private float brdf(Vector3 i, Vector3 o, OrthoNormalBasis basis)
{
float fr = 4 * (float)Math.PI * alphaX * alphaY;
fr *= (float)Math.Sqrt(basis.untransformZ(i) * basis.untransformZ(o));
Vector3 h = Vector3.add(i, o, new Vector3());
basis.untransform(h);
float hx = h.x / alphaX;
hx *= hx;
float hy = h.y / alphaY;
hy *= hy;
float hn = h.z * h.z;
fr = (float)Math.Exp(-(hx + hy) / hn) / fr;
return fr;
}
