关于GDI+图像处理的问题，为什么picturebox2显示的图像是全黑的呢

Question

我想截取一个图片的以正中心为中心，截取一个1000个像素的正方形区域的图片，显示在picture2控件上，结果全是黑的
 public static Bitmap Cut(Bitmap b, int StartX, int StartY,int endX,int endY, int iWidth, int iHeight)
        {
            if (b ==null)
            {
                return null;
 
            }
            int w = b.Width;
            int h = b.Height;
            
            StartX = (w / 2)-500;
            StartY = (h / 2)-500;
            endX = (w / 2) + 500;
            endY = (w / 2) + 500;
            iWidth = 1000;
            iHeight = 1000;
            Bitmap bitOut = new Bitmap(iWidth, iHeight, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
            Graphics g = Graphics.FromImage(bitOut);
            g.DrawImage(b, new Rectangle(StartX, StartY, endX,endY), new Rectangle(0, 0, iWidth, iHeight),GraphicsUnit.Pixel);
            g.Dispose();
            return bitOut; 
        }
        private void button1_Click(object sender, EventArgs e)
        {
            pictureBox1.Image = Image.FromFile(@"C:\r.jpg");
            Bitmap clean_bmp = new Bitmap(@"C:\r.jpg");
            textBox1.Text = clean_bmp.Width.ToString();
            textBox2.Text = clean_bmp.Height.ToString();
            int r = 200;
            int l = 100;
            int b = 200;
            int t = 100;
            Bitmap bmp = Cut(clean_bmp, r, l, b, t,1000,1000);
            pictureBox2.Image = bmp;
            
        }

Answer 1

 public static Bitmap Cut(Image bitmap,  int width, int height)
        {
            if (bitmap == null)
            {
                return null;            }            width = Math.Min(bitmap.Width, width);//确保图片够大
            height = Math.Min(bitmap.Height, height);
            width = height = Math.Min(width, height);//正方形
            Bitmap retBitmap = new Bitmap(width, height, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
            Graphics g = Graphics.FromImage(retBitmap);
            g.DrawImage(bitmap, 0,0, new Rectangle((bitmap.Width - width)/2,(bitmap.Height - height)/2, width, height)  , GraphicsUnit.Pixel);
            g.Dispose();
            return retBitmap;
        }
        private void button1_Click(object sender, EventArgs e)
        {
            pictureBox1.Image = Image.FromFile(@"C:/Documents and Settings/Administrator/My Documents/My Pictures/ReneLiu.jpg");
            Bitmap bmp = Cut(pictureBox1.Image, 1000, 1000);
            pictureBox2.Image = bmp;        }