| CPEN 461/661, Spring 1997 |
| OpenGL Tutorial |
In perspective projections, the farther an object is from the "camera", the smaller it appears in the final image. This is accomplished through the use of a frustum shaped viewing volume.
The command to create a perspective projection viewing volume is glFrustum(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top, GLdouble near, GLdouble far). The arguments left and right specify the coordinates for the left and right vertical clipping planes. The arguments bottom and top specify the coordinates for the bottom and top horizontal clipping planes. The arguments near and far specify the distances to the near and far depth clipping planes. Both distances must be positive.
With an orthographic projection, the viewing volume is shaped like a box. Unlike perspective projection, the size of the viewing volume does not change from one end to the other. Therefore, distance from the "camera" does not affect how large an object appears.
The command to create an orthographic viewing volume is glOrtho(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top, GLdouble near, GLdouble far). The arguments left and right specify the coordinates for the left and right vertical clipping planes. The arguments bottom and top specify the coordinates for the bottom and top horizontal clipping planes. The arguments near and far specify the distances to the nearer and farther depth clipping planes. These distances are negative if the plane is to be behind the viewer.