Scattering VDBs in Maya/Arnold

This is super easy via ASS files.
Arnold Scene Source (.ass) is the native scene definition format, stored in human-readable, editable ASCII files.

  1. Load your VDB file via an Arnold Volume container.
    1. Assign an aiStandardVolume shader.
    2. Set all settings for the container (don’t forget to set the step size and scale) and the volume shader as well.
    3. As always, test the results.
  2. Create a proxy geo for the VDB:
    1. You can do that with the help of the fairly new Bifrost Graph system.
      I wrote a simple script to do just that. Download here.
      Select the already set Arnold VDB container and run the script (you have to turn on the Bifrost plug-ins first).
    2. Or with Houdini/Houdini Engine (I recommend this method, more reliable, faster, more control).
    3. Make sure the proxy mesh is pretty low res.
  3. Select the VDB container and export as .ass.
    (Arnold -> Scene Export -> Export Selection to .ASS…)
  4. Select the proxy geo change the Arnold Translator type to procedural (on the mesh node)
  5. Link the exported .ass file in the Path section.
    1. If you want to use the shader inside the .ass file, make sure the Override Standin Shaders is turned off.
    2. If you want to use a different volume shader, you need a few things:
      1. Make sure the Override Standin Shaders is turned on.
      2. Assign a new simple shader (a Lambert will do) to the proxy geo
      3. Connect a new aiStandardVolume shader to the SG node’s Ai Surface Shader input (yes, not into the Volume Shader input, strange right?).

  6. Now you can use any scatter system to scatter the proxy geo (for example the MASH Network).
    Arnold will render the .ass file instead of the proxy mesh.
  7. Of course, you can use multiple proxy geos pointing to different .ass files (different clouds).

Also, you can just copy the proxy and put that manually wherever you see fit.

Cheers, D

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