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INTRODUCTION
TO
COMPUTER
GRAPHIC S
Introduction
Andries van Dam
September 4, 2008
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INTRODUCTION
TO
COMPUTER
GRAPHIC S
What is Computer Graphics?
•
•
(1/2)
Computer graphics generally means creation,
storage and manipulation of models and images
Such models come from diverse and expanding
set of fields including physical, mathematical,
artistic, biological, and even conceptual (abstract)
structures
Frame from animation by William Latham, shown
at SIGGRAPH 1992. Latham uses rules that govern
patterns of natural forms to create his artwork.
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
What is Computer Graphics?
•
•
(2/2)
William Fetter coined term “computer graphics” in
1960 to describe new design methods he was
pursuing at Boeing
Created a series of widely reproduced images on
pen plotter exploring cockpit design, using 3D
model of human body.
“Perhaps the best way to define computer graphics is to find
out what it is not. It is not a machine. It is not a computer,
nor a group of computer programs. It is not the know-how of
a graphic designer, a programmer, a writer, a motion picture
specialist, or a reproduction specialist.
Computer graphics is all these – a consciously managed and
documented technology directed toward communicating
information accurately and descriptively.”
Computer Graphics, by William A. Fetter, 1966
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
What is Interactive Computer
Graphics? (1/3)
•
•
User controls contents, structure, and appearance of
objects and their displayed images via rapid visual
feedback
Basic components of an interactive graphics system
–
–
–
•
input (e.g., mouse, tablet and stylus, force feedback device,
scanner, live video streams…)
processing (and storage)
display/output (e.g., screen, paper-based printer, video
recorder, non-linear editor…)
First truly interactive graphics system, Sketchpad,
pioneered at MIT by Ivan Sutherland for his 1963
Ph.D. thesis
Sketchpad in 1963. Note use of a CRT monitor, light
pen and function-key panel.
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
What is Interactive Computer
Graphics? (2/3)
Batch (1950s – now)
•
Before Sketchpad, output via plotters/printers,
input via keypunch, both in batch
Card punching (left). IBM 704 (right) took up a
whole room and was capable of about 4,000
arithmetic operations/second.
Cool facts: Whirlwind, built in early 50’s at MIT,
cost $4.5 million and could perform 40,000
additions/second. Mac 512K, list price $3,195 in
1984, could do 500,000. Today, commodity PCs
perform approximately two or three billion
operations/second.
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
What is Interactive Computer
Graphics? (3/3)
•
Almost all key elements of interactive graphics
system are expressed in first paragraph of
Sutherland’s 1963 Ph.D. thesis, Sketchpad, A
Man-Machine Graphical Communication System:
The Sketchpad system uses drawing as a novel
communication medium for a computer. The
system contains input, output, and
computation programs which enable it to
interpret information drawn directly on a
computer display. Sketchpad has shown the
most usefulness as an aid to the
understanding of processes, such as the
motion of linkages, which can be described
with pictures. Sketchpad also makes it easy to
draw highly repetitive or highly accurate
drawings and to change drawings previously
drawn with it…
•
Today, still use batch mode for
final production-quality video and
film (special effects – fx), where
one frame of a 24 fps movie may
take 8-24 hours to render on
fastest PC!
Render farm
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INTRODUCTION
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COMPUTER
GRAPHIC S
Environmental (R)evolution
(1/6)
• Graphics has been key to technology
growth in evolution of computing
environments:
– graphical user interfaces (GUIs)
– visual computing, e.g., desktop
publishing, scientific visualization,
information visualization
Apple iPhoneTM
• Hardware revolution drives everything
– every 12-18 months, computer power
improves by factor of 2 in price /
performance – Moore’s Law
• Palm TX™, HP I-Paq™ as full PC
• iPhone™, Blackberry™ for email/internet
• Hallmark singing card, LeapFrog Pad
HP I-PaqTM
Blackberry BoldTM
– graphics memory and network speeds
are on even faster exponentials
• Graphics chips in particular have major
improvements every six to nine months
(e.g. nVidia GeForce™ series, ATI
nVidia
GeForceTM
chip
Radeon™ series)
Andries van Dam
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Leapfrog PadTM
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GRAPHIC S
Environmental (R)evolution
(2/6)
Character Displays (1960s – now)
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•
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Display: text plus alphamosaic pseudo-graphics
Object and command specification:
command-line typing
Control over appearance: coding for text
formatting (.p = paragraph, .i 5 = indent 5)
Application control: single task
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INTRODUCTION
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COMPUTER
GRAPHIC S
Environmental (R)evolution
(3/6)
Vector (Calligraphic, Line Drawing)
Displays (1963 – 1980s)
•
•
•
•
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Display: line drawings and stroke text; 2D and
3D transformation hardware
Object and command specification:
command-line typing, function keys, menus
Control over appearance: pseudo-WYSIWYG
Application control: single or multitasked,
distributed computing pioneered at Brown via
mainframe host <-> minicomputer satellite
Term “vector” graphics survives as “scalable
vector graphics” library from Adobe and W3C –
shapes as transformable objects rather than just
bitmaps
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INTRODUCTION
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GRAPHIC S
Environmental (R)evolution
(4/6)
2D bitmap raster displays for PCs and workstations
(1972 at Xerox PARC - now)
•
Display: windows, icons, legible text, “flat earth”
graphics
Note: late 60’s saw first use of raster graphics, especially for
flight simulators
•
•
•
Object and command specification: minimal typing
via WIMP (Windows, Icons, Menus, Pointer) GUI: pointand-click selection of menu items and objects, widgets
and direct manipulation (e.g., drag and drop), “messy
desktop” metaphor
Control over appearance: WYSIWYG (which is really
WYSIAYG, What You See Is All You Get)
Application control: multi-tasking, networked clientserver computation and window management (even “X
terminals”)
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INTRODUCTION
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GRAPHIC S
Environmental (R)evolution
(5/6)
3D graphics workstations (1984 at SGI – now)
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Display: real-time, pseudo-realistic images of 3D
scenes
Object and command specification: 2D, 3D
and nD input devices (controlling 3+ degrees of
freedom) and force feedback haptic devices for
point-and-click, widgets, and direct manipulation
Control over appearance: WYSIWYG (still
WYSIAYG)
Application control: multi-tasking, networked
(client/server) computation and window
management
High-end PCs with hot graphics cards (nVidia
GeForce™, ATI Radeon™) have supplanted
graphics workstations
Such PCs are clustered together over high speed
buses or LANs to provide “scalable graphics” to
drive tiled PowerWalls, Caves, etc.
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INTRODUCTION
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GRAPHIC S
Environmental (R)evolution
(6/6)
Classical time-sharing is dead, 1:n g n:1
• PCs and Workstations merging in
distributed heterogeneous computer
networks (e.g., LANs, WANs, Internet
and clusters)
• But file-, print- and compute-servers
and network are still shared
• Client/server computing, component
software technologies are dominant
paradigms
• NCs (Network Computers), thin clients
attached to powerful servers reprise
dumb terminals and provide central
control – haven’t really taken off
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
Versus
Original
Macintosh
New iMac
24”
Date
1984
2008
+24
Price
$2500
$2200
x .88
CPU
8 MHz
3.06 GHz (Dual)
Memory
128KB RAM
Storage
400KB Floppy
Monitor
9” Black & White
512 x 342
68 dpi
Devices
GUI
Andries van Dam
Mouse
Keyboard
Desktop WIMP
x 15625
2.0GB DDR2
SDRAM
500GB Hard Disk
24” Color
1920 x 1200
100 dpi
x 1250000
x 2.6
x 13.2
x 1.5
Mouse
Keyboard
same
same
Desktop WIMP
September 4, 2008
x 382.5
same
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New Forms of Computing:
1990-2008
(1/9)
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Multimedia: text and graphics synchronized with
sound and video
Hypermedia: multimedia with hypertextual links
(also called Interactive Multimedia)
Discredited term: “Digital Convergence,” merging
of digital television and distributed computing,
consumer electronics: set-top computers (e.g.,
for Interactive TV, Video-On-Demand)
The Internet and Internet appliances
Embedded computing (information appliances,
Personal Digital Assistants)
Ubiquitous/pervasive/
invisible/nomadic
computing, “active
badges” a la Xerox
PARC, with hundreds
of devices per person
(appliances, clothing,
even body parts) ;
seamless computing
is the dream
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008
(2/9)
• Virtual Reality:
fully immersive VR
(via Head-mounted Displays,
Cave [180 George St.])
CAVE™
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008
(3/9)
VOX in Cave and Fishtank VR
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GRAPHIC S
New Forms of Computing:
1990-2008
(4/9)
ADVISER: Mars data visualization
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INTRODUCTION
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008
(5/9)
Arterial blood flow and bat flight visualization
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008
(6/9)
Cave Painting
Use feet for navigation,
freeing hands for other
uses
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INTRODUCTION
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008
(7/9)
semi-immersive VR
Elumens’
VisionStation™
Fishtank VR on a monitor
small volume, head-tracked
interactive stereo
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INTRODUCTION
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008
(8/9)
augmented VR
(via video see-through optics)
Video or optics superimposes computergenerated data on real world (e.g.,
Columbia’s MARS, led by our Ph.D., Steve
Feiner)
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INTRODUCTION
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COMPUTER
GRAPHIC S
New Forms of Computing:
1990-2008 (9/9)
New Interaction technology
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Inexpensive interaction devices from research lab
into marketplace : 2D and 3D graphics no longer
“special”
•
3D (even time-varying, “4D”) interactive
illustrations as clip art/clip models coming
•
Kids using computer graphics in rides (e.g.
Aladdin, Pirates of the Caribbean) and gaming
consoles (e.g. Nintendo Wii), with head-mounted
displays and/or force-feedback input devices
New forms of user-interface (UI Lecture)
•
3D Widgets; VR demands new interaction technology
•
Gesture-based Interaction (Brown’s “Sketch”)
•
Social interfaces (Microsoft’s Bob and Clippie
bombed, other avatars may not)
•
Agents for indirect control
– tablet PCs and electronic whiteboards present new
opportunities for pen-centric, gesture-based
computing
– Multi-touch (NYU’s Jeff Han, MS’ Surface, Apple’s
iPhone)
Andries van Dam
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COMPUTER
GRAPHIC S
Powerful, Inexpensive
Processing
Chips are Key in graphics subsystems
• Advances driven by Moore’s Law
– price/performance improves 2x every 18 months due
to doubling of number of transistors
– only exponential growth in technology except WWW
• CPU
– Newest processors are 64-bit, dual/quad/8 core
• Server: Dual-Core Intel Itanium 2™, Dual-Core Intel
Xeon™, Dual-Core AMD Opteron™, Sun UltraSPARC T1™
• Desktop: Intel Core 2 Duo™, AMD Athlon64 X2™, IBM
G5™, Mac ProTM Quad/8-Core
• Graphics subsystems (GPU)
– Commodity cards have taken over the mainstream
market (nVidia GeForce™, ATI Radeon™)
• Physics subsystems
•
– nVidia (formerly Ageia) PhysX PPU (Physics Processing
Unit)
– Offloads physics processing from the processor and
GPU
Artificial Intelligence subsystems
– AIseek Intia Processor
– Accelerated path finding, sensory simulation and
terrain analysis
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INTRODUCTION
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COMPUTER
GRAPHIC S
Chip Technology Advances in Games &
Commodity Graphics Cards
Last-generation game platforms and set-top
boxes use high-end processors (128-bit
architectures, great graphics capabilities)
–
–
–
Nintendo GameCube™ (powered by ATI)
Sony Playstation 2™
Microsoft XBoxTM
Current-generation platforms give us
unprecedented performance by utilizing multicore processors
– Microsoft Xbox 360™ (powered by ATI)
– Sony Playstation 3™
– Nintendo WiiTM
•
•
ATI Radeon HD 4800 and nVidia GeForce GTX
280 are the top graphics cards in 2008 (approx.
$200 - $300)
Significant advances in commodity graphics chips
every 6 months, outrunning CPU chip advances
–
–
•
•
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AMD Athlon 64 X2 processors have 243 million
transistors
Radeon HD 4800 chip has over 900 million!
GPUs are now arguably more powerful than CPUs
and programmable  CPU & OS losing their
hegemony as the GPU co-processor tail wags CPU
dog!
GPUs sometimes used instead of CPUs in clusters
for high-performance computing (e.g. bioinformatics algorithms)
Workstation market (and vendors) bowing to
commoditization (e.g., Sun’s losing client market)
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INTRODUCTION
TO
COMPUTER
GRAPHIC S
Application Distinctions
Two basic paradigms
•
Sample-based graphics (left): discrete samples are
used to describe visual information
– pixels can be created by digitizing images, using a samplebased “painting” program, etc.
– often some aspect of the physical world is sampled for
visualization, e.g., temperature across the US
– example programs: Adobe Photoshop™, GIMP™ , Adobe
AfterEffects™ (it came out of CS123/CS224!)
•
Geometry-based graphics (right): (also called
scalable vector graphics or object-oriented graphics)
geometrical model is created, along with various
appearance attributes, and is then sampled for
visualization (rendering a.k.a image synthesis);
– often some aspect of physical world is visually simulated,
or “synthesized”
– examples of 2D apps: Adobe Illustrator™, Adobe
Freehand™ (formerly by Macromedia), Corel CorelDRAW™
– examples of 3D apps: Autodesk’s AutoCAD2009™,
Autodesk’s (formerly Alias|Wavefront’s) Maya™,
Autodesk’s (formerly Discreet’s) 3D Studio Max™
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
Sampled-based Graphics
(1/2)
• Images are made up of grid of discrete
pixels, for 2D “picture elements”
CRT beam illumination pattern
light intensity
1 pixel
Mathematical
pixel grid
LCD display
NB: Can’t resolve
adjacent pixels on
CRT
• Pixels are point locations with
associated sample values, usually of
light intensities/colors, transparency,
and other control information. They
are not little circles or little
squares
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
Sampled-based Graphics
(2/2)
• Samples created directly in paint-type
program, or as sampling of continuous
(analog) visual materials. E.g., photograph can
be sampled (light intensity/color measured at
regular intervals) with many devices including:
– flatbed and drum scanners
– digital still and motion (video) cameras
– add-on boards such as frame grabbers
• Sample values can also be input numerically
(e.g., with numbers from computed dataset)
• Once an image is defined as pixel-array, it can
be manipulated
– Image editing: changes made by user, such
as cutting and pasting sections, brush-type
tools, and processing selected areas
– Image processing: algorithmic operations that
are performed on image (or pre-selected
portion of image) without user intervention.
Includes blurring, sharpening, edge-detection,
color balancing, rotating, and warping. Preprocessing step in computer vision
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INTRODUCTION
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COMPUTER
GRAPHIC S
Sampling an Image
•
Lets do some sampling of CIT building
3D
scene
•
A color value is measured at every grid point and
used to color corresponding grid square
0 = white, 5 = gray, 10 = black
•
Note: this poor sampling and image
reconstruction method creates blocky image
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COMPUTER
GRAPHIC S
What’s the Advantage?
•
•
•
Once image is defined in terms of colors at (x, y)
locations on grid, can change image easily by
altering location or color values
E.g., if we reverse our mapping above and make
10 = white and 0 = black, the image would look
like this:
Pixel information from one image can be copied
and pasted into another, replacing or combining
with previously stored pixels
Andries van Dam
September 4, 2008
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INTRODUCTION
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COMPUTER
GRAPHIC S
What’s the Disadvantage?
•
WYSIAYG (What You See Is All You Get): No
additional information
– no depth information
– can’t examine scene from different point of view
– at most can play with the individual pixels or
groups of pixels to change colors, enhance
contrast, find edges, etc.
•
But recently, strong interest in image-based
rendering to fake 3D scenes and arbitrary camera
positions. New images constructed by
interpolation, composition, warping and other
operations.
Photo Tourism: Exploring photo collections in 3D
(Siggraph 2006)
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COMPUTER
GRAPHIC S
Examples of 2D Image
Manipulation (1/3)
•
•
There are many things possible with
sampled images: uses of computer
imaging range from military
applications to entertainment,
medicine, art, and design.
The news: digitally “enhanced” images are more and more
common. Usually just sharpening, color balancing, but
sometimes much more. To “Photoshop something in”:
–
–
–
–
–
Time Magazine’s O.J. Simpson cover (below left)
Reuters’ doctored Beirut bombing photo (below right; note
cloned smoke)
National Geographic’s Pyramid cover
University of Wisconsin application
Faked NE blackout satellite photo
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INTRODUCTION
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COMPUTER
GRAPHIC S
Examples of 2D Image
Manipulation (2/3)
•
People can now mutate into other people or objects
through morphing, and can carry on conversations in
different times and places
– Interactive Digital Photomontage, Siggraph 2004
•
The belief in very strong connection between
photorealistic images, still or moving, and reality is being
severed
– no way to tell if news photos are “real”
photographs
– photographic evidence no longer considered
“proof” in court of law without clear provenance
of the image
– future of the family photo album?
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
Examples of 2D Image
Manipulation (3/3)
•
In artwork the processes and techniques of
photography and painting are merging in the art
of digital imaging
– Michele Turre: the artist, her daughter, and her
mother, all at 3 years of age
•
CS123 emphasizes geometry-based graphics but
we do two assignments on sample-based
graphics to learn simple image processing.
– image processing in general includes image
transformation: used for feature detection, pattern
recognition, machine/computer vision, and most
recently, for image-based rendering
– new field of "digital photography" (a.ka.
computational photography, fauxtography) brings
us many neat tricks with images, some now even
done in camera
Andries van Dam
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INTRODUCTION
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COMPUTER
GRAPHIC S
Geometry-Based Graphics
•
Geometry-based graphics applications store
mathematical descriptions, or “models,” of
geometric elements (lines, polygons,
polyhedrons…) and associated attributes (e.g.,
color, material properties). Elements are primitive
geometric shapes, primitives for short
•
Images created as pixel arrays (via sampling of
geometry) for viewing, but not stored as part of
model. Images of many different views are
generated from same model
•
Users cannot usually work directly with individual
pixels in geometry-based programs; as user
manipulates geometric elements, program
resamples and redisplays elements
•
Increasingly rendering combines geometric and
sample-based graphics, both as performance
hack and to increase quality of final product
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COMPUTER
GRAPHIC S
What is Geometric Modeling?
What is a model?
•
Captures salient features (data, behavior) of
thing/phenomenon being modeled
– data includes geometry, appearance attributes…
– note similarity to OOP notions
•
Real: some geometry inherent
– physical (e.g., actual object such as a pump)
– non-physical (e.g., mathematical function,
weather data)
•
Abstract: no inherent geometry, but for
visualization
– organizational (e.g., company org. chart)
– quantitative (e.g., graph of stock market)
•
•
•
Modeling is coping with complexity
Our focus: modeling and viewing simple everyday
objects
Consider this:
Through 3D computer graphics, first time in human
history we have abstract, easily changeable 3D forms.
This has revolutionized working process of many fields –
science, engineering, industrial design, architecture,
commerce, entertainment, etc. This has profound
implications for visual thinking and visual literacy
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Lecture Topics
•
•
•
•
We manipulated primitive shapes with geometric
transformations (translation, rotation, scale).
These transformations are essential for model
organization, process of composing complex
objects from simpler components.
Hierarchical models and same geometric
transformations are also essential for animation
Once object’s geometry is established, must be
viewed on screen: map from 3D to 2D for viewing
and from 2D to 3D for 2D input devices (e.g., the
mouse or pen/stylus)
While mapping from 3D to 2D, object (surface)
material properties and lighting effects are used
in rendering one’s constructions. This rendering
process is also called image synthesis
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Decomposition of a Geometric
Model
• Divide and Conquer
• Hierarchy of geometrical components
• Reduction to primitives (e.g., spheres,
cubes, etc.)
• Simple vs. not-so-simple elements (nail
vs. screw)
Head
Shaft
Point
composition
Andries van Dam
decomposition
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Hierarchical (Tree) Diagram of
Nail
•
Object to be modeled is (visually) analyzed, and
then decomposed into collections of primitive
shapes.
•
Tree diagram provides visual method of
expressing “composed of” relationships of model
root node
Nail
Head
Body
(cylinder)
Shaft
Point
(cylinder) (cone)
tree diagram
leaf nodes
•
Such diagrams are part of 3D program interfaces
(e.g., 3D Studio MAX, Maya)
•
As pointer data structure to be rendered, it is
called a scenegraph
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Composition of a Geometric
Model
Translate
Translate and Scale
Translate and Rotate
Primitives
in their own
modeling
coordinate system
Composition
in world (root)
coordinate
system
• Primitives created in decomposition process
must be assembled to create final object.
Done with “affine transformations,” T, R, S
(as in above example).
• Other composition operators will be briefly
discussed later (e.g., Constructive Solid
Geometry (CSG) with Boolean operators).
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GRAPHIC S
What Kind of Math do We Need?
Cartesian Coordinates
• Typically modeling space is floating point,
screen space is integer
x, y Cartesian grid
Integer Grid
NB:Often, screen coordinates are measured top to
bottom, based on raster scan
(0,0)
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Example 3D Primitives
Polyline
Sphere
Andries van Dam
Polyhedron
Patch
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GRAPHIC S
Conceptual Framework for
Interactive Graphics
•
•
•
•
•
Application
model
Graphics library/package is intermediary between
application and display hardware (Graphics
System)
Application program maps application objects to
views (images) of those objects by calling on
graphics library
User interaction results in modification of model
and/or image
Images are usually means to an end: synthesis,
design, manufacturing, visualization,…
This hardware and software framework is more
than 4 decades old but is still useful, indeed
dominant
Application
program
Andries van Dam
Graphics
Library
(GL)
Graphics
System
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Graphics Library
• Examples: OpenGL™, DirectX™, Windows
Presentation Foundation™
• Primitives
• Attributes
– color
– line style
– material properties for 3D
• Lights
• Transformations
• Immediate mode vs. retained mode
– immediate mode: no stored representation,
package holds only attribute state, and
application must completely draw each frame
– retained mode: library compiles and displays
from scenegraph, a complex DAG
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2D Hardware/Algorithms Outline
• Display Hardware: Raster scan vs. random
scan
• Drawing primitives by scan conversion
– Lines, polygons, circles and ellipses,
characters, attributes (color, line style, fill
pattern…)
• Clipping to clip rectangle: three
methods
– analytically compute
intersections and draw clipped
primitives
– test each pixel and write only if
inside
– compute spans inside the
primitive and fill entire span
without testing
• Color Table
– indirect specification of (pseudo) color
– color correction, simple types of animation
• BitBlt/RasterOp for operating on blocks of
pixels
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Graphics Display Hardware
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Vector (calligraphic, stroke, random-scan)
– still used in some plotters
Ideal Drawing
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Vector Drawing
Raster (TV, bitmap, pixmap), used in displays
and laser printers
Raster
Outline primitives
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Filled primitives
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Vector Architecture
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Vector display works with display list/file stored in
refresh buffer
Display controller draws all vectors at < 60 Hz
(often at variable rate); flicker is a problem
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2D Raster Architecture
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Raster display stores bitmap/pixmap in refresh
buffer, also known as bitmap, frame buffer; can
be in separate hardware (VRAM) or in CPU’s main
memory (DRAM)
Video controller draws all scan-lines at consistent
> 60 Hz; separates update rate of the frame
buffer and refresh rate of the CRT
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Drawing Lines
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For horizontal, vertical and diagonal lines all
pixels lie on ideal line: special case
For lines at arbitrary angle, pick pixels closest to
ideal line (Bresenham’s midpoint “scan
conversion” algorithm)
For thick lines, use multiple pixels in each column
or fill a rotated rectangle
Sampling continuous line on discrete grid
introduces sampling errors: “jaggies”
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Drawing Filled Polygons
1.
Find intersection of scanline with
polygon edges
2.
Sort intersections by increasing x
3.
Fill the polygon between pairs of
intersections (spans)
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Drawing Circles and Ellipses
circle outline
filled ellipse
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Thickness Attribute
• Thick circle drawing by tracing
rectangular pen
• Treat such primitives as regions and fill
them, e.g., approximate them as
sequences of connected quadrilaterals
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Drawing Characters
GRAPHIC S
(1/2)
• For characters defined by small bitmap
– selectively write pixels of refresh
buffer corresponding to “true”
bits of character bitmap
• Transparent mode: don’t write 0’s –
BitBlt with OR (explained later)
• Opaque mode: write background color
for 0’s
• Descenders & proportional spacing
easily accommodated
Base
line
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Drawing Characters
GRAPHIC S
(2/2)
• Outline fonts: defined in terms of
(mathematical) drawing primitives
(lines, arcs, splines) and thus scalable,
but more CPU intensive (e.g. Adobe
PostScript™, Microsoft TrueType™)
• Font design (typography is highly
skilled specialty, involving graphical and
algorithmic design)
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1-bit Bilevel Display
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Digital intensity value
Digital to Analog
Conversion (DAC)
analog signal to drive
electron beam
Black & White (or any 2 colors, depending on
monitor phosphor color)
Original Mac resolution was 512x384 pixels, now
from 640x480 up to 2560x1600
n-Bit Display
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2n intensities or colors: 1 (grayscale) or 3 (color)
DACs & guns
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Image Display System
Look-up Table
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Any specific 2n colors may be inadequate (n
typically 16-24 in low-end systems)
Look-up table allows 2n colors out of 224 colors to
be used in one image, some other 2n in another
image
– 224 = approx. 16.7 million, exceeds eye’s ability
to discriminate (somewhere between 7-10
million)
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Color table is resource managed (usually) by
window manager
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Color Look – Up Table Operation
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Pixel value is indexed to color look up table (CLUT)
where color is stored. Here we use only 12 bits
(4bits per color) for clarity – typically, 24 bits are
used
CLUT look up done at video rates, overlapped with
fetch and DAC!
In 24-bit “true” color systems, 3 x 8 bits for R, G,
B; each color has its own 8-bit CLUT (0-255)
CLUT allows variety of effects
– pseudo coloring (LandSat images, stress diagrams,
thermograms…)
– fast image changes: change table rather than
stored image
– multiple images: select or composite/blend
– animation hack
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BitBlt/RasterOp
GRAPHIC S
(1/3)
• Logically operate on each pixel in
rectangular source and destination
regions in same or different pixmaps to
achieve dynamics, e.g., to move/scroll
windows on screen
• RasterOp (Source, Destination) g
Destination
• In some implementations either S or D
may be masked, and need not be same
size
pixmap
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BitBlt/RasterOp
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GRAPHIC S
(2/3)
AND (S,D): S can mask out pixels in D
OR (S,D): S is non-destructively “added” to D;
used for painting, transparent and “kerned”
characters (where characters extend beyond their
boxes); not as useful in n-bit systems
Here’s how you can use them:
Let’s say you want to add some game sprites to background…
1. Mask out with black
AND
=
0 (black) AND anything is 0, 1 (white) AND anything is anything
2. Add sprites in
OR
=
0 OR anything is anything
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BitBlt/RasterOp
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GRAPHIC S
(3/3)
Replace (S,D): S destructively replaces D, i.e., is
deleted and copied on top of D (also called Move); used
for making opaque characters, icons, scroll
Copy (S,D) as above, but S is not deleted
XOR (S,D) S selectively inverts D; used in 1-bit systems
for rubber-banding/dragging, cheap cursors:
S XOR (S XOR D) = D 0 = W, 1 = B
XOR
S
D
D’
D’
D
XOR
S
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Note: effects in color systems for all but replace may be
weird
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