Oh, sorry...I have attached the pictures here. The first picture shows a cube rendered at [0,0,0], and selection markers which are rendered in ortho mode. They are rendered with z value 0.0, and the x and y values calculated using gluProject. This is working fine.
In the second picture I try to render a quad. I give gluProject x and z values of -10 and +10, and y value 0, so it should intersect the cube. The quad is textured with a cross, which should end up inside the cube, and depth testing is enabled (I hoped some would end up behind the cube). However, as you can see from the second picture, it is distorted.
The distortion seems to be less if I move the camera further away.
Any ideas what might cause this effect? Any help would be greatly appreciated.
I posted it here because I consider myself a beginner, at least to game programming, and I think this function (gluProject) is quite basic. If I posted it wrong, I appologise for that.