The Coordinate System

• Everything we have done so far has been 2D, but OpenGL is a 3D system.
• In the last program, we messed with the way world is set up with the glOrtho command
  • This command sets up a "viewing box", or view volume
    • Items inside of the view box are displayed
    • Items outside of the view box are clipped.
  • This command takes the left, right, bottom, top , near and far coordinates of a view box.
  • Coordinate System
    • The X axis is as expected,
    • The Y axis is up and down
    • The Z axis is into the screen, but we may change this.
    • 
  • Figures are rendered by projecting the images onto the near screen.
    • This is the view frame.
    • Much more on this later.
  • The view frame is then scaled to fit into the provided window.
    • This may involve skewing the final image.
    • Again, more on this later.
  • Sometimes we wish to keep the system square
    • In the Glut reshape routine, find the min of the width and height of the window
    • And set the viewport to this.
  • There is MUCH more on this to come in chapter 4.
  • It is worth playing with the examples pages 28-32
  • For now, keep your drawings in the view box.
• He discusses clipping
  • This is the process removing points outside of the view volume.
  • Or perhaps better, only drawing thins that are within the view volume
  • This is a complex process, again, more on this later.
• The OpenGL State Machine
  • OpenGL maintains a set of state variables
  • These variables control many portions of the drawing pipeline
    • The color things are drawn with.
    • The view volume
    • The width of lines and size of points
      • glPointSize(2);
      • glLineWidth()
    • Fill mode for polygons
      • glPolygonMode(GL_FRONT_AND_BACK, GL_LINE );
      • GL_FRONT, GL_BACK
      • GL_LINE, GL_FILL, GL_POINT
      • Fill works best with simple polygons.
      • Complex polygons confuse fill routines, but we will see that later too.