Abstract:
A method and an apparatus to divide image blocks to improve the quality of intermediate images by splitting each macro image block (macro block) in left-eye and right-eye views into sub image blocks (sub blocks) according to quadtree disparity estimation, and then splitting each sub block into smaller sub blocks. The method includes setting a plurality of splitting threshold values for a macro block in an image frame and determining whether to split the macro block into sub blocks, and setting a plurality of splitting threshold values for each sub block and determining whether to split each sub block into smaller sub blocks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from Korean Patent Application No. 2003-07118, filed Feb. 5, 2003, the contents of which are incorporated herein in their entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus to divide image blocks, and more particularly, to a method and apparatus to improve the quality of intermediate images. To achieve such improvements, this invention proposes a new standard to divide image blocks that can prevent flickering in synthesized intermediate images when splitting image blocks using quadtree disparity estimation. The image block splitting is followed by disparity estimation required for synthesis of intermediate views used to represent a three dimensional (3D) image. 
   2. Description of the Related Art 
   To realize an imaging and communication system that provides a high degree of realism and naturalness, it is necessary to develop 3D image processing technology that can naturally represent images according to human visual characteristics. 3D image processing employs binocular parallax, which is a difference in depth of an object perceived by left and right eyes. Processing and transmission of binocular images is of great concern in the field of next generation visual communication. 
   However, one significant problem associated with such image processing and transmission is a large amount of information contained in stereoscopic images, considering the fact that most images are color or moving images as well as transmission rate over a transmission line and processing rate of a transmission (or image processing) system. To overcome this problem, it is necessary to research a technique to efficiently and easily compress a large amount of information while maintaining the quality of a stereoscopic image. 
   Research is being conducted to develop a method that involves estimating variation in objects within an image using the fact that left and right views have high correlation, instead of independently encoding the two views, transmitting the variation information and either the left or right view, and compensating and restoring the transmitted view to binocular images at a receiving terminal. 
   Further, when the viewpoint of an observer moves or there are several observers, multi-view images are needed to create a natural stereoscopic image. However, since independent transmission of all multi-view images excessively increases the amount of information, reconstruction of multi-view images from binocular images restored at the receiving terminal of a binocular image transmission system, often called intermediate view reconstruction (IVR) or intermediate view synthesis, is used. In this case, reconstruction can be performed using intermediate view interpolation or extrapolation by obtaining variation information related to intermediate views from information on variation between the binocular images. 
   3D images are compressed and decompressed using an MPEG technique applied to two dimensional (2D) images. In particular, compression, transmission, and decompression of 3D images for digital broadcasting are performed using MPEG-2, which is a standard for digital broadcasting. 
   As is widely known in the art, MPEG-2 uses block-based compression schemes to compress 2D images. Currently, these schemes are also applied to compression of 3D images and are known as the most efficient method of 3D image compression. 
   Block-based compression is performed in blocks of a fixed size (for example, 16×16 pixels), each of which is called a macro block. Compression is achieved by motion estimation in units of a macro block and calculating a motion vector, which is the resulting value of estimation, and prediction error. When macro blocks of a fixed size are used in synthesizing intermediate views from binocular images, as in 2D images for stereoscopic depth perception by a viewer of 3D images as mentioned above, degradation in the quality of intermediate views may occur. In particular, quality degradation due to blurring of the edges of an image becomes a significant problem. 
   To overcome this problem, various quadtree disparity estimation approaches that can prevent quality degradation by splitting a macro block of a fixed size into smaller sub blocks near the edge of an image have been proposed (1998 SPIE Paper: Anthony Mancini and Janusz Konrad, “Robust Quadtree-based Disparity Estimation for the Reconstruction of Intermediate Stereoscopic Images”, and IEEE 0-7803-6685-9/01 Paper: D. R. Clewer, “Efficient Multiview Image Compression Using Quadtree Disparity Estimation”). 
   According to a quadtree disparity estimation approach, a block matching technique is used to calculate a mean absolute difference (MAD) of each macro block in left-eye and right-eye views and MADs of four sub blocks into which the macro block is divided. Then, if a ratio R madsub1  of the maximum sub block MAD to the minimum sub block MAD within a macro block is less than a predetermined threshold, disparity between binocular images is estimated in units of macro blocks. Conversely, if R madsub1  is greater than the threshold, disparity is re-estimated for each sub block. Since a method of calculating MADs is described in detail in literature including the above-cited references, a detailed description will be omitted. 
     FIG. 1A  shows a conventional block splitting algorithm using quadtree disparity estimation. As mentioned above, a quadtree disparity estimation approach involves splitting an N×N macro block into four (N/2)×(N/2) sub blocks and estimating disparity in units of sub blocks when the ratio of the maximum sub block MAD to the minimum sub block MAD within the macro block is greater than a predetermined threshold. The conventional block splitting process using the quadtree disparity estimation approach will now be described in detail with reference to  FIG. 1A . 
   If an N×N macro block that is the highest level block in disparity estimation (hereinafter called “Large Macro block (LMB)”) is input (operation S 10 ), block matching between binocular images is implemented to estimate disparity between the binocular images and verifies if the resulting estimate is correct. The adequacy and correctness of estimation is determined by R madsub1 , which denotes the ratio of the maximum MAD of any sub block (lower level block) within a macro block (higher level block) to the minimum MAD of any sub block within the same macro block. As R madsub1  becomes greater, the disparity difference between regions in a block becomes greater, and thus this estimation becomes less accurate. If the resulting estimate is verified as the most ideal one, which means there is little disparity difference between regions within the block, R madsub1  approximates 1, which is the minimum value. 
   Next, in operation S 11 , it is determined whether to split the LMB. This is done by determining whether R madsub1  is greater than a first threshold value R th1 , which is the reference value used to judge whether to split the LMB. R th1  is decided by experiment. 
   If R madsub1  is less than R th1 , which means disparity present between binocular images is low, LMB is not split (operation S 111 ) and disparity is estimated in units of LMBs (operation S 16 ). Then, in operation S 17  it is decided whether to split the next LMB. 
   If R madsub1  is greater than R th1 , the LMB is split into four (N/2)×(N/2) sub blocks (hereinafter called “Middle Sub Blocks (MSBs)”) (operation S 12 ), and it is determined whether to split each MSB (operation S 13 ). For this determination, the MAD of each (N/4)×(N/4) sub block (hereinafter called “Small Sub Block (SSB)”) within an MSB is calculated. Then, R madsub2 , the ratio of the maximum SSB MAD to the minimum SSB MAD within an MSB, is calculated and compared to a second threshold value R th2 , which is a reference value to determine whether to split the MSB. Like R th1 , R th2  is decided by experiment. 
   If R madsub2  is less than R th2 , the MSB is not split (operation S 14 ) and disparity is estimated in units of MSBs (operation S 16 ). Then, in step S 17  it is determined whether to split the next LMB. 
   If R madsub2  is greater than R th2 , the MSB is split into four (N/4)×(N/4) SSBs (operation S 15 ). Then, in operation S 17  it is determined whether to split the next LMB. 
     FIG. 1B  shows image blocks produced in a block splitting procedure using a quadtree disparity estimation approach. Each block is contained in a 720×288 image frame. A block splitting technique proposed by the present invention is applied to the image blocks shown in  FIG. 1B . For an image created by interlaced scanning, in particular, this splitting may be performed in frames or fields. As mentioned above, the conventional block splitting algorithm splits a higher level block into four lower level blocks in order to re-estimate disparity when the maximum to minimum MAD ratio of the lower level blocks exceeds a single threshold, thus obtaining more detailed intermediate views. 
   However, a little change can occur in disparity due to shaking of the camera, changes in lighting conditions, and noise components such as dust, even in a still image or low complexity image (with less motion). Since blocks in this kind of image may have MAD ratios near a threshold, the determination of whether these blocks are split is highly dependant on such changes in disparity. Thus, disparity estimates suffer from a significant change when synthesizing intermediate views for such an image block. In this case, since the conventional block splitting algorithm applies each single threshold when splitting an LMB into four MSBs and an MSB into four SSBs, flickering can occur in synthesized intermediate views. 
   Furthermore, the conventional block splitting algorithm in the quadtree disparity estimation approach cannot prevent degradation in quality of intermediate views, in particular, flickering near edges, by adopting a single threshold. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method and apparatus to prevent flickering when synthesizing intermediate views that are required to represent a three dimensional (3D) image. 
   Accordingly, it is an aspect of the present invention to provide a new method of splitting a macro image block (higher level block) into sub blocks (lower level blocks) according to a quadtree disparity estimation approach. 
   It is another aspect of the present invention to provide a new apparatus to split a macro image block (higher level block) into sub blocks (lower level blocks) according to a quadtree disparity estimation approach. 
   Additional aspects and/or advantages of the 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. 
   The foregoing and/or other aspects of the present invention are achieved by providing a method of splitting each of macro image blocks (hereinafter called “macro blocks”) in left-eye and right-eye views into sub image blocks (“sub blocks”) according to quadtree disparity estimation, and then splitting each sub block into smaller sub blocks. The method includes: setting a plurality of splitting threshold values for a macro block in an image frame and determining whether to split the macro block into the sub blocks; and setting a plurality of splitting threshold values for each sub block and determining whether to split each sub block into smaller sub blocks. 
   The foregoing and/or other aspects of the present invention may also be achieved by providing a method of splitting an image block the method comprising: setting a plurality of macro block splitting threshold values to split a macro block in an image frame into sub blocks and determining whether to split the macro block according to whether a macro block at the same location in a preceding image frame as the current macro block (hereinafter called “preceding macro block”) has been split; and setting a plurality of sub block splitting threshold value for splitting the sub block into smaller sub blocks and determining whether to split each sub block into smaller sub blocks according to whether a sub block at the same location in the preceding macro block as the current sub block (hereinafter called “preceding sub block”) has been split. 
   The foregoing and/or other aspects of the present invention may also be achieved by providing an apparatus to split an image block, the apparatus comprising: a macro block splitting determining unit that sets a plurality of macro block splitting threshold values to split a macro block in an image frame into sub blocks and determines whether to split the macro block; and a sub block splitting determining unit that sets a plurality of sub block splitting threshold values to split each sub block into smaller sub blocks and determines whether to split each sub block. 
   The foregoing and/or other aspects of the present invention may also be achieved by providing an apparatus to split an image block, the apparatus comprising: a macro block splitting determining unit that sets a plurality of macro block splitting threshold values to split a macro block in an image frame into sub blocks and determines whether to split the macro block according to whether a macro block at the same location in a preceding image frame as the current macro block (hereinafter called “preceding macro block”) has been split; and a sub block splitting determining unit that sets a plurality of sub block splitting threshold values to split each sub block into smaller sub blocks and determines whether to split each sub block according to whether a sub block at the same location in a preceding macro block as the current sub block (hereinafter called “preceding sub block”) has been split. 
   The foregoing and/or other aspects of the present invention may also be achieved by providing a recording medium on which a method is written as a program code that can be read and executed on a computer, the program coded method comprising: setting a plurality of splitting threshold value for a macro block in an image frame and determining whether to split the macro block into sub blocks; and setting a plurality of splitting threshold value for each sub block and determining whether to split each sub block into smaller sub blocks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1A  is a flowchart showing a conventional image block splitting method; 
       FIG. 1B  shows image blocks produced in a block splitting procedure according to a quadtree disparity estimation approach; 
       FIG. 2A  is a flowchart showing a block splitting method according to an embodiment of the present invention; 
       FIG. 2B  is a flowchart showing operation S 20  of the method of  FIG. 2A  according to another embodiment of the present invention; 
       FIG. 2C  is a flowchart showing operation S 21  of the method of  FIG. 2A  according to another embodiment of the present invention; 
       FIG. 3A  is a diagram showing a configuration of an image block splitting apparatus according to another embodiment of the present invention; 
       FIG. 3B  is a diagram showing the configuration of an LMB splitting determining unit shown in  FIG. 3A  according to another embodiment of the present invention; and 
       FIG. 3C  is a diagram showing the configuration of the MSB splitting determining unit shown in  FIG. 3A  according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the 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. 
   Referring to  FIGS. 2A-2C , in operation S 20 , a plurality of splitting threshold values are set for a first N×N large macro block (LMB) to be processed, and then splitting is determined depending on the set splitting thresholds. Hereinafter, to aid in understanding, it is assumed that N is 16 (generally, a macro block size is 16×16 or 8×8). 
   In operation S 201 , splitting threshold values are set for a 16×16 LMB by experiment. It is an aspect of the present invention to set two threshold values; a threshold value R thu1  to determine the possibility of splitting the LMB, and a threshold value R ths1  to determine whether to split the LMB. Since block splitting is performed to ensure the accuracy of estimation, the threshold values are preferably set based on mean absolute difference (MAD) ratios used in disparity estimation as described above. In this invention, MAD ratios calculated when splitting LMBs into MSBs and when splitting MSBs into SSBs are denoted by R madsub1  and R madsub2 , respectively. 
   Here, it is an aspect to determine whether to split the 16×16 LMB (operation S 20 ) by referring to whether the preceding 16×16 LMB has been split. 
   It is another aspect to determine whether to split the LMB by using only the above threshold values without referring to the result of processing the preceding 16×16 LMB. For example, while an LMB is not split if R madsub1  approximates R thu1 , the LMB may be split if R madsub1  approximates R ths1 . That is, whether to split the LMB can be determined by setting a predetermined reference value for R madsub1 . 
   Nevertheless, it is necessary to refer to splitting of the preceding 16×16 LMB due to ambiguity in the setting of the predetermined reference values and information about frames (blocks) in image data being similar. Thus, the image data may have hysteresis characteristics according to which processing of the current frame (block) is affected by the result of processing of the preceding frame (block). This invention determines whether to split an LMB to be processed using these characteristics of image data. The importance and reason for referring to splitting of the preceding LMB will be more clearly described infra. 
   Operation S 20  is divided into operations S 202  and S 203 . In operation S 202 , whether or not to split the LMB is determined by deciding whether R madsub1  is greater than R thu1 . Here, R madsub1  denotes the ratio of maximum MAD to minimum MAD of an 8×8 MSB within the 16×16 LMB. If R madsub1  is greater than R thu1 , in operation  203 , R madsub1 , R thu1 , and R ths1  are compared with one another to determine whether to split the LMB. 
   After the threshold values R thu1  and R ths1  of the LMB are set in the operation S 201 , the operation S 202  is performed to determine if R madsub1  is less than R thu1 . If R madsub1  is less than R thu1 , which means that disparity has been correctly estimated (there is no possibility of splitting the LMB), it is finally decided not to split the LMB. 
   Conversely, if R madsub1  is greater than R thu1  in the operation S 202 , which means there is a possibility of splitting the LMB, operation S 203  is performed to determine whether to split the LMB. Unlike conventional techniques, the present invention does not directly target the LMB for splitting even when it is determined that there is no possibility of splitting it. Instead, the present invention further includes the operation S 203  of determining whether to split the LMB. 
   When determining whether to split the LMB in the operation S 203 , first, it is determined in operation S 2031  whether R madsub1  is between R thu1  and R ths1 , which is the threshold value to determine whether to split the LMB. If R madsub1  is not between R thu1  and R ths1 , which means R madsub1  is greater than R ths1 , it is determined to split the LMB and the LMB is split into MSBs. Then, the process proceeds to operation S 21  of determining whether to split the MSBs. 
   If R madsub1  is between R thu1  and R ths1 , which means that the LMB will be split or there is a possibility of splitting the LMB, to ensure accuracy in determination of the splitting possibility, the present invention further includes operation S 2032  of determining whether an LMB placed at the same location in the preceding image frame as the LMB to be processed (hereinafter called “preceding LMB”) has been split. Thus, the LMB to be processed is split, or whether to split the LMB is determined by referring to whether the preceding LMB has been split. 
   As described supra, the reason for referring to whether the preceding LMB has been split becomes evident through operation S 203 . In particular, it is necessary to refer to splitting of the preceding LMB when the determination of whether to split the current LMB is ambiguous. That is, if R madsub1  is between R thu1  and R ths1 , a minute change in disparity may occur, as described in the conventional technique. In such an image block, since the MAD ratio is near the splitting threshold, there is often ambiguity as to whether to split. If intermediate views are synthesized by unconditionally either splitting or not splitting the image block, the intermediate views may exhibit flickering. Thus, the present invention assures accuracy in the determination of whether to split by referring to whether the preceding LMB has been split. 
   When R madsub1  is between R thu1  and R ths1  and it is ambiguous whether to split the current LMB, it is determined that the current LMB is to be split if the preceding LMB is found to have been split. If the preceding LMB has not been split, it is determined that the current LMB is not to be split. When R madsub1  is between R thu1  and R ths1  and the current LMB is split without referring to whether the preceding LMB has been split, unnecessary splitting may degrade the quality of intermediate views synthesized after splitting. Thus, unnecessary splitting can be prevented by referring to whether the preceding LMB has been split. 
   In order to refer to the splitting of the preceding LMB, a system implemented by this invention is required to have a separate information storage unit to store the result of splitting the preceding LMB. For example, the processing result may be stored in tabular form as shown in Table 1 below. Table 1 shows splitting information about the preceding block when an image of 720×288 pixels per frame is split into 810 macro blocks (MSBs) of 16×16 pixels. 
   
     
       
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Splitting information about preceding block 
             
           
        
         
             
               Macro block 
               LMB 
               MSB1 
               MSB2 
               MSB3 
               MSB4 
             
             
                 
             
             
                1 
               0 
               0 
               0 
               0 
               0 
             
             
                2 
               0 
               0 
               0 
               0 
               0 
             
             
               . . . 
             
             
               809 
               1 
               0 
               0 
               0 
               1 
             
             
               810 
               0 
               0 
               0 
               0 
               0 
             
             
                 
             
           
        
       
     
   
   In Table 1, ‘0’ means the preceding LMB has not been split while ‘1’ means it has been split. MSB1-MSB4 denote sub blocks in each LMB, and splitting information about the preceding block described above is updated with the result of processing the current LMB when starting the procedure to determine whether to split the next LMB. 
   If it is determined whether to split the LMB, then the process proceeds to operation S 21  of determining whether to split the resulting MSBs. 
   In operation S 21 , a plurality of splitting threshold values are set for sub blocks, and then it is determined whether to split each sub block into smaller sub blocks. Specifically, a plurality of splitting threshold values are set for an 8×8 sub block (MSB) into which the first 16×16 macro block was split in operation S 20 , and then it is determined whether to split each MSB according to the splitting threshold values. 
   Here, splitting threshold values of MSBs are set by experiment, like the LMB splitting thresholds. It is also preferable to set two threshold values: a threshold R thu2  to determine the possibility of splitting an MSB and a threshold R ths2  to determine whether to split an MSB. Since block splitting is performed to ensure the accuracy of estimation, the threshold values may be set based on MAD ratios used in disparity estimation, as in the case of splitting an LMB. 
   It is more preferable that whether to split an 8×8 MSB (operation S 21 ) is determined by referring to whether the preceding 8×8 MSB has been split, as in the case of LMB splitting. The background and reason for referring to whether the preceding 8×8 MSB has been split are the same as in the case of LMB splitting, so a detailed description thereof will be omitted. 
   Operation S 21  is divided into operations S 212  and S 213 . In operation S 212 , the possibility of splitting an MSB is determined by deciding whether R madsub2  is greater than R thu2 . Here, R madsub2  denotes the ratio of maximum MAD to minimum MAD of a smaller 4×4 sub block (SSB) within the 8×8 MSB. If R madsub2  is greater than R thu2 , in the operation  213 , R madsub2 , R thu2 , and R ths2  are compared with one another to determine whether to split the MSB. 
   After the threshold values R thu2  and R ths2  of MSB are set in operation S 211 , operation S 212  is performed to determine if R madsub2  is less than R thu2 . If R madsub2  is less than R thu2 , which means that disparity has been correctly estimated (there is no possibility of splitting the MSB), it is finally decided that the MSB is not to be split. 
   Conversely, if R madsub2  is greater than R thu2  in operation S 212 , which means there is a possibility of splitting the MSB, operation S 213  is performed to determine whether to split the MSB. Unlike conventional techniques, the present invention further involves step S 213  of determining whether to split the MSB, instead of directly targeting the MSB to split even when it is determined that the MSB has no possibility of being split. 
   When determining whether to split the MSB in operation S 213 , first, it is determined in operation S 2131  whether R madsub2  is between R th2  and R ths2 , which is the threshold value to determine the splitting of MSB. If R madsub2  is not between R thu2  and R ths2 , which means R madsub2  is greater than R ths2 , it is determined to split the MSB and the MSB is split into four SSBs. Then, the process proceeds to the operation of determining whether to split the next MSB. 
   If R madsub2  is between R thu2  and R ths2 , which means there is a possibility of splitting the MSB, to ensure accuracy in determination of the splitting possibility, the present invention further includes operation S 2132  of determining whether an MSB placed at the same location in the preceding LMB as the current MSB (hereinafter called “preceding MSB”) has been split, as in the case of splitting LMBs. Thus, splitting of the MSB currently being processed is determined by referring to whether the preceding MSB has been split. 
   As described above, the reason for referring to whether the preceding MSB was split becomes evident through operation S 213 . In particular, it is necessary to refer to splitting of the preceding MSB when the determination of whether to split the MSB currently being processed is ambiguous. That is, if R madsub2  is between R thu2  and R ths2 , a minute change in disparity may occur as described in the conventional technique. Since MAD ratio is near a splitting threshold in such an image block, an unambiguous determination of whether to split often cannot be made. If intermediate views are synthesized by splitting or not splitting the image block without referring to the result of processing the preceding block, the intermediate views exhibit flickering. Thus, the present invention ensures a reliable determination of whether to split by referring to whether the preceding MSB has been split. 
   When R madsub2  is between R thu2  and R ths2 , and the determination of whether to split the current MSB is ambiguous, it is determined to split the current MSB if the preceding MSB is found to have been split. If the preceding MSB has not been split, it is determined to not split the current MSB. When R madsub2  is between R thu2  and R ths2  and the current MSB is split without referring to whether the preceding MSB has been split, unnecessary splitting may degrade the quality of intermediate views synthesized after splitting. Thus, unnecessary splitting can be prevented by referring to whether the preceding MSB has been split. 
   In order to refer to the splitting of the preceding MSB, a system implemented by this invention is required to have a separate information storage unit to store the result of splitting the preceding MSB. For example, the processing result may be stored in tabular form as shown in Table 1 above. 
   Splitting information about the preceding block described in Table 1 is updated with the result of processing the current MSB when starting the procedure to determine whether to split the next MSB. 
   If it is determined to split the MSB, the procedure to determine whether to split the MSB is finished by splitting the MSB into four SSBs. Then, the same splitting process proceeds in order to determine whether to split the next LMB. 
     FIG. 3A  is a diagram showing the configuration of an image block splitting apparatus according to an embodiment the present invention,  FIG. 3B  is a diagram showing the configuration of an LMB splitting determining unit shown in  FIG. 3A  according to another embodiment of the present invention, and  FIG. 3C  is a diagram showing the configuration of an MSB splitting determining unit shown in  FIG. 3A  according to another embodiment of the present invention. 
   An LMB splitting determining unit  30  sets a plurality of splitting threshold values for a first N×N macro block (LMB) to be processed and determines whether to split the LMB according to the splitting threshold values. Hereinafter, it is assumed that N is 16, as is assumed in the above description of the method according to the present invention. 
   Splitting threshold values are set for a 16×16 LMB by experiment. As in the method according to the present invention, it is preferable to set two threshold values: a threshold value R thu1  to determine the possibility of splitting an LMB and a threshold value R ths1  to determine whether to split the LMB. Since block splitting is performed to ensure the accuracy of estimation, the threshold values are preferably set based on MAD ratios used in disparity estimation. To maintain consistency with the method of the present invention, MAD ratios calculated when splitting LMB into MSBs and when splitting MSB into SSBs are denoted by R madsub1  and R madsub2 , respectively. 
   Here, it is more preferable that whether to split the 16×16 LMB is determined by referring to whether the preceding 16×16 LMB has been split. The background and reason for this are the same as in the method of the present invention, so a detailed description thereof will be omitted here. 
   The LMB splitting determining unit  30  includes of an LMB splitting possibility determining portion  301  and an LMB splitting determining portion  302 . The LMB splitting possibility determining portion  301  determines the possibility of splitting LMB by deciding whether R madsub1  is greater than R thu1 . Here, R madsub1  denotes the ratio of maximum MAD to minimum MAD of an 8×8 sub block (MSB) within the 16×16 LMB. If R madsub1  is greater than R thu1 , the LMB splitting determining portion  302  compares R madsub1 , R thu1 , and R ths1  with one another and determines whether to split the LMB. 
   After the threshold values R thu1  and R ths1  of the LMB are set, the LMB splitting possibility determining portion  301  determines if R madsub1  is less than R thu1 . If R madsub1  is less than R thu1 , which means that there is no possibility of splitting the LMB, the LMB is not split. 
   Conversely, if the LMB splitting possibility determining portion  301  determines that R madsub1  is greater than R thu1 , i.e., that there is a possibility of splitting the LMB, the LMB splitting determining portion  302  finally determines whether to split the LMB. First, a preceding LMB splitting determiner  3021  determines whether R madsub1  is between R thu1  and R ths1 , which is the threshold value to determine splitting of the LMB, and then determines whether the preceding LMB has been split. If R madsub1  is not between R thu1  and R ths1 , which means R madsub1  is greater than R ths1 , an LMB splitting final determiner  3022  finally determines whether to split the LMB. 
   If the preceding LMB splitting determiner  3021  determines that R madsub1  is between R thu1  and R ths1 , the LMB splitting final determiner  3022  finally determines whether to split the LMB being processed by referring to the determination of whether the preceding LMB was split made by the preceding LMB splitting determiner  3021 . 
   As described above, in particular, it is necessary to refer to splitting of the preceding LMB when the determination of whether to split the LMB being processed is ambiguous. That is, if R madsub1  is between R thu1  and R ths1 , a minute change in disparity may occur, as described in the conventional technique. Since MAD ratio is near a splitting threshold value in this kind of an image block, an unambiguous determination of whether to split often cannot be made. If intermediate views are synthesized by splitting or not splitting image blocks without referring to whether the preceding block has been split, the intermediate views exhibit flickering. Thus, the present invention ensures a reliable determination of whether to split by referring to whether the preceding LMB has been split. 
   When R madsub1  is between R thu1  and R ths1 , the LMB splitting final determiner  3022  finally determines that the current LMB will be split if the preceding LMB is found to have been split by the preceding LMB splitting determiner  3021 . On the other hand, if the preceding LMB has not been split, the LMB splitting final determiner  3022  finally determines the current LMB will not be split. When R madsub1  is between R thu1  and R ths1  and the current LMB is split without referring to whether the preceding LMB has been split, unnecessary splitting may degrade the quality of intermediate views synthesized after splitting. Thus, unnecessary splitting can be prevented by referring to whether the preceding LMB has been split. 
   In order to refer to the splitting of the preceding LMB, the apparatus of the present invention is required to have a splitting information storage unit  32  to store the result of processing the preceding LMB. For example, the processing result may be stored in tabular form as shown in Table 1 above. 
   If it is determined whether to split the LMB, then an MSB splitting determining unit  31  determines whether to split resulting MSBs. 
   The MSB splitting determining unit  31  sets a plurality of MSB splitting threshold values, and then determines whether to split each MSB into smaller sub blocks. Specifically, the MSB splitting determining unit  31  sets a plurality of splitting threshold values for 8×8 MSBs into which the first 16×16 macro block has been split by the LMB splitting determining unit  30 , and then determines whether to split each MSB according to the splitting threshold values. 
   Here, splitting threshold values of the 8×8 MSB are set by experiment, like LMB splitting threshold values. As in the method according to the present invention, it is also preferable to set two threshold values: a threshold value R thu2  to determine the possibility of splitting an MSB and a threshold value R ths2  to determine whether to split the MSB. Since block splitting is performed to ensure the accuracy of estimation, the MSB splitting thresholds may be set based on MAD ratios as described above. 
   Here, it is more preferable that whether to split each 8×8 MSB is determined by referring to whether the preceding 8×8 MSB has been split. The background and reason for this are the same as in LMB splitting described above, so a detailed description thereof will be omitted here. 
   The MSB splitting determining unit  31  includes of an MSB splitting possibility determining portion  311  and an MSB splitting determining portion  311 . The MSB splitting possibility determining portion  311  determines the possibility of splitting an MSB by deciding whether R madsub2  is greater than R thu2 . Here, R madsub2  denotes the ratio of maximum MAD to minimum MAD of a 4×4 sub block (SSB) within the 8×8 MSB. If R madsub2  is greater than R thu2 , the MSB splitting determining portion  312  compares R madsub2 , R thu2 , and R ths2  with one another and determines whether to split the MSB. 
   After the threshold values R thu2  and R ths2  are set, the MSB splitting possibility determining portion  311  determines if R madsub2  is less than R thu2 . If R madsub2  is less than R thu2 , which means that there is no possibility of splitting the MSB, the MSB is not split. 
   Conversely, if the MSB splitting possibility determining portion  311  determines that R madsub2  is greater than R thu2 , i.e., that there is a possibility of splitting the MSB, the MSB splitting determining portion  312  finally determines whether to split the MSB. First, a preceding MSB splitting determiner  3121  determines whether R madsub2  is between R thu2  and R ths2 , which is the threshold value to determine splitting of an MSB, and then determines whether the preceding MSB has been split. If R madsub2  is not between R thu2  and R ths2 , which means R madsub2  is greater than R ths2 , an MSB splitting final determiner  3122  finally determines the splitting of the MSB. 
   If the preceding MSB splitting determiner  3121  determines that R madsub2  is between R thu2  and R ths2 , the MSB splitting final determiner  3122  finally determines whether to split the MSB being processed by referring to the determination of whether the preceding MSB was split by the preceding MSB splitting determiner  3121 . 
   As described above, it is necessary to refer to whether the preceding MSB was split when the initial determination of whether to split the MSB being processed is ambiguous. That is, if R madsub2  is between R thu2  and R ths2 , a minute change in disparity may occur as described in the conventional technique. Since MAD ratio is near a splitting threshold in this kind of an image block, a determination of whether to split may be ambiguous. If intermediate views are synthesized by splitting or not splitting image blocks without referring to whether the preceding block has been split, the intermediate views exhibit flickering. Thus, this invention ensures a reliable determination of whether to split by referring to whether the preceding MSB has been split. 
   When R madsub2  is between R thu2  and R ths2 , the MSB splitting final determiner  3122  finally determines that the current MSB will be split if the preceding MSB is found to have been split by the preceding MSB splitting determiner  3121 . On the other hand, if the preceding MSB has not been split, the MSB splitting final determiner  3122  finally determines that the current LMB will not be split. When R madsub2  is between R thu2  and R ths2  and the current LMB is split without referring to whether the preceding MSB has been split, unnecessary splitting may degrade the quality of intermediate views synthesized after splitting. Thus, unnecessary splitting can be prevented by referring to whether the preceding MSB has been split. 
   In order to refer to the splitting of the preceding MSB, the apparatus of the present invention is required to have the splitting information storage unit  32  to store the result of processing the preceding MSB. For example, the processing result may be stored in tabular form as shown in Table 1 above. 
   Splitting information about the preceding block described in Table 1 is updated with the result of processing the current MSB when starting the procedure to determine whether to split the next LMB. 
   If the MSB is determined to be split, the procedure to determine whether to split the MSB is finished by splitting the MSB into four SSBs. Then, the same splitting process proceeds in order to determine whether to split the next LMB. 
   The present invention is advantageous over conventional block splitting techniques in that it can synthesize intermediate views in such a way as to represent a high-equality three dimensional (3D) image, even over a very small change between blocks (frames), by setting two thresholds and using splitting information about a block preceding a block currently being processed. 
   Embodiments of the present invention can be written in computer-readable code on a computer-readable recording medium. Examples of the computer-readable recording medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage device. The code may also be transmitted via carrier waves, e.g., over the Internet. Furthermore, the computer-readable code may be stored on and executed from recording media distributed among computer systems connected to one other by a network. 
   Although the preferred embodiments of the present invention described above refer only to application to intermediate views for 3D image representation and a two-operation splitting procedure (LMB→MSB→SSB), the present invention may be embodied as a splitting procedure containing more than two operations, depending on the size of a macro block, and may be applied to general image processing. Therefore, the described embodiments should be considered not in terms of restriction but in terms of explanation. The scope of the present invention is not limited by the foregoing description but by the following claims, and all differences within the range of equivalents thereof should be interpreted as being covered by the present invention.