• The meaning of the term Graphics, is GraphicalTricks.
• Every image or picture is in fact a graph and when different
mathematical tricks are used to manipulate some change in its
properties like shape, size, motion etc., through the help of
computers then, the representation is nothing but computer
• so we can say that
• “Computer Graphics (CG) is the field of visual computing, where
one utilises computers both to generate visual images
synthetically and to integrate or alter visual and spatial
information sampled from the real world.” Or “
• Computer Graphics is the pictorial representation manipulation
of data by a computer” Or
• “Computer Graphics refers to any sketch, drawing, special
artwork or other material generated with the help of computer to
pictorially depict an object or a process or otherwise convey
information, as a supplement to or instead of written
Definition and Importance of Computer Graphics
Computer graphics can be broadly divided into the
• Business Graphics or the broader category of Presentation
Graphics, which refers to graphics, such as bar-charts (also
called histograms), pie-charts, pictograms (i.e., scaled
symbols), x-y charts, etc. used to present quantitative
information to inform and convince the audience.
• Scientific Graphics, such as x-y plots, curve-fitting, contour
plots, system or program flowcharts etc.
• Scaled Drawings, such as architectural representations,
drawings of buildings, bridges, and machines.
• Cartoons and artwork, including advertisements.
• Graphics User Interfaces (GUIs) which are the images that
appear on almost all computer screens these days, designed
to help the user utilise the software without having to refer to
manuals or read a lot of text on the monitor.
• Graphics can be two- or three-dimensional
• Computer Graphics is the creation and manipulation of images
or pictures with the help of computers.
• There are two types of computer graphics :
1) Passive Computer Graphics (Non-interactive Computer
2) Active Computer Graphics (Interactive Computer Graphics)
• Interactive Computer Graphics which is interactively
used by users e.g., games.
• Passive Computer Graphic which has no option for users
to interact or use computer graphics e.g., movies
• The major product of computer graphics is a picture. With
the help of CG, pictures can be represented in 2D and 3D
• Many applications show various parts of the displayed
picture changing in size and orientation. Such type of
transformations i.e. the pictures can be made to grow,
shrink, rotate and etc. can be achieved through CG.
• The display on them is often too big to be shown in their
entirety. Thus, with the help of CG, a technique called
clipping can be used to select just those parts of the
picture that lie on the screen and to discard the rest.
• CG is in daily use in the field of science, engineering,
medicine, entertainment, advertising, the graphic arts, the fine
arts, business, education etc.
• The electronic industry is more dependent on the technologies
provided by CG such as engineers can draw their circuit in a
much shorter time,
• architects can have alternative solution to design problems,
• the molecular biologist can display pictures of molecules and
can study on the structure,
• the town planners and transportation engineers use the
computer generated maps which display data useful to them in
their planning work etc.
Interactive Computer Graphics
• The Interactive computer graphics (ICG) provides two way
communications between the computer and the user.
• The various applications of ICG are as follows.
• Using ICG system the integrated electronic circuits which are
very complex can be drawn in a much shorter time.
• It is very useful in training of the pilots as they spend much of
their training on the ground at the controls of a flight simulator
and not in a real aircraft.
• There are many tasks that can be made easier & less expensive
by the use of ICG. The effectiveness of the ICG is the speed
with which the user can absorb the displayed information.
• The Interactive Graphics display consists of three major components
as follows & shown in Figure 1:
(1) Frame Buffer (2) T.V. Monitor (3)Display Controller
T. V. Monitor
Scan line Data
1) Frame Buffer
• The images that are to be displayed are stored in a frame buffer in the form of matrix
of intensity values.
• The frame buffer contains the image stored in binary form as a matrix of 0’s and 1’s
which represent the pixel. 0 indicates the darkness and 1 indicates the image.
• The Frame Buffer holds the set of intensity values for all the screen points.
• The intensity values stored in a Frame Buffer are retrieved and painted on a screen one
row at a time. This row is called as scan line.
2) Display Controller
• The Display Controller passes the contents of frame buffer to the T.V. Monitor.
• Display Controller reads successive bytes of data from the frame buffer & then
converts 0’s and 1’s into the corresponding video signal.
• These signals are fed to the T.V. Monitor.
3) T.V. Monitor
• The T.V. Monitor then produces black and white pattern on the screen.
• The frame Buffer contents are to be modified, in order to represent the new pattern of
pixels or if some changes are to be made on the displayed picture.
The following major areas of computer graphics are:
• Modeling deals with the mathematical specification of shape
and appearance properties in a way that can be stored on the
computer. For example, a coffee mug might be described as a
set of ordered 3D points along with some interpolation rule to
connect the points and a reflection model that describes how
light interacts with the mug.
• Rendering is a term inherited from art and deals with the
creation of shaded images from 3D computer models.
• Animation is a technique to create an illusion of motion
through sequences of images. Animation uses modeling and
rendering but adds the key issue of movement over time, which
is not usually dealt with in basic modeling and rendering.
There are many other areas that involve computer graphics.
• User interaction deals with the interface between input devices such as
mice and tablets, the application, feedback to the user in imagery, and other
• Virtual reality attempts to immerse the user into a 3D virtual world. This
typically requires at least stereo graphics and response to head motion. For
true virtual reality, sound and force feedback should be provided as well.
• Visualization attempts to give users insight into complex information via
• Image processing deals with the manipulation of 2D images and is used in
both the fields of graphics and vision.
• 3D scanning uses range-finding technology to create measured 3D models.
Such models are useful for creating rich visual imagery, and the processing
of such models often requires graphics algorithms.
• Computational photography is the use of computer graphics, computer
vision, and image processing methods to enable new ways of
photographically capturing objects, scenes, and environments.
Almost any field can make some use of computer graphics, but the major consumers of
computer graphics technology include the following industries:
• Video games increasingly use sophisticated 3D models and rendering algorithms.
• Cartoons are often rendered directly from 3D models. Many traditional 2D cartoons use
backgrounds rendered from 3D models, which allows a continuously moving viewpoint
without huge amounts of artist time.
• Visual effects use almost all types of computer graphics technology. Almost every
modern film uses digital compositing to superimpose backgrounds with separately filmed
foregrounds. Many films also use 3D modeling and animation to create synthetic
environments, objects, and even characters that most viewers will never suspect are not
• Animated films use many of the same techniques that are used for visual effects, but
without necessarily aiming for images that look real.
• CAD/CAM stands for computer-aided design and computer-aided manufacturing. These
fields use computer technology to design parts and products on the computer and then,
using these virtual designs, to guide the manufacturing process. For example, many
mechanical parts are designed in a 3D computer modeling package and then
automatically produced on a computer-controlled milling device.
• Simulation can be thought of as accurate video gaming. For example, a
flight simulator uses sophisticated 3D graphics to simulate the experience of
flying an airplane. Such simulations can be extremely useful for initial
training in safety-critical domains such as driving, and for scenario training
for experienced users such as specific fire-fighting situations that are too
costly or dangerous to create physically.
• Medical imaging creates meaningful images of scanned patient data. For
example, a computed tomography (CT) dataset is composed of a large 3D
rectangular array of density values. Computer graphics is used to create
shaded images that help doctors extract the most salient information from
• Information visualization creates images of data that do not necessarily
have a “natural” visual depiction. For example, the temporal trend of the
price of ten different stocks does not have an obvious visual depiction, but
clever graphing techniques can help humans see the patterns in such data.
• Presentation graphics: In applications like summarizing of data of
financial, statistical, mathematical, scientific and economic research reports,
presentation graphics are used. It increases the understanding using visual
tools like bar charts, line graphs, pie charts and other displays.
• In computer graphics, the graphics pipeline refers to a series of
interconnected stages through which data and commands related to a scene
go through during rendering process.
• It takes us from the mathematical description of an object to its
representation on the device. The figure 2 shown below illustrates a 3D
• The real world objects are represented in world coordinate system. It is then
projected onto a view plane. The projection is done from the viewpoint of
the position of a camera or eye.
• There is an associated camera coordinate system whose z axis specifies the
view direction when viewed from the viewpoint.
• The infinite volume swept by the rays emerging from the viewpoint and
passing through the window is called as view volume or view pyramid.
• Clipping planes (near and far) are used to limit the output of the object.
• The mapping of an object to a graphic device requires
the transformation of view plane coordinates to
physical device coordinates. There are two steps
involved in this process.
1) The window to a viewport transformation. The
viewport is basically a sub – rectangle of a fixed
rectangle known as logical screen.
2) The transformation of logical screen coordinates to
physical device coordinates.
Figure 3 (a): Sequence of transformation in viewing pipeline
Figure 3 (b): 2D coordinate system to physical device coordinates
Cathode RayTube (CRT)
• The most common graphics output device is the video
monitor which is based on the standard cathode ray tube
(CRT) design. Figure 4 illustrates the basic operation of a
Figure 4: Cathode Ray Tube (CRT)
• As shown in above figure, it consists of electron gun, focusing
system, deflection plates and a phosphor-coated screen.
• Electron gun is the primary component of a CRT. When the
heat is supplied to the electron gun by directing a current, a
beam of electrons emitted by an electron gun, passes through
focusing and deflection systems that direct the beam toward
specified positions on the phosphor-coated screen.
• The focusing system in a CRT is needed to force the electron
beam to converge into a small spot as it strikes the phosphor.
• There are two pairs of deflection plates - Horizontal
deflection plates and vertical deflection plates.
• One pair of plates is mounted horizontally to control the
vertical deflection, and the other pair is mounted vertically to
control horizontal deflection.
• The beam passes between the two pairs of deflection plates and
positioned on the screen.
• The phosphor then emits a small spot of light at each position
contacted by the electron beam.
• Because the light emitted by the phosphor fades very rapidly,
some method is needed for maintaining the screen picture.
• One Way to keep the phosphor glowing is to redraw the picture
repeatedly by quickly directing the electron beam back over the
same points. This type of display is called a refresh CRT.
• In CRT monitors there are two techniques of displaying
1) Raster scan displays
2) Random scan displays
• In Random-Scan Display electron beam is directed only to the
areas of screen where a picture has to be drawn. It is also called
vector display, as it draws picture one line at time. It can draw
and refresh component lines of a picture in any specified
sequence. Pen plotter is an example of random-scan displays.
• The number of lines regulates refresh rate on random-scan
displays. An area of memory called refresh display files stores
picture definition as a set of line drawing commands. The
system returns back to first-line command in the list, after all the
drawing commands have been processed. High-quality vector
systems can handle around 100, 00 short lines at this refresh
rate. Faster refreshing can burn phosphor. To avoid this every
refresh cycle is delayed to prevent refresh rate greater than 60
frames per second.
• Suppose we want to display a square ABCD on the
screen. The commands will be:
• Draw a line from A to B
Draw a line from B to C
Draw a line from C to D
Draw a line from D to A
Input in the form of an application program is stored in the
system memory along with graphics package. Graphics
package translates the graphic commands in application
program into a display file 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. Sometimes the
display processor in a random-scan is referred as Display
Processing Unit / Graphics Controller.
• Higher resolution as compared to raster scan display.
• Produces smooth line drawing.
• Less Memory required.
• Realistic images with different shades cannot be
• Colour limitations.
• Raster Scan Displays are most common type of graphics
monitor which employs CRT. It is based on television
technology. In raster scan system electron beam sweeps
across the screen, from top to bottom covering one row at
a time.A pattern of illuminated pattern of spots is created
by turning beam intensity on and off as it moves across
each row. A memory area called refresh buffer or frame
buffer stores picture definition. This memory area holds
intensity values for all screen points. Stored intensity
values are restored from frame buffer and painted on
screen taking one row at a time.Each screen point is
referred to as pixels.
• In raster scan systems refreshing is done at a rate of
60-80 frames per second. Refresh rates are also
sometimes described in units of cycles per second /
Hertz (Hz). At the end of each scan line, electron
beam begins to display next scan line after returning
to left side of screen. The return to the left of screen
after refresh of each scan line is known as horizontal
retrace of electron beam. At the end of each frame
electron beam returns to top left corner and begins
the next frame.
Raster-Scan Display Processor:
An important function of display process is to digitize a picture
definition given in an application program into a set of pixel-intensity
values for storage in refresh buffer. This process is referred to as scan
conversion. The purpose of display processors is to relieve the CPU
from graphics jobs.
Display processors can perform various other tasks like: creating
different line styles, displaying color areas, etc. Typically display
processors are utilized to interface input devices, such as mouse,
Raster Scan Display Processor
• Figure 9 shows one way to set up the organization of a raster system
containing a separate display processor, sometimes referred to as a
graphics controller or a display coprocessor.
Figure 9: Architecture of a raster-graphics system with a
• Real life images with different shades can be displayed.
• Color range available is bigger than random scan display.
• Resolution is lower than random scan display.
• More memory is required.
• Data about the intensities of all pixel has to be stored.
Difference between Raster Scan and Random Scan
Base of Difference Random Scan Raster Scan
1. Resolution The resolution of random scan is higher than raster scan.
While the resolution of raster scan
is lesser or lower than random
2. Cost It is costlier than raster scan.
While the cost of raster scan is
lesser than random scan.
3. Modification In random scan, any alteration is easy in comparison of raster scan.
While in raster scan, any alteration
is not so easy .
4. Interlacing In random scan, interlacing is not used.
While in raster scan, interlacing is
5. Line Drawings
In random scan, mathematical function is used for image or picture
rendering. It is suitable for applications requiring polygon drawings.
While in which, for image or
picture rendering, raster scan uses
pixels. It is suitable for creating
6. Motion of Electron Beam
Electron Beam is directed to only that part of screen where picture is
required to be drawn, one line at a time.
Electron Beam is directed from top
to bottom and one row at a time
on screen. It is directed to whole
7. Picture Definition
It stores picture definition as a set of line commands in the Refresh
It stores picture definition as a set
of intensity values of the pixels in
the frame buffer.
8. Refresh Rate
Refresh rate depends on the number of lines to be displayed i.e. 30 to 60
times per second.
Refresh rate is 60 to 80 frames per
second and is independent of
9. Solid Pattern In random scan, Solid Pattern is tough to fill.
In raster scan, Solid Pattern is easy
10. Example Pen Plotter TV Sets
HARD COPY DEVICES
INPUT DEVICES FOR OPERATOR INTERACTION
• Trackball and Spaceball
• Data Glove
• Touch Screens
• Light pen
• Voice Systems
• Image Scanners