A correction

If you were paying attention over the weekend, you noticed that I updated the screenshot for Assignment 4.If you compare it with the original screenshot, you’ll notice that the original wasn’t really a parallel projection.

The problem is that (as you should recall from your reading) OpenGL is a state machine. When you make a function call like gluPerspective() to update the current projection matrix, that change stays in effect until you reset the matrix (e.g., with glLoadIdentity().

My mistake was to draw the parallel projections (calling glOrtho()) without remembering to reset the projection matrix, which had already been set with gluPerspective. The result was that we took a cube in perspective, then drew a parallel projection of the perspective rendering. Oops. I didn’t notice because it looks “sort of” right — there was, in fact, a parallel projection happening.

To avoid making silly mistakes like me, do the following whenever you go to render a model (e.g., in your display callback):

  1. Set the viewport.
    glViewport(0, 0, width, height);
  2. Set up the projection.
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-2.0, 2.0, -2.0, 2.0, 0.0, 10.0);
  3. Set your viewing parameters.
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(0.0, 0.0, 3.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
  4. Draw the scene.
    glClearColor(0.0, 0.0, 0.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);
    
    glColor(0.0, 0.0, 1.0);
    glutWireSphere(1.0, 10, 10);

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