rgb2hsv.cpp

//
// RGB to HSV convert sample
//
//   see also : http://ja.wikipedia.org/wiki/HSV%E8%89%B2%E7%A9%BA%E9%96%93
//

#include <stdio.h>
#include <math.h>

#define min_f(a, b, c)  (fminf(a, fminf(b, c)))
#define max_f(a, b, c)  (fmaxf(a, fmaxf(b, c)))

void rgb2hsv(const unsigned char &src_r, const unsigned char &src_g, const unsigned char &src_b, unsigned char &dst_h, unsigned char &dst_s, unsigned char &dst_v)
{
    float r = src_r / 255.0f;
    float g = src_g / 255.0f;
    float b = src_b / 255.0f;

    float h, s, v; // h:0-360.0, s:0.0-1.0, v:0.0-1.0

    float max = max_f(r, g, b);
    float min = min_f(r, g, b);

    v = max;

    if (max == 0.0f) {
        s = 0;
        h = 0;
    }
    else if (max - min == 0.0f) {
        s = 0;
        h = 0;
    }
    else {
        s = (max - min) / max;

        if (max == r) {
            h = 60 * ((g - b) / (max - min)) + 0;
        }
        else if (max == g) {
            h = 60 * ((b - r) / (max - min)) + 120;
        }
        else {
            h = 60 * ((r - g) / (max - min)) + 240;
        }
    }

    if (h < 0) h += 360.0f;

    dst_h = (unsigned char)(h / 2);   // dst_h : 0-180
    dst_s = (unsigned char)(s * 255); // dst_s : 0-255
    dst_v = (unsigned char)(v * 255); // dst_v : 0-255
}

void hsv2rgb(const unsigned char &src_h, const unsigned char &src_s, const unsigned char &src_v, unsigned char &dst_r, unsigned char &dst_g, unsigned char &dst_b)
{
    float h = src_h *   2.0f; // 0-360
    float s = src_s / 255.0f; // 0.0-1.0
    float v = src_v / 255.0f; // 0.0-1.0

    float r, g, b; // 0.0-1.0

    int   hi = (int)(h / 60.0f) % 6;
    float f  = (h / 60.0f) - hi;
    float p  = v * (1.0f - s);
    float q  = v * (1.0f - s * f);
    float t  = v * (1.0f - s * (1.0f - f));

    switch(hi) {
        case 0: r = v, g = t, b = p; break;
        case 1: r = q, g = v, b = p; break;
        case 2: r = p, g = v, b = t; break;
        case 3: r = p, g = q, b = v; break;
        case 4: r = t, g = p, b = v; break;
        case 5: r = v, g = p, b = q; break;
    }

    dst_r = (unsigned char)(r * 255); // dst_r : 0-255
    dst_g = (unsigned char)(g * 255); // dst_r : 0-255
    dst_b = (unsigned char)(b * 255); // dst_r : 0-255
}


void test(const unsigned &r, const unsigned &g, const unsigned &b, const unsigned &h, const unsigned &s, const unsigned &v)
{
    unsigned char rv_h, rv_s, rv_v;
    unsigned char rv_r, rv_g, rv_b;

    rgb2hsv(r, g, b, rv_h, rv_s, rv_v);
    hsv2rgb(rv_h, rv_s, rv_v, rv_r, rv_g, rv_b);

    printf("rgb(%d, %d, %d) -> hsv(%d, %d, %d) -> rgb(%d, %d, %d) ", r, g, b, rv_h, rv_s, rv_v, rv_r, rv_g, rv_b);

    printf("\n");
}

int main(int argc, char *argv[])
{
    test(255,   0,   0,   0, 255, 255);
    test(255, 255,   0,  30, 255, 255);
    test(  0, 255,   0,  60, 255, 255);
    test(  0, 255, 255,  90, 255, 255);
    test(  0,   0, 255, 120, 255, 255);
    test(255,   0, 255, 150, 255, 255);
    test(255, 255, 255,   0,   0, 255);
    test(128, 128, 128,   0,   0, 128);
    test(  0,   0,   0,   0,   0,   0);
    test(255, 128,   0,   0, 255, 255);

    return 0;
}