Simon de Vet wrote:
> 
> Christoph Hormann wrote:
> 
> >
> > 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.
> 

Your description of the clouds lighting seems perfectly right, but i don't agree
about the distance.  Clouds like the ones in your scene are about 3-6 km high. 
In a large scale movement animation or a fast motion film you could perfectly
see the 3d structure of those clouds.  

> 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.
> 

I have seen that one before and i like it too, but NTL, the lighting used there
is not very convincing for roundish clouds, because it only makes clouds lighter
according to their general distance to the sun, while each cloud should be
somewhat lighter on it's sun side and dark on the opposite side.  

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

I agree, especially for high cirrus clouds the plain texture approach seems most
efficient.  

> 
> 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.
> 

That already looks quite promising for some kind of near horizon cumulus
clouds.  It only will be quite difficult to achieve realism on both near the sun
and on the opposite side.  

Christoph

--
Christoph Hormann <chr### [at] gmxde>
Homepage: http://www.schunter.etc.tu-bs.de/~chris/

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