Hi! I'm trying to simulate a real sky, i.e. using only scattering,
absorbtion, media densities.... no sky sphere or any artifact of that kind!
But I'm not a specialist in the behaviour of light in the atmosphere... I
already noticed that it is not easy! If you have any information about how
to do it correctly, or if you already made some work in that field I would
be very happy to hear from you! Thanks in advance. [Reynald]

Post a reply to this message

I just found an amazing site about an hour ago - don't recall the URL, but
he uses media density files (.DF3) to make the clouds... AND there is a
link on the site page (page = "MakeClouds" macro) to another site for a
freeware utility to make DF3 files from a series of .TGA files. Just look
at the demo images on the MakeClouds sub-page to see, you will be amazed.

You want to search the povray site, or perhaps these newsgroups for "Gilles
Trans" (the name of the person / artist that owns the site). When you get
to the site click "resources" and go to "MakeClouds" macro. (I would
download / archive the MakeClouds HTML page - thats what I did - I
downloaded his .ZIPS, and jumped to that other link on the page for that
TGA to DF3 tool).

If you can't find "Gilles Trans", try a websearch for "MakeClouds" and
"macro".

Post a reply to this message

> I just found an amazing site about an hour ago

www.oyonale.com

No idea what it means, but I have it memorized =)

 - Slime
 [ http://www.slimeland.com/ ]

Post a reply to this message

Jeff wrote:
> I just found an amazing site about an hour ago - don't recall the URL, but
> he uses media density files (.DF3) to make the clouds... AND there is a
> link on the site page (page = "MakeClouds" macro) to another site for a
> freeware utility to make DF3 files from a series of .TGA files. Just look
> at the demo images on the MakeClouds sub-page to see, you will be amazed.
> 
> You want to search the povray site, or perhaps these newsgroups for "Gilles
> Trans" (the name of the person / artist that owns the site). When you get
> to the site click "resources" and go to "MakeClouds" macro. (I would
> download / archive the MakeClouds HTML page - thats what I did - I
> downloaded his .ZIPS, and jumped to that other link on the page for that
> TGA to DF3 tool).
> 
> If you can't find "Gilles Trans", try a websearch for "MakeClouds" and
> "macro".
> 
> 
> 
> 
He's been amazing people for a while now.  He comes from France though.

Post a reply to this message

> "MakeClouds"

Thanks for the link. These "cloudscapes" are really great! I've seen that
"MakeClouds" uses sky spheres for the blue or the sunset background. I
would like to find a set of media that would scatter and absorb light the
same way the real sky do. I'm quit sure (but I'm a new user!) that it is
possible to do it with povray. I don't care about rendering time...

Till now I can get a "pseudo-real" blue sky, and "pseudo-real" sunset but
I'm using different sets of media. I'd like to find THE set that would
render blue when the "sun" is placed along the normal to the landscape,
reddish when the sun is close to the horizon etc...

[Reynald] - www.guidevtt.com

Post a reply to this message

> Till now I can get a "pseudo-real" blue sky, and "pseudo-real" sunset but
> I'm using different sets of media. I'd like to find THE set that would
> render blue when the "sun" is placed along the normal to the landscape,
> reddish when the sun is close to the horizon etc...

That's not possible with a reasonable PC. You're asking for actual
light-scattering according to wavelengths, which POV-Ray doesn't do unless
you use photons, and using photons to light up the sky is... well... out of
reach with current standard desktop PCs.

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

Post a reply to this message

in news:418e2658$1@news.povray.org Tim Nikias wrote:

> You're asking for actual
> light-scattering according to wavelengths, which POV-Ray doesn't do
> unless you use photons, and using photons to light up the sky is...
> well... out of reach with current standard desktop PCs.
> 

Sky-POV has (simulates) wavelength dependent raleigh scattering:
http://www.geocities.com/SiliconValley/Program/9231/povray.html

Ingo

Post a reply to this message

"Tim Nikias" <JUSTTHELOWERCASE:timISNOTnikias(at)gmx.netWARE> wrote:
> That's not possible with a reasonable PC. You're asking for actual
> light-scattering according to wavelengths, which POV-Ray doesn't do unless
> you use photons, and using photons to light up the sky is... well... out of
> reach with current standard desktop PCs.

Yes, I was exaggerating a little bit when asking for a real sky... I
understand that any simulation suppose some limitations. Let's see if my
understanding of "Photons" is correct:

We assume our scene is made of an atmosphere a sphere (earth) and a light
source. In that case the only elements we lose when rendering without
photons are some contrasts (in the media and on the sphere) due to
refraction of light in the atmosphere. Isn't it?

So, for a first simulation of earth atmosphere we could forget refraction
effects. I think it is reasonable since as far as I know the main physical
phenomena that determine the color of the sky are scattering and
absorbtion, but not refraction.

Now we could simulate the atmosphere using a first media with Rayleigh
scattering (chosing a higher coefficient in the blue)... and a second media
that would simulate the effects (absorbtion and diffusion) of aerosols...
maybe we need a third one...!

Limitations of these simulations are "single scattering", very simplified
wavelenght dependencies of scattering and absorbtion as well as simplified
atmosphere structure.

Please tell me if I missed something. At that point I still think we can
obtain somthing which is not too far from the real sky? I'm working on it,
and I'm learning some more about Povray, thanks Tim for your very
instructive website.

[Reynald] - www.guidevtt.com

Post a reply to this message

> Please tell me if I missed something.

No, I think you concluded the details very nicely. As POV-Ray doesn't
simulate actual wavelength scattering, you could only simulate the effect by
using pre-colored media, of which the color might be calculated via some
algorithm which works out the color in regard to the sun's position or so.

As for doing that entirely using media, I only see the advantage of this if
you plan to actually place objects visibly behind the atmosphere. For
example, for a scene I once did I used media to create the blue color of the
sky, so that I might place a moon behind the atmosphere. Thus, the black
shadow of the moon would be tinted blue as well and I got the same effect as
in real life.
For the major part of scenes, I simply use some distant ground fog to
simulate the distance-haze along with a properly colored sky-sphere. Clouds
are layered in shells above the scene. I seldomly use scattering media for
the entire atmosphere, it's often a local effect which I achieve by using a
large enough container and fading the scattering to the edges (e.g. using
the "spherical" pattern as a multiplier, or the "planar" pattern to simulate
volumetric ground fog).

> I'm working on it,
> and I'm learning some more about Povray, thanks Tim for your very
> instructive website.

Always a pleasure seeing that someone has a use for it! :-)

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

Post a reply to this message

> No, I think you concluded the details very nicely. As POV-Ray doesn't
> simulate actual wavelength scattering, you could only simulate the effect
by
> using pre-colored media

Couldn't it be said that POV-Ray does support wavelength-dependant
scattering, but that it only works with three wavelengths (R,G,B), and you
must specify the scattering for each one manually?

 - Slime
 [ http://www.slimeland.com/ ]

Post a reply to this message