You're explanations of scattering and refraction are physically correct (as
far as I can tell), but POV-Ray can't, not even with photons, create the
physical scattering effect you describe.

POV-Ray will only bend a ray or a photon's direction once it hits a surface
with a refractive value. When using photons and dispersion, a white photon
will be split into several photons with colors set according to the
wavelength, but still *only* rgb-value, not real wavelengths. The dispersion
itself is also just dependant of the refractive value and the dispersion
value, the color of the light will have no influence.

Scattering in POV-Ray will just "brighten" the media where light rays or
photons pass through. Neither the rays, nor the photons will be bent along
different paths when going from dense to sparse regions, it's always a
straight path. The scattering media types only *simulate* this bending by
brightening the samples according to the angle they are being hit by light
rays and where the camera is positioned.

To your technique with three hulls, varying extinction and absorption
values: it should work with some tweaking and the correct use of different
scattering types (or different values for the rayleigh scattering) so that
one or the other hull is more predominant when the sun is at a certain
position. I think you'd have to resort more to experimentation and tweaking,
rather than much mathematical research. Judging by your posts, the needed
background knowledge is present, it's just a matter of implementation and
artistic merit.

Regards,
Tim

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

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