Interactive Computer Graphics

1
Interactive Computer Graphics

• Instructor Amin Hammad
• E-mail hammad_at_cmu.edu
• URL hammad.ce.cmu.edu
• Office Hours Room 747
• Monday 900-1100 (or by
  appointment)

2
Administrivia of Course

• Website (hammad.ce.cmu.edu)
• TA Cholyeun Hongpisanwiwat
• cholyeun_at_sis.pitt.edu
• Office Hours Monday 900-1100 (or by
  appointment)
• Book Interactive Computer Graphics (Edward
  Angel)
• Collaboration Policy
• Grading Assignments (45) Tests (40)
• Quizzes (15)
• Difficulty/Time required
• Programming environment (mainly Visual C)
• Questions

3
What is Computer Graphics

• Anything to do with visual representations on a
  computer including
• Text (e.g. Japanese characters )
• GUIs
• Computer Images
• 3D Graphics CG special effects games
  animations
• Scientific Visualization
• Algorithms theory (physics math computation)
• Display devices hardware (graphics cards
  monitors)

4
Course Outline

• Graphics Systems and Models
• Graphics Programming
• Input and Interaction
• Geometric Objects and Transformation
• Viewing
• Shading
• Advanced Topics (Object-oriented graphics
  texture mapping curves etc.)

5
Contents

• General Overview History and applications of CG
• Graphics Systems
• Colors Perception
• Computer Colors RGB and HSL
• OpenGL Introduction
• Some CG demos.

Reading Angel 1 2.4
6
Historical Background Sketchpad

• Ivan Sutherland 1963 MIT
• Sketchpad A man machine
• Graphical communications system
• (first interactive graphics thesis
• included many GUI concepts)
• Inspired many early graphics researchers
• Sutherland also pioneered VR and hidden surface
  problems
• 1988 Turing award for his pioneering and
  visionary contributions to computer graphics
  starting with sketchpad

7
From Text to GUIs

• Xerox Star First workstation with GUI
• Invented at PARC in 1975. Used in the Apple
  Macintosh and now MS Windows.

Xerox Star
Windows 1.0
8
Graphical applications

• Presentations (bar charts graphs Powerpoint)
• Scientific Visualization

Compressible flow around Space shuttle science.gmu
.edu
9
3D Graphics

• Maybe what we generally consider CG
• Currently important in
• Design (CAD)
• Education Simulators VR
• Games
• Entertainment (Movies) Art

10
Landscape simulation of Los Angeles (UCLA)
11
Effects needed for Realism

• Shadows
• Reflections (Mirrors)
• Transparency
• Detail (Textures etc.)
• Complex Illumination
• Realistic Materials

The light of Mies van der Rohe Modeling Stephen
Duck Rendering Henrik Wann Jensen
12
Photorealistic Rendering
13
Interactive CG Systems
Input Devices
Cathode Ray Tube (CRT) Liquid Crystal Display
(LCD)
14
Cathode Ray Tube (CRT)
Electron Gun
Phosphor Field
15
Cathode Ray Tube (CRT)
Electron Gun
Phosphor Field
Deflectors
16
Phosphors Decay

• Light coming from a single phosphor does not last
  long.
• Human eye can detect a difference within 20
  milliseconds

• To appear without flicker the entire screen
  should be redrawn (Refreshed) at least 50 times
  per second (50 Hz)

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The Frame Buffer

• Phosphors are logically arranged into a 2D array
  of pixture elements (pixels)

• The Frame Buffer stores the array of pixels that
  are to be activated when the display is refreshed

• Resolution of Frame Buffer denotes the size of
  the array e.g. 1280 x 1024

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Frame Buffer
Monochrome System
Display
000000000000000000000000
000000000000000000000000
000000000000000000000000
000000000000000000000000
000011000011000000000000
000011000011000000000000
000000000000000000000000
000000000000000000000000
001000000000001000000000
000100000000010000000000
000010000000100000000000
000001111111000000000000
000000000000000000000000
000000000000000000000000
000000000000000000000000
000000000000000000000000
000000000000000000000000
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Interlacing

• Because phosphors are small it is difficult for
  the eye to distinguish from one phosphor to the
  next.

• Interlacing To refresh every other row of the
  display within 50Hz.

20
Colors in the Frame Buffer

• Depth refers to how many bits represent each
  pixel.
• Adding bits allows the frame buffer to store not
  only if a pixel is active but also what color it
  should be
• 8 bits 256 colors

21
Color CRT
Deflectors
Phosphor Field
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Color Displays

• Color code is stored in the frame buffer
• Display translates color code into intensities
  for three electron guns.
• Each gun fires into an appropriately colored
  phosphor in a group of three (known as a triad).
• Triad Pixel

23

Color Spectrum
Our sense of color is derived from a
physiological response to lightwaves in the
visible electromagnetic spectrum
24
Color Blending

• Light that we see is made up of photons moving at
  varying wavelengths

• A Color is actually defined by continuous
  function C() across all wavelengths of the
  visible spectrum

25
Human Visual System

• Retina is comprised of Rod cells and 3 types of
  Cone cells

Iris

• Rods sense dark/light

• Each type of the Cones responds best to a
  particular wavelength.

26
Sensitivity of Cones
27
Tri-color Model

• The eye breaks down incoming light into a triplet
  of values (RGB).
• The brain converts that triplet back into a
  single (perceived) color

• Instead of representing color as a continuous
  function we can simplify our representation to a
  triplet of Red Green Blue intensities.

28
RGB Color Cube
29
RGB Color Cube
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RGB Color Cube
What does this line represent
31
Alternative Color Model HSL

• Hue A specific color (usually selected on a
  continuum)
• Saturation How much is the dominant wavelength
  distinguished from the other colors (How much
  white has been added)
• Lightness How bright does the color appear.

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Understanding HSL
What is the darkest color possible
Black
What is the lightest color possible
White
What is the color if Saturation 0
Gray
What is the color if Saturation 1
Pure Color
33
HSL Cones
White
Black
34
HSL Cones
White
Grayscale
Black
35
HSL Cones
Pure Color
36
Color Coding

• Full spectrum can be specified using 3 numbers.
  (RGB or HSL)
• These values can be coded into a single number
  and stored in the frame buffer

37
The Pinhole Camera and Synthetic Camera Models
Image plane
Pinhole

• An ideal pinhole camera has an infinite depth of
  field (all points are in focus)
• The angle of view the angle made by the
  largest object that the camera can image on
  its film plane.

Illustration courtesy Greg Humphries
38
Ray Tracing

• Arbitrary paths realism
• Trace from light
• Most light rays dont hit eye
• Eye Ray tracing
• Primary Rays
• Shadow Rays
• Reflected/Transmitted Rays

Appel 68
39
Pipeline Architecture for Graphics APIs
(Application Programmers Interface)
Frame Buffer
Numeric Data
Clipper
Rasterizer
Projector
Transformer
Graphics Renderer
Numeric Data Mathematical Representations of
graphical shapes. Often in terms of vertices and
lines.
40
Pipeline Architecture for Graphics APIs
Frame Buffer
Numeric Data
Clipper
Rasterizer
Projector
Transformer
Transformer Mathematical operations are
performed on the data to handle movement and
positioning of the graphical representation
41
Pipeline Architecture for Graphics APIs
Frame Buffer
Numeric Data
Clipper
Rasterizer
Projector
Transformer
Clipper Limits the size of the dataset to
contain only information that should be displayed
for a given point-of-view
42
Pipeline Architecture for Graphics APIs
Frame Buffer
Numeric Data
Clipper
Rasterizer
Projector
Transformer
Projector Until this stage our numeric
representation has been dealing with a 3D model.
This stage reduces the dataset to 2D
43
Pipeline Architecture for Graphics APIs
Frame Buffer
Numeric Data
Clipper
Rasterizer
Projector
Transformer
Rasterizer The processed numeric information is
finally converted into individual pixels
44
Pipeline Architecture for Graphics APIs
Frame Buffer
Numeric Data
Clipper
Rasterizer
Projector
Transformer
Frame Buffer Pixels are sent to the Frame
Buffer to be displayed at the next refresh.
45
OpenGL

• OpenGL is a graphics API
• Software interface to graphics hardware
• Hardware independent
• No windowing or high-level tasks
• Higher level API utilities available (GLU)
• Application -gt Higher level -gt OpenGL -gt Hardware
• Window system separate (GLUT Motif etc.)
• Benefits
• Fast standard portable window-system
  independent
• Other APIs
• GKS(Graphical Kernel System)
• PHIGS (Programmers Hierarchical Interactive
  Graphics System)

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GL Related APIs

• GL implements the Graphics Renderer using
  pipeline architecture
• GLU (Utilities) provides some higher level
  interfaces for projections and viewing
• GLUT (Utility toolkit) provides window
  management and event handling

47
10 minutes break
48
Geographic Information System

• A system for processing digital thematic maps
• representing geographic features and their
  attributes.

49
Proposed Designs for a New Park
50
Case Study Area
51
Maps of Buildings Roads etc.
52
Add Buildings Heights
53
Add CAD Representation of the New Project
54
Automatically Add Image Mapping
I4
I3
Building layer (polygon) Attribute height LU
image etc.
I2
Image Layer (line) Attribute Base-height
height etc.
I1
I1
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Reality-Virtuality Continuum
(After Paul Milgram and Fumio Kishino).
57
Optical see-through HMD
Graphic Images
Head Tracker
Head Locations
Scene Generator
Monitors
Real World
Optical Combiners
58
Laser printer maintenance application
(Courtesy Steve Feiner Blair MacIntyre and
Dorée Seligmann Columbia University.)
59
Virtual fetus inside womb of pregnant patient
(Courtesy UNC Chapel Hill Dept. of Computer
Science.)
60
Simulation of London Millennium Bridge
(Courtesy Insitut Graphische Datenverarbeitung)
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Video and Graphics Mixing