Graphics Systems and OpenGL

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Graphics Systems and OpenGL
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Business of Generating Images

• Images are made up of pixels

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RGB
RGB Color cube (what we use in computer graphics)
Other color spaces include HSV, YUV, YCrCb, and
YIQ
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The goal of computer graphics

• Solve the function
• Red _at_ a pixel is f(i,j)
• Green _at_ a pixel is f(i,j)
• Blue _at_ a pixel is f(i,j)

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Two Dimensional Images
Y

• Images (at least the ones in this class) are two
  dimensional shapes.
• The two axes we will label as X (horizontal), and
  Y (vertical).

Y Axis
(0,0)
X Axis
X
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Hardware Pipeline
Input
Output
Computation
We want to draw a rectangle, how do we describe
it to a computer?
Model (n) - object description that a computer
understands.
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Partition the space
1. Define a set of points (vertices) in 2D space.
2. Given a set of vertices, draw lines between
consecutive vertices.
(7,9)
(14,9)
(7,3)
(14,3)
Vertex (pl. Vertices) - a point in 2 or 3
dimensional space.
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Record every position
Bitmap - a rectangular array of bits mapped
one-to-one with pixels.
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Representing Objects

• Most common method is the VERTEX method. Define
  the object as a set of points with connectivity
  information.
• Why is connectivity important?

Connectivity - information that defines which
vertices are connected to which other vertices
via edges. Edge - connects two vertices
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A Simple Program

• Generate a square on a solid background

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simple.c
include ltGL/glut.hgt void mydisplay()
glClear(GL_COLOR_BUFFER_BIT) glBegin(GL_QUADS)
glVertex2f(-0.5, -0.5)
glVertex2f(-0.5, 0.5)
glVertex2f(0.5, 0.5)
glVertex2f(0.5, -0.5) glEnd() glFlush()
int main(int argc, char argv) glutCreateW
indow("simple") glutDisplayFunc(mydisplay)
glutMainLoop()
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How do we do this?
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Practical Approach

• Process objects one at a time in the order they
  are generated by the application
• Pipeline architecture
• All steps can be implemented in hardware on the
  graphics card

Input
Output
Computation
application program
display
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simple.c
include ltGL/glut.hgt void mydisplay()
glClear(GL_COLOR_BUFFER_BIT) glBegin(GL_QUADS)
glVertex2f(-0.5, -0.5)
glVertex2f(-0.5, 0.5)
glVertex2f(0.5, 0.5)
glVertex2f(0.5, -0.5) glEnd() glFlush()
int main(int argc, char argv) glutCreateW
indow("simple") glutDisplayFunc(mydisplay)
glutMainLoop()
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Synthetic Camera Model
projector
p
image plane
projection of p
center of projection
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Vertex Processing

• Much of the work in the pipeline is in converting
  object representations from one coordinate system
  to another
• Object coordinates
• Camera (eye) coordinates
• Screen coordinates
• Every change of coordinates is equivalent to a
  matrix transformation
• Vertex processor also computes vertex colors

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Projection

• Projection is the process that combines the 3D
  viewer with the 3D objects to produce the 2D
  image
• Perspective projections all projectors meet at
  the center of projection
• Parallel projection projectors are parallel,
  center of projection is replaced by a direction
  of projection

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Primitive Assembly

• Vertices must be collected into geometric objects
  before clipping and rasterization can take place
• Line segments
• Polygons
• Curves and surfaces

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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 or object space
• Objects that are not within this volume are said
  to be clipped out of the scene

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Rasterization

• If an object is not clipped out, the appropriate
  pixels in the frame buffer must be assigned
  colors
• Rasterizer produces a set of fragments for each
  object
• Fragments are potential pixels
• Have a location in frame bufffer
• Color and depth attributes
• Vertex attributes are interpolated over objects
  by the rasterizer

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Fragment Processing

• Fragments are processed to determine the color of
  the corresponding pixel in the frame buffer
• Colors can be determined by texture mapping or
  interpolation of vertex colors
• Fragments may be blocked by other fragments
  closer to the camera
• Hidden-surface removal