Selection excluding occluded objects

Hi there,

I have a 3D scene and want to select some objects “under” a given rectangle. I’m finding that it is even selecting objects that are occluded which isn’t what I want. I’m programming to OpenGL 1.1 and have been told we can’t upgrade (I read in the Super Bible 4th ed’ chapter 13 that 1.5 has occlusion detection).

What techniques should I consider to achieve the selection of objects that are not occluded by others?