So I can't figure out how, or if it's possible, to have a matte object that reflects with an alpha. Useful especially when your reflection is not additive, like comping an object and it's reflection onto a SHINY floor, where it's reflection is actually occluding light coming in the window.
I managed to turn on Indirect Specular in the shadow/matte map which gives me a correct reflection in my matte object, but the alpha is not solid, it's only a greyscale of the reflection colours.
One solution is to do a pass where the reflecting object is solid white, and then use this as the reflection alpha...but I'd rather set this up to work in one render pass (with whatever AOV's are necessary) instead of rendering twice.
BTW I'm using Max 2021 and arnold 220.127.116.11 and maxtoa 18.104.22.168