Andries van Dam September 7, 2004 Introduction 1/56

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Andries van Dam September 7,
2004 Introduction 1/56

• Introduction

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What is Computer Graphics? (1/2)

• Computer graphics is commonly understood to mean
  the creation, storage and manipulation of models
  and images.
• Such models come from a diverse and expanding set
  of fields including physical, mathematical,
  artistic, biological, and even conceptual
  (abstract) structures.

Frame from an animation by William Latham, shown
at SIGGRAPH 1992. Latham uses rules that govern
patterns of natural forms to create his artwork.
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What is Computer Graphics? (2/2)

• The term computer graphics was coined in 1960
  by William Fetter to describe new design methods
  he was pursuing at Boeing.
• He created a series of widely reproduced images
  on a pen plotter exploring cockpit design, using
  a 3D model of a 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
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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 the use of a CRT monitor,
light pen and function-key panel.
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What is Interactive Computer
Graphics? (2/3)

• Before Sketchpad, output via plotters/printers,
  input via keypunch, both in batch
• Batch (1950s now)

Card punching (left). The IBM 704 (right) took up
a whole room and was capable of about 4,000
arithmetic operations/second. Cool facts
Whirlwind, built in the early 50s 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.
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What is Interactive Computer
Graphics? (3/3)

• Almost all the key elements of interactive
  graphics system are expressed in the first
  paragraph of Sutherlands 1963 Ph.D. thesis,
  entitled, Sketchpad, A Man-Machine Graphical
  Communication System
• 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 the fastest PC!

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
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Environmental (R)evolution (1/6)

• Graphics has been a key enabling technology in
  the evolution of computing environments
• graphical user interfaces
• visual computing, e.g., desktop publishing,
  scientific visualization, information
  visualization
• Hardware revolution drives everything
• every 12-18 months, computer power improves by
  factor of 2 in price / performance Moores Law
• 3Com Palm organizers, Compaq I-Paq as a full PC
• Hallmark singing card, LeapFrog Pad
• 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 Radeon series)

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Environmental (R)evolution (2/6)

• Character Displays (1960s now)
• 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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Environmental (R)evolution (3/6)

• Vector (Calligraphic, Line Drawing)
• Displays (1963 1980s)
• 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,
  mainframe host-minicomputer satellite distributed
  computing pioneered at Brown

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Environmental (R)evolution (4/6)

• 2D bitmap raster displays for PCs and
  workstations
• (1972 at Xerox PARC - now)
• Display windows, icons, legible text and flat
  earth graphics
• Note late 60s saw first use of raster
  graphics, especially for flight simulators
• Object and command specification minimal typing
  via WIMP (Windows, Icons, Menus, Pointer) GUI
  (Graphical User Interface) point-and-click
  selection of menu items and objects, widgets and
  direct manipulation (e.g., drag and drop), the
  messy desktop metaphor
• Control over appearance WYSIWYG (which is really
  WYSIAYG, What You See Is All You Get)
• Application control multi-tasking, networked
  client-server computation and window management
  (even X terminals)

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Environmental (R)evolution (5/6)

• 3D graphics workstations (1984 at SGI now)
• 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) are supplanting 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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Environmental (R)evolution (6/6)

• Classical time-sharing is dead, 1n g n1
• 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 havent really taken off

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New Forms of Computing1990-2004
(1/7)

• Multimedia text and graphics synchronized with
  sound and video
• Hypermedia multimedia with hypertextual links
  (also called Interactive Multimedia)
• Discredited term Digital Convergence, the
  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 seamless computing is the
  dream

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New Forms of Computing1990-2004
(2/7)

• Virtual Reality

fully immersive VR(via Head-mounted Displays,
Cave 180 George St.)
CAVE
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New Forms of Computing1990-2004
(3/7)
Cave Painting
Archeological research and analysis tool
CMU CUBE
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New Forms of Computing1990-2004
(4/7)
Blood flow through an arterial bypass graft
Use feet for navigation, freeing hands for other
uses
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New Forms of Computing1990-2004
(5/7)

• semi-immersive VR

Fishtank VR on a monitor
Barco, Immersadesk
GMDs Responsive workbench
Elumens VisionStation
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New Forms of Computing1990-2004
(6/7)

• augmented VR(via video see-through optics)

Video or optics superimposes computer-generated
data on real world (e.g., Columbias MARS, led by
our Ph.D., Steve Feiner)
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New Forms of Computing1990-2004
(7/7)

• New Interaction technology
• Inexpensive interaction devices from research lab
  into marketplace
• makes 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 gaming consoles
  VR games and rides (e.g. Aladdin, Pirates of the
  Caribbean, LBEs) with HMD and force-feedback
  input devices
• New forms of user-interface (UI Lecture)
• 3D Widgets gestures-based UI (Browns Sketch)
  VR demands new interaction technology
• Social interfaces (Microsofts Bob bombed, other
  avatars may not)
• Agents for indirect control

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Powerful, Inexpensive Processing

• Chips are Key in graphics subsystems
• Advances driven by Moores Law
• price/performance improves 2x every 18 months due
  to doubling of number of transistors
• only exponential in technology except growth of
  WWW
• CPU
• 64-bit computing enters the mainstream
• Server Intel Itanium, AMD Opteron
• Consumer IBM G5, AMD Athlon64
• AMD Athlon XP
• Intel Pentium 4
• Sun UltraSPARC IV
• Graphics subsystems
• Commodity cards have taken over the mainstream
  market (nVidia GeForce, ATI Radeon)

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Chip Technology Advances in
Games Commodity Graphics
Cards

• New 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 Xbox (powered by nVidia)
• nVidia GeForce 6800 Ultra and ATI Radeon X800
  XT are the top graphics cards (approx. 400 -
  500)
• Significant advances in commodity graphics chips
  every 6 months, outrunning CPU chip advances
• Intels newest Pentium 4 chip (Prescott) has 125
  million transistors, compared with only 55
  million in a previous Pentium 4 (Northwood)
• GeForce 6800 chip (NV40) has 222 million!
• GPUs are now arguably more powerful than CPUs and
  programmable ? CPU OS losing their hegemony as
  the GPU co-processor becomes the tail to wag the
  CPU dog!
• Workstation market (and vendors) bowing to
  commoditization (e.g., Suns losing client market)

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Application Distinctions

• Two basic paradigms
• Sample-based graphics discrete samples are used
  to describe visual information
• pixels can be created by digitizing images, using
  a sample-based 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 a geometrical model is
  created, along with various attributes, and is
  then sampled for visualization (this process is
  called rendering)
• often some aspect of the physical world is
  visually simulated, or synthesized
• example 2D vector graphics Adobe Illustrator,
  Macromedia Freehand, Corel CorelDRAW !
• example 3D programs AliasWavefronts Maya,
  Autodesks AutoCAD2005, Discreets 3D StudioMax

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Sampled-based Graphics (1/2)

• Images are made up of a grid of discrete pixels
  for 2D picture elements

light intensity
1 pixel

• 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, as
  well see.

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Sample-based Graphics (2/2)

• Samples created directly in paint-type program,
  or as a sampling of continuous (analog) visual
  materials. E.g., a 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 a computed dataset)
• Once an image is defined as a 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 an image (or pre-selected portion of
  an image) without user intervention. Includes
  blurring, sharpening, edge-detection, color
  balancing, rotating, and warping. A
  pre-processing step in computer vision.

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Sampling an Image

• Lets do some sampling of the CIT building
• A color value is measured at every grid point and
  used to color a corresponding grid square
• Note this poor sampling and image reconstruction
  method creates a blocky image

The 3D scene
0 white 5 gray 10 black
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Whats 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

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Whats the Disadvantage

• WYSIAYG (What You See Is All You Get) There is
  no additional information
• no depth information
• cant examine scene from a 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.

Capture of Hair Geometry from Multiple Images,
Siggraph 2004
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Examples of 2D Image
Manipulation (1/3)

• There are many things one can do 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 a lot more. To
  Photoshop something in
• National Geographics Pyramid cover
• TV Guides Oprah cover
• Time Magazines O.J. Simpson cover
• University of Wisconsin application

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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 a very strong connection between
  photorealistic images, still or moving, and
  reality is being severed

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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, and is used for feature
  detection, pattern recognition, and
  machine/computer vision, and most recently, for
  image-based rendering

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Geometry-Based Graphics

• Geometry-based graphics applications store
  mathematical descriptions, or models, of
  geometric elements (lines, polygons,
  polyhedrons) and their associated attributes
  (e.g., color, material properties). These
  elements are primitive geometric shapes,
  primitives for short.
• Images are created as pixel arrays (via sampling
  of the geometry) for viewing, but are not stored
  as part of the model. Images of many different
  views can be generated from the same model.
• One cannot usually work directly with the
  individual pixels in a geometry-based program
  one manipulates the geometric elements, then they
  are resampled and redisplayed.
• Increasingly rendering combines geometric and
  sample-based graphics, both as a performance hack
  and to increase quality of the final product

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What is Geometric Modeling?

• What is a model?
• Captures the salient features (data, behavior) of
  the 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, for the first time
  in human history we have abstract, easily
  changeable 3D forms. This has revolutionized the
  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

• Primitive shapes are manipulated with geometric
  transformations (translation, rotation, scale).
  These transformations are essential for model
  organization, the process of composing complex
  objects from simpler components.
• Hierarchical models and the same geometric
  transformations are also essential for animation.
• Once objects geometry is established, must be
  viewed on a screen. Must map from 3D to 2D for
  viewing and from 2D to 3D for 2D input devices
  (e.g., the mouse or pen/stylus).
• In the process of mapping from 3D to 2D, object
  (surface) material properties and lighting
  effects are used in rendering ones
  constructions. The 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
decomposition
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Hierarchical (Tree) Diagram of Nail

• The object to be modeled is (visually) analyzed,
  and then decomposed into collections of primitive
  shapes.
• The tree diagram provides a visual method of
  expressing the composed of relationships of the
  model.
• Such diagrams are part of 3D program interfaces
  (e.g., 3D Studio MAX, Maya).
• As a pointer data structure to be rendered, it is
  called a scenegraph

root node
Nail
Head (cylinder)
Body
Shaft (cylinder)
Point (cone)
leaf nodes
tree diagram
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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 the world (root) coordinate system

• The primitives created in the decomposition
  process must be assembled to create the final
  object. This is done with affine
  transformations, T, R, S (as in the example
  above).
• Other composition operators will be very briefly
  discussed later (e.g., Constructive Solid
  Geometry (CSG) with Boolean operators).

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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
• NBOften, screen coordinates are measured top to
  bottom, based on raster scan
• (0,0)

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Example 3D Primitives
Polyhedron
Polyline
Patch
Sphere
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Conceptual Framework for
Interactive Graphics

• Graphics library/package (e.g., OpenGL, DirectX
  ) is intermediary between application and the
  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 nearly 4
  decades old but is still useful, indeed dominant

Graphics Library (GL)
Application model
Application program
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Graphics Library

• 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
  a 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

• Vector (calligraphic, stroke, random-scan)
• still used in some plotters
• Raster (TV, bitmap, pixmap), used in displays and
  laser printers

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Vector Architecture

• Vector display works with a display list/file
  stored in refresh buffer
• Display controller draws all vectors at lt 60 Hz
  (often at variable rate) flicker is a problem

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2D Raster Architecture

• Raster display stores bitmap/pixmap in refresh
  buffer, also known as bitmap, frame buffer can
  be in separate hardware (VRAM) or in CPUs main
  memory (DRAM)
• Video controller draws all scan-lines at
  consistent

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Drawing Lines

• For horizontal, vertical and diagonal lines all
  pixels lie on the ideal line special case
• For lines at arbitrary angle, pick pixels closest
  to the ideal line (Bresenhams midpoint scan
  conversion algorithm)
• For thick lines, use multiple pixels in each
  column or fill a rotated rectangle
• Sampling a continuous line on a discrete grid
  introduces sampling errors the jaggies

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Drawing Filled Polygons

• Find intersection of scanline with polygon edges
• Sort intersections by increasing x
• 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 a 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 (1/2)

• For characters defined by small bitmap
• selectively write pixels of refresh buffer
  corresponding to true bits of character bitmap
• Transparent mode dont write 0s BitBlt with
  OR
• Opaque mode write background color for 0s
• Descenders proportional spacing easily
  accommodated

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Drawing Characters (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 a highly skilled
  specialty, involving graphical and algorithmic
  design)

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1-bit Bilevel Display

• Digital intensity value Digital to Analog
  Conversion (DAC) analog signal to drive
  electron beam
• Black White (or any 2 colors, depending on the
  monitor phosphor color)
• Original Mac resolution was 512x384 pixels, now
  from 640x480 up to 1280x1024, even 3Kx3K in IBMs
  Roentgen display. Our PCs are 1280x1024
• 1280x1024 memory 15 nanosec/pixel gt 32 pixel
  chunks stored in shift registers/buffers

Set or
X address
Address
increment
Raster scan generator
Frame buffer
Set or
Y address
Address
decrement
Deflection
Pixel value
Intensity
DAC
Data
Video Controller
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n-Bit Display

• 2n intensities or colors 1 (grayscale) or 3
  (color) DACs guns

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Image Display System
Look-up Table

• 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 eyes ability
  to discriminate (somewhere between 7-10 million)
• Color table is a resource managed (usually) by
  window manager

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Color Look Up Table Operation

• 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, 24bits 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 (1/2)

• 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 (2/2)

• 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
• 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
• AND (S,D) S can mask out pixels in D
• XOR (S,D) S selectively inverts D used in 1-bit
  systems for rubber-banding/dragging, cheap
  cursors
• Note effects in color systems for all but
  replace may be weird

S XOR (S XOR D) D 0 W, 1 B
XOR
S D D
XOR
S D D