Raster scan and random scan are two types of display systems that use CRT monitors. These are used to project or exhibit softcopy information in the form of alphanumeric or graphic symbology. The information that is displayed using these two techniques is not permanent which is why it is called softcopy information. All the information that is presented in graphical form can only be viewed as long as it is there on the display screen which is the CRT monitor.
Raster scan is based upon television technology which uses an electron beam that is swept across the screen and it creates a pattern of illuminated spots. In the case of random scan the electron beam is directed to only those parts of the screen where the picture is to be drawn. In random scan, one line of the picture is drawn at a time which is why it is also called a vector display. The display in random scan is basically a computer controlled oscilloscope.
For a layman, raster scan and random scan can be described in a very simple way of using a pencil to draw something on a screen. The first way would be to lift and lower the pencil and draw anything on the screen. This is a tedious procedure and looks old now.
Another way is to draw many parallel lines across the screen and using pressure the intensity can be varied to present different shades and thus achieving the graphical represent that you require on the CRT monitor. This makes it easy to draw horizontal and vertical lines at the same time and is called raster scan.
However, neither of the two types is being used these days as a new advanced method of individual pixels has been developed that can be used to turn on and off independently to emit and absorb light.
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Differences between raster and random scan • Raster scan system has less resolution while random display has higher resolution • While the lines produced in raster scan are zigzag, they are smooth in random scan as electron beam directly follows the line path • While it is difficult to achieve realism in random scan, a high degree of realism is achieved in raster scan with the help of advanced shading and hidden surface technique • While random scan is costlier, decreasing memory cost in raster scan has made it popular • Electron beam is swept across the screen, one row at a time from top to bottom in case of raster scan in the case of raster scan, while in the case of random scan electron beam directly to those parts of the screen where picture needs to be drawn • Random scan draws component lines at the rate of 30-60 times each second, while refreshing on raster scan is carried out at a rate of 60-80 frames per second. |