Abstract:
In a method and apparatus for decoding an image data block having multiple data lines compressed into codeword streams, when one of multiple different code types and a line-end code, which are defined by a run-length encoding algorithm, is detected in an N-bit part of an M-bit segment of the codeword streams, the detected one of the code types and the line-end code is interpreted to generate a decompressed output. The N-bit part is further analyzed to find a succeeding one of the code types and the line-end code therein. When an error is detected in the N-bit part, a miss code is outputted to stop interpretation and further generation of the decompressed output. Analysis of the N-bit part is continued until the line-end code is detected. The line-end code is then interpreted so as to complete decompression of one of the data lines.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a method and apparatus for decoding compressed image data, more particularly to a method and apparatus for decoding compressed image data and capable of preventing error propagation.  
           [0003]    2. Description of the Related Art  
           [0004]    Multimedia has become a widely used data format in computer and communication systems. However, multimedia data is very large in general so as to result in a heavy burden for storage and transmission. In order to reduce the memory space for storage and the channel bandwidth for transmission, data compression is necessary.  
           [0005]    In a conventional compression algorithm, such as a run-length encoding algorithm, different code types and a line-end code are defined according to the following rules:  
           [0006]    1. 1 to 3 continuous pixels with the same pixel data can be encoded into a code type  0  that has a 4-bit code length, as shown in Table 1 of FIG. 1.  
           [0007]    2. 4 to 15 continuous pixels with the same pixel data can be encoded into a code type  1  that has an 8-bit code length, as shown in Table 2 of FIG. 2.  
           [0008]    3. 16 to 63 continuous pixels with the same pixel data can be encoded into a code type  2  that has a 12-bit code length, as shown in Table 3 of FIG. 3.  
           [0009]    4. 64 to 255 continuous pixels with the same pixel data can be encoded into a code type  3  that has a 16-bit code length, as shown in Table 4 of FIG. 4.  
           [0010]    5. Continuous pixels with the same pixel data present at the end of a pixel line can be encoded into a line-end code that has a 16-bit code length, as shown in Table 5 of FIG. 5.  
           [0011]    6. After a data line compressed in accordance with the above rules has been completed, a 4-bit dummy code, such as “0000” or “1111”, may be inserted for byte alignment.  
           [0012]    In the above-described run-length encoding algorithm, a data line must have byte alignment, i.e., the data line has a code length that is a multiple of 8 bites. When a data line compressed according to the rules 1 to 5 includes x code type  0  (4-bit code length, x is an integer not less than zero), y code type  1  (8-bit code length, y is an integer not less than zero), z code type  2  (12-bit code length, z is an integer not less than zero), u code type  3  (16-bit code length, u is an integer not less than zero) and a line-end code (16-bit code length), the data line has a code length equal to 4x+8y+12z+16u+16=4*(x+2y+3z+4u+4) bits, and rule 6 can be applied to ensure that the code length of the data line is a multiple of 8 bits.  
           [0013]    According to the above rules, for example, if there are 2 continuous pixels with the same pixel data of “01” (0101), the pixels (0101) can be encoded into “1001” according to the code type  0  shown in FIG. 1. If there are 6 continuous pixels with the same pixel data of “01”, the pixels can be encoded into “00011001” according to the code type  1  shown in FIG. 2. If there are 63continuous pixels with the same pixel data of “01”, the pixels of 126 bits can be encoded into “000011111101” (12 bits) according to the code type  2  shown in FIG. 3. If there are 255 continuous pixels with the same pixel data of “01”, the pixels of 510 bits can be encoded into “0000001111111101”(16 bits) according to the code type  3  shown in FIG. 4. If there are a plurality of continuous pixels from a current pixel to the end of a pixel line with the same pixel data of “01”, the pixels can be encoded into “0000000000000001” (16 bits) according to the line-end code shown in FIG. 5.  
           [0014]    In summary, the aforesaid run-length encoding algorithm has the following properties:  
           [0015]    1. Each of the code types has a specific code length.  
           [0016]    2. Since a line-end code is present at the end of each data line, the compression algorithm is line-based.  
           [0017]    3. During the compression process, a proper code type can be used to represent a specific pixel combination. Accordingly, during the decompression process, an input codeword must be one of the code types and the line-end code when the input codeword is not corrupted.  
           [0018]    [0018]FIG. 6 illustrates a conventional run-length decoder  1  for decoding an image data block that has a plurality of data lines compressed into code word streams in accordance with the above-described run-length encoding algorithm. The conventional run-length decoder  1  includes a shifter  11 , a codeword comparator  12 , a code interpreter  13 , and a shifter controller  14 .  
           [0019]    The shifter  11  has a codeword input for receiving a 32-bit segment of the codeword streams, and a codeword output  112  for outputting a 16-bit part of the 32-bit segment received thereby.  
           [0020]    The codeword comparator  12  is connected to the codeword output  112  and is operable so as to orderly and continuously analyze the 16-bit part to verify presence of one of the code types (code type  0 , code type  1 , code type  2  and code type  3 ) and the line-end code therein. The codeword comparator  12  outputs one of the code types and the line-end code upon detecting the presence of said one of the code types and the line-end in the 16-bit part.  
           [0021]    The code interpreter  13  is connected to the codeword comparator  12  and is operable so as to interpret said one of the code types and the line-end code from the codeword comparator  12  in accordance with the run-length encoding algorithm to generate a decompressed output.  
           [0022]    The shifter controller  14  is connected to the shifter  11  and the codeword comparator  12 . The shifter controller  14  controls the shifter  11  to update the 16-bit part by discarding the detected one of the code types and the line-end code and by appending a succeeding P-bit part of the 32-bit segment to remainder of the 16-bit part. The P-bit part has a code length equal to that of the detected one of the code types and the line-end code. The shifter  11  generates a request for updating the 32-bit segment when an analyzed length thereof is equal to 16. As such, when the codeword comparator  12  outputs a code type  0 , the 16-bit part is updated by discarding the code type  0  therefrom and by appending a succeeding 4-bit part of the 32-bit segment to the remainder of the 16-bit part. When the codeword comparator  12  outputs a code type  1 , the 16-bit part is updated by discarding the code type  1  therefrom and by appending a succeeding 8-bit part of the 32-bit segment to the remainder of the 16-bit part. When the codeword comparator  12  outputs a code type  2 , the 16-bit part is updated by discarding the code type  2  therefrom and by appending a succeeding 12-bit part of the 32-bit segment to the remainder of the 16-bit part. When the codeword comparator  12  outputs a code type  3 , the shifter  11  generates a request for updating the 32-bit segment by discarding the analyzed first 16 bits therefrom and by appending 16 bits of the codeword streams to the second 16 bits of the 32-bit segment, and the 16-bit part is updated with a corresponding part of the updated 32-bit segment.  
           [0023]    In the following example, the conventional run-length decoder  1  is used for decoding a 100-pixel image data line compressed into a codeword stream. The codeword stream is “1100-00011101-000001000010-0000 000100000011-0000000000000010”.  
           [0024]    The decompression process of the data line is as follows:  
           [0025]    1. Initially, the first 32 bits (“1100-00011101-000001000010-00000001”) of the codeword stream are received by the shifter  11  and serves as the 32-bit segment. The shifter  11  outputs the first 16 bits (“1100-00011101-0000”) of the 32-bit segment that serves as the 16-bit part to the codeword comparator  12 . The codeword comparator  12  verifies the presence of a code type  0  (“1100”) in the 16-bit part, and outputs the code type  0  (“1100”). The code interpreter  13  interprets the code type  0  from the codeword comparator  12  in accordance with the above-described run-length encoding algorithm so as to generate a decompressed output corresponding to the code type  0  (“1100”) that has 3 continuous pixels (i.e., pixel length=3) with the same pixel data of “00”. The shifter controller  14  controls the shifter  11  to update the 16-bit part (“1100-00011101-0000”) by discarding the code type  0  (“1100”) from the 16-bit part (“1100-00011101-0000”) and by appending a succeeding 4-bit part (“0100”) of the 32-bit segment to the remainder of the 16-bit part (“00011101-0000”) such that the 16-bit part becomes “00011101-00000100”.  
           [0026]    2. Subsequently, the codeword comparator  12  continuous analysis of the 16-bit part (“00011101-0000 0100”), verifies presence of a code type  1  (“00011101”) in the 16-bit part, and outputs the code type  1  (“00011101”). The code interpreter  13  interprets the code type  1  from the codeword comparator  12  in accordance with the above-described run-length encoding algorithm so as to generate a decompressed output corresponding to the code type  1  (“00011101”) that has 7 continuous pixels (i.e., pixel length=7) with the same pixel data of “01”. The shifter controller  14  controls the shifter  11  to update the 16-bit part (“00011101-00000100”) by discarding the code type  1  (“00011101”) from the 16-bit part (“00011101-00000100”) and by appending a succeeding 8-bit part (“00100000”) of the 32-bit segment to the remainder of the 16-bit part (“00000100”) such that the 16-bit part becomes “000001000010-0000”.  
           [0027]    3. Then, the codeword comparator  12  continuous analysis of the 16-bit part (“000001000010-0000”) verifies presence of a code type  2  (“000001000010”) in the 16-bit part, and outputs the code type  2  (“000001000010”). The code interpreter  13  interprets the code type  2  from the codeword comparator  12  in accordance with the above-described run-length encoding algorithm so as to generate a decompressed output corresponding to the code type  2  (“000001000010”) that has 16 continuous pixels (i.e., pixel length=16) with the same pixel data of “10”. Since the analyzed length of the current 32-bit segment is equal to 24 (4+8+12), the shifter  11  generates a request for updating the 32-bit segment by discarding the first 16 bits (“1100-00011101-0000”) from the 32-bit segment and by appending a succeeding 16 bits (“00000011-00000000”) of the codeword stream to the remainder of the 32-bit segment (“01000010-00000001”) such that the 32-bit segment becomes “01000010-0000 000100000011-00000000”. The shifter controller  14  controls the shifter  11  to update the 16-bit part (“000001000010-0000”) by discarding the code type  2  (“000001000010”) from the 16-bit part (“000001000010-0000”) and by appending a succeeding 12-bit part (“000100000011”) of the 32-bit segment to the remainder of the 16-bit part (“0000”) such that the 16-bit part becomes “0000000100000011”.  
           [0028]    4. Thereafter, the codeword comparator  12  continuous analysis of the 16-bit part (“10000000100000011”), verifies presence of a code type  3  (“0000000100000011”) in the 16-bit part, and outputs the code type  3  (“0000000100000011”). The code interpreter  13  interprets the code type  3  from the codeword comparator  12  in accordance with the above-described run-length encoding algorithm so as to generate a decompressed output corresponding to the code type  3  (“0000000100000011”) that has 64 continuous pixels (i.e., pixel length=64) with the same pixel data of “11”. Since the analyzed length of the current 32-bit segment is equal to 16, the shifter  11  once again generates a request for updating the 32-bit segment according to the previously described manner such that the 32-bit segment becomes “00000011-0000000000000010-xxxxxxxx” (“x” means do not care). The shifter controller  14  controls the shifter  11  to update the 16-bit part (“0000000100000011”) by discarding the code type  3  (“0000000100000011”) from the 16-bit part such that the 16-bit part becomes “0000000000000010”.  
           [0029]    5. Eventually, the codeword comparator  12  verifies the presence of a line-end code (“0000000000000010”) in the 16-bit part, and outputs the line-end code (“0000000000000010”). The code interpreter  13  interprets the line-end code from the codeword comparator  12  in accordance with the above-described run-length algorithm so as to generate a decompressed output corresponding to the line-end code (“0000000000000010”) that has 10 (equal to 100-3-7-16-64) continuous pixels with the same pixel data of “10”. As such, decompression of the data line is thus completed.  
           [0030]    Due to the presence of noise during transmission or storage, the compressed codeword stream may be corrupted such that the codeword comparator  12  is unable to verify the presence of a correct one of the code types and the decoder  1  thus fails to output a correct decompressed output. It is noted that when the compressed data is corrupted, the conventional run-length decoder  1  cannot continuously decode the compressed data. Furthermore, when the codeword comparator  12  is unable to verify the presence of the correct one of the code types, the shifter controller  14  will be unable to properly control the shifter  11  to update the 16-bit part, thereby resulting in an adverse affect on a succeeding decompressed output, which arises in error propagation.  
         SUMMARY OF THE INVENTION  
         [0031]    Therefore, the object of the present invention is to provide a method and apparatus for decoding compressed image data and capable of preventing error propagation.  
           [0032]    According to one aspect of the present invention, a method is adapted for decoding an image data block that has a plurality of data lines compressed into codeword streams in accordance with a run-length encoding algorithm. The run-length encoding algorithm defines a plurality of different code types and a line-end code, each of which has a specific code length. Each of the code types contains a reference to number of continuous and repetitive pixel data. The method comprises the steps of:  
           [0033]    (a) receiving an M-bit segment of the codeword streams;  
           [0034]    (b) outputting an N-bit part of the received M-bit segment;  
           [0035]    (c) orderly analyzing the N-bit part to verify presence of one of the code types and the line-end code therein;  
           [0036]    (d) upon detecting the presence of one of the code types and the line-end code in the N-bit part, outputting said one of the code types and the line-end code, interpreting said one of the code types and the line-end code in accordance with the run-length encoding algorithm to generate a decompressed output, and continuing with analysis of the N-bit part to find a succeeding one of the code types and the line-end code therein; and  
           [0037]    (e) upon detecting the presence of an error in the N-bit part, outputting a miss code to stop interpretation and further generation of the decompressed output, continuing with analysis of the N-bit part until the line-end code is detected, outputting the line-end code when the line-end code is eventually detected, and interpreting the line-end code in accordance with the run-length encoding algorithm so as to complete decompression of at least one of the data lines.  
           [0038]    According to another aspect of the present invention, an apparatus is adapted for decoding an image data block that has a plurality of data lines compressed into codeword streams in accordance with a run-length encoding algorithm. The run-length encoding algorithm defines a plurality of different code types and a line-end code, each of which has a specific code length. Each of the code types contains a reference to number of continuous and repetitive pixel data. The apparatus comprises:  
           [0039]    a shifter having a codeword input for receiving an M-bit segment of the codeword streams, and a codeword output for outputting an N-bit part of the M-bit segment received thereby;  
           [0040]    a codeword comparator connected to the codeword output and operable so as to orderly and continuously analyze the N-bit part to verify presence of one of the code types and the line-end code therein,  
           [0041]    the codeword comparator outputting one of the code types and the line-end code upon detecting the presence of said one of the code types and the line-end code in the N-bit part,  
           [0042]    the codeword comparator subsequently outputting a miss code upon detecting presence of an error in the N-bit part,  
           [0043]    the output of said codeword comparator changing from the miss code to the line-end code when the presence of the line-end code is eventually detected thereby; and  
           [0044]    a code interpreter connected to the codeword comparator and operable so as to interpret said one of the code types and the line-end code from the codeword comparator in accordance with the run-length encoding algorithm to generate a decompressed output, and so as to stop interpretation and further generation of the decompressed output upon receipt of the miss code from the codeword comparator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]    Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which:  
         [0046]    [0046]FIG. 1 is a table illustrating a code type  0  defined by a run-length encoding algorithm;  
         [0047]    [0047]FIG. 2 is a table illustrating a code type  1  defined by the run-length encoding algorithm;  
         [0048]    [0048]FIG. 3 is a table illustrating a code type  2  defined by the run-length encoding algorithm;  
         [0049]    [0049]FIG. 4 is a table illustrating a code type  3  defined by the run-length encoding algorithm;  
         [0050]    [0050]FIG. 5 is a table illustrating a line-end code defined by the run-length encoding algorithm;  
         [0051]    [0051]FIG. 6 is a schematic circuit block diagram illustrating a conventional run-length decoder; and  
         [0052]    [0052]FIG. 7 is a schematic circuit block diagram illustrating the preferred embodiment of an apparatus for decoding compressed image data according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0053]    Referring to FIG. 7, according to the preferred embodiment of the present invention, an apparatus  2  is adapted for decoding an image data block (not shown) that has a plurality of data lines compressed into codeword streams in accordance with the previously described run-length encoding algorithm, and is shown to include a shifter  21 , a codeword comparator  22 , a code interpreter  23 , and a shifter controller  24 .  
         [0054]    The shifter  21  has a codeword input  211  for receiving a 32-bit segment of the codeword streams, and a codeword output  212  for outputting a 16-bit part of the 32-bit segment received thereby.  
         [0055]    The codeword comparator  22  is connected to the codeword output  212  and is operable so as to orderly and continuously analyze the 16-bit part to verify presence of one of different code types and a line-end code therein. The codeword comparator  22  outputs one of the code types and the line-end code upon detecting the presence of said one of the code types and the line-end code in the 32-bit part. In this embodiment, the code types include the code type  0  with a 4-bit code length (as shown in Table 1 of FIG. 1), the code type  1  with a 8-bit code length (as shown in Table 2 of FIG. 2), the code type  2  with a 12-bit code length (as shown in Table 3 of FIG. 3) and the code type  3  with a 16-bit code length (as shown in Table 4 of FIG. 4). The line-end code has a 16-bit code length (as shown in Table 5 of FIG. 5). The definitions of the different code types and the line-end code are the same as those described beforehand. The codeword comparator  22  subsequently outputs a miss code upon detecting the presence of an error in the 16-bit part. The error is detected when, due to the corruption of the 16-bit part, the codeword comparator  22  cannot verify the presence of any one of the code types and the line-end code in the 16-bit part. The output of the codeword comparator  22  changes from the miss code to the line-end code when the presence of the line-end code is eventually detected thereby.  
         [0056]    The code interpreter  23  is connected to the codeword comparator  22  and is operable so as to interpret said one of the code types and the line-end code from the codeword comparator  22  in accordance with the run-length encoding algorithm to generate a decompressed output, and so as to stop interpretation and further generation of the decompressed output upon receipt of the miss code from the codeword comparator  22 .  
         [0057]    The shifter controller  24  is connected to the shifter  21  and the codeword comparator  22 . The shifter controller  24  controls the shifter  21  to update the 16-bit part by discarding the detected one of the code types and the line-end code from the 16-bit part and by appending a succeeding P-bit part of the 32-bit segment to the remainder of the 16-bit part. The P-bit part has a code length equal to that of the detected one of the code types and the line-end code. In this embodiment, the shifter controller  24  further controls the shifter  21  to up date the 16-bit part when the codeword comparator  22  generates the miss code by discarding a predetermined code length of the 16-bit part and by appending a succeeding Q-bit part of the 32-bit segment to the remainder of the 16-bit part. The Q-bit part has a code length equal to that of the predetermined code length. The predetermined code length is a common factor of the code lengths of the code types and the line-end code. In this embodiment, to achieve an increased decoding speed, the predetermined code length is a highest common factor of the code lengths of the code types and the line-end code, i.e., the predetermined code length is 4.  
         [0058]    When an analyzed length of the32-bit segment is equal to 16, the shifter  11  generates a request for updating the 32-bit segment by discarding the analyzed first 16bits therefrom and by appending 16 bits of the codeword streams to the second 16 bits of the 32-bit segment. The 16-bit part is then updated with a corresponding part of the updated 32-bit segment.  
         [0059]    The following example is proposed to illustrate how the apparatus  2  of the present invention decompresses a 100-pixel image data line compressed into a codeword stream that is identical to that described beforehand. The original codeword stream is “1100-00011101-000001000010-0000000100000011-000000 0000000010”. Due to the presence of noise during transmission or storage, the original codeword stream is corrupted and becomes “1100-00011101-000001000010-0000000000010011-000000 0000000010” (the underlined portions are corrupted). The following describes the decompression procedure of the apparatus  2  of the present invention:  
         [0060]    1. Since a part (from the first bit to the 24 th  bit) of the corrupted codeword stream is not corrupted, the decompressed result is the same as that obtained in the aforesaid decoder of the prior art, and a detailed description will be omitted herein for the sake of brevity. As such, the code interpreter  23  orderly and continuously generates a first decompressed output (pixel length=3, pixel data=“00”), a second decompressed output (pixel length=7, pixel data=“01”), and a third decompressed output (pixel length=16, pixel data =“10”).  
         [0061]    2. Subsequently, with the 32-bit segment being “01000010-0000000000010011-00000000”, the codeword comparator  22  continuous analysis of the 16-bit part (“0000000000010011”), and detects the presence of an error in the 16-bit part. The error is detected due to the presence of  11  zeros that do not conform with the definition of any of the code types and line-end code. The codeword comparator  22  subsequently outputs a miss code. When the code interpreter  23  receives the miss code from the codeword comparator  22 , the code interpreter  23  stops interpretation and further generation of the decompressed output. The shifter controller  24  controls the shifter  21  to update the 16-bit part when the shifter controller  24  receives the miss code from the codeword comparator  22  by discarding the first 4 bits (“0000”) of the 16-bit part (“0000000000010011”) and by appending a succeeding 4-bit part (“0000”) of the 32-bit segment to the remainder of the 16-bit part (“000000010011”) such that the 16-bit part becomes “000000010011-0000”.  
         [0062]    3. Then, because the codeword comparator  22  has yet to detect the presence of the line-end code, the codeword comparator  22  continuous to output the miss code. Since an analyzed length of the 32-bit segment is 16 (equal to 8+4+4), the shifter  21  generates a request for updating the current 32-bit segment by discarding the first 16 bits (“01000010-00000000”) from the 32-bit segment and by appending a succeeding 16 bits (“00000010-xxxxxxxx”) of the codeword stream to the remainder of the 32-bit segment (“00010011-00000000”) such that the 32-bit segment becomes “00010011-0000000000000010-xxxxxxxx” (“x” means do not care). The shifter controller  24  controls the shifter  21  to update the 16-bit part when the shifter controller  24  receives the miss code from the codeword comparator  22  such that the 16-bit part becomes “00010011-00000000”.  
         [0063]    4. Thereafter, the codeword comparator  22  still has yet to detect the presence of the line-end code and continuous to output the miss code. The shifter controller  24  controls the shifter  21  to update the 16-bit part when the shifter controller  24  receives the miss code from the codeword comparator  22  by discarding the first 4 bits (“0001”) of the 16-bit part (“00010011-00000000”) and by appending a succeeding 4-bit part (“0000”) of the 32-bit segment to the remainder of the 16-bit part (“0011-00000000”) such that the 16-bit part becomes “0011-000000000000”.  
         [0064]    5. Thereafter, the codeword comparator  22  still has yet to detect the presence of the line-end code and continuous to output the miss code. The shifter controller  24  controls the shifter  21  to update the 16-bit part when the shifter controller  24  receives the miss code from the codeword comparator  22  by discarding the first 4 bits (“0011”) of the 16-bit part (“0011-000000000000”) and by appending a succeeding 4-bit part (“0010”) of the 32-bit segment to the remainder of the 16-bit part (“00000000000”) such that the 16-bit part becomes “0000000000000010”.  
         [0065]    6. Eventually, the codeword comparator  22  verifies the presence of a line-end code (“0000000000000010”) in the 16-bit part, and outputs the line-end code (“0000000000000010”) accordingly. The code interpreter  23  interprets the line-end code from the codeword comparator  22  in accordance with the above-described run-length algorithm so as to generate a decompressed output corresponding to the line-end code (“0000000000000010”) that has 74 (equal to 100-3-7-16) continuous pixels with the same pixel data of “10”. As such, the decompression of a data line of the codeword stream is completed.  
         [0066]    Since the codeword comparator  22  of this invention is designed to output a miss code when the 16-bit part contains a corrupted one of the code types and the line-end code, the code interpreter  23  can stop interpretation and further generation of the decompressed output, and the shifter controller  24  can control the shifter  21  to update the 16-bit part until the line-end code is detected. As such, even though a data line of the codeword streams is corrupted due to the presence of noise, the apparatus  2  of this invention can still decode a succeeding data line of the codeword streams, and the error can be limited to the corrupted data line such that error propagation is thus prevented.  
         [0067]    While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.