View source for Raster

A '''raster''' [[display]] is one which produces a rectangular array of parallel, closely-spaced lines of output (the rasters). It takes as its input [[signal]] a scan across the screen in precisely the form needed to produce that output in real time, i.e. it consists of a sequence of horizontal line segments, arranged from the top to the bottom of the display (with a short, constant vertical distance between them).

After scanning across the display on one raster, the means for producing the display of each line segment must then return to the other edge where it started, during which time nothing is shown on the display (it is said to be 'blanked'); this is known as the 'horizontal retrace interval'. Likewise, when it gets to the bottom of the display, it must return to the top, which is called the 'vertical retrace'.

This technique for displaying an image was first used in televisions using [[cathode ray tube|CRTs]], where the [[electron]] beam was scanned across the screen which formed the front face of the CRT, with the input signal being instantaneously used to control the intensity (i.e. brightness) of the beam as it scanned. This method was later adopted by [[video terminal]]s and other [[video display]]s; a similar technique is used by [[laser printer]]s. The general form of the interface (a series of line segments) is used in other display technologies.

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[[Category: Device Basics]]