Volume material alpha channel

[Matej Ovsenek]

I'm working on an animation which involves some storm clouds being formed over a surface and here's my problem.

The volume material doesn't appear in the alpha channel of the final render, and I need it to for post process purposes. The whole thing is gonna be in a virtual studio, so I need to key it exactly. I can't even get alpha from the volumetrics render element - it's the same as the beauty alpha.
I have tried with overriding it with the corona compositing tag, doesn't help.
I have tried with material properties/advanced/alpha mode/ always white, doesn't work
I have tried the mask render element with appropriate material ID's and it didn't show up.

I need soft alpha on the edges, so the element can be quickly selected and cut from the background, so rerendering with default material doesn't help me much.

The refraction and translucency options weren't satisfactory, since they produce a surface on the cloud.

I also tried Keying in after effects, putting a solid color in the background via rayswitch directly visible, but I loose all the details on the edges of the cloud during the keying process.

Please, make it happen :)

[maru]

Sorry for the delay.
I just checked this and I can confirm that alpha does not show up for volumetric materials.
It's also the same in 3ds Max version. I think it's a serious issue, because right now it seems that there is no way to composite fog/clouds on a background.

[Przemo]

Don't know if that's the related issue, but while rendering with Corona (C4D, Mac) it seems that the render is not showing the opacity of the alpha channel of the texture at all (see-through elements stay black). Works well with the physical render but cannot figure it out with Corona.

-
P.

[Eddoron]

Have you tried the standard corona shader BUT only refraction and volume channels checked?
refr: IOR 1, full white, gloss 1
vol: same settings used as in your volume material

The look should be the same and it appears in the alpha render.

I agree that pure volume materials should also appear in alpha & selections.

[Matej Ovsenek]

Yes, I managed to do it like this and it worked fine. Rendered preety fast too. refr IOR 1, gloss 1, volume only.

[houska]

Glad you figured it yourself, good job! :-)

Just out of curiosity - is the TV programme available through a website somewhere?

[Matej Ovsenek]

Thanks houska, I don't think it's available anywhere freely.
Some segments from the news programme can be found on the website, www.24ur.com but to watch whole shows you'd need a paid subscription.