> I do not think this behavior can be intended.
>
> From the documentation part 6.6.1
> "... It is totally illogical to have a ray enter an object with one index
> or refraction and then recalculate with another index. The result is not
an
> average of the two ior values. Similarly it makes no sense to have a ray
> enter at one ior and exit at a different ior without transitioning between
> them along the way. ..."
>
Read the next two sentences:
POV-Ray only calculates refraction as the ray enters or leaves. It cannot
incrementally compute a changing ior through the interior of an object.



>
> Another point to consider is that refraction is directional, so ior A ->
> ior B gives a different result than B -> A. That means it has to be
> distinguished between entering and leaving objects. Now at the back
surface
> of the box should the ray be refracted 1.0 -> 1.2 or 1.2 -> 1.0?

1.2 -> 1.0  where the 1.0 is the ior (implied) of the space outside the box
(as opposed to the applied ior 1.0).  The ray is considered leaving this
surface, the suface has an ior of 1.2 going into empty space, ior 1.0,
therefore it would act as though the interior of the object had an ior of
1.2 at this surface.  At the front surface it acts as if the interior has an
ior of 1.0, no refraction.  As stated (though perhaps not as clearly as it
could) in 6.6.1, the ior should vary alng the interior of this object but
that is almost impossible to calculate properly.  Having a CSG object with
different iors on different parts will not produce any kind of realistic
results and I would consider poor coding practice (unless it is intended).
This is just a limitation of POV, not a bug though.

-tgq

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