A pen plotter operates in a similar way and is an example of a randomscan, hard-copy device. When operated as a random-scan display unit, a CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays (or stroke-writing or calligraphic displays).
Here the electron gun of a CRT illuminate’s points and / or straight lines in any order. If we want a line connecting point A with point B on vector graphics display, we simply drive the beam reflection circuitry, which will cause beam to go directly from point A to point B. Refresh rate on a random-scan system depends on the number of lines to be displayed. Picture definition stored as a set of line drawing commands in an area of memory called “refresh display file” or also called as display list or display program or refresh buffer. To display a given picture, the system cycles through the set of commands in the display file, drawing each component line by line in turn. After all line drawing commands have been processed, the system cycles back to the first line drawing command in the list. And repeats the procedure of scan, display and retrace. This displays to draw all the component lines of picture 30 to 60 frames/second Random scan system is designed for line drawing applications; hence cannot display realistic shaded scenes. Vector displays produces smooth line drawings but raster produces jagged lines that are plotted points Random scan suitable for applications like engineering and scientific drawings Graphics patterns are displayed by directing the electron beam along the component lines of the picture A scene is then drawn one line at a time by positioning the beam to fill in the line between specified end points.