@TECHREPORT\{IMM2013-06646,
    author       = "J. R. Frisvad and T. Hachisuka and T. K. Kjeldsen",
    title        = "Directional Dipole for Subsurface Scattering in Translucent Materials",
    year         = "2013",
    month        = "aug",
    keywords     = "{BSSRDF,} radiative transfer, subsurface scattering, translucent materials",
    number       = "",
    series       = "",
    institution  = "",
    address      = "",
    type         = "",
    note         = "Manuscript",
    url          = "http://www2.compute.dtu.dk/pubdb/pubs/6646-full.html",
    abstract     = "Rendering translucent materials using Monte Carlo ray tracing is a
challenge. For such materials, rendering time often becomes prohibitively
long due to the large number of subsurface scattering
events. A faster approach is to use an analytical model derived from
diffusion theory. However, the efficiency of the analytical models
comes at the cost of losing translucency effects in the rendered result.
We present a new analytical model for subsurface scattering.
Our model captures translucency effects that are present in the reference
images but remain absent with existing analytical models.
The key difference is that our model is based on ray source diffusion,
rather than point source diffusion. A ray source corresponds
better to the light that refracts through the surface of a translucent
material. Using this ray source, we are able to take the direction
of the incident light ray and the direction toward the point of emergence
into account. We use a dipole construction similar to that of
the standard dipole model, but we now have positive and negative
ray sources instead of point sources. Our model is as efficient as
previous dipole models while the rendered images are significantly
closer to the references. Unlike some previous work, our model is
fully analytic and requires no precomputation."
}