Christoph Hormann wrote:

> Simon de Vet wrote:
>
> > I am planning, however, to create some POV code to generate semi-realistic
> > clouds procedurally. In my experimentation, I have made some progress, and I
> > think it should be do-able.
> >
>
> But not with that rendering time :-)

I'm not so sure about this.

I've looked at other people's cloud techniques, and they all use 3-d to generate
3-d looking clouds. Some use media, or isosurfaces, or stacked planes, but all use
actuall objects.

When I look at the clouds around me, most are way too far away to look 3-d. 3-d
objects are usually shaded based on the angle of the incomming light (diffuse
shading) and on self shadowing. Clouds, on the other hand, have a bit of this, but
the vast majority of the shading comes from the extinction of light as it passes
through the water vapour.

I've been looking at clouds not as 3-d objects, but as 2-d objects, painted onto a
giant sphere. If treated this way, I think that most of the effects can be
generated with complex textures on a skysphere, which are very quick to render.

http://freespace.virgin.net/hugo.elias/models/m_main.htm has a good (award
winning) example of a pigment based cloud render.

In addition, pigments can generate clouds other than puffy cumulus, like cirrus,
stratus, and realistic haze.


I'm attaching a test image I made before I gave up on this technique, due to time
limitations. It's a very early image, with no pseudo-3-d effects, but it shows how
pigments can be used to create the shape of distant thunderclouds. It's all one
pigment, generated with the 'pigment control trick' and MegaPov's spline.


Simon

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