render clouds as alpha mask

[zzu]

As titled, I'm still trying to learn the software, so any suggestion would be awesome. Also, my second question would be how to render out an object's shadow only? Guess all have to do with render passes . Thank you for your response.

[Oshyan]

Yes, all have to do with Render Passes. Please reference documentation here: http://planetside.co.uk/wiki/index.php?title=Render_Layers_and_Render_Elements
To render only the shadow of an object you would open the object's settings, go to the Rendering tab, then change "Render" from "Visible" to "Invisible", but keep Cast Shadows checked.

- Oshyan

[KyL]

Don't forget you can use the wonderful shadow catcher shader!

If you have several objects you can put them in a group and have the group invisible to camera in the render layer settings.
There is a similar topic here: http://www.planetside.co.uk/forums/index.php/topic,22424.0.html