cannot see reflection of light spot

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strawman
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cannot see reflection of light spot

Post by strawman » Sun Dec 20, 2020 12:59 pm

Hi, I'm trying to render a light spot where the light spot is created by a narrow-angle area light shining through a mirror. I can see the light spot directly with the camera. However, the light spot is not visible through the mirror which bounces the beam from the area light, nor is it visible through another mirror. I am attaching a test scene to reproduce this and linking to an annotated image showing the problem. The light spot falling on the objects on the ground should be visible through both mirrors, but it is not.

This is with the Bidir Engine, Metropolis Sampler, 20 Max bounces for both eye depth and light depth, Blender 2.83.20 win64 and BlendLuxCore 2.4 win64.

Is there something I can do to fix this? Or is there perhaps a bug?

EDIT: attached the annotated image directly rather than linking to imgur.
no-reflection-annotated.jpg
Attachments
moving-mirror7.blend
Sample scene showing problem.
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Last edited by strawman on Sun Dec 20, 2020 7:46 pm, edited 1 time in total.

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Dade
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Re: cannot see reflection of light spot

Post by Dade » Sun Dec 20, 2020 3:02 pm

strawman wrote:
Sun Dec 20, 2020 12:59 pm
https://imgur.com/a/hSBZmTG
They are called SDS paths (Specular-Diffuse-Specular) and they can be rendered only by using PhotonGI caustic cache. Normal BiDir can not render that kind of light path at all.

It is a feature introduced in v2.2:

Image
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strawman
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Re: cannot see reflection of light spot

Post by strawman » Sun Dec 20, 2020 7:35 pm

Thanks for the quick answer. If I use the PhotonGI cache and normal paths (not bidir) I get a different problem - I do not see any scattering from the volume (which is weakly scattering) immediately after the area source but before the narrow beam bounces off the mirror. I am attaching an image showing the problem. There should be a red beam going horizontally from the mirror to the small sphere.

Is there a way both to get the reflected light spot as well as the scattered beam in one image?
failed-early-light.jpg

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DionXein
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Re: cannot see reflection of light spot

Post by DionXein » Sun Dec 20, 2020 7:44 pm

Dade wrote:
Sun Dec 20, 2020 3:02 pm
They are called SDS paths (Specular-Diffuse-Specular) and they can be rendered only by using PhotonGI caustic cache. Normal BiDir can not render that kind of light path at all.
May I ask: In two words - why it's cannot be rendered even using BiDir?

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Sharlybg
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Re: cannot see reflection of light spot

Post by Sharlybg » Sun Dec 20, 2020 9:14 pm

DionXein wrote:
Sun Dec 20, 2020 7:44 pm
Dade wrote:
Sun Dec 20, 2020 3:02 pm
They are called SDS paths (Specular-Diffuse-Specular) and they can be rendered only by using PhotonGI caustic cache. Normal BiDir can not render that kind of light path at all.
May I ask: In two words - why it's cannot be rendered even using BiDir?
Simple Normal Bidir can't render SDS path No matter the render engine it is an inherent limitation.I know it is quite strange to say that about Bidir because most artists tend to think that Bidir can render anything.You need then a Much advanced Bidir to do that.It is like not having true spectral rendering capabilities.
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wasd
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Re: cannot see reflection of light spot

Post by wasd » Mon Dec 21, 2020 6:56 am

Of course Bidir can render SDS paths. It needs some time for it, though. SDS connections are purely random, so their probability is not very high.
Here you can see how bidir does sds. It took a week on Core i5-2400.
https://forums.luxcorerender.org/viewto ... f=3&t=2681
CPU Bidir + Metropolis | Core i5-4570

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Dade
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Re: cannot see reflection of light spot

Post by Dade » Mon Dec 21, 2020 9:53 am

wasd wrote:
Mon Dec 21, 2020 6:56 am
Of course Bidir can render SDS paths. It needs some time for it, though. SDS connections are purely random, so their probability is not very high.
Here you can see how bidir does sds. It took a week on Core i5-2400.
https://forums.luxcorerender.org/viewto ... f=3&t=2681
No, it can not render SDS path when the light source is not intersectable (point light, spot light, etc.).

It renders SDS paths of intersectable light source (area lights, HDR, etc.) exactly like a plain path tracer: it is useless.
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Dade
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Re: cannot see reflection of light spot

Post by Dade » Mon Dec 21, 2020 9:55 am

strawman wrote:
Sun Dec 20, 2020 7:35 pm
Thanks for the quick answer. If I use the PhotonGI cache and normal paths (not bidir) I get a different problem - I do not see any scattering from the volume (which is weakly scattering) immediately after the area source but before the narrow beam bounces off the mirror.
The scene is probably incorrectly built: you need to set materials outer volume or world volume. Like: https://github.com/LuxCoreRender/LuxCor ... -745247191
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strawman
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Re: cannot see reflection of light spot

Post by strawman » Mon Dec 21, 2020 10:34 pm

Thank you once again. However, despite playing around with your suggestion for several minutes, I still can't get this to work.

I had previously set the world volume to have homogeneous scattering. (That's how the beam was showing in the first and second images.) Now I deleted the the world volume and tried adding an external volume to the material of the mirror ("Mirror-Mat") in the blender file. I tried playing with various options, such as setting multiscattering on and playing with the parameters. Still, I cannot see the beams scattering in the air with the "Path" engine.

Do you have any further suggestion?

strawman
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Re: cannot see reflection of light spot

Post by strawman » Mon Dec 21, 2020 11:19 pm

Also, in case it is useful or relevant - in the github issue you linked https://github.com/LuxCoreRender/LuxCor ... -745247191, the sample code is using the bidir engine, which won't work for me due to my need for SDS (see the first posts in this topic).

I have a similar problem with that example if I switch it to use the path engine - no scattered beam is shown prior to the first reflection.

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