Color Lookup Table chunk


amiga.dev/iff message 1527
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TITLE: CLUT IFF chunk proposal

"CLUT" IFF 8-Bit Color Look Up Table

Date:	July 2, 1989
From:	Justin V. McCormick
Status:	Public Proposal
Supporting Software:  FG 2.0 by Justin V. McCormick for PP&S


Introduction:

  This memo describes the IFF supplement for the new chunk "CLUT".

Description:

  A CLUT (Color Look Up Table) is a special purpose data module
containing table with 256 8-bit entries.  Entries in this table
can be used directly as a translation for one 8-bit value to
another.

Purpose:

  To store 8-bit data look up tables in a simple format for
later retrieval.  These tables are used to translate or bias
8-bit intensity, contrast, saturation, hue, color registers, or
other similar data in a reproducable manner.

Specifications:

/* Here is the IFF chunk ID macro for a CLUT chunk */
#define ID_CLUT MakeID('C','L','U','T')

/*
 * Defines for different flavors of 8-bit CLUTs.
 */
#define CLUT_MONO	0L	/* A Monochrome, contrast or intensity LUT */
#define CLUT_RED	1L	/* A LUT for reds 		*/
#define CLUT_GREEN	2L	/* A LUT for greens 		*/
#define CLUT_BLUE	3L	/* A LUT for blues 		*/
#define CLUT_HUE	4L	/* A LUT for hues 		*/
#define CLUT_SAT	5L	/* A LUT for saturations 	*/
#define CLUT_UNUSED6	6L	/* How about a Signed Data flag */
#define CLUT_UNUSED7	7L	/* Or an Assumed Negative flag	*/

/* All types > 7 are reserved until formally claimed */
#define CLUT_RESERVED_BITS 0xfffffff8L

/* The struct for Color Look-Up-Tables of all types */
typedef struct
{
  ULONG type;		/* See above type defines */
  ULONG res0;		/* RESERVED FOR FUTURE EXPANSION */
  UBYTE lut[256];	/* The 256 byte look up table */
} ColorLUT;


CLUT Example:

  Normally, the CLUT chunk will appear after the BMHD of an FORM
ILBM before the BODY chunk, in the same "section" as CMAPs are
normally found.  However, a FORM may contain only CLUTs with no
other supporting information.

  As a general guideline, it is desirable to group all CLUTs
together in a form without other chunk types between them.
If you were using CLUTs to store RGB intensity corrections, you
would write three CLUTs in a row, R, G, then B.

  Here is a box diagram for a 320x200x8 image stored as an IFF ILBM
with a single CLUT chunk for intensity mapping:

      	+-----------------------------------+   
      	|'FORM'		64284		    |     FORM 64284 ILBM
      	+-----------------------------------+   
      	|'ILBM'				    |     
      	+-----------------------------------+   
      	| +-------------------------------+ |   
      	| | 'BMHD'	20		  | |     .BMHD 20
      	| | 320, 200, 0, 0, 8, 0, 0, ...  | |   
      	| | ------------------------------+ |   
      	| | 'CLUT'	264	          | |     .CLUT 264
      	| | 0, 0, 0; 32, 0, 0; 64,0,0; .. | |   
      	| +-------------------------------+ |   
      	| +-------------------------------+ |   
      	| |'BODY'		64000     | |     .BODY 64000
      	| |0, 0, 0, ...			  | |   
      	| +-------------------------------+ |   
      	+-----------------------------------+   


Design Notes:
-------------

  I have deliberately kept this chunk simple (KISS) to
facilitate implementation.  In particular, no provision is made
for expansion to 16-bit or 32-bit tables.  My reasoning is that
a 16-bit table can have 64K entries, and thus would benefit from
data compression.  My suggestion would be to propose another
chunk or FORM type better suited for large tables rather than
small ones like CLUT.