How to do 3D hit testing has come up a bit recently in the forums but essentially it isn’t any different than 2D hit testing which is described on MSDN here . You can either start with a 2D point on the Viewport3D or a 3D point on a Visual3D. Starting...

Many haven’t realized this, but we added the ability to transform between 2D and 3D Visuals back in 3.5. This is handy if you need to draw 2D content around your 3D object or if you want to know the 2D position of a 3D point without doing a hit test....