I applied some color to a model (Red), set the "Export Vertex Color" attribute to true in the Arnold Properties, created a aiUserDataColor node with a default color of Magenta, and plugged the color into the Scatter of a Standard Volume Shader. The result is the proper color is the Volume is blended with an object and the Default Color if the volume is blended with an empty background... Is this a bug?
What is the volume shader applied to?
Is the model being rendered as a volume (step size > 0) or is there an actual volume in the scene too?
There is a Center Sphere that is a normal mesh with Standard Material, there is a hollow sphere mesh with a Step Size set and Standard Volume Material applied. There is no background object, the black background is empty.
There isn't a Volume, just a Poly Mesh with a Volume Material applied (and the Step Size set).
And the hollow sphere has the user data?
or is it the Center sphere?
The Hollow Sphere has the Vertex Color applied as UserData.
It appears that when a Volume is rendered against the Void, it picks the Default Color. If the Volume is rendered against another object of any kind, it picks the Vertex Color.
In general, user data isn't supported for volumes. Polymesh as a volume is a special case, and I don't know what the expected result is (well, I expected to not work at all). We'll have to look into this. For now, it seems like the volume rays have to hit some object, either in or behind the volume.
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