...
> 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?

This is IMHO the best option, and the most physically based.
I've implemented in the past a particle system, and my bounces followed this
law.
A damping factor could be applied to the tangential component of the
movement factor to take into a first account friction between surfaces. A
more complex model would modulate this tangential damping factor according
to the angle of incidence as in your third option.

I found a lot of inspiration in the particle system library by David
McAllister found at www.cs.unc.edu/~davemc/Particle/, I suggest you to dig
into the source of that API which is available for free.

Alex

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