Imagic Film/Picture file format

Imagic Film/Picture    *.IC1 (low resolution)
                       *.IC2 (medium resolution)
                       *.IC3 (high resolution)

4 bytes         "IMDC"
1 word          resolution (0 = low res, 1 = medium res, 2 = high res)
16 words        palette
1 word          date (GEMDOS format)
1 word          time (GEMDOS format)
8 bytes         name of base picture file (for delta compression), or zeroes
1 word          length of data (?)
1 long          registration number
8 bytes         reserved
1 byte          compressed? (0 = no, 1 = yes)

If compressed {
1 byte          delta-compressed? (-1 = no, > -1 = yes)
1 byte          ?
1 byte          escape byte
}
--------
65 bytes        total for header (68 bytes if compressed)

? bytes         data

   Compressed data may be either stand-alone or delta-compressed (relative
to the base picture named in the header).  Delta compression involves
storing only how the picture differs from the base picture (i.e., only
portions of the screen that have changed are stored).  This is used to
to encode animated sequences efficiently.

Compressed data, stand-alone:

For each byte x in the data section:

        x = escape byte         Read one more byte, n.  (n is unsigned).

                                If n >= 2, use the next byte n times.
                                If n = 1, keep reading bytes until a
                                byte k not equal to 1 is encountered.
                                Then read the next byte d.
                                If the number of 1 bytes encountered is o,
                                use d (256 * o + k) times.  I.e.,

                                if (n == 1) {
                                        o = 0;
                                        while (n == 1) {
                                                o++;
                                                n = next byte;
                                        }

                                        k = n;
                                        d = next byte;

                                        Use d (256 * o + k) times.
                                }
                                else {
                                        d = next byte;
                                        Use d (n) times.
                                }

        x != escape byte        Use x literally.

Compressed data, delta compressed:

For each byte x in the data section:

        x = escape byte         Read one more byte, n.  (n is unsigned).

                                If n >= 3, use the next byte n times.
                                If n = 1, do the same as for n = 1 in
                                stand-alone compression (above).
                                If n = 2, then set n = next byte.
                                        If n = 0, end of picture.
                                        If n >= 2, take n bytes from base
                                        picture.
                                        If n = 1, do the same as for n = 1
                                        in stand-alone compression (above),
                                        but take (256 * o + k) bytes from 
                                        base picture.

        x != escape byte        Use x literally.

Note: 68K decompression code can be found here:
      http://gfa.atari-users.net/gfx/

Back to ST Picture Formats