Abstract:
A video decoding apparatus using an intra prediction, includes: a decoder to obtain mode information of a current block and residual sub-blocks corresponding to a plurality of sub-blocks included in the current block, from a bitstream; an intra predictor to select an intra prediction mode of the current block from among a plurality of intra prediction modes, based on the obtained mode information, identify the plurality of sub-blocks of the current block, and sequentially predict the plurality of sub-blocks by using neighboring pixels adjacent to each of the plurality of sub-blocks; an inverse quantization and transform unit configured to inversely quantize and then inversely transform each of the residual subblocks; and an adder configured to sequentially reconstruct the plurality of sub-blocks by adding each of the predicted sub-blocks to an inversely quantized and inversely transformed residual sub-block corresponding thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/330,156, filed Jul. 14, 2014, which is a continuation of U.S. patent application Ser. No. 13/122,123, filed Mar. 31, 2011, which is the national phase application of International Patent Application No. PCT/KR2009/005462, filed Sep. 24, 2009, which is based upon and claims priority to Korean Patent Application No. 10-2008-0096356, filed on Oct. 1, 2008. The disclosures of the above-listed application are hereby incorporated by reference herein in their entirely. 
    
    
     FIELD 
     The present disclosure relates to a method and apparatus for image encoding/decoding. 
     BACKGROUND ART 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Moving Picture Experts Group (MPEG) and Video Coding Experts Group (VCEG) have developed an improved and excellent video compression technology over existing MPEG-4 Part 2 and H.263 standards. The standard is named H.264/AVC (Advanced Video Coding) and was released simultaneously as MPEG-4 Part 10 AVC and ITU-T Recommendation H.264. Such H.264/AVC (hereinafter referred to as ‘H.264’) uses a spatial predictive coding method, which is different from known video coding-related international standards such as MPEG-1, MPEG-2, MPEG-4 Part 2 Visual and the like. 
     Known video coding methods use “intra prediction” for coefficients transformed in discrete cosine transform domain (or DCT transform domain) to seek higher encoding efficiency, resulting in degradation of the subjective video quality at low band transmission bit rates. However, H.264 adopts a method of encoding based on a spatial intra prediction in a spatial domain rather than in a transform domain. 
     An encoder using an encoding method based on the known spatial intra prediction predicts pixels of a block to currently encode from pixels in the previous blocks that have already undergone encoding and having been reproduced, encodes just the differences of the predicted block pixel values from pixel values of the actual block, and transmits the encoded difference information to a decoder. At this time, the encoder may either transmit parameters needed for prediction to the decoder or previously share the parameters needed for prediction with the decoder, so as to enable the decoder to predict the current block. Meanwhile, the decoder predicts information of a desired current block to be currently decoded by using information of neighbor blocks already decoded and reproduced, and reconstructs and reproduces the desired current block based on a sum of the predicted information and the difference information transmitted from the encoder. At this time also, if the decoder has received parameters need for the prediction from the encoder, the decoder uses the received parameters in predicting and decoding the current block. 
     However, in a prediction using the intra prediction according to the known video coding and video decoding, pixels of a current block to be currently encoded are predicted by referring to the already reconstructed information of adjacent pixels within a neighbor block around the current block (usually a block located at the left or above the current block). Here, the pixels of the current block to be encoded and the adjacent pixels of the neighbor block referred to in order to predict the current block may be spaced apart from each other by a considerably long distance. The inventor(s) has experienced that at this time, the accuracy of the prediction may be degraded too much, which may lower the quality of video encoding. The inventor(s) has experienced that as a result, the low quality video encoding may yield an unsatisfactory reproduction state when the image encoded with the low quality is restored and reproduced. 
     SUMMARY 
     The present disclosure has been made in an effort to improve the accuracy of prediction on a current block to be encoded or decoded in encoding or decoding an image, and thus provide a satisfactory image reproduction quality for the image by improved coding efficiency. 
     In accordance with some embodiments of the present disclosure, a video decoding apparatus using an intra prediction comprises a decoder, an intra predictor, an inverse quantization and transform unit, and an adder. The decoder is configured to obtain mode information of a current block and residual sub-blocks corresponding to a plurality of sub-blocks included in the current block, from a bitstream. The intra predictor is configured to select an intra prediction mode of the current block from among a plurality of intra prediction modes, based on the obtained mode information, identify the plurality of sub-blocks of the current block, and sequentially predict the plurality of sub-blocks by using neighboring pixels adjacent to each of the plurality of sub-blocks, wherein each of the plurality of sub-blocks is predicted by using the same intra prediction mode of the current block selected based on the obtained mode information. The inverse quantization and transform unit is configured to inversely quantize and then inversely transform each of the residual subblocks. And the adder is configured to sequentially reconstruct the plurality of sub-blocks by adding each of the predicted sub-blocks to an inversely quantized and inversely transformed residual sub-block corresponding thereto. Herein, one or more pixels of a pre-reconstructed sub-block within the current block are used as part of the neighboring pixels for predicting at least one non-reconstructed sub-blocks adjacent to the pre-reconstructed sub-block. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic block diagram of an image encoding apparatus according to at least one embodiment of the present disclosure; 
         FIG. 2  is a flowchart of an image encoding according to at least one embodiment of the present disclosure; 
         FIG. 3  is an example of a current block and a plurality of sub-blocks decomposed from the current block for application of an image encoding method according at least one embodiment of the present disclosure; 
         FIG. 4  is an example of an intra prediction encoding of a plurality of sub-blocks according to a sequence of a raster scan when the intra prediction mode of the current block is a horizontal mode; 
         FIG. 5  is an example of an intra prediction encoding of a plurality of sub-blocks according to a sequence of a raster scan when the intra prediction mode of the current block is a vertical mode; 
         FIG. 6  is an example of an intra prediction encoding of a plurality of sub-blocks according to a sequence of a raster scan when the intra prediction mode of the current block is a DC mode; 
         FIG. 7  is an example of an intra prediction encoding of a plurality of sub-blocks according to a zigzag sequence when the intra prediction mode of the current block is a horizontal mode; 
         FIG. 8  is an example of an intra prediction encoding of a plurality of sub-blocks according to a zigzag sequence when the intra prediction mode of the current block is a vertical mode; 
         FIG. 9  is an example of an intra prediction encoding of a plurality of sub-blocks according to a zigzag sequence when the intra prediction mode of the current block is a DC mode; 
         FIGS. 10A and 10B  are views of an example of a process of performing an intra prediction on one sub-block in a horizontal mode; 
         FIGS. 11A and 11B  are views of an example of a process of performing an intra prediction on one sub-block in a vertical mode; 
         FIGS. 12A and 12B  are views of an example of a process of performing an intra prediction on one sub-block in a DC mode; 
         FIG. 13  is a view for describing a process of generating a residual block for a certain sub-block on which an intra prediction has been performed; 
         FIG. 14  is a schematic block diagram of an image decoding apparatus  1400  according to at least one embodiment of the present disclosure; and 
         FIG. 15  is a flowchart of an image decoding method according to at least one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, aspects of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, the same elements will be designated by the same reference numerals although they are shown in different drawings. Further, in the following description of the present disclosure, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure rather unclear. 
     Also, in describing the components of the present disclosure, there may be terms used like first, second, A, B, (a), and (b). These are solely for the purpose of differentiating one component from the other but not to imply or suggest the substances, order or sequence of the components. If a component were described as ‘connected’, ‘coupled’, or ‘linked’ to another component, they may mean the components are not only directly ‘connected’, ‘coupled’, or ‘linked’ but also are indirectly ‘connected’, ‘coupled’, or ‘linked’ via a third component. 
       FIG. 1  is a schematic block diagram of an image encoding apparatus  100  according to at least one of the present disclosure. 
     Referring to  FIG. 1 , the image encoding apparatus  100  according to an aspect of the present disclosure includes a block decomposer  110  for decomposing a current block to be encoded in an input original image into a plurality of sub-blocks, and an intra prediction encoder  120  for generating a bit stream for the current block by performing an intra prediction encoding by referring to already encoded or decoded adjacent pixel information of each sub-block. Each of the block decomposer  110  and the intra prediction encoder  120  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs) specified for respectively corresponding operations and functions described herein. The image encoding apparatus  100  further comprises input units (not shown in  FIG. 1 ) such as one or more buttons, a touch screen, a mic and so on, and output units (not shown in  FIG. 1 ) such as a display, an indicator and so on. The image encoding apparatus  100  further comprises communication modem(s) to receive and/or communication signals to thereby communicate with an image decoding apparatus through wire or wireless networks (herein, the wire or wireless networks include, for example, one or more network interfaces including, but not limited to, cellular, Wi-Fi, LAN, WAN, CDMA, WCDMA, GSM, LTE and EPC networks, and cloud computing networks). 
     The block divider  110  may divide the current block into multiple sub-blocks based on the unit of frequency conversion. 
     Referring to  FIG. 1 , the intra prediction encoder  120  includes: an intra predictor  121  for selecting one sub-block among a plurality of sub-blocks according to a sequence of a raster scan or a zigzag sequence, and generating a predicted sub-block by performing an intra prediction for the selected sub-block with reference to adjacent pixel information of each corresponding sub-block based on an intra prediction mode equal to the intra prediction mode of the current block; a subtractor  122  for generating a residual sub-block by calculating a difference between the selected single sub-block and a corresponding predicted sub-block; a transform unit  123  for performing a Discrete Cosine Transform (DCT) on the generated residual sub-block; a quantizer  124  for quantizing the transformed residual sub-block; and an encoder  125  for encoding the quantized residual sub-block. Here, each of the divided sub-blocks is a transform-based block in the transform unit  123 , and may be a frequency transform-based block. Each of the intra predictor  121 , the subtractor  122 , the transform unit  123 , the quantizer  124 , and the encoder  125  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs) specified for respectively corresponding operations and functions described herein. 
     The sequence in which a plurality of sub-blocks are intra-prediction-encoded may be a sequence of a raster scan or a zigzag sequence. It may be more efficient to perform the intra-prediction-encoding by selecting one sub-block from a plurality of sub-blocks according to the sequence of a raster scan than the zigzag sequence. Although only two cases including the sequence of a raster scan and the zigzag sequence are discussed as the sequences by which a plurality of sub-blocks can be intra-prediction-encoded in the present disclosure, it is also possible to change the sequence according to the characteristic of the image. 
     The transform unit  123  described above may perform a Discrete Cosine Transform (DCT). The encoder  125  described above may perform an entropy encoding, in which the length of a code indicating a symbol changes according to the probability that the symbol occurs. 
     The intra prediction encoder  120  described above may further include a dequantizer  126  for dequantizing a quantized residual sub-block; an inverse transform unit  127  for performing an inverse transform on the dequantized residual sub-block; an intra-compensator  128  for performing an intra-compensation on the inverse-transformed residual sub-block, thereby generating a reference block; and a reference block storage unit  129  for storing the generated reference block. 
     The reference blocks stored in the reference block storage unit  129  described above contain adjacent pixel information of each sub-block, which will be referred to when an intra prediction is performed for a sub-block to be selected after the single sub-block selected from multiple sub-blocks according to the sequence of a raster scan or the zigzag sequence. 
     The intra prediction mode of the current block mentioned above may be, for example, one of a vertical mode, a horizontal mode, and a Direct Current (DC) mode in the 16×16 intra prediction mode. 
     The intra prediction of each sub-block employs the same intra prediction mode as the intra prediction mode of the current block. For example, based on an assumption that the current block is a 16×16 block and each of the sub-blocks decomposed from the current block is a 4×4 block, when the 16×16 intra prediction mode for the current block is a vertical mode, the 4×4 intra prediction mode for each sub-block is also a vertical mode. Further, when the 16×16 intra prediction mode for the current block is a horizontal mode, the 4×4 intra prediction mode for each sub-block is also a horizontal mode. When the 16×16 intra prediction mode for the current block is a DC mode, the 4×4 intra prediction mode for each sub-block is also a DC mode. 
       FIG. 2  is a flowchart of an image encoding according to at least one embodiment of the present disclosure. 
     Referring to  FIG. 2 , an image encoding method performed by an image encoding apparatus  100  according to an embodiment of the present disclosure includes: a block decomposing step (S 200 ) of decomposing a current block to be encoded in an input original image into a plurality of sub-blocks; and an intra prediction encoding step (S 202 ) of generating a bit stream for the current block by performing an intra prediction encoding by referring to reference pixel information for each sub-block already encoded and decoded. 
     The intra prediction encoding step (S 202 ) may include: a step (S 2020 ) of selecting a single sub-block among a plurality of sub-blocks according to a sequence of a raster scan or a zigzag sequence, and generating a predicted sub-block by performing an intra prediction for the selected single sub-block with reference to adjacent pixel information of each corresponding sub-block based on an intra prediction mode equal to the intra prediction mode of the current block; a step (S 2022 ) of generating a residual sub-block by calculating a difference between the selected single sub-block and a corresponding predicted sub-block; a step (S 2024 ) of transforming the generated residual sub-block; a step (S 2026 ) of quantizing the transformed residual sub-block; and a step (S 2028 ) of encoding the quantized residual sub-block. 
     After the quantization step (S 2026 ), the intra prediction encoding step (S 202 ) may further include the steps of: dequantizing a quantized residual sub-block; performing an inverse transform on the dequantized residual sub-block; performing an intra-compensation on the inverse-transformed residual sub-block, thereby generating a reference block; and storing the generated reference block. Those further included steps are performed in parallel with the intra prediction step (S 2020 ), the residual sub-block generating step (S 2022 ), the transform step (S 2024 ), the quantization step (S 2026 ), and the encoding step (S 2028 ). That is to say, the steps added in order to generate and store the reference block may be simultaneously performed with the intra prediction encoding process for the sub-block. 
     In order to apply the image encoding method according an embodiment of the present disclosure as described above, an original image is decomposed into Macro Blocks (MBs) each having a predetermined size. In the present disclosure, the decomposed macro block is called a “current block”. 
     Further, in the image encoding method according an embodiment of the present disclosure, a current block, which is a 16×16 macro block, is decomposed into a plurality of Sub-Blocks (SBs), each of which is a basic unit in the intra prediction encoding. Further, each sub-block includes a plurality of pixels, and each pixel contains information of a corresponding part in the original image. 
     The current block, sub-block, and pixel as described above will now be described again with reference to  FIG. 3  showing an example of a 16×16 block. 
       FIG. 3  illustrates an example of a current block and a plurality of sub-blocks decomposed from the current block for the application of an image encoding method according an embodiment of the present disclosure. 
     In  FIG. 3 , a current block applied to an image encoding according an embodiment of the present disclosure is a 16×16 macro block, which is identified by a box having a contour defined by a thick solid line. 
     Further, in  FIG. 3 , the current block, which is a 16×16 macro block defined by a thick solid line, is decomposed into 16 sub-blocks each having a 4×4 block size. The 16 decomposed sub-blocks are named “SB  1 , SB  2 , SB  3 , . . . , and SB  16 ” and are indicated by boxes defined by broken lines. 
     Referring to  FIG. 3 , each sub-block includes 16 pixels, and each pixel contains information on the image. 
     In  FIG. 3 , V( 0 ) to V( 15 ) and H( 0 ) to H( 15 ) correspond to pixel information on adjacent macro blocks of the current block (macro block), and the pixel information on the adjacent macro blocks correspond to pixel information of already encoded and decoded macro blocks. 
     By using the current block including the 16 sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) illustrated as an example in  FIG. 3 , an image encoding according to an embodiment of the present disclosure will now be described with reference to  FIGS. 1 and 2 . 
     In an image encoding according to an embodiment of the present disclosure, a current block having a 16×16 block size is decomposed into 16 sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ), and an intra prediction encoding is then performed for each of the sub-blocks according to a predetermined sequence. Here, the predetermined sequence may be a sequence of a raster scan or a zigzag sequence. 
     In other words, the 16 sub-blocks are scanned according to a sequence of a raster scan or a zigzag sequence, and an intra prediction encoding is then performed for a first sub-block among the scanned sub-blocks. Then, a next sub-block is determined among the 16 sub-blocks scanned according to the sequence of a raster scan or the zigzag sequence, and an intra prediction encoding is then performed for the next sub-block. Thereafter, the other sub-blocks are also subjected to the intra prediction encoding according to the sequence of a raster scan or the zigzag sequence. After the intra prediction encoding is completely performed for all the decomposed sub-blocks, a bit stream for the current block is generated by using the result of the intra prediction encoding for each sub-block. 
     The intra prediction encoding described above corresponds to a process performed for each sub-block, and may include a series of steps including an intra prediction, a transform, a quantization, and an encoding. 
     In the intra prediction step included in the intra prediction encoding performed for each sub-block, an intra prediction mode is selected for the intra prediction. In the present disclosure, the same intra prediction mode as the intra prediction mode of the current block is selected as an intra prediction mode for each sub-block. 
     For example, when the intra prediction mode of the current block is a horizontal mode, the intra prediction mode of the decomposed sub-blocks is also a horizontal mode. When the intra prediction mode of the current block is a vertical mode, the intra prediction mode of the decomposed sub-blocks is also a vertical mode. Also, when the intra prediction mode of the current block is a DC mode, the intra prediction mode of the decomposed sub-blocks is also a DC mode. 
     An image encoding through an intra prediction encoding of each sub-block when the intra prediction mode of the current block is a horizontal mode will be described in detail with reference to  FIGS. 4 and 7 , an image encoding through an intra prediction encoding of each sub-block when the intra prediction mode of the current block is a vertical mode will be described in detail with reference to  FIGS. 5 and 8 , and an image encoding through an intra prediction encoding of each sub-block when the intra prediction mode of the current block is a DC mode will be described in detail with reference to  FIGS. 6 and 9 . In the meantime, the intra prediction encoding shown in  FIGS. 4, 5, and 6  corresponds to an intra prediction encoding according to a sequence of a raster scan, while the intra prediction encoding shown in  FIGS. 7, 8, and 9  corresponds to an intra prediction encoding according to a zigzag sequence. 
       FIG. 4  illustrates an example of an intra prediction encoding of a plurality of sub-blocks according to a sequence of a raster scan when the intra prediction mode of the current block is a horizontal mode. 
     Referring to  FIG. 4 , in order to encode the current block, the 16 decomposed sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) are sequentially intra-prediction-encoded according to the sequence of a raster scan. That is, the intra prediction encoding is performed in the sequence of sub-blocks SB  1 , SB  2 , SB  3 , SB  4 , SB  5 , SB  6 , SB  7 , SB  8 , SB  9 , SB  10 , SB  11 , SB  12 , SB  13 , SB  14 , SB  15 , and SB  16 . 
     In  FIG. 4 , since it is assumed that the intra prediction mode of the current block is a horizontal mode, the intra prediction mode of the sub-block is also the horizontal mode. Therefore, the image encoding apparatus  100  performs the intra prediction in the horizontal mode on each sub-block by referring to the adjacent pixel information. 
     As shown in  FIG. 4 , by referring to adjacent pixel information (H 0 ), (H 1 ), (H 2 ), and (H 3 ), which are adjacent to the first sub-block (SB  1 ), among pixel information of macro blocks adjacent to the current block, the image encoding apparatus  100  performs an intra prediction encoding on the first sub-block (SB  1 ), and simultaneously generates and stores a reference block. The stored reference block includes adjacent pixel information, which is referred to during an intra prediction encoding process for another sub-block. 
     In  FIG. 4 , during the intra prediction encoding for the first sub-block (SB  1 ), pixel information included in the reference block, which has been already encoded, decoded, and stored, is indicated by indices “1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14, 1-15, and 1-16”. In each index of “first number-second number” indicating the pixel information, the first number identifies a corresponding sub-block, and the second number identifies a corresponding pixel within the corresponding sub-block. 
     Following SB  1 , the image encoding apparatus  100  performs an intra prediction encoding on SB  2  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  2  includes pixel information “1-4, 1-8, 1-12, and 1-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  1 . The reason why “1-4, 1-8, 1-12, and 1-16” are used for reference as adjacent pixel information from among the pixel information included in the reference block is that the intra prediction encoding on SB  2  is performed in the horizontal mode. 
     Following SB  2 , the image encoding apparatus  100  performs an intra prediction encoding on SB  3  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  3  includes pixel information “2-4, 2-8, 2-12, and 2-16”, which are adjacent to SB  3 . 
     Following SB  3 , the image encoding apparatus  100  performs an intra prediction encoding on SB  4  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  4  includes pixel information “3-4, 3-8, 3-12, and 3-16”, which are adjacent to SB  4 . 
     Following SB  4 , the image encoding apparatus  100  performs an intra prediction encoding on SB  5  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  5  includes pixel information “(H 4 ), (H 5 ), (H 6 ), and (H 7 )”, which are adjacent to SB  5 . 
     As in the intra prediction encoding process for SB  1 , SB  2 , SB  3 , SB  4 , and SB  5  described above, the image encoding apparatus  100  performs an intra prediction encoding on the remaining sub-blocks according to the sequence of a raster scan by referring to already encoded and decoded adjacent pixel information (pixel information having a second number of 4, 8, 12, or 16 in indices of pixel information of each sub-block) in an adjacent sub-block or already encoded and decoded adjacent pixel information in an adjacent macro block. 
       FIG. 5  illustrates an example of an intra prediction encoding of a plurality of sub-blocks according to a sequence of a raster scan when the intra prediction mode of the current block is a vertical mode. 
     Referring to  FIG. 5 , in order to encode the current block, the 16 decomposed sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) are sequentially intra-prediction-encoded according to the sequence of a raster scan. That is, the intra prediction encoding is performed in the sequence of sub-blocks SB  1 , SB  2 , SB  3 , SB  4 , SB  5 , SB  6 , SB  7 , SB  8 , SB  9 , SB  10 , SB  11 , SB  12 , SB  13 , SB  14 , SB  15 , and SB  16 . 
     In  FIG. 5 , since it is assumed that the intra prediction mode of the current block is a vertical mode, the intra prediction mode of the sub-block is also the vertical mode. Therefore, the image encoding apparatus  100  performs the intra prediction in the vertical mode on each sub-block by referring to the adjacent pixel information. 
     As shown in  FIG. 5 , by referring to adjacent pixel information (V 0 ), (V 1 ), (V 2 ), and (V 3 ), which are adjacent to the first sub-block (SB  1 ), among pixel information of macro blocks adjacent to the current block, the image encoding apparatus  100  performs an intra prediction encoding on the first sub-block (SB  1 ), and simultaneously generates and stores a reference block. The stored reference block includes adjacent pixel information, which is referred to during an intra prediction encoding process for another sub-block. 
     In  FIG. 5 , during the intra prediction encoding for the first sub-block (SB  1 ), pixel information included in the reference block, which has been already encoded, decoded, and stored, is indicated by indices “1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14, 1-15, and 1-16”. In each index of “first number-second number” indicating the pixel information, the first number identifies a corresponding sub-block, and the second number identifies a corresponding pixel within the corresponding sub-block. 
     Following SB  1 , the image encoding apparatus  100  performs an intra prediction encoding on SB  2  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  2  includes pixel information “(V 4 ), (V 5 ), (V 6 ), and (V 7 )”, which are adjacent to the SB  2 , from among the pixel information of the macro blocks adjacent to the current block. 
     Following SB  2 , the image encoding apparatus  100  performs an intra prediction encoding on SB  3  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  3  includes pixel information “(V 8 ), (V 9 ), (V 10 ), and (V 11 )”, which are adjacent to the SB  3 , from among the pixel information of the macro blocks adjacent to the current block. 
     Following SB  3 , the image encoding apparatus  100  performs an intra prediction encoding on SB  4  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  4  includes pixel information “(V 12 ), (V 13 ), (V 14 ), and (V 15 )”, which are adjacent to the SB  4 , from among the pixel information of the macro blocks adjacent to the current block. 
     Following SB  4 , the image encoding apparatus  100  performs an intra prediction encoding on SB  5  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  5  includes pixel information “1-13, 1-14, 1-15, and 1-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  1 , which is a sub-block adjacent to SB  5 . 
     As in the intra prediction encoding process for SB  1 , SB  2 , SB  3 , SB  4 , and SB  5  described above, the image encoding apparatus  100  performs an intra prediction encoding on the remaining sub-blocks according to the sequence of a raster scan by referring to already encoded and decoded adjacent pixel information (pixel information having a second number of 13, 14, 15, or 16 in indices of pixel information of each sub-block) in an adjacent sub-block or already encoded and decoded adjacent pixel information in an adjacent macro block. 
       FIG. 6  illustrates an example of an intra prediction encoding of a plurality of sub-blocks according to a sequence of a raster scan when the intra prediction mode of the current block is a DC mode. 
     Referring to  FIG. 6 , in order to encode the current block, the 16 decomposed sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) are sequentially intra-prediction-encoded according to the sequence of a raster scan. That is, the intra prediction encoding is performed in the sequence of sub-blocks SB  1 , SB  2 , SB  3 , SB  4 , SB  5 , SB  6 , SB  7 , SB  8 , SB  9 , SB  10 , SB  11 , SB  12 , SB  13 , SB  14 , SB  15 , and SB  16 . 
     In  FIG. 6 , since it is assumed that the intra prediction mode of the current block is a DC mode, the intra prediction mode of the sub-block is also the DC mode. Therefore, the image encoding apparatus  100  performs the intra prediction in the DC mode on each sub-block by referring to the adjacent pixel information. 
     As shown in  FIG. 6 , by referring to adjacent pixel information (H 0 ), (H 1 ), (H 2 ), (H 3 ), (V 0 ), (V 1 ), (V 2 ), (V 3 ) and M, which are adjacent to the first sub-block (SB  1 ), among pixel information of macro blocks adjacent to the current block, the image encoding apparatus  100  performs an intra prediction encoding on the first sub-block (SB  1 ), and simultaneously generates and stores a reference block. The stored reference block includes adjacent pixel information, which is referred to during an intra prediction encoding process for another sub-block. 
     In  FIG. 6 , during the intra prediction encoding for the first sub-block (SB  1 ), pixel information included in the reference block, which has been already encoded, decoded, and stored, is indicated by indices “1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14, 1-15, and 1-16”. In each index of “first number-second number” indicating the pixel information, the first number identifies a corresponding sub-block, and the second number identifies a corresponding pixel within the corresponding sub-block. 
     Following SB  1 , the image encoding apparatus  100  performs an intra prediction encoding on SB  2  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  2  includes pixel information “1-4, 1-8, 1-12, 1-16, (V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )”. From among the pixel information “1-4, 1-8, 1-12, 1-16, (V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )”, “1-4, 1-8, 1-12, and 1-16” correspond to pixel information in SB  1 , which is a sub-block adjacent to SB  2 , and “(V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )” correspond to pixel information adjacent to SB  2  from among the pixel information on the macro blocks adjacent to the current block. The reason why “1-4, 1-8, 1-12, 1-16, (V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )” are used for reference as adjacent pixel information in the intra prediction encoding on SB  2  as described above is that the intra prediction encoding on SB  2  is performed in the DC mode. 
     Following SB  2 , the image encoding apparatus  100  performs an intra prediction encoding on SB  3  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  3  includes pixel information “2-4, 2-8, 2-12, 2-16, (V 8 ), (V 9 ), (V 10 ), (V 11 ), and (V 7 )”. 
     Following SB  3 , the image encoding apparatus  100  performs an intra prediction encoding on SB  4  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  4  includes pixel information “3-4, 3-8, 3-12, 3-16, (V 12 ), (V 13 ), (V 14 ), (V 15 ), and (V 11 )”. 
     Following SB  4 , the image encoding apparatus  100  performs an intra prediction encoding on SB  5  according to the sequence of a raster scan. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  5  includes pixel information “(H 4 ), (H 5 ), (H 6 ), (H 7 ), 1-13, 1-14, 1-15, 1-16, and (H 3 )”. 
     As in the intra prediction encoding process for SB  1 , SB  2 , SB  3 , SB  4 , and SB  5  described above, the image encoding apparatus  100  performs an intra prediction encoding on the remaining sub-blocks according to the sequence of a raster scan by referring to already encoded and decoded adjacent pixel information (pixel information on pixels located at the right end and the lower end of each sub-block) in an adjacent sub-block or already encoded and decoded adjacent pixel information in an adjacent macro block. 
       FIG. 7  illustrates an example of an intra prediction encoding of a plurality of sub-blocks according to a zigzag sequence when the intra prediction mode of the current block is a horizontal mode. 
     Referring to  FIG. 7 , in order to encode the current block, the 16 decomposed sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) are sequentially intra-prediction-encoded according to the zigzag sequence. That is, the intra prediction encoding is performed in the sequence of sub-blocks SB  1 , SB  2 , SB  5 , SB  9 , SB  6 , SB  3 , SB  4 , SB  7 , SB  10 , SB  13 , SB  14 , SB  11 , SB  8 , SB  12 , SB  15 , and SB  16 . 
     In  FIG. 7 , since it is assumed that the intra prediction mode of the current block is a horizontal mode, the intra prediction mode of the sub-block is also the horizontal mode. Therefore, the image encoding apparatus  100  performs the intra prediction in the horizontal mode on each sub-block by referring to the adjacent pixel information. 
     As shown in  FIG. 7 , by referring to adjacent pixel information (H 0 ), (H 1 ), (H 2 ), and (H 3 ), which are adjacent to the first sub-block (SB  1 ), among pixel information of macro blocks adjacent to the current block, the image encoding apparatus  100  performs an intra prediction encoding on the first sub-block (SB  1 ), and simultaneously generates and stores a reference block. The stored reference block includes adjacent pixel information, which is referred to during an intra prediction encoding process for another sub-block. 
     In  FIG. 7 , during the intra prediction encoding for the first sub-block (SB  1 ), pixel information included in the reference block, which has been already encoded, decoded, and stored, is indicated by indices “1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14, 1-15, and 1-16”. In each index of “first number-second number” indicating the pixel information, the first number identifies a corresponding sub-block, and the second number identifies a corresponding pixel within the corresponding sub-block. 
     Following SB  1 , the image encoding apparatus  100  performs an intra prediction encoding on SB  2  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  2  includes pixel information “1-4, 1-8, 1-12, and 1-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  1 . The reason why “1-4, 1-8, 1-12, and 1-16” are used for reference as adjacent pixel information from among the pixel information included in the reference block is that the intra prediction encoding on SB  2  is performed in the horizontal mode. 
     Following SB  2 , the image encoding apparatus  100  performs an intra prediction encoding on SB  5  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  5  includes pixel information “H( 4 ), (H 5 ), (H 6 ), and (H 7 )”, which are adjacent to SB  5 , from among the pixel information on the macro blocks adjacent to the current block. 
     Following SB  5 , the image encoding apparatus  100  performs an intra prediction encoding on SB  9  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  9  includes pixel information “H( 8 ), (H 9 ), (H 10 ), and (H 11 )”, which are adjacent to SB  9 , from among the pixel information on the macro blocks adjacent to the current block. 
     Following SB  9 , the image encoding apparatus  100  performs an intra prediction encoding on SB  6  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  6  includes pixel information “5-4, 5-8, 5-12, and 5-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  5 . 
     As in the intra prediction encoding process for SB  1 , SB  2 , SB  5 , SB  9 , and SB  6  described above, the image encoding apparatus  100  performs an intra prediction encoding on the remaining sub-blocks according to the zigzag sequence by referring to already encoded and decoded adjacent pixel information (pixel information having a second number of 4, 8, 12, or 16 in indices of pixel information of each sub-block) in an adjacent sub-block or already encoded and decoded adjacent pixel information in an adjacent macro block. 
       FIG. 8  illustrates an example of an intra prediction encoding of a plurality of sub-blocks according to a zigzag sequence when the intra prediction mode of the current block is a vertical mode. 
     Referring to  FIG. 8 , in order to encode the current block, the 16 decomposed sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) are sequentially intra-prediction-encoded according to the zigzag sequence. That is, the intra prediction encoding is performed in the sequence of sub-blocks SB  1 , SB  2 , SB  5 , SB  9 , SB  6 , SB  3 , SB  4 , SB  7 , SB  10 , SB  13 , SB  14 , SB  11 , SB  8 , SB  12 , SB  15 , and SB  16 . 
     In  FIG. 8 , since it is assumed that the intra prediction mode of the current block is a vertical mode, the intra prediction mode of the sub-block is also the vertical mode. Therefore, the image encoding apparatus  100  performs the intra prediction in the vertical mode on each sub-block by referring to the adjacent pixel information. 
     As shown in  FIG. 8 , by referring to adjacent pixel information (V 0 ), (V 1 ), (V 2 ), and (V 3 ), which are adjacent to the first sub-block SB  1 , among pixel information of macro blocks adjacent to the current block, the image encoding apparatus  100  performs an intra prediction encoding on the first sub-block SB  1 , and simultaneously generates and stores a reference block. The stored reference block includes adjacent pixel information, which is referred to during an intra prediction encoding process for another sub-block. 
     In  FIG. 8 , during the intra prediction encoding for the first sub-block (SB  1 ), pixel information included in the reference block, which has been already encoded, decoded, and stored, is indicated by indices “1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14, 1-15, and 1-16”. In each index of “first number-second number” indicating the pixel information, the first number identifies a corresponding sub-block, and the second number identifies a corresponding pixel within the corresponding sub-block. 
     Following SB  1 , the image encoding apparatus  100  performs an intra prediction encoding on SB  2  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  2  includes pixel information “(V 4 ), (V 5 ), (V 6 ), and (V 7 )”, which are adjacent to the SB  2 , from among the pixel information of the macro blocks adjacent to the current block. 
     Following SB  2 , the image encoding apparatus  100  performs an intra prediction encoding on SB  5  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  5  includes pixel information “1-13, 1-14, 1-15, and 1-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  1 , which is a sub-block adjacent to SB  5 . The reason why “1-13, 1-14, 1-15, and 1-16” from among the pixel information included in the reference block stored for SB  1  is used for reference as adjacent pixel information is that the intra prediction encoding on SB  5  is performed in the vertical mode. 
     Following SB  5 , the image encoding apparatus  100  performs an intra prediction encoding on SB  9  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  9  includes pixel information of pixel information “5-13, 5-14, 5-15, and 5-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  5 , which is a sub-block adjacent to SB  9 . 
     Following SB  9 , the image encoding apparatus  100  performs an intra prediction encoding on SB  6  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  6  includes pixel information “2-13, 2-14, 2-15, and 2-16” from among the pixel information included in the reference block stored during the intra prediction encoding on SB  2 , which is a sub-block adjacent to SB  6 . 
     As in the intra prediction encoding process for SB  1 , SB  2 , SB  5 , SB  9 , and SB  6  described above, the image encoding apparatus  100  performs an intra prediction encoding on the remaining sub-blocks according to the zigzag sequence by referring to already encoded and decoded adjacent pixel information (pixel information having a second number of 13, 14, 15, or 16 in indices of pixel information of each sub-block) in an adjacent sub-block or already encoded and decoded adjacent pixel information in an adjacent macro block. 
       FIG. 9  illustrates an example of an intra prediction encoding of a plurality of sub-blocks according to a zigzag sequence when the intra prediction mode of the current block is a DC mode. 
     Referring to  FIG. 9 , in order to encode the current block, the 16 decomposed sub-blocks (SB  1 , SB  2 , SB  3 , . . . , and SB  16 ) are sequentially intra-prediction-encoded according to the zigzag sequence. That is, the intra prediction encoding is performed in the sequence of sub-blocks SB  1 , SB  2 , SB  5 , SB  9 , SB  6 , SB  3 , SB  4 , SB  7 , SB  10 , SB  13 , SB  14 , SB  11 , SB  8 , SB  12 , SB  15 , and SB  16 . 
     In  FIG. 9 , since it is assumed that the intra prediction mode of the current block is a DC mode, the intra prediction mode of the sub-block is also the DC mode. Therefore, the image encoding apparatus  100  performs the intra prediction in the DC mode on each sub-block by referring to the adjacent pixel information. 
     As shown in  FIG. 9 , by referring to adjacent pixel information (H 0 ), (H 1 ), (H 2 ), (H 3 ), (V 0 ), (V 1 ), (V 2 ), (V 3 ) and M, which are adjacent to the first sub-block (SB  1 ), among pixel information of macro blocks adjacent to the current block, the image encoding apparatus  100  performs an intra prediction encoding on the first sub-block (SB  1 ), and simultaneously generates and stores a reference block. The stored reference block includes adjacent pixel information, which is referred to during an intra prediction encoding process for another sub-block. 
     In  FIG. 9 , during the intra prediction encoding for the first sub-block (SB  1 ), pixel information included in the reference block, which has been already encoded, decoded, and stored, is indicated by indices “1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14, 1-15, and 1-16”. In each index of “first number-second number” indicating the pixel information, the first number identifies a corresponding sub-block, and the second number identifies a corresponding pixel within the corresponding sub-block. 
     Following SB  1 , the image encoding apparatus  100  performs an intra prediction encoding on SB  2  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  2  includes pixel information “1-4, 1-8, 1-12, 1-16, (V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )”. From among the pixel information “1-4, 1-8, 1-12, 1-16, (V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )”, “1-4, 1-8, 1-12, and 1-16” correspond to pixel information in SB  1 , which is a sub-block adjacent to SB  2 , and “(V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )” correspond to pixel information adjacent to SB  2  from among the pixel information on the macro blocks adjacent to the current block. The reason why “1-4, 1-8, 1-12, 1-16, (V 4 ), (V 5 ), (V 6 ), (V 7 ), and (V 3 )” are used for reference as adjacent pixel information in the intra prediction encoding on SB  2  as described above is that the intra prediction encoding on SB  2  is performed in the DC mode. 
     Following SB  2 , the image encoding apparatus  100  performs an intra prediction encoding on SB  5  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  5  includes pixel information “(H 4 ), (H 5 ), (H 6 ), (H 7 ), 1-13, 1-14, 1-15, 1-16, and (H 3 )”. From among the pixel information “(H 4 ), (H 5 ), (H 6 ), (H 7 ), 1-13, 1-14, 1-15, 1-16, and (H 3 )”, “(H 4 ), (H 5 ), (H 6 ), (H 7 ), and (H 3 )” correspond to pixel information adjacent to SB  5  from among the pixel information on the macro blocks adjacent to the current block, and “1-13, 1-14, 1-15, and 1-16” correspond to pixel information in SB  1 , which is a sub-block adjacent to SB  5 . 
     Following SB  5 , the image encoding apparatus  100  performs an intra prediction encoding on SB  9  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  9  includes pixel information “(H 8 ), (H 9 ), (H 10 ), (H 11 ), 5-13, 5-14, 5-15, 5-16, and (H 7 )”. 
     Following SB  9 , the image encoding apparatus  100  performs an intra prediction encoding on SB  6  according to the zigzag sequence. Here, the adjacent pixel information referred to during the intra prediction encoding on SB  6  includes pixel information “5-4, 5-8, 5-12, 5-16, 2-13, 2-14, 2-15, 2-16, and 1-16”. From among the pixel information, “5-4, 5-8, 5-12, and 5-16” correspond to pixel information in SB  5 , which is a sub-block adjacent to SB  6 , “2-13, 2-14, 2-15, and 2-16” correspond to pixel information in SB  2 , which is a sub-block adjacent to SB  6 , and “1-16” corresponds to pixel information in SB  1 , which is a sub-block adjacent to SB  6 . 
     As in the intra prediction encoding process for SB  1 , SB  2 , SB  5 , SB  9 , and SB  6  described above, the image encoding apparatus  100  performs an intra prediction encoding on the remaining sub-blocks according to the zigzag sequence by referring to already encoded and decoded adjacent pixel information (pixel information on pixels located at the right end and the lower end of each sub-block) in an adjacent sub-block or already encoded and decoded adjacent pixel information in an adjacent macro block. 
     The above description with reference to  FIGS. 4 to 9  presents embodiments of the present disclosure, in which the image encoding apparatus  100  intra-prediction-encodes sub-blocks of a current block in three types of intra prediction modes (horizontal mode, vertical mode, or DC mode) according to a predetermined sequence (sequence of a raster scan or a zigzag sequence). Hereinafter, the process of intra prediction encoding for each sub-block is described in more detail with reference to  FIGS. 10A to 13 .  FIGS. 10A to 12  are views for describing an intra prediction for a certain sub-block when the intra prediction mode of the current block is the horizontal mode, vertical mode, and DC mode, respectively, and  FIG. 13  is a view for describing a process of generating a residual block for a certain sub-block on which an intra prediction has been performed. 
       FIGS. 10A and 10B  are views illustrating an example of a process of performing an intra prediction on one sub-block in a horizontal mode. 
       FIG. 10A  illustrates a sub-block before the intra prediction is performed, and  FIG. 10B  illustrates a predicted sub-block generated through the intra prediction on the sub-block. It is assumed that the sub-block has a block size of 4×4, is marked by a thick solid line, and includes 16 pixels. 
     Referring to  FIG. 10A , each pixel included in a certain sub-block before the intra prediction contains corresponding pixel information relating to an original image. Each of 16 pixel information indices relating to the original image is expressed in the form of 4×4 coordinates, “M(i,j)”, including four rows (i) and four columns (j). For example, M(2,3) indicates pixel information of a pixel located at the third row (i=2) and the fourth column (j=3). 
     Further, since it is assumed that the intra prediction is performed in the horizontal mode, adjacent pixel information used for reference in an intra prediction of a certain sub-block is pixel information included in a sub-block (or macro block) adjacent to the left side of the certain sub-block and corresponds to “H( 0 ), H( 1 ), H( 3 ), and H( 3 )”. Here, “H( 0 ), H( 1 ), H( 3 ), and H( 3 )” refer to pixel information of four predetermined pixels horizontally adjacent to a certain sub-block being intra-predicted. 
     By intra-predicting a sub-block as shown in  FIG. 10A  in the horizontal mode, a predicted sub-block as shown in  FIG. 10B  is generated. The generated predicted sub-block includes 16 pixels, each of which contains predicted pixel information. The predicted pixel information is indicated by an index expressed in the form of 4×4 coordinates, “P(i,j)”, including four rows (i) and four columns (j). For example, P(2,3) indicates predicted pixel information of a pixel located at the third row (i=2) and the fourth column (j=3). 
     The predicted pixel information P(i,j) of each pixel included in a predicted sub-block, which is generated by performing an intra prediction on a certain sub-block in the horizontal mode, can be obtained by, for example, a method using the equation as defined below. In the equation defined below, H(i) indicates already encoded and decoded adjacent pixel information. 
     for (i=0; i&lt;4; i++)
         for (j=0; j&lt;4; j++)
           P(i,j)=H(i);   
               

     After the predicted sub-block is generated, a residual sub-block as shown in  FIG. 13  is generated by subtracting the predicted sub-block shown in  FIG. 10B  from the sub-block shown in  FIG. 10A . Each pixel included in the generated residual sub-block contains residual pixel information indicated by “R(i,j)”, which is obtained by subtracting P(i,j) from M(i,j). 
     The generated residual sub-block is then transformed, quantized, and entropy-encoded, so that the intra prediction encoding for the certain sub-block is completed. Further, by performing a dequantization, an inverse transform, and an intra-compensation on the quantized residual sub-block, a reference block is generated. Then, the generated reference block is used for a later intra prediction encoding of another sub-block. 
     The reference pixel information of each pixel included in the reference sub-block can be obtained by a method using the equation as defined below. In the equation, R′(i,j) indicates pixel information included in a residual sub-block obtained by dequantizing and inverse-transforming the quantized residual sub-block, and O′(i,j) indicates reference pixel information included in a reference sub-block obtained by intra-compensating and decoding such a residual sub-block. 
     for (i=0; i&lt;4; i++)
         for (j=0; j&lt;4; j++)
           O′(i,j)=H(i)+R′(i,j);   
               

     The above description discusses an example of an intra prediction encoding process including an intra prediction of a horizontal mode for one sub-block. Now, an example of an intra prediction encoding process including an intra prediction of a vertical mode for a certain sub-block will be discussed with reference to  FIGS. 11A and 11B . 
       FIGS. 11A and 11B  are views illustrating an example of a process of performing an intra prediction on one sub-block in a vertical mode. 
       FIG. 11A  illustrates a sub-block before the intra prediction is performed, and  FIG. 11B  illustrates a predicted sub-block generated through the intra prediction on the sub-block. It is assumed that the sub-block has a block size of 4×4, is marked by a thick solid line, and includes 16 pixels. 
     Referring to  FIG. 11A , each pixel included in a certain sub-block before the intra prediction contains corresponding pixel information relating to an original image. Each of 16 pixel information indices relating to the original image is expressed in the form of 4×4 coordinates, “M(i,j)”, including four rows (i) and four columns (j). For example, M(2,3) indicates pixel information of a pixel located at the third row (i=2) and the fourth column (j=3). 
     Further, since it is assumed that the intra prediction is performed in the vertical mode, adjacent pixel information used for reference in an intra prediction of a certain sub-block is pixel information included in a sub-block (or macro block) adjacent to the upper side of the certain sub-block and corresponds to “V( 0 ), V( 1 ), V( 3 ), and V( 3 )”. Here, “V( 0 ), V( 1 ), V( 3 ), and V( 3 )” refer to pixel information of four predetermined pixels vertically adjacent to a certain sub-block being intra-predicted. 
     By intra-predicting a sub-block as shown in  FIG. 11A  in the vertical mode, a predicted sub-block as shown in  FIG. 11B  is generated. The generated predicted sub-block includes 16 pixels, each of which contains predicted pixel information. The predicted pixel information is indicated by an index expressed in the form of 4×4 coordinates, “P(i,j)”, including four rows (i) and four columns (j). For example, P(2,3) indicates predicted pixel information of a pixel located at the third row (i=2) and the fourth column (j=3). 
     The predicted pixel information P(i,j) of each pixel included in a predicted sub-block, which is generated by performing an intra prediction on a certain sub-block in the vertical mode, can be obtained by, for example, a method using the equation as defined below. In the equation defined below, V(j) indicates already encoded and decoded adjacent pixel information. 
     for (j=0; j&lt;4; j++)
         for (i=0; i&lt;4; i++)
           P(i,j)=V(j);   
               

     After the predicted sub-block is generated, a residual sub-block as shown in  FIG. 13  is generated by subtracting the predicted sub-block shown in  FIG. 11B  from the sub-block shown in  FIG. 11A . Each pixel included in the generated residual sub-block contains residual pixel information indicated by “R(i,j)”, which is obtained by subtracting P(i,j) from M(i,j). 
     The generated residual sub-block is then transformed, quantized, and entropy-encoded, so that the intra prediction encoding for the certain sub-block is completed. Further, by performing a dequantization, an inverse transform, and an intra-compensation on the quantized residual sub-block, a reference block is generated. Then, the generated reference block is used for a later intra prediction encoding of another sub-block. 
     The reference pixel information of each pixel included in the reference sub-block can be obtained by a method using the equation as defined below. In the equation, R′(i,j) indicates pixel information included in a residual sub-block obtained by dequantizing and inverse-transforming the quantized residual sub-block, and O′(i,j) indicates reference pixel information included in a reference sub-block obtained by intra-compensating and decoding such a residual sub-block. 
     for (j=0; j&lt;4; j++)
         for (i=0; i&lt;4; i++)
           O′(i,j)=V(j)+R′(i,j);   
               

     The above description discusses an example of an intra prediction encoding process including an intra prediction of a vertical mode for one sub-block. Now, an example of an intra prediction encoding process including an intra prediction of a DC mode for a certain sub-block will be discussed with reference to  FIGS. 12A and 12B . 
       FIGS. 12A and 10B  are views illustrating an example of a process of performing an intra prediction on one sub-block in a DC mode. 
       FIG. 12A  illustrates a sub-block before the intra prediction is performed, and  FIG. 12B  illustrates a predicted sub-block generated through the intra prediction on the sub-block. It is assumed that the sub-block has a block size of 4×4, is marked by a thick solid line, and includes 16 pixels. 
     Referring to  FIG. 12A , each pixel included in a certain sub-block before the intra prediction contains corresponding pixel information relating to an original image. Each of 16 pixel information indices relating to the original image is expressed in the form of 4×4 coordinates, “M(i,j)”, including four rows (i) and four columns (j). For example, M(2,3) indicates pixel information of a pixel located at the third row (i=2) and the fourth column (j=3). 
     Further, since it is assumed that the intra prediction is performed in the DC mode, adjacent pixel information used for reference in an intra prediction of a certain sub-block is pixel information included in a sub-block (or macro block) adjacent to the left side and upper side of the certain sub-block and corresponds to “H( 0 ), H( 1 ), H( 3 ), H( 3 ), V( 0 ), V( 1 ), V( 3 ), V( 3 ), and M”. 
     Here, “H( 0 ), H( 1 ), H( 3 ), and H( 3 )” refer to pixel information of four predetermined pixels horizontally adjacent to a certain sub-block being intra-predicted, “V( 0 ), V( 1 ), V( 3 ), and V( 3 )” refer to pixel information of four predetermined pixels vertically adjacent to a certain sub-block being intra-predicted, and M refers to pixel information of a pixel adjacent in a leftward and upward diagonal direction to a certain sub-block being intra-predicted. 
     By intra-predicting a sub-block as shown in  FIG. 12A  in the DC mode, a predicted sub-block as shown in  FIG. 12B  is generated. The generated predicted sub-block includes 16 pixels, each of which contains predicted pixel information. The predicted pixel information is indicated by an index expressed in the form of 4×4 coordinates, “P(i,j)”, including four rows (i) and four columns (j). For example, P(2,3) indicates predicted pixel information of a pixel located at the third row (i=2) and the fourth column (j=3). 
     The predicted pixel information P(i,j) of each pixel included in a predicted sub-block, which is generated by performing an intra prediction on a certain sub-block in the DC mode, can be obtained by, for example, a method using the equation as defined below. In the equation defined below, H(i) indicates already encoded and decoded adjacent pixel information. 
     
       
         
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
             
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 for (j=0; j&lt;4; j++) 
               
             
          
           
               
                   
                 for (i=0; i&lt;4; i++) 
               
               
                   
                 { 
               
             
          
           
               
                   
                  k=3; 
                 //size of sub-block =(k+1) × (k+1) 
               
             
          
           
               
                   
                  Avg=Mean [H(0), H(1), ..., H(k), V(0), V(1), ..., V(k), M]; 
               
             
          
           
               
                   
                 // Mean[ ] refers to averaging of values 
               
             
          
           
               
                 within [ ]. 
               
             
          
           
               
                   
                  P(i,j) = Avg; 
               
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
     After the predicted sub-block is generated, a residual sub-block as shown in  FIG. 13  is generated by subtracting the predicted sub-block shown in  FIG. 12B  from the sub-block shown in  FIG. 12A . Each pixel included in the generated residual sub-block contains residual pixel information indicated by “R(i,j)”, which is obtained by subtracting P(i,j) from M(i,j). 
     The generated residual sub-block is then transformed, quantized, and entropy-encoded, so that the intra prediction encoding for the certain sub-block is completed. Further, by performing a dequantization, an inverse transform, and an intra-compensation on the quantized residual sub-block, a reference block is generated. Then, the generated reference block is used for a later intra prediction encoding of another sub-block. 
     The reference pixel information of each pixel included in the reference sub-block can be obtained by a method using the equation as defined below. In the equation, R′(i,j) indicates pixel information included in a residual sub-block obtained by dequantizing and inverse-transforming the quantized residual sub-block, and O′(i,j) indicates reference pixel information included in a reference sub-block obtained by intra-compensating and decoding such a residual sub-block. 
     for (j=0; j&lt;4; j++)
         for (i=0; i&lt;4; i++)
           O′(i,j)=Avg+R′(i,j);   
               

     The above description discusses an example of an intra prediction encoding process including intra predictions of a horizontal mode, a vertical mode, and a DC mode for one sub-block. 
     The image encoding according to the present disclosure as described above is based on H.264, and can also be performed according to a method defined in H.264 for another intra prediction mode other than the horizontal mode, the vertical mode, and the DC mode. 
       FIG. 14  is a schematic block diagram of an image decoding apparatus  1400  according to at least one embodiment of the present disclosure. 
     Referring to  FIG. 14 , the image decoding apparatus  1400  according to at least one embodiment of the present disclosure includes: a decoder  1410  for decoding a received bit stream, so as to extract residual sub-blocks and an intra prediction mode for a plurality of sub-blocks decomposed from a current block; a dequantizer  1420  for dequantizing the extracted residual sub-block; an inverse transform unit  1430  for performing an inverse transform on the dequantized residual sub-block; an intra predictor  1440  for generating predicted sub-blocks for a plurality of sub-blocks by performing an intra prediction according to an intra prediction mode by referring to adjacent pixel information for each sub-block already decoded and reconstructed; an adder  1450  for reconstructing a plurality of sub-blocks by adding the inverse-transformed residual sub-blocks and corresponding generated predicted sub-blocks; and a block combiner  1460  for reconstructing the current block by combining the plurality of reconstructed sub-blocks with each other. Each of the decoder  1410 , the dequantizer  1420 , the inverse transform unit  1430 , the intra predictor  1440 , the adder  1450 , and the block combiner  1460  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs) specified for respectively corresponding operations and functions described herein. The image decoding apparatus  1400  further comprises input units (not shown in  FIG. 14 ) such as one or more buttons, a touch screen, a mic and so on, and output units (not shown in  FIG. 14 ) such as a display, an indicator and so on. The image encoding apparatus  1400  further comprises communication modem(s) to receive and/or communication signals to thereby communicate with the image encoding apparatus  100  through wire or wireless networks (herein, the wire or wireless networks include, for example, one or more network interfaces including, but not limited to, cellular, Wi-Fi, LAN, WAN, CDMA, WCDMA, GSM, LTE and EPC networks, and cloud computing networks). 
     The intra prediction mode for the plurality of sub-blocks described above is the same as the intra prediction mode of the current block. 
     The block combiner  1460  may combine the plurality of reconstructed sub-blocks according to either a sequence of a raster scan or a zigzag sequence. 
     The received bit stream described above corresponds to a bit stream encoded according to an image encoding method according to an embodiment of the present disclosure. 
     The image decoding apparatus  1400  according to an embodiment of the present disclosure may share parameters or information necessary for decoding, dequantization, inverse-transform, and intra prediction, etc. with the image encoding apparatus  100  according to a predetermined scheme. For example, by receiving a bit stream including information on the intra prediction mode of the current block or information, by which it is possible to determined the intra prediction mode of the current block, the image decoding apparatus  1400  may share the information on the intra prediction mode with the image encoding apparatus  100 . 
       FIG. 15  is a flowchart of an image decoding method according to at least one embodiment of the present disclosure. 
     Referring to  FIG. 15 , an image decoding method according to an embodiment of the present disclosure includes the steps of: (S 1500 ) decoding a received bit stream, so as to extract residual sub-blocks and an intra prediction mode for a plurality of sub-blocks decomposed from a current block; (S 1502 ) dequantizing the extracted residual sub-block; (S 1504 ) performing an inverse transform on the dequantized residual sub-block; (S 1506 ) generating predicted sub-blocks for a plurality of sub-blocks by performing an intra prediction according to an intra prediction mode by referring to adjacent pixel information for each sub-block already decoded and reconstructed; (S 1508 ) reconstructing a plurality of sub-blocks by adding the inverse-transformed residual sub-blocks and corresponding generated predicted sub-blocks; and (S 1510 ) reconstructing the current block by combining the plurality of reconstructed sub-blocks with each other. 
     In step S 1510 , the plurality of reconstructed sub-blocks may be combined according to either a sequence of a raster scan or a zigzag sequence. 
     The intra prediction mode for the plurality of sub-blocks described above is the same as the intra prediction mode of the current block. 
     In the image encoding method and image decoding method according to an embodiment of the present disclosure, when a current block to be encoded or decoded is predicted, the current block is decomposed into sub-blocks and each of the sub-blocks is then intra-predicted by referring to adjacent pixel information of pixels adjacent to the corresponding sub-block, instead of performing an intra prediction on each decomposed sub-block by referring to adjacent pixel information of surrounding blocks adjacent to the current block. As a result, the image encoding method and image decoding method according to the present disclosure can improve the accuracy of the prediction, which can provide an image with a satisfactory reproduction quality by improved coding efficiency. 
     In the description above, although all of the components of the embodiments of the present disclosure have been explained as assembled or operatively connected as a unit, the present disclosure is not intended to limit itself to such embodiments. Rather, within the objective scope of the claimed invention, the respective components is able to be selectively and operatively combined in any numbers. Every one of the components is also implemented by itself in hardware while the respective ones can be combined in part or as a whole selectively and implemented in a computer program having program modules for executing functions of the hardware equivalents. Codes or code segments to constitute such a program is easily deduced by a person skilled in the art. The computer program may be stored in non-transitory computer readable recording medium, which in operation can realize the embodiments of the present disclosure. Examples of the non-transitory computer readable recording medium include magnetic recording media, such as a hard disk, a floppy disk, and a magnetic tape, and optical recording media, such as a compact disk read only memory (CD-ROM) and a digital video disk (DVD), magneto-optical media, such as a floptical disk, and hardware devices that are specially configured to store and execute program instructions, such as a ROM, a random access memory (RAM), and a flash memory. 
     In addition, terms like ‘include’, ‘comprise’, and ‘have’ should be interpreted in default as inclusive or open rather than exclusive or closed unless expressly defined to the contrary. All the terms that are technical, scientific or otherwise agree with the meanings as understood by a person of ordinary skill in the art unless defined to the contrary. Common terms as found in dictionaries should be interpreted in the context of the related technical writings not too ideally or impractically unless the present disclosure expressly defines them so. 
     Although exemplary embodiments of the present disclosure have been described for illustrative purposes, those of ordinary skill in the art will appreciate that various modifications, additions and substitutions are possible, without the spirit and scope of the claimed invention. Specific terms used in this disclosure and drawings are used for illustrative purposes and not to be considered as limitations of the present disclosure. Therefore, exemplary embodiments of the present disclosure have not been described for limiting purposes. Accordingly, the scope of the claimed invention is not to be limited by the above embodiments but by the claims and the equivalents thereof.