CSCI 346

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CSCI 346

• Chapter 1 and Chapter 2
• Graphics Systems and Models
• And Graphics Programming

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Agenda

• Homework - assignment 2 is due Friday 02/01/2002
• Finish chapter 1 and start chapter 2

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Homework 2

• Convert your 2D object from program 1 to a 3D
  object.
• Center it well in your 3D window.
• Use a meaningful window name (not "Susan's
  picture").
• Remember to document your code.
• Movement is not required for this assignment.
• Review your code before you submit it. If you see
  repeated chunks of code, create a function and
  use it.
• Display it using an orthographic projection.
• Use my program named 3D picture linked from the
  bottom of the syllabus as a guide.
• This programs source code is due before next
  weeks class 02/01/2002.

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glOrtho (GLdouble left, GLdouble right,
GLdouble, bottom, GLdouble top, GLdouble near,
GLdouble far)

• Creates a viewing volume with a box shape.
• Direction of projection is parallel to z axis.
• Viewpoint faces -z axis.
• glOrtho (0.0, (GLdouble) w, 0.0,
  (GLdouble) h, -500.0, 500.0)

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Imaging Systems

• The human vision system
• Pinhole camera
• Synthetic camera model

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The Human Vision System

• Very complex
• Light enters the eye through the lens and the
  cornea
• Iris opens and closes to regulate amount of light
• Lens forms an image on the retina
• Rods and cones are on the retina and are the
  light sensors and excitable by the visible light
  spectrum

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Color

• Color results from the interaction of light with
  the nervous system
• Lens
• Retina
• Color processing unit along the optic nerve

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Lens

• Focus the incoming light on the retina, which
  contains the photo receptors
• Lens must adjust to the focal length of the
  wavelength
• Red has the longest focal length
• Blue has the shortest focal length

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Lens

• Focus the incoming light on the retina, which
  contains the photo receptors
• Chromostereopsis
• Lens absorbs light, (blue 2x red)
• Fluid between the lens and the retina also absorb
  light

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Retina

• Photo receptors that absorb photons and transmit
  chemical signals to the brain.
• Rods.
• Night-vision receptors.
• No color dependency.
• Cones.
• Color senstitivity.
• Require a higher level of light intensity than
  the rods.
• Clustered in the center of retina,

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Retina

• We see objects by edge detection, where an edge
  can be created by a difference in color or
  brightness or both.
• Photoreceptors adjust their sensitivity to the
  overall light level.
• There is also a required minimum intensity level
  for the photoreceptors to respond.

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The Human Vision System

• Resolution
• The measure of what size objects we can see
• How close we can place two points and they remain
  distinct
• Intensity
• Physical measure of light energy
• Brightness
• Measure of how intense we perceive the light to be

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The Human Vision System

• VERY complex
• Automatic and instantaneous
• Pattern recognition
• Depth perception

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Pinhole Camera

• Simple model
• Box
• Small pin hole at one side
• Film at the opposite side

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Pinhole Camera

• Field or angle of view of the camera is the angle
  made by the largest object that our camera can
  image on its film plane
• Ideal pinhole camera has an infinite depth of
  view -- every point in the field of view is in
  focus

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Pinhole Camera

• Problems with pinhole camerathe pinhole
• The pinhole
• Cannot adjust field of view

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Sophisticated Cameras

• Lens replaces a pinhole
• Larger lenses admit more light
• Choosing a lens with the proper focal length
• Equiv to choosing d for the pinhole camera
• The angle of view can vary up to 180 degrees
• Objects in the field of view may or may not be in
  focus

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The Synthetic Camera Model

• Equivalent views of image formation

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The Synthetic Camera Model

• Imaging system in figure 1.17bellows camera
• Object and viewer are specified separately

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The Synthetic Camera Model

• Image or projection plane in front of the lens
• Image of the point on the object is located where
  the projector passes through the projection plane
• Limited size of the image
• Angle of view describes the limitation
• A clipping rectangle placed in the image plane to
  limit the size of the image
• Blinders on a horse
• Underwater goggles
• Window into the world

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Computer Graphics Programmers Interface

• Paint program
• User friendly interface
• Point and click, no programming
• Application programmers interface (API)
• Interface between an application program and a
  graphics system can be specified by graphics
  functions that reside in a graphics library.
  Specs called API

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Computer Graphics API - Specify

• Objects
• Vertices
• Set of primitives
• Viewer position
• Center of projection (center of the lens)
• Orientation -- may be rotated about the origin
  (center of projection or center of the lens)
• Focal length - the size of the image on the film
  plane
• Film plane -- back of the camera has a height
  and width

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A Sequence of Images

• Line drawing of a scene
• wireframe
• Flat shading
• One light source flat shading
• One light source smooth shading
• Texture

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Modeling and Rendering

• Modeling
• Rendering
• Sometimes it is good to separate the modeling of
  a scene from the production of the file (example
  ray tracing)

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Pipeline Architectures

• Pipeline architectures
• Benefit
• Translates well to computer graphics
• Four steps to producing an image
• Transformer
• Clipper
• Projector
• rasterizer

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Chapter 2

• Graphics Programming

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Sierpinski Gasket

• Pre-Mandelbrot classic
• Found by W. Sierpinski around world war I
• Generated by recursively dividing a triangle into
  four congruent smaller triangles
• Think of the interior triangles as "holes
• They occupy more and more of the total area,
  while the "solid" portion becomes as hopelessly
  fragile

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Pseudocode

• 1. Pick a point at random inside the triangle
• 2. Select one of the three vertices at random
• 3. Find a point halfway between the initial
  point and the randomly selected vertex
• 4. Display this new point by putting some sort
  of marker, such as a small circle, at its
  location
• 5. Replace the initial point with this new point
• 6. Return to step 2

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Pen Plotter

• moveto(x,y)
• lineto(x,y)

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Problems With Pen-plotter Model

• 3D difficult
• Must convert 3d world to 2d projection explicitly
• OpenGL allows us to focus on building 3D world
  and let computer handle projections

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Display Funtion

• void display( void )
• typedef GLfloat point22 / define a point
  data type /
• point2 vertices30.0,0.0,250.0,500.0,50
  0.0,0.0
• / A triangle /
• int i, j, k
• long rand() / standard random number
  generator /
• point2 p 75.0,50.0 / An arbitrary
  initial point / glClear(GL_COLOR_BUFFER_BIT)
  /clear the window /
• / computes and plots 5000 new points /
• for( k0 klt5000 k)
• jrand()3 / pick a vertex at random /
  
• / Compute pt halfway between vertex and
  old point /
• p0 (p0verticesj0)/2.0
• p1 (p1verticesj1)/2.0
• / plot new point /
• glBegin(GL_POINTS)
• glVertex2fv(p)
• glEnd()
• glFlush() / clear buffers /

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Coordinate System
3D COORDINATE SYSTEMS
Y
Y
Z
X
X
LEFT HANDED
RIGHT HANDED
Z
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Coordinate System in OpenGL

• What units are x, y, and z?
• Your choice
• Device independent
• World coordinate system
• Before displaying on output device, world
  coordinates must be converted to device or raster
  or screen coordinates

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Point To Remember

• We are studying computer graphics
• We are not studying OpenGL
• Wont cover all functions in OpenGL

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Graphics System As a Black Box
Function Calls
Output
User Program
Graphics System
Input/Output Devices
Data
Input
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API Functions

• Primitive functions
• Attribute functions
• Viewing functions
• Transformation functions
• Input functions
• Control functions

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3 D Gasket

• Use tetrahedons

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OpenGL - What Is It?

• A graphics rendering library
• API to produce high-quality, color images from
  geometric and raster primitives
• Window system and operating system independent
• OpenGL doesnt do windows

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OpenGL

• Most widely adopted graphics standard
• Introduced in 1992
• High visual quality and performance
• Industry standard
• Stable
• Reliable and portable
• Evolving
• Scalable
• Easy to use
• Well-documented

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Related APIs

• GLU (OpenGL utility library)
• Guaranteed to be available
• tesselators
• Quadrics
• NURBs, etc
• Some surprisingly common operations, such as
  projection transformations (such as
  gluPerspective)

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Related APIs

• GLX or WGL
• Bridge between window system and OpenGL
• Glut
• Portable bridge between window system and OpenGL
• Not standard, but uniformly popular

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Homework

• Finish reading chapter 2 - ICG
• Graphics programming
• Assignment 2 - program due 02/01/2002