Please,
Why renderID pass isn’t antialised ?
Because it is an integer channel and anti-aliasing does not really make much sense in that context. If you want pixel-based mattes you should go for MultiMatte elements.
Cheers,
Thorsten
What you could do, after picking the color your want from the renderID to get a quick mask for an object, is to multiply the Coverage pass on top of it, to add some “AA” to it.
It wont be pixel perfect like a multimatte, but it often works well enough for quick and dirty masks. Atleast for look dev compositing, before setting up proper mattes.