What is an easy way to work subsurface scattering into a raycaster?

Question

I've got some neat effects with path tracing:

I want to add in an ability to do subsurface scattering, but I'm unsure of the general algorithm. With path tracing, it's:

foreach pixel:
    trace( ray into scene )
        color = 0
        if( ray hits an object )
            if( that object is emissive )
                color += object.emissive_color
            if( that object is translucent )
                color += trace( object.refracted ray into scene )
            if( that object is specular )
                color += trace( object.specularly-reflected ray into scene )
            if( that object is diffuse )
                color += trace( object.random-direction reflection ray into scene )

        return color

That's the rough outline. Where does subsurface scattering go and how can I integrate it?

Answer 1

A common approach to subsurface scattering in offline rendering is the so-called "dipole approximation", which is described in the paper A Practical Model for Subsurface Light Transport by Henrik Wann Jensen and others. Unfortunately, I can't do much more than point you at the paper, as I haven't studied it enough to understand it in detail. Googling for "dipole subsurface scattering" will also find you a few more links.

Jensen also has a page with some more images and links to other papers on the subject here.

Finally, it's also possible to simulate subsurface scattering with a brute force Monte Carlo approach, using either path tracing or photon mapping, as long as your renderer supports volumetrics. You pretty much just make a volume that has an extremely high scattering coefficient (orders of magnitude more than you'd use for smoke/haze/etc.) This is very slow though, even by Monte Carlo standards.