I need masks for a project right now and I've come to the conclusion that Objects with Thin Walled SSS are receiving illumination from the mattes and/or are computing the subsurface effects, so the masks are unusable.
Once I turn thin walled off everything works as expected.
I've attached a screenshot showing how a matte looks with thin walled sss geometry in it.
I've attached a scenefile for you - I don't believe this is intentional behaviour. If it is, I think it would be nice to have something like a "Transmit AOV" checkbox for SSS. But as the standard SSS isn't showing this effect I think it might be a bug.
Could you please post me a screenshot of the issue in the shared scene? Only the camera rays are written to a custom AOV, no indirect, so it should look fine and I don't see any problems in your Multimatte AOV. Looks the same with or without Thin Walled enabled. Maybe I just overlook something.
OK, I've got it. It looks correct in the IPR, but not in the Picture Viewer. Which is odd. Seems like a bug, and reminds me of an issue with the object masks reported earlier. I'll take a look.
