This is a Deep Position Mask Tool I created at M.A.R.K.13 Stuttgart and they were so friendly to share them with everybody.
The Gizmos are designed to work with Depth/Position channels and Deep the same way. So you can tweak and mask as you like with good performance with the Depth or Position renderchannels and only switch to Deep for rendering and nice edges.

The heart of the tool is a Deep Expression that creates Deep Position values with the help of the Camera.

Copy the .gizmo to your .nuke folder and add the following line to your menu.py

nuke.menu('Nodes').addCommand('Deep/M13_DeepTools/M13_DeepPositionMask', lambda: nuke.createNode( 'M13_DeepPositionMask' ))

thanks to Falk Hofmann, Silvio Schwarz and Holger Weiss

cheers,

Vincent