static HWBType * RGB_to_HWB (RGBType RGB, HWBType * HWB)
{
	/*
	 * RGB are each on [0, 1]. W and B are returned on [0, 1] and H is
	 * returned on [0, 6]. Exception: H is returned UNDEFINED if W == 1 - B.
	 */

	float R = RGB.R, G = RGB.G, B = RGB.B, w, v, b, f;
	int i;

	w = MIN3 (R, G, B);
	v = MAX3 (R, G, B);
	b = 1 - v;
	if (v == w) {
		RETURN_HWB(HWB_UNDEFINED, w, b);
	}
	f = (R == w) ? G - B : ((G == w) ? B - R : R - G);
	i = (R == w) ? 3 : ((G == w) ? 5 : 1);

	RETURN_HWB(i - f / (v - w), w, b);
}