Hi,

I worked again on my media-clouds and i think it looks
much better than my last tries...
I mainly tuned the parameters of the medias for atmosphere
and the clouds, such as the density-functions and sampling-
options. Most difficult for me was to get the lighting and
brightness right. In the first tests the images either
suffered from too strong brightness of the clouds which lead
to color-clipping and therefore plain white spots in the clouds
or the image was too dark. I tried to compensate that with kind
of 'LightCompression' which means that i scaled all lights in
the scene down to prevent the clipping and i tried to abuse
assumed gamma to get this compression. Both ideas weren't
too successful though, so i adjusted the density-values once
again. That was the right idea :)
A small part of the image still has some white-clipping but i'll
try to fix this, too.
I plan to improve the shape of the clouds and the atmosphere further
for more realism.
What gives me headaches at the moment is the colouring-aspect.
In nature, the sky color depends on the position of the sun, because
the different wavelengths get scattered differently, but as povray
doesn't simulates this effect i have to fake it somehow...
Perhaps someone has a good idea to achieve this or how to improve
this scene in another way.
Comments and Criticism are always welcome.

Greetings,
Thies

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Attachments:
Download 'CloudTest_v4_800x600.jpg' (48 KB)

Preview of image 'CloudTest_v4_800x600.jpg'

Thies:

Your clouds look awesome when viewed from below.  From
the side, they are a little less awesome.  Is that some sort of
vertical stacking that is visible?

Also, some of your clouds seem to be lit from below, which
I think is a little unusual, considering that the sun is typically
located above the Earth's atmosphere.

Otherwise, this is really impressive!  I hope to see some sort
of full landscape using these soon.

Aaron

Aaron Gillies
New York City
x3rxes[@]yahoo.com

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> the sun is typically
> located above the Earth's atmosphere.

Yeah... usually. =)

But It looks to me like the light is from above.

> Your clouds look awesome when viewed from below.  From
> the side, they are a little less awesome.  Is that some sort of
> vertical stacking that is visible?

He's using media sampling method 3, with a container that's probably just
the intersection of two planes. Since method 3 samples at regular intervals,
this makes the media look like it was made with the "stacked planes" method.

I suggest cutting the media containter into two or three separate
containers, giving the farther ones better sampling parameters than the
closer ones.

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

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Thies Heidecke wrote:
 >
> What gives me headaches at the moment is the colouring-aspect.
> In nature, the sky color depends on the position of the sun, because
> the different wavelengths get scattered differently, but as povray
> doesn't simulates this effect i have to fake it somehow...
> Perhaps someone has a good idea to achieve this or how to improve
> this scene in another way.

I suppose you could use several atmospheric media, each one attuned to a 
different RGB value to simulate a frequency-dependent effect.. but the 
scenes would be horribly slow to render.

ISTR that someone was working on frequency-dependent scattering a long 
while back, but I don't remember if the work was ever finished, and if 
so, it wasn't incorporated into the Megapatch or 3.5.

-Xplo

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"Aaron Gillies" <no### [at] spamcom> wrote in news:3d86040f$1@news.povray.org...
> Thies:
Hi,

> Your clouds look awesome when viewed from below.  From
> the side, they are a little less awesome.  Is that some sort of
> vertical stacking that is visible?
Thanks. I know what you refer to, it has to do with the media-
sampling parameters. The effect gets stronger in the distance
because the distance between entrypoint and exitpoint of the
media container gets longer and at the same time the density
changes more often.

> Also, some of your clouds seem to be lit from below, which
> I think is a little unusual, considering that the sun is typically
> located above the Earth's atmosphere.
Nope, the clouds are definitely lit from above, the arrow points
towards the position of the lightsource. I think you mean the clouds
middle-left in the image. These are so bright because they are near
the direction to the 'sun'. This effect is actually quite realistic,
i think.

> Otherwise, this is really impressive!  I hope to see some sort
> of full landscape using these soon.
Thanks very much,
i'll certainly use the clouds in future-scenes and publish the code
so that others can play with it, too.

> Aaron
Greetings,
Thies

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"Slime" <slm### [at] slimelandcom> wrote in news:3d860c17@news.povray.org...
> > the sun is typically
> > located above the Earth's atmosphere.
>
> Yeah... usually. =)
>
> But It looks to me like the light is from above.
>
> > Your clouds look awesome when viewed from below.  From
> > the side, they are a little less awesome.  Is that some sort of
> > vertical stacking that is visible?
>
> He's using media sampling method 3, with a container that's probably just
> the intersection of two planes. Since method 3 samples at regular intervals,
> this makes the media look like it was made with the "stacked planes" method.
Yep, you're right.

> I suggest cutting the media containter into two or three separate
> containers, giving the farther ones better sampling parameters than the
> closer ones.
That's a good idea, i'll try that out for the next image.

>  - Slime
Greetings,
Thies

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"Xplo Eristotle" <xpl### [at] infomagicnet> wrote in news:3d861bc0@news.povray.org
> Thies Heidecke wrote:
>  >
> > What gives me headaches at the moment is the colouring-aspect.
> > In nature, the sky color depends on the position of the sun, because
> > the different wavelengths get scattered differently, but as povray
> > doesn't simulates this effect i have to fake it somehow...
> > Perhaps someone has a good idea to achieve this or how to improve
> > this scene in another way.
>
> I suppose you could use several atmospheric media, each one attuned to a
> different RGB value to simulate a frequency-dependent effect.. but the
> scenes would be horribly slow to render.
hmm, thanks for the idea, that would be a way to do it, but i don't know
if i can realize this in the near future. I'll try to get some research-
papers about the topic and then decide if it's feasible for me.

> ISTR that someone was working on frequency-dependent scattering a long
> while back, but I don't remember if the work was ever finished, and if
> so, it wasn't incorporated into the Megapatch or 3.5.
That would be interesting! Any links perhaps?

> -Xplo
Greetings,
Thies

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Thies Heidecke wrote:
>>ISTR that someone was working on frequency-dependent scattering a long
>>while back, but I don't remember if the work was ever finished, and if
>>so, it wasn't incorporated into the Megapatch or 3.5.
> 
> That would be interesting! Any links perhaps?

I don't have a link, but if you search in povray.general you might run 
across a thread or two about it.

-Xplo

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