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RGBN and RGB8 IFF Image Data

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RGB image forms, Turbo Silver (Impulse)

                    FORM RGBN and FORM RGB8

	RGBN and RGB8 files are used in Impulse's Turbo Silver and Imagine.
	They are almost identical to FORM ILBM's except for the BODY chunk
	and slight differences in the BMHD chunk.


	The BMHD chunk specfies the number of bitplanes as 13 for type RGBN
	and 25 for type RGB8, and the compression type as 4.

	The FORM RGBN uses 12 bit RGB values, and the FORM RGB8 uses
	24 bit RGB values.

	The BODY chunk contains RGB values, a "genlock" bit, and repeat
	counts.  In Silver, when "genlock" bit is set, a "zero color" is
	written into the bitplanes for genlock video to show through.
	In Diamond and Light24 (Impulse 12 & 24 bit paint programs),
	the genlock bit is ignored if the file is loaded as a picture
	(and the RGB color is used instead), and if the file is loaded
	as a brush the genlock bit marks pixels that are not part of
	the brush.

	For both RGBN and RGB8 body chunks, each RGB value always has a
	repeat count.  The values are written in different formats depending
	on the magnitude of the repeat count.

	For the RGBN BODY chunk:

		For each RGB value, a WORD (16-bits) is written: with the
		12 RGB bits in the MSB (most significant bit) positions;
		the "genlock" bit next; and then a 3 bit repeat count.  
		If the repeat count is greater than 7, the 3-bit count is
		zero, and a BYTE repeat count follows.  If the repeat count
		is greater than 255, the BYTE count is zero, and a WORD
		repeat count follows.  Repeat counts greater than 65536 are
		not supported.

	For the RGB8 body chunk:

		For each RGB value, a LONG-word (32 bits) is written:
		with the 24 RGB bits in the MSB positions; the "genlock"
		bit next, and then a 7 bit repeat count.

		In a previous version of this document, there appeared the
		following line:

		"If the repeat count is greater than 127, the same rules apply
		as in the RGBN BODY."

		But Impulse has never written more than a 7 bit repeat count,
		and when Imagine and Light24 were written, they didn't support
		reading anything but 7 bit counts.

	Sample BODY code:

			if(!count) {
				if (Rgb8) {
					fread (&w, 4, 1, RGBFile);
					lock = w & 0x00000080;
					rgb = w >> 8;
					count = w & 0x0000007f;
				} else {
					w = (UWORD) getw (RGBFile);
					lock = w & 8;
					rgb = w >> 4;
					count = w & 7;
				if (!count)
					if (!(count = (UBYTE) getc (RGBFile)))
						count = (UWORD) getw (RGBFile);

	The pixels are scanned from left to right across horizontal
	lines, processing from top to bottom.  The (12 or 24 bit) RGB
	values are stored with the red bits as the MSB's, the green
	bits next, and the blue bits as the LSB's.

	Special note:  As of this writing (Sep 88), Silver does NOT
	support anything but black for color zero.