In a raster scan display, the screen is divided into a grid of pixels that are scanned line by line from top to bottom. Each pixel is either on or off, controlled by values stored in a frame buffer. The electron beam scans across each line from left to right, then returns to the left side to draw the next line, in a process called horizontal retrace. After completing the frame, the beam returns to the top left corner for the next frame during vertical retrace. Interlacing displays every other line to reduce flicker.

1. Raster: A rectangular array of points or
dots.
Pixel (Pel): One dot or picture element of
the raster.
Scan Line: A row of pixels.
Frame: The image to be scanned out on
the CRT.

3.  In raster scan approach, the viewing
screen is divided into a large number of
discrete
phosphor picture elements, called pixels.
 In Raster scan display the electron beam
is swept across the screen one row at a
time from top to bottom.

5. ◦ Picture elements: screen point referred as
“Pixel”.
◦ Picture information stored in refresh (frame)
buffer.

6. •Beam intensity is turned on and off to create a
pattern.
•Frame buffer or refresh buffer holds the set of
all intensity values for all pixels.
•Stored intensity values are then retrieved
from the refresh buffer and painted on the
screen one row at a time.

7.  In simple black-and-white system, each pixel is
either on or off, so only one bit per pixel is needed
to control the intensity of screen positions.
 Buffer with 1 bit per pixel – Bitmap.
 Buffer with multiple bits per pixel –
Pixmap.

8. • Horizontal Retrace
 At the end of each scan line , the electron beam
returns to the left side of the screen to begin
displaying the next scan line. The return to the left
of the screen , after refreshing each scan line is
called horizontal retrace of electron beam.
• Vertical retrace
 At the end of each frame the electron beam
returns to the top left corner of the screen to begin
the next frame.
 Refresing rate->60 to 80 frame per second.

11. •A special purpose processor called video
controller is used to control the operation of the
display device.
• Scan lines are labelled from ymax at the top of
the screen to 0 at the bottom and screen pixel
positions from 0 to xmax

12. 0 xmax
ymax

13. • Advantages and Disadvantages of raster scan
system.
• Advantages
– Refresh rate independent of image complexity.
– Can handle colour and filled areas.
• Disadvantages
– Uses more memory

14.  Interlacing is a method of encoding a bitmap
image such that a person who has partially
received it sees a degraded copy of the entire
image.
 To reduce flicker, divide frame into two fields—
one consisting of the even scan lines and the
other of the odd scan lines.
 Even and odd fields are scanned out alternately to
produce an interlaced image.