Water-Glass-Air problem:

I've given this problem a lot of thought while trying to accurately model a
glass of Champagne. I've come up with four non-solutions.

Model with air-gap:
  -Doesn't work because it results in total reflection where
   there shouldn't be. (The air gap work the same way as the
   air gaps in optical fiber lines.)
  -Awful render times because of rays bouncing back and forth
   in the gap.

Union with overlap:
  -Not as much total reflection, but still incorrect.
  -Still awful render times, because of the two surfaces.

Merge with overlap:
  -Not really correct since merged objects shouldn't have different
   interiors.
  -The material is determined by the ray's point of entry, and
   doesn't change, because there is no surface at all where there
   were two surfaces before

Union + clipping.
  -Correct number of surfaces (one) between water and glass.
  -Problem when ray reenters air. (It doesn't, because it
     misses the surface where it should leave the water.)
  -Example:
    1st surface: Enter water (Ray is in water)
    2nd surface: Enter glass (Ray is in glass in water)
    - Surface to leave water is clipped
    3rd surface: Leave glass (Ray is in water, when it is actually in air.)

The only solution that I can think of would be to modify POV-ray itself.
The feature we need is a half-merge. That is a method which takes the
union of two objects, and keeps exactly one of the internal surfaces.

/Per

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