Computer Graphics

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Computer Graphics

• Richard Kelley

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Topics Covered

• Common uses for CG
• Types of CG
• Vector vs. Raster
• Programs
• Photoshop, Maya, Renderman, illustrator
• Ray tracing vs Scanline Algorithms
• Graphics Libraries
• OpenGL

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Common uses for CG

• CAD
• CAM -Computer Aided Manufacturing
• Virtual Reality
• Data Visualizations
• Scientific, Engineering, Medical
• Visualization of functions
• Medical Simulations
• Computer Art

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Common uses (cont)

• Entertainment
• Movies
• Image Processing
• GUI

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Types of CG

• Raster Graphics
• Vector Graphics

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Raster Graphics

• Most common form
• Bitmap
• Rectangular grid of pixels
• Each pixel is individually defined
• Generally in RGB color space
• Resolution defined by how many pixels are
  contained in the image
• Raster graphics CAN NOT be scaled to a higher
  resolution without loss in quality

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Raster Graphics (cont)
Resample from 100x100 to 200x200
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Note on Displays

• Virtually all modern video displays represent
  images in raster format
• Scans the screen displaying the color of each
  pixel
• Repaints the screen about 30 times a second
• Why
• Simplicity
• Easy to implement
• Cost effective

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Vector Graphics(Geometric Modeling)

• Uses geometric primitives to represent images
• points,
• Lines
• Curves
• Polygons
• Today vector graphics are used more in
  high-resolution line art and 2D graphics
• Almost all 3D rendering is done using a special
  subste of 2D graphics (including of a third point)

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Vector Graphics (cont)

• Suppose we wanted to display a circle in vector
  format.
• All we have to do is gather the parameters we
  want and plug them into the equation for a circle

• Radius r
• Location of the center j,k
• Stroke style
• Fill style (possibly empty)

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Vector Graphics (cont)

• Vector Displays - not that common
• Instead of displaying each pixel an electron beam
  is steered on the screen to trace out shapes
  leaving the rest of the screen black
• Process is repeated many times per second to get
  a flicker free image
• Dont confuse vector displays with the vector
  graphic format

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Vector Graphics (cont)

• VG store information (parameters) about the
  shapes contained in the images, and apply
  equations to reconstruct the them
• So why use vector graphic?
• Lets look at raster and vector side by side

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Vector Graphics (cont)
Vector
Bitmap
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Vector Graphics (cont)

• So this means that vector graphics can be resized
  without loss of quality!
• Advantages over Raster graphics
• Minimal amount of information translates to
  smaller file size
• Can zoom in and still have smooth lines
• Parameters are stored and can be changed
• Really any affine transform can be applied easily
• Rotation
• Skew

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Programs Used

• There are many graphics programs to choose from
  depending on your needs (and how much money you
  have)
• Photoshop
• GIMP
• Adobe Illustrator
• Maya
• 3D Studio Max

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Adobe Photoshop

• Bitmap graphics editor
• Initially created to edit scanned images
• Because of its capabilities it has quickly
  dominated the market
• Powerful because of its layering system and
  filters
• Disadvantage costly! 650
• GIMP - Open Source Alternative (Free)

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Adobe Illustrator

• Vector based graphics program
• Power lies in its use of Bézier curves

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Maya

• 3D modeling and animation studio
• Extremely powerful (and expensive)
• Used in many movies
• Full version 2000
• Personal Learning Edition (free)
• You better have some serious RAM

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3D Studio Max

• 3D modeling and animation studio
• Similar to Maya
• Used mostly by game developers because of its
  integrated character studio, but also used in
  movies and TV
• 3495

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Ray Tracing

• Basic principal
• A ray is traced backwards from the eye through a
  pixel in the image plane
• Sometimes the ray misses all the objects
• Sometimes it hits something
• When this happens the object has to be evaluated
  to determine how the ray should react and
  otherwise be displayed

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Ray Tracing (cont)

• If the ray hits a shadow
• A secondary ray (shadow ray) is shot backwards
  towards the light source
• If the shadow ray hits another object before it
  hits a light source, then the first intersection
  is in the shadow of the second object
• If the ray hits a reflective object
• A secondary ray is bounced off the object and
  everything that ray hits will be displayed as a
  reflection on that object

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Ray Tracing (cont)

• If the ray hits on object that is transparent
• A secondary ray is sent out and everything that
  it hits is displayed on the object
• Heres an example to help clear things up

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• Example of some of the cool things you can do
  with ray tracing techniques

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Scan line Algorithms

• A method of rendering 3D graphics on a
  point-by-point basis rather than
  polygon-by-polygon basis
• So some point on a line is calculated, followed
  by each successive points in the line
• After one line is finished the algorithm moves on
  to the next line

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Scan line (cont)

• The advantage of scan line rendering is that is
  not necessary to translate the coordinates of all
  vertices from main memory into working memory
  each time a new polygon is encountered
• Its quicker to access the cache than HD
• Scan line rendering is used by most modern
  graphics cards
• Can be accessed by programmers through an API
  such OpenGL

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Graphics Libraries

• What is a GL anyway?
• Basically just a set of function calls to add
  features not normally available
• Types
• OpenGL
• Java 2D, 3D
• Direct3D
• Renderman
• How do they work?
• Binding

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OpenGL

• First developed by Silicon Graphics as a platform
  independent graphics API
• Provide routines to easily build high-level
  rendering and modeling
• Didnt work though without hardware support
• Needed the right graphics card etc.
• Eventually became so popular that it was
  developed into a platform independent package
• Today the package it maintained by OpenGL
  Architecture Review Board

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OpenGL (cont)

• OpenGL is mostly used for 3D graphics but can be
  used for 2D just by zeroing out the z-variable
• Internally
• A collection of hundreds of functions to provide
  direct access to your graphics card
• Can control
• Current color
• Lighting
• Blending
• etc

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OpenGL (cont)

• Code example
• http//www.opengl.org/resources/code/basics/redboo
  k/aaindex.c
• Result

Click Me
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Other Libraries

• Java 2D, 3D
• Direct3D
• Subset of DirectX
• DirectX controls other things besides graphics
  such sound
• Renderman
• Developed by Pixar for their movie
• Toy Story, Monsters Inc, Finding Nemo
• Included in Matlab, Mathematica, and Maple
• To display 2D, 3D models of functions

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Summary

• Common uses for CG
• Types of CG
• Vector vs. Raster
• Programs
• Photoshop, Maya, Renderman, illustrator
• Ray tracing vs Scanline Algorithms
• Graphics Libraries
• OpenGL

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Sources

• OpenGL Programming Guide. http//www.opengl.org/
  resources/code/basics/redbook/. March 6 2005
• Raster Graphics. http//en.wikipedia.org/wiki/Ra
  ster_graphics. March 6 2005
• Vector Graphics. http//en.wikipedia.org/wiki/Ve
  ctor_graphics. March 6 2005
• Bezier Curves. http//en.wikipedia.org/wiki/Bezi
  er_curves. March 6 2005

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More Sources

• Litt, J. Vector vs Bitmap Graphics.
  http//graphicdesign.about.com/library/weekly/aa04
  2398.html. March 6 2004
• Hearn D. Baker P. Computer Graphics with
  OpenGL. New York, 2003