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Grouping points in cached FLIP sim?

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I have a pre-simmed/cached FLIP sim. I would like to create a group of a section of the particles of the sim so I can mesh them separately and apply different shaders to them later. Re-simming is difficult and slightly impractical, so I would love to be able to do this with the cached points as-is(/are).

I can create a group of points, but it looks like point numbers are not constant in FLIP sims, as the group changes over the course of the sim (frame to frame, actually). Is there a way to create a group that then remains constant and flows with the existing sim? Or even a way to create a new group of particles that is advected by the existing cached sim? (I'm pretty sure it's no longer a "volume" by then, so I'm guessing a straightforward advection by volume POP wouldn't work.) Or a way to do some sort of "sort" operation to compel points to retain point numbers, which I could then group? Or advecting another form to follow the sim, and I could then use that as a bounding volume to create my group? (Point deforms with FLIP sims as the deformer seem to get really messy really quickly.)

Thanks for any help!

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if you have id attribute on the points you can select by id which will be stable, it's best if the reseeding and narrow band is off as otherwise the particles are constantly being created and deleted so your selected ids may not survive for too long

you can also try advecting group of points from previous frame and transfer that group to current using Group Transfer SOP, if you don't have original vel volume you can just try advecting them using their existing v attribute. v for FLIP may be backward vel so maybe advecting in reverse may be more precise

Edited by anim

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