Your example is interesting, but it does not actually snap to OpenGL objects.
Your code just happens to draw OpenGL “peru” colored squares at the same locations as the Geom::Point3d points in a “virtual” curve vertex array, which is what PickHelper is snapping to. (Ie, virtual in that an actual curve object does not exist. This is what I find most interesting about the example.)
The point of this topic and the related topic is that the OP (is saying) he does not have access to “read” the point positions of the “special” point clouds created by the Trimble Scan Essentials extension. In that topic, Christina said …