/**
 * Converts an RGB color to CIE Yxy.
 * <p>The conversion is described in
 * {@link http://content.gpwiki.org/index.php/D3DBook:High-Dynamic_Range_Rendering#Luminance_Transform|Luminance Transform}
 * </p>
 * 
 * @name czm_RGBToXYZ
 * @glslFunction
 * 
 * @param {vec3} rgb The color in RGB.
 *
 * @returns {vec3} The color in CIE Yxy.
 *
 * @example
 * vec3 xyz = czm_RGBToXYZ(rgb);
 * xyz.x = max(xyz.x - luminanceThreshold, 0.0);
 * rgb = czm_XYZToRGB(xyz);
 */
vec3 czm_RGBToXYZ(vec3 rgb)
{
    const mat3 RGB2XYZ = mat3(0.4124, 0.2126, 0.0193,
                              0.3576, 0.7152, 0.1192,
                              0.1805, 0.0722, 0.9505);
    vec3 xyz = RGB2XYZ * rgb;
    vec3 Yxy;
    Yxy.r = xyz.g;
    float temp = dot(vec3(1.0), xyz);
    Yxy.gb = xyz.rg / temp;
    return Yxy;
}