Gaffer/Arnold Volume Lights

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Sumit Suri

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Nov 8, 2018, 2:33:50 AM11/8/18
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Hi,
I am trying to create a volume with quad lights. I am unable to figure this out. The atmosphere shader behaves like an atmosphere fog, but not sure how can I have an arnold light (quad/spot...) to create a volume effect.
any help appreciated
thanks
Sumit

John Haddon

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Nov 8, 2018, 4:27:03 AM11/8/18
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Hi Sumit,
It sounds like you are using Arnold's fog shader, which is not affected by lighting. If you use Arnold's atmosphere_volume shader instead, then I think you will get the effect you want. I've attached an example.
Cheers...
John

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arnoldVolume.gfr

Sumit Suri

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Nov 8, 2018, 7:36:02 AM11/8/18
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Hi John,
thanks for the prompt response. I have the same setup. It works with quad light, but not with distant light. I am not sure if it is suppose to work with a distant light although the light has a parameter called volume & volume samples.
regards
Sumit



On Thursday, November 8, 2018 at 4:27:03 AM UTC-5, John Haddon wrote:
Hi Sumit,
It sounds like you are using Arnold's fog shader, which is not affected by lighting. If you use Arnold's atmosphere_volume shader instead, then I think you will get the effect you want. I've attached an example.
Cheers...
John
On 8 November 2018 at 07:33, Sumit Suri <animat...@gmail.com> wrote:
Hi,
I am trying to create a volume with quad lights. I am unable to figure this out. The atmosphere shader behaves like an atmosphere fog, but not sure how can I have an arnold light (quad/spot...) to create a volume effect.
any help appreciated
thanks
Sumit

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dan...@image-engine.com

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Nov 8, 2018, 1:40:35 PM11/8/18
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Nope, doesn't work with distant_light.  See the top of here:

https://docs.arnoldrenderer.com/display/A5AFMUG/Atmosphere+Volume

Kinda makes sense - if the atmosphere volume is infinite in size, and a distant light is infinitely far away, then it will always be attenuated to 0 before reaching the scene.

If you want to use a distant light, you would have to create a big cube and assign a standard_volume to it, and set the shape step_size so that it gets rendered as a volume.

Sumit Suri

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Nov 8, 2018, 5:44:33 PM11/8/18
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thanks Daniel, very helpful
regards
Sumit

Riccardo Cecchinato

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Sep 10, 2024, 12:52:07 PMSep 10
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Hello,

Using this topic to avoid creating a new one and polluting the group.

As Andrew Kawfman stated in this topic a while back https://groups.google.com/g/gaffer-dev/c/9i7ksyxadVg/m/9_B_1mICBAAJ :

" [...] In Gaffer, you'd assign a shader to a location, via a ShaderAssigment node and a Filter. This would be the same for any shader, not just aiAtmosphereVolume. However, for that shader specifically, Gaffer needs some way of knowing the location in question contains a volume rather than a mesh, which you can accomplish using an ArnoldAttributes node, filtered to the same location, enabling the Volume Step Size plug, and setting it to some non-zero value. [... ]."

This is exactly what I am trying to do, assigning the vol shader to a cube, so to have a RenderPass "Haze_Vol" containing only that cube with everything else in Matte/flat black. But I am not able to succeed.

Therefore the question: Is it still possible to assign an AtmosphereVolume shader to a primitive (eg. a cube)?

Or - since the existance of the node "ArnoldAtmosphere" - the AtmosphericVolume shader assignment is deprecated?

Thank you in advance for your always Valuable help!

Riccardo

Daniel Dresser

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Sep 10, 2024, 2:01:06 PMSep 10
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"This is a scene-wide volume shader (or an atmosphere shader in Arnold's terms)."

That seems to mean that it cannot apply to individual objects, and you would have to apply it with the ArnoldAtmosphere node in Gaffer.

Riccardo Cecchinato

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Sep 11, 2024, 3:40:11 AMSep 11
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Thank you for your quick reply! Applying the atmosphere shader to a sizable cube is a common pipeline and it works in Maya with the same Arnold version I have in Gaffer.

Infact I found out, while playing around with the ArnoldAttributes node, that giving a non-zero value to "Shape Step Size", not to the "Volume Step Size", does it!

However, I am wondering if in Gaffer this workflow may be less optimized than dropping the purposefully-created "ArnoldAtmosphere" node in a specific Render Pass sub-net, and assigning to the atmosphere a specific AOV.
It would be nice to have some Devs advice about this!

I will surely conduct some tests!

Thank you again,

Riccardo

Riccardo Cecchinato

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Sep 11, 2024, 11:05:15 AMSep 11
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After a good amount of testing I came to the conclusion that both methods are valid for different purposes.

This being said, having the shader applied to a box instead of the world allows access to volumetric refinement and optimization (stepsize etc.), which render this method more suitable for sampling refinement.

Also, there are usecases which seems to be impossible to obtain through the "scene-wide volume" method, since the stepsize is too large by default to obtain certain looks (glows, lights eccentricity, etc).

Riccardo





Daniel Dresser

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Sep 11, 2024, 12:06:45 PMSep 11
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Hmm, if you can provide a simple .ass file that shows an atmosphere shader working correctly when applied to an object, we can try to figure out how to export this from Gaffer.

The Arnold docs pretty explicitly claim that this can't work ... I certainly wouldn't rule out that the docs are wrong, but in that case we need an example to know what we're supposed to be doing.

-Daniel

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