1. Display Techniques
 Raster Scan Displays
 Random Scan Displays
1.) Raster Scan Displays: In this, the
electron beam is swept across the screen,
one row at a time from top to bottom.
While moving the beam intensity is turned
on and off to create a pattern of illuminated
spots. Picture definition is stored in a
memory area called the refresh buffer or
frame buffer.

2. Raster-scan system displays

3. The capacity of storing information makes this
system well suited for realistic display of scenes.
The example of raster scan is Home televisions.
 For B&W TV systems, each screen point is either
on or off, and only 1bit/pixel is needed to control
the intensity. So it is called bilevel and the frame
buffer is called bitmap.
 For Color TV systems, up to 24 bits/pixel are
used. And it also required several megabytes of
storage for the frame buffer, which is known as
pixmap.

4. Refreshing on rater scan displays is carried
out at the rate of 60-80 frames/sec. Its units
are cycles/sec or Hertz.
Horizontal retrace of the electron beam is the
return of the beam to the left of the screen,
after refreshing each scan line.
Vertical retrace is that when electron beam
returns to the top left corner of the screen to
begin the next frame.

5. Interlacing in raster scan displays
In interlaced procedure each frame is
displayed in two passes.
In the first pass, the beam sweeps across
every other scan line from top to bottom.
And after Vertical retrace the remaining
where sweeps out.
The entire screen displayed in one-half
time and it will avoid flicker.

6. 2.) Random Scan displays: In this, a CRT has the
electron beam directed only to the parts of the
screen where a picture is to be drawn. It displays
picture one line at a time so it is called vector
displays, e.g of this is pen plotters.
 Picture definition is stored as a set of line
drawing commands in the memory area called
refresh display file or refresh buffer.
 Refresh rate depends upon no. of line to be
displayed and all the component lines are drawn
30-60 times per second.

7. Random-scan system displays

8. Difference b/w Raster & Random scan display:
Well suited for realistic Design for line drawing
display scenes. applications.
 Picture definition is Picture definition is
stored in frame buffer as a stored in refresh buffer as
set of intensity value for a set of line drawing
all screen points. instructions.
Low resolution High resolution
Produces jagged lines Produces smooth line
drawing