C#彩色图像转灰度图

C#彩色图像转灰度图,picturebox加载一副图片(本地或者网络图片均可):pic.Load(URL)
使用下面的代码转为灰度图:

private void btnRec_Click(object sender, EventArgs e)
        {
            Bitmap picImage = (Bitmap)pic.Image.Clone();
            int rows = picImage.Height;
            int cols = picImage.Width;
            Rectangle rect = new Rectangle(0, 0, cols, rows);
            BitmapData picData = picImage.LockBits(rect, ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
            byte temp = 0;
            unsafe
            {
                byte* ptr = (byte*)(picData.Scan0);
                for (int i = 0; i < rows; i++ )
                {
                    for (int j = 0; j < cols; j++ )
                    {
                        temp = (byte)(0.299 * ptr[2] + 0.587 * ptr[1] + 0.114 * ptr[0]); // get gray value
                        ptr[0] = ptr[1] = ptr[2] = temp;        // set current pixel
                        ptr += 3;                               // point to next pixel
                    }
                    ptr += picData.Stride - picData.Width * 3;  // row data offset
                }
            } // end of unsafe code
            picImage.UnlockBits(picData);

            //pic.Image = picImage;
        }