Thanks Tim for your explanations. Sorry for waiting so much since your last
post... I made some more tests, I'm quite happy with the results but they
are some aspect of the behaviour of POV light in the atmosphere that I
don't understand. First, and since english is not my mother tongue, let me
fix some keywords...

SCATTERING: according to my understanding it corresponds the probability
that, withing a given volume of the media, a photon will change his
direction of flight. This probability is given by the Rayleigh function
(molecular scattering). It has nothing to do with refraction. It is (to my
understanding) the origin of the blue sky and the reddish sunset.

REFRACTION: this is the deviation of rays due to a change in the n index
(refraction index) according to the law of Snell-Descartes. Since this
deviation  is wavelength dependant it will split a real white light into
the whole spectrum (like a glass prism that separates a white ray into
different colors from red to violet). In povray, refraction is only
effective if we use photons. As pointed out by Tim it is the origin of the
bending of light in the atmosphere (since n, which is related to density,
is smoothly increasing).

As a first approximation (I think) we can neglect the refraction effects and
only consider scattering.

So if we simplify the atmosphere with 3 layers (according to the paper I
mentioned in a previous post) and if (by trials and errors) we adjust the
coefficients for scattering color, extinction and absorbtion, then we can
get something which is not too far from reality. I have some exemples here:
www.guidevtt.com/GPS/softs_povray.php. The only difference between these 4
images is the position on the light source.

O.K. but as I mentioned before I'm still confused. Why does the color of
light change that way in the media when varying the scattering and/or the
extinction coefficient. I have added new pictures
(http://www.guidevtt.com/Z_Povland/PovRay_atmosphere/SphereAtmosphere3.php)
with a huge set of media and varying thickness, scattering coefficient and
extinction. This set was very useful to improve my atmosphere... I don't
have the code in this computer but I will add the code tomorrow. As a
summary I would like to understand the colors of this set of images...
probably it has something to do with a clear understanding of the *rgb*
function.

As usual any information is welcome!

Well, enough for today...
[Reynald] - www.guidevtt.com

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