Computer Graphics for Begginers

1. Raster Images
 Picture is made up of small cells
called pixels
 Stored in computer memory as
array of numeral values called pixel
map or bitmap
 Raster images are created suing
 Hand designed images
 Computed images (Mathematical
model of a scene)
 Scanned images (support scaling)

2. Gray scale raster images
 Pixel dept (memory required /pixel)
 n-bit memory generates 2n
color values
 Effect of pixel dept
 Quality of image
 Avoid color banding
 Size of image

3. Color Raster images
 Three numeral value for each pixel
 3-bytes true colors

4. Flat Panel Displays
 Emissive
 Flat screen CRT
 Plasma
 LCD with backlight
 Non-Emissive
 LCD

5. Display technologies
 Cathode Ray Tubes
(CRTs)
 Most common display
device today.
 Evacuated glass
bottle
 Extremely high
voltage
 Heating element
(filament)
 Electrons pulled
towards
anode focusing
cylinder
 Vertical and
horizontal deflection
plates
 Beam strikes
phosphor coating on
front of tube

6. CRT

7. Electron gun Contains a filament that, when heated, emits a stream of
electrons
 Electrons are focused with an electromagnet into a sharp
beam and directed to a specific point of the face of the
picture tube
 The front surface of the picture tube is coated with small
phosphor dots
 When the beam hits a phosphor dot it glows with a brightness
proportional to the strength of the beam and how long it is hit

8. CRT characteristics
 What’s the largest (diagonal) CRT you’ve
seen?
 Why is that the largest?
 Evacuated tube == massive glass
 Symmetrical electron paths (corners vs. center)
 How might one measure CRT capabilities?
 Size of tube
 Brightness of phosphers vs. darkness of tube
 Speed of electron gun
 Width of electron beam
 Pixels?

9. Random Scan Display (Vector
Displays) Early computer displays, Control X,Y with
vertical/horizontal plate voltage
 Often used intensity as Z (close things were
brighter)
 Name two disadvantages
Just does wireframe
Complex scenes cause visible flicker

10. Vector Display draws a set of lines in
any order

11. Vector Scanning

12. Vector Displays
 How to generate an image using vectors
 A line is represented by endpoints (10,10) to (90,90)
 The points along the line are computed using a line
equation
y = mx + b
 If you want the image larger, no
problem…

13. Raster Displays
 Raster: A rectangular array of points or dots
 Pixel: One dot or picture element of the
raster
 Scan line: A row of pixels

14. Raster Displays
 Black and white television: A fixed scan pattern: left to
right, top to bottom
 As beam sweeps across entire face of CRT, beam
intensity changes to reflect brightness
 Must synchronize
 Your program makes decisions about the intensity signal
at the pace of the CPU…
 The screen is “painted” at the pace of the electron gun
scanning the raster
 Solution: special memory to buffer image with scan-out
synchronous to the raster. We call this the frame buffer.

15. Raster-scan display system
draws a discrete set of points

16. Line Generation on Raster Display
 How to generate a line using rasters
 A line is represented by assigning some pixels a
value of 1
 The entire line is specified by the pixel values
What do we do to make image larger?

17. Phosphers
 Fluorescence
 Light emitted while the phosphor is being struck
by electrons()
 Phosphorescence
 Light emitted once the electron beam is
removed
 Persistence
 The time from the removal of the excitation to
the moment when phosphorescence has
decayed to 10% of the initial light output

18. Refresh
 Frame must be “refreshed” to draw new images
 As new pixels are struck by electron beam,
others are decaying
 Electron beam must hit all pixels frequently to
eliminate flicker
 Critical fusion frequency
 Typically 60 times/sec
 Varies with intensity, individuals, phospher
persistence, lighting...

19. CRT Timing

20. Color CRT
 Color CRTs are much more complicated
 Requires manufacturing very precise geometry
 Uses a pattern of color phosphors on the screen:
 Why red, green, and blue phosphors?
Delta electron gun arrangement In-line electron gun arrangement

21. Color CRT
 Color CRTs have
 Three electron guns
 Beam penetration and shadow mask methods
 A metal shadow mask to differentiate the beams

22. CRT
 Raster CRT pros:
 Allows solids, not just wireframes
 Leverages low-cost CRT technology (i.e., TVs)
 Bright! Display emits light
 Cons:
 Requires screen-size memory array
 Discrete sampling (pixels)
 Practical limit on size (call it 40 inches)
 Bulky

23. CRTs – A Review
 CRT technology hasn’t changed much in 50 years
 Early television technology
 high resolution
 requires synchronization between video signal and electron
beam vertical sync pulse
 Early computer displays
 avoided synchronization using ‘vector’ algorithm
 flicker and refresh were problematic

24. CRTs – A Review
 Raster Displays (early 70s)
 like television, scan all pixels in regular pattern
 use frame buffer (video RAM) to eliminate sync
problems
 RAM
 ¼ MB (256 KB) cost $2 million in 1971
 Do some math…
- 1280 x 1024 screen resolution = 1,310,720
pixels
- Monochrome color (binary) requires 160 KB
- High resolution color requires 5.2 MB

25. Liquid Crystal Displays (LCDs)
 LCDs: organic molecules, naturally in crystalline state, that liquefy
when excited by heat or Electric field
 Crystalline state twists polarized light 90º.

26. Polarizer
A wire-grid polarizer converts an unpolarized beam
into one with a single linear polarization. Red arrows
depict the electric field vector.

27. Liquid Crystal Display (LCD)

29. Liquid Crystal Displays (LCDs)

30. Display Technology: Plasma
 Plasma display panels
 Similar in principle to
fluorescent light tubes
 Small gas-filled capsules
are excited by electric field,
emits UV light
 UV excites phosphor
 Phosphor relaxes, emits
some other color

31. Plasma Display

32. Thin Film
Electroluminescent Display
 Same as plasma display
 Has phosphor coating instead of gas
 High voltage applied on phosphor
 Require more power
 Good colors are hard to generate
 Another emissive display is LED

33. Direct View Storage Tube
(DVST) Instead of refreshing screen, it Keeps information inside CRT
 Picture information is stored as charge distribution behind the phosphorous
screen
 Two electron guns are used in DVST, the primary gun and flood gun
 Primary helps is storing picture patterns and flood gun maintain picture
display
 No refresh requires, can display complex high resolution picture without flicker
 Usually doesn’t display colors
 Part of a picture can’t be refreshed (redrawn)
 Erasing old picture and redrawing modified picture could take seconds
 No replaced by raster displays

34. Raster Scan System
 Special purpose processor called video/display
controller used to control operations of display
 Video controller accesses frame buffer from
system memory
 Some systems also use graphics/display processor
to relieve CPU of core graphic activities
 Frame buffer locations and corresponding screen
positions are referenced in Cartesian coordinates
 Coordinate origin is decided

35. Raster Scan System
Architedure of a simple raster graphics system.
Architecture of a raster system with a fixed portion of the system
memory reserved for the frame buffer.

36. Raster Scan System
intensity
Basic video-controller refresh operations
The origin of the coordinate
system for identifying screen
positions is usually specified
in the lower-left corner.

37. Raster Scan System
 High quality system provides two frame buffers
 Video controller also transform screen
 Some video controllers use lookup tables (color
palets) and frame buffer values are indexed into
it to retrieve intensities
 Some controllers allows mixing frame buffer’s
image with camera picture

38. Raster Scan Display
Processor
Architecture of a raster-graphics system with a display processor.

39. Raster Scan Display
ProcessorDisplay processors scan convert, lines, curves and other
objects into intensity values
Characters are specified by rectangular grid or curved lines
Character grid can be 5 by 7 or 9 by 12 or more in high quality displays
Character grid is superimposed and character curves are scan converted

40. Raster Scan Display
Processor Digitize/scan-conversion picture definition provided by
application program
 Display processor also performs additional tasks e.g. changing line
styles, coloring areas, transformation on objects etc
 Efforts are made to shorten frame buffer
 One way is to store intensity information as link list and apply run-
length encoding
 Similar method applies when intensities change linearly
 Short runs can increase storage requirements
 Short runs bring difficulty in processing
 Another approach encode raster as set of rectangular areas,
called cell encoding

41. Random Scan System
Architecture of a simple random scan system.

42. Random Scan System
 Display processor also called display processing unit or
graphics controller
 Application program along with graphic package
stored in memory
 Graphics commands of application are transformed by
graphic package into display file, stored in memory
 Display processor goes through each line in display file in
a refresh cycle
 Lines are specified by end points
 Scene is drawn one line at a time and deflection
voltages are directed to fill region b/w end points

43. Hard Copy Devices
 Purpose is to put our graphics/picture on
paper
 Quality of pictures: Dots/inch also known
as resolution
 High quality printing: Adjacent dots
overlaps
 Two type of printers: Impact and non-
impact
 Impact: press formed character faces
against ink ribbon
 Non-impact: Use laser technique, inkjet
spay, xerographic process, electrostatic
or electro-thermal methods

44. Hard Copy Devices
Dot-matrix Printer
 Impact printer
 Dot-matrix head with rectangular array of
protruding wire pins
 Number of pins depends on quality of printer
 Character face or graphic patterns drawn by
retracting certain pins
 Single column vs multi column printing head

45. Hard Copy Devices
Laser Printer
 Have photoelectric,
e.g. selenium, coated
drum
 Graphics are drawn by
creating charge
distribution on drum
using laser beam
 Toner is applied to the
drum
 Toner then applied to
paper to draw graphics
Xerographic Process

46. Hard Copy Devices
Ink-jet Printer
 Paper wrapped on a drum
 Ink is squirted horizontally across the length of paper
 Ink-stream is deflected by electric field
 Produce dot-matrix patterns
 Three types: Thermal ink-jet, Piezoelectric ink-jet and continuous ink-jet
 Thermal inkjet send current through heating element in heating
chamber and make steam explosion
 Current send to piezoelectric material in ink-filled chamber which
changes shape & size and produce pressure
 High-pressure pumps direct ink through a gunbody, microscopic nozzle,
piezoelectric crystal produce accoustic waves which breaks ink stream
into droplets at continuous intervals
 Heating element contract on cooling and suck further ink from reserviors

47. Hard Copy Devices
Pen Plotter
 A plotter is a vector graphics printing device to print
graphical plots
 Move pen across the piece of paper
 Can draw complex line art and text but very slowly
 Can draw filled regions by complex closed lines
 Printer control languages created to send more detailed
command for text
 Paper placed over a roller which moves paper back &
forth for x motion and pen moved back & forth for y
motion
 Fast computing, inexpensive memory and quality raster
graphics with laser and inkjet have replaced pen-plotters

48. Hard Copy Devices
Electrostatic Device
 Place negative charge on the length of paper one
row at a time
 Paper is then exposed to toner
 Toner is positively charged
 Negative and positive charge produce adhesion

49. Hard Copy Devices
Electro-thermal Device
 Heat sensitive coated thermal paper
 Apply selective heating
 Produce output patterns using dotmatrix printing
head
 Image is produced in the area where it is heated
 Two color direct thermal printer produce black and
red color images when heated on two different
temperatures

50. Input Devices
Keyboard
 Used for entering text
 Extra features: screen coordinates, object selection,
menu selection and graphic functions
 Function keys used to activate frequently used
operations
 Cursor control keys used to relocate screen cursor
 Can accompany track ball or joy stick
 Additional numeric keyboard used for fast entry of
numeric data
 Specialized application: buttons, switches and dials

51. Input Devices
Mouse
 Another major input device
 Small handheld box used to reposition screen cursor
 Movement is recorded using wheel or roller in
bottom
 Movement is also detected with optical sensor with
special mouse pad having horizontal and vertical
line grid
 Not fixed, Used for relative changes in screen cursor
 Also accompany two or three buttons to activate
certain function
 Also Z mouse for 3D application

52. Input Devices
Track ball and space ball
 A ball with potentiometer, produce screen
movements
 Unlike track ball, the Space ball doesn’t actually
move
 Strain gauges measure the amount of pressure and
the direction of pressure
 Used for three dimensional positioning in virtual
reality, animation and CAD

53. Input Devices
Joy Stick
 A small vertical lever which steers screen cursor
 Some record physical movement of the stick while
others record only pressure on the stick
 In first movement and direction and in second
pressure and direction is gauged (strain gauge)
 Potentiometer is used
 Might be mounted on the keyboard
 Can also use one or more buttons

54. Input Devices
Image scanner
 Pictures, graphs, charts, text etc can be used as bitmap in
computer memory
 They are stored as color or grayscale grades
 Purpose is to process image objects
 Transform objects
 Crop different areas (Picture editing)
 Image processing
 Extract text
 Available in variety of sizes, from hand held to huge

55. Input Devices
Touch Panels
 Select processing options, displayed
objects or screen positions with touch of
screen
 Touch inputs are devised with optical,
electrical or acoustic methods
 In optical method, infrared LEDs are fixed
along vertical and horizontal edges of the
frame (light detectors on opposite sides)
 Detector detects which horiz and vert
beams are interrupted
 In case of closed LEDs, Avg is taken when
two or more beams are interrupted

56. Input Devices
Touch Panel (continued…)
 Should match the color and contour of system
 In electrical method, two transparent plats, one
conductor and second resister, placed on small
distance
 Both plates touches when pressure is applied
 Voltage drop in resister plates is recorded as screen
coordinates
 In acoustic method, acoustic waves are generated
across horiz and vertical directions
 Interruption causes reflection of waves
 Time interval is calculated to find screen
coordinates

57. Input Devices
Light Pen
 Record screen location when electron been passes
to that point
 Don’t detect environment light
 Can’t detect black spots
 Application must keep non-zero intensity for black
screen position
 Few other disadvantages

58. Voice System
 System is trained for a particular operator
 Frequencies are recorded for different words for the
operator
 System record word and operation performed on
those words
 On use, operator voice is searched in dictionary
and operation for that word is performed
 User don’t have to switch among devices

59. Visible spectrum
(Continued)

60. Visible spectrum
(Continued)

61. Human vision system

62. Human vision system
 Human eye is a spherical camera with 20mm focal length
 Lens focus the image on retina
 Iris control the amount of light passing through pupil
 Each eye is populated unevenly with 100m receptor cells
 Near the fovea dense concentration of color receptor called
cons
 Density of black & white receptors, called rods, increases
away from the center and cons decreases
 Three different types of cons (with different chemical
properties) are sensitive to light of three different wavelengths
 human is tri-chromate

63. Color model
 Provide mechanism for encoding of color in
visible spectrum
 Mathematical model for describing colors as
tuple of numbers
 Numbers are usually three or four
 Gives precise definition of how components are
interpreted
 Usually define some primary colors

64. RGB Color Model

65. CMY Color Model
 Cyan, Magenta, Yellow
 Subtractive color scheme (Subtract from
white, transparent inks)
 Colors which are not absorbed, determine
color
 Cyan absorb red, magenta absorb green
and yellow absorbs blue
 White(0,0,0): No component of light is
absorbed
 Black(255,255,255): ???

66. CMYK Color Model
 Cyan, Magenta, Yellow, Key
(Black)
 Same as CMY
 Black is added to improve
reproduction of some dark
colors
 To save money on ink, and to
produce deeper black tones,
unsaturated and dark colors are
produced by substituting black
ink for the combination of cyan,
magenta and yellow

67. HSI or HSV Color Model
 Hue, Saturation,
Intensity or Value

68. HSI or HSV Color Model
 Separate Intensity from chromacy – Hue and
Saturation
 Hexagon or hexacon is formed by projecting
RGB color cube along its major diagonal
 Vertical intensity axes
 H is defined angle between 0 and 2Π relative
to a-axes (0 angle mean pure red)
 S is defined as color purity (1 pure i.e.
saturated and 0 for unsaturated i.e. some
shades of gray)
 Convenient for graphics designers, provide
direct control over hue and brightness

69. 2D Trasformation
 Changes in the object orientation, size and shape
is accomplished by 2D transformation
 Basic transformations
 Translation
 Rotation and
 Scaling

70. 2D Trasformation
Translation
 Object is moved along a line path from one
coordination to another
 It’s a rigid body transformation (no deformation)
 (x, y) Original coordinate location
 (tx, ty) translation distance, also called the translation
vector or shift vector
 (x’
,y’
) new coordinate location

71. 2D Trasformation
Translation
 Can be expressed as
 X’
= x + tx y’
= y + ty
 Can also be expressed in matrix form

72. 2D Trasformation