BiDir darkening surface at glancing angles

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Martini
Posts: 8
Joined: Fri Nov 23, 2018 11:36 am

BiDir darkening surface at glancing angles

Post by Martini » Wed Sep 11, 2019 6:01 pm

Hi,

I have been struggling for a few days to identify if I am doing something wrong or if this is a bug. The only thing I have changed between renders is the integrator from Path to BiDir.

What you can see is that at more extreme angles (on the floor near the back of the room, and on the columns where it meets the wall), the BiDir integrator is darkening the surface. Even in direct light. This does not happen with regular path (either OpenCL or CPU).

Edit: The room is made of solid meshes with flat shading applied. The floor is separate from the walls and the walls are separate from the ceiling. The room is "open" to the sky to the above right. I have checked all the normals with both overlay showing direction and face colour mode in Blender 2.80. I am using BlendLuxCore-v2.2beta4-blender2.80-win64-opencl but originally noticed the issue on beta 3.

Things tested with BiDir all exhibit the same problem:
- Metropolis vs. Sobol sampler
- Glossy vs. matte vs. disney shaders
- Sky envionment vs. flat colour environment vs. mesh emitter vs. sun lamp
- Rendering to insane sample count (< 3/256 noise level took 36 hours at 4K scene and yielded 12,336 samples per pixel)
- Using an image texture on surfaces. With bump vs no bump vs solid colour with bump vs no bump
- Hiding and unhiding different meshes (walls, ceiling)

All these things yielded no fix for BiDir, yet could never reproduce it with Path.

Is this a known issue, or am I doing something silly?

Edit 2: Attached sample .blend file in case it's useful
Attachments
Control-2.8b4-matte-bidir-metropolis-1024spp.png
BiDir with Metropolis sampler
Control-2.8b4-matte-path-metropolis-1024spp.png
Path with Metropolis sampler
Control - simple.blend
(1.43 MiB) Not downloaded yet
Last edited by Martini on Thu Sep 12, 2019 9:09 am, edited 3 times in total.

Martini
Posts: 8
Joined: Fri Nov 23, 2018 11:36 am

Re: BiDir darkening surface at glancing angles

Post by Martini » Wed Sep 11, 2019 6:16 pm

Additional test images. Not recommended for mobile users as they are 9.5 MB files and rendered at 2160p.

Shows the issue with also reflective surfaces, bump mapping and textures. Not really all that useful in helping with the issue but it's what I'm working on. Both images rendered with stop condition of < 3/256 noise level 8-)

Problem should be obvious enough, but if not, load them each in separate browser tabs and switch back and forth.
Attachments
Control-bidir-metropolis-12336spp-36hr.png
CPU BiDir with Metropolis. 12,336 spp (36 hr)
Control-path-sobol-13161spp-2hr.png
OpenCL Path with Sobol. 13,161 spp (2 hr)

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Dade
Developer
Developer
Posts: 2933
Joined: Mon Dec 04, 2017 8:36 pm

Re: BiDir darkening surface at glancing angles

Post by Dade » Thu Sep 12, 2019 10:02 am

I should have fixed the problem. The fix will be included in the daily build (when it is ready).

For the record, this was a regression of a Jun 10 commit (https://github.com/LuxCoreRender/LuxCor ... a646e420f1) for fixing this (http://forums.luxcorerender.org/viewtop ... 6&start=10) problem ... I was so sure that fix was going to bite back, it was still fresh in my memory :roll:
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Martini
Posts: 8
Joined: Fri Nov 23, 2018 11:36 am

Re: BiDir darkening surface at glancing angles

Post by Martini » Thu Sep 12, 2019 6:12 pm

Thank you Dade!

We can now rely on BiDir being the "reference" renderer once again :lol:

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