rgbhsv.h

struct r_g_b {
	double r;       // a fraction between 0 and 1
	double g;       // a fraction between 0 and 1
	double b;       // a fraction between 0 and 1
};

struct h_s_v {
	double h;       // angle in degrees
	double s;       // a fraction between 0 and 1
	double v;       // a fraction between 0 and 1
};

static inline struct h_s_v rgb2hsv(const struct r_g_b in)
{
	struct h_s_v out;
	double      min, max, delta;

	min = in.r < in.g ? in.r : in.g;
	min = min  < in.b ? min  : in.b;

	max = in.r > in.g ? in.r : in.g;
	max = max  > in.b ? max  : in.b;

	out.v = max; // v
	delta = max - min;
	if (delta < 0.00001)
	{
		out.s = 0;
		out.h = 0; // undefined, maybe nan?
		return out;
	}
	if ( max > 0.0 ) { // NOTE: if Max is == 0, this divide would cause a crash
		out.s = delta / max; // s
	} else {
		// if max is 0, then r = g = b = 0
		// s = 0, v is undefined
		out.s = 0.0;
		out.h = NAN;                            // its now undefined
		return out;
	}
	if (in.r >= max)                           // > is bogus, just keeps compilor happy
		out.h = ( in.g - in.b ) / delta;   // between yellow & magenta
	else
		if (in.g >= max)
			out.h = 2.0 + ( in.b - in.r ) / delta;  // between cyan & yellow
		else
			out.h = 4.0 + ( in.r - in.g ) / delta;  // between magenta & cyan

	out.h *= 60.0;                              // degrees

	if( out.h < 0.0 )
		out.h += 360.0;

	return out;
}

static inline struct r_g_b hsv2rgb(const struct h_s_v in)
{
	double      hh, p, q, t, ff;
	long        i;
	struct r_g_b out;

	if (in.s <= 0.0) {       // < is bogus, just shuts up warnings
		out.r = in.v;
		out.g = in.v;
		out.b = in.v;
		return out;
	}
	hh = in.h;
	if (hh >= 360.0)
		hh = 0.0;
	hh /= 60.0;
	i = (long)hh;
	ff = hh - i;
	p = in.v * (1.0 - in.s);
	q = in.v * (1.0 - (in.s * ff));
	t = in.v * (1.0 - (in.s * (1.0 - ff)));

	switch (i) {
	case 0:
		out.r = in.v;
		out.g = t;
		out.b = p;
		break;
	case 1:
		out.r = q;
		out.g = in.v;
		out.b = p;
		break;
	case 2:
		out.r = p;
		out.g = in.v;
		out.b = t;
		break;

	case 3:
		out.r = p;
		out.g = q;
		out.b = in.v;
		break;
	case 4:
		out.r = t;
		out.g = p;
		out.b = in.v;
		break;
	case 5:
	default:
		out.r = in.v;
		out.g = p;
		out.b = q;
		break;
	}
	return out;
}