Not sure if it applies, but isn't plus values the z-axis towards the viewer and minus values away from the center.
This would mean that unless you translate the viewpoint away from the center, objects moved too far out on the plus range of the z-axis will not be seen.
[edit] you can use glTranslatef 0.0,0.0,0.0 to move the viewpoint.
This would mean that unless you translate the viewpoint away from the center, objects moved too far out on the plus range of the z-axis will not be seen.
[edit] you can use glTranslatef 0.0,0.0,0.0 to move the viewpoint.