Bitmap LockBits Example

 byte[] rgbValues;
 private void openFileDialog1_FileOk(object sender, CancelEventArgs e)
        {
            paintWind.Image = Bitmap.FromFile(openFileDialog1.FileName);
            g = Graphics.FromImage(paintWind.Image);
            Bitmap bmp = paintWind.Image as Bitmap;

            // Lock the bitmap's bits.
            Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
            System.Drawing.Imaging.BitmapData bmpData =
                bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,
                bmp.PixelFormat);

            // Get the address of the first line.
            IntPtr ptr = bmpData.Scan0;

            // Declare an array to hold the bytes of the bitmap.
            int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
            rgbValues = new byte[bytes];

            // Copy the RGB values into the array.
            System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);

            // Set every third value to 255. A 24bpp bitmap will look red.
            for (int counter = 0; counter < rgbValues.Length; counter+=3)
            {
                //rgbValues[counter+1] = 255;
            }
            // Copy the RGB values back to the bitmap
            System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, bytes);

            // Unlock the bits.
            bmp.UnlockBits(bmpData);

            int i = 0;
            for (int counter = 0; counter < rgbValues.Length; counter += 3)
            {
                bmp.SetPixel(i % bmp.Width, i / bmp.Width,
                    Color.FromArgb(rgbValues[counter], rgbValues[counter + 1], rgbValues[counter + 2]));
                i++;

            }
            paintWind.Refresh();

        }