Hi guys,

I have been bumping into a peculiar issue on the whitewater setup from the shelf tools, where I seem to have white water particles that ‘stick’ in the air for a moment before coming down.

I have tried a number of ways to tackle this issue but have not been successful. Mainly, I have isolated the issue down to when the foam particles transit into spray. That seems to be when they hang around for a bit before gravity brings them down.

I'm probably not doing something right, as I am relatively new to using the white water setup from the shelf.

Has anyone bumped into something like this before? I have a scene file where I have this issue replicated.

Anyone has a clue what is going on here? Appreciate any help

I havnt looked at your scene but it sounds like maybe you are trying to read the velocity field to far from the surface field. I dont use the whitewater shelf tool, so not sure how sesi blend between spray and foam velocities but there must be some kind of distance threshold somewhere.

I posed my question to sidefx and John Lynch gave me a good reply that diagnosed what the problem was, how to fix it proper and how to fix it quickly in my situation.

I thought I would post it hear for anyone else facing this issue in the future as it is fairly easy to fall into this problem if using custom emitters for your flip sims.

I believe your issue is with your emission geometry.

Normally if you use something like FluidSource, you will be creating both particles and a “surface” SDF that represents the emission geometry. The FLIP Solver setup uses this via the SourceVolume DOP to merge in the new particles, *and* to merge in the surface SDF of the emission geometry so that the final FLIP surface field represents the full fluid including the newly emitted fluid. This last step is necessary because emission happens last in each timestep.

Having an accurate surface SDF coming out of FLIP is important for whitewater since it uses it for depth testing. Unfortunately it's a little too easy to provide incorrect (or missing) values when creating a custom emitter, as you have done here. You are only creating particles, which is generally fine, but since you don't actually create a surface SDF, you are giving the FLIP solver invalid values for the surface field in the SourceVolume DOP. These flow through to the whitewater depth testing and lead to stuck particles when it can't figure out where in the fluid the whitewater particles are.

There's an easy fix: just set the Source Volume operation on source_surface_from_splash_emitter to None, which will disable the merging of the (non-existing) surface SDF field. We should probably handle this situation better by default.

A couple of useful debugging tips for these kind of issues: turn on the Surface guide for the FLIP sim. It should be a smooth (if low-res) representation of the fluid. Without this fix it's blocky due to the invalid values.

Also you can clear the Delete Internal Attributes parameter on the Whitewater Solver and look at the values for “depth” on all the particles. If you see a lot of particles with a constant value of zero, that's usually a sign of a bad surface SDF.