Some BiDir + SDS-cache issues

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Some BiDir + SDS-cache issues

Post by epilectrolytics » Wed Sep 15, 2021 1:49 pm

I'm testing BiDir with SDS-cache on a scene derived from Charlie's Cold Fusion.

6000 samples rendered without denoising, no clamping.
All lights, volumes and materials color neutral.
Windows 10 PC, Blender 2.93 with LuxCore 2.6alpha.

Several issues came up.
1. color tint
In the first image the inside of the prism has a reddish tint except for the right face which appears blueish.
This should not be the case when rendered without clamping (the tint is even stronger with PathCPU, not shown).
The second image has the cache disabled, now the volumetric SDS paths inside the prism are not rendered, but there seems to be no tint (also not in the reflections), so I assume the tint is introduced by the cache.

2. missing reflections
In the first image the volumetric beams are not reflected in the mirrors and the ground.
In the second image I changed all mirrors to metal material with roughness 0.005 and also disabled the caustic cache.
Suddenly all SDS-reflections are rendered by BiDir Metro even from non-intersectable laser lights!
Only SDS-refractions in the prism (delta glass) don't show. Therefore I make a third test (3.image) with rough glass and now all SDS-paths are rendered without cache, but of course dispersion is now missing resulting in a greyscale image.

I think BiDir+SDS-cache should show all reflections on delta mirrors (in the first image).

3. the second image is much darker (no idea why).

4. The prismatic beams only show the three RGB colors without any intermediate tones like yellow, orange or purple.
This is not a cache problem but a general LuxCore dispersion problem.
Sometimes I find some yellow in dispersion caustics, sometimes not (like here).

Closing remarks:
I'm amazed how well glossy materials help with the SDS problem, if there was rough glass with dispersion available in LuxCore this scene would render well without caustic cache.
An alternative would be the implementation of a glossy filter (simulating glossy reflections internally) as a lightweight SDS solution, probably it could also make SDS cache more efficient when combined.

Rendering dispersion with LuxCore is always tedious because clamping introduces color tint and thus cannot be used.
Now the SDS-cache introduces color shifts even without clamping which is very unfortunate because no workaround exists.

Two blendfiles from the first and second image:
(225.35 KiB) Downloaded 13 times

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