I’m very excited to see that 2 volume passes have been implemented in master.
However I have one question:
Shouldn’t there also be a volume alpha map so we could use these passes to build the final image back ?
I’ve been making my own volume pass with this trick I came up with and so far it seems like this newly implemented pass doesn’t give a result as complete and usable as the workaround I use.
However I’m eager to try the 2.8 override system to make my trick entirely dynamic !
Ok sorry, I found how to properly build back the final image from the passes so, nevermind my question.
I didn’t think about the fact that absorption is not something you put on top of the “surface” image, it’s simply an absence of light on the surfaces so you can’t extract it in a pass (which is sad because it could’ve given more control for post-production).
Yes, the Cycles render passes are additive. Alpha blending does not work in general because there can be volumes behind transparent surfaces for example, or within the same pixel due to antialiasing.