HtoA: 5.5.0_r1582e12_houdini-18.0.597
When writing out .ass files consisting of multiple density grids (each with a unique shader assignment), we've noticed that Arnold combines the grids in the ass file into a single volume definition and applies only the first shader it finds to the result. Thus, if you have three clouds, and cloud A has a red shader, cloud B has a blue shader, and cloud C has a green shader (applied with Material SOPs), and this merged result is written to a single .ass file, the render in Maya will show three red clouds.
However, if you render three unique Arnold volume objects to the .ass file (not merged in SOPs), each cloud is then treated as a separate volume with a unique shader, and Maya will render the three colors properly.
We'd like to take a single SOP output with multiple merged density fields (each with a unique value in 'shop_materialpath') and export them to an .ass file in a way that preserves the unique materials. Is there an attribute or some other Arnold property that would allow us to force the .ass file to keep each grid in a separate volume definition with it's own shader?
Can you upload a simple example scene?