Bidirectional path tracing. You send rays from the sun, rays from the camera, and try to connect them. The ones that connect are the ones that get computed for illumination.
Bidirectional path tracing is one way of sampling paths (actually it combines many techniques for sampling paths and weights those using something called Multiple Importance Sampling). It's not the only way of doing it. Disney most likely uses path tracing with next event estimation. This means that they start a path as explained in the video and they end a path by sampling a point on a light and then connecting the start of the path with the point to form a full path. This is one the techniques used by bidirectional path tracing (bdpt), this technique uses n vertices on the camera path and 1 vertex on the light path, but bdpt also uses techniques with s vertices on the camera path and t vertices on the light path. This means that there are multiple ways to sample the same path, so these techniques need to be weighted using something called Multiple Importance Sampling.
How is that even possible? The beam incidence angular calculation (and multi path dispersion) creates insane complexity that must be computed on the fly to even make sense.
For instance: a beam hits some material and needs to reflect or worse, pass through via transparency. Another issue: If we are calculating on a per pixel basis, that means bundling multiple paths together to figure out what the weighted return will look like. How can this all be computed with any kind of efficiency without cheating?
Bidirectional path tracing doesn't do anything clever to "try to connect" paths from light sources and cameras. The approach is just
- Trace random paths from light sources until they terminate (usually decided with Russian roulette).
- Trace random paths from the camera (usually N per pixel, or you can use more paths in noisy areas) until they terminate.
- Try to connect each point in a camera path with each point in a light path, using a simple line test. If it succeeds, that color is added to the pixel from which the camera path originated.
At least that's my understanding; I've only implemented simpler algorithms and read a bit about bidirectional path tracing.
> If we are calculating on a per pixel basis, that means bundling multiple paths together to figure out what the weighted return will look like. How can this all be computed with any kind of efficiency without cheating?
Right, we still need to consider many paths per pixel to get a high quality image. But it converges faster than most other Monte Carlo techniques.
