Calculate correct cursor position with pan and zoo

Question

I've been messing around with this problem for about an hour now.

I have a viewport which supports panning and zooming, the panning is handled by storing an offset for the X and Y axis. The zoom is just a float going from 0.2 to 14.

My problem is that I need to be able to place stuff where the user clicks in the viewport but when I zoom and pan the mouse coordinates are not correct. I haven't been able to figure out how to properly calculate the mouse coordinates.

Here's an image showing what I have so far: http://i.imgur.com/WQSXKJ2.png

As you can see the mouse origin is always at the top-left of the viewport component. You can see the pan X and Y offset as well as the zoom value in the bottom-left corner of the image. I've also added an example of the mouse coordinates in relation to the top-left of the viewport.

Now since in that image it's currently zoomed in the objects I place will be offset.

Thank you for your time!

EDITED WITH SOLUTION THAT WORKS FOR MY CASE:

void Viewport_MouseClick(object sender, MouseEventArgs e){
    Point mousePosition = new Point((int)((e.X - Pan.X) / Zoom),
                                    (int)((e.Y - Pan.Y) / Zoom));
}

This calculates a correct "screen-space" mouse position whilst taking the pan and zoom into account. I got that solution by playing around with TaWs answer. Thank you for the help! :)

Answer 1

Using a TrackBar for the Zoom and two more for the offsets this seems to work:

private void panel1_Paint(object sender, PaintEventArgs e)
{
    using (Bitmap bmp = new Bitmap(filename))
    {
        e.Graphics.ScaleTransform(zoom, zoom);
        e.Graphics.DrawImage(bmp, trb_offsetX.Value, trb_offsetY.Value);
    }
}

float zoom = 1f;

private void panel1_MouseMove(object sender, MouseEventArgs e)
{

  Point mouseLocation = e.Location;
  Point imageLocation = new Point((int)((mouseLocation.X / zoom - trb_offsetX.Value)),
                                  (int)((mouseLocation.Y / zoom - trb_offsetY.Value)));

   st_mousePos.Text = "   "  +  imageLocation.ToString();

}


private void trackBar1_Scroll(object sender, EventArgs e)
{
    zoom = trackBar1.Value;
    panel1.Invalidate();
}

I have added the code for the Paint event, so you can see if that's the way your are handling it, too.

Answer 2

Put you picture in to PictureBox and then calculate the relative position of the mouse. That is

    double Pic_width = orginal_image_width/ pictureBox.width;
    double Pic_height = orginal_image_height/ pictureBox.height;
    var mouseArgs = (MouseEventArgs)e;
    int xpoint = Convert.ToInt16(mouseArgs.X * Pic_width);
    int ypoint = Convert.ToUInt16(mouseArgs.Y * Pic_height);

this code will give you the position of the cursor in the image.