How to retrieve zoom factor of a WinForms PictureBox?

Question

I need the precise position of my mouse pointer over a PictureBox.

I use the MouseMove event of the PictureBox.

On this PictureBox, I use the "zoom" property to show an image.

What is the correct way for getting the position of the mouse on the original (unzoomed) image?

Is there a way to find the scale factor and use it?

I think need to use imageOriginalSize/imageShowedSize to retrieve this scale factor.

I use this function:

float scaleFactorX = mypic.ClientSize.Width / mypic.Image.Size.Width;
float scaleFactorY = mypic.ClientSize.Height / mypic.Image.Size.Height;

Is possible to use this value to get the correct position of the cursor over the image?

Answer 1

I had to solve this same problem today. I wanted it to work for images of any width:height ratio.

Here's my method to find the point 'unscaled_p' on the original full-sized image.

            Point p = pictureBox1.PointToClient(Cursor.Position);
            Point unscaled_p = new Point();

            // image and container dimensions
            int w_i = pictureBox1.Image.Width; 
            int h_i = pictureBox1.Image.Height;
            int w_c = pictureBox1.Width;
            int h_c = pictureBox1.Height;

The first trick is to determine if the image is a horizontally or vertically larger relative to the container, so you'll know which image dimension fills the container completely.

            float imageRatio = w_i / (float)h_i; // image W:H ratio
            float containerRatio = w_c / (float)h_c; // container W:H ratio

            if (imageRatio >= containerRatio)
            {
                // horizontal image
                float scaleFactor = w_c / (float)w_i;
                float scaledHeight = h_i * scaleFactor;
                // calculate gap between top of container and top of image
                float filler = Math.Abs(h_c - scaledHeight) / 2;  
                unscaled_p.X = (int)(p.X / scaleFactor);
                unscaled_p.Y = (int)((p.Y - filler) / scaleFactor);
            }
            else
            {
                // vertical image
                float scaleFactor = h_c / (float)h_i;
                float scaledWidth = w_i * scaleFactor;
                float filler = Math.Abs(w_c - scaledWidth) / 2;
                unscaled_p.X = (int)((p.X - filler) / scaleFactor);
                unscaled_p.Y = (int)(p.Y / scaleFactor);
            }

            return unscaled_p;

Note that because Zoom centers the image, the 'filler' length has to be factored in to determine the dimension that is not filled by the image. The result, 'unscaled_p', is the point on the unscaled image that 'p' correlates to.

Hope that helps!