Game Editor - When screen is clicked, how do you identify what coordinate you clicked?

Question

This is a follow-up to my earlier question:

Game Editor - When screen is clicked, how do you identify which object that is clicked?

Something I've noticed is that picking only calculates a ray and checks for the closest objects that intersects it, but since this method isn't working for placing object in a 3D World what are the best way of doing that? A ray is just a line that goes from one point in a certain direction, it checks the first object that intersects it that is closest. But not if you don't have any objects.

I've used one method for calculating a position with an Isometric Camera, but currently I have an arc ball camera and it doesn't produce the same results. Is there a general way that actually produces a good coordinate?

This is currently the method I'm trying to understand, but I've found no tutorials/information on how to actually translate numbers between 0 and 1 to real coordinates on the screen.

3D Arc Ball Camera with this method:

public static Vector3 GetScreenCoordinates(MouseState Mouse, Viewport viewport, Matrix Projection, Matrix View, Matrix World)
{
    Vector3 pos1 = viewport.Unproject(new Vector3(Mouse.X, Mouse.Y, 0), Projection, View, World);
    Vector3 pos2 = viewport.Unproject(new Vector3(Mouse.X, Mouse.Y, 1), Projection, View, World);
    Vector3 dir = Vector3.Normalize(pos2 - pos1);
    dir.Normalize();

    return dir;
}

Answer 1

Use a plane or three planes to check collision against them.

If the world is empty you need to collide with something... one easy solution is setting a reference plane... usually the ground... and check ray collision against the plane to get coordinates in world space...

when you work with objects in an editor, the easier is working in world coordinates... and being independent of screen coordinates...

Here is a video of the editor i'm doing now... where you can see how it is done...