How to composite volumetrics render element.

[JoachimArt]

I'm working on a scene where the fog changes from bright to dark over time and I would like to composite the fog if possible by rendering the volumetric and component as render elements. But I have not found a better way to composite the fog than having the layer additive. the volumetric doesn't seem to have an alpha. How can I composite the volumetric and get it looking exactly the same as in the beauty pass no matter what color the fog has? Any tips?

[Avi]

I am not sure what do you mean by Volumetric does not have alpha. Are you doing something different than me?

[JoachimArt]

I am rendering the image and save it as exr with volumetrics and composite as layers. Is there a custom alpha rendered for volumetrics? In that case I have not found it. I am getting a volumetric pass/element which I can apply to the image as a additive. But then I cannot add a dark volumetric because it only apply it as additive.

[JoachimArt]

I am not sure what do you mean by Volumetric does not have alpha. Are you doing something different than me?

Didn't notice your video. thanks. But I don't mean that the render doesn't have an alpha. I mean the volumetric should have it's own alpha for compositing it back onto the image, if not you will not be able to change fog from bright to dark. But if the volumetric had a alpha multiplied with black, you would be able to make the fog exactly as in the beauty pass.   ---unless there's a way to composite volumetrics which I have not cought.