float Vis_SmithJointApprox(float Roughness, float NoV, float NoL) {
	float a = Roughness * Roughness;
	float Vis_SmithV = NoL * (NoV * (1.0 - a) + a);
	float Vis_SmithL = NoV * (NoL * (1.0 - a) + a);
	return 0.5 / (Vis_SmithV + Vis_SmithL);
}
Vector2 IntegrateBRDF(Vector2 Random, float Roughness, float NoV){
	Vector3 V;
	V.x = sqrt(1.0 - NoV * NoV);// sin;
	V.y = 0;
	V.z = NoV;// # cos

	float A = 0;
	float B = 0;

	int NumSamples = 128;
	for (int i = 0;i < NumSamples;i++){
		Vector2 E = Hammersley(i, NumSamples, Random);
		Vector3 H = ImportanceSampleGGX(E, Roughness);
		Vector3 L = H * (2 * V.dot(H)) - V;

		float NoL = saturate(L.z);
		float NoH = saturate(H.z);
		float VoH = saturate(V.dot(H));

		if (NoL > 0){ 
			float Vis = Vis_SmithJointApprox(Roughness, NoV, NoL);
			//#Incident light = NoL
			//#pdf = D * NoH / (4 * VoH)
			//#NoL * Vis / pdf
			float NoL_Vis_PDF = NoL * Vis * (4 * VoH / NoH);
			float Fc = pow(1 - VoH, 5);
			A += (1 - Fc) * NoL_Vis_PDF;
			B += Fc * NoL_Vis_PDF;
		}
	}
	return Vector2(A / NumSamples, B / NumSamples);
}