Volume Intersect Problem

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DionXein
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Re: Volume Intersect Problem

Post by DionXein » Tue Dec 01, 2020 6:57 pm

Dade wrote:
Tue Dec 01, 2020 1:47 pm
It is working fine here:
and it wasn't before (the posted scene doesn't include smoke data so I'm using an homogeneous volume both cases).

What kind of result do you get ?
I'm still on last automatic build (4 days ago)
And problem still there, see pics - with heterogeneous and homogeneous effect are the same.

Try to bake a few frames in smoke domain data (even 5 would be enough) to get some defined density for heterogeneous volume.
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Hetero.jpg
Homo.jpg

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Dade
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Re: Volume Intersect Problem

Post by Dade » Fri Dec 04, 2020 9:35 am

It is still working fine but your problem (under Windows, not Linux like me) may have been a result of this regression introduced with the fix: https://forums.luxcorerender.org/viewto ... f=4&t=2859
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B.Y.O.B.
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Re: Volume Intersect Problem

Post by B.Y.O.B. » Fri Dec 04, 2020 1:44 pm

@Dade, I have simplified the scene a bit, can you do a test with this?
https://github.com/LuxCoreRender/BlendL ... -738789935

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Dade
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Re: Volume Intersect Problem

Post by Dade » Sun Dec 06, 2020 2:56 pm

B.Y.O.B. wrote:
Fri Dec 04, 2020 1:44 pm
@Dade, I have simplified the scene a bit, can you do a test with this?
https://github.com/LuxCoreRender/BlendL ... -738789935
This is something I have already talked in the past: mixing a transmitting material with transparency is a can of worm you don't wont to open.

Transparency makes the object (and the associate volume) literally to not exist. LuxCore can maintain consistency while tracing a single ray: if the ray intersect a sphere, the sphere is solid or transparent both for the entry point and the exit.

However it is impossible to maintain consistency over a chain of rays (i.e. a path). In your case, the path hit the sphere surface and a shadow ray is traced. The material can transmit so the shadow ray can be traced trough the back of the sphere surface. However the shadow ray has a new random value to decide if the sphere is solid or not.

The sphere can become suddenly transparent for the shadow ray because of the new random value so it lighted by the light source. This causes also a huge mess with volumes because you entered in a volume but you never exited

Short version: I mark this as "wontfix" (more correctly it is a "cantfix").

You can probably show a big warning in BlendLuxCore for all transmitting materials if the transparency socket is used. However can be hard to say for recursive materials like Mix or GlossyCoatting.

Transmitting materials are:

- archglass
- glass
- glossytranslucent
- mattetranslucent
- null
- roughglass
- roughmattetranslucent
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B.Y.O.B.
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Re: Volume Intersect Problem

Post by B.Y.O.B. » Sun Dec 06, 2020 3:26 pm

It sounds like the best course of action would be to remove the opacity/transparency socket completely from these materials.

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Sharlybg
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Re: Volume Intersect Problem

Post by Sharlybg » Sun Dec 06, 2020 9:37 pm

B.Y.O.B. wrote:
Sun Dec 06, 2020 3:26 pm
It sounds like the best course of action would be to remove the opacity/transparency socket completely from these materials.
It make sense from my pov. Don't remenber any use of opacity for them in my daily work.
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DionXein
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Re: Volume Intersect Problem

Post by DionXein » Mon Dec 07, 2020 5:57 pm

Sharlybg wrote:
Sun Dec 06, 2020 9:37 pm
It make sense from my pov. Don't remenber any use of opacity for them in my daily work.
I grab this marble material from example scene, where dish behind is a marble, and tried to combine it in complex scene with smoke around it. It's not a daily work, yes, but it's an artistic way to doin things. I just want to test abilities of LuxCore and make new interesting example scene with SSS and smoke around it.
Dade wrote:
Sun Dec 06, 2020 2:56 pm

Short version: I mark this as "wontfix" (more correctly it is a "cantfix").
Sad to hear it, but I know it's quite fair, because volumetrics are complex thing. Maybe the right way is to make different render passes and combine them later.

And I still not sure how properly must be realised SSS here, because opacity is really strange - how could object be (not)opaque to see it inner volume? Shadows, glossiness, other details are lost due using opacity slider.
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juangea
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Re: Volume Intersect Problem

Post by juangea » Tue Dec 08, 2020 10:35 am

This is something I have already talked in the past: mixing a transmitting material with transparency is a can of worm you don't wont to open.
Mmm this makes me think ni subsurface scattering, since as of today we need transparency to handle subsurface scattering, I wonder if this may help bump the Disney shader sub surface scattering to have it like the cycles one, with random walk and inside the Uber node, since it’s not weird to have something with SSS and smoke / volumes

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Re: Volume Intersect Problem

Post by B.Y.O.B. » Tue Dec 08, 2020 1:25 pm

B.Y.O.B. wrote:
Sun Dec 06, 2020 3:26 pm
It sounds like the best course of action would be to remove the opacity/transparency socket completely from these materials.
I did this, but then it occured to me that it actually might not be a good idea.

The problem can just as well be produced by using a non-transmissive material with lowered opacity, e.g. matte with 80% opacity:
matte_80percent_opacity.jpg
Capture.PNG
It seems to me like it is actually a problem created from combining multiple volumes with partially transparent materials.

Note: transmissive materials with lowered opacity are used in the "Canelle et Fromage" scene on the marble plate in the background, and on the cheese. No artifacts show up in these cases as long as only a single interior volume is used for each material.

I'll have to check if I can detect possibly artifact-prone volume setups during export and show a warning.

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Re: Volume Intersect Problem

Post by B.Y.O.B. » Tue Dec 08, 2020 9:14 pm

The Blender addon now shows the warning "Combining volumes and materials with opacity < 1 can lead to artifacts!" if a material combines volumes and at least one material node with opacity < 1.

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