Plan 9 from Bell Labs’s /usr/web/sources/plan9/sys/src/libdraw/rgb.c

 ``` #include #include #include /* * 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; } ```