float ModifiedKelemenGeometricShadowingFunction (float NdotV, float NdotL,
 float roughness)
{
	float c = 0.797884560802865;    // c = sqrt(2 / Pi)
	float k = roughness * roughness * c;
	float gH = NdotV  * k +(1-k);
	return (gH * gH * NdotL);
}