Mark Spindler demonstrates another of his Gizmo creations for Nuke called Position to Mask which creates a spherical mask in 3D space perfect when rendering with a position pass in Nuke. MArk notes that this tool was inspired by a fxguidetv episode about District 9, in which Shervin Shoghian from Image Engine shows a similar tool that was developed for this film.
Mark Spindler writes: This is a short tutorial on my Position to Mask gizmo for Nuke. If you would like to download the gizmo, come and visit my website