1. Interactive Computer Graphics Introduction
James Gain and Edwin Blake
Department of Computer Science University of Cape Town
July 2002
Collaborative Visual Computing Laboratory

2. Interactive Computer Graphics Contents
Map of the Lecture
Applications of Computer Graphics
Graphics Systems
Evolution of Computer Imagery
Course Information
Topics
Practicals
Background Reading
Marks
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3. Applications of Computer Graphics I
A driving force behind CG innovation
Display of Information
Floor plans in Architecture
Maps in GIS
Imaging in Medicine
Scientific and Financial Visualization
Digital Publishing
Design
Architecture
Mechanical Parts
VLSI Circuits
Display - exploit the human visual systems ability to process data and recognize patters
Design - from specification, through iterative design, to automated analysis
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4. Applications of Computer Graphics II
Simulation
Virtual Reality Training (flight, surgery, etc.)
Robotics Research
Games
Special Effects and Animation for Film
User Interfaces
WIMP
Internet Browsers
Innovative Interfaces
VR Training - replaces hazardous or expensive training situations
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5. Interactive Computer Graphics Contents
A Graphics System
Elements of a CG System:
Processor, Memory, Frame Buffer
Input Devices, Output Devices
Most PC systems have all these components, but often have specialized graphics processors
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6. Pixels and the Frame Buffer
Raster (as opposed to vector) Display
Picture swept out in horizontal rows
Pixels (picture elements)
Squares of a uniform colour on a grid
Frame buffer
Memory store of pixels
Depth
Number of bits used for a pixel
True colour has >= 24 bits
Often use RGB, a combination of red, green, blue primaries
Rasterization (scan conversion)
converting geometric primitives (lines, polygons) to pixels
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7. Interactive Computer Graphics Contents
Output Devices
Cathode-Ray Tube (CRT)
Electron beam steered by deflection plates to strike and activate phosphors
Only emits for milliseconds and needs to be refreshed
Refresh Rate
Sufficlently high redisplay to avoid flicker
Interlacing displays odd and even rows alternately
Colour Displays
a triad of coloured phosphors
A shadow mask ensures only the correct phosphors are excited
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8. Interactive Computer Graphics Contents
Input Devices
Keyboard
Input characters
“QWERTY” originally designed to slow typists down
2-D Locators:
Mouse, light pen, data tablet
3-D Locators
Trackers, 3-D Mouse
Haptics (Force Feedback)
Phantom
Data can also be input from dials, switches, digital camera
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9. Interactive Computer Graphics Contents
A Sequence of Images I
Wireframe
Flat Shaded
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10. Interactive Computer Graphics Contents
A Sequence of Images II
Textured
Smooth Shaded
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11. A Sequence of Images III
Modelling: Set Operations
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12. Interactive Computer Graphics Contents
Final Image
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13. Interactive Computer Graphics Contents
Exercise: Realism
Question: Consider some recent computer animated films (Shrek, Toy Story 2, Final Fantasy, Ice Age). Which aspects do you feel where particularly realistic and which unrealistic? Try to name particular scenes.
Answer: - +
Subsurface lighting (skin, milk, etc)
Hair
Facial animation and lip synching
Complex shapes
Perfection (no dust, dirt and dents)
Physical effects (wind, water, fire, cloth)
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14. Interactive Computer Graphics Contents
Image Formation
Combining objects and viewers to produce an image
Objects:
Exist independently of image formation
Constructed from geometric primitives (vertices)
Viewers:
Form the 2-D image of the objects from a particular perspective
Object and viewer are 3-D, image is 2-D
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15. Interactive Computer Graphics Contents
Light and Images
Light interacts with objects and a portion enters the camera lense or eye
Visible light has wavelengths in the range nm
CG often assumes point light sources that emit constant intensity light from a single location
CG often ignores the wave nature of light
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16. Interactive Computer Graphics Contents
Ray Tracing
Rays (semi-infinite lines) are traced from the light source. Some of them eventually strike the image plane
Light may be reflected, scattered or refracted as it strikes objects
Ray tracing is based on this principle but is (arguably) too computationally costly in many cases
A simple approximation is to assume uniform ambient light levels
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17. Interactive Computer Graphics Contents
Human Visual System
Cones:
Central high resolution colour
Three types, roughly r, g, b
Rods:
peripheral low resolution monochromatic
Visual acuity measures how closely two point can be placed and still distinguished
Respond differently to different wavelengths according to the CIE standard observer curve
Signal sent along optic nerves to visual cortex for very sophisticated high-level processing
16000 pixels across high at 15 cm range
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18. The Synthetic Camera Model
Computer generated and optical (camera) image formation are equated
Image of a point is found by drawing a line from the point through the centre of projection of the lense onto the projection plane
CG moves the projection plane in front of the camera and uses a clipping window to limit the size of the image
Together the centre of projection, projection plane and clipping window define the image
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19. Application Programmer’s Interface
Shield a programmer from the underlying hardware and software
Provide a system metaphor (the synthetic camera model)
Examples: OpenGL, PHIGS, Direct3D, Java-3D
API needs to allow specification of:
Objects
Viewer
Light Sources
Material Properties
OpenGL code for a triangle
glBegin(GL_POLYGON);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 1.0);
glEnd();
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20. The Modelling-Rendering Paradigm
Modelling of the scene can be separated from production of the image
Different requirements (modelling - interactive, rendering - high quality)
Objects are viewer independent
Leads to specialisation (e.g. RenderMan renderer and FreeForm modeller)
Dominant in CAD and SFX
Approach adopted in this course (rendering first half, modelling second half)
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21. Graphics Architectures
For efficiency implement much of the graphics API in hardware
Display processors:
Unburden the host processor
Compile primitives as a display list rendered by the display processor
Pipeline Architectures:
Increases throughput where a sequence of identical operations are applied to a stream of data
Geometry has this property
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22. Interactive Computer Graphics Contents
Geometric Pipeline
Transformations:
Need to transform from object to camera to screen coordinates
Encoded as a 4x4 matrix
Clipping:
Limited field of view so cut geometry against the clipping rectangle
Projection:
Flatten objects from 3-D to 2-D
Rasterization:
Convert objects to pixels in the frame buffer
But need to balance latency (time for a single datum to pass) against throughput (rate at which datum are output)
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23. Interactive Computer Graphics Contents
Course Information
Interdisciplinary (little bits of physics, maths, psychology, engineering)
Sufficient to be an independent practitioner
3rd period, Room 302 (alternates with CPL/PLT)
30 lectures
Course information on the WWW:
Lecturers:
Dr James Gain
Prof Edwin Blake
Textbook:
“Interactive Computer Graphics: A Top-Down Approach with OpenGL” (2nd ed.), E. Angel, Addison-Wesley, 2000
Automobile analogy
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24. Interactive Computer Graphics Contents
Topics
Introduction - ch. 1
Graphics Programming (Basic OpenGL) - ch. 2
Geometric Objects and Transformations - ch. 4
Viewing - ch. 5
Shading - ch. 6
Implementation of a Renderer - ch. 7
Hierarchical and Object-Oriented Graphics - ch. 8
Input and Interaction - ch. 3
Discrete Techniques - ch. 9
Curves and Surfaces - ch. 10
Procedural Methods - ch. 11
Modelling - additional topic
Special Effects - additional topic
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25. A Guide to Surviving the Mathematics
Contrary to expectations CG does not require advanced maths
Master the mathematics before the course (do exercise 0)
Mostly Linear Algebra
Vectors
addition
dot product, cross product
Line equations, plane equations, normals
Matrices, transformations
Curves and approximations
polynomials
circles, ellipses, parabolas
parameters, functions
1. Find the dot products for the following pairs of vectors:
a. (1,1,1) • (1,1,-1)
b. (2,5,3) • (2,5,3)
c. (1,0,0) • (10,1,5)
2. Find the length of the vector (2,5,3).
3. Let i = (1,0,0), j = (0,1,0) and k = (0,0,1). By considering the meaning of cross product (´) write down the answers to:
i ´ i, j ´ j, k ´ k, i ´ j, j ´ i , i ´ k , k ´ i, j ´ k, k ´ j
4. Using the result in 3. and the fact that (x,y,z) = xi + yj + zk find an expression for p ´ q where p = (x1, y1, z1) and q = (x2, y2, z2). Hence find the cross products for each of (a), (b), (c) in 1.
5. The equation of a plane is ax + by + cz = d. (Under what circumstances will d=0?). Find a,b,c,d for each of the cases:
a. The plane is the XY axis.
b. The plane is the XZ axis.
c. The plane is the YZ axis.
d. The plane is parallel to the XY axis and of distance D from the origin.
6. Describe the geometric meaning of the planes:
a. x = y
b. x+y = 0
c. x+y+z = 0
d. x+y+z = 1
7. Find the equation of the plane which passes through the following three points: (1,0,0), (0,1,0), (0,0,1).
8. In general given the points (x1, y1, z1), (x2, y2, z2), (x3, y3, z3) find the equation of the plane which passes through these points, by using the plane equation ax + by + cz = d.
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26. Interactive Computer Graphics Contents
Further Reading
The Graphics Bible:
“Computer Graphics: Principles and Practice” (2nd ed), J.D. Foley, S.K. Feiner, A. van Dam, J.F. Hughes, Addison-Wesley, 1996
ACM Digital Library
Computer Bibliography Database
SIGGRAPH Conference Proceedings
Journals (Transactions on Graphics, etc)
IEEE Computer Society:
Computer Graphics and Applications Magazine
Transactions on Computer Graphics and Visualization
Eurographics:
Computer Graphics Forum
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27. Interactive Computer Graphics Contents
Practicals
Available on the WWW:
3 Practicals:
Shading: Glut, Phong & Gouraud Shading (3 weeks)
Scene Graph: OpenGL, Coordinate Transformations, Perspective (4 weeks)
Animation (3 weeks)
Time estimate depends on your knowing the material!
Tutor:
Matthew Hampton
Copying:
All practicals must be entirely your own work
You may not copy from previous years
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28. Interactive Computer Graphics Contents
Mark Breakdown
3 practicals, 1 test, 1 final exam
Practical work 35%
Class test 15%
Examination 50%
Final exam:
One and a half hours
Open Book
1 compulsory question
choice of 2 out of 3 remaining
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29. Interactive Computer Graphics Contents
Life After the Exam
Hey, I want to do more Computer Graphics!
Honours
Visualization
Advanced Graphics
Useability
Virtual Reality
Image Processing
Outside
Small firms (WEB !)
Further study overseas
Masters — Collaborative Visual Computing Lab:
Collaborative Virtual Environments;
Computer Graphics;
Cooperative Visualization;
Usability and Human Computer Interaction
Web-based applications;
Medical Imaging
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