Alain <aze### [at] qwertyorg> wrote:

> > clipka wrote:
> >
> >>> Maybe that's a part for media, not pigment.
> >>
> >> Birefringence is definitively neither a media nor pigment thing, but a
> >> straightforward interior thing, and would have to join ranks with ior
> >> and dispersion (as a matter of fact it's a difference in ior depending
> >> on polarization with respect to the "optical axis" of the (AFAIK
> >> necessarily) crystalline material).
> >
> > How hard would it be to implement birefringence ...? Seems like it could
> > easily be faked
> >
> > Syntax would be trivial:
> >
> > ior 1.5, 1.7 //(birefringent material)
> >
> > You don't really need to care about the polarization of light, since
> > light in POVRay doesn't have polarity anyway.
> >
> > I would imagine tracing a second ray at a different IOR would be rather
> > simple to add.
> >
> In addition of the second ior, you also need an axis or "direction".
> If you take a sphere of birefringent material and make it turn, the
> difference in refractions changes. It also changes depending on the
> distance from the view axis.
> There is a plane where both iors are the same and an axis where they are
> the most different, usualy. They are frequently perpendicular, but I
> think that there are cases where the axis is not normal to the plane...

If the "axis" isn't normal to the plane, you're working with something with at
least two axes of anisotropy.  (This would be a _much_ harder case to work with.
 Uniaxial birefringence is probably doable; trying to write it to handle two or
more axes will result in programmers chasing you with hatchets. :-D)

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