Darkness in transparent surface ?

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zooropa
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Darkness in transparent surface ?

Post by zooropa » Tue Oct 06, 2020 3:55 pm

Hi. I am working with an acrylic material and I am wondering if the strong dark line is part of the accurate calculation or I might be missing some checkbox or properties to get rid of it.
artifact.jpg
Thank you

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B.Y.O.B.
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Re: Darkness in transparent surface ?

Post by B.Y.O.B. » Tue Oct 06, 2020 4:36 pm

Impossible to say without having access to the scene.

You could try to rule out some possibilities first:
For example, the path depth might be too low - to rule that out, set the path depths to something very high (like 100) and compare it with your current render.

zooropa
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Re: Darkness in transparent surface ?

Post by zooropa » Tue Oct 06, 2020 4:46 pm

path.jpg
Let me know if I can send you the file privately.

zooropa
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Re: Darkness in transparent surface ?

Post by zooropa » Tue Oct 06, 2020 5:17 pm

B.Y.O.B. wrote:
Tue Oct 06, 2020 4:36 pm
Impossible to say without having access to the scene.

You could try to rule out some possibilities first:
For example, the path depth might be too low - to rule that out, set the path depths to something very high (like 100) and compare it with your current render.
Sent you a private message

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B.Y.O.B.
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Re: Darkness in transparent surface ?

Post by B.Y.O.B. » Tue Oct 06, 2020 7:28 pm

Following the advice I gave before:

This is with your path depths:
a.jpg
and this is with depths 100:
b.jpg
So you will have to try different path depths to find one that is sufficient (which will be between 12 and 100).

zooropa
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Re: Darkness in transparent surface ?

Post by zooropa » Tue Oct 06, 2020 8:15 pm

Thank you, I thought the path depth is just limiting the amount of bounces on the other three variables: Diffuse, glossy and specular. I did not know in itself will be a value that luxcore will compute, but more like an overall limit to the scene. So you did not tweak the other bounces, just the path depth ?

I can not find information in the manual.

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Re: Darkness in transparent surface ?

Post by B.Y.O.B. » Tue Oct 06, 2020 8:56 pm

I set all 4 depths to 100 (total, diffuse, glossy and specular).
It's probably only necessary to raise the total and specular depths, though.

zooropa
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Re: Darkness in transparent surface ?

Post by zooropa » Wed Oct 07, 2020 8:50 am

It worked at 100, I tried with 60 and 30 and not good. Might be a bit extreme just to get that surface clean. But worked. Thank you

CodeHD
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Re: Darkness in transparent surface ?

Post by CodeHD » Wed Oct 07, 2020 12:36 pm

zooropa wrote:
Wed Oct 07, 2020 8:50 am
Might be a bit extreme just to get that surface clean. But worked.
It's not really extreme when you consider the geometry. Since the term hasn't been written yet: it is the consequence of total internal reflection (TIR), i.e. light bouncing between the surfaces. You can actually quite easily calculate how many bounces you will need for a given perspective.

See the following picture for an example calculation. I have assumed table dimension of 160cm x 80cm x 1cm and the default refractive index of n=1.5

You then get 90 bounces if you look along the short side, and 200 bounces along the diagonal. This is at the limiting angle though, so if you look at more shallow angles it will be a bit lower.
20201007_134320_3.jpg
In addition you must also consider that the light ray from the outside is refracted when it enters the side of the acrylic. This makes it even shallower, thus requiring further bounces - between 72 and 162 in my example.
(Side note: For realistic index of refraction, the angle inside will then always be shallower than the TIR-angle. If you check out a rectangular piece of glass, like a trophy, you will notice that it is impossible to see through the corner ;) )

You image looks like like you are looking pretty diagonally, so a number around 100 doesn't seem unrealistic.

zooropa
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Re: Darkness in transparent surface ?

Post by zooropa » Wed Oct 07, 2020 12:45 pm

amazing insights. Then I might even be able to use more and have an even clenaer acrylic!

Such a niche sketches bringing information! I really appreciate it!

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