Raster scan displays use a sweeping electron beam to illuminate spots on the screen in a row-by-row fashion to generate an image. Intensity values for each pixel are stored in a refresh buffer and read out row-by-row to control the beam. Raster scan allows for photorealistic color images but requires high memory and has lower resolution than random scan displays. Random scan displays draw images by directing the electron beam only to locations with picture elements, tracing out lines specified by commands in a refresh buffer. This allows for higher resolution with less memory usage but limits images to simple drawings with few colors.

1. introduction

2.  Raster scan display is the most common type
of display used in CRT televisions and
monitors.
 In a raster-scan system, the electron beam is
swept across the screen, one row at a time
from top to bottom.
 As the beam is swept across the row, the
beam intensity is turned on and off to create
a pattern of illuminated spots.

3.  The picture definition that is to be drawn is
stored in a memory area called the refresh
buffer or frame buffer.
 The refresh buffer holds the set of intensity
values for all the screen points.
 Stored intensity values are the retrieved from
the refresh buffer and “painted” on the screen
one row at a time .

4.  In a simple black & white raster scan display,
the intensity value is either is only 1 bit per
pixel as it’s either on or off.
 But in color raster scan display up to 24 bits
per pixel are included in high-quality
systems.
 In a black and white raster scan display
system with 1 bit per pixel the frame buffer is
called “BITMAP”.
 Whereas is color system/multiple bit per pixel
system the frame buffer is called “PIXMAP”

5.  Refresh rate of raster scan display scan can
be 60 to 80 frames per second.
 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 which is
known as “horizontal retrace”
 At the end of each frame the electron beam
returns to the top left corner of the screen to
display new frame.
 This process is called “ vertical retrace”.

6.  We can display real life images which have
different shades.
 The color range available is way bigger than
random scan display.

7.  The resolution is comparatively lower than
random scan display.
 High memory is required.
 Data about intensities of all pixel over the
screen has to be stored rather than only the
area where picture is displayed.

8.  In random scan display the electron beam is
directed to the area on the screen only where
the picture is to be drawn.
 Random scan display are also known as
vector display or stroke-writing or
calligraphic display.
 The reason its also called vector display is
because random scan display draws the
picture one line at a time.

9.  Refresh rate on a random-scan system
depends on the number of lines to be
displayed.
 The picture definition is stored as a set of line
drawing commands in an area of memory
called the refresh display file.
 The refresh display file is also called the
display list or simply the refresh buffer.

10.  To display a specified picture, the system
cycles through the set of commands in the
display file, drawing each component line in
turn.
 After all line-drawing commands have been
processed, the system cycles back to the first
line command in the list.
 The refresh rate of random scan display is 30
to 60 frames per second (fps)

11.  For example if we want to display and
maintain a picture of a triangle ABC on the
screen then the display list will have
commands like this:
 Draw a line from A to B.
 Draw a line from B to C.
 Draw a line from C to A.

12.  Random scan display have higher resolution
compared to raster scan display as the
picture is stored as line drawing instructions
rather than intensity values.
 It produces smooth line drawing.
 It also requires less memory as we storing
information about only the picture to be
drawn rather than information about full
screen.

13.  We cannot draw realistic images like with
different shades
 There is limitation to the colors that can be
drawn for example when we draw images
with beam penetration display its actually
random scan display and we can draw only
maximum 4 colors which are based on how
deep the beam penetrates the phosphor
layer.