But I don't see anything!

The first (and second and maybe third) time one tries out a 3D application, even after having checked the code very carefully, there may be nothing on the screen. The following is a list of possible pitfalls:

1. Is the View Reference Point really on or near the object to be viewed?
2. Is it defined in WC or NDC, and does this match the coordinate switch?
3. Is the View Plane Normal pointing the correct way? A classical bug is to look 180 degrees in the wrong direction. GKS-3D uses a Right-Handed system, so if the object to be viewed is at the origin, and is viewed along the Z axis, then one looks in the negative Z direction, but the VPN points in the positive Z direction (0.0, 0.0, 1.0). As only the direction and not the length of View Plane Normal vector matters, one can simply use the coordinates of the eye point to specify this vector, so long as the View Reference Point is placed at the origin.

4. Assuming one wants an orthographic projection, is the Projection Reference Point in the middle of the View Window?
5. Has the View Window (UMIN, VMIN), (UMAX, VMAX) really been set around the object?
6. Have the Front Plane and Back Plane distances been set correctly in front and behind the object?
7. Is the aspect ratio of the Normalization Viewport volume the same as that of View volume?
8. Are the Clipping Limits which define the projection parallelepiped correctly set within the NPC system limits? Do the Clipping Limits and Projection Viewport Limits match? It is always safer to start with clipping switched off!
9. Have the Workstation Window and Workstation Viewport been set to the same aspect ratio?