Per Pixel Alpha Blend in C#

...

class PerPixelAlphaForm : Form
{
  Timer _RefreshFormTimer = new Timer();

  public AlphaForm()
  {
    // This form should not have a border or else Windows will clip it.
    FormBorderStyle = FormBorderStyle.None;

    _RefreshFormTimer.Interval = 10;
    _RefreshFormTimer.Tick += new EventHandler(RefreshFormTimer_Tick);
    _RefreshFormTimer.Enabled = true;
  }

  private Bitmap _FormBitmap;
  private byte _FormOpacity;
  private void RefreshFormTimer_Tick(object sender, EventArgs e)
  {
    if (_FormBitmap != null)
      SetBitmap(_FormBitmap, _FormOpacity);
  }

  /// Changes the current bitmap.
  public void SetBitmap(Bitmap bitmap)
  {
    SetBitmap(bitmap, 255);
  }

...

...
	/// Changes the current bitmap with a custom opacity level.  Here is where all happens!
	public void SetBitmap(Bitmap bitmap, byte opacity)
	{
		if (bitmap.PixelFormat != PixelFormat.Format32bppArgb)
			throw new ApplicationException("The bitmap must be 32ppp with alpha-channel.");

    _FormBitmap = bitmap;
    _FormOpacity = opacity;

		// The ideia of this is very simple,
		// 1. Create a compatible DC with screen;
		// 2. Select the bitmap with 32bpp with alpha-channel in the compatible DC;
		// 3. Call the UpdateLayeredWindow.

		IntPtr screenDc = Win32.GetDC(IntPtr.Zero);
		IntPtr memDc = Win32.CreateCompatibleDC(screenDc);
		IntPtr hBitmap = IntPtr.Zero;
		IntPtr oldBitmap = IntPtr.Zero;

		try {
			
                        foreach (Control ctrl in this.Controls)
                          ctrl.DrawToBitmap(bitmap, ctrl.Bounds);

                        hBitmap = bitmap.GetHbitmap(Color.FromArgb(0));  // grab a GDI handle from this GDI+ bitmap

                        oldBitmap = Win32.SelectObject(memDc, hBitmap);

			Win32.Size size = new Win32.Size(bitmap.Width, bitmap.Height);
			Win32.Point pointSource = new Win32.Point(0, 0);
			Win32.Point topPos = new Win32.Point(Left, Top);
			Win32.BLENDFUNCTION blend = new Win32.BLENDFUNCTION();
...

_FormBitmap = bitmap;

_FormBitmap