Yep! Rendering is very parallelizable, thank goodness. And at more than one level: You can assign different regions of a single frame to different processors, and you can also assign different frames to different processors. It's one of those special computing problems that really can be solved by throwing more silicon at it. Which is a real blessing, considering how unworkably slow it would be otherwise.
I understand how you would break up an individual frame if you are using the 'local illumination' described in the ancestor post, but if the 'global illumination' has interactions across the entire frame, how is that compatible with parallel processing?
It sounds a bit like the n-body problem, which has parallel approximation algorithms, but nothing terribly straightforward.
In global illumination there are lighting interactions between objects in the scene, but not between pixels. Each pixel is independent of the others, and so can (in theory) be processed in parallel.
In another way of thinking about it, raytracing simulates photons. Photons don't interact with each other, so the problem of simulating photons is massively parallel.
I'm not an expert on how the GI algorithms are parallelized. First, it's worth noting there are a lot of them, so the strategy probably varies. But here's a guess for a popular one known as photon mapping. With that technique, you bounce around virtual photons, and they contribute to the lighting of each point they hit (to simplify a bit). AFAIK, each photon's path is only a function of the light source, the scene geometry, and the scene materials. I.e. it's not a function of what other photons are doing. So I think you can parallelize individual photons bouncing around. As a final step, you have to fold all the light contributions together, which I believe could in turn be parallelized per polygon.
Generally you're using some kind of sampling algorithm to randomly sample from the light distribution in the scene, so it's easy to calculate N different images of the same scene, and then average them together. If it's not the whole image that's sampled at once, then you can still parallelize the sampling step.
I believe the photons are simulated backwards; they emanate from the camera.
So you don't have to simulate the same photons lots of times for different parts of the frame. You just simulate the photons that will eventually end up in a certain part of the image.
