Transforming mouse coordinates

Question

Im making a graph program and im stuck at where I need to get mouse coordinates to equal graphic scale. With picturebox I use transform to scale my graphic:

RectangleF world = new RectangleF(wxmin, wymin, wwid, whgt);
        PointF[] device_points =
            {
                new PointF(0, PictureBox1.ClientSize.Height),
                new PointF(PictureBox1.ClientSize.Width, PictureBox1.ClientSize.Height),
                new PointF(0, 0),
            };
        Matrix transform = new Matrix(world, device_points);
        gr.Transform = transform;

Im using MouseMove function. Is there a way to transform mouse coordinates? When I put my mouse on x=9 I need my mouse coordinate to be 9.

private void PictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        Console.WriteLine(e.X);
    }

Answer 1

As Hans' comment implies, you can use a second Matrix to accomplish this. You can obtain it either by copying the original Matrix and calling the copy's Invert() method, or you can create the new Matrix from scratch by reversing your input rectangles from the original.

IMHO inverting is easier, but it does mean you'll need to create the inverse matrix and store it somewhere. E.g.:

    Matrix transform = new Matrix(world, device_points);
    gr.Transform = transform;
    inverseTransform = transform.Clone();
    inverseTransform.Invert();

where inverseTransform is a field in your class rather than a local variable, so that your mouse-handling code can use it later.

If you must construct the Matrix later, you could do it like this:

RectangleF device = new RectangleF(new Point(), PictureBox1.ClientSize);
PointF[] world_points =
    {
        new PointF(wxmin, wymin + whgt),
        new PointF(wxmin + wwid, wymin + whgt),
        new PointF(wxmin, wymin),
    };
Matrix inverseTransform = new Matrix(device, world_points);

In either case, you'd simply use the Matrix.TransformPoints() method in your mouse-handling code to apply the inverse transform to the mouse coordinates to get back to your world coordinates.