So far in my particle system I have used a simple collision model where
the outcoming angle is equal to the incoming angle and the outcoming
velocity is smaller than the incoming velocity due to the energy loss at
the collision. ( See my illustration I in povray.binaries.images in the
message "Collision with energy loss" ) But I wonder if that is a very
good model to use...

The energy loss is often quite big, like 80%, because many things when
they bounce only reach a small fraction of the height they were dropped
from.

When the incoming angle is close to 90 degrees the model can look quite
fine. But imagine that the incoming angle is very small. Then the energy
loss of 80% will seem very unnatural, like the particle slows down to
20% speed for no reason.

So how should energy loss be applied? One Idea I had was that the energy
loss should only apply to that part of the movement vector that is
perpendicular to the surface the particle collides against. ( See
illustration II in the before-mentioned image. ) But this would mean
that the outcoming angle would not be equal to the incoming angle. In
reality is the incoming angle always equal to the outcoming angle, also
when energy loss is taken into consideration?

A third option would be to still use the same model as in illustration
I, but vary the energy loss so that it is greatest when the
incoming=outcoming angle is close to 90 degrees and so that there's
almost no energy loss when the incoming=outcoming angle is close to 0
degrees.

What are your thoughts on this?

Collisions in the real world are probably way more complicated, but I'm
looking for a very simplistic model which is still as realistic as
possible...

Rune
--
3D images and anims, include files, tutorials and more:
Rune's World:  http://rsj.mobilixnet.dk (updated May 20)
POV-Ray Users: http://rsj.mobilixnet.dk/povrayusers/
POV-Ray Ring:  http://webring.povray.co.uk

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