The document discusses different display technologies used for computer graphics, including cathode ray tubes (CRTs), liquid crystal displays (LCDs), and light emitting displays (LEDs). It focuses on CRTs and flat panel displays. CRTs use an electron gun to fire a beam onto a phosphor-coated screen, but have issues like large size, weight, and flickering. Flat panel displays like LCDs overcame these issues, providing clearer images while using less energy. The document explains raster scanning and pixel matrices used in displays to generate graphics from geometric information.

1. GRAPHICS DISPLAY Aisurya Kumar Nayak

2. INTRODUCTION
The Graphics Display is considered important because:
 Quality of image influences the perception of designs on CAD/CAM systems.
 It enables user to communicate with the displayed image by adding, deleting, blanking and
moving graphics entities on the display screen.
Various display technologies are available to choose from which are based
on the concept of converting the computer’s electrical signals, controlled by
digital information, into visible images at high speeds.
Some of the available technologies are:
 Cathode Ray Tube (CRT)
 Liquid Crystal Display (LCD)
 Light Emitting Display (LED)

3. CATHODE RAY TUBE CRTs could disappear in the
near future.

4. SCHEMATIC DIAGRAM
OF A CRT
Figure 1

5. CATHODE RAY TUBE (CRT)
Figure 1 shows a schematic diagram of a typical CRT.
The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a
source of electrons) and a fluorescent screen, with internal or external
means to accelerate and deflect the electron beam, used to create images in
the form of light emitted from the fluorescent screen. The image may
represent electrical waveforms (oscilloscope), pictures (television, computer
monitor), radar targets and others.

6. OVERVIEW
The CRT uses a vacuum tube that provides high vacuum and voltage.
An electron gun is used to fire a beam of electrons onto a colour phosphor
coating, in the back of the display surface, at very high speed.
The energy transfer from the electron to the phosphor due to impact, causes
it to illuminate and glow.
The electrons are generated by the electron gun, which contains the
cathode, and they are focused into a beam by the focusing unit.
The control of the beam’s direction & intensity is related to graphics
information generated in computer allows us to display meaningful & desired
graphics on the screen.

7. TYPES OF SCAN
TECHNOLOGY
The graphic display can be divided into following two types based on the
scan technology used to control the electron beam to generate graphics on
the screen:
 Random Scans
 Raster Scans
The word ‘random’ indicates that the screen is not scanned in a particular
order
In the raster scan system, the screen is scanned from left to right, top to
bottom all the time to display graphics, similar to the home TV scan system.
Figure 2 shows the two types of scans.

8. TYPES OF SCAN
TECHNOLOGY
Figure 2
CRT
screen
CRT
screen
Previous beam position 1
Current
beam
position
Beam
movement
(a) Random scan (b) Raster scan

9. SHORTCOMINGS OF CRT
A CRT is big because of the space required by the electron gun and its
focusing units.
It is heavy and suffers from flickers.
The actual viewable screen size is smaller.
It consumes too much of electricity because of high voltage that is required
by its tube.
Its single electron beam is prone to misfocus.
The displayed colours exhibit variations across the screen.
It may create harmful electromagnetic variations.
The quality of image display suffers from the curved screen due to reflection
& glare.

10. FLAT SCREEN CRT
Flat CRT is the last attempt to
survive competition from
LCDs

11. FLAT SCREEN CRT
Flat screen CRTs have flat screens.
The flat display screen makes the monitors inherently comfortable to look at
from wide viewing angles with the same bright, clean screen images
associated with typical flat panel displays.
A flat screen CRT catches fewer reflections.
It offers large viewable area than equivalently size curved CRT.
It is well suited for touchscreen applications.
Except for screen curvature, the flat screen CRTs suffer from most of the
problems of CRTs.
However it improves the image quality and reduces the screen glare.

12. ANALOG FLAT PANEL
DISPLAY
An analog flat panel has to
convert video signals twice,
not a good idea.

13. ADVANTAGES OVER CRT
Flat panel displays overcome all shortcomings of CRTs.
They eliminate distortions caused by the curved CRTs.
They provide a larger viewable screen area because the edges of the frame
do not overlap with the glass screen.
They use little desk space.
They provide clearer, brighter & crisper images than CRTs & are flicker free.
They emit less radiation & use less energy than CRTs.
They can display digital signals, thus eliminating the need to convert analog
signals to digital ones.

14. DISADVANTAGES
The screen’s soft surface is easily damageable.
They have no built-in speakers, poorer display in high speed action & fixed
native resolution.
Resetting the resolution of a flat panel degrades its image quality.
Key difference between CRT & LCD is that LCD is a ‘fixed-panel’ display.

15. APPLICATIONS
Flat panel displays are used in many devices such as
 Desktop display monitors
 Laptop
 Palm pilots
 Pocket PCs
 Personal digital assistants (PDA)
 Cell phones
 Wall-mounted screens etc

16. OVERVIEW
Flat panel displays do not use high voltage or high vacuum tubes.
Most commonly available LCD use TFT (thin film transistor) technology to
produce high quality images.
A liquid crystal consists of long rod-like molecules which, in their natural
state, arranges themselves with their long axes roughly parallel.
The molecules are almost transparent substances, exhibiting the properties
of both solid & liquid matter.
The property of solid matter makes the light passing through the molecules
follow the alignment of the solid molecules.
The property of liquid molecules change their molecular alignment when they
are charged with electricity which changes the way light passes through
them.

17. SCHEMATIC DIAGRAM OF A FLAT
PANEL DISPLAY
Figure 3
Screen
Voltage
Vertical
polarizing filter
Horizontal
polarizing filter
Horizontal-
grooves glass
panel
Vertical -grooves
glass panel
Backlight
Vertical
molecules
Twisted
molecules
Horizontal
molecules
Colour
filter
Liquid crystal
molecules

18. DIGITAL FLAT PANEL
DISPLAYS
This is the monitor from
heaven.

19. VIDEO INTERFACE
Computers are inherently digital & produce digital signals that displays
monitors use as input.
In analog monitors, the signals are converted via DAC (digital-to-analog
converter) into analog signals before monitor can display them.
The DAC is part of the monitor interface known as video interface.
There are two types of hardware interfaces:
 Analog video interface (AVI)
 Digital video interfaces (DVI)

20. PROBLEMS IN ANALOG
DISPLAYS
Analog flat panel displays use AVIs.
After a display receives the receives the analog signal from AVI, it must
convert it back to digital before it can display it.
The two conversions result in pixel-colour error.
Pixel-colour error is inherent in analog data, but analog displays can conceal
errors by blurring or blending the phosphor in CRTs or neighbouring pixels in
flat panel displays.

21. DIGITAL FLAT PANEL
DISPLAYS
Digital flat panel displays do not suffer from pixel-colour error because they
can use DVIs.
Maintaining the digital integrity of the signal from start to finish by DVIs
ensures that the digital-to-analog & analog-to-digital conversions do not
produce pixel-colour error or signal degredation.
Therefore, they provide crisp & sharp line edges & colour values that are true
to the original data at high resolutions.

22. RASTER DISPLAYS All types of monitors use a
pixel matrix

23. RASTER DISPLAYS
In raster displays, the display screen is divided horizontally and vertically into
matrix of small elements called picture elements or pixels.
A pixel is the smallest addressable area on the screen.
An N x M resolution defines a screen with N rows & M columns, where each
row defines a scan line.
The creation of raster-format data from geometric information is called scan
conversion or rasterization.
A rasterization process is needed in order to display graphic entities.
A rasterizer that forms the image creation system as shown in Figure 5 for a
CRT is mainly a set of scan-conversion algorithms.
N
M
Pixel
y
x
Figure 4

24. COLOUR RASTER DISPLAY WITH
EIGHT PLANES
Figure 5
Rasterizer
Image creation
system Cell
P
Pixel
values in
memory
(bit map)
Image refresh system
Image display system
Plane 1
Plane 8
Colour
map
CPU (host)
Input devices
R
G
B
Display
Processo
r
Deflectio
n and
colour
system
Input dataDisplay
commands
CR
T
Phosphor
pixels