/**
 * @private
 */
float czm_alphaWeight(float a)
{
    float z = (gl_FragCoord.z - czm_viewportTransformation[3][2]) / czm_viewportTransformation[2][2];

    // See Weighted Blended Order-Independent Transparency for examples of different weighting functions:
    // http://jcgt.org/published/0002/02/09/
    return pow(a + 0.01, 4.0) + max(1e-2, min(3.0 * 1e3, 0.003 / (1e-5 + pow(abs(z) / 200.0, 4.0))));
}