1. Introduction to Computer Graphics
Lecture 1

2. Objectives
We explore what computer graphics is about and survey some application areas
We start with a historical introduction
Course Instructor: Sadia Arshid

3. Computer graphics deals with all aspects of creating images with a computer
Hardware
Software
Applications
Course Instructor: Sadia Arshid

4. Example Where did this image come from?
What hardware/software did we need to produce it?
Course Instructor: Sadia Arshid

5. Preliminary Answer Application:
The object is an artist’s rendition of the sun for an animation to be shown in a domed environment (planetarium)
Software: Maya for modeling and rendering but Maya is built on top of OpenGL
Hardware: PC with graphics card for modeling and rendering
Course Instructor: Sadia Arshid

6. Use of Computer Designed Pictures
Art, Entertainment and publishing
Movie production, Animation and special effects
Computer Games
Browsing on world wide web
Paint System
Course Instructor: Sadia Arshid

7. Use of Computer Designed Pictures
Monitoring a process
Displaying simulations
Computer aided designs
Scientific analysis and visualization
Course Instructor: Sadia Arshid

8. Computer Graphics & Image Processing
create pictures and images
Synthesize them on basis of some description
Model
Image Processing->
Improve or alter the images
Processing can remove noise or enhance contrast
Enhance certain features.
Course Instructor: Sadia Arshid

9. Computer Graphics & Image Processing
Out put
Input
Image
description
image
Image Processing
Pattern Recognition
Description
Computer Graphics
Data Processing
Course Instructor: Sadia Arshid

10. Elements of pictures Created in Computer Graphics
Basic objects are
Poly line
Text
Filled regions
Raster images
Course Instructor: Sadia Arshid

11. Elements of pictures Created in Computer Graphics
Poly Lines
Connected sequence of straight lines
Line Drawings
Pictures made up of poly lines
If several lines in poly line
Each line is called an edge
Two adjacent lines meet at vertex
A polyline is closed if it ends where it starts called polygon
It is simple if it does not self-intersect
simple polygon(Jordan polygons)
Well defined two regions(inside and out side)
convex polygon.
Course Instructor: Sadia Arshid

12. Course Instructor: Sadia Arshid

13. Attributes of Lines & Poly Lines
Color
Thickness
Type( dashed, solid, dotted)
Manner in which thick edges are blend
Course Instructor: Sadia Arshid

14. Filled Region Shape filled with some color or pattern.
Boundary of filled shape is a polygon
Attributes
Enclosing border
Pattern
color
Course Instructor: Sadia Arshid

15. Course Instructor: Sadia Arshid

16. Text Text can be thought of as a sequence of characters in some font.
Some Graphic devices has two modes
Text Mode
Graphic Mode
Use for simple input & out put Characters to control operating system or edit code in program
Built in character generator
Capable of drawing Alphabets, Numeric & special characters
Place data on pre defined grid
Do not support images or tables
Course Instructor: Sadia Arshid

17. Graphic Mode Attributes: Richer set of character shapes
Characters can be placed arbitrarily
Attributes:
Font Face
Weight
Size
Style
Spacing
Orientation
Course Instructor: Sadia Arshid

18. Course Instructor: Sadia Arshid

19. Raster Images Image is made up of many cells, in different shades.
Each cell is known as pixel.
Stored in memory as 2-dimensional array of square/ rectangle cells
pixel maps/bitmap
Course Instructor: Sadia Arshid

20. How Raster Images are created
Hand Designed Images
Designer figure out value of each pixel and store it in memory
Paint program
Computed Images
Algorithm is used to render a scene.
Scanned Images
Digitization
Course Instructor: Sadia Arshid

21. Representation of Shades in Raster images
Monochrome raster images
Pixel can contain one of two values in a raster image(bi-level).
Grey level images
Pixel depth( number of bits needed to represent grey levels)
n bit quantity has 2n possible values
Course Instructor: Sadia Arshid

22. Grey Scale Quantization
Let we have original image using 8 bit per pixel
Display it or alter it with some less value per bit
Either display image is not capable of displaying 256 values
Image takes too much memory
This loss is hardly noticeable if we use 5 or 6 bits per pixel
Banding
Course Instructor: Sadia Arshid

23. Color Raster Images
Commonly each pixel is Ordered triple with red, green and blue value
Number of bits used to represent color of each pixel is called Color depth.
Color depth of 3 allows one bit for each color.
True color image- color depth of 24 bit
Course Instructor: Sadia Arshid

24. Digitization Digitization involves Quantization Sampling
Converting set of “continues values” to discrete
Decide number of bits required to save each pixel value
Sampling
Digitizing coordinate values
Course Instructor: Sadia Arshid

25. Quantization Effect
Original image 256 levels, 128 levels,64levels ,32levels,16levels,8levels, 4levels, 2levels
Course Instructor: Sadia Arshid

26. Quantization Effect 8 bits 4 bits 2 bits 1 bit
Course Instructor: Sadia Arshid

27. Sampling Effect
(a) 1024*1024, 8-bit image. (b) 512*512 image resampled into 1024*1024 pixels by row and column duplication. (c) through (f) 256*256, 128*128, 64*64, and 32*32 images resampled into 1024*1024 pixels.
Course Instructor: Sadia Arshid

28. Resolution Number of pixels per inch(PPI) or DPI
Higher the resolution, greater number of pixels
Smaller resolution, smaller digital Image
Keep balance between resolution and image size.
Course Instructor: Sadia Arshid

29. Vector graphics
Geometrical primitives such as points, lines, curves, and shapes or polygon(s),
Based on mathematical equations, to represent images in computer memory
They can be enlarged, rotated, stretched without loss of picture resolution
Course Instructor: Sadia Arshid

30. Vector Images vs Raster images
Vector Drawing Applications(Path Based)
Create and modify technical diagrams ie houses, cars
Resolution independent
While zooming edges are always sharp
Raster Painting Applications(Pixel Based)
Create art work
Zooming based on interpolation
Course Instructor: Sadia Arshid

31. Vector images Raster images Usually have no back ground
In appropriate for photo realistic images
Raster images
Pixels in grid
Resizing reduce quality
Minimal support for transparency
Course Instructor: Sadia Arshid

32. Course Instructor: Sadia Arshid

33. Graphics Display Devices
Course Instructor: Sadia Arshid

34. Vector Displays Early computer displays: basically an oscilloscope
Control X,Y with vertical/horizontal plate voltage
Intensity or brightness of the display is sometimes called the Z axis.
Course Instructor: Sadia Arshid

35. Vector Drawing’s Commands
Course Instructor: Sadia Arshid

36. Each line segment to be displayed takes very few data values
Vector displays can draw a picture very rapidly
Each XY pair is used to generate deflection voltages
which will sweep the beam across the face of screen
When the beam strike the phosphor,
visible light is produced for certain time
Course Instructor: Sadia Arshid

37. Vector Displays: Refresh Rate
Screen refresh every 1/30th of a second
assume approximately 100 microseconds per command 1/30th second = 33 milliseconds = microseconds
Therefore this Vector Display can only process
approximately 330 commands before flicker starts.
Course Instructor: Sadia Arshid

38. Vector Displays Disadvantages
Complex object drawing is difficult
list of commands becomes very long
Complex scenes
visible flicker
first line may have faded out before last is drawn
Vector displays can not show
smoothly shaded regions
Scanned images
Cross hatching effect
Course Instructor: Sadia Arshid

39. Raster Displays Pen plotters Types
move the pen invisibly on the papers on the spots specify by computer
Types
Flatbed Plotters
Move pen in two directions on stationary sheet
Drum Plotters
Move paper back and forth to provide one direction in motion
Move pen back and forth to provide other direction.
Course Instructor: Sadia Arshid

40. 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
Course Instructor: Sadia Arshid

41. Examples Video Monitor Flat panel Laser printer
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42. Dot matrix Ink jet plotter Film recorder
Course Instructor: Sadia Arshid

43. Line/vector display raster display
no pixels array of pixels
Course Instructor: Sadia Arshid

44. Raster Displays Fixed scan pattern
left to right, top to bottom
To paint the screen, computer needs to synchronize with the scanning pattern
Framebuffer
special memory to buffer image with scan-out synchronous to the raster display device.
Region of memory should be large enough to hold all pixel values
Graphic card
House the memory require for frame buffer
Course Instructor: Sadia Arshid

45. Scanning process
RGB Frame buffer
Course Instructor: Sadia Arshid

46. Display Technologies Cathode Ray Tubes (CRTs)
Most common display device today
Evacuated glass bottle
Electrons pulled towards anode focusing cylinder
Vertical and horizontal deflection plates
Beam strikes phosphor coating on front of tube
Course Instructor: Sadia Arshid

47. Course Instructor: Sadia Arshid

48. Color CRTs have Three electron guns
A metal shadow mask to differentiate the beams
Course Instructor: Sadia Arshid

49. Phosphers Flourescence: Phospherescence: Persistence:
Light emitted while the phospher is being struck by electrons
Phospherescence:
Light emitted once the electron beam is removed
emits visible light for a certain period after the bombardment has ceased
Persistence:
Time from the removal of the excitation to the moment when phosphorescence has decayed to 10% of the initial light output
Long persistence -> image smears as picture is moved on screen.
Short persistence -> no smearing as image moves but more refreshing needed
Course Instructor: Sadia Arshid

50. Raster Displays 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, phosphor persistence, lighting...
Scan controller scans quickly through the entire memory of frame buffer
Sending each pixel value to its proper spot on screen surface
Course Instructor: Sadia Arshid

51. Index Colour and lookup table
Suppose we have Color depth 6
Each pixel go through an intermediate step
Before they drive CRT
These bits are used as an index in to a table of 2n values
Course Instructor: Sadia Arshid

52. Color is expensive …
The more color you want, the more bits you will need for each pixel
Exercise: 1024 x 1280 screen with
24 bits per pixel, how large is the frame buffer?
1024 x 1280 x 24 / 8 = 4M Byte
Course Instructor: Sadia Arshid

53. Color Lookup Table only 3 bits per pixel
But I insist on having high quality pictures …
Use Color Look Up Table (LUT)
3 bits/pixel frame buffer
R G B
You can still have 24 bits
in each of the color table
entries 1 2 3 4 5 6 7
24 bit wide
Course Instructor: Sadia Arshid

54. Colour Index Framebuffer
Course Instructor: Sadia Arshid

55. LUT memory If Raster display has color depth= b bits
Each LUT entry is w bits wide then
System can display
2w colors 2b at one time
So it require very little memory
Only 2b words of w bits each
System have capability of 24 bit colors
Frame Buffer
1024 x 1280 x 8 / 8 = 1M Byte
LUT
768 bytes
Course Instructor: Sadia Arshid

56. Summarizing LUT Relationship b/w bitplanes and width of LUT
System has color depth of b bits.
LUT entry is w bit wide
System will display: 2w colors 2b at one time
Relationship b/w bitplanes and width of LUT
W is multiple of 3 derive each DAC.
Memory required by LUT
2b *w/8
Storage required by System
b/3*rows *cols + size of LUT
Course Instructor: Sadia Arshid

57. Advantage To reduce cost of memory
Increasing b increases amount of memory needed for the frame buffer
Course Instructor: Sadia Arshid

58. Display Technology: LCDs
Liquid Crystal Displays (LCDs)
organic molecules
naturally in crystalline state
that liquefy when excited by heat or E field
Crystalline state twists polarized light 90º.
Course Instructor: Sadia Arshid

59. Display Technology: LCDs
Converts electric field to visible dots
Each pixel is addressed by horizontal and vertical grid wire
Course Instructor: Sadia Arshid

60. Active matrix panels Are LCD panels
Have tiny transistors at each pixel
Transistors responds to electric field
adjusts the liquid crystal by an amount proportional to the field
Allow the display of different level of brightness
Course Instructor: Sadia Arshid

61. Transistors provide some “memory”
CRT uses 2 patterns
Left to right
Top to bottom
LCD don’t use it
Display whole screen at once
Transistors provide some “memory”
that holds the crystals in their adjusted states
so that the display need not be refreshed
This produce much brighter display
Course Instructor: Sadia Arshid

62. Display Technology Plasma DMD / DLP Organic LED Arrays
Display geometry
Neon bulb turn on and off by electric field
DMD / DLP
optical semiconductor, called a Digital Micromirror Device (DMD),
uses mirrors made of aluminum to reflect light to make the picture
Organic LED Arrays
It consists of a series of organic layers between two electrical contacts (electrodes).
Each layer is deposited on the other, creating a single unit.
Course Instructor: Sadia Arshid

63. Hard Copy Raster Devices
Film Recorder
Strip of photographic film
electron beam exposes as it sweeps over it in raster pattern
Have their own frame buffer or camera mounted on CRT
Electronic Hard copy of Image
Laser Printer
Sweep a laser beam on internal defined buffer
Surface becomes electronically charged causing toner to adhere to spots.
Course Instructor: Sadia Arshid

64. InkJet Plotter Post Script Language
A tiny nozzle sweeps over the paper and squirts proper color of ink at each pixel position.
Post Script Language
Printer are equipped with micro processor.
Generate high quality text and graphics
Course Instructor: Sadia Arshid

65. References Computer Graphics using open GL( chapter 1) by F.S Hill
InterNET Resources
Howstufff works.com/lcd.htm
& A lot More
Course Instructor: Sadia Arshid