r/Houdini • u/tk421storm • 4d ago

Help Split MPM Surface into two steps - why?

The MPM surface node has a second input for surface and vel, and the tooltip IDs it as:

MPM Surface and Vel
The output of the MPM Surface. Useful to split the SDF creation from the polygon mesh creation in two separate steps.

What is the advantage of splitting the operation up into two steps? So I can perform operations on the SDF first, and then feed that into the polygon surface operation?

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u/DavidTorno Houdini Educator & Tutor - FendraFx.com 4d ago

The SDF and Velocity help define where a fluid surface mesh is, and informs where it’s going. That can be useful to fill gaps, smooth the surface results of the meshing step.

Splitting not only lightens the memory footprint a bit on the main stream, it also makes it easier to post process as you said.

FLIP outputs the same volumes, but all in the primary stream, so if you’ve ever used are doing your own meshing step you will have to split and cleanup manually.

It’s part convenience, and part requirement because of the prefab post steps looking for specific inputs at specific output indices.

There may be a deeper reason why, but that’s how I have seen it.