Picking a Point on an object

I am trying to pick a point on a given object and display that point on the object.
I have looked through the forum and found some things to try out, but I fear that I am mis-using one functions that I found.
I have implemented the function that was posted by: Stephane Routelous from http://www.exotk.org, called "Convert2dPntTo3dPnt".
What is happening is: the point that I pick seems to be on the object from the active view point, but when I rotate the object sometimes that points are completly off of the object.
Furthermore I read through the explaination of how this function works from another post by Stephane
Routelous. I am relatively new to 3D graphics development, so I can't be sure if the explaination makes perfect sense to me.
I would have thought: That you would project a line from the EyePoint to the PickedPoint. Intesect that line with the object. The intesection that is the shortest distance to the EyePoint is the user's picked point on the object.
If I'm out to lunch I appologize.
But either way can someone offer a bit more light on this issue. I can't seem to follow it properly.


Jonathan Hill's picture