Picking 2D objects after transforming camera in XNA

Question

I have an XNA application (a force based visualisation of some circles) that I've implemented zooming and scrolling in, thanks to this guide.

Before I had the camera implemented, I had picking implemented (click and drag a circle to move it around). However, after the camera transformation I'm struggling to match up my mouse X,Y coordinates with the post-transformation coordinates of my circles.

I attempted to multiply my mouse X,Y vector with my camera transformation matrix. Given a mouse click at (356, 492), after translation I get (713,300), but my objects are around (-1000,900) or so.

So, given a mouse click in local X,Y coordinates and circle locations in post-transformation X,Y coordinates, how do I match them up?

(If it's needed, my VS2010 project is here.)

Answer 1

The view matrix transforms from world space to view space. Your mouse is in view space and you want to transform it into world space. That's the opposite of what the view matrix does. But the solution is simple - just multiply by the inverse view matrix instead.

Matrix inverseViewMatrix = Matrix.Invert(camera.ViewMatrix);
Vector2 worldMousePosition = Vector2.Transform(mousePosition, inverseViewMatrix);

You can then match that location against your world space circles.

Answer 2

You can also use method Viewport.Unproject(...).