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> Alain<aze### [at] qwertyorg>  wrote:
>> Anyway, I find it dificult to aprehend the reason why, in this case, you
>> may need to use variable ior.
>>
> I'll take a stab in the dark (matter) and guess gravitational lensing.
>
> On a somewhat related note, I'm starting to wonder why scattering media seems to
> be the go-to media for visualizing fields.  I've had good luck applying the much
> less intensive emission media for this purpose to get a good look at some fairly
> gnarly fractal-generating functions.
>
>
>
>
>
Emissive media is visible by itself and is prety fast.
On a minus note, it don't self shadow. The result is that, if it gets 
very thick or have a very high density, it's effect can become to large.

Scattering media behave more like most medias you encounter everywhere. 
It requires some light source to become visible. It self shadow and 
absorbs the light that illuminate it. It's also the slowest kind.

Then, some peoples use a combination of emissive and absorbing media. 
The absorbing media will limit the contribution of the emissive one.

A few application of variable ior:
Gravitational lensing, can be simulated with a properly shaped lense.
Some substance having a non-uniform temperature or salinity. Heated air 
over a flame; fresh water "floating" on sea water; some plastics, mostly 
under stress; fiber optics...

The case of a magnet and it's field don't even come close to those 
cases. Progressive lenses definetly don't use that, they use a complex 
curvature.




Alain

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