computer graphics unit 1.ppt

Assignment—10 marks
 With suitable diagrams write a note on the
working of following input/output interactive
graphics devices
 Touch Screen
 Light Pen
 Joystick
 Scanner
 Mechanical and optical mouse
 Touch Pad

Assignment—10 marks
contd…
 WAP to draw basic shapes in C
 WAP program in C to animate a ball in circular motion
using putimage/getimage functions and without
putimage/getimage functions.
 Implement bresenham’s midpoint line drawing algorithm
in C
 Implement bresenham’s midpoint circle drawing
algorithm in C.
 WAP in C to perform free hand drawing using mouse
functions.

Computer Graphics
 Till recently the output obtained from a computer
has been alphanumeric.
 It is well known that anything presented in graphical
and pictorial form tremendously aids human
comprehension.
 Examples:
 bar charts, pie charts sales graphs are commonly used by
managers.
 Building plans, perspective views are used by architects.
 Engineers use mechanical drawings, circuit diagrams in
their work.

Chinese proverb
“a picture is worth ten thousand words”

Computer Graphics
 Computer graphics is concerned with the
generation, representation, manipulation and
display of pictures with the aid of a computer.
 The Process transforms and presents
information in a visual form.
 Image created by computer is called
computer graphics.

Computer Graphics Applications
 Computer Graphics may be subdivided into
three broad areas:
 Generative Graphics
 Cognitive Graphics
 Image Processing

Generative Graphics
Generative Graphics system objectives:
 It deals with the creation of 2D and 3D pictures.
 Computer aided generation of pictures from
mathematical representation
 For display animations, pictures may be transforms,
rotated, contracted & expanded.

Applications Of Generative Graphics
Pilot Training: In pilot training the graphic display may be
used to mimic the approach of an aircraft to an airstrip as
the pilot manipulates the landing controls.
CAD: Design Electrical, Electronic, Mechanical and
Structural components and systems. Design Circuits port
layouts, VLSI chips, Aircrafts structures and Buildings
structures.
CAM: Manufacturing information (such as parts list etc)
are automatically generated. The great advantage of
computer graphics is the possibility of very quickly
modifying drawings based on drawing changes

COGNITIVE GRAPHICS
 It deals with algorithms to recognize and classify
pictures.
 Applications are :
 Classify peoples’ faces ,fingerprints is commonly used now a
days in criminal investigation.
 Microscope pictures of bacterial colonies
 ECG patterns and x-rays of different parts of the body to perform
medical diagnosis of patient by using computers.
 etc.
 In such applications, the pictures are converted to digital
form by devices called optical scanners
 The digital information is transformed and classified by
specially written pattern recognition programs.

IMAGE PROCESSING GRAPHICS
 Image processing apply techniques to modify or interpret
existing pictures, such as photographs and TV scans.
 It is used to improve and modify the quality of picture.
 First image is digitized and there it is given a color
pattern for improving color quality.
 For example : Boundaries making, if it is not visible.
 Two principle uses of image processing are:
Ø Improving picture quality.
Ø Machine perception of visual information as used in robotics.

IMAGE PROCESSING GRAPHICS
 Image processing are used to analyze satellite photos of
earth and photos of galaxies.
 Medical applications also use extensive use of image
processing techniques for picture enhancement, in
tomography, in simulation of operation.
 Image processing has the sub areas like image
enhancement, pattern detection and recognition and
scene analysis and computer vision.
 Application of image processing are: face detection,
feature detection, medical image processing, microscope
image processing, remote sensing and many others.

Image processing
 Image processing applies techniques to modify or
interpret existing pictures ,such as photographs and TV
scans.
 Principle applications of image processing is in following
areas
 To improve picture quality
 To extract information from pictures
 To help in m/c perception of visual information e.g. in robotics
 To apply image processing , the picture of first digitized
into an image file then digital methods are used to
rearrange the picture parts, enhance color
separation or to improve quality of shading

 Computer graphics is concerned with the
generation, representation, manipulation and display
of pictures with the aid of a computer.
 Typical graphics system comprises of a host
computer with support of fast processor,large
memory, frame buffer and
 Display Devices (color monitors)
 Input Devices (mouse, keyboard, joystick, trackball, touch
screen etc.)
 Output Devices (LCD panels, printers, plotters etc)

Conceptual frame work for
interactive graphics
Application
Model
Application
Program
Graphics
System

Representative
Uses & advantages
of
computer graphics

GUI Interfaces
 Graphical User Interface
 Typical components used
 Menus
 Icons
 Cursors
 Dialog boxes
 Scroll bars
 Buttons
 Valuators
 Grids
 Sketching( used to draw lines, polylines, arcs, ellipses )

GUI Interfaces
 Most applications have user interfaces on desktop
window system to manage multiple simultaneous
activities.
 point – and –click facilities to allow users to select
menu items, icons , and objects on the screen
 Icons and menus take up less space and are also
intuitive.
 E.g. Word-processing, spread sheet and desktop
publishing etc.

(Interactive) plotting in business, science, & technology
 Today graphics is used to create 2D and 3D graphs
of mathematical, physical and economic functions-
 Histograms
 bar and pie chart
 task scheduling charts
 Inventory and production charts
 All these are used to represent meaningfully and
concisely the trends from data
 That clarify complex phenomena and ease the
decision making

Office automation and electronic
publishing
 Office automation and electronic publishing can
produce both printed( hardcopy) documents and
electronic (softcopy) documents that contain
 Text
 Tables
 Graphs
 Other forms of drawn or scanned –in graphics

Computer –aided design and drafting
 Computer aided design (CAD) is the use of computer
technology for the process of design. or,
 Computer Aided Designing describes the process of
designing with a computer.
 CAD software, or environments, provide the user with
input-tools for the purpose of design processes,
drafting, documentation, and manufacturing
processes.
 It helps to perform design adjustments and quickly
visualize effects
 Some applications also allow real time animation to
test performance of the product 3D realistic images,
multiple angle views , greatly simplifies the design
process.

Computer –aided design and drafting
 It is used to design
components and systems of
 After objects are designed the
utility programs can post
process the design database
to make part lists, to process
bills of materials etc.
Mechanical Electrical
Electronic devices
Electromechanical
Structures such as
 Buildings
 Automobiles bodies
 Airplane
 Ship hulls
 VLSI chips
 Optical systems
 Computer network

Computer graphics in CAD
 In CAD graphics is used to design electrical, electronic,
mechanical and structural components and systems.
 It is extensively used in designing printed circuit board
layouts, design of VLSI chips.
 It is also used in making aircraft structures, building
structures based on the drawing.
 A common problem in visualization applications is the
display of one surface overlying another. These
systems are used to visualize any 3D objects by the use
of computer soft -wares and is used in major designing
such as car designing , machine designing etc.

Simulation
 To study abstract mathematical models
of such phenomena as fluid flow ,
relativity, nuclear and chemical reactions
etc.
 Pilot training

Entertainment
 Computer graphic is extensively used in the
production of motion pictures, music, videos and TV
shows.
 Sometimes graphic scenes are displayed by
themselves and sometimes graphic objects are
combined with actors and live scenes
 Image processing techniques such as morphing
can be used to produce transformation of one
person or object in another
 A graphics scenes generated for the movie AVTAAR.

Education and Training
 Computer generated model of physical,
financial and economic system are useful
educational aid.
 They find wide use in education and
training institution.
 Special simulation aids are also made for
specialized area for providing training for
aircraft pilot, ship captains etc.

Presentation Graphics
 Reports can be presented on slides and
transparencies.
 They can be used to present scientific,
mathematical and economic data using
different kind of figures, graphics and
charts.
 3D graphics can also be used to provide
special effects and more attractive
presentations.

Pixels & Resolution
 Pixels
 Graphic images are made up of tiny dots called
pixels.
 Each pixel has a particular address on the screen.
 Resolution
 It is defined as the maximum number of pixels or
dots can be displayed on the screen.
 Examples:800 by 600 pixels,1024 by 768 pixels,1152
by 864 pixels etc.

Code Name
Aspect
ratio
Width Height
XGA eXtended Graphics Array 4:3 1024 768
XGA+ eXtended Graphics Array Plus 4:3 1152 864
WXGA Widescreen eXtended Graphics Array 16:9 1280 720
WXGA Widescreen eXtended Graphics Array 16:10 1280 800
SXGA
(UVGA)
Super eXtended Graphics Array 4:3 1280 960
SXGA Super eXtended Graphics Array 5:4 1280 1024
HD High Definition 16:9 1360 768
HD High Definition 16:9 1366 768
WXGA+
Widescreen eXtended Graphics Array
Plus
16:10 1440 900
HD+ High Definition Plus 16:9 1600 900
UXGA Ultra eXtended Graphics Array 4:3 1600 1200
WSXGA+
Widescreen Super eXtended Graphics
Array Plus
16:10 1680 1050
FHD (Full
HD)
Full High Definition 16:9 1920 1080
WUXGA
Widescreen Ultra eXtended Graphics
Array
16:10 1920 1200
QFHD Quad Full High Definition 16:9 2560 1440
Other

Aspect Ratio
 The aspect ratio of an image describes the proportional relationship
between its width and its height.
 It is represented as two numbers separated by a colon as
4(Width):3(Height), 16:9 etc.
 A square has the smallest possible aspect ratio 1:1.
For an ellipse, the aspect ratio
denotes the ratio of the major axis
to the minor axis.
An ellipse with an aspect ratio 1:1
is a circle

BIT DEPTH COLOR RESOLUTION CALCULATION
1-bit 2 colors 21 (2)
2-bit 4 colors 22 (2x2)
3-bit 8 colors 23 (2x2x2)
4-bit 16 colors 24 (2x2x2x2)
5-bit 32 colors 25 (2x2x2x2x2)
6-bit 64 colors 26 (2x2x2x2x2x2)
7-bit 128 colors 27 (2x2x2x2x2x2x2)
8-bit 256 colors 28 (2x2x2x2x2x2x2x2)
16-bit 65,536 colors 216
24-bit 16,777,215 colors 224

Display Devices
CRT,EGA/CGA/VGA/SVGA monitors,
plotters, laser printers, films, flat-panel
devices, video digitizers, scanners,
LCD panels, keyboard, joystick,
mouse, touch screen, track ball etc.
Commonly used display device is the MONITOR or CRT MONITOR

Types of CRT Display Devices
 DVST( Direct View Storage Tube)
 Calligraphic or Random Scan Display System
 Refresh and Raster Scan Display System
Commonality between these three devices is they all are based on
CRT technology

CRT Technology
Operation an electron gun with an accelerating anode

In a CRT (Cathode Ray Tube), you have heating filament which is
responsible to heat up the cathode element of the CRT that is what
generates the electrons or you can say when heating filament heat
up the cathode electrons simply boil off from the cathode and these
electrons are guided by the set of devices which are cylindrical in
nature and helps the electron beam to reach to the screen.
Emitted electron beam must have the following properties:
1. It must be accurately focused so that it produce the sharp spot of
light where it strikes the phosphor.
2. It must have high velocity since brightness depends on the velocity of
the beam.

We have three functions
Control Grid
Focusing Anode
Accelerating Anode

What does the control grid do?
 When you observe a picture on screen in some
parts of the picture may be bright or some picture
may be dark.
 This brightness or darkness is the illumination or
intensity on the screen is basically controlled by the
intensity of the beam which strikes the particular
point on the screen
 This intensity of the screen is controlled by
controlling intensity of the electrons or electron
beam coming out of the cathode.

 Electron beam coming out of the cathode is
negatively (-ve) charged.
 Control grid is also negatively (-ve) charged. It is
a high –ve voltage applied to the control grid
 If both are –vely charged so they repel each
other.
 The amount of voltage at the control grid will
essentially allow a certain amount of electrons to
pass through the control grid
 So, we can say the number of electrons pass
through the control grid is controlled by the
voltage applied on the control grid.

 So, if you reduce the amount of voltage on
the control grid , you will allow large number
of electrons to pass through the control grid
and the intensity of beam will be higher so,
the illumination or intensity on the screen
will also be higher.
 and vice-versa

Focusing Anode & Accelerating
Anode ,unlike the control grid
these are +vely charged or +ve
voltage

Focusing Anode & Accelerating Anode
 Focusing Anode
 helps us to focus the electron beam on the
particular location of the screen
 Accelerating Anode
 is required to strike the electron beam on to the
screen with a very high speed or velocity to emit
light.

 Till now you can see the electron beam is
going straight and will be able to hit the
centre of the screen
 But the screen is the rectangular matrix.
 To create a picture, this beam should move
all over the screen.
 So, you need horizontal deflection as well as
vertical deflection so that you can cover the
entire screen.

Basic Design of Magnetic Deflection CRT

Magnetic Deflection
 This beam is deflected by magnetic deflection
coils which is available on the top and bottom
and also on other side of the CRT
 Magnetic deflection coils generates the
magnetic field which deviate the electron
beam.

NOTE:
The voltages used for deflection are
generated by the display controller from
digital values provided by the computer.
These values normally represent co-
ordinates that are converted into voltages by
digital-to-analog converter

Oscilloscope (CRO) uses capacitive
deflection plates
TV system uses electromagnetic
deflection system

Basic Operation
 The beam of electrons emitted by an electron
gun passes through focusing and deflection
system that directs the beam toward specified
positions on the phosphor coated screen
 The phosphor glows when electron beam strikes
it.
 Because light emitted by the phosphor fades
very rapidly, a steady picture is maintained by
tracing it out repeatedly .This is called
refreshing

Refreshing
Redraw the picture repeatedly by quickly
directing the electron beam back over the
same points. This is called refreshing

Properties of phosphor
 Persistence:-it is defined as time taken for brightness
to drop to 1/10th of its initial value that means how
long they continue to emit light.
 Or, It is defined as the time from the removal of
excitation to the moment when phosphorescence has
decayed to 10 % of the initial light output.
 E.g. most phosphors used in graphics equipment it is
usually 10 to 60 microseconds.
 A phosphor with low persistence is useful for
animation.
 A phosphor with high persistence is useful for
displaying highly complex, static pictures

Properties of phosphor
 Color:-phosphors are generally available in
blue, green & white colors.
 Grain size:-grain size is the size of pixels
 e.g. if grain size is small pixel will be more.
To produce higher resolution grain size must
be small.
 Efficiency:-it is defined as the amount of
electrical energy converted into light energy in
which some energy is lost as heat.
 Resistance:- To burning under long excitation
is an important property of phosphor

 Note: Any given phosphor has several
different quantum levels to which electrons
can be excited, each corresponding to a color
associated with the return to an unexcited
state.
 Further , electrons on some levels are less
stable and return to unexcited state more
rapidly than others.

PHOSPHORENCE
Phosphorescence is the light given off by the
return of relatively more stable(elements that have full outer shell)
excited electrons to their unexcited state once the
electron beam excitation is removed.
With typical phosphors, most of the light emitted
is phosphorescence.
It is a specific type of photoluminescence related
to fluorescence.
A phosphorescent material does not immediately
re-emit the radiation it absorbs.

FLUORESCENCE
A phosphor’s fluorescence is the light emitted as
these very unstable electrons lose their excess
energy while the phosphor is being struck by
electrons.
Fluorescence occurs where the energy is supplied
by electromagnetic radiation, usually ultraviolet light.
The energy kicks an electron of an atom from a
lower energy state into an “excited” higher energy
state, then the electron releases energy in the form
of light when it falls back to a lower energy state.

Refresh Rate
 Refresh rate of a CRT is the number of times per
second the image is redrawn.
 It is typically 60 per second for raster displays.
 Refresh rate to avoid flickering – 60 Hz.
 A flicker free picture appears constant or steady to the
viewer.
 Reducing the refresh rate increases flicker
 As the refresh rate decreases, flicker develops
because the eye can no longer integrate the
individual light impulses coming from a pixel.

Critical Fusion Frequency
 The refresh rate above which a picture stops
flickering and fuses into a steady image is
called Critical Fusion Frequency (CFF).
 Very high intensity images requires refresh
rate of 80 to 90 Hz.

 Note: Refresh rate of raster scan display is
usually at least 60 frames per second and is
independent of picture complexity
 Refresh rate of vector systems depends
directly on the picture complexity (no. of lines
,points and characters)
 Greater the complexity, longer the time taken
by a single refresh cycle and lower the
refresh rate.

DVST
 The DVST behaves like a CRT with long persistence.
 A line remains visible for up to an hour before it starts to fade.
 The writing beam does not write directly on the phosphor but a
fine mesh wire grid coated with dielectric( electrical insulator)
 A pattern of +ve charges is deposited on the grid
 A continuous flood of electrons is emitted by the electron gun.
 These flood electron transfers the pattern of charges from the grid
to the phosphor.
 Just behind the storage mesh is a second grid which is called a
collector which makes the flow of flood electrons smooth.
 Where there is +ve charge on the mesh the electrons are
attracted and pass through it to strike the phosphor.

Advantages
 It offers the flat screen
 It is a CRT with long persistence phosphor
 It provides flicker free display
 No refreshing is necessary

Limitations
 Very limited interactive support
 Modifying any part of the image requires redrawing the entire
image. So, animation is ruled out in case of DVST.
 Change in the image requires to generate a new charge
distribution in the DVST
 Slow processing of drawing-(typically a few seconds are necessary for a complex picture)
 Erasing takes about 0.5 seconds .Selected parts of the
picture can't be erased. All lines and characters must be
erased.
 Only single intensity is possible. The DVST can’t display
multiple colors.

Raster Scan Display
 Used in television screens
 Unlike DVST and random scan display which were line
drawing ,refresh CRT is point- plotting device.
 Raster displays stores the display primitives (lines,
characters, shaded & patterned areas) in refresh buffer.
 The image is displayed on the screen as a succession of scan
lines where each scan line is made up of several pixels.
 The picture definition is stored in the memory are called the
Refresh Buffer or Frame Buffer as rectangular matrix
containing intensity value for each dot or pixel on the screen.
 Or, you can say the refresh buffer stores the drawing
primitives in terms of points and pixel components.

Raster Scan Display
 As the electron beam moves across each row, the beam intensity is
turned on or off depending on the value of each pixel.
 In a simple black and white system , each screen point is either on or
off i.e. only one pixel is needed.
 For complex systems displaying several colors, additional bits are
required i.e. high color quality system use 24 bits per pixel.
 Frame buffer which uses one bit per pixel is called bitmap
 Frame buffer which uses multiple bits per pixel is called pix-map
 Entire image is scanned out sequentially by the video controller( one
raster line at a time)
 Raster lines are scanned from top to bottom and then back to top
 Causes the effect of jaggies or staircase effects
 It is suitable for realistic display of scenes containing different color
patterns and shading.

HORIZONTAL RETRACE & VERTICAL 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 the
horizontal retrace.
 And at the end of each frame, the electron
beam returns to the top left corner of the
screen to begin the next frame is called
the vertical retrace

Fig. shows horizontal and vertical retrace
 Starting at the top-left of the screen and
going to the bottom-right, the electron beam
is turned on a line at a time (1), then turned
off to go back to the next line (2), then off
once again to go back up to the top (3).

Raster scan display system draws a discrete set of points

Rasterisation
(a) General Lines (b) Special Cases

CPU Peripherals
System
memory
Video
controller
CRT
System
bus
Architecture of a Simple Raster Graphics System

CPU Peripherals
System
memory
Video
controller
CRT
System
bus
Frame
buffer
Architecture of a Simple Raster Graphics System with a fixed portion
of the system memory reserved for the frame buffer

Y-Register X-Register
Memory Address
Scan
Conversion
horizontal/
vertical
deflection
Pixel
value(s)
CRT
Intensity
MEMORY
DATA
Logical Organization of the video controller
Raster Scan
Generator

 Assume that the frame buffer is addressed in x 0 to
xmax and in y from 0 to ymax.
 Then ,at the start of a refresh cycle, the X register is set
to zero and Y register is set to ymax (the top scan line).
 As the first scan line is generated, the X address is
incremented up through xmax.
 Each pixel value is fetched and is used to control the
intensity of the CRT beam.
 After the first scan line, the X address is reset to zero
and the Y address is decremented by one.
 The process continues until the last scan line (y=0) is
generated
Logical Organization of the video controller

ARCHITECTURE
OF
RASTER SCAN DISPLAY
(WITH DISPLAY PROCESSOR)

PERIPHERAL DEVICE
VIDEO CONTROLLER CRT
DISPLAY
PROCESSOR
MEMORY
FRAME
BUFFER
DISPLAY
PROCESSOR SYSTEM MEMORY
CPU

A single bit-plane black & white frame buffer raster
CRT graphics device.

An N-bit plane gray level frame buffer

A 24-bit-plane color frame buffer

Memory Usage
 If one uses a 512ˣ512 element, then 218 bits
are necessary in a single bit plane.
 Memory size required = 32 KB
 A DAC (digital to analog converter) is used to
convert the bit value (0,1) to analog signals
for refreshing the screen

Memory Size Required for N-bit plane
N Display Color Memory Size
1 Black & White 128KB
8 256 1MB
24 16 million colors 3MB
32 16 million colors 4MB
If one uses 1024ˣ1024 high resolution CRT

Random Scan Display
 Also called vector, stroke, line drawing displays.
 Characters are also made of sequences of strokes (or short lines).
 Vectored-because electron beam is deflected from end point( vector type of
movement )
 In random scan display the electron beam is directed only to those parts of
screen where picture is to be drawn.
 Order of deflection is dictated by the arbitrary order of the display commands.
 The picture is drawn one line at a time and therefore these monitors are also
called as “Vector Displays”.
 The picture definition is stored as a set of line drawing commands in an area of
memory called as “Refresh Buffer” or “Refresh Display File”.
 To display a specified picture the system cycles through a set of commands
stored in a display file.
 After all commands are processed , the system cycle back to the first line
command to refresh cycle.
 Refresh rate is 30-60times/sec.
 It is suitable for line drawing applications but can’t display realistic shaded
pictures or scenes.

Host CPU Display Buffer
Display
Controller
Display
Processor
Vector
Generator
CRT
Block Diagram of Calligraphic
Refresh Display

 The display processor sends digital and point
co-ordinate values to vector generator.
 Vector generator needs the co-ordinate
values as well as the intensity value to draw
any object.
 The vector generator converts the digital co-
ordinates values to analog voltages for the
beam deflection circuits.
 Scope of animation is available.

Images
 An image is a spatial representation of an object, a
two dimensional or three dimensional scene or
another image.
 In computer graphics ,an image is always a digital
image
 When we generate with the help of computer
graphics then two types of images are
available:-
 BITMAP IMAGES
 VECTOR IMAGES

Bitmap Images
 Bitmaps are an image type most appropriate for photo-realistic
images and complex drawings require fine details.
 A bitmap image is simple information matrix describing the
individual dots of an image called pixels.
 The bit-depth determines the number of colors that can be
displayed by an individual pixel.
 We can grab a bitmap image from a screen, scan it with a
scanner , download it from a website, or capture it from a video
capture device.
 Limitations:
 Large file sizes
 Inabilty to scale or resize the image easily while maintaining
quality.

Vector Images
 Vector images are most appropriate for lines ,boxes, circles,
polygons and other graphic shapes that can be mathematically
expressed in angles, co-ordinates and distances.
 A vector images can be filled with color and patterns and we can
select it as a single object.
 Vector drawn objects use a fraction of memory space required to
describe and store the same object in bitmap form.
 For the web, pages that use the vector graphics use plug-ins
such as Flash, and download faster when used for animation
draw faster than bitmaps.
 Most drawing programs can export a vector drawing as a bitmap
but converting bitmap to vector drawn object is difficult.
 Limitations:
 Vector images cannot be used for photo-realistic images.
 Plug-ins are required.

 There is a need of certain set of rules, constraints,
syntax, procedures and function calls to built a
Graphics Package System.
 If there are no standards, then everyone will come up
with his/her own standards.
 So there is a need to define standards , if not defined
then, we will not be able to understand one’s software
or graphical package.
 Certain Compilers provide their own graphical libraries
such as Visual Basic, C, C++, Java, Turbo C, Visual C
etc.

 These systems can be DEVICE DEPENDENT
and DEVICE INDEPENDENT.
 Device Dependent- Assembly language and
Solaris.
 Device Independent- Open- GL, PHIGS.
 Standard Graphics Packages are-
 GKS (Graphics Kernel system).
 first ISO standard low level computer graphics for 2D
vector graphics
 Applications written to use GKS portable to many
platforms and devices
 Suitable for chart drawings

 SRGP (Simple Raster Graphics Package).
 PHIGS (Programmers’ Hierarchical Interactive
Graphics system).
 Standard for rendering 3D computer graphics,support
features like transformations, texture mapping, shading,
light models
 OPEN-GL-is currently used.

 Computer Graphics system could be active or
passive
 In both cases ,the input to the system is
scene description and output is a static or
animated scene to be displayed
 In case of active system the user controls the
display with the help of GUI, using an input
device

COLOR CRT MONITORS
• Colored pictures can be displayed using a
combination of phosphors that emit different color
light.
• Commonly used techniques for display of colors
are:
-> Beam Penetration
-> Shadow Mask

BEAM PENETRATION
• Uses multilayer phosphor.
• Used with random scan display.
• Two layers of phosphor (usually red and
green) are coated on inner side of CRT
screen.

• Electron Beam intensity decides the displayed
color.
• High potential electron beam excite the green
phosphor.
• Low potential electron beam excite red phosphor.
• Intermediate beam gives combinations of green and
red light i.e. orange and yellow.

ADVANTAGE
• Inexpensive Method.
DISADVANTAGES
• Limited colors are possible.
• Poor picture quality.
• Difficulty in changing electron beam potential
by large amount.

Shadow Mask
Shadow mask method is used in majority of color
TV set and computer monitors.
It can display wide range of colors.
This method is commonly used in raster scan
displays.
Construction:
•It has red, green & blue color dots at each pixel
position on the screen (forms a delta)
It also has three electron guns one for each color dot
(forms a delta)
A shadow-mask grid is placed just behind the
phosphor-coated screen with holes, corresponding to
each pixel on screen.

Shadow Mask
 A metal plate or shadow mask is placed just behind
the phosphor screen.
 The plate has small holes.
 The guns placed shape of a triangle.
 These electron guns group in delta
 They are responsible for RED, BLUE & GREEN
components of light i.e. output of the CRT.
 The electron beam from all the three guns are
brought to the same point of focus on the shadow
mask
 The phosphor is laid down in groups of three spots
red ,blue and green such that each spot is struck
by electrons from appropriate gun.

 By modulating the beam current the light
output in each of the three color components
can be controlled
 Different phosphor light can be combined to
get a range of colors

 Advantage:
 produce realistic images
 also produced different colors
 and shadows scenes.
 Disadvantages
 low resolution
 expensive
 electron beam directed to whole screen

Inherent Memory Devices
 To avoid repeated refreshing, inherent
memory contents are used.
 These devices store picture information
inside the CRT.
 There are three types of inherent memory
devices
 DVST
 Plasma Panel display
 Laser Scan Display

Flat Panel Displays
 Less volume, weight and power requirement
 Used in flat and thin TV monitors , laptops,
calculators ,handheld devices, advertising boards
and notice boards etc.
 Flat panel displays comes in two categories:
 Emissive Displays: They convert electrical energy into
light energy. E.g. Plasma Panel Display.
 Non Emissive Displays: They use optical effects to
convert sunlight or light from other sources into graphical
patterns. E.g. Liquid Crystal Display

•Electroluminescent display is an example of emissive
displays.
•It consists of two glass sheets placed very close to each
other.
•A series of vertical electrodes are attached on inner glass
sheet and horizontally on other sheet.
•Electroluminescent substances are a kind of phosphor.
Electroluminescent phosphors emit light after absorbing
electricity.
•The region between glass plates is filled with phosphor
such as-:
• Phosphor doped with manganese.
• Zinc sulphide doped with manganese.
Electroluminescent Display

Thin film Electroluminescent display
device

Electroluminescent Display
• When a sufficient high voltage is applied to a pair of
crossing electrodes, the phosphor becomes a
conductor in the area of intersection of the
two electrodes.
• Electric energy is then absorbed by the
manganese atoms, which then releases
energy as a spot of light.

ADVANTAGES:
 Recently blue, red, and green electroluminescent
materials have been developed that offer the potential
for long life and full color electroluminescent displays.
 It has high resolution display.
DISADVANTAGES:
 High power consumption.
 Expensive and bulky.

Plasma panel Display
 It consists of two glass sheets placed face to face and very close
to each other.
 The region between them is filled with a mixture of neon based
gas.
 Thin and closely spaced gold electrodes are attached vertically
on the inner side of one sheet and horizontally on another sheet
When a sufficiently high voltage is applied the gas dissociates,
i.e. electrons are stripped from the atoms. The dissociated gas is
called a plasma, hence the name plasma display.
When the electrons recombine, energy is released in the form of
photons; and the gas glows .
This glow can be sustained by maintaining a high frequency
alternating voltage across the cell.

 A cell can be switched on by momentarily
increasing the sustaining voltage and
switched off by lowering the signal.
 The signal amplitude is chosen so correctly ,
cells that have not been fired will not be
affected.

 Advantages:
 It allows both selective writing and selective erase
 It produces a steady image which is free from flicker.
 It is less bulky than CRT.
 Disadvantages:
 It is monochromatic
 It has poor resolution
 It has complex addressing and wiring requirements

LIQUID CRYSTAL DISPLAY (LCDs)
 LCDs are commonly used in small systems,
such as calculators, laptop computers. these
are the non emissive devices.
 The term liquid crystal refers to the fact that
these compounds have a crystalline
arrangement of molecules, yet they flow like a
liquid.
 Flat panel displays commonly used nematic
(thread like) liquid crystal compounds that tend
to keep the long axes of the rod shape
molecules aligned.

 Two glass plates, each containing a light – polarizer
at right angles to the other plate, sandwich the liquid
crystal material.
 Rows of horizontal transparent conductors are built
into one glass plate and columns of vertical
conductors are put into the other plate
 The intersection of two conductors defines a pixel
position.
 The polarized light passing through the material is
twisted so that it will pass through the opposite
polarizer.
 The light is then reflected back to the viewer.

 To turn off the pixel we apply the voltage to
the two intersecting conductors to align the
molecules so that the light is not twisted.
 This type of flat panel displays is referred to
as passive-matrix LCD.
 Colors can be displayed by using different
materials and by placing a triad of color pixels
at each screen loactions.

 Another method for construction LCDs is to
place a transistor at each pixel location using
thin- film transistor technology.
 The transistors are used to control the
voltage at pixel locations and prevent charge
from gradually leaking out of the liquid crystal
cells.
 These devices are called Active Matrix
Displays

construction
 LCD is made up of six layers.
 The front layer is the first layer is a vertical
polarizer.
 Second layer with thin grid wire electrode
deposited on the surface adjoining the crystals.
 Third is a thin liquid crystal layer .
 Fourth is a layer with horizontal grid wires on
the surface.
 Fifth layer is the horizontal polarizer .
 Last one is reflector.

.
Matrix form
.
.
Pixel
Reflector Vertical
grid
wire
.
Horizontal
polarizer
Horizontal
grid wire
Liquid crystal
layer
Vertical
polarizer
.

LASER SCAN DISPLAY
• Laser scan display is one of the few high resolution large screen
display devices.
•It has been used in displaying maps, high quality text and elaborate
ckt diagrams.
• In laser scan display , a laser is deflected by a pair of mirrors so that
it traces out the desired image on a sheet of photo chromic film. The
material usually transparent but light from the laser leaves a dark
trace on it.
•A light projection system is used to project light onto a large screen.
The image thus deposited on the film.
•To produce a fresh image the display simply winds the role of film to
bring a blank region under the laser, the mirrors deflecting the laser
are extremely small and are controlled by the electrical signals
received from the display controller.

Interlacing
 Each frame has two fields, each containing half the
picture.
 Fields are interlaced and interwoven.
 Fields are presented alternately every other 1/60th of
a second.
 One field contain odd lines(1,3,5,7,…….)
 One field contain even lines(0,2,4,6,…….)
 Interlacing scan lines on a raster scan display.
 First all points on even numbered (solid) scan lines
are displayed, then all points along the odd numbers
(dashed) lines are displayed.

computer graphics unit 1.ppt

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