Overview-of-GraphicsSystem@2334444_5.ppt

Overview of Graphics System
Dr. V. Subba Ramaiah
Assistant Professor
Dept. of CSE
MGIT, Hyderabad
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Video Display Devices
 Primary output device : Video monitor (VM).
 Operation of most VM’s: Standard CRT design
CRT: Cathode-Ray tube.

Cathode-Ray Tube (CRT)
(cathode ray)
Fig. 1: Basic design of a magnetic-deflection CRT.
3

Basic operations of a CRT
Steps:
1. The electron gun emits a beam of electrons (cathode rays).
2. The electron beam passes through focusing and deflection
systems that direct it towards specified positions on the
phosphor-coated screen.
3. When the beam hits the screen, the phosphor emits a small
spot of light at each position contacted by the electron
beam.
4. Redraw the picture repeatedly by quickly directing the
electron beam back over the same screen points.
This type of display is called a refresh CRT.
4

Operation of an electron gun with an accelerating anode
Fig. 2: Operation of an electron gun with an accelerating anode
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Electron gun
1. primary components: Heated metal cathode and a control grid.
2. Heat is supplied to the cathode by directing a current through a coil of wire,
called Filament.
3. On the application of heat, all the negatively charged electrons are then
accelerated toward the phosphor coating by a high positive voltage.
4. The electron beam intensity is controlled by setting voltage levels on the
control grid..
5. A high negative voltage applied to the control grid will shut off the beam by
repelling electrons and stopping them from passing through the small hole
at the end of the control grid structure.
6. The amount of light emitted by the phosphor coating depends on the
number of electrons striking the screen, we control the brightness of a
display by varying the voltage on the control grid.
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Focusing system
1. Purpose: to force the electron beam to converge into a
small spot as it strikes the phosphor.
2. Can be electrostatic (lens) or magnetic (field)
 Electrostatic focusing:
• used in television & computer graphics monitors.
• Action: focuses the electron beam at the center of the screen, in
exactly the same way that an optical lens focuses a beam of light
at a particular focal distance.
 Magnetic lens: produces the smallest spot size on the
screen and is used in special purpose devices.
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Electron beam deflection
 The deflection of the electron beam can be controlled by
horizontal and vertical deflection plates.
 The voltage applied to vertical plates controls the vertical
deflection of the electron beam and the voltage applied to
horizontal plates controls the horizontal deflection of the
electron beam.
 The frequency (or color) of the light emitted by the
phosphor is proportional to the energy difference between
the excited quantum state and the ground state.
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Important characteristics of video display devices
1. Persistence
2. Resolution
3. Aspect ratio
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1. Persistence
Definition:
 The time it takes the emitted light from the screen
to decay to one-tenth of its original intensity.
 Lower persistence requires high refresh rate & it is
good for animation
 High persistence is useful for displaying highly
complex static picture.
 Graphics monitors are usually constructed with
10 to 60 microseconds.
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2. Resolution
• The maximum number of points that can be displayed without
overlap on a CRT. (or)
• The number of points per inch or centimeter that can be plotted
horizontally & vertically.
• The smaller the spot size, the higher the resolution.
• The higher the resolution, the better is the graphics system
• High quality resolution is 1280x1024
• The intensity distribution of spots on the screen have Gaussian
shape.
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 The intensity is greater at the center of the spot, an
d it decreases with Gaussian distribution out to the
edges of the spot.
 Two illuminated phosphor spots are distinguishable
when their separation is greater than the diameter
at which a spot intensity has fallen to 60% of
maximum (at the center of the spot).
Brightness (Intensity) distribution
of a phosphor spot on the screen

Type of Display Device
 Type of CRT
 Raster-Scan Displays
 Random-Scan Displays
 Color CRT Monitors
 Direct-View Storage Tube
 Flat-Panel Displays
 Emissive displays
 Non-emissive displays
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1. Raster-Scan Displays
 Raster: 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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Raster-Scan Displays
 Based on Television technology
 Electron beam swept across screen one row at a time from
top to bottom
 Each row is referred to as a scan line
 Examples: Home television sets and printers
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 Pixel or pel (Picture Element): Smallest addressable screen
element
 Picture definition is stored in a memory area called the called
Frame buffer (Refresh Buffer)
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 The number of bits per pixel in the frame buffer is
called depth or bit planes
 Buffer with 1 bit per pixel – Bitmap
 Buffer with multiple bits per pixel – Pixmap
 Refreshing on raster-scan displays is carried out
at the rate of 60 to 80 frames per second
 On some raster-scan systems, each frame is
displayed in two passes using an interlaced
refresh procedure.
 In the first pass, the beam sweeps across every
other scan line from top to bottom. Then after the
vertical retrace, the beam sweeps out the
remaining scan lines. 18

 Retrace
 Horizontal retrace/Vertical retrace
 Horizontal retrace – beam returns to left of screen
 Vertical retrace – beam returns to top left corner of screen
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2. Random-Scan Display
 Vector Display (vector, stroke-writing, or calligraphic displays)
 stored as a set of line-drawing commands in an area of memory
(refresh display file, display list, display program)
 draw a picture one line at a time
 Example: A pen plotter
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Random-scan Displays
 The electron beam directly draws the picture in
any specified order.
 Picture is stored in a display list, refresh display
file as a set of line drawing commands.
 Refreshes by scanning the list 30 to 60 times per
second.
 More suited for line-drawing applications such as
architecture and manufacturing.

Opcode Operand
X Y
Command Opcode
MOVE 1
LINE 2
PLOT 3
Opcode X Y
1 30 30
2 50 80
2 70 30
2 40 55
2 60 55
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Random-scan Displays
 Advantages:
 High resolution
 Easy animation
 Requires little memory
 Disadvantages:
 Requires intelligent electron beam (processor controlled)
 Limited screen density, line-based images
 Limited color capability.
 Improved in the 1960’s by the Direct View Storage Tube
(DVST) from Tektronix.

Raster-Scan
System
Random-Scan
System
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Raster Scan Vs Random Scan
Random scan
Raster scan
Ideal line drawing

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The Bit-map
1-bit-deep frame-buffer
1 1 1 1 1 1 0 0
0 1 1 0 0 1 1 0
0 1 1 0 0 1 1 0
0 1 1 1 1 1 0 0
0 1 1 0 0 1 1 0
0 1 1 0 0 1 1 0
1 1 1 1 1 1 0 0
0 0 0 0 0 0 0 0
Frame buffer Pixels

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Gray Level images
n-bits per pixel produce 2n gray levels.
Many images use 8-bits per pixel, i.e. 256 gray l
evels since it gives acceptable quality.
01 10 11
00
Color image 8-bit per pixel
256 gray level
1-bit per pixel
2 gray levels
Black & White
2-bit per pixel
4 gray level

Raster Displays (Bitmap)
 Intensity for each pixel depends on the size of
frame buffer
 ex) Black & White system
one bit per pixel is needed
the frame buffer is commonly called Bitmap
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Raster Displays (Pixmap)
 With multiple bits per pixel, we can display gray-scale or
color pictures
the frame buffer is commonly called pixmap
Ex) Size of Frame Buffer when N=3, with 512 X 512
Size of Frame buffer = 3 X 512 X 512 = 3 X 256k = 768k 29

Type of Display Device
 Type of CRT
 Raster-Scan Displays
 Random-Scan Displays
 Color CRT Monitors
 Direct-View Storage Tube
 Flat-Panel Displays
 Emissive displays
 Non-emissive displays
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3. Color CRT Monitors
 A CRT monitor displays color pictures by
using a combination of phosphors that emit
different colored light.
 By combining the emitted light from the
different phosphors, a range of colors can be
generated.
 Two basic techniques:
1. Beam Penetration
2. Shadow Mask
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1. Beam Penetration Method
 For displaying color pictures used with random-scan monitors.
 Two layers of phosphor (red and green) are coated onto the
inside of the CRT screen.
 The displayed color depends on how far the electron beam
penetrates into the phosphor layers.
 A beam of slow electrons excites only the outer red layer.
 A beam of very fast electrons penetrates through the red layer
and excites the inner green layer.
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1. Beam Penetration Method
 At intermediate beam speeds, combinations of red
and green light are emitted to show two additional
colors, orange and yellow.
 The speed of the electrons, and hence the screen
color at any point, is controlled by the beam
acceleration voltage.
Advantages:
 Inexpensive way to produce color in random-scan
monitors.
Disadvantages:
 Only limited number of colors (4 ) are produced.
 The quality of pictures is not as good as with other
methods.
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2. Shadow-mask Method
 Commonly used in raster-scan systems (including
color TV).
 The color CRT has:
 Three color phosphor dots (red, green and blue) at
each pixel position.
 One phosphor dot emits a red light, another emits a
green light, and the third emits a blue light.
 Three electron guns, each controlling the display of
red, green and blue light and a shadow-mask grid
just behind the phosphor-coated screen.
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Shadow Mask Method
The delta-delta method:

delta-delta shadow-mask method
 Commonly used in color CRT systems.
 The three electron beams are deflected and focused as
a group onto the shadow mask, which contains a series
of holes aligned with the phosphor-dot patterns.
 When the three beams pass through a hole in the
shadow mask, they activate a dot triangle, which
appears as a small color spot on the screen.
 The phosphor dots in the triangles are arranged so that
each electron beam can activate only its corresponding
color dot when it passes through the shadow mask.
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In-line Method
 Another configuration for the three electron guns is an
in-line arrangement in which the three electron guns,
and the corresponding red-green-blue color dots on the
screen, are aligned along one scan line instead of in a
triangular pattern.
 This in-line arrangement of electron guns is easier to
keep in alignment and is commonly used in
high-resolution color CRTs.
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Shadow Mask Method
Delta Method:
In-line Method:

Shadow-mask Method
 We obtain color variations by varying the intensity
levels of the three electron beam.
 The output color generated depends on the amount of
excitation of the RGB colors.
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Red Green Blue Resultant color
1 1 1 White
1 0 0 Red
0 1 0 Green
0 0 1 Blue
1 0 1 Magenta
0 1 1 Cyan
1 1 0 Yellow
0 0 0 Black

Composite monitors
 Adaptations of TV sets that allow bypass of the
broadcast circuitry.
 These display devices still require that the picture
information be combined, but no carrier signal is
needed.
 Picture information is combined into a composite
signal and then separated by the monitor, so the
resulting picture quality is still not the best attainable.
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RGB monitors
 RGB monitors use shadow-mask methods and take
the intensity level for each electron gun(red, green,
and blue) directly from the computer system without
any intermediate processing.
 An RGB color system with 24 bits of storage per pixel
is generally referred to as a full-color system or a
true-color system.
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Beam penetration vs Shadow-mask methods
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Direct View Storage Tube (DVST)
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Direct-View Storage Tube
 For maintaining a screen image is to store the picture
information inside the CRT instead of refreshing the
screen.
 Stores the picture information as a charge distribution
just behind the phosphor-coated screen.
 Function of guns: Two guns are used in DVST
 Primary gun: It is used to store the picture pattern
 Flood gun or Secondary gun: It is used to maintain
picture display.
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DVST
 Advantages:
 Because of the absence of refreshing, highly complicated pictures
can be displayed without any flickering there by providing high
resolution.
 Disadvantages:
 The chosen parts of the picture cannot be erased. In order to
remove a part, the entire screen must be deleted, and the modified
picture must be redrawn.
 Large amount of time is wasted in deleting and redrawing a picture.
 DVST’s usually do not display colors.
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Flat-Panel Displays
 A class of video devices that have reduced volume,
weight and power required compared to a CRT.
 Significant feature: Thinner than CRT
 Currently used in TV monitors, Calculators, pocket video
games, laptop computers & advertisement boards etc.
 Two categories of Flat-panel displays are:
1) Emissive Displays
2) Non-emissive Displays
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Emissive Displays
 Emissive Displays are devices that convert electrical
energy into light.
 Examples:
 Plasma panels
 Thin-film electroluminescent displays and
 Light-emitting diodes (LED).
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Non-emissive Displays
 Non-emissive Displays use optical effects to convert
sunlight or light from some other source into graphics
patterns.
 Example: Liquid crystal display (LCD)
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1. Emissive Displays: Plasma Panel Display
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Emissive Displays: Plasma Panel Display
 Also called gas discharge displays
 Constructed by filling the region between two glass
plates with a mixture of gasses specially Neon.
 A set of vertically conducting ribbons is placed on one g
lass panel and a set of horizontally conducting ribbons
on other glass panel.
 An appropriate voltage is applied to a pair of horizontal
and vertical conductors cause the gas at intersection of
two conductors to break down into a glowing plasma of
electrons and ions.
 Picture definition is stored in a refresh buffer and the firi
ng voltages are applied to refresh the pixel positions 60
times per second.
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Plasma Panel Display
 Advantages:
• Refreshing is not required.
• Produce very steady image.
• Light weight than CRT
• Allow selective writing and selective erasing.
• Flat screen and is transparent.
 Disadvantages:
• Poor resolution.
• Complex addressing and wiring
• Costly than CRTs.

2. Thin-film electroluminescent displays
 Similar to plasma panel except that they contain
phosphor such as zinc sulphide doped with manganese
between two gas plates.
 The phosphor acts as a conductor between the two
electrodes.
 Electrical energy is then absorbed by manganese
atoms, which then release the energy as a spot of light.
 Disadvantages:
 They require more power than plasma panels.
 Hard to achieve good color and grey scale displays.
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3. Light emitting diode (LED)
 A device that contains a matrix (grid) of diodes is
arranged to form the pixel positions in the display.
 Picture definition is stored in refresh buffer.
 As in scan-line refreshing of CRT, information is
read from refresh buffer and converted to voltage
levels that are applied to the diodes to produce light
patterns in the display.
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Non-Emissive Displays:
Liquid Crystal Display (LCD)
 Commonly used in calculators, laptops, and portable
computers.
 Produce pictures by passing polarized light through liquid
crystal material that can be aligned to either block or
transmit the light.
 Advantages:
 Low cost
 Low weight
 Small size
 Low power consumption
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Raster Scan System
 In addition to the CPU, a special-purpose processor, called the video
controller or display controller, is used to control the operation of
display device.
 Frame buffer can be anywhere in the system memory.
 Video controller access the frame buffer to refresh the screen.
 Other processors such as coprocessors & accelerators to implement
various graphics operations
Fig.7: Architecture of a simple Raster graphics system

Architecture of a raster system with a fixed portion
of the system reserved for frame buffer
 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.

Video controller
 Frame-buffer locations, and the corresponding screen
positions, are referenced in Cartesian coordinates.
 For many graphics monitors, the coordinate origin is
defined at the lower left screen corner.
 The screen surface is then represented as the first
quadrant of a two-dimensional system, with positive x
values increasing to the right and positive y values
increasing from bottom to top.
 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. 60

Basic video-controller refresh operations

Basic refresh operations
 Two registers are used to store the coordinates of the screen
pixels (x register, y register).
 Initially x register is set to zero & y register is set ymax.
 The Value from frame buffer for this pixel position is
retrieved & used to set the intensity value of the CRT beam.
 x register is incremented by 1 and process repeated for each
pixel along the scan line.
 Then x register is reset to zero & y register is decremented by
one.
 The process is repeated till y=0, x=xmax

 This procedure is repeated for each pixel along the scan
line.
 After the last pixel on the top scan line has been
processed, the x register is reset to 0 and the y register
is decremented by 1.
 Pixels along this scan line are then processed in turn,
and the procedure is repeated for each successive scan
line.
 After cycling through all pixels along the bottom scan
line (y = 0), the video controller resets the registers to
the first pixel position on the top scan line and the
refresh process starts over.
 Refresh rate is 60 frames/seconds.
63

 To speed up pixel processing, video controllers can
retrieve multiple pixel values from the refresh buffer on
each pass.
 The multiple pixel intensities are then stored in a separate
register and used to control the CRT beam intensity for a
group of adjacent pixels.
 When that group of pixels has been processed, the next
block of pixel values is retrieved from the frame buffer.
 A number of other operations can be performed by the
video controller, besides the basic refreshing operations.
 For various applications, the video controller can retrieve
pixel intensities from different memory areas on different
refresh cycles.
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 In high quality systems, for example, two frame buffers are often
provided so that one buffer can be used for refreshing while the
other is being filled with intensity values.
 Then the two buffers can switch roles. This mechanism is useful in
real time animation because it does not waste time in reloading the
buffer.
 Also, some transformations can be accomplished by the video
controller.
 Areas of the screen can be enlarged, reduced, or moved from one
location to another during the refresh cycles.
 In addition, the video controller contains a lookup table, so that pixel
values in the frame buffer are used to access the lookup table
instead of controlling the CRT beam intensity directly.
 This provides a fast method for changing screen intensity values.
 Finally, some systems are designed to allow the video controller to
mix the frame-buffer image with an input image from a television
camera or other input device.
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Architecture of Raster scan graphics system
with a display processor

Raster scan display processor
 Display processor or graphics controller or display
coprocessor
 Purpose of Display processor – free the CPU from
the graphics chores
 In addition to the system memory, a separate
display processor memory area can be provided
 Major task of Display processor: digitizing a picture
definition in an application program into a set of
pixel intensity values for storage in frame buffer is
called scan conversion.

 Characters defined with rectangular grids of pixel positions
(rectangular grid pattern – frame buffer).
 The array size for character grids can vary from about 5 by 7 to
9 by 12 or more for higher-quality displays.
 A character grid is displayed by superimposing the rectangular grid
pattern into the frame buffer at a specified coordinate position.
 Character defined as a curve outline (curve outlines, character
shapes – frame buffer)

Display processor operation
 Generating various line styles (dashed, dotted or solid)
 displaying color areas
 performing certain transformations
 manipulations on displayed objects
 designed to interface with interactive input devices, such as
a mouse.
 To reduce memory requirements
 Frame buffer is organized as linked list
 Encoding intensity information

1. Run-length encoding
 Store each scan line as a set of integer pairs
 First number of each pair indicates intensity value
 Second number specifies the number of adjacent pixels
on the scan line with that intensity value
 Advantage: save storage space if a picture is to be
constructed with long runs of single color

2.Cell encoding
 Encode the raster as set of rectangular areas.
 Disadvantages:
 Encoding runs are that intensity changes are difficult to make
and storage requirements actually increase as the length of
the runs decreases.
 It is difficult for the display controller to process the raster
when many short runs are involved.

Random scan systems
(Vector systems)
Fig : Architecture of random scan system

Random scan systems
 An Application program is input & stored in system
memory along with a graphics package.
 Graphics commands in application program are
translated to display file by graphics package & stored in
system memory.
 This display file is then accessed by the display
processor to refresh the screen.
 The display processor cycles through each command in
the display file program once during every refresh cycle.
 Sometimes the display processor in a random-scan
system is referred to as a display processing unit or a
graphics controller.

Random scan systems
 Graphics patterns are drawn on a random-scan system
by directing the electron beam along the component lines
of the picture.
 Lines are defined by the values for their coordinate
endpoints, and these input coordinate values are
converted to x and y deflection voltages.
 A scene is then drawn one line at a time by positioning
the beam to fill in the line between specified endpoints.

Raster Scan System vs Random Scan system
76

Input
 Input is any data or instructions entered into the
computer in the form of signals
 The input into the computer can be entered:
 Through keyboard
 By selecting commands on the screen and then clicking with mouse
 By pressing finger on touch screen
 By speaking into a microphone
 By sending image through digital camera
 By scanning data printed on paper through scanner

Input Devices
1. Keyboard
2. Mouse
3. Trackball and Spaceball
4. Joystick
5. Data Glove
6. Digitizer
7. Image Scanners
8. Touch Panels
9. Light Pens
10. Voice System

Input Devices
mouse trackball
light pen
data tablet joy stick space ball

Digitizer
Desktop tablet
Artistic Digitizer System
3D digitizing system

Input Devices
81
Light pen Voice system
Data glove

Input Device
 Image Scanners
Hand-held scanner
flatbed scanner
Wide-format scanner

1. Keyboard
 The keyboard has alphabetic as well as numeric keys.
Some special keys are also available.
 Numeric Keys: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
 Alphabetic keys: a to z (lower case), A to Z (upper case)
 Special Control keys: Ctrl, Shift, Alt
 Special Symbol Keys: ; , " ? @ ~ ? :
 Cursor Control Keys: ↑ → ← ↓
 Function Keys: F1 F2 F3....F12.
 Numeric Keyboard: It is on the right-hand side of the
keyboard and used for fast entry of numeric data.
83

Keyboard
http://menotek.com/101899.html

Keyboard
http://computerhws.blogspot.com/2011/08/parts-of-computer-keyboard.html

1. Keyboard
 An alphanumeric keyboard on a graphics system is used primarily
as a device for entering text strings.
 Keyboard is an efficient device for inputting such as nongraphic
data as picture labels associated with a graphic display.
 Cursor keys and function keys are common features on general
purpose keyboards.
 Cursor control keys can be used to select displayed objects or
coordinate positions by positioning the screen cursor.
 Function keys allow users to enter frequently used operations in a
single keystroke.
86

1. Keyboard
 Other types of cursor pointing devices such as track ball or
joystick are included on some keyboards.
 A numeric keypad is often included on the keyboard for fast
entry of numeric data.
 For specialized applications, input to a graphics application
may come from a set of buttons, dials or switches that select
data values or customized graphics operations.
 Buttons and switches are often used to input predefined
functions and dials are common devices for entering scalar
values.
 Real numbers within some defined range are selected for
input with dial rotations.
 Potentiometers are used to measure dial rotations, which are
then converted to deflection voltages for cursor movement.
87

Input Device
 Input Device
 Keyboard, Mouse
 Button Boxes, Dials
88

2. Mouse
 A mouse is small hand-held box used to position the screen cursor wheels
or rollers on the bottom of the mouse can be used to record the amount and
direction of movement.
 Another method for detecting mouse motion is with an optical sensor.
 For these systems, the mouse is moved over a special mouse pad that has a
grid of horizontal and vertical lines.
 The optical sensor detects movement across the lines in the grid.
 Since a mouse can be picked up and put down at another position without
change in cursor movement, it is used for making relative changes in the
position of the screen cursor.
90

2. Mouse
 One, two or three buttons are usually included on the top of the
mouse for signaling the execution of some operation such as
recording cursor position or invoking a function.
 Additional devices can be included in the basic mouse design to
increase the number of allowable input parameters.
 The z mouse includes 3 buttons, a thumb wheel on the side, a track
ball on the top and a standard mouse ball underneath.
 With z mouse, we can pick up an object, rotate it and move it in any
direction or we can navigate our viewing position and orientation
through a 3-D scene.
 Applications of z-mouse include virtual reality, CAD and animation
.
91

3. Trackball and Spaceball
 A track ball is a ball that can be rotated with fingers or palm
of the hand to produce screen cursor movement.
 Potentiometers, attached to the ball measure the amount and
direction of rotation.
 Track balls are often mounted on keyboards or other devices
such as the z-mouse.
 While a track ball is a 2-D positioning device, a space ball
provides six degrees of freedom.
 Space ball does not actually move strain gauges measure the
amount of pressure applied to the space ball to provide input
for spatial positioning and orientation as the ball is pushed or
pulled in various directions.
 Space balls are used for 3-D positioning and selection
operations in virtual reality systems, modeling, animation,
CAD and other applications.
93

4. Joystick
 A Joystick has a small, vertical lever (called the stick) mounted
on the base and used to steer the screen cursor around.
 Consists of two potentiometers attached to a single lever.
 Moving the lever changes the settings on the potentiometers.
 The left or right movement is indicated by one potentiometer
and forward or back movement is indicated by other
potentiometer
94

5. Data Glove
 The data glove is used to grasp a virtual object.
 constructed with a series of sensors that detect hand and
finger motions.
95

5. Data Glove
 Electromagnetic coupling between transmitting antennas and receiving
antennas is used to provide information about the position and
orientation of the hand.
 The transmitting and receiving antennas can each be structured as a set
of three mutually perpendicular coils, forming a three-dimensional
Cartesian coordinate system.
 Input from the glove can be used to position or manipulate objects in a
virtual scene.
 A two-dimensional projection of the scene can be viewed on a video
monitor, or a three-dimensional projection can be viewed with a
headset.
96

6. Digitizer
 A common device for drawing, painting, or interactively
selecting coordinate positions on an object is a digitizer.
 These devices can be used to input coordinate values in
either a two-dimensional or a three-dimensional space.
 Typically, a digitizer is used to scan over a drawing or
object and to input a set of discrete coordinate positions,
which can be joined with straight-Iine segments to
approximate the curve or surface shapes.
97

Data tablet
 One type of digitizer is the graphics tablet (also referred to
as a data tablet), which is used to input two-dimensional
coordinates by activating a hand cursor or stylus at
selected positions on a flat surface.
 A hand cursor contains cross hairs for lighting positions,
while a stylus is a pencil-shaped device that is pointed at
positions on the tablet.
 Many graphics tablets are constructed with a rectangular
grid of wires embedded in the tablet surface.
 Electromagnetic pulses are generated in sequence along
the wires, and an electric signal is induced in a wire coil in
an activated stylus or hand cursor to record a tablet
position.
99

7.Image Scanners
 Drawings, graphs, color and black-and-white photos, or text
can be stored for computer processing with an image scanner
by passing an optical scanning mechanism over the
information to be stored.
 The gradations of gray scale or color are then recorded and
stored in an array.
 Once we have the internal representation of a picture, we can
apply transformations to rotate, scale, or crop the picture to a
particular screen area.
 We can also apply various image-processing methods to
modify the array representation of the picture.
 For scanned text input, various editing operations can be perfo
rmed on the stored documents.
 Some scanners are able to scan either graphical representatio
ns or text, and they come in a variety of sizes and capabilities.
100

7.Image Scanners
 Image Scanners
Hand-held scanner
flatbed scanner
Wide-format scanner

8. Touch Panels
 Touch panels allow displayed objects or screen positions to be
selected with the touch of a finger.
 A typical application of touch panels is for the selection of
processing options that are represented with graphical icons.
 Optical touch panels employ a line of infrared light-emitting
diodes (LEDs) along one vertical edge and along one horizontal
edge of the frame.
 The opposite vertical and horizontal edges contain light
detectors.
 These detectors are used to record which beams are interrupted
when the panel is touched.
102

9. Light Pens
 pencil-shaped devices are used to select screen
positions by detecting the light coming from points on
the CRT screen.
 They are sensitive to the short burst of light emitted
from the phosphor coating at the instant the electron
beam strikes a particular point.
103

9. Light Pens
 Other Light sources, such as the background light in the
room, are usually not detected by a light pen.
 An activated light pen, pointed at a spot on the screen as
the electron beam lights up that spot, generates an
electrical pulse that causes the coordinate position of the
electron beam to be recorded.
 As with cursor-positioning devices, recorded Light-pen
coordinates can be used to position an object or to select
a processing option.
104

9. Light Pens
Disadvantages:
 When a light pen is pointed at the screen, part of the
screen image is obscured by the hand and pen.
 Prolonged use of the light pen can cause arm fatigue.
 Light pens require special implementations for some
applications because they cannot detect positions
within black areas.
 Sometimes give false readings due to background
lighting in a room.
105

10. Voice System
 Speech recognizers are used in some graphics workstations
as input devices to accept voice commands
 The voice-system input can be used to initiate graphics
operations or to enter data.
 These systems operate by matching an input against a
predefined dictionary of words and phrase.
 A dictionary is set up for a particular operator by having, the
operator speak the command words to be used into the
system.
 Each word is spoken several times, and the system
analyzes the word and establishes a frequency pattern for
that word in the dictionary along with the corresponding
function to be performed.
106

10. Voice System
 Later, when a voice command is given, the system
searches the dictionary for a frequency-pattern match.
 Voice input is typically spoken into a microphone
mounted on a headset.
 The microphone is designed to minimize input of other
background sounds.
 If a different operator is to use the system, the dictionary
must be reestablished with that operator's voice patterns.
 Voice systems have some advantage over other input
devices, since the attention of the operator does not have
to be switched from one device to another to enter a
command.
107

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