a

libdraw/rgb.c

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+#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;
+}