Midterm Review

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Midterm Review
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Examples of Graphics
Icons and Symbols
Charts and Graphs
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More Examples
Photographs
Rendered Images
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More Examples
Scientific Visualization
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More Examples
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How? On Computer

• Set up virtual camera
• Set up virtual world
• Adjust virtual lighting
• Render image
• Display the image

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Rendering

• Almost all modern graphics devices are raster
  devices, meaning that the graphics surface
  (screen, page, etc) is divided into a discrete
  grid of cells called pixels
• The job of the graphics programmer is to select
  which pixels to fill in with what colors

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The Frame Buffer

• Is an array the raster that holds graphics
  related information
• Mostly, it holds pixels
• Converting geometric entities to pixel
  information is called rasterization
• Each pixel in a picture requires color data
• The number of bits used is called its depth
• The depth determines how many different colors an
  image can have

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Color Pallet

• Non-true color images store the color values in
  pallets
• Pallets are an array of colors stored, usually,
  at the beginning of an image file
• The color information of a particular pixel is
  stored as an index reference to the pallet

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True Color

• 24-bit images
• use 3, 8-bit segments to encode the three
  primaries
• Red, green, and blue (RGB)
• Each 8-bit segment can, therefore, hold 256
  shades of that color
• This results in a possible 16,777,216 different
  colors

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The human visual system

• The sizes of the rods and cones along with
  properties of the lens and cornea determine our
  visual resolution
• Resolution is the measure of how close we can
  place two points and still recognize that there
  are two distinct points

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Object Specification

• Most APIs support a limited set of primitives
  including
• Points (1D object)
• Line segments (2D objects)
• Polygons (3D objects)
• Some curves and surfaces
• Quadrics
• Parametric polynomials
• All are defined through locations in space or
  vertices

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Synthetic Camera Model
projector
p
image plane
projection of p
center of projection
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Advantages

• Separation of objects, viewer, light sources
• Two-dimensional graphics is a special case of
  three-dimensional graphics
• Leads to simple software API
• Specify objects, lights, camera, attributes
• Let implementation determine image
• Leads to fast hardware implementation

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Clipping

• Just as a real camera cannot see the whole
  world, the virtual camera can only see part of
  the world space
• Objects that are not within this volume are said
  to be clipped out of the scene

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OpenGL Functions

• Primitives
• Points
• Line Segments
• Polygons
• Attributes
• Transformations
• Viewing
• Modeling
• Control
• Input (GLUT)

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OpenGL State

• OpenGL is a state machine
• OpenGL functions are of two types
• Primitive generating
• Can cause output if primitive is visible
• How vertices are processed and appearance of
  primitive are controlled by the state
• State changing
• Transformation functions
• Attribute functions

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OpenGL Primitives
GL_POINTS
GL_POLYGON
GL_LINE_STRIP
GL_LINES
GL_LINE_LOOP
GL_TRIANGLES
GL_QUAD_STRIP
GL_TRIANGLE_FAN
GL_TRIANGLE_STRIP
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Polygon Issues

• OpenGL will only display polygons correctly that
  are
• Simple edges cannot cross
• Convex All points on line segment between two
  points in a polygon are also in the polygon
• Flat all vertices are in the same plane
• User program must check if above true
• Triangles satisfy all conditions

nonconvex polygon
nonsimple polygon
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RGB color

• Each color component is stored separately in the
  frame buffer
• Usually 8 bits per component in buffer
• Note in glColor3f the color values range from 0.0
  (none) to 1.0 (all), while in glColor3ub the
  values range from 0 to 255

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Indexed Color

• Colors are indices into tables of RGB values
• Requires less memory
• indices usually 8 bits
• not as important now
• Memory inexpensive
• Need more colors for shading