/**
 * Converts an RGB color to HSL (hue, saturation, lightness)
 * HSL <-> RGB conversion: {@link http://www.chilliant.com/rgb2hsv.html}
 *
 * @name czm_RGBToHSL
 * @glslFunction
 * 
 * @param {vec3} rgb The color in RGB.
 *
 * @returns {vec3} The color in HSL.
 *
 * @example
 * vec3 hsl = czm_RGBToHSL(rgb);
 * hsl.z *= 0.1;
 * rgb = czm_HSLToRGB(hsl);
 */
 
vec3 RGBtoHCV(vec3 rgb)
{
    // Based on work by Sam Hocevar and Emil Persson
    vec4 p = (rgb.g < rgb.b) ? vec4(rgb.bg, -1.0, 2.0 / 3.0) : vec4(rgb.gb, 0.0, -1.0 / 3.0);
    vec4 q = (rgb.r < p.x) ? vec4(p.xyw, rgb.r) : vec4(rgb.r, p.yzx);
    float c = q.x - min(q.w, q.y);
    float h = abs((q.w - q.y) / (6.0 * c + czm_epsilon7) + q.z);
    return vec3(h, c, q.x);
}

vec3 czm_RGBToHSL(vec3 rgb)
{
    vec3 hcv = RGBtoHCV(rgb);
    float l = hcv.z - hcv.y * 0.5;
    float s = hcv.y / (1.0 - abs(l * 2.0 - 1.0) + czm_epsilon7);
    return vec3(hcv.x, s, l);
}