Merging your velocity vel.x, vel.y, and vel.z grids into a single vector grid using a Vdb Vector Merge node can speed up your renders.
You can safely ignore the “component grids have different transforms” warning on the VDB Vector Merge. The x, y, and z components of the velocity have slightly different transforms because they located on the center of the cell faces in each direction; that’s coming from the marker-and-cell (MAC) grid used in the simulation.
If you really want to be precise with the velocities, you need to resample them on the same grid, say the density grid:
Hat tips to Saber, Fred
Arnold 220.127.116.11 added support for per-light AOVs for volumetrics.
Here’s how to set up per-light AOVs in MtoA 18.104.22.168:
- In the light Attribute Editor, enter a name for the light group.
- Create a custom AOV for the light group. Give the AOV a name that starts with “volume_”. For example, if the light group name is “red”, then the AOV name is “volume_red”.