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Fix #114515: Cycles: Numerical precision issues in triangle light sampling #119224

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Lukas Stockner wants to merge 1 commits from LukasStockner/blender:triangle-sampling-precision into main
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Lukas Stockner commented 2024-03-08 21:02:50 +01:00
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The refactor in 97d9bbbc97 changed the way q is computed in the spherical triangle sampling code. While the new approach is more efficient and saves a few operations, it introduces numerical precision issues for skinny/small (spherical) triangles.

Therefore, this change moves the computation of q back to the method from the paper, while keeping the more efficient solid angle computation.

Maybe there's a more elegant way to do this, @weizhen probably knows better. The main problem from what I can see are the cosine terms, since for small triangles cos_a/b/c will all be 0.9999..., which causes precision loss due to cancellation in many operations. To be honest, I'm surprised that this change is enough to make it work, I'd have expected all the other sin_from_cos etc. to also be problematic.

For example, here's a modified version of same glitch.blend from #114515:

Method main PR
Render main.png pr.png
Time 4m46s 4m55s
The refactor in 97d9bbbc97f8 changed the way `q` is computed in the spherical triangle sampling code. While the new approach is more efficient and saves a few operations, it introduces numerical precision issues for skinny/small (spherical) triangles. Therefore, this change moves the computation of `q` back to the method from the paper, while keeping the more efficient solid angle computation. Maybe there's a more elegant way to do this, @weizhen probably knows better. The main problem from what I can see are the cosine terms, since for small triangles `cos_a/b/c` will all be `0.9999...`, which causes precision loss due to cancellation in many operations. To be honest, I'm surprised that this change is enough to make it work, I'd have expected all the other `sin_from_cos` etc. to also be problematic. For example, here's a modified version of `same glitch.blend` from #114515: | Method | `main` | PR | |-|-|-| | Render | ![main.png](/attachments/1e3f814c-36b0-4546-b640-0d4a7e3238e8) | ![pr.png](/attachments/5ca1e2e1-0fd0-429f-83c1-8bfa0f24dbfe) | | Time | 4m46s | 4m55s |
pr.png
127 KiB
main.png
157 KiB
Brecht Van Lommel was assigned by Lukas Stockner 2024-03-08 21:02:50 +01:00
Weizhen Huang was assigned by Lukas Stockner 2024-03-08 21:02:50 +01:00
Lukas Stockner added 1 commit 2024-03-08 21:03:01 +01:00
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f5f248649b Fix #114515: Cycles: Numerical precision issues in triangle light sampling 
The refactor in 97d9bbbc97 changed the way q is computed in the spherical
triangle sampling code. While the new approach is more efficient and saves
a few operations, it introduces numerical precision issues for skinny/small
(spherical) triangles.

Therefore, this change moves the computation of q back to the method from
the paper, while keeping the more efficient solid angle computation.
Lukas Stockner added this to the Render & Cycles project 2024-03-08 21:03:06 +01:00
Brecht Van Lommel requested review from Brecht Van Lommel 2024-03-08 22:47:38 +01:00
Brecht Van Lommel requested review from Weizhen Huang 2024-03-08 22:47:38 +01:00
Brecht Van Lommel removed their assignment 2024-03-08 22:47:46 +01:00
Weizhen Huang was unassigned by Brecht Van Lommel 2024-03-08 22:47:46 +01:00
Weizhen Huang approved these changes 2024-03-08 23:28:50 +01:00
Weizhen Huang left a comment
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You should remove the computation of sin_b_sin_c_2 now that it's unused. Otherwise looks good.

Good that you figured out. The computation of sin_b_sin_c_2 and q had numerical issues. The other sin_from_coss are not called on small values so that seems fine.

You should remove the computation of `sin_b_sin_c_2` now that it's unused. Otherwise looks good. Good that you figured out. The computation of `sin_b_sin_c_2` and `q` had numerical issues. The other `sin_from_cos`s are not called on small values so that seems fine.
Brecht Van Lommel commented 2024-03-11 13:40:43 +01:00
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Can a test be added for this?

It could also go into 4.1.

@blender-bot build

Can a test be added for this? It could also go into 4.1. @blender-bot build
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