clinch

Hi. I'm wondering if this sort of thing is possible, and how it is achieved:

I would like to create a 3D image in Houdini, say a coffee cup, and export it for use in a web site. If the steam coming off the cup of coffee is transparent in Houdini, is there a way to render the scene with an alpha channel that can then be imported into Adobe Photoshop and placed as a new layer.

I want the steam of the cup to be transparent in photoshop so that the background layers show through. I hope this makes sense, it was the best example I could think of. This would also help for renders that use depth of field and I need a blurry, antialiased edge in photoshop.

If there's a tutorial describing this type of thing, please post the link. I'm sure it has a technical name that I'm not familiar with.