jehanne/sys/src/lib/draw/rgb.c

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/*
* This file is part of the UCB release of Plan 9. It is subject to the license
* terms in the LICENSE file found in the top-level directory of this
* distribution and at http://akaros.cs.berkeley.edu/files/Plan9License. No
* part of the UCB release of Plan 9, including this file, may be copied,
* modified, propagated, or distributed except according to the terms contained
* in the LICENSE file.
*/
#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;
uint32_t 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;
}