float DuerGeometricShadowingFunction (float3 lightDirection,float3 viewDirection, 
float3 normalDirection,float NdotL, float NdotV){
    float3 LpV = lightDirection + viewDirection;
    float Gs = dot(LpV,LpV) * pow(dot(LpV,normalDirection),-4);
    return  (Gs);
}