Raster and random scan displays
In Raster scan displays, whole screen is refreshed a number of times in a second to keep the picture visible on the screen. This is called refresh rate of the system. In random scan display line drawing commands are used to plot a graphics that is every graphics object is saved as a sequence of line segments. This is also refreshed a number of times in a second, but its refresh rate depends on the number of lines to be displayed. If the drawing is complex, the refresh rate is different from when the drawing is small and simple.