OpenGL Programming Guide

Chapter 3Viewing

Manipulating the Matrix Stacks

The Projection Matrix Stack

The projection matrix contains a matrix for the projection transformation, which describes the viewing volume. Generally, you don't want to compose projection matrices, so you issue glLoadIdentity() before performing a projection transformation. Also for this reason, the projection matrix stack need be only two levels deep; some OpenGL implementations may allow more than two 4 × 4 matrices. (You can use glGetIntegerv() with GL_MAX_PROJECTION_STACK_DEPTH as the argument to find the stack depth.)

One use for a second matrix in the stack would be an application that needs to display a help window with text in it, in addition to its normal window showing a three-dimensional scene. Since text is most easily drawn with an orthographic projection, you could change temporarily to an orthographic projection, display the help, and then return to your previous projection:

glMatrixMode(GL_PROJECTION);
glPushMatrix();                        /*save the current projection*/
    glLoadIdentity();
    glOrtho(...);                        /*set up for displaying help*/
    display_the_help();
glPopMatrix();

Note that you'd probably have to also change the modelview matrix appropriately.

Advanced

If you know enough mathematics, you can create custom projection matrices that perform arbitrary projective transformations. For example, the OpenGL and its Utility Library have no built-in mechanism for two-point perspective. If you were trying to emulate the drawings in drafting texts, you might need such a projection matrix.

OpenGL Programming Guide