Capture opacity in beauty render

[GodToldMeToDoIt]

How can I set up Arnold to capture the opacity in a render?

I've got two renders to comp, background and foreground.

The forground has an Atmosphere Volume with a noise texture feeding the volumes RGB Density channel.

This is what I need:

... but this is what I get when I save to any format:

[hamsterHamster]

Can you be more specific, what is the issue you refer to? It is hard to tell once you place such faint alpha over checker in small resolution.

In the beauty render I see some kind of fog/smoke, which might be in alpha too. I'd either render atmosphere in a separate pass, or tershold-key out semitransparent alpha parts in comp software, maybe some premult tricks.

[GodToldMeToDoIt]

Sorry about that.  Maybe this'll help.

Here's the background:

Due to motion blur applied to the stars I have to crank the sampling to clean it up.

Here's the foreground:

Quarter of the samples.

Here's how I want things to look (this scene's far from finished):

I'm a noob to all this.  Started playing around at the start of the pandemic, so if I'm not describing stuff well, it's probably cause I don't know the proper terminology.

Maybe I'm misunderstanding or over estimating the alpha channels uses.

I just want to comp the foreground over the background so you can see the star field through the smog, without having to render it all in one pass with high enough samples to clean up the motion blur, cause even with adaptive sampling that'll take a very long time on my PC.

I'm using GIMP by the by, so may have something to do with it.

[GodToldMeToDoIt]

This is what I always end up with:

[hamsterHamster]

Is this a still, or an animation? For single still you have more flexibility to paint some things manually, as for sequence you'd need some sophisticated approach to automate.

In any way, it seems that in your final image you get the fog/cloud part completely ignored. I'd use existing alpha/matte of the skyline inverted and mask out the stars, then compose those masked stars over the smokey buildings with something like Overlay/Lighten/etc, see what fits better your reference image.

I can't tell anything about how Gimp works, haven't touched that.