OpenGL Programming Guide

Chapter 3Viewing

Viewport Transformation

Recalling the camera analogy, the viewport transformation corresponds to the stage where the size of the developed photograph is chosen. Do you want a wallet-size or a poster-size photograph? Since this is computer graphics, the viewport is the rectangular region of the window where the image is drawn. Figure 3-14 shows a viewport that occupies most of the screen. The viewport is measured in window coordinates, which reflect the position of pixels on the screen relative to the lower left corner of the window. Keep in mind that all vertices have been transformed by the modelview and projection matrices by this point, and vertices outside the viewing volume have been clipped.

Figure 3-14 : A Viewport Rectangle

Defining the Viewport
The Transformed z Coordinate

OpenGL Programming Guide