以下基于CompuPhase - Color metric的色差“低成本近似” ，但避免了平方根函数（从而测量色差的平方，这对于比较来说很好），并缩放结果避免整数除法造成的精度损失。它将一组颜色元组按从“最红”到“最红”的顺序排序。
比较使中灰色比黑色更红，黑色比白色更红，这似乎相当随意！
#include <stdio.h>
#include <stdlib.h>
/*
* Sorts an array of RGB color values (each component from 0 to 255)
* in descending order of redness.
*
* Uses a low-cost approximation of color distance from
* <https://www.compuphase.com/cmetric.htm>. It uses a weighted Euclidean
* distance function to measure the distance between colors, where the weight
* factors depend on the mean level of "red":
*
* rmean = (c1.r + c2.r) / 2
* dr = c1.r - c2.r
* dg = c1.g - c2.g
* db = c1.b - c2.b
*
* dc = sqrt(((2 + (rmean / 256)) * dr * dr) + (4 * dg * dg) +
* ((2 + ((255 - rmean) / 256)) * db * db))
*
* Uses a modified version of the above which returns the square of the
* distance between two colors scaled by a silly amount to avoid loss of
* precision caused by integer division.
*/
struct rgb {
unsigned char r; /* red range 0..255 */
unsigned char g; /* green range 0..255 */
unsigned char b; /* blue range 0..255 */
};
/* distance squared between two colors, scaled by some silly amount. */
long color_dist_squared(const struct rgb *c1, const struct rgb *c2)
{
long rsum = c1->r + c2->r;
long dr = (long)c1->r - (long)c2->r;
long dg = (long)c1->g - (long)c2->g;
long db = (long)c1->b - (long)c2->b;
return (((1024 + rsum) * dr * dr) + (2048 * dg * dg) +
((1534 - rsum) * db * db));
}
/* distance squared from pure red, scaled by some silly amount. */
long antiredness_squared(const struct rgb *c)
{
const struct rgb pure_red = { .r = 255, .g = 0, .b = 0 };
return color_dist_squared(&pure_red, c);
}
/*
* qsort() comparison function.
* a and b point to struct rgb values.
* Returns 1 if *a is more anti-red (less red) than *b.
* Returns 0 if *a and *b are equally (anti-)red.
* Returns -1 if *a is less anti-red (more red) than *b.
*/
int compar_antiredness(const void *a, const void *b)
{
const struct rgb *ca = (const struct rgb *)a;
const struct rgb *cb = (const struct rgb *)b;
long ara = antiredness_squared(ca);
long arb = antiredness_squared(cb);
long diff = ara - arb;
return (diff > 0) - (diff < 0);
}
int main(void)
{
struct rgb colors[] = {
{ .r = 125, .g = 0, .b = 0 },
{ .r = 150, .g = 255, .b = 0 },
{ .r = 100, .g = 20, .b = 21 },
{ .r = 255, .g = 21, .b = 22 },
{ .r = 0, .g = 0, .b = 0 },
{ .r = 255, .g = 255, .b = 255 },
{ .r = 128, .g = 128, .b = 128 },
{ .r = 255, .g = 0, .b = 0 },
};
size_t num_colors = sizeof(colors) / sizeof(colors[0]);
size_t i;
printf("Unsorted colors:\n");
for (i = 0; i < num_colors; i++) {
printf("[%zu] R=%u, G=%u, B=%u\n", i, (unsigned)colors[i].r,
(unsigned)colors[i].g, (unsigned)colors[i].b);
}
printf("\n");
qsort(colors, num_colors, sizeof(colors[0]), compar_antiredness);
printf("Colors sorted from most red to least red:\n");
for (i = 0; i < num_colors; i++) {
printf("[%zu] R=%u, G=%u, B=%u\n", i, (unsigned)colors[i].r,
(unsigned)colors[i].g, (unsigned)colors[i].b);
}
return 0;
}
上述输出：
Unsorted colors:
[0] R=125, G=0, B=0
[1] R=150, G=255, B=0
[2] R=100, G=20, B=21
[3] R=255, G=21, B=22
[4] R=0, G=0, B=0
[5] R=255, G=255, B=255
[6] R=128, G=128, B=128
[7] R=255, G=0, B=0
Colors sorted from most red to least red:
[0] R=255, G=0, B=0
[1] R=255, G=21, B=22
[2] R=125, G=0, B=0
[3] R=100, G=20, B=21
[4] R=128, G=128, B=128
[5] R=0, G=0, B=0
[6] R=150, G=255, B=0
[7] R=255, G=255, B=255
编辑：当然，使用以下函数从最绿色到最不绿色或从最蓝色到最不蓝色排序同样容易：
/* distance squared from pure green, scaled by some silly amount. */
long antigreenness_squared(const struct rgb *c)
{
const struct rgb pure_green = { .r = 0, .g = 255, .b = 0 };
return color_dist_squared(&pure_green, c);
}
/*
* qsort() comparison function.
* a and b point to struct rgb values.
* Returns 1 if *a is more anti-green (less green) than *b.
* Returns 0 if *a and *b are equally (anti-)green.
* Returns -1 if *a is less anti-green (more green) than *b.
*/
int compar_antigreenness(const void *a, const void *b)
{
const struct rgb *ca = (const struct rgb *)a;
const struct rgb *cb = (const struct rgb *)b;
long aga = antigreenness_squared(ca);
long agb = antigreenness_squared(cb);
long diff = aga - agb;
return (diff > 0) - (diff < 0);
}
/* distance squared from pure blue, scaled by some silly amount. */
long antiblueness_squared(const struct rgb *c)
{
const struct rgb pure_blue = { .r = 0, .g = 0, .b = 255 };
return color_dist_squared(&pure_blue, c);
}
/*
* qsort() comparison function.
* a and b point to struct rgb values.
* Returns 1 if *a is more anti-blue (less blue) than *b.
* Returns 0 if *a and *b are equally (anti-)blue.
* Returns -1 if *a is less anti-blue (more blue) than *b.
*/
int compar_antiblueness(const void *a, const void *b)
{
const struct rgb *ca = (const struct rgb *)a;
const struct rgb *cb = (const struct rgb *)b;
long aba = antiblueness_squared(ca);
long abb = antiblueness_squared(cb);
long diff = aba - abb;
return (diff > 0) - (diff < 0);
}