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									Introduction to Computer Graphics

                 R. J. Renka

  Department of Computer Science & Engineering
            University of North Texas


                 01/16/2010




             R. J. Renka   Introduction to Computer Graphics
Introduction


  Computer Graphics is a subfield of computer science concerned
  with the creation and manipulation of images. It differs from
  image processing in that the emphasis is on image generation.

  Modern graphics API’s include OpenGL, Direct3D, Java3D,
  Matlab, and others. Our programming environment will
  include C or C++, OpenGL, and GLUT (system-independent
  interface to OpenGL) on a workstation or laptop computer
  running Windows, Linux, or OS X. Graphics lends itself well to
  object-oriented programming, but since OpenGL is not object
  oriented, and we do not want to hide low level details,
  procedural code is preferred.



                         R. J. Renka   Introduction to Computer Graphics
OpenGL

 OpenGL (Open Graphics Library) is a software interface to
 graphics hardware.
     OpenGL is hardware independent and must therefore
     be implemented on different hardware platforms. Since it
     is independent of the windowing system, it requires either
     platform-dependent function calls or a
     platform-independent interface such as GLUT (GL Utility
     Toolkit).
     OpenGL includes 3-D, realism, and animation, but it is
     low-level in the sense that models must be built from
     simple geometric primitives (points, lines, and polygons).
     The functions in GLU (OpenGL Utility Library) are higher
     level.


                        R. J. Renka   Introduction to Computer Graphics
OpenGL continued



     In an X Windows implementation, OpenGL is designed for
     a network environment in which the client and server
     may run on different machines with the server running
     locally.
     OpenGL is a state machine. The state or mode is
     changed by changing state variables such as the current
     drawing color, geometric transformations, etc. All state
     variables have default values.
     OpenGL has bindings to C/C++, Fortran 90, Ada, Java,
     Perl, and Python.




                       R. J. Renka   Introduction to Computer Graphics
Graphics Subfields


  The study of computer graphics can be partitioned into three
  subfields.

    Modeling Mathematical specification of shape and
             appearance, such as a triangle mesh surface and
             reflection model.
   Rendering Creation of shaded images from 3-D computer
             models.
   Animation A technique to create the illusion of motion by
             time-sequencing rendered images.




                        R. J. Renka   Introduction to Computer Graphics
Related Fields


  Related fields include the following.

  User Interaction Interface between user input devices and an
               application program.
  Virtual Reality Attempt to immerse the user in a 3-D virtual
               world using stereo graphics, response to head
               motion, sound, haptics (force feedback), etc.
  Visualization Provide insight into data via visual display.
  Image Processing Manipulation of 2-D images.
  3-D Scanning Use of range-finding to create 3-D models.




                         R. J. Renka   Introduction to Computer Graphics
Applications
      Video games: simulations without the need for high accuracy.
      Movie special effects such as digital compositing
      (superimposed backgrounds with separately filmed
      foregrounds) or computer-generated foregrounds. The first
      full-length computer-generated film was Toy Story in 1994.
      CAD/CAM (Computer-aided Design/Manufacturing):
      mechanical parts and products are designed by a 3-D
      modeling package and produced by a computer-controlled
      milling machine.
      Simulation
      Medical imaging: creation of shaded images from scanned
      patient data.
      Visualization
      Paint programs, Art
      Word processing and desktop publishing
      Business graphics: graphs and charts
      GUI’s
                         R. J. Renka   Introduction to Computer Graphics
A Brief History of Graphics Hardware
  1950’s: Line printers and pen plotters
  Both are limited to monochrome hard copy in batch mode. The
  line printer is a low-resolution raster scan device characterized by
  sequential access. Gray shades are produced by overstriking. The
  pen plotter is a high-resolution vector graphics or random-scan
  device in which the primitives (lines or vectors) are stored as a
  sequence of commands and endpoint coordinates.
  mid 1960’s: Vector system consisting of a CRT, display buffer,
  and display processor
  late 1960’s: Direct-view storage tube (DVST) (Tektronics
  terminal) — a monochrome CRT with long-persistence phosphor
  which eliminated the buffer and refresh process
  early 1970’s: CRT-based raster systems based on TV technology
  late 1970’s: PC, Dot matrix printer, and light pen
  1980’s: Graphics workstations, coprocessors, laser printers, mouse,
  audio
                           R. J. Renka   Introduction to Computer Graphics
CRT-based Raster System

  The advantages of CRT-based raster systems over CRT-based
  vector systems are the following:
    1   Lower cost
    2   Color-filled polygons (essential for realism)
    3   A simple refresh process independent of image complexity

  The disadvantages are as follows:
    1   Scan conversion of lines is expensive and must be done every
        time the image is transformed
    2   Aliasing (jaggies, staircasing)

  The aliasing problem is mitigated by antialiasing techniques, such
  as varying pixel intensity values by distance from a line segment or
  polygon edge. This requires a gray-scale or color system with
  several bits per pixel.

                             R. J. Renka   Introduction to Computer Graphics
Raster Display Hardware
  A bilevel monochrome CRT (Cathode Ray Tube) is a glass tube
  containing an electron gun, a phosphor-coated negatively-charged
  surface, and a yoke — a system of electromagnetic coils which
  deflect the electron beam horizontally and vertically.
  A color TV or RGB monitor is a CRT with three electron guns, a
  shadow mask to improve focus, and discrete sets of red, green, and
  blue phosphor dots in place of a uniform phosphor coating.
  Raster scan is a scanning method in which the electron beam
  sweeps out a fixed path at fixed speed controlled by two oscillators
  — horizontal and vertical.
  Horizontal scanning frequency (scan rate, synchronization rate)
  is typically 12 to 120 Khz (15.75 Khz for TV), where a cycle
  includes a forward sweep (left to right) in which the electron
  beams are switched on and off for each pixel, and a horizontal
  retrace with the beams off.
                          R. J. Renka   Introduction to Computer Graphics
Raster Display Hardware continued
  Vertical scan frequency, including a vertical retrace, is nominally
  60 Hz (NTSC: National Television Standards Committee) or higher
  for monitors (70 or 72).
  Standard TV and some monitors use interlacing in which only half
  of each frame (refresh buffer contents) is displayed in each vertical
  cycle, resulting in a refresh rate of 30 frames/sec. This doubles the
  number of scan lines that can be displayed to less than (15750
  h-cycles/sec)/(30 frames/sec) = 525 h-cycles. In order to reduce
  flicker, the scan lines are interlaced — all even numbered lines in
  one vertical cycle followed by all odd numbered lines in the next.
  The alternative (noninterlaced mode) requires higher frequency
  which implies more expensive hardware.
  The vertical resolution (number of scan lines or rows of pixels) is
  less than 525 because some of the horizontal cycles occur during
  the vertical retrace. Also, there is a border or overscan area —
  pixels and scan lines not mapped to memory.
                           R. J. Renka   Introduction to Computer Graphics
Raster Display Hardware continued

  Phosphor persistence is the time from removal of excitation to
  the moment when phosphorescence (intensity) drops to 10% of its
  initial value (exponential decay): typically 10 to 60 ms or, roughly,
  short, medium, or long persistence.
  Critical fusion frequency (CFF) is the refresh rate (number of
  image redraws per second) at which the image stops flickering and
  fuses into a steady image. It depends on
    1   phosphor persistence (nonlinearly): high persistence reduces
        CFF or flicker at a given refresh rate, but is problematic for
        animation
    2   image intensity and ambient room light: CFF increases with
        both
    3   wavelength
    4   the observer — by up to 20%

                            R. J. Renka   Introduction to Computer Graphics
Raster Display Hardware continued




  Video bandwidth (pixel rate) is the frequency with which the
  electron beam is switched on and off. It determines the resolution
  rather than speed. Standard TV frequency is 14.318 Mhz, limiting
  the horizontal resolution to less than (14318000 pixels/sec)/
  (15750 scanlines/sec) = 909 pixels per scan line.




                          R. J. Renka   Introduction to Computer Graphics

								
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