Sounds simple: How do I render an object so it appears in the beauty pass but does not contribute to the alpha channel? I am doing Pyro tests and I want to see smoke over a light background, but still be able to see the density of the smoke looking at its alpha.

The background is just a big sphere with a constant material. Turning the Opacity to zero makes the background object disappear, logically. So did attaching a matte node to the Constant Material. So how can I get something like the colored background object to appear in the render but not have an alpha associated with it?

(Yes, I know this would mess up a comp but these are tests…)

in your BG object shader connect Constant 0 to Af

That got it.

Initially I made the mistake of plugging the Constant (0) into the Alpha on the Material which just made the object invisible. It worked once I jumped down a level into the Constant shader Output VOP. Now it makes much more sense…

Many thanks!