Graphics Reference
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deleted, and otherwise if the grid cell has fewer than 64 particles it is topped
up with new randomly seeded particles. This kind of reseeding becomes
especially important, and delicate, when solids enter into the picture: to
allow water to separate from a solid surface (allowing air to fill the newly
created gap) particles will have to be seeded as soon as the water moves
away, but when water remains in contact with a solid we
don't
want any
particles as they will interfere with the proper extrapolation of the water
φ
into the solid. Rasmussen et al. [Rasmussen et al. 04] discuss this issue to
some extent, though a completely robust treatment still remains a research
problem.
The one final comment is the status of truly escaped particles. The
simplest thing that makes sense is to just delete them: otherwise they run
the risk of later flickering back into being when they pass close enough to a
grid point, and then disappearing again, giving rise to severely distracting
grid-aligned strobing artifacts. They no longer give information that can
be reliably represented on the grid and thus have no more direct use in the
simulation. However they do still represent
some
information, though it's
perhaps dicult to interpret strictly physically. Foster and Fedkiw [Foster
and Fedkiw 01] suggest instead of deleting them, transforming them into
a secondary particle system such as tiny water droplets following ballistic
motion until they hit the level set again; many subsequent authors and
practitioners have found similar uses for the escaped particles.
