Is it possible to remove somehow an atmosphere from the alpha ?

[KirillK]

Without doing a second render.  I can't reach a natural look of clouds and repaint them by hand a lot.  A separate clear alpha for a cloud layer only would be very helpful.  Or maybe there is a way to speed up the second alpha render? Also it would be good to have unpremultiplied result with alpha. I see some nodes which probably could be used for that but can't find how to connect them to an output image.

[Matt]

Without doing a second render.  I can't reach a natural look of clouds and repaint them by hand a lot.  A separate clear alpha for a cloud layer only would be very helpful.  Or maybe there is a way to speed up the second alpha render?

The only way to render an alpha of just the clouds is to render without the atmosphere. The alpha of the atmosphere is tied to its opacity I'm afraid. However, this might be something we can do as we look at multi-layered output in future.

Also it would be good to have unpremultiplied result with alpha. I see some nodes which probably could be used for that but can't find how to connect them to an output image.

I don't think there's any way you could do this with the nodes - we'd probably have to do a final unpremultiply step on the image. I suspect doing it on the final anti-aliased image might be the only way to have it match the anti-aliased alpha, but maybe unpremultiplied samples might work, I'm not sure. ("Premultiplying" is sort of inherent in the volumetric integration, but there might be a way to cancel it out.) Can you unpremultiply in post? I know that doesn't always work very well due to numerical imprecision, but isn't it qualitatively equivalent to what you're asking for?

Matt

[KirillK]

Thanks for the reply. The multi-layered output would be really great for composing with real pictures. The necessity  to render several times to get clouds on separate layers looks  sometimes longer than just paint everything by hand from a beginning.

Unpremultipling is possible  in composing programs with exr saved output.  The problem is  that those programs are quite expensive and not always necessary (in gamedev for example). Would be nice to just have something similar like a check-box from Maya and 3ds max where you  can get the non-premultiplied tga or png  with alpha against of a black background. There is a special background switcher shader in Mental Ray that allow to render against black background( without premultipling) while having all the influence from the sky on. Would be nice also.