sheen and sheen tint

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lacilaci
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sheen and sheen tint

Post by lacilaci » Fri Jan 03, 2020 3:49 pm

those two sliders on disney material don't do anything at all... do they?

juangea
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Re: sheen and sheen tint

Post by juangea » Fri Jan 03, 2020 8:09 pm

I also noticed this, in Cycles they give like the velvet effect.

Usually we need to force it more than 1.0, to 3 or 4, but I also noticed that in Lux it's capped at 1.0, depending on the effect it should give may be reasonable to remove that upper limit.

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lacilaci
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Re: sheen and sheen tint

Post by lacilaci » Fri Jan 03, 2020 8:45 pm

juangea wrote:
Fri Jan 03, 2020 8:09 pm
I also noticed this, in Cycles they give like the velvet effect.

Usually we need to force it more than 1.0, to 3 or 4, but I also noticed that in Lux it's capped at 1.0, depending on the effect it should give may be reasonable to remove that upper limit.
Well yes it should create a bit of backscatter effect for quick cheap cloth look but in luxcore it doesn't seem to do anything.

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Re: sheen and sheen tint

Post by juangea » Fri Jan 03, 2020 11:02 pm

Yes, I see the same, no effect at all in lux.

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lacilaci
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Re: sheen and sheen tint

Post by lacilaci » Sat Jan 04, 2020 8:22 am

hm. so it does seem to work, but very weak.
no sheen
A.jpg
sheen
B.jpg

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Dade
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Re: sheen and sheen tint

Post by Dade » Sat Jan 04, 2020 1:44 pm

Always refer the original Disney material to know how it is supposed to work:

param1.jpg
param2.jpg

If Lux doesn't work like that, it is a bug.

As you can see the expected Sheen behavior is a bit subtle but it is there.
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Dade
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Re: sheen and sheen tint

Post by Dade » Sat Jan 04, 2020 1:48 pm

juangea wrote:
Fri Jan 03, 2020 8:09 pm
I also noticed this, in Cycles they give like the velvet effect.

Usually we need to force it more than 1.0, to 3 or 4, but I also noticed that in Lux it's capped at 1.0, depending on the effect it should give may be reasonable to remove that upper limit.
All Disney material parameters are, by design, between 0.0 and 1.0. It is the concept around which the material has been developed. If Cycles accepts values over 1.0, it is doing something different from the original Disney concept (and it may not be a good idea because Disney BRDF is now a de-facto industrial standard available across many applications).
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Re: sheen and sheen tint

Post by juangea » Sat Jan 04, 2020 4:11 pm

But we don’t have to loose sight that it was an artist driven BRDF, and even from Disney they say that some values can surpass the 1.0 limit to achieve different effects, like the specular one:
The specular parameter may be pushed beyond one to reach higher ior values but should be done with caution.
This is from this document:
https://disney-animation.s3.amazonaws.c ... tes_v2.pdf

Sheen is not mentioned explicitly, but due to its behaviour it’s clear that for it to be useful in many situations we need to be able to push the 1.0 limit, that’s why in Cycles is not limited, to keep in mind that it’s not a "physics" shader, but an "artists" shader :)

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Re: sheen and sheen tint

Post by Dade » Mon Jan 06, 2020 10:39 am

juangea wrote:
Sat Jan 04, 2020 4:11 pm
Sheen is not mentioned explicitly, but due to its behaviour it’s clear that for it to be useful in many situations we need to be able to push the 1.0 limit, that’s why in Cycles is not limited, to keep in mind that it’s not a "physics" shader, but an "artists" shader :)
You can't use values over 1.0 for Sheen, it would break energy conservation law (i.e. it would create light from nowhere). Check the code: https://github.com/LuxCoreRender/LuxCor ... y.cpp#L209

Sheen parameter is used for a linear interpolation between 2 colors so if a value > 1.0 it will start to glow. It something can give you a lot of problems. As far as I remember, there was a thread on BlenderArtists about this problem :?:

I could allow values > 1.0 and than clip, between 0.0 and 1.0, the result of the linear interpolation; this solution would not break energy conservation law and allow the user to force the result more toward the sheen tint :idea:
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Re: sheen and sheen tint

Post by juangea » Mon Jan 06, 2020 11:01 am

That could be enough, in the end we just need some artistic control to give sheen more appearance, if it’s easy to implement maybe you can add it to a 2.3 build, we can test it and if it’s good enough it can stay, if it looks great it could just be removed :)

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