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<pre>
Interchange File Format *.IFF

4 bytes "FORM" (FORM chunk ID)
1 long length of file that follows
4 bytes "ILBM" (InterLeaved BitMap file ID)

4 bytes "BMHD" (BitMap HeaDer chunk ID)
1 long length of chunk [20]
20 bytes 1 word = image width in pixels
1 word = image height in lines
1 word = image x-offset [usually 0]
1 word = image y-offset [usually 0]
1 byte = # bitplanes
1 byte = mask (0=no, 1=impl., 2=transparent, 3=lasso)
1 byte = uncompressed [0], packbits [1], vertical RLE [2]
1 byte = unused [0]
1 word = transparent color (for mask=2)
1 byte = x-aspect [5=640x200, 10=320x200/640x400, 20=320x400]
1 byte = y-aspect [11]
1 word = page width (usually the same as image width)
1 word = page height (usually the same as image height)

4 bytes "CMAP" (ColorMAP chunk ID)
1 long length of chunk [3*n where n is the # colors]
3n bytes 3 bytes per RGB color. Each color value is a byte
and the actual color value is left-justified in the
byte such that the most significant bit of the value
is the MSB of the byte. (ie. a color value of 15 ($0F)
is stored as $F0) The bytes are stored in R,G,B order.

4 bytes "CRNG" (Color RaNGe chunk ID)
1 long length of chunk [8]
8 bytes 1 word = reserved [0]
1 word = animation speed (16384 = 60 steps per second)
1 word = active [1] or inactive [0]
1 byte = left/lower color animation limit
1 byte = right/upper color animation limit

4 bytes "CAMG" (Commodore AMiGa viewport mode chunk ID)
1 long length of chunk [4]
1 long viewport mode bits:
bit 2 = interlaced
bit 7 = half-bright
bit 11 = HAM
bit 15 = high res

4 bytes "BODY" (BODY chunk ID)
1 long length of chunk [# bytes of image data that follow]
? bytes actual image data

NOTES: Some of these chunks may not be present in every IFF file, and may
not be in this order. You should always look for the ID bytes to find a
certain chunk. All chunk IDs are followed by a long value that tells the
size of the chunk (note that "ILBM" is not a chunk ID). This is the number of
bytes that FOLLOW the 4 ID bytes and size longword. The exception to this is
the FORM chunk. The size longword that follows the FORM ID is the size of the
remainder of the file. The FORM chunk must always be the first chunk in an
IFF file.

The R,G,B ranges of AMIGA and ST are different (AMIGA 0...15, ST 0...7),
as is the maximum number of bitplanes (AMIGA: 5, ST: 4).

Format of body data

An expanded picture is simply a bitmap. The most common packing method is
PackBits (see below), and is identical to MacPaint and DEGAS Elite compressed.

The (decompressed) body data appears in the following order:

line 1 plane 0 ... line 1 plane 1 ... ... line 1 plane m
[line 1 mask (if appropriate)]
line 2 plane 0 ... line 2 plane 1 ... ... line 2 plane m
[line 2 mask (if appropriate)]
...
line x plane 0 ... line x plane 1 ... ... line x plane m
[line x mask (if appropriate)]

The FORM chunk identifies the type of data:

"ILBM" = interleaved bit map
"8SVX" = 8-bit sample voice
"SMUS" = simple music score
"FTXT" = formatted text (Amiga)

The ST version of DPAINT always uses the vertical RLE packing format in the body data.
Within the BODY chunk there is one VDAT chunk per bit plane (four in total). Every VDAT chunk
is laid out in columns, similar to the Tiny format. Only the compression scheme is slightly
different, command 0, and 1 are flipped, and extra count words store in data list.
4 bytes 'VDAT' vertical bitplane data
4 bytes length of cunk
?? bytes compressed data

Data compression format:
2 bytes cmd_cnt command bytes count - 2
? bytes cmd_cnt - 2 command bytes.
? words data words

Note that the number of commands is 2 more than actual commands, and that the data
words may start on an odd address if the number of command bytes are odd.

while(cmd_cnt>2)
1 byte cmd 0: read one data word as count
output count literal words from data words.
1: read one command word as count, then one data word as data
output data count times
<0: -cmd is count, output count words from data words
>2: cmd is count, read one data word as data
output data count times
</pre>
See also [[PackBits Compression Algorithm]]

Back to [[ST Picture Formats]]

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