Bidirectional strange with glossy material

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Burk
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Bidirectional strange with glossy material

Post by Burk » Mon Jul 02, 2018 5:54 pm

(Windows 10, Blender 2.79b, LuxCoreRender 2.0)

For checking light paths in scientific experiments, I examine a polystyrene glass by directing an area light on it.
My polystyrene material is a mix of 95% glass and 5% glossy.

Many of my setups are working very fine, but I have difficults whith the final render in the following situation:
When the area light passes through the polystyrene, the entrance surface reflects the beam much weaker than the exit surface. That's physically wrong.

See example:
Bidirectional strange with glossy material.blend
(676.11 KiB) Downloaded 66 times
Correct reflections (where both surfaces reflect approximately equally strong) occur with the following setups:
- If I use the matt material instead of glossy material for the mix, the reflection is ok.
- If I put the mix to 1.0 (so it's only glass material), the reflection is ok too.

Is there a reason why the glossy material distort the reflections? Is there a remedy? Or am I doing something wrong?

neo2068
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Re: Bidirectional strange with glossy material

Post by neo2068 » Mon Jul 02, 2018 7:14 pm

IIRC, the glossy material isn't the same for the frontface and the backface, i.e. the backface is matte and the frontface is glossy. So the reflection on the left interface (air->glass) and the reflection on the right interface (glass->air) aren't the same.

Why do you use a mix material for you polystyrene glass?
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Dade
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Re: Bidirectional strange with glossy material

Post by Dade » Mon Jul 02, 2018 8:53 pm

neo2068 wrote:
Mon Jul 02, 2018 7:14 pm
Why do you use a mix material for you polystyrene glass?
Exactly, isn't rough glass (https://wiki.luxcorerender.org/LuxCoreR ... ough_Glass) better ?
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Burk
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Re: Bidirectional strange with glossy material

Post by Burk » Mon Jul 02, 2018 8:55 pm

neo2068 wrote:
Mon Jul 02, 2018 7:14 pm
Why do you use a mix material for you polystyrene glass?
My real polystyrene container specimen is not crystal clear like real glass but a little turbid.
For cycles (which I used in first place) there was the recommandation for plastic glass to add some glossy material.
It worked very good in cycles. The mixed material looks really like the real thing.
I reproduced that material in LuxCoreRender and it looked exactly like the the cycles renderings.

Thanks for your explanation. I've learned that I cannot use glossy for light path analysis.

But I can make a work around for my problem.
- Either I try to build a new material in LuxCoreRender without glossy
- or more simple I use a mixed material with matte material (for a little diffusion to lighten the polystyrene) that will correctly display the light paths in the bidirectional rendering.
For the light path analysis I only need schematic pictures in black and white. Therefore, the exact material is not so important

Burk
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Re: Bidirectional strange with glossy material

Post by Burk » Mon Jul 02, 2018 8:59 pm

Dade wrote:
Mon Jul 02, 2018 8:53 pm
Exactly, isn't rough glass (https://wiki.luxcorerender.org/LuxCoreR ... ough_Glass) better ?
Rough glass is not the exact material I need. My polystyrene container specimen is'nt really rough. It is only a little cloudy in the volume.
And roughness interferes with the light paths, I think. There is absorption and less reflection.

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B.Y.O.B.
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Re: Bidirectional strange with glossy material

Post by B.Y.O.B. » Mon Jul 02, 2018 9:10 pm

Burk wrote:
Mon Jul 02, 2018 8:55 pm
For cycles (which I used in first place) there was the recommandation for plastic glass to add some glossy material.
But the "glossy" material in Cycles would be the "Mirror" or "Metal" materials in LuxCore (mirror if roughness == 0, metal otherwise).
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wasd
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Re: Bidirectional strange with glossy material

Post by wasd » Wed Jul 04, 2018 9:54 pm

This is my experiments with a container (HDRI not included):

There's homogeneous volume above "table" to make beam visible.
With nothing on the its path, laser is clearly seen.
1.png
Laser is seen in the "air" past container, but not before.
2.png
With polystyrene sphere on laser path, beam is not visible.
3.png
Attachments
Bidirectional strange with glossy material-3.blend
(738.07 KiB) Downloaded 33 times
Last edited by wasd on Wed Jul 04, 2018 10:25 pm, edited 1 time in total.
CPU Bidir + Metropolis | Log Power | Core i5-4570

wasd
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Re: Bidirectional strange with glossy material

Post by wasd » Wed Jul 04, 2018 10:25 pm

It's even funnier with mirrors: the beam is seen only after mirror 3, which is the third mirror on its path.
Attachments
beam.png
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B.Y.O.B.
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Re: Bidirectional strange with glossy material

Post by B.Y.O.B. » Wed Jul 04, 2018 10:41 pm

Try setting the camera volume to the same you used as the world volume.
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wasd
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Re: Bidirectional strange with glossy material

Post by wasd » Thu Jul 05, 2018 12:27 am

B.Y.O.B. wrote:
Wed Jul 04, 2018 10:41 pm
Try setting the camera volume to the same you used as the world volume.
Beam splitter! (with beam operator)
Mirror is half-translucent.

Camera volume set to world volume:
4.png
Camera volume set to dust volume (the volume which makes beam visible):
5.png
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