METHOD FOR ENCODING AND DECODING A DIGITAL IMAGE

Abstract: A method for encoding and decoding digital image data, includes the steps of testing the data for a vertical relation­ ship between adjacent lines of the image, generating directly a reference code word representative of the vertical relation­ ship for each successful test, generating by table lookup a run code word for a stream of data bits of common value for each unsuccessful test, wherein a data element representing a number of bits in a code word is stored at a predetermined offset from a table entry for the code word, merging the vertical reference code words and the run code words generated by the above steps to form an encoded data stream, testing an encoded data stream for a reference code, decoding directly the vertical relationship from the reference code for each successful test: decoding by table lookup a run of bits of common value by serial examination of said encoded data stream for each unsuccessful test and storing data decoded by the above steps in a predetermined relationship

Description: METHOD FOR ENCODING AND DECODING A DIGITAL IMAGE The present invention relates to digital image processing methods and more particularly to methods for encoding and decoding image data. The following are systems representative of the prior art. R. Hunter and A. H. Robinson, "International digital facsimile coding standards," CCITT Proceedings of the IEEE, Vol 68, No 7, 854-867, (July 1980). D. A. Huffman, "A method for the construction of minimum redundancy codes," Proc. IRE, vol. 40, PP. 1097-1101, (Sept. 1952) A. J. Frank, "Uniform decoding of minimum-redundancy codes," U.S. Patent 3,883,847, May 13, 1975. H. Meyr, H. G. Rosdolsky, and T. S. Huang, "Optimum Run Length Codes," IEEE Trans. Comm. Vol COM-22, No 6, p826-835 (June 1974). G. Goertzel and J. L. Mitchell, "Two-Dimensional Image Data Compression and Decompression System," U. S. Defensive Publication > T985,005 August 7, 1979. D. C. Van Voorhis, "Variable-length to fixed-length conversion of minimum-redundancy codes,' U.S. Patent 4,044,347, August 23, 1977. W. S. Michel, W. O. Fleckenstein and E. R. Kretzmer, "A coded Fac simil-e- System,!' IRE Wescon Conv Rec p. 84-93 (1957). Bahl, L.B., D.I. Barnea, and H. Kobayashi, Image Compaction System, U.S. Patent 3,833,900 (1974). DELTA INFORMATION SYSTEMS, Inc., "Group 3 Facisimile Apparatus for Document Transmission" Appendix B, Final Report to National Communications System, August 16, 1982. US Patent 3,927,251, shows apparatus and method for detection and control of errors in two dimensionally compressed image data. US Patent 4,121,258 shows a method for coding a facimile signal employing two dimensional coding. US Patent 4,040,093 shows a method for coding a facimile signal employing two dimensional coding. US Patent 4,103,287 shows a method for encoding an image signal employing image partitioning. US Patent 3,927,251 shows a method for coding a facimile signal employing detection and control of errors. US Patent 3,830,965 shows a method for coding a facimile signal employing one of two preselected encoding modes. US Patent 4,028,731 shows a method for encoding an image signal employing cross array correlation. US Patent 3,675,211 shows a method for data compaction employing a three state associative memory. Huffman describes a simple way to construct a minimum redundancy variable length code. The code word lengths are related to the frequencies elf the source messages, the more frequent messages being assigned shorter code words. Such Huffman codes have been shown to be the optimum coding for an ensemble of messages. However, they have one major disadvantage over other (suboptimum) techniques for variable-length codes in that they are random patterns. This complicates the decoding process. There is usually a tradeoff between speed of decoding and the size of the decode tables. Frank describes a table lookup for parallel decoding which is a compromise between speed and table storage. Myer et al looked for special classes which could be easily implemented by having the code words be multiples of fixed length blocks. The CCITT one-dimensional (described in Hunter et al) run length standard uses a Modified Huffman code. The runs from 0-63 are individually coded while longer runs are constructed from codes for the multiples of 64 followed by a code for the remaining length of 0-63. Separate tables are used for the white and black runs since they have rather different distributions. One way to implement a serial decode of the CCITT runs lengths is by table lookup a bit at a time. In Goertzel et al the least significant bit of the address into the ROM table was the next code bit. One of the output bits flagged whether to continue decoding using the remaining output bits as the next table address or to stop because a complete code had been encountered. Then the output gave the run length size. The table could have just as easily had the next code bit as the most significant bit. That case is equivalent to separate tables according to whether the next bit was a '0' or '1'. CCITT ALGORITHMS The International Telegraph and Telephone Consultative Committee (CCITT) has a standard one-dimensional data compression scheme (Hodified Huffman) and an optional two-dimensional data compression scheme (Modified READ) for black-white facsimile images (see Hunter et al, above). Host-attached facsimile machines are available which can transfer compressed images to a computer. Methods for quickly decoding compressed images are needed on the host machine. Conversely, it is desirable to enable the host machine to encode image data and transfer it in compressed form to the facsimile unit. It is also useful to be able to extract parts of images (in either compressed or raster scan format) and recombine them, possibly with some padding, and either leave them in raster scan format for further processing on the host or encode them for transmission to the facsimile unit. The ability to decode and reencode without going to a raster scan format allows a compressed image to be conveniently converted from one compression algorithm to another. The International Telegraph and Telephone Consultative Committee (CCITT) has standardized two facsimile data compression techniques. In November, 1977, CCITT Study Group XIV standardized a one-dimensional data compression scheme for facsimile images. Two years later this standard was incorporated in a two-dimensional coding scheme. Data compression (coding) of images is essential if high quality digital facsimile is to be practical. A standard facsimile document has 3.85 scan lines/mm vertically. However, an optional vertical resolution of 7.7 lines/mm can be used for images with fine details. The standard horizontal resolution is fixed at 8.04 picture elements/mm (1728 picture elements in 215 mm) to avoid excessive "stair stepping on curved or diagonal edges of the reproduced images. Almost a quarter million bytes are required to store the raw digitized data of a 215 mm x 280 mm page at the normal resolution. At the higher resolution, close to a half million bytes are needed. Run length coding schemes, such as the CCITT one-dimensional standard, reduce the storage requirement by factors of 5 to 15 for typical images. Two-dimensional schemes take advantage of correlation between successive scan lines to obtain better compression. Fixed codes assume that average statistics apply to all documents. However, some documents do not fit these statistics. When the assumptions are completely invalid, regions of the "com pressed11 image may require many more bits than the original uncompressed image. The uncompressed mode option in the two dimensional CCITT recommendation can be used to minimize the worst case local expansion. The black and white picture elements in typical images are not randomly distributed but come with a high degree of correlation or redundancy. In each horizontal scan line the pels of a given color (black or white) tend to come in groups or runs. Run length coding takes advantage of this horizontal correlation in a practical way. The number of pels between color changes is coded instead of each pel. The runs alternate in color so that as long as the first run in a line is guaranteed to be a white run no extra bits are needed to specify the color. A length of zero can be used if the line actually starts black. The CCITT one-dimensional standard Modified Huffman Code is a run length coding scheme in which the black and white runs have separate tables. Runs longer than 63 are coded in two pieces in order to decrease the siza of the code tables by an order of magnitude. Multiples of 64 are coded first as a Makeup Code and then the remainder follows as a Terminating Code. Short runs (0 - 63) only need the Terminating Code. Since the CCITT standard is for telephone transmission, the standard also includes a unique end of line (EOL) code for resynchronization after transmission errors. It is a unique pattern of eleven zeros followed by a one. Extra zeros can precede the EOL as fill bits to maintain a minimum transmission time per line. Run length coding can only take advantage of the horizontal redundancy. There is also strong vertical correlation in most images which comes from the vertical continuity of objects, strokes, or lines. Vertical reference coding codes a run as the difference between the run length and the distance to the same color change on the history line (preceding scan line). Generally about half of all the runs vertically align exactly and another 25Z are within one pel of the appropriate transition. Larger vertical differences are much less frequent. The CCITT two-dimensional Modified Relative Element Address Designate (Modified READ) coding scheme uses vertical reference codes out to plus or minus three. If a run does not end with a vertical reference code, the horizontal mode (JIM) code precedes two Modified Huffman run length codes for that run and the next run. Coding a pair of runs with one prefix is a way of taking advantage of the high probability that a run length code is immediately followed by another run length code. Sometimes a run on the preceding line comes from an object which terminated on that line. A pass mode skips over such runs on the preceding line as is shown below. History line 1 1 1 0 0 0 1 1 0 0 0 0 0 1 1 Current line 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 < -PASS mode- > l < --V0--- > l The coding of the PASS mode indicates that the vertical reference code which follows it refers to the third black (1) run on the history line, rather than the second, as would normally be expected. It is used if another transition is encountered to the right of the first vertical reference transition (before the end of the run). A new run is started at that transition. Pass mode can be coded several times within a single run. In order to limit the effect of transmission errors, at least every Kth line is coded one-dimensionally. The value of K is set as 2 for the normal resolution and 4 for the higher resolution. The Modified READ code table is given below. Modified READ Code Table RLPREFIX 001 PASS 0001 VO 1 VL1 010 VR1 011 VL2 000010 VR2 000011 VL3 0000010 VR3 0000011 EOL 000000000001T Extensions 2-D 0000001xxx l-D 000000001xxx T on the EOL denotes a tag bit which tells whether the next line is code one-dimensionally (T = 1) or two-dimensionally (T = 0). One of the future extensions has been identified as uncompressed mode. The xxx bits are '111' for this mode. This mode can improve compression significantly for scanned halftones and images with many one or two pel runs. Uncompressed Mode Code Table Icage Pattern Code Word 1 1 01 001 001 0001 0001 00001 00001 00000 000001 Exits 0000001T 0 00000001T 00 000000001T 000 0000000001T 0000 00000000001T The exit codes signal when to resume normal coding. The T denotes a tag bit which tells the color of the next run (Black = 1, White = 0). The present invention i disclosed in the attached claims. It is an object of the present invention to encode and decode digita data by a method including the steps of: testing said data for a predetermined relationship between adjacent lines of said data; generating--directly a reference code word represen tative of said predetermined relationship for each successful test; generating by table lookup a run code word for a stream of data bits of common value for each unsuccessful test, wherein a data element representing a number of bits in a code word is stored at a predetermined offset from a table entry representing said code word; merging said reference code words and said run code words generated by the above steps to form an encoded data stream; testing an encoded data stream for a reference code; decoding directly said predetermined relationship from said reference code for each successful test; decoding by table lookup a run of bits of common value by serial examination of said encoded data streak for each unsuccessful test storing data decoded by the above steps in a predetermined relationship. Accordingly, a method for encoding and decoding digital image data, includes the steps of testing the data for a vertical relationship between adjacent lines of the image, generating directly a reference code word representative of the vertical relationship for each successful test, generating by table lookup a run code word for a stream of data bits of common value for each unsuccessful test, wherein a data element representing a number of bits in a code word is stored at a predetermined offset from a table entry for the code word, merging the vertical reference code words and the run code words generated by the above steps to form an encoded data stream, testing an encoded data stream for a reference code, decoding directly the vertical relationship from the reference code for each successful test; decoding by table lookup a run of bits of common value by serial examination of said encoded data stream for each unsuccessful test and storing data decoded by the above steps in a predetermined relationship. The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of the preferred embodiments of the convention, as illustrated in the accompanying drawing. FIG. 1 is a flow diagram showing an overview of the encoding method according to the present invention. FIG. 2.1 is a flow diagram showing the initialization process for the encoding of one line of image data. FIG. 2.2 is a flow diagram showing the inner loop for the line encoding procedure. FIG. 2.3 is a flow diagram describing the generation of vertical reference codes. FIG. 2.4 is a flow diagram describing the encoding of run lengths. FIG. 2.5 is a flow diagram showing the process used to encode a single run length. FIG. 3 is a flow diagram showing an overview of the decoding method according to the present invention. FIG. 4.1 is a flow diagram showing the initialization process for the decoding of one line of image data. FIG. 4.2 is a flow diagram showing the inner loop for the line decoding procedure. FIG. 4.3 is a flow diagram describing the decoding of some vertical reference codes. FIG. 4.4 is a flow diagram describing the decoding of additional vertical reference codes. FIG. 4.5 is a flow diagram describing the decoding of run lengths. FIG. 4.6 is a flow diagram showing the process used to decode a single run length. In the drawing, like elements are designated with similar references, and identical elements in different specific embodiments are designated by identical references. The invention as described can encode and decode either the Modified RED or Modified Huffman CCITT standard algorithms, as well as variations of these algorithms which do not require end-of-line codes on every line. The method of encoding a digital image in accordance with the present invention will be described first. The encoder works in two stages. First the input is converted from the input buffer into a buffer containing the color change positions (i.e. transitions or run ends). In the preferred embodiment of the invention, the first halfword in each end point buffer contains the number of bytes used. The next three halfwords are set to zero. The run ends are stored in subsequent halfwords, begin- ning with a white run end (which will be O if the first run in the line is black). The last run end (i.e. the line length) is replicated at least two extra times. Differences of adjacent halfword entries are the run lengths for the black and white runs. The second stage of the encoding process codes each run either as a vertical reference code or as a run length. The vertical reference codes are generated with inline code. The run length code words are looked up in tables. The run length code tables are set up as follows. The 8 least significant bits of the code words are stored right justified in a byte table. (Only the 8 least significant bits are non-zero if more than 8 bits are valid.) The number of valid bits is stored in another byte with a fixed offset from the code bits. During the encoding process, the compressed data is accumulated in a 32-bit register. Each time there are more than 16 valid bits in the register, the high-order 16 valid bits are output into the compressed buffer. This guarantees that there is always room for the next code word, since no code word is longer than lS bits. To add a code word to the compressed data, the current compressed data is shifted left by the number of Ylid bits (introducing zeros in the rightmost bits). Then the byte containing the right justified bits of the code word is ADDed/ORed into the register. (The code word will only have non-zero bits where the zeros were int[oduced during the previous shift.) After each line is encoded, that endpoint buffer is saved to use as a history line for the next line. The program which calls the encoder can force it to ping-pong between two buffers, or alternatively it can cause the encoding of a sequence of run end buffers already in storage. In both cases, the previously encoded line must be saved for use as a history line in the two-dimensional encoding algorithm. The variables referred to in the detailed description of the encoder are as follows: FLAG1D - A flag which specifies whether the current line is to be encoded/decoded one-dimensionally (1) or two-dimensionally (O) . MH - A flag which indicates Modified Huffman encoding/decoding (1). In Modified Huffman coding no tag bit is expected after the EOL code. RP - A pointer into the buffer of run end points. RP points at the (halfword) run end which is currently being processed. This run end is referred to as RO. Successive run ends are referred to as R2, R4, and R6. RPST - Address of the current buffer of run end points. This address will be saved as the address of the history buffer for two-dimensional processing of the next line before setting RPST to point to the next buffer of run end points. HP - A pointer into the buffer of run end points for the history line. HP points at the (halfword) run end used as the history point for the run end currently being encoded. This history run end is referred to as HO. The following history run end is referred to as H2. HPST - Address of the buffer of run end points for the line preceding the line currently being encoded/decoded. This history buffer is used only for two-dimensional encoding/decoding. After each line is completed, RPST is saved in HPST. RL - Run length size for run length coding. ROLD - Position on the line which has already been encoded/decoded. During most of the coding ROLD contains the position of the previous run. DIF - Number of pels between the edges on the history line (HO) and the current line (RO). TP - The table pointer points at the top of the correct run length table during encoding of a run length. During decoding it points at the current table entry. NXITP - The next table pointer stores the start. of the opposite color run length tables. TO - Byte value referenced by TP. OFFSET - The relative distance from the beginning of the run length tables pointed to by TP to the beginning of the run length tables for the opposite color. This value is stored along with the run length tables, based on TP. If the white run length tables are first in storage, then OFFSET is positive when TP points to the white tables and negative when TP points to the black tables. HICODE - Code tables for the run length multiples of 64. The eight least significant bits of each code word are stored (the additional leading bits in the code word, if any, are zero), and the number of bits in each code word is stored at a fixed offset from the code word. This table is addressed using the pointer TP; there are separate tables for white and black run lengths. LOCODE - Code tables for the run length from 0 to 63. The eight least significant bits of each code word are stored (the additional leading bits in the code word, if any, are zero), and the number of bits in each code word is stored at a fixed offset from the code word. This table is addressed using the pointer TP; there are separate tables for white and black run lengths. BITIM - The flag which indicates that the next line of input for the encoder is already in the buffer pointed to by RPST as a list of change positions (run ends). Thus, the output of the decoder can be directly re-encoded (with a different algorithm, set of options, or sub-section) without first creating the raster scan bit image. ETOPPAD - The number of blank lines to pad in the compressed image before compressing input lines. EBOTPAD - The number of blank lines to be compressed at the bottom of the image. EYSKIP- Number of input lines to skip before starting encoding. PADCLR - Blank lines and left and right padding can be either white (0) or black (1) according to the desired padding color. EVSCALE - The vertical scale allows high (vertical) resolution facsimile image to be converted to a lower (vertical) resolution image by skipping lines during the encoding process. LII - The line length is the number of pels per line to be compressed. CNUM - The number of valid bits encoded bits in the register R. R - Register used to assemble the compressed bits. RUN2 - A flag which indicates whether the first or second run of the required pair is being processed during two-dimensional run length encoding/decoding. RLEN(BASE RPST) - The number of bytes used in the buffer starting at the address contained in RPST is stored in the first two bytes of the buffer. The compressed data is considered to be a continuous stream of bits. In the preferred embodiment of the encoding/decoding algorithms, however, this stream is divided into buffers (e.g. records of a compressed file on disk, or blocks of data to be sent over a transmission line). For computational convenience, the preferred embodiment of the algorithms divides the buffers into halfwords (two-byte units). The encoder operation referred to as "coding" adds bits to the bit stream. In our implementation, the bit stream is managed as follows. The most recently coded 0-15 bits are kept in a 32-bit register R. A counter, CNUM, gives the number of code bits in R at any given time. Bits are added to the code stream by shifting the contents of R left by the length (in bits) of the code to be added, adding or ORing the new bits into the low-order bits of R, and incrementing CNUM by the number of bits added. If after this is done there are more than 15 code bits in R, the high-order 16 code bits are transferred to the buffer in storage and the counter for the number of bits in R is reduced by 16. Since no code word in any of the encoding algorithms is more than 13 bits in length, this method guarantees that no data is shifted out of R before it has been stored. In the preferred embodiment of the invention, the high-order 16 bits of code are transferred from R to the code buffer by shifting R left bg 32-CNUM bits (positioning the high-order bits beginning with the most significant bit in R), storing all 32 bits of R in the buffer, and then shifting R back 32-CNUM bits to the right. The low-order 16 bits stored are overwritten by the next storage operation. ihen storing the last halfword in the buffer, a copy is made of R, shifted right by CNUM-16, and the low-order half of the result is stored to avoid writing beyond the end of the buffer. A counter keeps track of the next halfword in the buffer to be written; when the end of the buffer has been reached, it is sent out (written to disk, sent over the transmission line, etc.) and a new buffer (possibly the same one) is obtained into which to write additional halfwords of compressed data. Referring to FIG. 1, the method of encoding image data according to the present invention will be described. INfTIALIZATION FOR ENCODING METHOD The initialization process is described below. It occurs only once for each image encoded. Artificial all white and all black lines are created. A run end point buffer representing a white blank line is created and stored. It has the following sequence: 16 0 0 0 LL LL LL LL where LL is the line length. A run end point buffer representing a black blank line is also created and stored. It has the following sequence: 16 0 0 0 O LII LL LII These buffers will be used as blank line end points to be en coded-when ETOPPAD and EBOTPAD are non-zero.- HPST is initialized to point at the first white run (first LL) of the blank white line run buffer, in case the first line of the image is to be encoded two-dimensionally and the convention is adopted of a blank white history line. CNUM is initialized to O. Other initialization for managing the compressed data should be done at this time. Other variables such as MH, FLAGID, BITIM, and VSCALE are operator controlled to select the desired options. Both the one-dimensional and two-dimensional CCITT standards require that the first line be encoded one-dimensionally; therefore, if the CCITT standard is selected, FLAGID must be set to 1 initially. PAD LINES If ETOPPAD is positive, then extra blank lines should be encoded at the top of the image. First RPST must be set to either the black or white blank line according to PADCLR. Then the subroutine to encode a line from run ends must be called once for each extra line. SKIP LINES If EYSKIP is positive, then the appropriate number of input image lines must be skipped. LOOP TO ENCODE ONE LINE The main encoder loop encodes one line. It first tests to see whether there are any additional lines to be encoded; if not, an exit from the loop occurs. The test for additional lines can be done in many ways. One way is to preset the number of input lines desired, decrement a counter for each input line processed, and stop processing input lines when the counter reaches zero. The vertical scale parameter EVSCALE, if greater than one, allows input lines to be skipped and therefore for a lower resolution to be encoded than the original image. Here the lines are shown as being skipped first, but this is not essential. The input image would frequently be a raster scan image. If so, then a line must be converted into a run end representation. A preferred method for converting from a bit map representation to run representation is given in US Patent 4,xxx,xxx. Alternately, the input image might be in the form of many lines already in run end point format. One such source would be the decoder. This makes conversion between the one and two-dimensional algorithms and the various options easier since the conversion to and from the raster image can be avoided. The run ends are encoded a line at a time from the run ends. This process will be described in greater detail with reference to Figures 2.1 to 2.5. PAD LINES Once the desired number of input lines have been processed, extra lines are padded if EBOTPAD is positive. The same process of pointing to the desired color blank line used with ETOPPAD is used at the end of the image. EBOTPAD may be calculated at this point to maintain a fixed size image. After all of the lines have been processed and before exiting completely, the Return to Control sequence of 6 EOLs may be coded. PROCEDURE TO ENCODE ONE LINE Figures 2:1 through 2.5 illustrate the steps required to encode one line from a run end point representation. Figure 2.1 shows the initialization required before encoding the data. Some selection criteria are used to determine whether the next line is to be encoded one- or two-dimensionally. For the two-dimensional standard, a one-dimensional line must be encoded at least once every K lines, where K is a constant (2 or 4 for the CCITT standard). For the one-dimensional case, an end-of-line code is first coded; the bit pattern is '00000000C)001' if the Modified Huffman algorithm is being used and '0000000000011' otherwise. RP is set to point to the first run end to be encoded; TP is set to point to the run length code tables for white runs; RL (the length of the next run to be encoded) takes the value of the first run end of the line; and FLAGlD is set to 1, indicating that the current line is being encoded one-dimensionally. Control then passes to the loop which encodes runs using the run length coding procedure. If the line is to be encoded using the two-dimensional algorithm, the end-of-line (EOL) code is optional; a flag is tested to determine whether it should be coded. In an error-free environment it may be desirable to omit the EOL code. However, under many circumstances (when encoding using the CCITT standards, when switching from one-dimensional to two-dimensional encoding, or when a minimum number of bits per compressed line must be maintained), the EOL code will precede a two-dimensionally encoded line. If the EOL is coded, the bit pattern is '0000000000010'. RP is set to point to the first run end to be encoded; HP is set to point to the first run end in the run end buffer describing the history line; ROLD (the run end of the last run encoded) is set to 0; and FLAG1D is set to 0, indicating that the current line is being encoded two-dimensionally. Control then passes to the loop which encodes runs using either vertical reference codes or run length coding. Figure 2.2 shows the inner loop for the two-dimensional scheme. This loop calculates the difference (DIF) between the run end being encoded and a given run end in the history buffer. If this difference is zero, a vertical reference 0 (binary code '1' for VO) is encoded and the history buffer pointer is incremented to point to the next run end in the history line. At this point the history run end pointed to by HP (which will be used to encode the next run) is guaranteed to be to the right of the end of the run just encoded. A test is performed to determine whether the run end just encoded was at the right edge of the line being encoded; if so, the entire line has been encoded and an exit from the line encoding procedure occurs. Otherwise, the run end just encoded is saved as ROLD, the run end pointer is updated to point to the next run to be encoded, and the process repeats. The main loop is reentered in three places. The loop is entered at point A immediately after initialization, after a pair of runs has been run-length coded in the two-dimensional algorithm, or after the PASS mode has been coded. The other entry points are used after a nonzero vertical reference has been coded. In those cases the run end pointer still points to the last run en coded; it must be updated before calculating DIF. If the coded ru- ended to the right of the history run, the loop is entered at point C, before the test to see if the right edge has been reached. If the coded run ended to the left of the history run, then clearly the right edge has not been reached (since the line to be encoded has the same length as the history line), so the test for the right edge can be skipped and the loop reentered at B. iMen the right edge of the image is reached, immediately before control is returned to the main loop in the encoder (described in Figure 1), the pointer to the beginning of the run end buffer for the line just encoded (RPST) is copied to HPST so that the line will be available for use as a history line if the next line is to be encoded two-dimensionally. All other exits from the loop to encode a line also update HPST. If the value of DIF calculated in the main loop is nonzero, then tests are performed to see whether another vertical reference or a PASS code can be generated. This procedure is described in Figure 2.3. If DIF is less than zero (i.e. the current run end is to the left of the history run end), then we test to see if a vertical left reference is to be coded.' If the current run ends one pel te the left of the history run, we code a vertical left refer elce 1 (bit pattern '010' for VL1) and increment the history pcinter HP to point to the next run in the history line, since t-at must be the first history run end of the opposite color which is to the right of the run end just encoded. The exist eoce of extra copies of the last run end (line length) in each lue guarantees that there will be usable history data at HP even if the right edge has been reached. Control then returns to the main loop. If the current run ends two or three pels to the left of the history run, we code a vertical left reference 2 or 3 (bit patterns '000010' and '0000010' for VL2 and VL3, respectively). The history pointer is backed up, in case the previous history run ended to the right of the end of the run just encoded. A test is performed to see if this is in fact the case; if not, the history pointer is incremented to point to the next run of the same color. Control returns to the main loop. If no vertical reference code can be generated, the run length portion of the code is entered to encode a pair of runs. If DIF is greater than zero, we may be able to code the run as a vertical right reference or a PASS. If the current run ends one pel to the right of the history run, we code a vertical right reference 1 ('011' for VR1), and increment the history pointer to point to the next history run. This history run end is not to the left of the end of the run just encoded. A test must be made to ensure that it is in fact to the right of that run end. If it is not, then HP is incremented again. Control then returns to the main loop. If a VR1 is not coded, we determine whether the current run end is to the right of H2, the next run (of the opposite color) in the history buffer. If it is, then we code a PASS ('0001'), store the end point H2 as ROLD, point the history pointer to the run end of the next history run of the same color as the run being encoded, and return control to the main loop. If neither a VR1 or a PASS can be coded, we determine if the current run ends 2 or 3 pels to the right of the history run. If so, we code the appropriate vertical right reference ('000011' or '0000011' for VR2 or VR3), and increment the history pointer to point to the next history run. A test must be made to ensure that it is to the right of the run end just en coded. If it is not, then HP is incremented again and the new H. is tested. If it is still not to the right of the end of the run just encoded, HP is incremented again. Control then returns to the main loop. If a vertical reference cannot be coded, the run length coding portion of the algorithm is used to encode a pair of run lengths. Figure 2.4 shows the procedure used to encode run lengths. This procedure may be entered from one of two places: either immediately after the initializations for one-dimensional encoding of a line (point Y), or from the portion of the two-dimensional algorithm which attempts vertical reference coding, if such coding is not appropriate (point X). If the run length encoder is entered from the two-dimensional algorithm, some initialization is required. A prefix indicating that a pair of runs is encoded using the run length algorithm (binary code '001' for the RL prefix) is coded. TP must then be set to point to the run length tables for the color of run which is to be encoded first. This color may be determined by taking the difference of RP and RPST and determining whether or not it is a multiple of four. Since there are four entries in the run end buffer before the run end data begins, and since each entry requires two bytes, the white run ends are stored at offsets of 8, 12, 16, ... in the run end buffer while the black run ends are stored at offsets 10, 14, 18 .... Thus, if RP-RPST is a multiple of four, TP is set to point to the white tables; otherwise TP is set to point to the black tables. After TP is set, a flag (RUN2) is set to O to indicate that the first run of the pair is being encoded, and the run length of the run to be encoded is calculated as the difference between the current run end and the value saved as ROLD (which is either the end of the previous run or the position coded using PASS mode). After the initialization (if any) has been performed, a run length is encoded. This process will be described in greater detail with reference to Figure 2.5. After the run length has been encoded, some updating must be done and a decision must be made as to whether to encode another run length. FLAG1D is first tested to determine whether the current line is being encoded one-dimensionally. If it is, then the run end of the last run encoded is tested to determine whether the right edge of the image line has been reached; if it has, then the entire line has been encoded, and exit from the line encoder occurs. Otherwise, the table pointer TP is set to point to the run length tables for the opposite color (the color of the next run to be encoded), the run end just coded is saved, the run end pointer is incremented to point to the run end of the next run to be encoded, and the run length to be encoded is calculated as the difference between the old and new run ends. Control then goes to the top of the loop to encode the new run length. If the current line is being encoded two-dimensionally, the flag RUN2 is tested to determine if the run just coded was the first or second run of a pair. If it was the first, then another run must be encoded using run lengths. RUN2 is set to one to indicate that the second run of the pair is being encoded, and then the same setup procedure used for one-dimensional coding is performed: TP is set to point to the run length tables for the opposite color, the run end just coded is saved, the run end pointer is incremented to point to the run end of the next run to be encoded, and the run length to be encoded is calculated as the difference between the old and new run ends. Control then -goes to the top of the loop to encode the new run length. If the current line is being encoded two-dimensionally and the last run coded was the second run of a pair, then RO is tested to determine if the right edge of the line has been reached. If it has, an exit from the line encoder occurs. Otherwise the history pointer must be updated. Since it already points to a run of the correct color, updating consists of testing to see if the history run end pointed to by HP is to the right of the end of the run just encoded. If it is not, then HP is incremented to point to the next history run end of the same color, and that run end is tested. This process repeats until a history run end which is to the right of the end of the run just encoded is found. We are guaranteed that such a history run end exists since the run just encoded ends to the left of the right edge and we must eventually encounter a history run end which is on the right edge. Control then returns to the main loop for two-dimensional encoding. Figure 2.5 shows the procedure for encoding a run length. A run length is encoded using zero or-more "makeup" codes (encoding multiples of 64 which are no greater than 2560) and one "terminating" code giving the number of leftover bits (0-63). The run length is the sum of the values corresponding to the code words. Thus a run length of 5150 would be encoded as 2560+2560+30; a run length of 4000 would be encoded as 2560+1408+32; a run length of 2000 would be encoded as 1984+16; and a run length of 10 would be encoded using only the terminating code for 10. The loop which encodes a run length first tests the run length to see if it is less than 64. If it is, then the appropriate terminating code LOCODE(RL) can be coded, and encoding of the run length is complete. Otherwise, the run length is tested to see if it is greater than or equal to 2560. If so, then the makeup code for 2560 ('000000011111') is coded, the run length to be encoded is decremented by 2560, and control goes to the top of the loop. If the run length is not greater than 2560, then it is divided by 64 (this operation is implemented as a right shift) to obtain a value which is used to index into the table (HICODE) of makeup codes. The appropriate makeup code is coded, and the remainder from the division replaces the previous value of RL. This is accomplished by masking out the portion of RL which was used to generate the index for the makeup code. Control then returns to the top of the loop to encode the remainder. DECODING METHOD A method of decoding an encoded compressed digital image in accordance with the present invention will be described next with reference to Figures 3 through 4.6. The following list sets forth some of the features of the present invention which allow fast decoding. 1. Simple tables are used for fast serial decoding of Huffman variable length code words. The intermediate nodes specify the relative distance to the next node for a O (or 1) bit next in the compressed data stream. The other color sense has its next node as the next entry in a table. Negative entries flag terminating nodes and contain information about the code word found. For the Modified Huffman tables if branching is done for a '0', the relative distance is to the next node in the table is never more than 78, so a byte table is adequate even though 221 entries are used. 2. The same register is used to sequentially look at the compressed bits and to keep track of when all bits have been examined. The test for whether more data is needed is only necessary if a '1' compressed bit was apparently found since this might be the marker bit which flags the end of the data. Only in that special case will the rest of the register be zero. 3. The vertical reference codes are decoded with in-line code so that the location in the program indicates the code word found. Minimum tests are made to update the pointers into the history data buffers. The run end buffer is constructed so that even on the left and right edges looking forward or backwards up to three runs will still give correct information. 4. Fast code is used to create a bit map from the positions of color changes. (Described in U S Patent 4,xxx,xxx) 5. One subroutine is used to decode all three algorithms with almost no extra overhead. This minimizes the object code size. 6. Most of variables needed in the inner loop can be kept in registers. DECODER OVERVIEW The basic process to decode an image line is done in two steps. First, each line is decompressed into a buffer of run end positions in a subroutine. The run end buffer is saved at least until after the next line is decoded because it is needed to supply the history line for two-dimensional decoding. The run end buffer has the same format as the run end buffer for the encoder. As a consequence of this structure, even on the left and right image edges looking forward or backwards up to three runs will still give correct information. This is accomplished by replicating the first and last runs at least two more times. The advantage of at least three copies of the final runend value is that one can always move the pointer two runs ahead and still have valid data. Many less tests are thus needed during the decoding process. The compressed data stream can be correctly interpreted by decoding only into run ends. Thus, if some lines are to be skipped at the top of the image, these can be removed by only decoding into the runends until the requisite number of lines have been ignored. Second, if a bit image is desired, then the run end positions are converted to a bit map by a method described in U S Patent 4,aaa,aaa. One subroutine is used to decode all three algorithms into the buffer of run ends with almost no extra overhead. This minimizes the object code size. All three algorithms start with an EOL followed by a one-dimensional line. Therefore, the decoder must be able to decode one-dimensional lines. These lines occur at least every 2 to 4 lines for the MR algorithm. During two-dimensional coding the Horizontal Mode prefix code of '001' is followed by a pair of runs which must be decoded with the one-dimensional algorithm. A few simple switches are set so that tests against zero will keep the one-dimensionally coded lines always inside the run length decoding loop. One main difference between the MR code and the MMR code is the fact that the MR code has an EOL on every line. An EOL consists of at least eleven zeros followed by a 1 followed for two-dimensional decoding by a tag bit which specifies whether the next line is one- or two-dimensionally coded. It is needed at the end of each line in order to stop error propagation in Group III machines. By checking for the right edge of the compressed image after each run, the EOLs are not needed to flag the end of a line. The same code can be used for both algorithms by restarting the loop to decode a line if an EOL is found first. A flag is set so if a second EOL is decoded next, the RTC sequence is recognized. DECODE TABLES FOR HUFF}1AN VARIABLE LENGTH CODES. Simple tables for serial decoding of Huffman variable length code words are easily constructed and fast to decode. For N code words, N-l entries are intermediate nodes which specify the relative distance to the next node for either the 0 or 1 compressed bit. The other color sense does not need an explicit entry because its next node is the next entry in the table. N entries are terminating nodes. These are distinguished from intermediate nodes by a flag bit (in this embodiment the most significant bit). The remainder of the terminal node entry is the information about the code word. The entry at intermediate nodes can also be the absolute index into the table. For the Modified Huffman tables if branching is done for a 'O', the relative distance is to the next node in the table is never more than 78, so a byte table is adequate even though 221 entries are used. (It is ideal for 8 bit microprocessors.) Table of byte patterns Intermediate Nodes OXXXXXXX Add OXXXXXXX to current pointer to get to next entry if '0' next. Add 1 to get to next entry if '1' next. Terminal Nodes lOXXXXXX Run lengths 0-63. llXXXXXX if XXXXxX < 41 then it is the high order 6 bits for the run lengths which are larger than 63. 11111100 Uncompressed mode 11111101 Illegal bit patterns 11111110 10 zeros followed by a 1 (maybe EOL) 11111111 11 zeros of EOL The entry for 11 zeros (221st entry in the table) is made an intermediate node with a relative distance to the next node of 0. Then any zeros following the 11 zeros will not cause a movement to a new entry. The EOL terminal node of x'FF' is moved to the 222nd byte. The first '1' following the indefinite string of more than 10 zeros will be identified as the EOL. A method to generate these decode tables is given next. 1. List all codes words with a field containing the information needed for these terminal nodes. 2. List all prefixes to these codes including the null code and indicate that they are intermediate nodes. 3. Sort as an ascending binary sequence and remove duplicates. 4. To generate the intermediate node table entries append the bit which will be used to branch and find the match to the longer bit pattern. Fill in the relative distance in the desired units. In the simple example below the intermediate nodes are flagged with an I and the terminal nodes with a T. Original Node Code Words Info. O T 1 10 T 2 111 T 3 1100 T 4 1101 T 5 Table in ascending order of codes and prefixes for branching on 1. Code I 2 O T 1 1 12 10 T 2 11 I 4 110 1 2 l T 4 llC1 T 5 111 T 3 If the I/T bit were converted into the MSB of a byte, then the final table would be 2 129 2 130 4 2 132 133 131. Each byte either indicates the relative distance to go if the next bit decoded is a '1' or gives the result of the decoding. The most significant bit (MSB) steers the decoding process. If the MSB is zero, then continue decoding by skipping to the next byte if the next compressed bit is a 'O' or skipping the number of bytes specified in the byte table entry if a '1' is next. The flag for complete decoding is the MSB set to a 1. The process of decoding an image into storage is now described. In addition to the variables defined for the encoding process, the decoder uses the following variables: DTOPPAD - The number of blank lines to pad at the top of the output image before decoding input lines. DBOTFED - The number of blank lines to pad in the output image after finishing decoding. DOBOTPAD can be used to create a fixed size output image. DYSKIP- Number of input (compressed) lines to skip before outputting lines into the output buffer. DVSCALE - The vertical scale allows low (vertical) resolution facsimile image to be converted to a higher (vertical) resolution image by duplication during the decoding process. In the decoder, the basic operation performed on the compressed data stream is a test to determine whether the next bit is a zero or a one. In the preferred embodiment the compressed data stream in the decoder is managed by keeping 0-16 bits of compressed data left-justified in the 32-bit register R. The bit immediately beyond the end of the compressed data is a 1; all remaining bits in R are zero. We can then test the next compressed bit (the high-order bit in R) by the following procedure. R is added to itself. This shifts all bits of R left one bit. In the IBM 370 series the ADD sets condition codes which specify whether a carry/overflow occurred from the high-order bit of R (the bit to be tested) and whether the remainder is zero. If no carry occurs, the bit tested was a zero, and this information can be returned to the decoder. If an overflow did occur, then either a one bit occurred in the compressed data stream or the one bit which flags the end of the compressed data has been shifted out. Which of these is the case can be determined by examining R. If R is nonzero, then the flag bit is still in R, and consequently the one which was shifted out was a bit of compressed data, so this information can be returned to the decoder. If R is zero, then the one which was shifted out was the flag bit. In this case we must read in another byte or halfword of compressed data, left-justify it in R (introducing zero bits in the low-order portion or R), and insert a flag bit immediately beyond the end of the nèw data. The next bit can be tested, and the result returned to the decoder. A counter keeps track of the next halfword of compressed data to be used; when the buffer is exhausted, more data is requested from the operating environment. For machines other than the IBM 370 series, the most significant bit can be tested by determining whether the register is negative and then adding it to itself. Only on the negative path will it be necessary to test if the register is now 0 and therefore the last bit was the flag bit. Wheys decoding the Modified Huffman run lengths, if 16 new (not all zero) compressed bits are loaded into the compressed data register then testing for the end of valid data will not be needed for the rest of that code word. The longest code is 13 bits, except for the extra zeros added by FILL to the EOL. Thus as long as the new bits are not all zero, the code is guaranteed to end before the marker bit is reached. In addition, when decoding the vertical reference codes, once new data has been moved into the register, there are guaranteed to be 8 valid bits, which is the maximum number needed to decode any of the veritical reference codes as well as the first 8 zeros for the EOL. In this case a separate path which does not check for valid data could be followed. Referring to FIG. 3, the method of decoding compressed image data according to the present invention will be described. INITIALIZATION FOR DECODING METHOD The initialization process is described below. It occurs only once for each image decoded. It is similar to the initialization required for the encoder. Artificial all white and all black lines are created. If the actual line length is not known, then LL is set to X'FFFF' and the blank lines are recalculated after the first one-dimensional line has been decoded to determine the real line length. A ru end point buffer representing a white blank line is created and stored. It has the following sequence: 16 0 0 0 LL LL IL LL where LL is the line length. A run end point buffer representing a black blank line is also created and stored. It has the following sequence: 16 0 0 0 0 LL LL LL These buffers will be used as blank line end points to be decoded when DTOPPAD and DBOTPAD are non-zero. HPST is initialized to point at the first white run (first LL) of the blank white line run buffer, in case the first line of the image is to be decoded two-dimensionally and the convention is adopted of a blank white history line. The register used to examine the compressed data a bit at a time is initialized to X'80000000'. This will cause compressed data to be read t-he first time that an attempt is made to test a bit. Other variables such as MH, BITIM, and VSCALE are operator controlled to select the desired options. FLAG1D will normally be set by the leading EOL in the compressed data. It must be preset if there is none. SKIP LINES If DYSKIP is positive, then the appropriate number of compressed lines must be decoded into run ends and discarded. PAD LINES If DTOPPAD is positive, then extra blank lines must be generated at the top of the image. Since the first line is always decoded one-dimensionally and followed by an End of Line (EOL) code, the number of bits per line are known at the end of the first decoded line. If no lines are skipped, and extra lines should be padded, then the size of the output image must be preset. (This can be done by decoding just one line to find the original line size and then restarting the process). To generate blank lines, RPST is set to either the black or white blank line according to PADCLR. If a run end represen tat ion of the image is desired, then the 16 bytes are copied the correct number of times into the output buffer. Otherwise, the procedure to convert from run ends to bits is applied to convert the first blank line and the resulting bit image line is copied the desired number of times. LOOP TO DECODE ONE LINE The main decoder loop decodes one line. It first tests to see whether any additional lines are to be decoded; if not, an exit from the loop occurs. This can occur because the compressed data stream was exhausted, the return to control sequence of several EOLs in a row was encountered, or the number of output lines requested has been decoded. The procedure to decode a line into run ends is described in greater detail with reference to Figures 4.1 to 4.6. If a bit image is desired, then the decoded run end representation must be converted to raster scan format. A preferred method for this conversion is given in US Patent 4,xxx,xxx. Alternately, the output image might be left in the form of many lines in run end point format. Such a representation could be readily re-encoded. This makes conversion between the one and two-dimensional algorithms and the various options easier since the conversion to and from the raster image can be avoided. The vertical scale parameter DVSCALE, if greater than one, causes duplicate copies of the output line to be created and therefore for a higher resolution to be decoded than the original image. PAD LINES Once the desired number of compressed lines have been decoded, extra lines are padded if DBOTPAD is positive. The same process of pointing to the desired color blank line used with DTOPPAD is used at the end of the image. DBOTPAD may be calculated at this point to maintain a fixed size image. PROCEDURE TO DECODE ONE LINE Figures 4.1 through 4.6 illustrate the steps required to decode one line into a run end point representation. Figure 4.1 shows the initialization required before decoding the data. RPST is assumed to be pointing to the place when the run end buffer is to be created. The three halfwords following the halfworc where the length is to be put are zeroed. RF is then reset tc point to the place where the first decoded run end is to be stored. ROLD, the last run, is initialized to O. FLAGlD is examined to determine whether the next line is expected to be one- or two-dimensionally coded. (If the compressed data has an EOL code next, then either path may be chosen for the same result.) If the FLAGID indicates one-dimensional coding (1), TP is set to point to the run length decode tables for white runs, and NXTTP is set to the run length decode tables for black runs. Control then passes to the loop which decodes runs using the run length decoding procedure. If the line is to be decoded using the two-dimensional algorithm, HP is set to point to the first run end in the run end buffer describing the history line. Control then passes to the loop which decodes runs according to the two-dimensional al algorithm Figure 4.2 shows the inner loop for the two-dimensional scheme. This loop examines the next compressed data bit. If this bit is '1', a vertical reference 0 (VO) has been decoded. The history run end is saved in ROLD and the history buffer pointer is incremented to point to the next run end in the history line. A test is performed to determine whether the run end just decoded was at the right edge of the line being decoded; if so, the en tire line has been decoded and an exit from the line decoding procedure occurs. Otherwise, the ROLD is saved as the run end just decoded, the run end pointer is updated to point to the next run to be decoded, and the process repeats. The main loop is reentered in three places. The loop is entered at point A immediately after initialization, after a pair of runs has been run-length decoded in the two-dimensional algorithm, or after the PASS mode has been decoded. The other entry points are used after a nonzero vertical reference has been decoded. In those cases the last run end decoded is in ROLD; it must be saved at RO and RP must be updated before testing the next bit. If the decoded run ended to the right of the history run, the loop is entered at point C, before the test to see if the right edge has been reached. If the decoded run ended to the left of the history run, then clearly the right edge has not been reached (since the line to be decoded has the same length as the history line), so the test for the right edge can be skipped and the loop reentered at B. When the right edge of the image is reached, immediately before control is returned to the main loop in the decoder (described in Figure 3), the pointer to the beginning of the run end buffer for the line just decoded (RPST) is copied to HPST so that the line will be available for use as a history line if the next line is to be decoded two-dimensionally. All other exits from the loop to decode a line also update HPST. Extra copies of the last (right edge) end point are made in the run end buffer at this time. The number of bytes in the run end buffer may optionally be stored at RLEN, the halfword pointed to by RPST. If the bit tested in the main loop is nonzero, then additional bits are tested to identify the code word. This procedure for the vertical reference codes and PASS code is-described in Fig ures 4.3 and 4.4. Successive bits are examined until a complete code word is identified. When VL1 is decoded, ROLD is set one bit to the left of the history run end. The history pointer can be incremented (as in the encoder for VL1) and control returned to the main loop. When VRl is decoded, ROLD is set one bit to the right of the history run end. The history pointer is incremented to point to the next history run. This history run end is not to the left of the end of the run just decoded. A test must be made to ensure that it is in fact to the right of that run end. If it is not, then HP is incremented again. Control then returns to the main loop. If the run length prefix is decoded, the table pointer is set to the correct color decode table using the same procedure to determine the color as was described in the encoder. NXTTP is set to the run length decode tables for the opposite color. A flag (RUN2) is set to O to indicate that the first run of the pair is being decoded. Control then passes to the loop which decodes runs using the run length decoding procedure. If we decode a PASS ('0001'), we save the end point H2 as ROLD, point the history pointer to at the next history end point of the same color as the run being decoded, and return control to the main loop. Figure 4.4 shows the continuation of the vertical reference decoding procedure. If one of the four remaining vertical reference codes (VL2, VL3, VR2, or VR3) is decoded, then ROLD is calculated as the correct relationship to HO and the history pointer is updated in the same way as during the encoding process. Control returns to the main loop. The bit pattern of '0000001' is the prefix to the two-dimensional code extensions. The pattern for the same extensions during one-dimensional decoding is '000000001'. The table pointer for the '0000001' path is set to the point it would have reached if the one-dimensional decoder had processed the extension prefix and control is passed to the run length decode loop. The bit pattern of '0000000' is expected to produce an EOL code. The table pointer for the '0000000' path is set to the point it would have reached if the one-dimensional decoder had processed these seven zeroes and control is passed to the run length decode loop. Figure 4.5 shows the procedure used to decode run lengths. This procedure is entered at the point Y immediately after the initializations for one-dimensional decoding of a line, after decoding a run length prefix, after decoding the prefix to the extensions, or after decoding the first seven zeros of the EOL. This loop begins by decoding a run length. This process will be described in greater detail with reference to Figure 4.6. If an EOL is decoded at the start of the line, then control is returned to point Z during the initialization process (Figure 4.1). If an EOL is decoded at the end of a line, then exit from the line decoder occurs. Otherwise, after the run length has been decoded, ROLD has been incremented by the run length. Some updating must then be done and a decision made as to whether to decode another run length. FLAGlD is tested to determine whether the current line is being decoded one-dimensionally. If it is, then the run end of the last run decoded is tested to determine whether the right edge of the image line has been reached; if it has, then the entire line has been decoded, and exit from the line decoder occurs. Otherwise, the table pointer TP is set to point to the run length tables for the opposite color (the color of the next run to be decoded), the run end just decoded and saved in ROLD is stored at RO, and the run end pointer is incremented to point to the next (unused) space in the run end buffer. Control then goes to the top of the loop to decode the new run length. If the current line is being decoded two-dimensionally, the flag RUN2 is tested to determine if the run just decoded was the first or second run of a pair. If it was the first, then another run must be decoded using run lengths. RUN2 is set to one to indicate that the second run of the pair is being decoded, and then the same setup procedure used for one-dimensional coding is performed: TP is set to point to the run length tables for the opposite color, the run end just decoded and saved in ROLD is stored at RO, and the run end pointer is incremented to point to the next (unused) space in the run end buffer. Control then goes to the top of the loop to decode the new run length. If the current line is being decoded two-dimensionally and the last run coded was the second run of a pair, then ROLD is tested to determine if the right edge of the line has been reached. If it has, an exit from the line decoder occurs. Otherwise the history pointer must be updated. Since it already points to a run of the correct color, updating consists of testing to see if the history run end pointed to by HP is to the right of the end of the run just decoded. If it is not, then HP is incremented to point to the next history run end of the same color, and that run end is tested. This process repeats until a history run end which is to the right of the end of the run just decoded is found. We are guaranteed that such a history run end exists since the run just decoded ends to the left of the right edge and we must eventually encounter a history run end which is on the right edge. Control then returns to the main loop for two-dimensional decoding. Figure 4.6 shows the procedure for decoding a run length. The bits are examined one at a time. If the next bit is a '1', then the table pointer is incremented to the next entry. If this entry (TO) is less than X'80' then a complete code has not yet been identified and control returns to the top of the loop to test the next bit. If the bit tested is a '0', then the entry value (TO) is added to the table pointer to move it to the subtree of the code table for codes which begin with the bits already decoded, and the loop repeats. If on either path, the table entry is not less than X'80', then a complete code word has been decoded. If the second most significant bit of the table entry is zero, then a terminating code has been decoded. The number of image bits represented by this code is obtained from the. low order 6 bits of the table entry and added into ROLD. At this point, the decoding of the run length is complete. Otherwise, TO is tested to see if it is greater than X'E8'. If not, then a makeup code has been decode. The number of image bits represented by this code is obtained by multiplying the value in the low order 6 bits of the table entry by 64 (this operation is implemented as a left shift) and added into ROLD. The table pointer has to be reset to the top of the same color run length tables. This is done by setting it to NXTTP and adding in the offset to get back to the original table. Control then returns to the top of the loop to decode another makeup or terminating code word. If the table entry is greater than X'E8', then an EOL has been decoded. If the Modified Huffman algorithm is not being used then it is necessary to decode the tag bit and set FLAGID to indicate whether the next line is one- or two-dimensionally coded. ROLD is then tested to determine whether the EOL was encountered at the start or end of the line. If it occurred at the end of the line, then exit occurs from the line decode procedure. If it occurred at the start of a line, control returns to point Z in the initialization process (Figure 4.1). This description assumes that no invalid code words will occur in the compressed data and that uncompressed mode was not used in the encoder. Instead of interpreting all table entries greater than XtE8' as EOLs, the appropriate tests and error handling procedures can be implemented to detect invalid codes flagged by X'Ex'. If the uncompressed mode extension X'Fx' is found, a person skilled in the art could extend the procedure to decode uncompressed mode code words.

Claims: CLAIMS 1. A method for encoding and decoding digital data, wherein parameters are initialized, comprising the steps of: test ing said data for a predetermined relationship between ad jacent lines of said data; generating directly a reference code word representative of said predetermined relationship for each successful test; generating by table lookup a run code word for a stream of data bits of common value for each unsuccessful test, wherein a data element representing a number of bits in a code word is stored at a predeter mined offset from a table entry representing said code word; merging said reference code words and said run code words generated by the above steps to form an encoded data stream; testing an encoded data stream for a reference code; decoding directly said predetermined relationship from said reference code for each successful test; decoding by table lookup a run of bits of common value by serial ex amination of said encoded data stream for each unsuccessful test storing data decoded by the above steps in a predeter mined relationship. 2. A method for encoding digital data, wherein parameters are initialized, comprising the steps of: testing said data for a predetermined relationship between adjacent lines of said data; generating directly a reference code word repre sentative of said predetermined relationship for each suc cessful test; generating by table lookup a run code word for a stream of data bits of common value for each unsuc cessful test, wherein a data element representing a number of bits in a code word is stored at a predetermined offset from a table entry representing said code word; merging said reference code words and said run code words generated by the above steps to form an encoded data stream. 3. A method according to claims 1 or 2, further comprising the step of encoding pad lines as specified by operator control. 4. A method according to claims 1 or 2, further comprising the step of skipping image lines as specified by operator con -trol. 5. A method according to claims 1 or 2, further comprising the step of changing a vertical scale of an encoded image as specified by operator control. 6. A method for decoding digital data, wherein parameters are initialized, comprising the steps of: testing an encoded data stream for a reference code; decoding directly said predetermined relationship from said reference code for each successful test; decoding by table lookup a run of bits of common value by serial examination of said encoded data stream for each unsuccessful test storing data decoded by the above steps in a predetermined relationship. 7. A method according to claims 1 or 6, further comprising the step of decoding pad lines. 8. A method according to claims 1 or 6, further comprising the step of decoding skipped image lines. 9. A method according to claims 1 or 6, further comprising the step of decoding a change in a vertical scale of encoded data.

Inventors: ANDERSON, KAREN LOUISE, GOERTZEL, GERALD (NMN), MITCHELL, JOAN LA VERNE
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1984
Main classification: H04N1
IPC codes: G06T9, H04N1