// todo: make this abstract?
public virtual Spectrum Sample_Wi(
Interaction it,
Point2D u,
out Vector3D wi,
out double df,
out Point2D pRaster,
out VisibilityTester vis)
{
throw new NotImplementedException();
}
public abstract Spectrum Sample_Li(Interaction it, Point2D u, out Vector3D wi, out double pdf, out VisibilityTester vis);
