Coming from Mental Ray, I was used to do things in a different way.
I used to render my scene with regular opaque alpha for hero objects, and zero alpha for background (surface background, like a photography studio background).
But I can't find a way to do this with Arnold.
I know there is a "matte" option in the object's shape, but that forces the alpha channel to be zero, but makes the object completely black too, and that's not what I want.
I want the object to render as normal, but write black alpha channel.
Is that possible?.
Thanks in advance.