Andries van Dam September 5, 2000 Introduction to Computer Graphics 17

1
Introduction
2
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.
3
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 co
nsciously managed and documented technology
directed toward communicating information
accurately and descriptively.
Computer Graphics, by William A. Fetter, 1966
4
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.
5
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 Whirlwi
nd, built in 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.
6
What is Interactive Computer Graphics? (3/3)

• Almost all key elements of interactive graphics
  system are expressed in first paragraph of
  Sutherlands 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
7
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
  
• Hardware revolution drives everything
• every 12-18 months, computer power improves by
  factor of 2 in price / performance Moores Law
  
• Palm TX, HP I-Paq as full PC
• iPhone, Blackberry for email/internet
• 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)
  

8
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

9
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,
  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

10
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 60s saw first use of raster
  graphics, especially for flight simulators
  
• Object and command specification minimal typing
  via WIMP (Windows, Icons, Menus, Pointer) GUI
  point-and-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
  client-server computation and window management
  (even X terminals)

11
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) 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.

12
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

13
Versus
Original Macintosh
New iMac 24
2008
1984
Date
24
x .88
2500
2200
Price
CPU
x 382.5
8 MHz
3.06 GHz (Dual)
Memory
x 15625
128KB RAM
2.0GB DDR2 SDRAM
Storage
400KB Floppy
500GB Hard Disk
x 1250000
24 Color
9 Black White
x 2.6
Monitor
1920 x 1200
512 x 342
x 13.2
100 dpi
x 1.5
68 dpi
Mouse
Mouse
same
Devices
Keyboard
Keyboard
same
Desktop WIMP
Desktop WIMP
same
GUI
14
New Forms of Computing1990-2008 (1/9)

• 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

15
New Forms of Computing1990-2008(2/9)

• Virtual Reality

fully immersive VR(via Head-mounted Displays,
Cave 180 George St.)
CAVE
16
New Forms of Computing1990-2008 (3/9)
VOX in Cave and Fishtank VR
17
New Forms of Computing1990-2008 (4/9)
ADVISER Mars data visualization
18
New Forms of Computing1990-2008 (5/9)
Arterial blood flow and bat flight visualization
19
New Forms of Computing1990-2008 (6/9)
Cave Painting
Use feet for navigation, freeing hands for other
uses
20
New Forms of Computing1990-2008 (7/9)

• semi-immersive VR

Elumens VisionStation
Fishtank VR on a monitor small volume, head-track
ed interactive stereo
21
New Forms of Computing1990-2008 (8/9)

• 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)

22
New Forms of Computing1990-2008 (9/9)

• New Interaction technology
• 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 (Browns Sketch)
• tablet PCs and electronic whiteboards present new
  opportunities for pen-centric, gesture-based
  computing
  
• Multi-touch (NYUs Jeff Han, MS Surface, Apples
  iPhone)
  
• Social interfaces (Microsofts Bob and Clippie
  bombed, other avatars may not)
  
• Agents for indirect control

23
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 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

24
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 multi-core
  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
  
• 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.
  bio-informatics algorithms)
  
• Workstation market (and vendors) bowing to
  commoditization (e.g., Suns losing client market)

25
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 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 (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 Autodesks AutoCAD2009,
  Autodesks (formerly AliasWavefronts) Maya,
  Autodesks (formerly Discreets) 3D Studio Max

26
Sampled-based Graphics (1/2)

• Images are made up of 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

27
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. Pre-processing
  step in computer vision

28
Sampling an Image

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

3D scene
0 white, 5 gray, 10 black
29
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

30
Whats the Disadvantage?

• WYSIAYG (What You See Is All You Get) No
  additional information
  
• no depth information
• cant 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)
31
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 Magazines O.J. Simpson cover (below left)
• Reuters doctored Beirut bombing photo (below
  right note cloned smoke)
  
• National Geographics Pyramid cover
• University of Wisconsin application
• Faked NE blackout satellite photo

32
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?

33
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

34
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

35
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

36
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 objects 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 ones constructions. This rendering
  process is also called image synthesis

37
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
decomposition
composition
38
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
  
• 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

root node
Nail
Head (cylinder)
Body
Shaft (cylinder)
Point (cone)
leaf nodes
tree diagram
39
Composition of a GeometricModel
Translate
Translate and Scale
Translate and Rotate
Primitives in their own modeling coordinate syst
em
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).

40
What Kind of Math do We Need?

• Cartesian Coordinates
• Typically modeling space is floating point,
  screen space is integer
  
• NBOften, screen coordinates are measured top to
  bottom, based on raster scan
  

Integer Grid
x, y Cartesian grid
(0,0)
41
Example 3D Primitives
Polyhedron
Polyline
Patch
Sphere
42
Conceptual Framework for Interactive Graphics

• 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

Graphics Library (GL)
Application model
Application program
43
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

44
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

45
Graphics Display Hardware

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

46
Vector Architecture

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

47
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
  
• 60 Hz separates update rate of the
  frame buffer and refresh rate of the CRT

48
Drawing Lines

• 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 (Bresenhams 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

49
Drawing Filled Polygons

• Find intersection of scanline with polygon edges
• Sort intersections by increasing x
• Fill the polygon between pairs of intersections
  (spans)

50
Drawing Circles and Ellipses
circle outline
filled ellipse
51
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

52
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 (explained later)
  
• Opaque mode write background color for 0s
• Descenders proportional spacing easily
  accommodated

Base line
53
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 highly skilled
  specialty, involving graphical and algorithmic
  design)

54
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
  monitor phosphor color)
  
• Original Mac resolution was 512x384 pixels, now
  from 640x480 up to 2560x1600
  

n-Bit Display

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

55
Image Display SystemLook-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 resource managed (usually) by
  window manager

56
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, 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

57
BitBlt/RasterOp (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
58
BitBlt/RasterOp (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

Heres how you can use them Lets say you want t
o add some game sprites to background
59
BitBlt/RasterOp (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
  
• Note effects in color systems for all but
  replace may be weird