"on twos"
The artists made a bigger decision to break with the way most computer-animated motion is achieved. Usually movements are created by advancing the image — say, a character raising his arm — in each frame, 24 times per second. It’s called “animating on one’s.” The resulting motion is fluid and smooth, but it can look too regular, even stolid.
Having worked at Disney with the Oscar-winning animator Glen Keane (whose characters include Aladdin, Beast and Tarzan), Persichetti wanted to borrow ideas from hand-drawn techniques. In traditional animation, much of the movement is done “on two’s”: A new drawing is made or the image shifted every second frame. Using animation on two’s gave the artists more control over the speed and power of the movements. Much of the animation in classic Disney features and Warner Bros. cartoons was done on two’s.
Working on one’s and two’s let the artists vary the rhythms of movements. When a scared Miles dashes through a snowy forest, his run is animated on one’s to emphasize his speed. When he stumbles and falls, he rises on two’s as he slowly pushes against gravity to get back on his feet. And when he leaps from skyscraper to skyscraper, the animation crackles with an energy it might otherwise lack. The motions themselves become exciting to watch.
The animation also allows the filmmakers to stress dynamic poses that telegraph how Miles is leaping and spinning through Manhattan.
