Memo for myself: how to convert from/to RGB and HSV. It is explained on this Wikipedia page, and can be implemented in C as follow (given equalf, modulof are respectively the equality and modulo operators for floating point values):

// Convert a color from RGB to HSV. // Input: // rgb: RGB values in [0.0, 1.0] // Output: // hsv: result HSV values in [0.0, 1.0] void rgb2hsv(float const* const rgb, float* const hsv) { float max = rgb[2]; loop (i, 2) if (rgb[i] > max) max = rgb[i]; float min = rgb[2]; loop (i, 2) if (rgb[i] < min) min = rgb[i]; float range = max - min; if (equalf(range, 0.0) == true) hsv[0] = 0.0; else if (equalf(max, rgb[0])) hsv[0] = modulof(((rgb[1] - rgb[2]) / range), 6.0); else if (equalf(max, rgb[1])) hsv[0] = (rgb[2] - rgb[0]) / range + 2.0; else if (equalf(max, rgb[2])) hsv[0] = (rgb[0] - rgb[1]) / range + 4.0; hsv[0] /= 6.0; if (hsv[0] < 0.0) hsv[0] = 1.0 + hsv[0]; hsv[2] = max; if (equalf(hsv[2], 0.0) == true) hsv[1] = 0.0; else hsv[1] = range / hsv[2]; }

and:

// Convert a color from HSV to RGB // Input: // hsv: HSV values in [0.0, 1.0] // Output: // rgb: result RGB values in [0.0, 1.0] void rgb2hsv(float const* const hsv, float* const rgb) { float c = hsv[1] * hsv[2]; float x = c * (1.0 - fabsf(modulof(hsv[0] * 6.0, 2.0) - 1.0)); float m = hsv[2] - c; if (hsv[0] < 1.0 / 6.0) { rgb[0] = c; rgb[1] = x; rgb[2] = 0; } else if (1.0 / 6.0 <= hsv[0] && hsv[0] < 2.0 / 6.0) { rgb[0] = x; rgb[1] = c; rgb[2] = 0; } else if (2.0 / 6.0 <= hsv[0] && hsv[0] < 3.0 / 6.0) { rgb[0] = 0; rgb[1] = c; rgb[2] = x; } else if (3.0 / 6.0 <= hsv[0] && hsv[0] < 4.0 / 6.0) { rgb[0] = 0; rgb[1] = x; rgb[2] = c; } else if (4.0 / 6.0 <= hsv[0] && hsv[0] < 5.0 / 6.0) { rgb[0] = x; rgb[1] = 0; rgb[2] = c; } else if (5.0 / 6.0 <= hsv[0]) { rgb[0] = c; rgb[1] = 0; rgb[2] = x; } loop (i, 3) rgb[i] += m; }

If you want to know more about other color spaces, read this article.