float MixFunction(float i, float j, float x)
{
	 return  j * x + i * (1.0 - x);
}

float SchlickFresnel(float i)
{
    float x = clamp(1.0-i, 0.0, 1.0);
    float x2 = x*x;
    return x2*x2*x;
}

//normal incidence reflection calculation
float F0 (float NdotL, float NdotV, float LdotH, float roughness)
{
    float FresnelLight = SchlickFresnel(NdotL);
    float FresnelView = SchlickFresnel(NdotV);
    float FresnelDiffuse90 = 0.5 + 2.0 * LdotH*LdotH * roughness;
    return  MixFunction(1, FresnelDiffuse90, FresnelLight) * MixFunction(1, FresnelDiffuse90, FresnelView);
}