component that are calculated by the motion estimation unit  103 . Alternatively, the inter prediction mode selection unit  102  selects a single motion vector to be commonly applied to all the R component, G component, and B component from among a motion vector of the R component, a motion vector of the G component, and a motion vector of the B component that are calculated by the motion estimation unit  103 . 
       FIG. 3  illustrates the selection of a motion vector in the moving picture coding apparatus of  FIG. 1 . 
     Referring to  FIGS. 1 and 3 , the inter prediction mode selection unit  102  selects a single motion vector that is optimal to the blocks of all the color components from among the motion vector for a block of the R component, the motion vector for a block of the G component, and the motion vector for a block of the B component. This selection can apply to the YCgCo color space. 
     The motion estimation unit  103  estimates motion in the current picture for each color component based on a reference picture according to an inter prediction mode selected by the inter prediction mode selection unit  102 . In particular, the motion estimation unit  103  selects motion vectors to be independently applied to the color components constituting the color space in units of blocks that are independent for the color components or selects a motion vector to be commonly applied to the color components constituting the color spaces in units of blocks that are common to all the color components according to the inter prediction mode selected by the inter prediction mode selection unit  102 . 
     A case where the motion estimation unit  103  estimates motion according to a single mode will be described. The motion estimation unit  103  calculates a motion vector corresponding to the displacement in the current picture with respect to the reference picture in units of blocks corresponding to the size of blocks selected by the inter prediction mode selection unit  102 . In the present embodiment, the current picture refers to an object picture on which coding and decoding are performed. The reference picture indicates a picture referred to when coding or decoding the current picture. In general, a picture preceding the current picture is used as a reference picture. However, a picture following the current picture also can be sued as a reference picture. Alternatively, a plurality of reference pictures can be used. 
     For example, the motion estimation unit  103  calculates a motion vector of the Y component, a motion vector of the Co component, and a motion vector of the Cg component by estimating motion in the current picture for each of the Y component, Co component, and Cg component based on the reference picture according to the inter prediction mode selected by the inter prediction mode selection unit  102 . Alternatively, the motion estimation unit  103  calculates a motion vector of the R component, a motion vector of the G component, and a motion vector of the B component by estimating motion in the current picture for each of the R component, G component, and B component based on the reference picture according to the inter prediction mode selected by the prediction mode selection unit  102 . 
     The motion compensation unit  104  compensates for the motion between the current picture and the reference picture according to the inter prediction mode selected by the inter prediction mode selection unit  102 . In particular, the motion compensation unit  104  generates a predicted picture in the current picture from the reference picture in units of blocks corresponding to the size of blocks selected by the inter prediction mode selection unit  102  using the motion vector selected by the inter prediction mode selection unit  102 . The inter prediction performed by the motion estimation unit  103  and the motion compensation unit  104  is for removing temporal redundancy between the current picture and the reference picture. 
     For example, the motion compensation unit  104  generates a predicted picture for the Y component, a predicted picture for the Co component, and a predicted picture for the Cg component by compensating for the motion between the current picture and the reference picture according to the inter prediction mode selected by the inter prediction mode selection unit  102 . Alternatively, the motion compensation unit  104  generates a predicted picture for the R component, a predicted picture for the G component, and a predicted picture for the B component by compensating for the motion between the current picture and the reference picture according to the inter prediction mode selected by the inter prediction mode selection unit  102 . 
     The intra prediction mode selection unit  105  selects intra prediction modes to be independently applied to the color components constituting the color space selected by the color space selection unit  101  or a single intra prediction mode to be commonly applied to all the color components constituting the color space selected by the color space selection unit  101 . According to MPEG-4 AVC/H.264 video coding schemes, there are 9 intra prediction modes in units of 4×4 blocks and 4 intra prediction modes in units of 16×16 blocks for a luma component, and 4 intra prediction modes in units of 8×8 blocks for a chroma component. In general, since prediction modes for the luma component and the chroma component are different, the correlation between the color components is low, which is disadvantageous in RCT, IPP, or other similar transforms. The coding efficiency for moving pictures can be increased by applying a single intra prediction mode to all the color components. Therefore, in the present embodiment, an intra prediction mode in units of 4×4, 8×8, or 16×16 blocks for the luma component is applied to the chroma component. 
     The intra prediction mode selection unit  105  selects prediction directions to be independently applied to the color components constituting the color space, or selects a prediction direction to be commonly applied to all the color components constituting the color space from among prediction directions for the color components constituting the color space according to the intra prediction mode selected by the intra prediction mode selection unit  105 . A case where the intra prediction mode selection unit  105  selects a single mode will be described in detail. The intra prediction mode selection unit  105  selects a size of blocks to be commonly applied to all the color components constituting the color space selected by the color space selection unit  101 . The sizes of blocks constituting the current picture can be 16×16, 8×8, 4×4, etc. The selection of a size of blocks for intra prediction is performed in the same manner as for the inter prediction illustrated in  FIG. 2 . 
     Next, the intra prediction mode selection unit  105  selects a single prediction direction to be commonly applied to all the color components constituting the color space in units of blocks corresponding to the size of blocks selected above from among prediction directions of the color components constituting the color space. For example, the intra prediction mode selection unit  105  selects a single prediction direction to be commonly applied to the Y component, Co component, and Cg component from among a prediction direction of the Y component, a prediction direction of the Co component, and a prediction direction of the Cg component. Alternatively, the intra prediction mode selection unit  105  selects a single prediction direction to be commonly applied to the R component, G component, and B component from among a prediction direction of the R component, a prediction direction of the G component, and a prediction direction of the B component. 
       FIG. 4  illustrates the selection of a prediction direction in the moving picture coding apparatus of  FIG. 1 . 
     Referring to  FIGS. 1 and 4 , the intra prediction mode selection unit  105  selects a single prediction direction that is optimal to the blocks of all the color components and is to be commonly applied to all the color components from among a prediction direction for a block in the R component, a prediction direction for a block in the G component, and a prediction direction for a block in the B component. This selection can apply to the YCgCo color space. 
     The intra prediction unit  106  estimates blocks constituting the current picture from adjacent pixels in a picture reconstructed by an adder  115 , according to the intra prediction mode selected by the intra prediction mode selection unit  105 , and generates a predicted picture constituted by the predicted blocks. In particular, the intra prediction unit  106  estimates blocks constituting the current picture from adjacent pixels indicated by the prediction directions, which are independently applied to the color components of the color space, in units of blocks that are independent for the color components, and generates a predicted picture constituted by the predicted blocks. Alternatively, the intra prediction unit  106  estimates blocks constituting the current picture from adjacent pixels indicated by the prediction direction, which is commonly applied to all the color components of the color space, in units of blocks that are common to all the color components, and generates a predicted picture constituted by the prediction direction. The intra prediction performed by the intra prediction unit  106  is for removing spatial redundancy in the current picture. 
     For example, the intra prediction unit  106  predicts blocks constituting the current picture from adjacent pixels for each of the Y component, Co component, and Cg component indicated by the prediction direction selected by the intra prediction mode selection unit  105  and generates a predicted picture for the Y component, a predicted picture for the Co component, and a predicted picture for the Cg component. Alternatively, the intra prediction unit  106  predicts blocks constituting the current picture from adjacent pixels for each of the R component, G component, and B component indicated by the prediction direction selected by the intra prediction mode selection unit  105  and generates a predicted picture for the R component, a predicted picture for the G component, and a predicted picture for the B component. 
     The subtractor  107  generates first residual data corresponding to differences between the current picture and the predicted picture for each of the color components generated by the motion compensation unit  104  or by the intra prediction unit  106 . For example, the subtractor  107  generates the first residual data for the Y component, the first residual data for the Co component, and the first residual data for the Cg component by calculating differences between the current picture and the predicted picture for each of the Y component, Co component, and Cg component that are generated by the motion compensation unit  104  or by the intra prediction unit  106 . Alternatively, the subtractor  107  generates the first residual data for the R component, the first residual data for the G component, and the first residual data for the B component by calculating differences between the current picture and the predicted picture for each of the R component, G component, and B component that are generated by the motion compensation unit  104  or by the intra prediction unit  106 . 
       FIG. 5  illustrates changes in correlation between residual data when a single prediction mode is applied to different color components in an embodiment according to the present invention. 
     Referring to  FIG. 5 , plots  501  and  502  illustrate the correlation between residual data obtained by applying independent prediction modes to the color components. In particular, plot  501  illustrates the correlation between residual data for the R component and the G component, and plot  502  illustrates the correlation between residual data for the B component and the G component. Meanwhile, plots  503  and  504  illustrate the correlation between residual data obtained by applying a signal prediction mode to all the color components. In particular, plot  503  illustrates the correlation between residual data for the R component and G component, and plot  504  illustrates the correlation between residual data for the B component and the G component. As is apparent from the plots  501  through  504  in  FIG. 5 , the correlation between residual data for color components is higher when a single prediction mode is applied to all the color components than when independent prediction modes are applied to the all the color components. 
     The residue transform unit  108  of  FIG. 1  generates second residual data corresponding to differences between the first residual data generated by the subtractor  107  of  FIG. 1 . In the present embodiment, to increase the coding efficiency for moving pictures, redundancy between the color components that remains after the prediction or intra prediction is utilized. The inter prediction utilizes temporal redundancy, and the intra prediction utilizes spatial redundancy. However, redundancy between the color components still remains after the inter prediction or intra prediction. 
       FIG. 6  illustrates the correlation between color components when a single prediction mode is applied to all the color components in an embodiment of the present invention. 
     Referring to  FIGS. 1 and 6 , plot  601  illustrates the correlation between the R component and the G component after intra prediction. Plot  602  illustrates the correlation between the B component and the G component after intra prediction. Plot  603  illustrates the correlation between the R component and the G component after inter prediction, and plot  604  illustrates the correlation between the B component and the G component after inter prediction. As is apparent from the plots  601  through  604  in  FIG. 6 , there still remains a strong correlation between the residual data for the color components after the inter prediction or intra prediction. 
     In other words, the residue transform unit  108  generates second residual data corresponding to differences between the first residual data for each of the Y component, the Co component, and the Cg component in the YCgCo color space using Equation Set (1) below. In particular, Y=(R+2G+B)&gt;&gt;2, Co=(R−B)&gt;&gt;1, and Cg=(−R+2G−B)&gt;&gt;2.
 
Δ 2   B=ΔR−ΔB  
 
 t=ΔB+ (Δ 2   B&gt;&gt; 1)
 
Δ 2   R=ΔG−t  
 
Δ 2   G=t+ (Δ 2   R&gt;&gt; 1)  Equation Set (1)
 
Here, ΔX denotes first residual data, Δ 2 X denotes second residual data, the notation “&gt;&gt;” denotes right shift operation, which approximates a division by 2, and variable t is used to the purpose of temporary calculation.
 
     Alternatively, the residue transform unit  108  generates second residual data corresponding to differences between the first residual data for each of the R component, the G component, and the B component in the RGB color space using Equation Set (2) below.
 
Δ 2   G=ΔG′ 
 
Δ 2   R=ΔR−ΔG′ 
 
Δ 2   B=ΔB−ΔG′   Equation Set (2)
 
Here, ΔX denotes first residual data, Δ 2 X denotes second residual data, and ΔX′ denotes reconstructed first residual data. Equation (2) is effective when the G component has a large amount of picture information. The second residual data can be calculated using the R component or B component as a dominant component.
 
     The frequency domain transform unit  109  transforms the second residual data in the color space generated by the residue transform unit  108  into second residual data in a frequency domain. According to H.264/MPEG-4 AVC, discrete hadamard transform (DHT), discrete cosine transform (DCT)-based integer transform, etc., are used as schemes for transform from the color space to the frequency domain. 
     The quantization unit  110  quantizes the frequency component values transformed by the frequency domain transform unit  109 . In particular, the quantization unit  110  divides the frequency component values transformed by the frequency domain transform unit  109  by a quantization parameter and approximates the divided results to integer values. 
     The entropy coding unit  111  generates a bitstream by entropy-coding the quantized values obtained by the quantization unit  110  and outputs the bitstream. According to H.264/MPEG-4 AVC, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), etc., are used as entropy coding schemes. 
     The dequantization unit  112  dequantizes the quantized values obtained by the quantization unit  110 . In particular, the dequantization unit  112  reconstructs the frequency domain values by multiplying the integer values approximated by the quantization unit  110  by the quantization parameter. 
     The frequency domain inverse transform unit  113  reconstructs the second residual data by transforming the frequency component values in the frequency domain reconstructed by the dequantization unit  112  into data in the color space. 
     The residue inverse transform unit  114  generates first residual data corresponding to the sum of the second residual data reconstructed by the frequency domain inverse transform unit  113 . In particular, the residue inverse transform unit  114  generates first residual data corresponding to the sum of the second residual data for each of the Y component, Co component, and Cg component in the YCgCo color space using Equation Set (3) below.
 
 t=Δ   2   G′− (Δ 2   R′− 1)
 
Δ G′=Δ   2   R′+t  
 
Δ B′=t− (Δ 2   B′− 1)
 
Δ R′=ΔB′+Δ   2   B′   Equation Set (3)
 
Here, ΔX′ denotes reconstructed first residual data and Δ 2 X′ denotes reconstructed second residual data.
 
     Alternatively, the residue inverse transform unit  114  generates first residual data corresponding to the sum of the second residual data for each of the R component, G component, and B component in the RGB color space, using Equation Set (4) below.
 
Δ G′=Δ   2   G′ 
 
Δ R′=Δ   2   R′+ΔG′ 
 
Δ B′=Δ   2   B′+ΔG′   Equation Set (4)
 
Here, ΔX′ denotes reconstructed first residual data, and Δ 2 X′ denotes reconstructed second residual data.
 
     The adder  115  generates a reconstructed picture corresponding to the sum of the predicted pictures generated by the motion compensation unit  104  or the intra prediction unit  106  and the first residual data generated by the residue inverse transform unit  114 . For example, the adder  115  generates a reconstructed picture in the YCgCo color space by calculating the sum of the predicted pictures for the Y component, Co component, and Cg component generated by the motion compensation unit  104  or the intra prediction unit  106  and the first residual data generated by the residue inverse transform unit  114 . Alternatively, the adder  115  generates a reconstructed picture in the RGB color space by calculating the sum of the predicted pictures for the component, G component, and B component generated by the motion compensation unit  104  or the intra prediction unit  106  and the first residual data generated by the residue inverse transform unit  114 . 
     The filter  116  improves the quality of the reconstructed picture by smoothing distortion at block boundaries in the reconstructed picture generated by the adder  115 . However, the filter  116  is not used for a high bit rate moving picture because high frequency components are likely to be lost. 
     Although the moving picture coding apparatus in  FIG. 1  uses a scheme of adaptively applying a color space and a single mode scheme, it is to be understood that a moving picture coding apparatus using one of the two schemes is also possible. 
       FIG. 7  is a block diagram of a moving picture decoding apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 7 , a moving picture decoding apparatus according to an embodiment of the present invention includes an entropy decoding unit  701 , a dequantization unit  702 , a frequency domain inverse transform unit  703 , a residue inverse transform unit  704 , a motion compensation unit  705 , an intra prediction unit  706 , an adder  707 , and a filter  708 . 
     The entropy decoding unit  701  reconstructs integer values by entropy-decoding an input bitstream output from, by way of a non-limiting example, the moving picture coding apparatus of  FIG. 1 . 
     The dequantization unit  702  dequantizes the integer values reconstructed by the entropy decoding unit  701  to reconstruct frequency component values. In particular, the dequantization unit  702  reconstructs the frequency component values by multiplying the integer values reconstructed by the entropy decoding unit  701  by a quantization parameter. 
     The frequency domain inverse transform unit  703  transforms the frequency component values in the frequency domain reconstructed by the dequantization unit  702  into data in the color space used in the moving picture coding apparatus to reconstruct second residual data corresponding to the differences between the residual data for each of the color components constituting the color space. 
     The residue inverse transform unit  704  generates first residual data corresponding to the sum of the second residual data in the color space used in the moving picture coding apparatus that are reconstructed by the frequency domain inverse transform unit  703 . In particular, the residue inverse transform unit  704  generates first residual data corresponding to the sum of the second residual data for each of the Y component, Co component, and Cg component in the YCgCo color space, using Equation set (3). Alternatively, the residue inverse transform unit  704  generates first residual data corresponding to the sum of the second residual data for each of the R component, G component, and B component in the RGB color space using Equation Set (4). 
     The motion compensation unit  705  compensates for the motion between the current picture and the reference picture according to the inter prediction mode used in the moving picture coding apparatus. In other words, the motion compensation unit  705  compensates for the motion between the current picture and the reference picture according to inter prediction modes independently applied to the color components of the color space or an inter prediction mode commonly applied to all the color components of the color space. 
     In other words, the motion compensation unit  705  compensates for the motion between the current picture and the reference picture according to a single inter prediction mode commonly applied to all the color components. In particular, the motion compensation unit  705  generates predicted pictures for the current picture using motion vectors, which are independently applied to the color components in units of blocks that are independent for the color components. Alternatively, the motion compensation unit  705  generates predicted pictures for the current picture using a single motion vector, which is commonly applied to all the color components in units of blocks that are common to the color components. In other words, the motion compensation unit  705  generated predicted pictures for the current picture from the reference picture in units of blocks corresponding to the size of blocks commonly applied to all the color components using the motion vector commonly applied to all the color components. 
     The intra prediction unit  706  predicts blocks constituting the current picture from adjacent pixels in a picture reconstructed by the adder  707 , according to the intra prediction mode used in the moving picture coding apparatus, and generates predicted pictures constituted by the predicted blocks. In other words, the intra prediction unit  706  predicts blocks constituting the current picture from adjacent pixels in a picture reconstructed by the adder  707 , according to the intra prediction modes independently applied to the color components or a intra prediction mode commonly applied to all the color components, and generates predicted pictures constituted by the predicted blocks. In particular, the intra prediction unit  706  predicts blocks constituting the current picture from adjacent pixels indicated by the prediction direction used in the moving picture coding apparatus and generates predicted pictures constituted by the predicted blocks. In other words, the intra prediction unit  706  predicts blocks constituting the current picture from adjacent pixels indicated by the prediction directions, which are independently applied to the color components in units of blocks that are independent for the color components, and generates predicted pictures constituted by the predicted blocks. Alternatively, the intra prediction unit  706  predicts blocks constituting the current picture in units of blocks corresponding to the size of blocks commonly applied to all the color components, from adjacent pixels for each of the color components indicated by the prediction direction commonly applied to all the color components. 
     The adder  707  generates a reconstructed picture corresponding to the sum of the predicted pictures generated by the motion compensation unit  705  or the intra prediction unit  706  and the first residual data generated by the residue inverse transform unit  704 . For example, the adder  707  generates a reconstructed picture in the YCgCo color space by calculating the sum of the predicted pictures for the Y component, Co component, and Cg component generated by the motion compensation unit  705  or the intra prediction unit  706  and the first residual data generated by the residue inverse transform unit  704 . Alternatively, the adder  707  generates a reconstructed picture in the RGB color space by calculating the sum of the predicted pictures for the R component, G component, and B component generated by the motion compensation unit  705  or the intra prediction unit  706  and the first residual data generated by the residue inverse transform unit  704 . 
     The filter  708  improves the quality of the reconstructed picture by smoothening distortion at block boundaries in the reconstructed picture generated by the adder  707 . However, the filter  708  is not used for a high bit rate moving picture because high frequency components are likely to be lost. 
       FIGS. 8A and 8B  are flowcharts of a moving picture coding method according to an embodiment of the present invention. 
     Referring to  FIGS. 8A and 8B , a moving picture coding method according to an embodiment of the present invention includes operations performed in time-series in the moving picture coding apparatus of  FIG. 1 . The descriptions of the moving picture coding apparatus with reference to  FIG. 1  apply to the moving picture coding method in the present embodiment, and thus the descriptions thereof will not be repeated here. 
     In operation  801 , the moving picture coding apparatus selects a color space from among a plurality of color spaces based on the characteristics of a current picture. 
     In operation  802 , the moving picture coding apparatus determines whether to perform inter prediction or intra prediction. Operation  803  is performed for inter prediction, or operation  806  is performed for intra prediction. 
     In operation  803 , the moving picture coding apparatus selects inter prediction modes to independently applied to the color components constituting the color space selected in operation  801  or an inter prediction mode to be commonly applied to all the color components constituting the color space selected in operation  801 . 
     In operation  804 , the moving picture coding apparatus selects motion vectors to be independently applied to the color components constituting the color space or selects a motion vector to be commonly applied to all the color components constituting the color space according to the inter prediction mode selected in operation  803 . 
     In operation  805 , the moving picture coding apparatus generates predicted pictures for the current picture from a reference picture using the motion vector selected in operation  804 . 
     In operation  806 , the moving picture coding apparatus selects intra prediction modes to independently applied to the color components constituting the color space selected in operation  801  or a intra prediction mode to be commonly applied to all the color components constituting the color space selected in operation  801 . 
     In operation  807 , the moving picture coding apparatus selects prediction directions to be independently applied to the color components constituting the color space, or selects a prediction direction to be commonly applied to all the color components constituting the color space from among predicted directions for the color components, according to the intra prediction mode selected in operation  806 . 
     In operation  808 , the moving picture coding apparatus predicts blocks constituting the current picture from adjacent pixels indicated by the predicted direction selected in operation  807  and generates predicted pictures constituted by the predicted blocks. 
     In operation  809 , the moving picture coding apparatus generates first residual data corresponding to differences between the current picture and the predicted picture for each of the color components generated in operation  805  or  808 . 
     In operation  810 , the moving picture coding apparatus generates second residual data corresponding to differences between the first residual data. 
     In operation  811 , the moving picture coding apparatus transforms the second residual data in the color space generated in operation  810  into values in a frequency domain. 
     In operation  812 , the moving picture coding apparatus quantizes the values transformed in operation  811 . 
     In operation  813 , the moving picture coding apparatus generates a bitstream by entropy-coding the values quantized in operation  812  and outputs the bitstream. 
     In operation  814 , the moving picture coding apparatus reconstructs frequency component values by dequantizing the values quantized in operation  812 . 
     In operation  815 , the moving picture coding apparatus reconstructs the second residual data by transforming the frequency component values reconstructed in operation  814  into data in the color space. 
     In operation  816 , the moving picture coding apparatus generates first residual data corresponding to the sum of the second residual data reconstructed in operation  815 . 
     In operation  817 , the moving picture coding apparatus generates a reconstructed picture corresponding to the sum of the predicted pictures for the color components generated in operation  805  or  808  and the first residual data generated in operation  816 . 
     In operation  818 , the moving picture coding apparatus improves the quality of the reconstructed picture by smoothening distortion at block boundaries in the reconstructed picture generated in operation  816 . 
       FIG. 9  is a flowchart of a moving picture decoding method according to an embodiment of the present invention. 
     Referring to  FIG. 9 , a moving picture decoding method according to an embodiment of the present invention includes operations performed in time-series in the moving picture decoding apparatus of  FIG. 7 . The descriptions of the moving picture decoding apparatus with reference to  FIG. 7  apply to the moving picture decoding method in the present embodiment, and thus the descriptions thereof will not be repeated here. 
     In operation  901 , the moving picture decoding apparatus reconstructs integer values by entropy-decoding the bitstream output from a moving picture coding apparatus such as, by way of a non-limiting example, that of  FIG. 1 . 
     In operation  902 , the moving picture decoding apparatus reconstructs frequency component values by dequantizing the integer values reconstructed in operation  902 . 
     In operation  903 , the moving picture decoding apparatus reconstructs the second residual data corresponding to the differences between the first residual data constituting the color space used in the moving picture coating apparatus among a plurality of color spaces by transforming the frequency component values in the frequency domain reconstructed in operation  902  into data in the color space used in the moving picture coding apparatus. 
     In operation  904 , the moving picture decoding apparatus generates first residual data corresponding to the sum of the second residual data reconstructed in operation  903  in the color spaced used in the moving picture coding apparatus. 
     In operation  905 , the moving picture coding apparatus determines whether to perform inter prediction or intra prediction. Operation  906  is performed for inter prediction, or operation  907  is performed for intra prediction. 
     In operation  906 , the moving picture decoding apparatus compensates for the motion between a current picture and a reference picture according to inter prediction modes independently applied to the color components constituting the color space or an inter prediction mode commonly applied to all the color components. 
     In operation  907 , the moving picture decoding apparatus predicts blocks constituting the current picture from adjacent pixels in a reconstructed picture according to the inter prediction modes independently applied to the color components or the single inter prediction mode commonly applied to the color components, and generates a predicted picture constituted by the predicted blocks. 
     In operation  908 , the moving picture decoding apparatus generates a reconstructed picture corresponding to the sum of the predicted pictures generated in operation  906  or  907  and the first residual data generated by the residue inverse transform unit  704 . 
     In operation  909 , the moving picture decoding apparatus improves the quality of the reconstructed picture generated in operation  908  by smoothening distortion at block boundaries in the reconstructed picture. 
       FIG. 10  is the results of a simulation test according to embodiments of the present invention. 
     As is apparent from Table  1001  and Table  1002  in  FIG. 10 , in the simulation test, RCT and IPP were performed in a single prediction mode. The results were compared with the results of coding according to H.264/MPEG-4 AVC FRExt. JM9.3 reference software was used for the simulation test. The test conditions are as follows. Film and Viper sequences (1920×1088@24 Hz, progressive) were used as a test material, the search range was 64, quantization parameters were 12, 18, 24, 30, and 36, the number of reference pictures, i.e., reference frames, was 3, an entropy coding scheme was CABAC, the RD-optimized mode selection was “ON”, the GOP (Group of Pictures) structure was IBBP, and the number of slice groups was 1. 
     The methods used in the simulation test are as follows. IPP is a method according to an embodiment of the present invention using IPP and single mode prediction, RCT is a method according to an embodiment of the present invention using RCT and single mode prediction, RCT (FREXT) is RGB coding using YCgCo for residual color transform, and YCgCo is external YCgCo coding in which RGB input data are converted to YCgCo before being coded. In particular, to evaluate the fidelity of the reconstructed RGB images, the average of peak signal to noise rates (PSNR) of all the color components, i.e., the average of RGB components, was measured. The results are shown in Table  1001  of  FIG. 10 . In addition, the PSNR gain in each of the methods over the YCgCo method at two high bit-rates (20 Mbps and 60 Mbps) appears in Table  1001 . As shown in Table  1002 , the results of Y PSNR in YCgCo domain with approximately equal fidelity for chroma channels were compared in order to isolate the effect on a luma channel (most important channel). 
       FIG. 11  is a graph comparatively showing the coding efficiencies of various methods according to embodiments of the present invention in a lossless mode. 
     In the near future, it will be important to support lossless and near-lossless video compression. In H.264/MPEG-4 AVC FRExt, lossless coding can be achieved by skipping frequency domain transform and quantization. The methods according to en embodiment of the present invention can be applied to lossless coding when frequency domain transform and quantization are skipped. As is apparent from  FIG. 11 , the efficiency of the method using IPP and single mode prediction is highest in lossless coding. 
       FIGS. 12A and 12B  are rate distortion (RD) curves obtained through simulation of embodiments of the present invention. 
     In this simulation, only intra coding was performed. In particular, to evaluate the coding performance when independent intra prediction modes are used, all the color components were treated as monochromatic images. 
     Embodiments of the present invention can be written as computer programs and can be implemented in general-use digital computers that execute the programs using a computer readable recording medium. In addition, the data structures used in the embodiments of the present invention can be written to a computer readable medium by various means. 
     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), and storage media such as carrier waves (e.g., transmission through the Internet). 
     According to the above-described embodiments of the present invention, since residual data are generated according to a single prediction mode commonly applied to all the color components, the correlation between the residual data is high. As a result, the coding efficiency of moving pictures increases. In addition, according to the present invention, the coding efficiency of a moving picture can be increased using a single color space adaptively selected among a plurality of color spaces according to the characteristics of the moving picture. Furthermore, according to the present invention, the coding efficiency of moving pictures can be maximized by applying all the methods described above. 
     Although a few embodiments of the present invention have been shown and described, the present invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.
An image decoding method is provided. The image decoding method includes obtaining information about a color representation from a bitstream, entropy decoding a residue, which corresponds to a difference between a current image and a predicted image of the current image, and reconstructing the current image by using the entropy decoded residue and the predicted image, based on the information about the color representation.