frame rates

Film takes advantage of persistence of vision by providing limited samples of live action movement, in essense a series of still photographs run consecutively, letting the mind fill in the gaps to create the illusion of smooth motion. The receptors in the eye continually sample light in the environment. The only limitation on motion detection is the reaction time of those sensors and on certain mechanical limitations such as blinking and tracking. If an object moves fast enough, then the receptors in the eye will not be able to respond fast enough for the brain to distinguish a sharply defined, individual detail; motion blur results.

In either film or video, a sequence of images is recorded which can be played back at rates fast enough to fool the eye into interpreting them as continuous motion. Of course, in order to save resources, this rate is kept as low as possible while still maintaining the persistence of motion. Under some viewing conditions such as room lighting and viewing distance, the rate at which single images must be played back in order to maintain the perception of motion varies. The image is said to flicker when the perception of continuous motion fails to be created. The object appears as a rapid sequence of still images to the eye-brain.

There are actually two rates that are of concern. One is the number of images per second that are displayed in the viewing process. The other, is the number of different images that occur per second. The former is the playback rate; the latter is the sampling rate or update rate. For example, images are always played back at 30 images per second on a TV, but in some Saturday morning cartoons there may be only six different images per second with each image repeated five times.

Animation mimics live action footage by changing images quickly enough to also take advantage of persistence of vision, also giving the illusion of movement. This is very important. If your images change too slowly, they will no longer appear to move. Below a certain frames per second (f.p.s) threshold (which varies somewhat with the conditions of the image), the illusion of motion stops, and all you have is a slide show.

Media vary in how many images they have per second:

• 8mm and Super 8mm have 18 f.p.s.;
• larger film stocks (16mm, 35mm, etc.) generally run at 24 f.p.s. (old silent films, though 16mm, were usually shot at 18 f.p.s. speed, which is why they appear comically fast when projected at 24 f.p.s.);
• video is 30 f.p.s. for NTSC (North American Standard) and 25 f.p.s. for PAL (British and European).

With film, most animators will generally 'shoot on twos', meaning that they will take two frames for every change in image (always with exceptions: camera moves require shooting on ones, some painterly images can go longer than twos), so that most film animators will usually work in increments of 1/12 second (about 8/100). With N.A. video, you can get away with shooting on threes: 1/10 of a second is still acceptable and is the usual frame rate for GIF animations. Making your shots any slower than this can often destroy the illusion of movement. There are different conditions that require faster or allow slower times that will still give the illusion of movement.

To 'fudge' your movement, so you can get away with slower times, you can try the following:

• Decrease contrast along the moving edges, to reduce flicker;
• Draw a "motion blur' on succeeding frames, which acts as a bridge to the eye between two positions;
• Draw "motion lines" along the path of movement to help the eye follow the correct path--but be a little subtle here; keep some overlap between positions;
• Make your animated image relatively small; test out and correct for your timing for the best effect.