What is the difference between real source and virtual source? They both are technically "virtual" as the true light source should be the one emitting the light down to the surface, right?
@wvshelu According to Prof. Ng, the result of subsurface scattering by a ray from a light source incident upon a SS surface can be heuristically approximated as the result of placing an isotropic light source in the location of the "real" source. However, given that most renderers prefer light source not embedded in the surface, we can make the same result by flipping to the virtual source.
I think the real and virtual labels are separate than the "true" light source you are talking about. They're called real and virtual to differentiate from each other, not the ray.
Real corresponds to the positive part of a dipole, and virtual is the negative part. Modeling it like this gives the nice property of diffusion away from the two sources.