Per Pixel Alpha Blend in C#

Bitmap temp_bmp = new Bitmap(bitmap);
foreach (Control ctrl in this.Controls)
{
                    ctrl.DrawToBitmap(temp_bmp, ctrl.Bounds);
}

...
 
class PerPixelAlphaForm : Form
{
  <code>Timer _RefreshFormTimer = new Timer();</code>
 
  public AlphaForm()
  {
    // This form should not have a border or else Windows will clip it.
    FormBorderStyle = FormBorderStyle.None;
 
    <code>_RefreshFormTimer.Interval = 10;
    _RefreshFormTimer.Tick += new EventHandler(RefreshFormTimer_Tick);
    _RefreshFormTimer.Enabled = true;</code>
  }
 
  <code>private Bitmap _FormBitmap;
  private byte _FormOpacity;
  private void RefreshFormTimer_Tick(object sender, EventArgs e)
  {
    if (_FormBitmap != null)
      SetBitmap(_FormBitmap, _FormOpacity);
  }</code>
 
  /// <para>Changes the current bitmap.</para>
  public void SetBitmap(Bitmap bitmap)
  {
    SetBitmap(bitmap, 255);
  }
 
...

...
	/// <para>Changes the current bitmap with a custom opacity level.  Here is where all happens!</para>
	public void SetBitmap(Bitmap bitmap, byte opacity)
	{
		if (bitmap.PixelFormat != PixelFormat.Format32bppArgb)
			throw new ApplicationException("The bitmap must be 32ppp with alpha-channel.");
 
    <code>_FormBitmap = bitmap;
    _FormOpacity = opacity;</code>
 
		// 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 {
			<code>
                        foreach (Control ctrl in this.Controls)
                          ctrl.DrawToBitmap(bitmap, ctrl.Bounds);
</code>
                        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