Compression and decompression methods on two-dimensional image data

• US 5,883,975 A
• Filed: 09/11/1995
• Issued: 03/16/1999
• Est. Priority Date: 09/12/1994
• Status: Expired due to Fees

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation
• Inventors: Matsuura, Kazunori, Narita, Yoshinori
• Primary Examiner(s): Couso, Jose L.
• Application Number:
  US08/526,663
• Time in Patent Office: 1,282 Days
• Field of Search: 382/232, 382/233, 382/238, 382/239, 382/240, 382/243, 382/245, 382/246, 358/261.1, 358/261.2, 358/261.3, 341/51, 341/55, 341/79, 341/106
• US Class Current: 382/232
• CPC Class Codes: G06T 9/005 Statistical coding, e.g. Huffman, run length coding
• CPC Class Codes: H03M 7/3084 using adaptive string matching, e.g. the Lempel-Ziv method
• CPC Class Codes: H03M 7/46 Conversion to or from run-length codes, i.e. by representing the number of consecutive digits, or groups of digits, of the same kind by a code word and a digit indicative of that kind

Abstract

A data compression apparatus includes a dictionary buffer for holding data of the immediately preceding line of successively inputted two-dimensional image data, a buffer for extracting a plurality of continuous data strings from the inputted data of the current line to store them as a partial data string to be compressed, a scanning comparison section for detecting whether or not the same data string as the partial data string to be compressed exists in the data stored in the dictionary buffer, and a code generator by which in the case where the presence of the same data string is detected by the scanning comparison section, data of the position and the length of a partial data string on the data of the immediately preceding line stored in the dictionary buffer is outputted as data of that partial data string of the current line.

Claims

1. A data compression apparatus comprising:

reference memory means for holding data of the immediately preceding line for successively inputted two-dimensional image data;

buffer means for extracting a plurality of continuous data strings from an inputted signal of the current line to store them as a partial data string to be compressed;

scanning comparison means for detecting whether or not the same data string as a data string continuing from a pixel of interest of said partial data string to be compressed exists in the data stored in said reference memory means; and

code generating means by which in the case where the presence of the same data string is detected by said scanning comparison means, data of the position of a leading pixel of said partial data string relative to said pixel of interest and the length of said partial data string on the data of the immediately line stored in said reference memory means is determined and a corresponding code is outputted as data of that partial data string of the current line by referring to a table in which a different code is allotted in accordance with said relative position and the length of said partial data string for each position of the pixel of interest on the current line.

2. A data compression apparatus according to claim 1, wherein variable-length codes in at least three different cases including the case where the relative position of a pixel of interest of input data of the current line and a leading pixel of the same partial data string of the immediately preceding line is the same pixel position, the case where the relative position is shifted to the left by one pixel and the case where the relative position is shifted to the right by one pixel are stored in said table.

3. A data compression apparatus according to claim 1, wherein in the case where there is inputted data for which the same partial data string does not exist in the immediately preceding line, a corresponding code is outputted by referring to a run-length table in which a different variable-length code is allotted in accordance with the position of that data on the current line and the length of the same continuous data.

4. A data compression apparatus according to claim 2, wherein in the case where there is inputted data for which the same partial data string does not exist in the immediately preceding line, a corresponding code is outputted by referring to a run-length table in which a different variable-length code is allotted in accordance with the position of that data on the current line and the length of the same continuous data.

5. A data compression apparatus according to claim 3, wherein the variable-length codes stored in said run-length table are allotted with variable-length codes which are different from the variable-length codes stored in said table.

6. A data compression apparatus according to claim 4, wherein the variable-length codes stored in said run-length table are allotted with variable-length codes which are different from the variable-length codes stored in said table.

7. A data decompression apparatus for compressed two-dimensional image data in which in the case where there exists in two-dimensional image data a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there is successively inputted, as data of the overlapping portion, data encoded by referring to a table in which a different code is allotted in accordance with the relative position and the length of a partial data string on the immediately preceding line for each leading position of the compressed data on the current line, the apparatus comprising:

buffer memory means for holding data of the immediately preceding line;

inverse transformation tables provided for the respective positions of a pixel of interest on the current line for outputting data of the relative position and the length of a partial data string on the immediately preceding line in accordance with an inputted code; and

decoding means for selecting an inverse transformation table corresponding to the position on the current line of a pixel of interest to be subjected to the decoding of the inputted data, extracting said code from the inputted data to refer to the selected table on the basis of said code so that data of the relative position and the length of a partial data string on the immediately preceding line is derived from said inverse transformation table, and extracting, from said buffer memory means, data which continues from a bit corresponding to the relative position of said partial data string to the extent of the length of said partial data string, the extracted data being outputted as decompressed data.

8. A data decompression apparatus for compressed two-dimensional image data in which in the case where there exists in two-dimensional image data a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, data allotted with a code differing in accordance with the relative position and the length of a partial data string on the immediately preceding line for each leading position of the compressed data on the current line is successively inputted as data of the overlapping portion, and in which in the case where the same partial data string does not exist, that data is successively inputted together with data allotted with a code which differs in accordance with the position of that data on the current line and the length of the same continuous data, the apparatus comprising:

a counter for counting the position on the current line of a pixel of interest to be subjected to the decoding of the inputted data;

buffer memory means for holding data of the immediately preceding line;

inverse transformation tables provided for the respective positions of a pixel of interest on the current line for outputting data of the relative position and the length of a partial data string on the immediately preceding line or a run length in accordance with an inputted code;

a decoder for selecting an inverse transformation table corresponding to the count value of said counter, and extracting said code from the inputted data to refer to the selected table on the basis of said code so that a mode in which the partial data string exists or a mode in which the partial data string does not exist is outputted while data of the relative position and the length of a partial data string on the immediately preceding line or data of the length of the same continuous data is derived from said inverse transformation table;

first decoding means by which in the case of the mode in which the partial data string exists, data continuing from a bit corresponding to the relative position of said partial data string to the extent of the length of said partial data string is extracted from said buffer memory means and the extracted data is outputted as decompressed data; and

second decoding means by which in the case of the mode in which the partial data string does not exist, data succeeding said code is continuously outputted to the extent of the length of the same continuous data.

9. A data compression method for two-dimensional image data in which a compression processing is performed in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, the method comprising:

preparing tables which are provided for the respective values of T, T being T=0 when x=0, T=1 when 1≦

x≦

H-N, and T=x-H+N+1 when H-N+1≦

x≦

H-1 where H is the number of pixels in one line of said two-dimensional image data, N is the maximum detection string length of said partial data string (1≦

N≦

H) and x is the position of a pixel of interest on the current line from a leading pixel thereof (0≦

x≦

H-1) and in which a different code is allotted in accordance with the relative position of a leading pixel of said partial data string and said pixel of interest and the length of said partial data string on the immediately preceding line; and

determining, in the case where there is detected in the current line a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, the value of T on the basis of the position x of said pixel of interest on the current line and selecting the corresponding table on the basis of the determined value of T to output a code corresponding to the leading position and the length of the partial data string on the current line.

10. A data compression method according to claim 9, wherein codes in at least three different cases including the case where the relative position of a pixel of interest of input data of the current line and a leading pixel of the same partial data string of the immediately preceding line is the same pixel position, the case where the relative position is shifted to the left by one pixel and the case where the relative position is shifted to the right by one pixel are stored in said table in the form of variable-length codes.

11. A data compression method according to claim 9, wherein in the case where there is not detected in the current line a partial data string which is the same as the immediately preceding line, a corresponding code is outputted by referring to a run-length tables which are prepared for the respective values of T and in which a different code is allotted in accordance with the length of the same continuous data.

12. A data compression method according to claim 10, wherein in the case where there is not detected in the current line a partial data string which is the same as the immediately preceding line, a corresponding code is outputted by referring to a run-length tables which are prepared for the respective values of T and in which a different code is allotted in accordance with the length of the same continuous data.

13. A data compression method according to claim 11, wherein the codes stored in said run-length table are allotted with codes which are different from the codes stored in said table.

14. A data compression method according to claim 12, wherein the codes stored in said run-length table are allotted with codes which are different from the codes stored in said table.

15. A data compression method according to claim 13, wherein said table and said run-length table are united into one so that codes for the length of the same partial data string in three different cases including the case where the relative position is the same pixel position, the case where the relative position is shifted to the left by one pixel and the case where the relative position is shifted to the right by one pixel and codes for the length of the same continuous data in the case of run length are stored in the form of different variable-length codes.

16. A data compression method according to claim 14, wherein said table and said run-length table are united into one so that codes for the length of the same partial data string in three different cases including the case where the relative position is the same pixel position, the case where the relative position is shifted to the left by one pixel and the case where the relative position is shifted to the right by one pixel and codes for the length of the same continuous data in the case of run length are stored in the form of different variable-length codes.

17. A data decompression method for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there is successively inputted as data of the overlapping portion a code which differs in accordance with the relative position of a leading pixel of said partial data string and a pixel of interest and the length of said partial data string on the immediately preceding line for each value of T, T being T=0 when x=0, T=1 when 1≦

x≦

H-N, and T=x-H+N+1 when H-N+1≦

x≦

H-1 where H is the number of pixels in one line of said two-dimensional image data, N is the maximum detection string length of said partial data string (1≦

N≦

H) and x is the position of said pixel of interest on the current from a leading pixel thereof (0≦

x≦

H-1), the method comprising;

holding at least decompressed data of the immediately preceding line in a buffer;

determining the value of T on the basis of the position x of said pixel of interest to be decompressed on the current line to select a corresponding table on the basis of the determined value of T from tables which are prepared for the respective values of T and in which a different code is allotted in accordance with said relative position and said length; and

extracting said code from the inputted compressed data to refer to the selected table on the basis of the extracted code so that data continuing from a bit corresponding to the leading position of said partial data string to the extent of the length of said partial data string is extracted from said buffer for the inputted data of the current line and the extracted data is outputted as decompressed data.

18. A data compression/decompression method in which on a transmitting side, in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, the relative position of a leading pixel of said partial data string on the immediately preceding line and a pixel of interest on the current and the length of said partial data string are determined, and a corresponding code is outputted as compressed data by referring to tables which are prepared for the respective values of T, T being T=0 when x=0, T=1 when 1≦

x≦

H-N, and T=x-H+N+1 when H-N+1≦

x≦

H-1 where H is the number of pixels in one line of said two-dimensional image data, N is the maximum detection string length of said partial data string (1≦

N≦

H) and x is the position of a pixel of interest on the current line (0≦

x≦

H-1) and in which a different code is allotted in accordance with said relative position and said length, and

in which on a receiving side, the method comprises;

holding at least decompressed data of the immediately preceding line in a buffer;

determining the value of T on the basis of the position x of a pixel of interest to be decompressed on the current line from a leading pixel thereof to select the table corresponding to the determined value of T; and

extracting said code from the inputted compressed data to refer to the selected table on the basis of the extracted code so that data continuing from a bit corresponding to the leading position of said partial data string to the extent of the length of said partial data string is extracted from said buffer for the inputted data of the current line and the extracted data is outputted as decompressed data.

19. A data compression/decompression method according to claim 18, wherein codes in at least three different cases including the case where the relative position of a pixel of interest of input data of the current line and a leading pixel of the same partial data string of the immediately preceding line is the same pixel position, the case where the relative position is shifted to the left by one pixel and the case where the relative position is shifted to the right by one pixel are stored in said table.

20. A data compression apparatus comprising:

reference memory means for holding data of the immediately preceding line for successively inputted two-dimensional image data;

buffer means for extracting a plurality of continuous data strings within the range of a predetermined length from an inputted signal of the current line to store them as a partial data string to be compressed;

scanning comparison means for detecting whether or not the same partial data string as a partial data string continuing from a pixel of interest of said partial data string to be compressed exists in the data stored in said reference memory means; and

code generating means by which in the case where said scanning comparison means detects the same partial data string, the relative position of a leading pixel of said partial data string and said pixel of interest and the length of said partial data string on the data of the immediately line stored in said reference memory means are determined, and a corresponding code is outputted as data of that partial data string on the current line by referring to tables which are prepared for the respective values of T, T being T=0 when x=0, T=1 when 1≦

x≦

H-N, and T=x-H+N+1 when H-N+1≦

x≦

H-1 where H is the number of pixels in one line of said two-dimensional image data, N is the maximum detection string length of said partial data string (1≦

N≦

H) and x is the position of a pixel of interest on the current from a leading pixel thereof (0≦

x≦

H-1) and in which a different code is allotted in accordance with said relative position and said length of the partial data string.

21. A data compression apparatus according to claim 20, wherein variable-length codes in at least three different cases including the case where the relative position of a pixel of interest of input data of the current line and a leading pixel of the same partial data string of the immediately preceding line is the same pixel position, the case where the relative position is shifted to the left by one pixel and the case where the relative position is shifted to the right by one pixel are stored in said table.

22. A data compression apparatus according to claim 20, wherein in the case where there is not detected in the current line a partial data string which is the same as the immediately preceding line, a corresponding code is outputted by referring to a run-length tables which are prepared for the respective values of T and in which a different variable-length code is allotted in accordance with the length of the same continuous data.

23. A data compression apparatus according to claim 21, wherein in the case where there is not detected in the current line a partial data string which is the same as the immediately preceding line, a corresponding code is outputted by referring to a run-length tables which are prepared for the respective values of T and in which a different variable-length code is allotted in accordance with the length of the same continuous data.

24. A data compression apparatus according to claim 22, wherein the variable-length codes stored in said run-length table are allotted with variable-length codes which are different from the variable-length codes stored in said table.

25. A data compression apparatus according to claim 23, wherein the variable-length codes stored in said run-length table are allotted with variable-length codes which are different from the variable-length codes stored in said table.

26. A data decompression apparatus for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there is successively inputted as data of the overlapping portion a code which differs in accordance with the relative position of a leading pixel of said partial data string and a pixel of interest and the length of said partial data string on the immediately preceding line for each value of T, T being T=0 when x=0, T=1 when 1≦

x≦

H-N, and T=x-H+N+1 when H-N+1 ≦

x ≦

H-1 where H is the number of pixels in one line of said two-dimensional image data, N is the maximum detection string length of said partial data string (1≦

N≦

H) and x is the position of said pixel of interest on the current from a leading pixel thereof (0≦

x≦

H-1), the apparatus comprising;

buffer memory means for holding decompressed data of the immediately preceding line;

inverse transformation tables which are prepared for the respective values of T and in which the relative position of a leading pixel of said partial data string and said pixel of interest and the length of said partial data string on the immediately preceding line are allotted in accordance with said code; and

decoding means for determining the value of T on the basis of the position x of a pixel of interest of the compressed data to be decompressed on the current line from a leading pixel thereof to select the inverse transformation table corresponding to the determined value of T, extracting said code from the inputted data to refer to the selected inverse transformation table on the basis of said code so that data of the leading position and the length of said partial data string on the immediately preceding line is derived from said inverse transformation table, and extracting, from said buffer memory means, data which continues from a bit corresponding to the leading position of said partial data string to the extent of the length of said partial data string, so that the extracted data is outputted as decompressed data.

27. A data decompression apparatus for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there is successively inputted as data of the overlapping portion a code which differs in accordance with the relative position of a leading pixel of said partial data string and a pixel of interest and the length of said partial data string on the immediately preceding line for each value of T, T being T=0 when x=0, T=1 when 1≦

x≦

H-N, and T=x-H+N+1 when H-N+1≦

x≦

H-1 where H is the number of pixels in one line of said two-dimensional image data, N is the maximum detection string length of said partial data string (1≦

N≦

H) and x is the position of said pixel of interest on the current from a leading pixel thereof (0≦

x≦

H-1), and in which in the case where there is not detected in the current line a partial data string which is the same as the immediately preceding line, a code differing in accordance with the length of the same continuous data for each value of T and the same data are successively inputted as compressed data of run length, the apparatus comprising;

a counter for counting the position x of a pixel of interest of compressed data to be decompressed on the current line from a leading pixel thereof;

buffer memory means for holding decompressed data of the immediately preceding line;

inverse transformation tables which are prepared for the respective values of T and in which data of the relative position of a leading pixel of said partial data string and said pixel of interest and the length of said partial data string on the immediately preceding line or a run length are allotted in accordance with an inputted code;

a decoder for determining the value of T on the basis of the position x of said pixel of interest on the current line from the leading pixel thereof counted by the count value of said counter to select the inverse transformation table corresponding to the determined value of T, and extracting said code from the inputted data to refer to the selected inverse transformation table on the basis of said code so that a mode flag for discriminating whether or not there is detected in the current line a partial data string which is the same as the immediately preceding line is outputted while data of said relative position and said length or data of the length of the same continuous data is derived from said inverse transformation table;

first decoding means by which in the case where there is detected in the current line a partial data string which is the same as the immediately preceding line, data continuing from a bit corresponding to said relative position to the extent of the length of said partial data string is extracted from said buffer memory means and the extracted data is outputted as decompressed data; and

second decoding means by which in the case where there is not detected in the current line a partial data string which is the same as the immediately preceding line, data succeeding said code is continuously outputted to the extent of the length of the same continuous data.

28. A data compression method in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string to be compressed which is a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the position of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string for each value of the position x₁ of a pixel of interest as a leading pixel of said partial data string to be compressed from a leading pixel of the current line and/or the position x₂ of a leading pixel of said immediately preceding partial data string from a leading pixel of the immediately preceding line.

29. A data compression method according to claim 28, wherein the code corresponding to the length of said immediately preceding partial data string and outputted as compressed data of said partial data string to be compressed is allotted with a code which differs in accordance with the length of said immediately preceding partial data string for each value of T which is a larger one of said position x₁ and said position x₂.

30. A data compression method according to claim 28, wherein information concerning the position of said immediately preceding partial data string is represented as the relative position on a two-dimensional space of a pixel of interest on the current line and a leading pixel of an immediately preceding partial data string on the immediately preceding line.

31. A data compression method according to claim 29, wherein information concerning the position of said immediately preceding partial data string is represented as the relative position on a two-dimensional space of a pixel of interest on the current line and a leading pixel of an immediately preceding partial data string on the immediately preceding line.

32. A data compression method according to claim 30, wherein the range of reference to the immediately preceding line is limited to fall within a predetermined range of said relative position.

33. A data compression method according to claim 31, wherein the range of reference to the immediately preceding line is limited to fall within a predetermined range of said relative position.

34. A data decompression method for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string to be compressed which is a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there are successively inputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the position of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string for each value of the position x₁ of a pixel of interest as a leading pixel of said partial data string to be compressed from a leading pixel of the current line and/or the position x₂ of a leading pixel of said immediately preceding partial data string from a leading pixel of the immediately preceding line, the method comprising:

holding at least decompressed data of the immediately preceding line;

extracting information concerning the position of said immediately preceding partial data string from the inputted compressed data while determining the position x₂ of the leading pixel of said immediately preceding partial data string from the leading pixel of the immediately preceding line on the basis of the extracted information to extract information concerning the length of said immediately preceding partial string on the basis of the position x₁ of a pixel of interest to be decompressed from a leading pixel of the current line and/or said position x₂ ; and

extracting said immediately preceding partial data string from the held decompressed data of the immediately preceding line on the basis of the information concerning the position of said immediately preceding partial data string and the information concerning the length of said immediately preceding partial string to output said immediately preceding partial data string as decompressed data of the inputted compressed data.

35. A data compression/decompression method in which on a transmitting side, in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string to be compressed which is a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the position of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string for each value of the position x₁ of a pixel of interest as a leading pixel of said partial data string to be compressed from a leading pixel of the current line and/or the position x₂ of a leading pixel of said immediately preceding partial data string from a leading pixel of the immediately preceding line, and

in which on a receiving side, the method comprises holding at least decompressed data of the immediately preceding line, extracting information concerning the position of said immediately preceding partial data string from the inputted compressed data while determining the position x₂ of the leading pixel of said immediately preceding partial data string from the leading pixel of the immediately preceding line on the basis of the extracted information to extract information concerning the length of said immediately preceding partial string on the basis of the position x₁ of a pixel of interest to be decompressed from a leading pixel of the current line and/or said position x₂, and extracting said immediately preceding partial data string from the held decompressed data of the immediately preceding line on the basis of the information concerning the position of said immediately preceding partial data string and the information concerning the length of said immediately preceding partial string to output said immediately preceding partial data string as decompressed data of the inputted compressed data.

36. A data compression apparatus comprising:

reference memory means for holding data of the immediately preceding line for successively inputted two-dimensional image data;

buffer means for extracting a plurality of continuous data strings from an inputted signal of the current line to store them as a partial data string to be compressed;

scanning comparison means for detecting whether or not the same partial data string as said partial data string to be compressed exists in the data stored in said reference memory means; and

code generating means by which in the case where said scanning comparison means detects the same partial data string, a code differing in accordance with the position of an immediately preceding partial data string corresponding to said partial data string to be compressed on the data stored in said reference memory means and a code differing in accordance with the length of said immediately preceding partial data string for each value of the position x₁ of a pixel of interest as a leading pixel of said partial data string to be compressed from a leading pixel of the current line and/or the position x₂ of a leading pixel of said immediately preceding partial data string from a leading pixel of the immediately preceding line are outputted as compressed data of said partial data string to be compressed.

37. A data compression apparatus according to claim 36, wherein said buffer means successively increases the number of bits of the data extracted from the signal of the current line in the case where said scanning comparison means detects the same data string, the increase being repeated until said scanning comparison means ceases to detect the same data string, and wherein said buffer means stores, as the partial data string to be compressed, a partial data string detected by said scanning comparison means immediately before said scanning comparison means ceases to detect the same data string.

38. A data compression apparatus according to claim 36, wherein said scanning comparison means makes comparison by extracting that portion of the data of the immediately preceding line stored in said reference memory means which is within a predefined pixel range from a pixel of interest.

39. A data compression apparatus according to claim 37, wherein said scanning comparison means makes comparison by extracting that portion of the data of the immediately preceding line stored in said reference memory means which is within a predefined pixel range from a pixel of interest.

40. A data decompression apparatus for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected in the current line a partial data string to be compressed which is a partial data string which is the same as the immediately preceding line to the extent of at least one or more pixels, there are successively inputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the position of an immediately preceding partial data string corresponding to said partial data string to be compressed on the immediately preceding line and a code which differs in accordance with the length of said immediately preceding partial data string for each value of the position x₁ of a pixel of interest as a leading pixel of said partial data string to be compressed from a leading pixel of the current line and/or the position x₂ of a leading pixel of said immediately preceding partial data string from a leading pixel of the immediately preceding line, the apparatus comprising:

buffer memory means for holding at least decompressed data of the immediately preceding line;

extracting means for extracting information concerning the position of said immediately preceding partial data string from the inputted compressed data while determining the position x₂ of the leading pixel of said immediately preceding partial data string from the leading pixel of the immediately preceding line on the basis of the extracted information to extract information concerning the length of said immediately preceding partial string on the basis of the position x₁ of a pixel of interest to be decompressed from a leading pixel of the current line and/or said position x₂ ; and

decoding means for extracting said immediately preceding partial data string from the decompressed data of the immediately preceding line stored in the buffer memory means on the basis of the information concerning the position of said immediately preceding partial data string and the information concerning the length of said immediately preceding partial data string to output said immediately preceding partial data string as decompressed data of the inputted compressed data.

41. A data compression method in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data-string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string,

wherein in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and outputted as compressed data of the partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each value of T which is the relative position of compressed data previously outputted.

44. A data compression method according to claim 41, wherein the range of reference to the immediately preceding line is limited to the case where said relative position is the same pixel position and the case where said relative position is a position shifted by one pixel.

42. A data compression method in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on an immediately preceding line corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of the pixel of interest from a leading pixel of the current line (0≦

x≦

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string.

43. A data compression method according to claim 42, wherein in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and outputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each of the value of T, T being the relative position of compressed data previously outputted and the value of said S.

45. A data compression method according to claim 42, wherein the range of reference to the immediately preceding line is limited to the case where said relative position is the same pixel position and the case where said relative position is a position shifted by one pixel.

46. A data compression method according to claim 43, wherein the range of reference to the immediately preceding line is limited to the case where said relative position is the same pixel position and the case where said relative position is a position shifted by one pixel.

47. A data decompression method for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are successively inputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string, and in which in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and inputted as compressed data of the partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each value of T, T being the relative position of compressed data previously inputted, the method comprising:

holding at least decompressed data of the immediately preceding line;

extracting information concerning said relative position and the length of said immediately preceding partial data string from the inputted compressed data and extracting, in the case where the inputted compressed data is compressed data of a partial string data to be compressed which is subjected to the second or subsequent detection, information concerning said relative position on the basis of the value of T; and

extracting said immediately preceding partial data string from the held compressed data of the immediately preceding line on the basis of the extracted information concerning said relative position and said length of said immediately preceding partial data string to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

48. A data decompression method for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are successively inputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on an immediately preceding line corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of the pixel of interest from a leading pixel of the current line (0≦

x<

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string, the method comprising;

holding at least decompressed data of the immediately preceding line;

determining the value of S on the basis of the position x of a pixel of interest to be compressed on the current line to extract information concerning said relative position on the basis of the determined value of S and information concerning the length of said immediately preceding partial data string from the inputted compressed data; and

extracting said immediately preceding partial data string from the held compressed data of the immediately preceding line on the basis of the extracted information concerning said relative position and the length of said immediately preceding partial data string to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

49. A data decompression method according to claim 48, wherein in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and inputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each the value of T, T being the relative position of compressed data previously inputted and the value of said S, and

wherein in the case where the inputted compressed data is compressed data of a partial string data to be compressed which is subjected to the second or subsequent detection, information concerning said relative position is extracted on the basis of the value of T and the value of S.

50. A data compression/decompression method in which on a transmitting side, in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are successively outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string, and in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and outputted as compressed data of the partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each value of T, T being the relative position of compressed data previously inputted, and

in which on a receiving side, the method comprises holding at least decompressed data of the immediately preceding line, extracting information concerning said relative position and the length of said immediately preceding partial data string from the inputted compressed data, extracting, in the case where the inputted compressed data is compressed data of a partial string data to be compressed which is subjected to the second or subsequent detection, information concerning said relative position on the basis of the value of T, and extracting said immediately preceding partial data string from the held compressed data of the immediately preceding line on the basis of the extracted information concerning said relative position and the length of said immediately preceding partial data string to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

51. A data compression/decompression method in which on a transmitting side, in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string on an immediately preceding line corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of said pixel of interest from a leading pixel of the current line (0≦

x≦

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string, and

in which on a receiving side, the method comprises holding at least decompressed data of the immediately preceding line, determining the value of S on the basis of the position x of a pixel of interest to be compressed on the current line to extract information concerning said relative position on the basis of the determined value of S and information concerning the length of said immediately preceding partial data string from the inputted compressed data, and extracting said immediately preceding partial data string from the held compressed data of the immediately preceding line on the basis of the extracted information concerning said relative position and the length of said immediately preceding partial data string to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

52. A data compression/decompression method in which on a transmitting side, in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string on an immediately preceding line corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of said pixel of interest from a leading pixel of the current line (0≦

x≦

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string, and in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and outputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each the value of T, T being the relative position of compressed data previously outputted and the value of said S, and

in which on a receiving side, the method comprises holding at least decompressed data of the immediately preceding line, determining the value of S on the basis of the position x of a pixel of interest to be compressed on the current line to extract information concerning said relative position on the basis of the determined value of S and information concerning the length of said immediately preceding partial data string from the inputted compressed data, extracting, in the case where the inputted compressed data is compressed data of a partial string data to be compressed which is subjected to the second or subsequent detection, information concerning said relative position on the basis of the value of T and the value of S, and extracting said immediately preceding partial data string from the held compressed data of the immediately preceding line on the basis of the extracted information concerning said relative position and the length of said immediately preceding partial data string to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

70. A data compression/decompression method according to claim 52, wherein codes in at least three different cases are stored in said table in the form of variable-length codes, said three different cases including a case where said relative position represents that said leading pixel is positioned at the same pixel position as said pixel of interest, a case where said relative position represents that said leading pixel shifts to the left by one pixel to said pixel of interest, and a case where said relative position represents that said leading pixel shifts to the right by one pixel to said pixel of interest.

53. A data compression apparatus comprising:

reference memory means for holding data of the immediately preceding line for successively inputted two-dimensional image data;

buffer means for extracting from an inputted signal of the current line a plurality of partial data strings having a pixel of interest as its leading pixel and a length corresponding to one or more pixels to store them;

scanning comparison means for detecting a partial data string to be compressed which is that one of said partial data strings stored in said buffer means which matches data stored in said reference memory means to the longest extent; and

code generating means by which in the case where said scanning comparison means detects said partial data string to be compressed, a code differing in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on the data stored in said reference memory means and corresponding to said partial data string to be compressed and a code differing in accordance with the length of said immediately preceding partial data string are outputted as compressed data of said partial data string to be compressed, and by which in the case where said partial data string to be compressed is detected by said scanning comparison means on the same line continuously two or more times, a code corresponding to said relative position and outputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each value of T, T being the relative position of compressed data previously outputted.

56. A data compression apparatus according to claim 53, wherein in the case where a partial data string matching the partial data string stored in said buffer means is detected by said scanning comparison means from the data stored in said reference memory means, data succeeding said partial data string stored in said buffer means is extracted by said buffer means from the signal of the current line to successively increase the length of the partial data string stored in said buffer means, the increase being repeated until said scanning comparison means ceases to detect the matching partial data string, and

said scanning comparison means detects, as said partial data string to be compressed, a partial data string stored in said buffer means immediately before said scanning comparison means ceases to detect the matching partial data string.

59. A data compression apparatus according to claim 53, wherein said scanning comparison means makes comparison by extracting that partial data string from the data stored in said reference buffer means which has, as its leading pixel, data for which said relative position is the same pixel position or a position shifted by one pixel.

62. A data compression apparatus according to claim 56, wherein said scanning comparison means makes comparison by extracting that partial data string from the data stored in said reference buffer means which has, as its leading pixel, data for which said relative position is the same pixel position or a position shifted by one pixel.

54. A data compression apparatus comprising:

reference memory means for holding data of the immediately preceding line for successively inputted two-dimensional image data;

buffer means for extracting from an inputted signal of the current line a plurality of partial data strings having a pixel of interest as its leading pixel and a length corresponding to one or more pixels to store them;

scanning comparison means for detecting a partial data string to be compressed which is that one of said partial data strings stored in said buffer means which matches data stored in said reference memory means to the longest extent; and

code generating means by which in the case where said scanning comparison means detects said partial data string to be compressed, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string on the data stored in said reference memory means and corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of said pixel of interest from a leading pixel of the current line (0≦

x≦

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string.

57. A data compression apparatus according to claim 54, wherein in the case where a partial data string matching the partial data string stored in said buffer means is detected by said scanning comparison means from the data stored in said reference memory means, data succeeding said partial data string stored in said buffer means is extracted by said buffer means from the signal of the current line to successively increase the length of the partial data string stored in said buffer means, the increase being repeated until said scanning comparison means ceases to detect the matching partial data string, and

said scanning comparison means detects, as said partial data string to be compressed, a partial data string stored in said buffer means immediately before said scanning comparison means ceases to detect the matching partial data string.

60. A data compression apparatus according to claim 54, wherein said scanning comparison means makes comparison by extracting that partial data string from the data stored in said reference buffer means which has, as its leading pixel, data for which said relative position is the same pixel position or a position shifted by one pixel.

63. A data compression apparatus according to claim 57, wherein said scanning comparison means makes comparison by extracting that partial data string from the data stored in said reference buffer means which has, as its leading pixel, data for which said relative position is the same pixel position or a position shifted by one pixel.

55. A data compression apparatus comprising:

reference memory means for holding data of the immediately preceding line for successively inputted two-dimensional image data;

buffer means for extracting from an inputted signal of the current line a plurality of partial data strings having a pixel of interest as its leading pixel and a length corresponding to one or more pixels to store them;

scanning comparison means for detecting a partial data string to be compressed which is that one of said partial data strings stored in said buffer means which matches data stored in said reference memory means to the longest extent; and

code generating means by which in the case where said scanning comparison means detects said partial data string to be compressed, there are outputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string on the data stored in said reference memory means and corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of said pixel of interest from a leading pixel of the current line (0≦

x≦

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string, and by which in the case where said partial data string to be compressed is detected by said scanning comparison means on the same line continuously two or more times, a code corresponding to said relative position and outputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each of the value of said S and the value of T, T being the relative position of compressed data previously outputted.

58. A data compression apparatus according to claim 55, wherein in the case where a partial data string matching the partial data string stored in said buffer means is detected by said scanning comparison means from the data stored in said reference memory means, data succeeding said partial data string stored in said buffer means is extracted by said buffer means from the signal of the current line to successively increase the length of the partial data string stored in said buffer means, the increase being repeated until said scanning comparison means ceases to detect the matching partial data string, and

said scanning comparison means detects, as said partial data string to be compressed, a partial data string stored in said buffer means immediately before said scanning comparison means ceases to detect the matching partial data string.

61. A data compression apparatus according to claim 55, wherein said scanning comparison means makes comparison by extracting that partial data string from the data stored in said reference buffer means which has, as its leading pixel, data for which said relative position is the same pixel position or a position shifted by one pixel.

64. A data compression apparatus according to claim 58, wherein said scanning comparison means makes comparison by extracting that partial data string from the data stored in said reference buffer means which has, as its leading pixel, data for which said relative position is the same pixel position or a position shifted by one pixel.

65. A data decompression apparatus for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are successively inputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of said pixel of interest and a leading pixel of an immediately preceding partial data string on the immediately preceding line corresponding to said partial data string to be compressed and a code which differs in accordance with the length of said immediately preceding partial data string, and in which in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and inputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each value of T, T being the relative position of compressed data previously inputted, the apparatus comprising:

buffer memory means for holding at least decompressed data of the immediately preceding line;

extracting means by which information concerning said relative position and the length of said immediately preceding partial data string is extracted from the inputted compressed data, and by which when the inputted compressed data is compressed data of a partial string data to be compressed which is subjected to the second or subsequent detection, information concerning said relative position is extracted on the basis of the value of T; and

decoding means for extracting said immediately preceding partial data string from the compressed data of the immediately preceding line stored in said buffer memory means on the basis of the information concerning said relative position and the length of said immediately preceding partial data string extracted by said extracting means to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

66. A data decompression apparatus for compressed two-dimensional image data in which in the case where the reference to data of the immediately preceding line for successively scanned two-dimensional image data results in that there is detected from the current line a partial data string to be compressed which has a pixel of interest as its leading pixel and a length corresponding to one or more pixels and which matches data of the immediately preceding line to the longest extent, there are successively inputted, as compressed data of said partial data string to be compressed, a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string on an immediately preceding line corresponding to said partial data string to be compressed for each value of S, S being S=0 when x=0, S=1 when 1≦

x≦

H-2, and S=2 when x=H-1 where H is the number of pixels in one line of the two-dimensional image data and x is the position of said pixel of interest from a leading pixel of the current line (0≦

x≦

H-1) and a code which differs in accordance with the length of said immediately preceding partial data string, the apparatus comprising;

buffer memory means for holding at least decompressed data of the immediately preceding line;

extracting means for determining the value of S on the basis of the position x of a pixel of interest to be compressed on the current line to extract information concerning said relative position on the basis of the determined value of S and information concerning the length of said immediately preceding partial data string from the inputted compressed data; and

decoding means for extracting said immediately preceding partial data string from the compressed data of the immediately preceding line stored in said buffer memory means on the basis of the information concerning said relative position and the length of said immediately preceding partial data string extracted by said extracting means to output the extracted immediately preceding partial data string as decompressed data of the inputted compressed data.

67. A data decompression apparatus according to claims 66, wherein in the case where the detection of said partial data string to be compressed continues on the same line two or more times, a code corresponding to said relative position and inputted as compressed data of a partial data string to be compressed which is subjected to the second or subsequent detection, is allotted with a code which differs in accordance with the relative position of a pixel of interest and a leading pixel of an immediately preceding partial data string for each of the value of T, T being the relative position of compressed data previously inputted and the value of said S, and

wherein when the inputted compressed data is compressed data of a partial string data to be compressed which is subjected to the second or subsequent detection, said extracting means extracts information concerning said relative position is extracted on the basis of the value of T and the value of S.

68. A data decompression method for decompressing compressed two-dimensional image data which are produced by a data compressing method using first line data of a first line and second line data of a second line, said first line immediately preceding said second line, wherein

said data compressing method comprising the steps of:

preparing tables which are provided every position of a pixel of interest on said second line and in each of which codes are allotted so as to be different in accordance with a relative position of a leading pixel of said first partial data string to said pixel of interest and a length of said first partial data string;

comparing a first partial data string among said first line data and a second partial data string among said second line data; and

selecting, when said second partial data string is the same as said first partial data string, one of said tables in accordance with a position of said pixel of interest on said second line to output one of said codes which is allotted corresponding to a leading pixel of said first partial data string to said pixel of interest and the length of said first partial data, instead of said second partial data string,

said data decompressing method comprising the stops of;

preparing said tables;

holding at least said first line data included in said compressed two-dimensional image data;

selecting one of said tables in accordance with a position of a pixel to be decompressed on said second line;

extracting said one of codes included in said compressed two-dimensional image data;

referring said selected one of tables in accordance with said extracted one of codes to extract data, which comprises a corresponding bit corresponding to the relative position of the leading pixel of said first partial data string to said pixel to be decompressed and at least one bit following said corresponding bit by the length of said first partial data string, from said held first line data; and

reproducing said second line data by using said extracted data instead of said extracted one of codes.

69. A data compression/decompression method comprising:

a data compression method performed in a transmitting side and for compressing two-dimensional image data using first line data of a first line and second line data of a second line, said first line immediately preceding said second line, comprising the steps of;

preparing tables which are provided every position of a pixel of interest on said second line and in each of which codes are allotted 90 as to be different in accordance with a relative position of a leading pixel of said first partial data string to said pixel of interest and a length of said first partial data string;

comparing a first partial data string among said first line data and a second partial data string among said second line data; and

selecting, when said second partial data string is the same as said first partial data string, one of said tables in accordance with a position of said pixel of interest on said second line to output one of said codes which is allotted corresponding to a leading pixel of said fires partial data string to said pixel of interest and the length of said first partial data, instead of said second partial data string, and

a data decompressing method performed in a receiving side comprising the steps of;

preparing said tables;

holding at least said first line data included in said compressed two-dimensional image data;

selecting one of said tables in accordance with a position of a pixel to be decompressed on said second line;

extracting said one of codes included in said compressed two-dimensional image data;

referring said selected one of tables in accordance with said extracted one of codes to extract data, which comprises a corresponding bit corresponding to the relative position of the leading pixel of said first partial data string to said pixel to be decompressed and at least one bit following said corresponding bit by the length of said first partial data string, from said held first line data; and

reproducing said second line data by using said extracted data instead of said extracted one of codes.