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Aaron Ross avatar image
Aaron Ross asked ·

Render a material to an AOV? Nothing fancy!

Hi,

I cannot figure out how to simply output a particular material to its own AOV. I read the documentation, but it's all about how to replace one thing with another thing. I.e. how to create an AOV for object IDs, overriding the object material with a Utility shader. Well that is fine, but it does not help me understand how AOV Write nodes work in general. So I can't apply those general principles to my specific situation. The documentation just leads me down the garden path to a place I don't want to be.

What I need is to simply render out a particular material to its own AOV. I need to do this with emissive materials because they are not showing up in my light group AOVs.

The thing is, I thought I could just create an AOV Write RGBA and connect the output of an Arnold Standard Surface to both the passthrough and the AOV_input of the AOV Write RGBA. So the material will show up in the beauty pass as well as the AOV. But I cannot connect the output of Arnold Standard Surface to the AOV_Input slot. It accepts a map, but not a material? This is very strange.

Please tell me how I can output a particular material to its own AOV.

Thank you,

Aaron

rendershaderaov
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Stephen Blair avatar image
Stephen Blair answered ·

Standard Surface returns a closure, not a color, that's why you cannot connect to the AOV Input.

Can you use the Emission AOV? That should have all emissive materials.

If you need just one emissive material, and not the others, then perhaps you could use cryptomatte and a material ID pass to get the specific material.

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Aaron Ross avatar image
Aaron Ross answered ·

Yeah, I still don't know how to output a material to its own AOV.

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Jim Polk avatar image
Jim Polk answered ·

SolidAngle, are you listening? Not being able to do this is a huge issue.

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However in our case we're just trying to generate a simple holdout matte AOV of a specific material. If no longer possible, then A5 just broke an important part of our pipeline.

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@Jim Polk You can still plug a Standard Surface into the Beauty port of a Write Color node. The color for the holdout matte goes into the Input, and that is what is written to the AOV

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Jim Polk avatar image Jim Polk Stephen Blair ♦♦ ·

Hmm, I don't see a 'beauty' param on the MtoaWriteColor node...but we're a few versions behind the curve..

arnold-5.1.0.1:-version option
node: MtoaWriteColor
type: shader
output: RGBA
parameters: 5
filename: /muse/prod/arnold-5.0.2.4/mtoa-maya20165-2.1.0.2/shaders/mtoa_shaders.so
version: 5.0.2.3

Type Name Default
------------ -------------------------------- --------------------------------
RGBA passthrough 0, 0, 0, 1
RGB aov_input 0, 0, 0
STRING aov_name
BOOL blend_opacity false
STRING name

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Aaron Ross avatar image
Aaron Ross answered ·

So we used to be able to do this in Arnold 4? Sounds like a feature regression to me. In this case I was barking up the wrong tree to solve my problem; AOV write wasn't the correct approach. But I can easily imagine situations in which it would be useful to output specific materials to their own AOVs.

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What was your solution, @Aaron Ross?

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@Ben Perkins

There wasn't a solution to the problem of rendering a material out to its own AOV. Apparently it is not possible anymore; that's progress.
My actual production challenge was to render the light coming from an emissive material as its own AOV. And that works fine with the built-in Emission AOV, as Stephen mentioned.

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But my emissive material was outside a window. The Emission AOV did not work through a transmissive object, i.e. window glazing. I guess I could have tried a Transmission AOV, but I don't think that would include the incident light from the emissive material onto the scene geometry. In the end, I gave up and just turned off all lights and rendered a beauty pass. So much for trying to do things the clever way.

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By the way, the 600-character limit on replies is lame sauce.

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Stephen Blair avatar image
Stephen Blair answered ·

That looks like an old Arnold 4 screenshot, with the old Standard shader. I'll have that updated, thanks

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Aaron Ross avatar image
Aaron Ross answered ·

OK, thanks Stephen! I am very confused by the illustration on the docs page showing an Arnold Standard Surface material connected to the AOV Input of an AOV Write node.

https://docs.arnoldrenderer.com/display/A5AF3DSUG/AOV+Write


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