INDIGO GRAPHIC SYSTEMS



T A B L E   O F   C O N T E N T S

1. COMPRESSED IMAGE FORMAT ................................... 1

1.1. Data Bytes .............................................. 2

1.2. The Control Code Byte ................................... 3

2. Example ................................................... 4





1. COMPRESSED IMAGE FORMAT

   The compressed image data consists of four-byte words. The
MSByte contains the op-code which defines the type of data
contained by the other three bytes.

Data format:

     D31        D24 D23        D16 D15         D8 D7        D0
        control          byte2          byte1         byte0

   Byte0 is the first byte of the word to be decompressed and
plotted. Byte2 is the last one. Each compressed line begins in a
new word.


1.1. Data Bytes

The Data Bytes may contain 3 types of data:

RASTER : The byte contains uncompressed image data. The least
         significant bit of a RASTER byte is the first to be
         plotted.

RUNLENGTH : The value of the data byte represents the number of
         bytes in the runlength (zeroes or ones).

REPEAT RASTER : The value in the data byte represents the number
         of times the previous data byte (which must be a RASTER
         byte) is to be repeated. For example : a RASTER that
         appears 100 times will be compressed as the RASTER byte
         itself followed by 99 in the next data byte.

PI COMMAND : The byte contains either EOL (End Of Line) or EOI
         (End Of Image) command. This byte is always the last
         data byte in its word.

         bit 7 - not used
         bit 6 - not used
         bit 5 - not used
         bit 4 - not used
         bit 3 - not used
         bit 2 - not used
         bit 1 - EOL
         bit 0 - EOI


1.2. The Control Code Byte

   The Control Code Byte defines the type of data in each data byte.

         bit 7 - C1
         bit 6 - C0
         bit 5 - O/Z_REP2
         bit 4 - O/Z_REP1
         bit 3 - O/Z_REP0
         bit 2 - RL/RAS2
         bit 1 - RL/RAS1
         bit 0 - RL/RAS0


C1,C0 - 00 - Data Byte 0 contains PI_COMMAND
        01 -  "    "   1    "         "
        10 -  "    "   2    "         "
        11 - No data byte contains PI_COMMAND

RL/RASi - 1 - Data byte i contains a runlength
          0 -  "    "   "    "     " raster

O/Z_REPi - 0 - if RL/RASi=1 then
                    Data byte i contains a runlength of zeroes
               else
                    Data byte i is a regular RASTER
           1 - if RL/RASi=1 then
                    Data byte i contains a runlength of ones
               else
                    Data byte i is a REPEAT RASTER



2. Example

Lets take for example the following image raster line:

00000000 00000000 00000000 00000000 11111111 11111111 11111111
10101010 10101010 10101010 00001111

This line containes:

1. A RUNLENGTH of 4 bytes of zeroes.
2. A RUNLENGTH of 3 bytes of ones.
3. Three repetetive identical RASTER bytes.
4. One different RASTER byte.

The hexadecimal compressed raster format is following:

control   byte2    byte1    byte0
11010011 01010101 00000011 00000100
10X01X00 00000000 11110000 00000010
   MSB                        LSB


Note that:
1. X means "DONT CARE"
2. A RASTER byte appears in the compressed raster format as "lsb
         first".
3. The compression ratio in this case is not very good but it
         becomes better when the RUNLENGTHs are longer.


                                                  EDDI LITVAK .