private void button1_Click(object sender, EventArgs e)
        {
            GraphicsPath path = new GraphicsPath();
            Point[] p = {
                            new Point(424,244),
                            new Point(240,440),
                            new Point(340,552),
                            new Point(550,526),
                            new Point(478,366),
                            new Point(348,660),
                            new Point(424,244)
                        };
            path.AddLines(p);
            OpenFileDialog openFileDialog = new OpenFileDialog();

            openFileDialog.Filter = @"Bitmap文件(*.bmp)|*.bmp|Jpeg文件(*.jpg)|*.jpg|所有合適文件(*.bmp,*.jpg)|*.bmp;*.jpg";

            openFileDialog.FilterIndex = 3;

            openFileDialog.RestoreDirectory = true;

            if (DialogResult.OK == openFileDialog.ShowDialog())
            {

                Bitmap bit = (Bitmap)Bitmap.FromFile(openFileDialog.FileName, false);

                Bitmap newBit = null;
                BitmapCrop(bit, path, out newBit);
                newBit.Save(@"C:/Users/Public/Pictures/Sample Pictures/沙漠22.jpg");
            }
        }

/// <summary>
        /// 圖片截圖
        /// </summary>
        /// <param name="bitmap">原圖</param>
        /// <param name="path">裁剪路徑</param>
        /// <param name="outputBitmap">輸出圖</param>
        /// <returns></returns>
        public static Bitmap BitmapCrop(Bitmap bitmap, GraphicsPath path, out Bitmap outputBitmap)
        {
            RectangleF rect = path.GetBounds();
            int left = (int)rect.Left;
            int top = (int)rect.Top;
            int width = (int)rect.Width;
            int height = (int)rect.Height;
            Bitmap image = (Bitmap)bitmap.Clone();
            outputBitmap = new Bitmap(width, height);
            for (int i = left; i < left + width; i++)
            {
                for (int j = top; j < top + height; j++)
                {
                    //判斷坐標是否在路徑中  
                    if (path.IsVisible(i, j))
                    {
                        //復制原圖區域的像素到輸出圖片  
                        outputBitmap.SetPixel(i - left, j - top, image.GetPixel(i, j));
                        //設置原圖這部分區域為透明  
                        image.SetPixel(i, j, Color.FromArgb(0, image.GetPixel(i, j)));
                    }
                    else
                    {
                        outputBitmap.SetPixel(i - left, j - top, Color.FromArgb(0, 255, 255, 255));
                    }
                }
            }
            bitmap.Dispose();
            return image;
        }