This document provides information about different types of display devices used in computer graphics. It discusses cathode ray tube (CRT) displays, including how CRTs work using an electron gun and accelerating electrons to excite phosphors to emit light. It describes raster scan displays, which draw images as a grid of pixels by sweeping an electron beam across the screen, and random scan displays, which draw images line by line. The document also covers color CRT displays using beam penetration or a shadow mask to combine red, green, and blue phosphors at each pixel location.

1. Display Devices
Organized By: Vinay Arora
Assistant Professor, CSED
Thapar University, Patiala

2. Disclaimer
This is NOT A COPYRIGHT MATERIAL
Content has been taken mainly from the following books & websites:
Computer Graphics C Version By Donald Hearn, M. Pauline Baker
Schaum's outline of theory and problems of computer graphics By Zhigang Xiang, Roy A. Plastock
Computer Graphics: Principles and Practice By James D. Foley
http://en.wikipedia.org/wiki/Computer_graphics
http://www.howstuffworks.com/3dgraphics.htm
http://www.graphics.cornell.edu/online/tutorial/
http://www.cgarena.com/
etc…
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3. Display Devices
A Display Device is an Output device for presentation of Information
for Visual reception.
The Display Systems are often referred to as Video Monitor or Video
Display Unit (VDU).
Display Devices having CRT:-
DVST – Direct View Storage Tube
Random Scan
Raster Scan
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4. Computer Monitor - External View
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5. Computer Monitor - Internal View
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6. CRT
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7. Electron Gun
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8. Electron Gun
Heat is supplied to the cathode by the Filament.
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9. CRT - Operation of Electron Gun
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10. Positively Charged Metal (No Accelerating Anode)
A positively charged metal coating on the inside of the CRT envelope
near the phosphor screen.
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11. Electrostatic Deflection
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12. Magnetic Deflection
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13. Magnetic Coil
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14. CRT
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15. Brands of CRT
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16. Resolution
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17. Resolution
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18. Related Terms
Fluorescence – Is the light emitted as electrons lose their excess energy
while the Phosphor is being struck by electrons.
Phosphorescence – Is the light given off by the return of the relatively
more stable excited electrons to their unexcited state, once the electron
beam excitation is removed.
Persistence – Time from the removal of the excitation to the moment
when Phosphorescence has decayed to 10% of the Initial Light Output.
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19. Raster Scan Display
Raster Scan is the representation of images as a collection of pixels/dots
(A Rectangular array of Points or Dots)
Pixel: One Dot or Picture element of the Raster
Scan Line: A Row of Pixels
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20. Raster Scan Display (contd.)
In a Raster Scan System, the electron beam is swept across the screen,
one row at a time from top to bottom.
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21. Raster Scan Display (contd.)
Picture Definition is stored in a memory area called the Refresh Buffer
or Frame Buffer.
Refresh Buffer or Frame Buffer This memory area holds the set of
intensity values for all the screen points.
Stored Intensity values then retrieved from refresh buffer and “painted”
on the screen one row (scan line) at a time.
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22. Raster Scan Display (contd.)
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23. Raster Drawing
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24. Architecture - Raster System
In addition to the Central Processing Unit (CPU), a special processor,
called the Video Controller or Display Controller, is used to control the
operation of the Display Device.
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25. Architecture - Raster System (Conti…)
A fixed area of the system memory is reserved for the frame buffer, and
the video controller is given direct access to the frame buffer memory.
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26. Pixel Coordinate System
In a Pixel Coordinate System the Pixel at the Lower Left corner of
an image is considered to be at the origin (0,0).
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27. Cartesian Coordinates
Screen Positions, are referenced in Cartesian Coordinates.
Scan Lines are then labeled from ymax at the top of the screen to 0 at the
bottom. Along each scan line, screen pixel positions are labeled from 0
to xmax.
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28. Registers for Storing Coordinates
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29. Random Scan Display
In a Random Scan Display, a CRT has the electron beam directed only
to the parts of the screen where a picture is to be drawn.
Random Scan monitors draw a picture One Line at a time (Vector
Display, Stroke – Writing or Calligraphic Displays).
Refresh rate depends on the number of lines to be displayed.
Picture Definition is now stored as a Line-Drawing Commands an
area of memory referred to as refresh display file (display list).
To Display a Picture, the system cycle through the set of commands in
the display file, drawing each component line in turn.
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30. Random Scan Display
The component lines of a picture can be drawn and refreshed.
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31. Random Scan Drawing
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32. Architecture - Random System
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33. Random Scan Display
Random Scan Displays are designed to draw all the component lines of
a picture 30 to 60 times each second.
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34. Vector Drawing
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35. Random Scan Drawing
Random scan displays are designed for line drawing applications and
can not display realistic shaded scenes.
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36. View of Raster/Random
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37. Bitmap / Pixmap
A Black-and-White System: Each Screen Point is either ON or OFF,
so only one bit per pixel is needed to control the intensity of screen
positions.
On a Black-and-White System with one bit per pixel, the Frame Buffer
is called BITMAP.
For system with multiple bits per pixel, the Frame Buffer is called
PIXMAP.
Sometimes, refresh rates are described in unit of cycles per second, or
Hertz (HZ)
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38. Color CRT Monitors
A CRT monitor displays color pictures by using a combination of
phosphors that emit different COLOR lights.
Methods:-
1. Beam Penetration
2. Shadow Mask
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39. Beam Penetration Method
Used with Random Scan monitors.
Two layers of phosphor (red and green) are coated onto the inside of the
CRT screen.
The display color depends on how far the electron beam penetrates
into the phosphor layers.
The speed of the electrons, and the screen color at any point, is
controlled by the beam acceleration voltage.
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40. Beam Penetration Method (contd.)
Only 4 colors are possible (red, green, orange, and yellow).
Quality of pictures is not as good as with other methods.
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41. Shadow Mask Method
The color CRT has:-
Three color phosphor dots (red, green and blue) at each point on the
screen.
Three electron guns, each controlling the display of red, green and
blue light.
2 Methods for Implementation:-
1. Delta Method.
2. Inline Method.
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42. Shadow Mask Method (contd.)
Used in raster scan system (including color TV)
We obtain color variations by varying the intensity levels of the three
electron beam.
Designed as RGB monitors.
High quality raster graphics system have 24 bits per pixel in the frame
buffer (a Full Color System or a True Color System)
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43. Shadow Mask – Delta Method
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44. Delta Method
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45. Shadow Mask – Inline Method
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46. In Line Method
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47. Dot Pitch
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48. Flat Panel Displays
A class of video devices that have reduce volume and weight compared
to a CRT.
A significant feature of Flat Panel Displays is that they are thinner than
CRTs.
Current uses for Flat Panel Displays:
Small TV monitors
Calculators
Pocket video games
Laptop computers
Advertisement board in elevators
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49. Flat CRT
Electron beams are accelerated parallel to the screen, then deflected 90º
to the screen.
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50. Plasma Panel
A layer of gas (usually neon) is sandwiched between two glass plates.
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51. Plasma Panel (Conti…)
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52. Plasma Panel (Conti…)
By applying high voltage to a pair of horizontal & vertical conductors, a
small section of the gas at the intersection of the conductors break down
into glowing plasma of electrons and ions.
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53. Thin Film Electroluminescent
The region between the glass plates is filled with a phosphor, such as
zinc sulfide doped with manganese.
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54. Light Emitting Diode (LED)
A matrix of diodes is arranged to form the pixel positions in the display,
and picture definition is stored in a refresh buffer.
Information is read from the refreshed buffer and converted to voltage
levels that are applied to the diodes to produce the light patterns in the
display.
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55. Liquid Crystal Displays (LCD)
Used in small systems, such as calculators, laptop computers.
Produce a picture by passing polarized light (from the surrounding
or from an internal light source) through a liquid-crystal material that
can be aligned to either block or transmit the light.
Two glass plates, each containing a light polarizer at right angles to the
other plate, sandwich the liquid crystal materials.
Rows of horizontal transparent conductor & columns of vertical
conductors (put into glass plates)
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56. LCD Display
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57. Off State
To turn off the pixel, we apply a voltage to the two intersecting
conductor to align the molecules so that the light is not twisted.
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58. On State
Polarized light passing through the material is twisted so that it
will pass through the opposite polarizer.
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59. Vinay Arora
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60. Color Model
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61. The RGB Color Model
R, G, and B represent the colors produced by red, green and blue
phosphors, respectively.
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62. Color Depth, Bit Depth
The number of discrete intensities that the video card is capable of
generating for each color determines the maximum number of colors
that can be displayed.
The Number of Memory Bits required to store color information
(intensity values for all three primary color components) about a pixel is
called COLOR DEPTH or BIT DEPTH.
A minimum of one memory bit (color depth=1) is required to store
intensity value either 0 or 1 for every screen pixel.
If there are n pixels in an image a total of n bits memory used for storing
intensity values (in a pure black & white image)
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63. Bit Plane
The block of memory which stores (or is mapped with) intensity values
for each pixel (B& W image) is called a Bit Plane or Bitmap.
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64. 3 Bit Color Display
Color or gray levels can be achieved in the display using additional bit
planes.
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65. N Bit Planes & True Color
The result for n bits per pixel (color depth=n) is a collection of n bit
planes (2n colors or gray shades at every pixel)
For TRUE COLOR three bytes of information are used, one for each
of the red, blue and green signals that make a pixel.
A byte can hold 256 different values and so 256 intensities setting are
possible for each electron gun which mean each primary color can have
256 intensities (256*256*256 color possible)
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66. True Color
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67. Frame Buffer
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68. Look Up Table (LUT)
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69. Frame Buffer with LUT
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70. Frame Buffer with LUT (Conti…)
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71. General Terms
Modeling Coordinates
World Coordinates
Device Coordinates
Geometric Transformations
Viewing Transformations
GKS (Graphics Kernel System)
PHIGS/PHIGS+ (Programmer’s Hierarchical Interactive Graphics Standard)
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72. Transformation Sequence
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73. Horizontal/Vertical Retrace
Horizontal Retrace: The return to the left of the screen, after refreshing
each scan line.
Vertical Retrace: At the end of each frame the electron beam returns to
the top left corner of the screen to begin/start again.
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74. Interlacing
On some raster systems (TV), each frame is displays in two passes using
an interlaced refresh procedure.
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75. Interlacing - Example
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76. Aspect Ratio
The ASPECT RATIO of an image is the ratio of the width of the image
to its height, measured in Unit length or number of pixels. e.g. 2 x 2 inch
image and a 512 x 512 image would have an Aspect ratio of 1
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77. Resolution
Can be defined as the Number of Pixels per unit Length.
Normally Image Size is given as Total Number of pixels in Horizontal
direction times the Total number of pixels in Vertical direction.
e.g. 512 x 512, 1024 x 768 etc.
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78. Thnx…
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