Random Scan Display

This device by using CRT directs the electron beam only to the parts of the screen where a picture is to be drawn. This kind of device draws only a line on the screen given by its two end points as illustrated in Figure. That is why these devices are referred like as vector displays (or calligraphic displays). In case of display device, this is not only an object which ought to be drawn but it is also important to refresh it before it disappears in a given period of time depending on the persistence. No frame buffer present for this kind of systems. While the object consists of too many lines, refreshing time might be longer than the persistence of the phosphor. This can lead to flickering of the picture on screen. In such systems, there can be a display processor that reads a display file. Display file consists of a sequence of graphics commands produced by the application program to refresh the screen. Advantages and disadvantages of the random scan display are as follows:

                                                          Figure 1.3 : Random Scan Display

Advantages

- Early computer displays: fundamentally an oscilloscope.

- Control X, Y along vertical/horizontal plate voltage.

-  Often utilized intensity as Z.

Disadvantages

-  Just by wire-frame, this is almost impossible to create images with shaded objects or areas filled with a given colour.

- In case of complex scenes flickering is visible.