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Comment by uryga | Hacker News Reader

> your render will never finish with physically correct subsurface scattering

did you mean "will take a very long time", or "won't actually terminate"? if the latter, why? that sounds really interesting!

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Path tracing in general do not technically terminate since it's trying to approach an integral (the rendering equation) with sums. It's up to the user to determine the sample per pixel threshold with which he is satisfied. The goal is often to not have too much noise for the time spent. It's not easy because noise reduction over time tends to be logarithmic.

This. Doing “physical” SSS means shooting rays stochastically into the medium (skin, milk, marble) and having them bounce around inside, emerging at a random location, but more likely close to where it entered.

That type of tracing will simply not produce a reasonable result in a reasonable time. You have to cheat.

That's not completely true - at least in the VFX industry we've been doing brute-force SSS (with multiple scattering) for more than two years now in some cases. Yeah, it can be slow depending on the MFP, but it's usable...

Cheats like Dipole/Dwivedi are still used when they can be, but they often have artefacts and aren't completely correct, so they're starting to be avoided now.

Interesting, any particular renderers that do this?

Dwivedi is brute force. It’s just a sampling strategy.

True, but it's biased, which still causes trouble with some edge cases, depending on how/where you bias towards - biasing towards point-of-entry isn't always better than not biasing depending on where the lights are, and it's cumbersome to set up for artists (depending on where you bias towards).

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