Persistence of vision
According to the theory of persistence of vision, the perceptual processes of the brain or the retina of the human eye retains an image for a split second. This theory supposedly accounts for the fact that when a motion picture flashes a series of progressive images, instead of the mind seeing the flashing of a series of images, it sees the illusion of motion.
In actuality, psychologists and physiologists have long ago abandoned this theory's applicability to film viewership, though film textbooks, film professors, and film theorists have largely not.
Persistence of vision should be compared with the related phenomena of beta movement and phi movement. A critical part of understanding these visual perception phenomena is that the eye is not a video camera: there is no "frame rate" or "scan rate" in the eye: instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience.
The frequency at which flicker becomes invisible is called the flicker fusion threshold, and is dependent on the level of illumination.
Through experience in the early days of film innovation, it was determined that a frame rate of less than 16 frames per second caused the mind to see flashing images. Audiences still interpret motion at rates as low as ten frames per second or slower (as in a flipbook), but the flicker caused by the shutter of a motion picture projector is distracting below the 16-frame threshold.
It is important to distinguish between the frame rate, and the flicker rate, which are not necessarily the same. In physical film systems, it is necessary to pull down the film frame, and this pull down needs to be obscured by a shutter to avoid the appearance of blurring, there needs to be at least one flicker per frame in film. To reduce the appearance of flicker, virtually all modern projector shutters are designed to add additional flicker periods, typically doubling the flicker rate to 48 Hz, which is less visible. (Some newer projector shutters even triple it to 72 Hz.)
In digital film systems, the raster scan rate may be decoupled from the image update rate. In some systems, such as the DLP system, there is no flying spot or raster scan at all, so there is no flicker other than that generated by the temporal aliasing of the film image capture.
The new film system MaxiVision 48 films at 48 frames per second, which, according to film critic Roger Ebert, offers even a flickerless tracking shot past picket fences. The flickerless shot is due to the higher sampling rate of the camera.
Again, with video, the flicker rate is not the same as the frame rate. Each complete frame is divided into two "video fields" of alternate lines, and the two fields are shown consecutively to make up a frame. Thus, the field rate of video is twice the frame rate.
Some modern video systems also decouple display from image update, for example systems using LCD monitors or intermediate frame buffers to increase the display rate.
In drawn animation, moving characters are often drawn "on twos", that is to say, one drawing for every two frames. However, even though the image update rate is so low, the fluidity is quite satisfactory for most subjects. However, when a character is required to perform a quick movement, it is usually necessary to revert to animating "on ones" as "twos" are too slow to convery the motion adequately. A blend of the two techniques keeps the eye fooled and the animation company in business!
- Persistence of Vision
- The Myth of Persistence of Vision Revisited - commentary on whether the concept is really a myth.
Persistence of vision is also:
- the name of a short story by John Varley, and the title of one of his later anthologies.
- the name of a demo/cracking crew involved in the Atari ST Demo Scene
- the name of a freely available ray tracing software package, also known as POV-Ray