Volumetric Clouds

Last summer I did some experiments with an unusual way of rendering volumetric clouds. The usual way to render volumetric clouds is with a bunch of quads/voxels/slices. But this approach is much simpler, and at least for the exterior, gives really good results.

The problem came when rendering multiple clouds in a field. Due to the way the cloud is rendered, overlapping clouds would produce artifacts, and short of rendering each cloud by itself I couldn't figure out how to eliminate the artifacts. But then again, I didn't spend too much time on them either.

The general approach is as follows:

• Draw your scene as usual
• Then render a low poly cloud mesh - this could be created in Max/Maya with a group of spheres/ellipsoids in the shape of a cloud
• Copy the backbuffer to a render target, and clear the alpha channel
• Render the clouds with full alpha and your lighting of choice (I only implemented simple phong shading, a better lighting algorithm would have helped). We'll call this the cloud map.
• Blur the cloud map using a Gaussian filter, or another if you like.
• After blurring we need to distort the blurred cloud map. To do this, we place a quad at the center of each cloud and billboard each vertex to the camera (make sure the billboard covers the entire cloud from any angle).
• Now we render this quad to distort the cloud map. In the pixel shader for the quad we use the projected position as the texture coordinates and shift the texture coordinates based on the angles to the X and Y axis. We then sample a 2-channel fractal/noise texture with these texture coordinates to obtain our distortion offset. Next, we sample the blurred cloud map using the texture coordinates distorted by the distortion offset and the distance from the quad to the camera:
• float4 distortedColor = tex2D(BlurredCloudSampler, texC + offset/ dist);
• Optionally, after we have distorted our cloud map, we can perform a radial blur for a softer look.
  
• Finally we merge our render target with the back buffer.

A visual representation of the process:


A really simple algorithm that produces a pretty good result. And as you can see, it is essentially a post processing technique. However, it seems that this approach is more suitable to large volcanic or nuclear plumes rather than a dense field of many cumulus clouds (e.g. a large cloud model with many mega particles, as can be seen in the author's images).

My experiment:



Author's images:




There's an article on this technique in Shader X5 and you can find the slides here. If you're thinking of trying this technique out, you really don't need the book. The slides are more than you need to implement it. The only detail is that you don't actually use a fractal cube like the slides say, you really just use a billboarded quad.