如何在strechmode-c#中裁剪和保存图片

我无法在strechmode下裁剪图片框中的图像，我尝试了几个星期，但没有得到解决方案。问题是我的图像大小是说(1200,750)和图片大小是说(450,300)，但当我尝试在strechmode下在picturebox中裁剪它时，它需要图像的原始尺寸(1200,750)的坐标，但我需要用picturebox(450,300)中显示的图像的坐标进行裁剪，因此我在裁剪中得到错误的结果。我在c# winforms中使用visual studio 2013。请给我一个解决方案。提前谢谢你。

    private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        // Starting point of the selection:
        if (e.Button == MouseButtons.Left)
        {
            _selecting = true;
            _selection = new Rectangle(new Point(e.X, e.Y), new Size());
        }
    }
    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        // Update the actual size of the selection:
        if (_selecting)
        {
            _selection.Width = e.X - _selection.X;
            _selection.Height = e.Y - _selection.Y;
            // Redraw the picturebox:
            pictureBox1.Refresh();
        }
    }
    private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
    {
        // Make a note that we "have the mouse". 
        bHaveMouse = true;
        // Store the "starting point" for this rubber-band rectangle. 
        ptOriginal.X = e.X;
        ptOriginal.Y = e.Y;
        // Special value lets us know that no previous 
        // rectangle needs to be erased. 
        // Display coordinates 
        //lbCordinates.Text = "Coordinates  :  " + e.X.ToString() + ", " + e.Y.ToString();
        //ptLast.X = -1;
        //ptLast.Y = -1;
        //rectCropArea = new Rectangle(new Point(e.X, e.Y), new Size()); 
        if (e.Button == MouseButtons.Left && _selecting)
        {
            // Create cropped image:
            Image img = pictureBox1.Image.Crop(_selection);
            // Fit image to the picturebox:
            pictureBox1.SizeMode = PictureBoxSizeMode.StretchImage;
            pictureBox1.Image = img.Fit2PictureBox(pictureBox1);
            _selecting = false;
        }
        button5.Enabled = true;
    }
    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        if (_selecting)
        {
            // Draw a rectangle displaying the current selection
            Pen pen = Pens.GreenYellow;
            e.Graphics.DrawRectangle(pen, _selection);
        }
    }

我使用的命名空间类如下

namespace gfoidl.Imaging
{
    public static class ImageExtension
    {
        public static Image Crop(this Image image, Rectangle selection)
        {
            Bitmap bmp = image as Bitmap;
            // Check if it is a bitmap:
            if (bmp == null)
                throw new ArgumentException("Kein gültiges Bild (Bitmap)");
            // Crop the image:
            Bitmap cropBmp = bmp.Clone(selection, bmp.PixelFormat);
            // Release the resources:
            image.Dispose();
            return cropBmp;
        }
        public static Image Fit2PictureBox( this Image image, PictureBox picBox)
        {
            Bitmap bmp = null;
            Graphics g;
            // Scale:
            double scaleY = (double)image.Width / picBox.Width;
            double scaleX = (double)image.Height / picBox.Height;
            double scale = scaleY < scaleX ? scaleX : scaleY;
            // Create new bitmap:
            bmp = new Bitmap(
                (int)((double)image.Width / scale),
                (int)((double)image.Height / scale));
            // Set resolution of the new image:
            bmp.SetResolution(
                image.HorizontalResolution,
                image.VerticalResolution);
            // Create graphics:
            g = Graphics.FromImage(bmp);
            // Set interpolation mode:
            //g.InterpolationMode = InterpolationMode.HighQualityBicubic;
            // Draw the new image:
            g.DrawImage(
                image,
                new Rectangle(          // Ziel
                    0, 0,
                    bmp.Width, bmp.Height),
                new Rectangle(          // Quelle
                    0, 0,
                    image.Width, image.Height),
                GraphicsUnit.Pixel);
            picBox.SizeMode = PictureBoxSizeMode.StretchImage;
            // Release the resources of the graphics:
            g.Dispose();
            // Release the resources of the origin image:
            image.Dispose();
            return bmp;
        }
    }
}   //invoked it in the main program with the header using gfoid1.imaging;

private Point mouseDown;
private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            _selecting = true;
            mouseDown = new Point(e.X, e.Y);
            _selection = new Rectangle(mouseDown, new Size());
        }
    }
private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        if (_selecting)
        {
            Point mousePos = new Point(e.X, e.Y);
            if(mousePos.X < 0)
            {
                mousePos.X = 0;
            }
            if(mousePos.X >= pictureBox1.Width)
            {
                mousePos.X = pictureBox1.Width - 1;
            }
            if(mousePos.Y < 0)
            {
                mousePos.Y = 0;
            }
            if (mousePos.Y >= pictureBox1.Height)
            {
                mousePos.Y = pictureBox1.Height - 1;
            }

            _selection.X = Math.Min(mouseDown.X, mousePos.X);
            _selection.Y = Math.Min(mouseDown.Y, mousePos.Y);
            _selection.Width = Math.Abs(mousePos.X - mouseDown.X);
            _selection.Height = Math.Abs(mousePos.Y - mouseDown.Y);
            // Redraw the picturebox:
            pictureBox1.Invalidate();
        }
    }

private Point mouseDown;
private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        _selecting = true;
        mouseDown = new Point(e.X, e.Y);
        _selection = new Rectangle(mouseDown, new Size());
    }
}
private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    if (_selecting)
    {
        Point mousePos = new Point(e.X, e.Y);
        if(mousePos.X < 0)
        {
            mousePos.X = 0;
        }
        if(mousePos.X >= pictureBox1.Width)
        {
            mousePos.X = pictureBox1.Width - 1;
        }
        if(mousePos.Y < 0)
        {
            mousePos.Y = 0;
        }
        if (mousePos.Y >= pictureBox1.Height)
        {
            mousePos.Y = pictureBox1.Height - 1;
        }

        _selection.X = Math.Min(mouseDown.X, mousePos.X);
        _selection.Y = Math.Min(mouseDown.Y, mousePos.Y);
        _selection.Width = Math.Abs(mousePos.X - mouseDown.X);
        _selection.Height = Math.Abs(mousePos.Y - mouseDown.Y);
        // Redraw the picturebox:
        pictureBox1.Invalidate();
    }
}