Geometry

OpenGL Geometry

OpenGL includes functions to draw points, lines, and triangles.

All other shapes are made up of these elements.

A basic shape is entirely described by its vertices, which are connected by straight edges.

The graphics hardware fills in all the necessary pixels.

OpenGL Geometry

GL_POINTS	GL_LINES	GL_LINE_STRIP	GL_LINE_LOOP
GL_TRIANGLES	GL_TRIANGLE_STRIP	GL_TRIANGLE_FAN

OpenGL Geometry

Using a Pyglet vertex_list:

    vlist = pyglet.graphics.vertex_list(3, ('v2f', [0,0, 400,50, 200,300]))

    ...

    vlist.draw(GL_TRIANGLES)

OpenGL Geometry

For older, fixed-function pipeline, the basic method for drawing a shape is:

glBegin(...)
glVertex(...)
glVertex(...)
glVertex(...)
...

glEnd()

Always be sure to have a matching glEnd() for each glBegin()

Coordinate Systems

Coordinate Systems

A coordinate system is needed for measuring objects' positions

It allows us to describe any location by a set of numbers - 2 numbers when working in 2 dimensions, 3 numbers for 3 dimensions.

A coordinate system has an origin (a reference point) and coordinate axes

In 2D we have an X axis and a Y axis. In 3D, we add a Z axis.

The axes are perpendicular - they are independent

Coordinate Systems

Some coordinate systems used:

• Desktop coordinates
  • Origin at upper left corner of screen
  • +X points right, +Y points down
  • 1 unit = 1 pixel
  
  
• Pyglet drawing & pointer coordinates
  • Origin at lower left corner of window
  • +X right, +Y up
  • 1 unit = 1 pixel
  
  
• Default OpenGL drawing coordinates ("normalized device coordinates"), except in Pyglet
  • Origin at center of window
  • +X right, +Y up
  • 1 unit = 1/2 width or height of window

gluOrtho2D

For older, fixed-function pipeline

The default drawing coordinate system can be changed with the function gluOrtho2D

e.g.:

    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    gluOrtho2D(0.0, 10.0, 0.0, 5.0)
    glMatrixMode(GL_MODELVIEW)

This document is by Dave Pape, and is released under a Creative Commons License.