The intensity is greatest at the center of the spot, and
decrease with Gaussian distribution out to the
edges of the spot.
The maximum number of points displayed without
overlap on a CRT is referred to as the resolution.
A more precise definition of resolution is the number
of points per centimeter that can be plotted
horizontally and vertically, although it is often simply
stated as the total number of points in each direction.
This depends on the type of phosphor used and the
focusing and deflection system & the intensity to be
Another property of video monitors .
This number gives the ratio of vertical points to
horizontal points necessary to produce equal-length
lines in both directions on the screen.
An aspect ratio of 3/4 means that a vertical line plotted
with three points has the same length as a horizontal
line plotted with four points.
Persistence is defined as the time it takes the
emitted light from screen to decay to one-tenth of
its original intensity.
These monitors displays color pictures by using
combination of phosphors that emit different
There are two techniques for producing color
displays on CRT. They are :
Beam Penetration Method
Shadow Mask Method
Similar to simple CRT but contains different colored Phosphor
layers(Red and Green) coated onto inside of the screen.
The displayed color depends on how far the electron beam
penetrates into the layers.
The speed of electrons – beam acceleration voltage.
Slow Electrons Red color
Fast Electrons Green color
Intermediate Speed Electrons Yellow and orange colors
Quality of Picture is not so good and is inexpensive.
Works on the principle of combining the basic colors - Red, green
and Blue - in suitable proportions to get a combination of colors.
Uses 3 electron gun placed one by the side of the other to form a
triangle or a "Delta".
Each pixel point on the screen is also made up of 3 types of
phosphors to produce red, blue and green colors. Before the
phosphor screen is a metal screen, called a "shadow mask".
This plate has holes placed strategically, so that the beams from the
three electron guns are focused on a particular pixel.
When the beam passes through the mask they form a dot
Another configuration is inline arrangement of electron
guns and RGB dots on screen are aligned on one scan line.
By varying intensity , we get different colors
Beam Penetration method
Shadow Mask method
It is used with Random Scan
System to display color.
It is Used With Raster Scan System to
It can displays Only four colors i.e. Red
it can display Millions of colors.
, Green, Orange and Yellow.
Less colors are available because the
Millions of colors are available because
colors in Beam Penetration depends the colors in Shadow Mask depends
on the speed of the electron beam.
on the type of the ray.
It is Less Expensive as compared to
It is More Expensive than other
Quality of picture is not so good i.e.
Poor with Beam Penetration Method.
Shadow Mask gives realism in picture
with shadow effect and millions of
It gives Low Resolution.
It gives High Resolution.
In Beam Penetration method, Color
In Shadow Mask Method, there are no
display depends on how far electron
such criteria for producing colors. It is
excites outer Red layer and then Green
used in computers, in color TV etc.
An alternative method for maintaining a screen image is to store the picture
information inside the CRT instead of refreshing the screen.
A direct-view storage tube (DVST) stores the picture information as a charge
distribution just behind the phosphor-coated screen.
Two electron guns are used in a DVST. One, the primary gun, is used to store the
picture pattern; the second, the flood gun, maintains the picture display .
A DVST monitor has both disadvantages and advantages compared to the refresh
CRT. Because no refreshing is needed, very complex pictures can be displayed at
very high resolutions without flicker.
Disadvantages of DVST systems are that they ordinarily do not display color and
that selected parts of a picture cannot be erased. To eliminate a picture section,
the entire screen must be erased and the modified picture redrawn. The erasing
and redrawing process can take several, seconds for a complex picture. For these
reasons, storage displays have been largely replaced by raster systems.
Most common method
Beam is swept across the screen one line at a time,
pixel by pixel
Uses Frame Buffer (an array ,on and off state)
Screen is refreshed 60 times/sec –Interlacing (Odd
numbered and Even numbered lines)
Progressive Scanning – alternate to interlacing,reduces
Flickers- refreshes every line on screen
Also called Vector/Stroke/Calligraphic Displays
Beam is swept across the screen one line at a time, only
Picture Definition – Display file (set of line drawing
Only area where picture is drawn is refreshed
It has Poor or Less Resolution
It has High Resolution
Electron beam is directed from top
to bottom at a time on screen , but
electron beam is directed to whole
Electron beam is directed only to
that part of screen is required to be
It is less expensive
It is expensive
Refresh Rate is 60 to 80 frames /
Refresh Rate depends on no.of
lines to be displayed ,30 to 60 times/
Stores Picture Definition in Frame
Stores Picture Definition in Display
Zig Zag line is produced
Smooth line is produced
Interactive raster-graphics systems typically employ
several processing units :
Special purpose register – Video/Display Controller
It consists of the three components :
(1) Frame Buffer
(2) Monitor like a TV set without the tuning and
(3) Display Controller It passes the contents of the
frame buffer to the monitor.
Raster Scan Generator
Application programs are stored in system memory. Graphics commands
in the program are translated by the graphics package into a display file
stored in the system memory. This display file is accessed by the display
processor to refresh the screen. Display processor in a random scan system
is referred to as a display processing unit or graphics controller.
It is a ball that is a ball that can be rotated with
fingers to produce cursor movements.
Potentiometers attached to the ball , measure the
amount and direction of rotation.
Normally mounted on KB.
2d positioning device.
Spaceball six-degree-of-freedom input devices.
The Spaceball may be used to rotate, translate, and scale
molecules, using up to 6 control axes simultaneously.
The Spaceball can be used
simultaneously with the mouse.
Spaceball are used for 3D positioning and selection
operations in virtual reality systems, modeling, animation,
CAD, and other application.
A joystick is a input device consisting of a stick that pivots
on a base and reports its angle or direction to the device it
Used to select screen positions with actual stick
movement, others respond to pressure on the stick.
A glove equipped with sensors that
sense the movements of the hand and
interfaces those movements with a
Data gloves are commonly used in
virtual reality environments where the
user sees an image of the data glove
and can manipulate the movements of
the virtual environment using the
A device for drawing , painting , or interactively
selecting co-ordinate positions on an object.
Conversion of a typically analog object, image or a
signal into digital form.
It is a type of Digitizer.
It enables a user to hand-draw images,
animations and graphics, similar to the
way a person draws images with a pencil
These tablets may also be used to capture
data or handwritten signatures. It can also
be used to trace an image from a piece of
paper which is taped or otherwise secured
to the surface. Capturing data in this way,
either by tracing or entering the corners
of linear poly-lines or shapes is called
Is a device that optically scans images, printed
text, handwriting, or an object, and converts it to
a digital image.
Smart phone Scanner
Touch panels allow displayed objects or screen position s to
be selected with touch of a finger.
Types of Touch Panels :
a. Resistive Film Touch Panels
b. Capacitive Touch panels
c. Surface Capacitive Touch Panels
d. Surface Acoustic Wave Touch
e. Optical Touch Panels
f. Electromagnetic Induction
It is in the form of a light-sensitive
wand used in conjunction with a
computer's CRT display.
It allows the user to point to displayed
objects, or draw on the screen, in a
similar way to a touch screen but with
greater positional accuracy.
A light pen can work with any CRTbased
with LCD screens, projectors and
other display devices.
A device in which speech is used to input data or
system commands directly into a system.
Such equipment involves the use of speech recognition
processes, and can replace or supplement other input
devices. Some voice input devices can recognize spoken
words from a predefined vocabulary, some have to be
trained for a particular speaker.
The quality of the picture obtained from the device depends on dot
size and no of dots/inch.
a) General Programming Package
Provides functions that can be used in HLP Lang.
Eg : Graphics Library
b)Special Purpose Application Package
Provides functions for graphic Design
For Non Programmers
Eg : CAD
5 Coordinate Representations in Graphics :
General graphics packages are designed to be used with
Cartesian coordinate representations (x,y,z).
Usually several different Cartesian reference frames are used to
construct and display a scene:
Modeling coordinates are used to construct individual object
World coordinates are computed for specifying the placement
of individual objects in appropriate positions.
Normalized coordinates are converted from world coordinates,
such that x,y values are ranged from
0 to 1.
Device coordinates are the final locations on the output devices.
(xmc,ymc) (xwc,ywc) (xnc,ync) (xdc,ydc)
The basic building blocks for pictures are referred to as Output
Primitives. They include character strings, geometric entities.
Attributes are properties of O/P primitives. It describes how a
primitive should be displayed.
Geometric transformation is changing the size ,position,
orientation of an object within a scene.
Viewing transformations are used to specify the view that is to
be presented & the portion of the o/p display area that is to be
Pictures can be sub divided into small parts called Structures or
segments or objects. Each structure defines one logical unit of
I/P functions are used to control and process the data flow from
these interactive devices.
Control operation is responsible for house keeping tasks.
The primary goal of standardized graphics software is
First Graphic S/W standard : Graphical Kernel System (GKS)
Second Graphic S/W standard : Programmers Hierarchical
Interactive Graphics Standard (PHIGS)
Third Graphic S/W standard : PHIGS+
These standards were adopted by ISO and ANSI.
Fourth Graphic S/W standard : Computer Graphics Interface
Fifth Graphic S/W standard : Computer Graphic Metafile (CGM
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