"Jim Holsenback" <jho### [at] hotmailcom> wrote:
> "Bruno Cabasson" <bru### [at] alcatelaleniaspacefr> wrote in message
> news:web.4668773643579e8de5a817210@news.povray.org...
> > I am trying to find general pinciples for good skies, especially sunsets,
> > by
> > imitating phyical phenomena instead of faking them (eg use of a
> > earth-sized
> > spherical container with Rayleigh scattering of blue color with 'real'
> > density of the air particles, instead of a sky_sphere with a color_map).
> > As
> > the renders are quite long, I can only do 1 or 2 tries a day ...
>
> hmmmm ..... interesting. I'm working on an approach that involves the
> sky_sphere as that outer most layer (gradient blues) and concentric spheres
> with largely transparent portions. I've declared pigments (PinkClouds,
> DarkClouds, DarkerClouds) based on wrinkles pattern then textured the
> spheres using bozo and texture_map. I'm having moderate success varying the
> finish parameters from sphere to sphere to give it depth. I agree simple
> sky_sphere with a color_map falls short of the look I'm trying to achieve.
> I've avoided media solutions up until now because of the render time but
> your approach has me reconsidering that, as I've not yet achieved the look I
> want .... thanks for the nudge!

I think the use of media can be OK for some reasons:

  -) Generally, the sky is not the main subject of outdoor scenes (but
contributes a lot to the visual effect)
  -) If so, the artist might do an effort for patience to get what he wants,
if it is good quality and more or less automatic (through settings)
  -) I imagine that the skyscape can be rendered with a semi-spherical
camera in order to produce a sky-dome image, then map it onto a sphere, and
then find a trick to fake the lighting. I wonder anyway whether it can
easily be made suitable for radiosity and for the clouds to cast shadows on
the terrain.

My work so far tells me that when the sun is not too near from the horizon,
I don't need that many samples to get a near-photo result, and the render
time is acceptable. BUT for low elevations (I focus on sunsets), I
encounter sampling problems and perhaps numerical artifacts, probably due
to the small angles involved and the length of the path through the
container, and I need to increase the samples and/or intervals. However,
the colors I get are OK and quite realistic: the blue of the sky is
obtained by Rayleigh scattering and a density function for the atmosphere,
and the colors redden while the sun approaches the horizon. My main concern
by now is to get good cloud sampling at low/negative elevations.

Maybe your previous works with media can help and give information. I still
feel that I am not that far from a complete solution for skyscapes, but I
bang my head on the walls for low elevations until I find the trick (if
any): happy me if I find it, shame on me otherwise ...


   Bruno

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