Making things move--the options for computer-based displays.

Two methods for producing moving stimuli on computer-controlled displays are discussed. The first method, memory manipulation, can take several forms: moving a memory window over a fixed memory image (memory windowing); moving the memory associated with objects and background for such objects (memory movement); and changing all the memory within the image on every frame of the display (memory animation). Memory windowing and movement are suitable for low memory or low processing-capacity computer systems, but are limited in their range of temporal stimulus properties. Memory animation is the most flexible of all display techniques available but requires higher memory or processing capacity or both. The second method, palette-manipulation, is akin to being able to change the contents of a book simply by changing the page numbers in its index. Look-up-table splitting, look-up-table bit-splitting and look-up-table rotation are all suitable for low memory or low processing-capacity computing systems. Look-up-table splitting allows independent control of the motion of one or several objects but severely limits the spatio-temporal properties of those objects. Look-up-table bit-splitting gives much more flexible control of the spatio-temporal properties of the stimulus but is useful only for very brief stimulus presentations. Look-up-table rotation provides good control of stimulus temporal properties but limits the spatial properties of the stimuli. Look-up-table animation provides excellent spatial and temporal control of periodic stimuli but, unless stimuli are presented in spatio-temporal quadrature, requires high memory or processing capacity or both.