Christopher James Huff wrote:
>You really need to define your problem more clearly: what do you mean by
>"proper reflections"? What is your goal, a picture that looks like the
>interior of the tokamak, or an actual simulation? If the latter is the
>case, a custom raytracer will probably be needed.

OK, here's an attempt at a reasonable definition:

The plasma is optically thin (at visible wavelengths anyway), so I don't
need to worry about absorption.  Scattering is also negligible.

Imagine if you will two types of toroidally symmetric emission regions: the
first is a simple filled torus inside which the emission is constant (and
let's say its minor radius is much smaller than the major radius); the
second type is a hollow torus, of which only a thin shell at its surface
emits light (and in this case let's the minor radius is nearly as large as
the major radius).

Now, a camera looking into the scene will see a very different result from
these two distributions, and one can take advantage of the toroidal
symmetry to convert the 2D camera picture into a pretty good estimate of
the emissivity profile of the toroids in question.

Then consider an enclosure around the plasma.  This will reflect light into
the camera view which would not otherwise be there.  Some of the light
reaching the camera will have been multiply reflected - in some cases from
parts of the enclosure which aren't even in the camera's view.  Some of the
reflecting surfaces are made of graphite (weak, mostly diffuse reflection)
while others are made of machined stainless steel (fairly strong, mostly
specular but the machining causes anisotropy of the specular part, possibly
averaging out to something close to diffuse).  These reflections will make
it harder to determine the light distribution within the plasma from camera
pictures.

What I'm hoping to do is model the amount of reflected light, and then study
the effects of adding absorbers - chiefly within certain limited regions of
the camera view, but also away from the direct view to minimise multiple
reflections.

Now, I'm not expecting to be able to model the processes perfectly - for
example to accurately model the specular reflections from the stainless
steel you'd need to know the direction of the machining for every part of
the surface.  (And then add to that the fact that POV doesn't do specular
reflections!)  Nonetheless it seems reasonable to hope that POV could do
usefully close approximations to reality...

Are multiple reflections handled by the radiosity & emitting media code?  (I
mean multiple reflections from regular objects btw.)

>POV is currently only capable of handling diffuse reflections of light
>emitted by media, such as light bouncing off a sheet of paper, or media
>seen directly. Specular reflections, like light bouncing off a mirror,
>are not handled.

I've read the FAQ's on this before, but I must admit I'm still somewhat
puzzled.  A perfect mirror (unlike machined metal) should be trivial to
handle - the direction of the ray is simply reflected about the normal to
the mirror, after which you track it until it hits something else.  Is it
that people want to handle imperfect mirrors?  Or is it simply that the
angular density of the rays becomes an issue?
(On re-reading your paragraph above, I realise you meant specular
reflections of light emitted from media.  However, I understood from the
docs that specular reflections aren't really handled at all - correct?)

thanks for your feedback...
Neil

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