free-form crop selection doesn't work properly

Question

I made a program that lets you do a random selection then the selected area to save into a new image, but I got a problem it doesn\'t work how it\'s supposed to... I will po

Answer 1

You need to calculate the zoom and the offset of the image when it is zoomed.

Here is how to do that; this assumes the PictureBox is indeed in Zoom mode, not in Stretch mode. If you stretch it you need to calculate the zooms for x and y separately..

SizeF sp = pictureBox5.ClientSize;
SizeF si = pictureBox5.Image.Size;    
float rp = sp.Width / sp.Height;   // calculate the ratios of
float ri = si.Width / si.Height;   // pbox and image   

float zoom = (rp > ri) ? sp.Height / si.Height : sp.Width / si.Width;

float offx = (rp > ri) ? (sp.Width - si.Width * zoom) / 2 : 0;
float offy = (rp <= ri)? (sp.Height - si.Height * zoom) / 2 : 0;
Point offset = Point.Round(new PointF(offx, offy));

You calculate this after setting the Image and after resizing the PictureBox..

Now you can transform each drawn point into a zoomed or an unzoomed coordinate:

    PointF zoomed(Point p1, float zoom, Point offset)
    {
        return (new PointF(p1.X * zoom + offset.X, p1.Y * zoom + offset.Y));
    }

    PointF unZoomed(Point p1, float zoom, Point offset)
    {
        return (new PointF((p1.X - offset.X) / zoom, (p1.Y - offset.Y) / zoom));
    }

Here is a demo the draws on to either a normal (left) or a zoomed in (middle) image. To the right is the result of placing your GetSelectedArea bitmap onto a PictureBox with a checkerbox background:

Case 1: If you store the points as they come in: In your GetSelectedArea method use this point list instead:

    private Bitmap GetSelectedArea(Image source, Color bg_color, List<Point> points)
    {
        var unzoomedPoints = 
            points.Select(x => Point.Round((unZoomed(Point.Round(x), zoom, offset))))
                  .ToList();
        // Make a new bitmap that has the background

After this replace each reference to points in the method by one to unzoomedPoints. Actually there are just two of them..

Case 2: If you store the points already 'unZoomed' :

Points.Add(unZoomed(e.Location, zoom, offset));

you can use the list directly..