I have always followed the crude logic that such objects can either
touch or be or bit apart, but they can never intersect (in real life).
In the glass and water example it probably doesn't matter much (Ken, I
believe the water and glass would interreflect either way?)
I think the cylinder should not extend into the plane (esp. when the
plane is opaque and the cylinder is transparent but tinted) Otherwise
the part of the cylinder inside the plane is invisible and its colour is
not substracted. This looks unnatural.

Margus

Ken wrote:
> 
> Hi Micha,
> 
>    I have run into this problem on occasion and what I do is
> for the plane problem I extend it slightly into the plane. This
> helps get rid of multiple reflections between the bottom of
> the cylinder and the surface of the plane.
>    For water in a glass I always scale the water a little larger.
> If you make it smaller you have an unnatural reflection situation
> between the glass surface and the water surface. Just keep your
> over scaling to a minimum amount - something like .0001 is
> enough. I believe this is the recommended method somewhere
> in the docs but I'm unsure which section. Probably under cgs
> operations.
> 
>   Other people may handle these situations differently but
> they have worked well for me.
> 
> Ken Tyler

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