Abstract:
A method is provided for detecting an error in an image data stream in a device environment where digital image data is reproduced, especially, in a wired/wireless network such as a personal portable device. The method comprises determining whether or not an error is present in an input image data stream; determining a similarity between patterns of transform coefficients that are generated in the course of decoding the input image data stream; and detecting the starting position of the error based on the similarity determination. Accordingly, a decoder can independently detect the position of the error accurately, and the quality of an image which is replaced and restored using error concealment can be improved.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 10-2007-0031136, filed on Mar. 29, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Methods and apparatuses consistent with the present invention relate to detecting an error in an image data stream, and more particularly, to an apparatus for and a method of determining whether or not an error is present in an input image data stream and detecting the starting position of the error more accurately if an error is present. 
     2. Description of the Related Art 
     In a wired/wireless network environment where TVs, DVD players, devices supporting digital multimedia broadcasting (DMB), and especially, personal portable devices play digital moving pictures, communication channels for picture streaming vary according to time, and burst errors occur frequently due to poor conditions of transmission and receipt of data. For this reason, the quality of service (QoS) of and quality of experience (QoE) of moving pictures cannot be ensured. 
     A real-time system of streaming, in which procedures for inputs and outputs and the interaction with the surrounding environment should be performed within limited time, depends on the logical functional features and temporal features of the accuracy of the system operation. In such a system of streaming, since a receiving terminal can display moving pictures only for limited time, generally, it is hard to ensure the QoS of the moving pictures by retransmitting. 
     Therefore, the receiving terminal needs to determine if there are errors in the moving pictures and detect and conceal the errors. 
       FIG. 1  is a functional block diagram of procedures of detecting an error in a received image data stream and decoding the image data according to the conventional art. 
     Referring to  FIG. 1 , a channel decoder  110  receives image signals from an external source. An image decoding system includes a transport stream (TS) demultiplexing unit  120  that demultiplexes an image signal (TS packets), which the channel decoder  110  that is a signal receiving unit uses to extract an elementary stream (ES), and an image data decoder  130  that reproduces an image signal by decoding a reduction encoded image data stream. 
     The channel decoder  110  receives TS packets from the image signal of a channel chosen by a user. Channel coding and cyclic redundancy check (CRC) is performed on the received TS packets to detect errors, and in the case of a TS packet where an error occurs, a flag of a transport error indicator in a TS header of the TS packet is set to 1. In the case of a TS packet which goes missing during transmission, since the values of continuity counters of packets before and after the omitted packet are not continuous, the existence of a missing packet can also be detected. 
     The TS demultiplexing unit  120  includes a TS program ID (PID) filter  121  and an error detector  122 . The TS PID filter  121  filters image packets using program ID information in a header of the received TS packets, and makes an ES consisting of TS image packets. 
     The error detector  122  inserts information about loss or damage of the packets which has been checked by the channel decoder  110  into the ES as error information in the course of producing the ES. The error information includes a predetermined bit sequence which informs of starting of the error and the number of damaged or lost TS packets to inform of the size of the error. 
     The image data decoder  130  includes a bit parsing unit  131 , a macroblock decoder  132 , and a slice decoder  133 . The bit parsing unit  131  transmits bits necessary to the macroblock decoder  132  from among bit sequences of a video stream received from the TS demultiplexing unit  120 . Also, during real-time decoding, the bit parsing unit  131  checks the error information that the TS demultiplexing unit  120  detected and displayed. The macroblock decoder  132  and the slice decoder  133  decode an image while performing concealment or restoring of the error on a macro basis and on a slice basis, respectively. 
     However, the conventional art has a problem in that it cannot be used when the transmission standard does not support CRC or the TS demultiplexing unit  120  is not able to detect an error in an elementary stream. 
       FIG. 2  is a diagram showing images to which error detection and error concealment methods are applied according to the conventional art. 
     In  FIG. 2 , the left drawing  210  shows an image without an error concealment (restoring) process, and the right drawing  220  shows an image which is reproduced after an error concealment process according to the conventional art. However, it is noticeable that the right drawing still has a macroblock in which an error  221  is present, and this problem occurs because the accurate starting point of the error cannot be detected during the error detection process. 
     According to the moving picture experts group (MPEG) standard, a decoding procedure includes variable length decoding, and during this decoding, error detection can fail if a symbol matches another symbol in a range of defined bit sequences even when the received bit sequence includes an error bit. For example, if bit sequences assigned to letters ‘A’, ‘B’, ‘C’, and ‘D’ are 00, 01, 10, and 11, respectively, when the bit sequence of 00 is received wrongly instead of the bit sequence of 01 (=B), a decoder in a receipt side continues decoding, while recognizing the bit sequence of 00(=A), since the symbol ‘A’ corresponding to the wrong bit sequence, that is, 00 is still present in a range of the defined bit sequences. As such, this method (range check method) cannot detect the actual position of occurrence of an error, and can cause repetition of errors which may be determined as errors at incorrect locations afterwards, and thus the determination of accurate error detection points cannot be ensured. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of and an apparatus for detecting an error in an image data stream, which determine if there is an error present in the image data stream, and allow a decoder to detect an accurate error position by detecting the starting position of the error if an error is present. 
     According to an aspect of the present invention, a method is provided for detecting an error in an image data stream, the method comprising: determining whether or not an error is present in an input image data stream; determining similarity between patterns of transform coefficients that are generated in the course of decoding the input image data stream; and detecting the starting position of the error based on the similarity determination. 
     Determining the similarity between the patterns of transform coefficients may include determining a spatial similarity between patterns of transform coefficients of adjacent macroblocks and a current macroblock in a current image frame of the input image data stream, or determining a temporal similarity between patterns of transform coefficients of a current macroblock of the current image frame and a macroblock which corresponds to the current macroblock and belongs to a reference frame which the current image frame refers to. 
     In the determination of the similarity between the patterns of transform coefficients, the pattern of transform coefficients may be divided into one or more areas to determine the spatial or temporal similarity of the patterns. 
     In the determination of the similarity between the patterns of transform coefficients, the difference between transform coefficients which have non-zero values may be compared to a predetermined threshold value. 
     In the determination of the similarity between the patterns of transform coefficients, a weighted value may be applied to each divided area, a value obtained after applying the weighted value may be compared to the threshold value, and the transform coefficients may be discrete cosine transform coefficients. 
     According to another aspect of the present invention, an apparatus is provided for detecting an error in an image data stream, the apparatus comprising: an error determination unit which determines whether or not an error is present in an input image data stream; and an error detection unit which detects the starting position of an error if it is determined that an error is present, wherein the error detection unit includes a pattern determination unit which determines a similarity between patterns of transform coefficients, which are generated in the course of decoding the input image data stream, when an error occurs. 
     The pattern determination unit may determine a spatial similarity between patterns of transform coefficients of adjacent macroblocks and a current macroblock in a current image frame of the input image data stream, or a temporal similarity between patterns of transform coefficients of a current macroblock of the current image frame and a macroblock which corresponds to the current macroblock and belongs to a reference frame which the current image frame refers to. 
     The pattern determination unit may divide each pattern of transform coefficients into one or more areas to determine the spatial or temporal similarity of the patterns. 
     The pattern determination unit may compare the difference between the transform coefficients which have non-zero values to a predetermined threshold value to determine the similarity, and may apply a weighted value to each divided area and compare a value obtained after applying the weighted value to the threshold value. 
     The apparatus may further comprise a pattern analyzing unit which reads the pattern of transform coefficients of a macroblock in the current image frame and stores the read pattern of transform coefficients to determine the spatial or temporal similarity of the patterns. 
     According to yet another embodiment of the present invention, an image data decoder is provided which employs therein an error detection apparatus. 
     According to still another embodiment of the present invention, a computer readable recording medium is provided, having embodied thereon a computer program for executing an error detection method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent through a detailed description of exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a functional block diagram of procedures of detecting an error in a received image data stream and decoding the image data according to the conventional art; 
         FIG. 2  shows drawings depicting images to which error detection and error concealment methods are applied according to the conventional art; 
         FIG. 3  is a flowchart of a method of detecting an error in an image data stream according to an exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart for explaining in detail the method of detecting an error in the image data stream shown in  FIG. 3 , according to an exemplary embodiment of the present invention; 
         FIGS. 5A-5D  show drawings for explaining the spatial similarity determination in the course of detecting an error in an image data stream according to an exemplary embodiment of the present invention; 
         FIGS. 6A-6D  show drawings for explaining the temporal similarity determination in the course of detecting an error in an image data stream according to an exemplary embodiment of the present invention; and 
         FIG. 7  is a functional block diagram of an apparatus for detecting an error in an image data stream according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The attached drawings for illustrating exemplary embodiments of the present invention are referred to in order to gain a sufficient understanding of the present invention, the merits thereof, and the objectives accomplished by the implementation of the present invention. 
     Hereinafter, the present invention will be described in detail by explaining exemplary embodiments of the invention with reference to the attached drawings. Like reference numerals in the drawings denote like elements. In other instances, the detailed description of well-known features is omitted or simplified when being considered to obscure the present invention. 
       FIG. 3  is a flowchart of a method of detecting an error in an image data stream according to an embodiment of the present invention. 
     Referring to  FIG. 3 , the method includes the main operations of, determining whether an error is present in an input image data stream ( 310 ); and according to the result of the determining ( 320 ), detecting a starting point of the error ( 330 ) if an error occurs. 
     The error occurrence may be determined by performing a range check to check corresponding symbols, but this determination result may be for errors that have occurred previously due to the characteristics of variable length coding. That is, once a received bit sequence corresponds to any symbol in a range of defined symbols, even when the received bit sequence is different from the original bit sequence, the error would not be detected immediately. Therefore, the error detection might be performed at the next bit sequence not corresponding to any symbol in the range of defined symbols, and thus an error is detected not for the symbol where the error actually occurs, but for the next symbol. 
     Therefore, the error detection method cannot accurately detect an error occurrence position which is necessary to restore or conceal an error, and even though an image is reproduced through the concealment or restoring process, the quality of service (QoS) of the image is not satisfactory. Thus, when an error occurs, the detection of an accurate starting position of the error is required. 
       FIG. 4  is a flowchart for explaining in detail the method of detecting an error in the image data stream shown in  FIG. 3 . 
     Referring to  FIG. 4 , detailed procedures for detecting the error are described if an error occurs ( 420 ), after it is determined whether an error is present in the input image data stream ( 410 ). 
     In operation  430 , it is determined whether patterns of transform coefficients generated during the decoding of the input image data stream are similar to one another when it is determined that an error occurs ( 420 ). In other words, the similarity between the patterns of the transform coefficients is determined using the transform coefficients generated during the decoding of the input image data stream. The transform coefficients, which are coefficients of several frequency components ranging from direct current (DC) components to high-frequency components, show a definite pattern in which the transform coefficients are concentrated towards a low-frequency energy, the transform coefficients being generated on a 16×16 basis for each macroblock in the case of the discrete cosine transform, because this transform is performed on a macroblock basis. Thus, in a macroblock where an error occurs, the transform coefficients show an abnormal pattern, and more accurate error detection can be performed by determining if the patterns are similar spatially or temporally to one another, which will be described later. 
     The similarity determination may be performed on the entire macroblock, or performed on a plurality of sections into which the macroblock is divided. 
     As an example of the similarity determination, the pattern of the transform coefficients may be divided into sections, and according to the difference in the number of the transform coefficients which have a non-zero value in each section, the similarity of the patterns can be determined. This similarity determination will be described in detail from the point of view of space or time with reference to  FIGS. 5A-5D  and  6 A- 6 D. 
     The pattern similarity determination in each section can be performed by comparing a threshold value with the sum of weighted values which are set as experimental values in each section. 
     In operation  440 , the starting position of the error is detected based on the determination of the similarity between patterns of the transform coefficients. Specifically, by comparing a value obtained by the similarity determination with a predetermined threshold value that is determined as an experimental value, a corresponding macroblock can be determined as the starting position of the error when the value is greater than the threshold value. 
       FIGS. 5A-5D  show drawings for explaining the spatial similarity determination in the course of detecting an error in an image data stream according to an embodiment of the present invention. 
     Referring to  FIGS. 5A-5D , a spatial pattern similarity between a macroblock and surrounding macroblocks in a current image frame can be determined. In  FIG. 5A , the current macroblock “current MB” and the surrounding macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C” which have been already decoded in the same frame (or slice) are shown. 
     To improve the reliability of error detection based on the spatial repetition that forms a screen, the similarity between the patterns of the transform coefficients of surrounding macroblocks adjacent to the current macroblock in the same screen is determined. 
     Before the pattern similarity determination, the macroblock may be divided into a predetermined number of areas as shown in  FIGS. 5B ,  5 C and  5 D, and this is because the characteristics of the common pattern shown in each area differ from each other even in the same macroblock. By dividing the macroblock into a plurality of areas, the accuracy of the determination can be enhanced. 
     More specifically, in  FIG. 5B , the transform coefficients are compared with each other in a single zone, and the comparison method is as follows:
         (i) “Curr”, the number of transform coefficients which have a non-zero value in the current macroblock, is obtained,   (ii) “Prev”, the average value of the numbers of transform coefficients which have a non-zero value in the previous macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C”, is obtained, and   (iii) When the value of (Curr−Prev) is greater than a threshold value, the current macroblock may be considered as the starting position of the error occurrence. The threshold value is obtained by experiment, and can be appropriately selected according to the reliability necessary for the error detection.       

     Therefore, if the error occurrence is detected by the range check method thereafter in the course of the decoding process, in regard to the position where the error actually occurs in the current image frame, it can be determined that the corresponding error occurrence time is the point of time determined in the above process (iii). 
     In  FIG. 5C  in which the macroblock is divided into two zones, the comparison method is as follows. In  FIG. 5C , the left upper corner is a low-frequency area, and the right lower corner is a high-frequency area.
         (i) “Curr_low”, the number of transform coefficients which have a non-zero value in the low-frequency area of the current macroblock, is obtained,   (ii) “Prev_low”, the average value of the numbers of transform coefficients which have a non-zero value in the low-frequency area of the previous macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C”, is obtained,   (iii) “Curr_high”, the number of transform coefficients in the high-frequency area of the current macroblock, is obtained,   (iv) “Prev_high”, the average value of the numbers of transform coefficients which have a non-zero value in the high-frequency area of the previous macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C”, is obtained, and   (v) if the value of {w 1 ×(Curr_low−Prev_low)}+{w 2 ×(Curr_high−Prev_high)} is greater than a threshold, it can be determined that the current macroblock is the starting position of the error occurrence.       

     In the process (v), w 1  and w 2  are weighted variables which are determined through an experiment; for example, the values of w 1  and w 2  may be w 1 =0.3, and w 2 =0.7. The values of the weighted variables should be decided appropriately according to the distribution pattern of the transform coefficients after a transform algorithm is applied, for example, a discrete cosine transform (DCT). 
     In  FIG. 5D  in which the macroblock is divided into three zones, the left upper corner is a low-frequency area, the middle zone is a middle-frequency area, and the right lower corner is a high-frequency area. The comparison method is as follows:
         (i) “Curr_low”, the number of transform coefficients which have a non-zero value in the low-frequency area of the current macroblock, is obtained,   (ii) “Prev_low”, the average value of the numbers of transform coefficients which have a non-zero value in the low-frequency area of the previous macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C”, is obtained,   (iii) “Curr_mid”, the number of transform coefficients which have a non-zero value in the middle-frequency area of the current macroblock, is obtained,   (iv) “Prev_mid”, the average value of the numbers of transform coefficients which have a non-zero value in the middle-frequency area of the previous macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C”, is obtained,   (v) “Curr_high”, the number of transform coefficients in the high-frequency area of the current macroblock, is obtained,   (vi) “Prev_high”, the average value of the numbers of transform coefficients which have a non-zero value in the high-frequency area of the previous macroblocks “previous MB-A”, “previous MB-B”, and “previous MB-C” is obtained, and   (vii) if the value of {w 1 ×(Curr_low−Prev_low)}+{w 2 ×(Curr_mid−Prev_mid)}+{w 3 ×(Curr_high−Prev_high)} is greater than a threshold, it can be determined that the current macroblock is the starting position of the error occurrence.       

     Like  FIG. 5C , in the process (vii), w 1 , w 2 , and w 3  are weighted variables which are determined through experiment; for example, the values of w 1 , w 2 , and w 3  may be w 1 =0.1, w 2 =0.3, and w 3 =0.6. 
       FIGS. 6A-6D  show drawings for explaining the temporal similarity determination in the course of detecting an error in an image data stream according to an embodiment of the present invention. 
     Referring to  FIGS. 6A-6D , the temporal similarity can be determined between a current macroblock and a corresponding macroblock in a reference frame which a current image frame refers to.  FIG. 6A  shows a reference macroblock “reference MB” which the current macroblock “current MB” refers to. 
     To improve the reliability of error detection in image data sequences, which are input time-sequentially, based on the temporal repetition which forms a screen, the similarity of patterns of transform coefficients is determined from the reference macroblock which the current macroblock that is predictively coded refers to. 
     Like in  FIGS. 5A-5D , the macroblock can be divided into a predetermined number of zones as shown in  FIGS. 6B ,  6 C and  6 D, which can enhance the accuracy of the determination, because characteristics of the common patterns differ from each other according to frequency in the same macroblock. 
     In  FIG. 6B , the macroblock is a single zone, and the comparison method is as follows:
         (i) “Curr”, the number of transform coefficients which have a non-zero value in the current macroblock, is obtained,   (ii) “Prev”, the number of transform coefficients which have a non-zero value in the reference macroblocks, is obtained, and   (iii) if the value of (Curr−Prev) is greater than a threshold, it can be considered that the current macroblock is the starting position of the error occurrence.       

     Therefore, if the error occurrence is detected by the range check method thereafter in the course of the decoding process, in regard to the position where the error actually occurs in the current image frame, it can be determined that the corresponding error occurrence time is the point of time determined in the above process (iii). 
     In  FIG. 6C  in which the macroblock is divided into two zones, wherein the left upper corner is a low-frequency area and the right lower corner is a high-frequency area, the comparison method is as follows;
         (i) “Curr_low”, the number of transform coefficients which have a non-zero value in the low-frequency area of the current macroblock, is obtained,   (ii) “Prev_low”, the average value of the numbers of transform coefficients which have a non-zero value in the low-frequency area of the reference macroblocks, is obtained,   (iii) “Curr_high”, the number of transform coefficients in the high-frequency area of the current macroblock, is obtained,   (iv) “Prev_high” the average value of the numbers of transform coefficients which have a non-zero value in the high-frequency area of the reference macroblocks, is obtained, and   (v) if a value of {w 1 ×(Curr_low−Prev_low)}+{w 2 ×(Curr_high−Prev_high)} is greater than a threshold, it can be determined that the current macroblock is the starting position of the error occurrence.       

     The variables w 1  and w 2  are weighted variables which are determined through experiment; for example, the values of w 1  and w 2  may be w 1 =0.3, and w 2 =0.7. 
     In  FIG. 6D  the macroblock is divided into three zones, wherein the left upper corner is a low-frequency area, the middle zone is a middle-frequency area, and the right lower corner is a high-frequency area. The comparison method is as follows:
         (i) “Curr_low”, the number of transform coefficients which have a non-zero value in the low-frequency area of the current macroblock, is obtained,   (ii) “Prev_low” the average value of the number of transform coefficients which have a non-zero value in the low-frequency area of the reference macroblocks, is obtained,   (iii) “Curr_mid” the number of transform coefficients which have a non-zero value in the middle-frequency area of the current macroblock, is obtained,   (iv) “Prev_mid”, the average value of the number of transform coefficients which have a non-zero value in the middle-frequency area of the reference macroblocks, is obtained,   (v) “Curr_high”, the number of transform coefficients in the high-frequency area of the current macroblock, is obtained,   (vi) “Prev_high”, the average value of the number of transform coefficients which have a non-zero value in the high-frequency area of the reference macroblocks, is obtained, and   (vii) if the value of {w 1 ×(Curr_low−Prev_low)}+{w 2 ×(Curr_mid−Prev_mid)}+{w 3 ×(Curr_high−Prev_high)} is greater than a threshold, it can be determined that the current macroblock is the starting position of the error occurrence.       

     Likewise, w 1 , w 2 , and w 3  are weighted variables which are determined through experiment; for example, the values of w 1 , w 2 , and w 3  may be w 1 =0.1, w 2 =0.3, and w 3 =0.6. 
       FIG. 7  is a functional block diagram of an apparatus  700  for detecting an error in an image data stream according to an embodiment of the present invention. 
     Referring to  FIG. 7 , the error detection apparatus  700  includes mainly an error determination unit  710  which determines whether an error is present in an input image data stream, and an error detection unit  720  which detects the starting position of an error if it is determined that an error occurs. 
     The error detection unit  720  includes a pattern determination unit  722  which determines similarity between patterns of transform coefficients, which are generated in the course of decoding of the input image data stream, if the error occurs. Moreover, the error detection unit  720  may further include a pattern analyzing unit  721  which reads the patterns of transform coefficients of a macroblock in a current image frame and stores the read patterns of transform coefficients in order to determine the spatial or temporal similarity between patterns. 
     As such, the error detection apparatus  700  may be employed in an image decoder, and may further include a storage medium, such as external DDR memory that stores transform coefficients of previous macroblocks, a syntax parser, or a macroblock/slice decoder. 
     The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, and optical data storage devices. The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     As described above, according to the present invention, a method of and an apparatus for detecting an error in an image data stream allow a decoder to independently detect an accurate position of the error, and improve the quality of an image which is replaced and restored using an error concealment method. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.