Csharp:白底圖片透明處理

Bitmap bmp = new Bitmap(pictureBox1.Image, pictureBox1.Image.Size);
                BitmapData data = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadWrite, bmp.PixelFormat);
                int length = data.Stride * data.Height;
                IntPtr ptr = data.Scan0;
                byte[] buff = new byte[length];
                Marshal.Copy(ptr, buff, 0, length);
                for (int i = 3; i < length; i += 4)
                {
                    if (buff[i - 1] >= 230 && buff[i - 2] >= 230 && buff[i - 3] >= 230)
                    {
                        buff[i] = 0;
                    }
                }
                Marshal.Copy(buff, 0, ptr, length);
                bmp.UnlockBits(data);