# The Industry's Foundation for High Performance Graphics

1. ## volumetric clouds

Hello,

I am working on a program that has to render volumetric clouds with impostors (like in http://www.cs.unc.edu/~harrism/SkyWorks/).

So far, everything works fine expect one thing: the cloud itself, the arrangement of the particles that make up the cloud. I am looking for an algorithm that creates clouds out of particles. Does anyone know about that?

Thanks
Jan

2. ## Re: volumetric clouds

"except" not "expect"

3. ## Re: volumetric clouds

Hi,

You could generate the clouds as perlin noise, sample it on some regular grid and create particles where the noise function exceeds some isovalue. You can also vary the particle size, opacity etc. according to the noise function.

-Ilkka

4. ## Re: volumetric clouds

sounds interesting.. do you have an example and/or information page about perlin noise (never heard of that before)?

Jan

5. ## Re: volumetric clouds

Never heard of Perlin noise? Well, this one looks ok: http://freespace.virgin.net/hugo.eli...s/m_perlin.htm

-Ilkka

6. ## Re: volumetric clouds

I Guess i just didn't know the word as I am from germany. Of course i know weisses rauschen etc. But the url seems great, thanks .

Jan

8. ## Re: volumetric clouds

Why not looking at the real thing: http://www.kenperlin.com/

9. ## Re: volumetric clouds

Originally posted by JanHH:
I Guess i just didn't know the word as I am from germany. Of course i know weisses rauschen etc. But the url seems great, thanks .

Jan
Perlin noise is not the same as white noise. It's a type of noise generated in a very special way. This is done to ensure that it has certain properties, most importantly the 'bumps' in the noise all have roughly the same size. This is very important for compositing.

Though perlin noise is more or less random, it has a specific frequency. You can then compose multiple 'octaves' of perlin noise to achieve a desired spectrum. This is actually very similar to audio processing.

10. ## Re: volumetric clouds

Originally posted by Relic:
Why not looking at the real thing:
Nothing to do with the real thing. If the real thing is what you want, look out of your window