- There is no such thing as a perfect matte. Rotoscoping is an art form that takes into account the background image, the movement of the object, and the new elements to be composited in the background.
- Try to start your shape at its most complex point in time, where it will need the most control points.
- Break a complex shape into multiple simple shapes. If you are rotoscoping a humanoid form and an arm becomes visible, consider rotoscoping the arm as its own element, rather than adding extra points on the body that will serve no purpose when the arm is obscured.
- Imagine you are the animator who created the shot. What would your dope sheet look like? No matter the medium, whether CG, live action or otherwise, most movements are rarely linear. They normally move in arcs; they normally accelerate in and out of stopped positions. Try and understand the mechanics behind how things are moving in your shot. This will help you to minimize keyframes.
- Watch and study the shot before you start working. Where are the changes in directions? These will normally have keyframes. Where are the starts and stops? Are there camera moves that can be stabilized to make your work easier?
- Don’t be afraid to trash your work and start over. Beginning roto artists often make the mistake of trying to fix a flawed approach by adding more and more keyframes. Experienced roto artists learn to quickly identify an inferior approach and are unashamed to trash their work and start over, often many, many times. It is very difficult to get a good matte without a conscious effort to keep the keyframes to a minimum.
And from the same talent pool is a handy summary of tips in Confessions of a Roto Artist: Three Rules For Better Mattes (PDF) by Scott Stewart (now a movie director), who did the roto tape for the out-of-print Masters of Visual Effects:
While still missing key features, there's plenty of rotoscoping done in AE, and many good After Effects tutorials. Sean Kennedy takes you through Rotoscoping Tools in After Effects, and Pete O'Connell (now found under Nuke) has a tutorial movie Rotoscoping in AE and a DVD, Advanced Rotoscoping Techniques for Adobe After Effects, at Creative Cow. Also, there are several tutorials involving rotoscoping with Imagineer Mocha & Mocha Shape, some noted in AEP's More mocha tracking & roto, as well as several by Mathias Möhl using his AE script MochaImport. Digital Tutors has a course for CS4 too, Rotoscoping Techniques in After Effects.
Todd Kopriva has a few more tips (from Pete O'Connell) in his Rotoscoping in After Effects and in After Effects Help. And of course there were previous posts here at AEP, tagged roto.
Update: The Art of Roto: 2011 by Mike Seymour at Fxguide is a super update to previous surveys of rotoscoping. It includes an interview with Scott Squires, who created Commotion (a desktop roto/paint/com tool) while at ILM.