Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!sgi!paul@manray.sgi.com
From: paul@manray.sgi.com (Paul Haeberli)
Newsgroups: comp.graphics
Subject: conversion from RGB to CMYK
Message-ID: <50957@sgi.sgi.com>
Date: 16 Feb 90 10:02:17 GMT
Sender: news@sgi.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 90

Here is a little code fragment that converts from rgb to CMYK.
We have used this transformation for several years as part of
the process of creating digital color separations for various 
publications. 

Paul Haeberli 
paul@sgi.com

/*
 *	rgb_to_cmyk -
 *		Convert from rgb to cmyk.  This implements 100% grey component
 *	replacement. 
 *
 *	Inputs:
 *		r intensity 0 to 255
 *		g intensity 0 to 255
 *		b intensity 0 to 255
 *
 *	Outputs:
 *		c coverage 0 to 255
 *		m coverage 0 to 255
 *		y coverage 0 to 255
 *		k coverage 0 to 255
 *
 *
 *	An input value of rgb=[255,255,255] represents white.  When this is
 *	transformed to cmyk, we get cmyk=[0,0,0,0] which represents zero 
 *	coverage. 
 *
 *	An input value of rgb=[128,128,128] represents 50 percent grey.  When
 *	this is transformed to cmyk, we get cmyk=[0,0,0,127] which represents 
 *	zero coverage by cmy, and 50 percent coverage by black. 
 *
 *				Paul Haeberli - 1988
 *		
 */
#define ULTRAC		160
#define ULTRAM		160
#define ULTRAY		0
#define ULTRAK		255
#define BLACKLERP	4

rgb_to_cmyk(r,g,b,c,m,y,k)
register int r, g, b;
int *c, *m, *y, *k;
{
    register int i;
    float param; 

/* i is the max of r, g, and b */
    i = 0;
    if(r>i)
	i = r;
    if(g>i)
	i = g;
    if(b>i)
	i = b;

/* if r, g and b are all zero then print full k plus some m and c */
    if(i == 0) {
	*c = ULTRAC;
	*m = ULTRAM;
	*y = ULTRAY;
	*k = ULTRAK;
	return;
    }
    r = (255*r)/i;
    g = (255*g)/i;
    b = (255*b)/i;
    *c = 255-r;
    *m = 255-g;
    *y = 255-b;
    *k = 255-i;

/* if i is less than blacklerp start lerp towards ultra-black */
    if(i<BLACKLERP) {
	param = ((float)i)/((float)BLACKLERP);
	*c = lerp(ULTRAC,*c,param);
	*m = lerp(ULTRAM,*m,param);
	*y = lerp(ULTRAY,*y,param);
	*k = *k;
    }
}

lerp(i0,i1,p)
int i0, i1;
float p; 
{
    return ((i0*(1.0-p))+(i1*p));
}