Volumes: non-convex boundaries and variable density

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Look into techniques for rendering volumes with a non-convex boundary, and for variable density.

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Ideas:

• Look into ray-marching
• For variable density, maybe enough to just sample the density from a solid texture when choosing whether to scatter or not?

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For non-convex boundaries: rather than checking for entry and exit boundary hits,

1. check for as many hits as there are
2. build intervals of t that are inside the object
3. select a random interval and a random point in that interval