I'm new in 3D game developing.
I'm developing a plane game using JSR 184 like classic game 1942.
I'm using 2D tiled background and a 3D plane. The plane moves correctly, it turns ok, and it goes up and down.

I have this problem,
I'm trying to paint a 2D shadow under the plane in a x,y screen coordinate. I don't know how to make the shadow follow the plane, i.e., the x.y screen coordinate where my 3d plane is.

The same problem appears in the moment i'm trying to detect when the plane goes out of the screen or colision detection between 3D plane (with a x,y,z coordinates) and 2D objects (with a x,y screen coordinates).

I think "picking techniques" will not be very usefull in this case.

Looking forward your reply.

Sorry for my english I hope you understand it