/**
 * Computes the luminance of a color. 
 *
 * @name czm_luminance
 * @glslFunction
 *
 * @param {vec3} rgb The color.
 * 
 * @returns {float} The luminance.
 *
 * @example
 * float light = czm_luminance(vec3(0.0)); // 0.0
 * float dark = czm_luminance(vec3(1.0));  // ~1.0 
 */
float czm_luminance(vec3 rgb)
{
    // Algorithm from Chapter 10 of Graphics Shaders.
    const vec3 W = vec3(0.2125, 0.7154, 0.0721);
    return dot(rgb, W);
}