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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?

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up vote 3 down vote accepted

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.

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Yes, I came across this paper. It looks interesting, but I was actually interested in the Monte Carlo approach (the actual raycasting physics, without the dipole precompute). "Volumetric raytracing" is the answer I was looking for, it seems! – bobobobo Jul 23 '12 at 20:08
@bobobobo Ahh, I see. "Single scattering" and "multiple scattering" are also good keywords to search for. If you don't have it already, PBR is also a great reference on volumetric raytracing as well as all other aspects of raytracing. – Nathan Reed Jul 23 '12 at 20:15

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