I'd say the easiest way currently would be to use an image-based mask/density shader and through-camera projection. You have to decide whether you want to create the mask for a specific angle, or try to create a decent mask that will work from above.
If you do it from your camera position just render a low detail version of your scene then open it in an image editor and paint white where you want the clouds to be, black everywhere else. Then load your image mask with an Image Map shader, attach that to your Density Shader input on the cloud layer (or better yet, for some realistic randomness, to the Blend shader input of your existing Density Shader), select Through Camera for projectiont ype, specify your main camera as the projection camera, and then try a test render. Depending on whethere there were already clouds there it might work. You may have to shift the existing Density Shader toward white quite a bit to get enough coverage. The realism of this will depend largely on the quality and shape of your mask, so ideally you'd use something like a cloud photo source to define the edges. You could even make a mask out of an existing real-life volcano eruption photo if you wanted to.
The alternative is to try to do a mask from above. I'm not sure how well this would work (I'm honestly not sure how well the other approach would work either, hehe), but it may be worth a try. Essentially you'd use the same technique as above except you'd make a 2nd camera and position it over your volcano, then make your render, and then in creating the mask make more like a circle over the point you want the clouds to cover. The rest is more or less the same.
Give that a shot and let us know what comes out. It'll probably start out pretty ugly but it might be workable. Just for some example I did manage to render some clouds based on a mask I created by adjusting levels in a photo of clouds and they looked pretty decent. But it's more problematic with really 3D clouds; mine were more like cirrus.
We do hope to provide better explicit cloud manipulation tools in the future which should make stuff like this easier. A classic particle-based system would probably be ideal but I don't think we'll manage to get a full particle system into TG2. Maybe TG3.
- Oshyan
