Raster Scan - Perception

Perception

Raster scan on CRTs produces both the impression of a steady image from a single scanning point (only one point is being drawn at a time) through several technical and psychological processes. These images then produce the impression of motion in largely the same way as film – a high enough frame rate of still images yields the impression of motion – though raster scans differ in a few respects, particularly interlacing.

Firstly, due to phosphor persistence, even though only one "pixel" is being drawn at a time (recall that on an analog display, "pixel" is ill-defined, as there are no fixed horizontal divisions; rather, there is a "flying spot"), by the time the whole screen has been painted, the initial pixel is still relatively illuminated. Its brightness will have dropped some, which can cause a perception of flicker. This is one reason for the use of interlacing – since only every other line is drawn in a single field of broadcast video, the bright newly-drawn lines interlaced with the somewhat dimmed older drawn lines create relatively more even illumination.

Second, by persistence of vision, the viewed image persists for a moment on the retina, and is perceived as relatively steady. By the related flicker fusion threshold, these pulsating pixels appear steady.

These perceptually steady still images are then pieced together to produce a moving picture, similar to a movie projector. However, one must bear in mind that in film projectors, the full image is projected at once (not in a raster scan), uninterlaced, based on a frame rate of 24 frames per second. By contrast, a raster scanned interlaced video produces an image 50 or 60 fields per second (a field being every other line, thus corresponding to a frame rate of 25 or 30 frames per second), with each field being drawn a pixel at a time, rather than the entire image at once. These both produce a video, but yield somewhat different perceptions or "feel".