Persistence of vision

The idea of persistence of vision forms one of the basics of all kinds of film. The general idea is that, as the human eye can retain an image for a fraction of a second, what we see with our eyes is effectively a combination of both the present and a fraction of a second past. In traditional film-like media, what we perceive as fluid motion is actually a series of stills, or frames, being displayed at a high enough frame-rate (usually 24 frames per second, or FPS) in order to achieve the feeling of a smooth image, and thus persistence of vision. In animation, frames work a little differently: moving images and characters are "shot on twos", which means that one drawing is shot for two frames, effectively halving the frame-rate from, for example, 24fps to 12fps. However, some drawings are animated "on ones" when required to animate fast motion, which means one drawing per frame.

Above: An animated image comparing the film-standard 24fps (with and without motion blur added in post-processing) with a 60fps refresh rate.

Above: This horse is animated using the aforementioned "shot on twos" method, meaning it is being displayed at only 12fps, half the frame-rate of standard film. Compare this with the above 24fps image.