harvey/sys/src/libdraw/rgb.c

#include <u.h>
#include <libc.h>
#include <draw.h>

/*
 * This original version, although fast and a true inverse of
 * cmap2rgb, in the sense that rgb2cmap(cmap2rgb(c))
 * returned the original color, does a terrible job for RGB
 * triples that do not appear in the color map, so it has been
 * replaced by the much slower version below, that loops
 * over the color map looking for the nearest point in RGB
 * space.  There is no visual psychology reason for that
 * criterion, but it's easy to implement and the results are
 * far more pleasing. 
 *
int
rgb2cmap(int cr, int cg, int cb)
{
	int r, g, b, v, cv;

	if(cr < 0)
		cr = 0;
	else if(cr > 255)
		cr = 255;
	if(cg < 0)
		cg = 0;
	else if(cg > 255)
		cg = 255;
	if(cb < 0)
		cb = 0;
	else if(cb > 255)
		cb = 255;
	r = cr>>6;
	g = cg>>6;
	b = cb>>6;
	cv = cr;
	if(cg > cv)
		cv = cg;
	if(cb > cv)
		cv = cb;
	v = (cv>>4)&3;
	return ((((r<<2)+v)<<4)+(((g<<2)+b+v-r)&15));
}
*/

int
rgb2cmap(int cr, int cg, int cb)
{
	int i, r, g, b, sq;
	ulong rgb;
	int best, bestsq;

	best = 0;
	bestsq = 0x7FFFFFFF;
	for(i=0; i<256; i++){
		rgb = cmap2rgb(i);
		r = (rgb>>16) & 0xFF;
		g = (rgb>>8) & 0xFF;
		b = (rgb>>0) & 0xFF;
		sq = (r-cr)*(r-cr)+(g-cg)*(g-cg)+(b-cb)*(b-cb);
		if(sq < bestsq){
			bestsq = sq;
			best = i;
		}
	}
	return best;
}

int
cmap2rgb(int c)
{
	int j, num, den, r, g, b, v, rgb;

	r = c>>6;
	v = (c>>4)&3;
	j = (c-v+r)&15;
	g = j>>2;
	b = j&3;
	den=r;
	if(g>den)
		den=g;
	if(b>den)
		den=b;
	if(den==0) {
		v *= 17;
		rgb = (v<<16)|(v<<8)|v;
	}
	else{
		num=17*(4*den+v);
		rgb = ((r*num/den)<<16)|((g*num/den)<<8)|(b*num/den);
	}
	return rgb;
}

int
cmap2rgba(int c)
{
	return (cmap2rgb(c)<<8)|0xFF;
}