Mask for light material

[jojorender]

I use a Corona sky w/ HDRi for lighting and a c4d-sky w/ light material -no emit light -tonemap shader sky image (Comp tag: only seen by cam, buffer 1). See attached. ** I need a mask for the c4d-sky image.
Double-checked all checkboxes…
I get the mask layer but no mask. What am I missing?

[jojorender]

Nothing? Nobody?

[burnin]

much seems unclear... what do you want to achieve?
care to share a scene?

[jojorender]

Basically the comp tag gets ignored on a sky object. Tried with corona sky and c4d sky.
When I put the same light mat and comp tag on a regular sphere I get a mask.
Is this a known bug? When I search the forum for “composition tag on sky object ignored” I get nothing.

[burnin]

I think it's by design... cuz well, sky isn't really an "object"

[jojorender]

Lol, that’s pretty bad design then.
Not arguing here with you, but hover with your mouse pointer over corona sky or c4d sky… both are “objects” and nothing is real…

So, there is no way to mask out a sky object? Seems pretty basic and sky replacement is a daily thing in archviz/ photo.
I always used the prehistoric technique of an upscaled sphere object with tons of unnecessary polys and phong tag.
I’m looking for a better way.   

[Nejc Kilar]

Have you tried unchecking the Sky (in your case the Light material) from affecting the Alpha? In that case you might be able to use the Alpha channel to mask our your sky. Alternatively, you can also set up an ID mask where the sky will always turn out black - you  can then try using the black color to mask things out.

[jojorender]

Hi Nejc,
inverted alpha, that could work. Will give it a try. Thanks.