"Mark Hanson" <mar### [at] comcastnet> wrote in
news:3fb11715$1@news.povray.org: 

> "Alain" <noe### [at] onca> wrote in message
> news:3f9a7e1b@news.povray.org... 
>>     - The red wall is bouncy, the blue wall is not.
> 
> I suppose that accounts for what to my eyes seems as though the balls
> are bouncing slightly *through* the blue wall before rebounding? Like
> the wall is actually a little further away than it seems. Or maybe my
> brain expects the balls to bounce harder off the blue wall than they
> actually are. Hmm. 
> 
> Nifty animation -- both as a test, and as an inadvertent optical
> illusion! 

I'm not sure.  There shouldn't be any penetration.  But the blue wall 
does have high friction, as do the balls, therefore the balls don't 
bounce much off of the blue wall, but they do pick up a lot of spin as 
they fall downwards and rub against the wall.  This means that when they 
hit the ground, which has high friction on the right side as well, the 
balls' spin will make them move away from the blue wall, perhaps 
appearing to have bounced off of it.

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