Hi!
I need to combine some Vue metaclouds (the ones which got real geometry) with others pictures, so I need to have an alpha channel for this clouds, but I doesn’t find this in the render window…is there a way to got the alpha channel of metaclouds?
Thank
Christophe
Nope, I fear there is no way. Metaclouds are part of the atmosphere and can not be separated from that, in terms of masking.
The only way I can see is #1 use the old spere scheme with volumetric materials instead or #2 try to render the metaclouds in front of a neutral background and mask that background out in postwork.
I’ll have a try with the neutral background solution 
but I don’t know the sphere method? Are you speaking of taking a sphere and give it a cloud material?
I’ll have a try too by rendering the scene in external application, maybe it will understand the alpha channel?
Sphere method is the method we all had to use before metaclouds existed. You can see an example when you load an object exterior/nature/cloud (fourth from left in the top row) and exchange the material.
That you will have more alpha channel options in other applications I doubt heavily. But I can be wrong of course!
Yes the sphere method is working.
I’ve try to render in external applications but you’re right, no alpha channel available.
Thank you for you time Wabe
I’ve only been using Vue now for the day but the best way I’ve figured out is to do it just like a mask pass in say Mental Ray or Brazil.
Set the background to black with no specular. Turn off all atmospheric effects.
Renders your clouds as 100% self illuminated (aka ambience to the max). Save this out as your alpha.
Render again with a black fog. (Difference of this will be your atmosphere adjust mask)
Set your clouds back to their desired density and what not.
Render again.
Set the first render as your alpha and you have a pre-multiplied cloud layer with an alpha as if you had rendered it straight. And you have a fog pass.
Good tips!
I’ve already setup a black plane in Vue, no shadow of course, no diffuse, all black. and render the clouds like that. But the approximative “alpha” channel I get there was not really good, I think with you 100% self illuminated tips, it will be far better 
Oh one thing I forgot to add. You also need to uncheck “self shadow” when doing the alpha. Otherwise it still will shade the cloud.
Here are two clouds I did real quick on black and then put onto Fuchsia for demonstration purposes.
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