> How do you guys do it?

Well, I start with three rays and shoot along the direction which I
calculate from
(new_position - former_position)
One ray is shot from the center of the particle, the other in front of the
particle along the velocity-vector and one from below the particle, along
the gravity-vector.
Additionally, when a particle moves, I spread a User-Defined amount of
trace-origins on the frontern hemisphere of the particle.

From all these samples I choose the one closest to any origin and check for
collisions from there. To properly place the particle, I keep track where
the sample-origin is in relation to the particle's center and add that
relation to the collision-point.

Of course, there are still inaccuracies, but as long as you trace samples
there's always a possibility that the samples don't hit the object. Same
goes for raytracing, BTW, when rays shoot just aside of small objects, they
won't be visible in the final image.

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>
Email: tim.nikias (@) nolights.de

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