Calculate a zoom

Zoom.cs

// sourceSize = source control size
// targetSize = target control size
// center of the zoom
// zoom radius

static Rectangle Zoom(Size sourceSize, Size targetSize, Point center, int radius)
{
    var f = Math.Min((double)targetSize.Width / ((double)radius * 2d), (double)targetSize.Height / ((double)radius * 2d));

    var loc = new Point((int)((-center.X + radius) * f), (int)((-center.Y + radius) * f));
    var size = new Size((int)(sourceSize.Width * f), (int)(sourceSize.Height * f));

    loc.X += (int)(((double)targetSize.Width - (double)radius * f * 2d) / 2d);
    loc.Y += (int)(((double)targetSize.Height - (double)radius * f * 2d) / 2d);

    return new Rectangle(loc, size);
}