A neat feature I added to an animation program I was working on; on macs you can change the transparency of the application window so you can see through to a movie playback program positioned underneath.
In my case I added that option, and a few hot keys to execute unix commands that would send frame advance/frame reverse commands to the movie player, in addition to adding a new drawing. This way one could just roto the frames, hitting the ‘next frame’ button each time which would automatically advance the frame in the movie underneath, and create a new drawing in the animation program.
I found I could do this easily with Mac’s own “DVD Player”.. a simple shell script I called ‘dvdplayer-step’ would send the frame advance command to DVD Player, e.g.
osascript > /dev/null << EOF
tell app “DVD Player”
To add a feature in animation paper to allow the user to run external unix commands (might be useful for other purposes..!), just allow the user to define simple strings that get executed with system(), and attach them to a hotkey.