One tool that is very useful in light and comp pipeline is creating mattes for diverse tasks later in comp, sometimes there is a need to do specific mattes for certain objects or do a lot of mattes at once.
One tool we build with my friend and now sup in Bardel (Mike Tran), was the multi-matte to manage in a fast way this task. It began as a contest who can make the best matte tool for Vray and at the end we fusion both in this one :)
This tool will work using Vray render engine only.
There are diverse ways to generate mattes with this tool, but all will at the end create passes in the layer that you are with the names "Multi_Matte_A" (A-B-C to H) depending the amount of objects you select.
Auto MultiMatte is the automatic button, just select a bunch of objects or groups, for "each" selection will create a pass to the total of 24 passes. so maximum of 24 objects.
Take in mind that ObjectID is used to tag the proper pass later on, but is parent-child hierarchy driven, so if there an object or a group that has already assigned an ObjectID on top of your selection, that object rules.
Sometimes this could create a mess specially if you didn't model/layout your sets, the next button "remove ObjectID form Geo" will remove it or set to "0", if don't work you will need to remove it manually (reference files do this).
The next tab is to manually assign mattes to objects, to passes from A to H, is very straightforward, select the object or objects then select the pass name you want to assign it and color. done.
The pass will be created automatically if is not present.
Finally the grid matte
Is for fast assign mattes to objects, just select the objects and press any color of the pass you want to be assigned, for simplicity there is A-B-C Multi_Matte passes.
This tool was very useful in the pipeline of some shows I work on last year.
(this is a Python script)