hsltorgb.c

Graphics Gems 源码 a collection of algorithms, programs, and mathematical techniques for the computer

/*
A Fast HSL-to-RGB Transform
by Ken Fishkin
from "Graphics Gems", Academic Press, 1990
*/

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

    /*
     * RGB-HSL transforms.
     * Ken Fishkin, Pixar Inc., January 1989.
     */

    /*
    * given r,g,b on [0 ... 1],
    * return (h,s,l) on [0 ... 1]
    */
void
RGB_to_HSL	(r,g,b,h,s,l)
double 	r,g,b;
double *h, *s, *l;
{
    double v;
    double m;
    double vm;
    double r2, g2, b2;

    v = MAX(r,g);
    v = MAX(v,b);
    m = MIN(r,g);
    m = MIN(m,b);

    if ((*l = (m + v) / 2.0) <= 0.0) return;
    if ((*s = vm = v - m) > 0.0) {
		*s /= (*l <= 0.5) ? (v + m ) :
			(2.0 - v - m) ;
    } else
	return;


    r2 = (v - r) / vm;
    g2 = (v - g) / vm;
    b2 = (v - b) / vm;

    if (r == v)
		*h = (g == m ? 5.0 + b2 : 1.0 - g2);
    else if (g == v)
		*h = (b == m ? 1.0 + r2 : 3.0 - b2);
    else
		*h = (r == m ? 3.0 + g2 : 5.0 - r2);

    	*h /= 6;
	}

    /*
     * given h,s,l on [0..1],
     * return r,g,b on [0..1]
     */
void
HSL_to_RGB(h,sl,l,r,g,b)
double 	h,sl,l;
double 	*r, *g, *b;
{
    double v;

    v = (l <= 0.5) ? (l * (1.0 + sl)) : (l + sl - l * sl);
    if (v <= 0) {
		*r = *g = *b = 0.0;
    } else {
		double m;
		double sv;
		int sextant;
		double fract, vsf, mid1, mid2;

		m = l + l - v;
		sv = (v - m ) / v;
		h *= 6.0;
		sextant = h;	
		fract = h - sextant;
		vsf = v * sv * fract;
		mid1 = m + vsf;
		mid2 = v - vsf;
		switch (sextant) {
			case 0: *r = v; *g = mid1; *b = m; break;
			case 1: *r = mid2; *g = v; *b = m; break;
			case 2: *r = m; *g = v; *b = mid1; break;
			case 3: *r = m; *g = mid2; *b = v; break;
			case 4: *r = mid1; *g = m; *b = v; break;
			case 5: *r = v; *g = m; *b = mid2; break;
		}
    }
}