LiAR isn't a raytracer

… are really good fun. And a huge performance killer too. Especially if you enable in-scattering on your final gather rays. Oh boy, that really hurts! Still playing around with the Sponza atrium, I wanted to do a render of sunlight being casted through the arches on the first floor.

Anyway, to cut things short, here’s the final render. It only took a ridiculous long time to compute (12 hours on a quadcore). Click on the image for full resolution. Tonemapping has been done in photoshop. Model & textures courtesy of Marko Dabrovic.

Adding single scattering was easy enough. Just ray march or sample some points along your camera rays and add light source contributions to each of them using a suitable phase function. Don’t forget to add some attenuation – Beer’s law comes to mind – to your camera rays, light rays and photon paths. Basically on anything that travels through your medium. As phase function, I’ve chosen to use the widely used Henyey-Greenstein model. It isn’t particularly the fastest one around, but as I don’t have a good profiler (yet ;), I don’t really know its impact on the render time anyway.

Multiple scattering is somewhat of a different beast, though it’s not too difficult either. In the photon mapping world, it means augmenting the renderer with a volumetric photon map that records all scatter events during the photon trace pass. For each photon that travels through a medium, you sample a possible scattering location. In case of a homogenous medium, this is as simple as feeding a uniform variate in the inverse cumulative of the exponential distribution, which can yield a travel distance from zero to infinity. If it is nearer than the first surface intersection, you store the photon and sample the phase function for a new direction. Some more details can be found in Lafortune and Willems (1996) and Raab, Seibert and Keller (2008).

In the rendering pass, I used the beam radiance estimate from Jarosz, Zwicker and Jensen (2008) to collect the in-scattered light from the volumetric map on the camera and final gather rays. This method represents the volumetric photons as spheres, and all photons intersected by a ray contribute to it. In case of camera rays, I ignore “single scattered” photons, as I account for single scattering seperately.

Because this operation weights very heavy on the final gather step, I stochastically skip the in-scattering estimate for a number of gather rays. For each ray, a uniform variate is compared to a quality factor. Only if it is lower, volumetric photons are collected. The result is divided by the quality factor to compensate. That way I can trade speed for accuracy.

This entry was posted on Tuesday, May 4th, 2010 at 9:07 pm and is filed under bramz' diary. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.