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FLIP Gravity field - gasexternalforces takes forever


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I'm using a SOP Vector Field DOP, plugged into a Mask Field DOP, plugged into a Gravity Force DOP, to create a spatially varying gravity force.


I've noticed that the sim stalls on the gasexternalforces node for ages applying that gravity to the FLIP sim.  It's frustratingly single-threaded, and bizarrely, appears to take progressively longer with each substep, so it would appear there's some majorly redundant processing going on (and is probably the reason I never spotted it before, as the first timestep isn't so bad, but when I get up to 5 or 6 for a fast-moving sim, it spends about 60-70% of each frame's total sim time just applying gravity!)


I have tried directly caching the SOP input to disk, and loading straight from there, and it still does exactly the same thing.



Does anyone know what might be causing it?  Or alternately, does anyone know a way to source a gravity field directly into a FLIP sim without using "Apply External Forces"?  Every other force or POP node seems to apply some sort of drag function or mass relative effect that destroys the fluid dynamics... Is the Gravity Force DOP the only way to apply a straight uniform acceleration to a FLIP sim?

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Aha!  Answered my own question it seems...


I can actually use a POP Attrib From Volume node, plugged into the FLIP Particle Velocity input, and "Add" my gravity field directly to the "force" particle attribute, and it gives an identical result to applying it via an external Gravity Force DOP.

It also happens to be almost-instantaneous, rather than clocking up 2-3 minutes per frame!

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