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 known as a vector display (vector displays), a stroke display (stroke-writing display), or a handwriting display (callgraphic display). The composition lines of the graph are drawn and refreshed in any given order by the random scanning system (Figure 2.9). Pen plotters also work in a similar way, which is an example of the most taboo scanning and hard copy devices. [Figure 2.9] The flush rate of the random scan system depends on the number of lines displayed. At this point the definition of the graph is to search for a set of underlined commands called refreshing the display file store. The refresh display file is called the display list, the display file is refreshed (the refresh displays files), the vector file (display program) is displayed. 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 20~60 times per second. The high-performance vector system can handle about 100,000 multiple 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 line drawing applications, such as architectural and engineering layout diagrams, and it cannot display realistic shadow 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 creating a smooth line. 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" 2.1.3 Random scan display
