Dumpduck post on forums.thefoundry.co.uk:
Hi, I'm working with footage of an eye and I'm trying to dilate the pupil ever so slightly. The challenge for me is getting the iris to look as it is contracting accordingly. By rendering out a normal shader from Maya (one channel from top to bottom, one from right to left and one with facing ratio) and using it with an IDistort (same method as this guy fakes refraction in Nuke: vimeo.com, I've managed to push the pixels around the pupil. It works to some extent, but doesn't seem like a good way of achieving this.
Good problem solving.