Computer Graphics

Monday, November 28, 2016CTAE, Udaipur 1
Hemwati Kumawat

 Activity of drawings and making pictures composed using simple
lines and strong colors through computer.
 Involves display, manipulation, storage of pictures and experimental
data for proper visualization using a computer .
 System could be active or passive: In both cases, input to the system
is the scene description and output is a static or animated scene to be
displayed.
 Passive Systems: No control over the pictures
 Active systems: control the display with the help of GUI using input device
 Complex & diversify technology.

 Computer Aided Design (CAD)
 Graphical User Interfaces (GUI)
 Desktop Publishing
 Scientific visualization and Business Plotting
 Entertainment
 Process Monitoring
 Cartography
 Multimedia
 Digital Image Processing
 Education & Training

 Computer Aided Design (CAD) used for
 Designing components of mechanical, electrical and electronics devices
 Building Structures
 Automobiles Parts
 Telephone & Computer Networks
 Circuits & Networks of water Supply
 Helps the designer to choose the optimize design with minimum
no. of components, smaller size, efficient and cost effective
systems.
 Helps to study the appearance of buildings, industrial complex
before giving them a final look.
 Animation can be used to test the performance of vehicle or other
systems.

 Graphical User Interfaces (GUI) used to
 Interact with an Application in easier way.
 Components of GUI
 Menus – helps the user to reach the application
 Icons – Symbols , represents the option
 Cursors
 Dialog boxes
 Scroll bars – To slide Up and Down, Right and Left.
 Buttons
 Valuators – Resembles volume control dial on traditional radio sets
 Grids
 Sketching
 3D Interface

 Desktop Publishing window system help in multiple Activities:
 Word Processing
 Spread Sheets
 Incorporating
 Texts
 Pictures
 Generating
 Cutting and Pasting from documents
 Various fonts and styles and colors to make the documents more
presentable.
 Scientific visualization and Business Plotting used to study
scientific phenomenon like:
 Fluid flow
 Nuclear
 Chemical reaction
 Physiological System
 Organ Function

 Business Analysits need to analyze large amount of data.
 If numerical data is converted into visual form (charts & graphs), analyzing
the same data becomes easier.
 Entertainment Computer Graphics include:
 Motion Pictures
 Music Videos
 Television Shows
 Movement of the cartoons figures are controlled by computer
commands.
 Process Monitoring Used in displaying the status of
 Refineries
 Power Plants
 Visualizing data values from sensors attached to critical system
components.
 Proves helpful in defence fields, flight control, space craft
controlling.
 Space craft controllers monitor telemetry data and take corrective
action is needed.

 Cartography used to produce Accurate and Schematic
Representations of Geographical and natural Phenomenon from
measured data
 Example:
 Geographic maps
 Relief maps
 Exploration chats
 Weather maps
 Population Density maps
 Multimedia Concerned with the computer controlled integration
of :
 Text
 Graphics
 Drawing
 Still & Moving Images(video)
 Animation audio
 Other media where energy type of information can be represented, stored,
Transmitted & Processed Digitally.

 Digital Image Processing
 Concerned with the modification of the picture Quality.
 Picture quality Improvement or enhancement includes elimination
of noise(missing pixel data) & enhancing contrast to improve
visibility.
 Education And Training Used in
 Pie Charts
 Bar Charts
 Birth Rate
 Helps them to link to the real world scenario
 Example:
 Simulators for practice sessions
 Training of ship captains or aircrafts Pilots
 Heavy Equipment
 Various utilities & tools available for web baesd design
include: Java, XML, GIF animators.

 Display Devices depends on CRT
 CRTs used for display purposes is same as used in black and white home
TV sets.
DESIGN OF CRT
 The electron gun emits a beam of electrons or cathode rays which
passes through focusing system and magnetic deflection coils and
finally hits the phosphor coated screen.
 Phosphor then emits small spots of light at point where electron
beam strikes.
 The emitted light fades away very rapidly, so picture is not
maintained for long on the screen.
 The solution to this problem is, repeatedly strike on the beam on
the same point. Thus, phosphor keeps glowing and picture is
visible on the screen for long time. This type of display is called
Refresh CRT .

 Magnetic Deflection: Achieved using two pairs of magnetic coils within
the CRT , one pair is on the top and below any one pair is on the opposite
side of CRT
 Electric Deflection: Achieved by using two pairs of deflection places
inside the CRT . One pair is mounted vertical and other is mounted
horizontal.
 Horizontal Deflection : Plates provide vertical deflection to the electron
beam and vertical deflection plates provides horizontal deflection to the
electron beam.

 The cathode rayed tube can easily increase the monitor’s
brightness by reflecting the light.
 They produce more colours
 The Cathode Ray Tube monitors have lower price rate than the
LCD display or Plasma display.
 The quality of the image displayed on a Cathode Ray Tube is
superior to the LCD and Plasma monitors.
 The contrast features of the cathode ray tube monitor are
considered highly excellent.

 They have a big back and take up space on desk.
 The electromagnetic fields emitted by CRT monitors constitute a health
hazard to the functioning of living cells.
 CRTs emit a small amount of X-ray band radiation which can result in a
health hazard.
 Constant refreshing of CRT monitors can result in headache.
 CRTs operate at very high voltage which can overheat system or result
in an implosion
 Within a CRT a strong vacuum exists in it and can also result in a
implosion
 They are heavy to pick up and carry around

 Dot Pitch : Represents the distance with separate two phosphor dots to
the same color
 The lower value of the dot pitch picture provide better image quality
 Dot pitch =< 0.025 mm Comfortable to use
 While monitor with dot => 0.28 mm should be avoided
 Persistence: Time it takes the emitted light from the phosphor screen to
decay the 10% of the original light output
 Refresh Rate: Represents the number of image which are displayed per
second or the number of times the image is reproduced each second
 Also called vertical scan rate, expressed in hertz(Hz) or Frames per sec
 Minimum refresh rate is 60 frames per sec
 Resolution: Represents the number of pixels per surface unit
 Definition: Number of pixels that the screen can display
 This number is between 640x 480 and 1600 x 1200, but higher resolutions are
technically possible.

 Size Calculated by measuring the diagonal of the screen and is expressed
in inches
 Aspect Ratio : Ratio of vertical points to the Horizontal points necessary
to produce equal –length lines in both direction on the screen
 Aspect Ratio = (Yp/Ys) / (X p / Xs )
 Y p = Total number of vertical points
 Ys = Maximum vertical height
 X p = Total number of horizontal points
 Xs = Maximum horizontal height
 Types of CRT display devices
 Direct view storage Tube
 Random Scan Display
 Refresh and Raster Scan Display System

 Simplest CRT display devices.
 Also known as Bistable Storage Tube
 Display System is a line drawing display
 Line can be drawing directly from any addressable point to any other addressable point
 Two electron guns are used in DVST
 Primary Gun: To store the Picture pattern
 Flood gun: Maintains the picture display
 To draw a line or character on display, the electron beam intensity is
increased to cause the phosphor to assume its bright storage stage.
 The display is erased by flooding the entire tube with a specific voltage,
which cause the phosphor to assume its dark state.

 CRT with long Persistence Phosphor
 Means light emitted by Phosphor doesn’t decay fast.
 Provides flicker free display and capable of displaying an unlimited
number of vectors or any complex pictures.
 Screen has a Storage mesh in which the phosphor is embedded
 Stores the picture information as a charge distribution just behind the
Phosphor coated screen
 No Refreshing is required
 Easier to program
 Limited interactive Support

 Slow moving electron beam take few seconds to draw complex picture
 Complex picture erase
 Dynamic motion & Animation is not possible with DVST
 Can’t display color

 Electron beam directly strikes those parts of the screen where a picture has to
be drawn
 Also known as Vector Displays or Stroke - writing Displays or
Calligraphic Displays
 As it draws one line at a time
 The lines can be drawn in any Arbitrary order of the display commands, i.e it
is called Random Scan System.
 The display commands or the display list (or programs) is stored in refresh
buffer to draw the picture.
 Every time we refresh the screen, we have to cycle through this refresh
buffer, one line at a time
 The number of lines in the buffer decides the refresh rate
 In general, refresh rate is 60 frames per sec

 Pen plotter devices based on Random System of display picture
 If the number of lines to display in refresh buffer is small then refresh
cycle is decayed . So as to maintain the refresh rate of 60 frames per sec.
 Otherwise refresh rate can burn out the phosphor
 A random scan system draws the component lines of an object in any
order specified
 Phosphor in random system has Short persistence of 10-100 micro second
 Animation is possible with segmentation
 Mixture of static and dynamic parts of picture
 Produce smooth line drawing

 Advantages:
 This minimal amount of information translates to a much smaller
file size. (file size compared to large raster images)
 On zooming in, and it remains smooth
 The parameters of objects are stored and can be later modified
(transformation).
 High resolution since picture definition is stored as line drawing
commands, easy animation, and requires little memory (just display
program)
 Disadvantages : Limited colour capability and flicker occurs as
complexity of image increases
I/O Devices
System Bus
CPU
System
Memory
Display
Processor
Monitor

 Raster: A rectangular array of points or dots
 Pixel: One dot or picture element of the raster
 Scan Line: A row of pixels

 In a raster scan system, the electron beam is swept across the
screen, one row at a time from top to bottom.

 As the electron beam moves across each row, the beam intensity is turned on
and off to create a pattern of illuminated spots.
 Picture definition is stored in a memory area called the refresh buffer or
frame buffer.
 Refresh buffer or frame buffer: This memory area holds the set of intensity
values for all the screen points.
 Stored intensity values then retrieved from refresh buffer and “painted” on
the screen one row (scan line) at a time.
 Intensity range for pixel positions depends on the capability of the raster
system.
 A black-and-white system: each screen point is either on or off, so only one
bit per pixel is needed to control the intensity of screen positions.
 On a black-and-white system with one bit per pixel, the frame buffer is called
bitmap.
 For system with multiple bits per pixel, the frame buffer is called pixmap.
 Sometimes, refresh rates are described in unit of cycles per second, or Hertz
(HZ)
 Refreshing on raster scan displays is carried out at the rate 60 to 80 frame per
second.

 Horizontal retrace: The return to the left of
the screen, after refreshing each scan line
 Vertical retrace: At the end of each frame (displayed
in 1/80th
to 1/60th
of a second) the electron beam returns
to the top left corner of the screen to begin the next
frame.

 A raster system containing a separate display processor (graphics
controller, display coprocessor)
 The purpose of the DPDP is to free the CPU from the graphics chores.

 Display Processor
 A major task of the display processor is Scan Conversion.
 Scan Conversion: is digitizing a picture definition given in an
application program into a set of pixel intensity values for storage
in the frame buffer.( scan conversion straight line segment,
Character )
 Generation various line styles (dashed, dotted, or solid)
 Displaying color areas
 Performing certain transformation and manipulation on display
objects.

 Interlacing: On some raster systems (TV), each frame is displays in two
passes using an interlaced refresh procedure.
 On an older, 30 frame per-second, noninterlaced display, some flicker is
noticeable.
 With interlacing, each of the two passes can be accomplished in 1/60th
of
a second.
 An effective technique for avoiding flicker

 Scan frame 30 times per second
 To reduce flicker, divide frame into two fields—one consisting of the
even scan lines and the other of the odd scan lines.
 Even and odd fields are scanned out alternately to produce an interlaced
image.

 Raster Image: The quality of a raster image is determined by the total
number pixels (resolution), and the amount of information in each pixel
(color depth)
 Raster graphics cannot be scaled to a higher resolution without loss of
apparent quality.
 Advantages
 Low cost (memory has become cheap)
 Refresh rate independent of image complexity
 Can handle colour and filled areas
 Disadvantages
 Uses more memory
 Mathematical models of objects must be scan converted by the host
processor
 aliasing
 jagged lines that are plotted as discrete points

 A class of video devices that have reduce volume ,weight and power
requirement compared to a CRT.
 A significant feature of flat panel displays is that they are thinner
than CRTs.
 Two main categories:
 Emissive Displays
 Non-emissive Displays
 Emissive or Emitters Displays
 Emissive displays convert electrical energy into light energy.
 Examples: Plasma panel, thin-film electroluminescent displays, Light-Emitting
Diodes (LED) and flat CRT.
 Non-emissive or Non-emitters Displays
 Use optical effects to convert sunlight or light from some other source into
graphics pattern.
 Example: Liquid-Crystal Device (LCD)

 Current uses for flat panel displays:
 Small TV monitors
 Calculators
 Pocket video games
 Laptop computers
 Advertisement boars in elevators
 Flat CRT : Electron beams are accelerated parallel to the screen, then
deflected 90º
to the screen.
 Flat-panel displays are commonly of two types:
 Plasma displays
 LCD displays

 Plasma panels (gas-discharge display)
 A layer of gas (usually neon) is sandwiched between two glass plates.
 By applying high voltage to a pair of horizontal and vertical conductors,
a small section of the gas (tiny neon bulb) at the intersection of the
conductors break down into glowing plasma of electrons and ions.
 Contracted by filling the region between two glass plates
with a mixture of gases
 Refresh buffer used to store picture information
 Firing voltages applied to refresh the pixel positions

 Used in small systems, such as calculators, laptop computers.
 Produce a picture by passing polarized light (from the surrounding or from an
internal light source) through a liquid-crystal material that can be aligned to
either block or transmit the light.
 Uses small amount of power, so is good for battery-operated devices
 Liquid crystal: These compounds have a crystalline arrangement of molecules,
yet they flow like a liquid.
 Two glass plates, each containing a light polarizer at right angles to the other
plate, sandwich the liquid crystal materials.
 Rows of horizontal transparent conductor & columns of vertical conductors
(put into glass plates)

 Polarized light passing through the material is twistedtwisted so that it will
pass through the opposite polarizer.
 The light is then reflected back t the viewer.
 To turn off the pixel, we apply a voltage to the two intersecting
conductor to align the molecules so that the light is not twisted.
 Different intensities
 Voltage controls amount of twist, and therefore the amount of light
passing through.
 Color displays
 Each pixel has three sub-pixels: Red, Green and Blue
 Different intensities of the sub-pixels give perception of whole
spectrum of colors

On StateOn State
Off StateOff State

 Transmissive and reflective displays
 Transmissive display: light source is from a backlight, and viewed from the
front. Example: Computer display
 Reflective display: Use external light reflected by a reflector behind the
display. Example: watch, calculator
 Transmissive displays are brighter
 Passive and active matrix
 Passive matrix:
 Pixels are addressed one at a time by row and column matrix
 Pixels must maintain state between refresh without steady electric
charge
 Only used in low-resolution displays (such as watch, calculator)
 Slow response time, poor contrast
 Active matrix:
 Each pixel has its own transistor
 Each row line is activated sequentially
 When one row line is activated, all column lines are connected to this
row
 Used in computer displays
 Disadvantages of LCD
 Lower contrast ratio than plasma or CRT
 Only crisp images in native resolution or whole fraction
 Small viewing angle

 A CRT monitor displays color pictures by using a combination of
phosphors that emit different colorcolor lights.
 An electron gun for each colour – red, green and blue
 Color CRTs are much more complicated
 Requires manufacturing very precise geometry
 Uses a pattern of color phosphors on the screen
 Methods Of Color CRT Monitor
 Beam Penetration
 Shadow Mask

 Two layers of phosphorphosphor (red and green) are coated onto the inside of the
CRT screen.
 The display color depends on haw far (length) the electron beam
penetrates into the phosphor layers.
 The speed of the electrons, and the screen color at any point, is controlled
by the beam acceleration voltage
 The beam penetration method:
 Used with random scan monitors
 Only four colors are possible (red, green, orange, and yellow).
 Quality of pictures is not as good as with other methods.

 Three color phosphorphosphor dots (red, green and blue) at each point on the
screen
 Three electron gunselectron guns, each controlling the display of red, green and blue
light
 Used in raster-scan systems
 Produce wide range of color with RGB color model
 The delta-delta method:

 The in-line method:
 We obtain color variations by varying the intensity levels of the three
electron beam.
 Shadow mask methods are:
 Used in raster scan system (including color TV)
 Designed as RGBRGB monitors.
 High quality raster graphics system have 24 bits per pixel in the frame
buffer (a full color system or a true color system)

 Commonly used today
 Mouse-like devices
 mouse
 wheel mouse
 Trackball
 A ball device that can be rotated with fingers
 Spaceballs
 Provide six degrees of freedom
 Joysticks
 Data grove
 Keyboards
 Button Boxes, Dials
 Digitizer
 Device for drawing, painting, or interactively selecting position
 Graphics tablet
 3D digitizer

 Pen-based devices
 pressure sensitive
 absolute positioning
 tablet computers
 IPAQ, WinCE machines
 Microsoft eTablet coming soon
 palm-top devices
 Handspring Visor, PalmOS™
 3D Input Device
 Electromagnetic trackers
 can be attached to any head, hands, joints, objects
 Polhemus FASTRAK™(used in Brown’s Cave)
 Gloves
 attach electromagnetic tracker to the hand
 Pinch gloves
 contact between digits is a “pinch” gesture
 in CAVE, extended Fakespace PINCH™ gloves with extra contacts

 There are a range of output devices currently available:
 Printers/plotters
 Cathode ray tube displays
 Plasma displays
 LCD displays
 3 dimensional viewers
 Virtual/augmented reality headsets

 Desktop
 Vector display
 CRT
 LCD flatpanel
 Plasma
 workstation displays(Sun Lab)
 PC and Mac laptops
 Tablet computers
 Wacom’s display tablet
 Digital Micromirror Devices (projectors)
 Field Emission Devices (FEDs)
 Organic Light-Emitting Diode (OLED) Arrays
 Immersive
 Head-mounted displays (HMD)
 Stereo shutter glasses
 Virtual Retinal Display (VRD)
 CAVE™

 Classifications
 Special purpose package
 Designed for non-programmers
 No graphics procedures involved
 Communicate with a set of menus
 General programming package
 Designed for programmers
 Provide library of graphics functions used in programming language
 Two general classifications
 general programming package
 provides a library of graphics functions that can be used in a
programming language such as C, C++, Java, or Fortran
 CG API
 Computer-graphics application programming interface
 Ex) GL, OpenGL, VRML, Java2D, Java3D, Direct3D

 Special purpose package
 design for nonprogrammers who want to generate picture, graphs
 ex) 3D MAXstudio, Autocad, MAYA, WAVEFRONT, ALIAS,
SOFTIMAGE for Workstations
 Software standard
 GKS (Graphical Kernel System) 1984
− 2-D oriented package
 PHIGS (Programmer’s Hierarchical Interactive Graphics Standard)
− extension of GKS
− Increased Capabilities for modeling, rendering
 OpenGL
 GL (Graphics Library)  Hardware Independent
 OpenGL Architecture Review Board
 Script Language
 Render Man, Radiance, PovRay

 Digital Differential Analyzer is a scan conversion line algorithm based on
calculating either dy or dx.
 DDA Algorithm is used to plot line between two nodes i.e two end points
in computer system.
 While our computer understand pixels, if we want to plot a line, we
should have maximum intermediate vertices of the line i.e intermediate
points, so as to generate a straight line. Here DDA does the same.
 Used for linear interpolation of variables over an interval between start
and end point. DDAs are used for rasterization of lines, triangles and
polygons.

 Define the nodes, i.e end points in form of (x1,y1) and (x2,y2).
 Calculate the distance between the two end points vertically
and horizontally, i.e dx=|x1-x2| and dy=|y1-y2|.
 Define new variable name ‘pixel’, and compare dx and dy
values,
if dx > dy then
pixel=dx
else
pixel =dy.
 dx=dx/pixel
and dy=dy/pixel
 x=x1;
y=y1;
 while (i<=pixel) compute the pixel and plot the pixel with
x=x+dx and y=y+dy.

THANKS!☻

Computer Graphics

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