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PBR / physical shaders questions

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PBR / physical shaders questions

Unread postby kai01w » Mon Aug 01, 2016 8:20 pm

Hi,

after playing around with 3.0 I still got quite a few open questions on the new PBR and Physical shaders.
I really think the documentation needs more work as many things are not really touched upon.

1) In the documentation, I could find no info whatsoever on the Brdf attribute of the "reflection" shader and its options. Obviously there
is some info on the web but it is sometimes a little overwhelming and if the Docs can explain the "Color" attribute
of a shader using some wedge images, I would think some info on the Brdf and how it works in Clarisse would be very welcome.

2) The "Fresnel" options in the reflective shader, I also struggle with. While the "Fresnel Incident Tint" seems to do what I thought it would
(control the color of the reflection on areas facing the camera), the "Fresnel Edge Tint", does not really seem to do anything.
Certainly setting it to some really strong colors does not seem to have any corresponding effect to the areas facing away, or actually to anything at all.
Do I miss something here?

3) the "diffuse" shader has a "Back Color" attribute for translucency like effects.
The "reflection" shader does not. Is there a technical/logical reason for this?

4) I am not sure I really understand what the Tint/Absorption/Incidence Parameters are supposed to do visually in a layered shader. Again that little introduction video was very brief on these.

5) I also would like to see some more info on best practices, optimization, etc.: Will using the layered shader have a performance impact compared to trying to achieve similar things with just one shader but using a more complex shading network? Should I create fresnel like effects through the layered shader or would a single reflection shader
with correct settings render faster?

thanks

-k
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Re: PBR / physical shaders questions

Unread postby halim » Fri Aug 05, 2016 12:53 pm

Hi,

You will find a lot of PBR information on the blog, and an introduction video:

http://blog.isotropix.net/
http://blog.isotropix.net/?p=224

https://www.youtube.com/watch?v=UigldhV_xTM

More will come later.

Regarding your specific questions, I need to wait for one of my teammates feedback since I'm not sure I have the right answers.

Hope this helps.

Halim.
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Re: PBR / physical shaders questions

Unread postby kai01w » Sun Aug 07, 2016 3:50 pm

Hi Halim,

thanks for your reply. I could find no or only little information in both the documentation as well as the blog post on the things that I mentioned. Would be awesome if as you said some of your colleagues could post a little more on these topics.

thanks

-k
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Re: PBR / physical shaders questions

Unread postby halim » Tue Aug 09, 2016 11:13 am

Hello

Please find bellow some answers to your questions

1) In the documentation, I could find no info whatsoever on the Brdf attribute of the "reflection" shader and its options. Obviously there
is some info on the web but it is sometimes a little overwhelming and if the Docs can explain the "Color" attribute
of a shader using some wedge images, I would think some info on the Brdf and how it works in Clarisse would be very welcome.

As for BRDF:
Beckmann is the most physically accurate
GGX is best suited for metals
Phong / Ward - fits better to soft surfaces (rubber, skin etc)

3) the "diffuse" shader has a "Back Color" attribute for translucency like effects.
The "reflection" shader does not. Is there a technical/logical reason for this?

The back color of a diffuse is similar to a "diffuse tranparency". The equivalent on a reflection would be a Transmission which is a feature of DIeletric Material.

Hope this helps,
Halim Negadi
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Re: PBR / physical shaders questions

Unread postby isoyann » Tue Aug 09, 2016 11:49 am

1) Each BRDF corespond to a different microfacet model. Usually, GGX is a good all around one, personally I don't bother with the 2 others (many softwares only have GGX), but in some cases, if you need to match a very specific look, ward or beckman react differently with the roughness attribute.

2) The Fresnel Edge tint is broken. It will be fixed, but the best is to use the fresnel integrated to the layered material. If you want to manually control this, the best is to connect a gradient to the color attribute.


4) Tint/Abosrption/Incidence are here to simulate clearcoat (best used with a reflection layer). No effect if the tint color is white.
The color control the color of the clearcoat.. the complemetary (opposite) color will be absorbed by the clearcoat.
The Absorption control how much the color is absorbed .
The Incidence control multiply the absorption depending on the incidence angle.. it only works with low absorption values (10 is a good value), if you use it with a high absorption value, the maximum will be reach everywhere and the incidence have no effect.

5)The best is to use the layered shader with integrated fresnel, and layer simple materials (diffuse, reflection etc..). The speed is almost the same if you use separate reflection mixed together inside a layered shader and a single reflection material directly applied (and same speed if there is only one reflection connected to the layered material).

Thanks,

Yann
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Re: PBR / physical shaders questions

Unread postby xilofoton » Tue Aug 09, 2016 2:10 pm

Hi!

Nice explanations, would be nice in the docs too!

Yes, GGX is good and has a low but permanent quasi diffuse lobe starting at low roughness values, which I found more realistic in most situations. Sometimes I use it for diffuse too, because there is no native correct roughness in the diffuse material(disatvantage: AOV mismatch). The roughness (as the popup help tells it) is a grazing angle boost which is a nice feature but not the "real" roughness. Sometimes I modulate the diffuse with random/noise bump to get proper roughness... it seems using GGX with more than 90% roughness does a good job with less noise than bump, and there is an easy way to add some "invisible" glossy just lowering slightl below 90%.

I ask here what I planned to ask in the feature request forum:

Is there any way to use custom BRDF (with OSL maybe) or will be a feature in the future? There was a request about the Disney all-inclusive shader, but a full freedom to create a custom would be very nice.

http://brdflab.sourceforge.net/

http://www.disneyanimation.com/technology/brdf.html

There are a lot brdf-s shipped with it.

A long time ago this was my favorite approach:
http://www.plunk.org/~gdb/publications/ ... -s2003.pdf

Would be nice to tweak the BRDF values watching it in 3D in a context which updates the render scene context/image.
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Re: PBR / physical shaders questions

Unread postby isoyann » Wed Aug 17, 2016 9:46 am

Hi,

Yes, uber shaders similar to Disney is on the roadmap.. about a way to define you own BSDF , currently it is only possible using the SDK , OSL closure are not supported by Clarisse (it was implemented for pattern generation only, like in most renderers).

Thanks,

Yann
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Re: PBR / physical shaders questions

Unread postby RokhAV1 » Sat Feb 22, 2020 1:24 am

isoyann wrote:define you own BSDF , currently it is only possible using the SDK

this is something i found zero info about. i have an eye on clarisse for a while now but custom shaders and utility nodes would be mandatory.
so any info or code examples or anything else in that regard would be very welcome.
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