Hello,

I just experimented a bit with glass, water and photons. For making
a glass filled with water, the documentation recommends to make a
hollow container using CSG, and filling it with a liquid object
scaled down a bit to avoid coincident surfaces. I'm a bit unsure
about this, as the extra transitions through a thin air slice would
seem to cause unnecessary calculations, and yield incorrect results
as well (e.g., I think it is possible that the expected refraction
at a water/glass transition turns into a total internal reflection
at a water/air transition). A problem like this was discussed some
time ago in povray.newusers regarding "Refraction in the eye":

http://news.povray.org/povray.newusers/thread/%3Cweb.45642420c0c6e74c618bf8930@news.povray.org%3E/

So, I made three images:

1. Uses the recommended technique
2. Embeds the liquid in a solid glass block (in the hope that
    the IOR specified in the second object takes precedence)
3. Is a variation of 1., with the liquid scaled *up* a tiny
    bit and the "merge"-d with the glass container to remove
    the inner surface (but no idea how POV-Ray handles the
    differing IORs in a merge).

As a result, the total brightness of the glass seems to be a
bit exaggerated in the first version. I like the second result
best, but the behavior appears to be rather undefined (when
combining two objects with differing IORs, what is the IOR in
the intersection supposed to be?). Also note that there seems
to be problem with seeing the lemon slice through the water
when using this method. The third image looks rather dull.

Also, the version with the embedded liquid took a lot longer
to render (although it should be much less work calculating
the refraction).

And no praise for the lemon texture please,
it's a photo - of an orange, actually ;)

You know you've been raytracing to long when ...

... you start photographing your breakfast with a
mobile phone to reuse it as a texture.

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