Random Scan Display
When the CRT is used to randomly scan a display (Random-scan display), its electron beam only shows part of the graphic on the screen . The electron beam tracks the composition lines of the graph, creating a line chart. Thus, a random-scan display is also called a vector display , a stroke display (stroke-writing display), or a handwriting display (calligraphic display). The constituent lines of the graph the random scan system is drawn and refreshed in any given order (see Figure 2.9). Pen plotters also work in a similar way, which is an example of a randomly scanned, hard-copy device.
The refresh frequency of the random scan system depends on the number of lines displayed. The graph is defined as a paintings line command that is stored in a display file store called refresh. Refresh display file is called
display list ,
refresh display file ,
vector file , or
display program (Display program.). To display the specified graphic, the system periodically draws its constituent lines sequentially by a set of commands in the display file. When all the draw line commands have been processed, the system periodically returns to the first line of the list command. The random-scan display is designed to draw all the lines of the graph 30,160 times per second. the high-performance vector system can handle about 100 000 short lines in such a refresh rate. When the displayed lines are small, each refresh cycle is deferred to avoid a refresh rate of more than 60 frames per second. Otherwise, the line refreshes too quickly and may burn out the fluorescent layer.
The random scanning system is used for drawing line applications, such as architectural and engineering layout diagrams, and it does not display realistic shaded scenes. because the graphical definition is stored as a paintings line command rather than as a strength value for all screen points, the vector display generally has a higher resolution than the raster system. In addition, the CRT electron beam of the vector display line is drawn directly by the line path, thus producing smooth lines. In contrast, the raster system draws lines by displaying a set of discrete points,
thus creating jagged lines . However, the great flexibility of the grating system and the improved line drawing ability still eliminated the vector technology.
Computer graphics (i) video display equipment _3_ random scan display
