On IRC someone suggested using rgb.txt in the reverse direction: when you have a color and want to know what it is called. The short C program below illustrates the idea.
As some experimentation will show, it does not work too well. I suspect this is due to two factors: that rgb.txt isn't the most suitable sample of names, and that the program measures distances in RGB-space. (the better alternative would be to convert to L*u*v*-space, using the xlib color management functions, and compare distances there). It did find the name turquoise, though, which was the original question. :)
/* colorname.c - find the closest color in rgb.txt, in the sense of
smallest sum of squares of differences of components.
(that is, shortest distance in the RGB cube). */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define RGBFILE "/usr/X11R6/lib/X11/rgb.txt"
int main (int argc, char *argv[]) {
int R, G, B, r, g, b;
FILE *f;
unsigned dist, bestdist = -1;
char name[42], bestname[42] = "none";
char buf[100];
if (argc!=4) {
fprintf(stderr, "Usage: %s <R> <G> <B>\n", argv[0]);
exit(1);
}
R = atoi(argv[1]); G = atoi(argv[2]); B = atoi(argv[3]);
if (!(f = fopen(RGBFILE, "r"))) {
perror(RGBFILE);
exit(1);
}
while (fgets(buf, sizeof(buf), f)) {
if (sscanf(buf, "%d %d %d %42[^\n]\n", &r, &g, &b, name) != 4)
continue;
dist = (R-r)*(R-r) + (G-g)*(G-g) + (B-b)*(B-b);
if (dist < bestdist) {
bestdist = dist;
strcpy(bestname, name);
}
}
printf("Best match: %s (distance %.1f)\n", bestname, sqrt(bestdist));
return 0;
}
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