Alpha transparent window form - vb.net

Question

I got following code to load a PNG with alpha channel on to a form and reshape a form to match alpha channel.
Public Function applyAlphaForm(ByVal f As Form, ByVal bitmap As Bitmap, Optional ByVal opacity As Byte = 255) As Boolean
f.FormBorderStyle = FormBorderStyle.None
Dim style As Long
style = Win32.GetWindowLong(f.Handle, Win32.GWL_EXSTYLE)
If Not (style And Win32.WS_EX_LAYERED = Win32.WS_EX_LAYERED) Then
style = style Or Win32.WS_EX_LAYERED
Win32.SetWindowLong(f.Handle, Win32.GWL_EXSTYLE, style)
End If
Return SetBitmap(f, bitmap, opacity)
End Function
Public Function SetBitmap(ByVal f As Form, ByVal bitmap As Bitmap, ByVal opacity As Byte) As Boolean
f.Height = bitmap.Height
f.Width = bitmap.Width
If bitmap.PixelFormat <> PixelFormat.Format32bppArgb Then
f.BackgroundImage = bitmap
f.TransparencyKey = bitmap.GetPixel(0, 0)
Return True
End If
Dim screenDC As IntPtr = Win32.GetDC(IntPtr.Zero)
Dim memDC As IntPtr = Win32.CreateCompatibleDC(screenDC)
Dim hBitmap As IntPtr = IntPtr.Zero
Dim oldBitmap As IntPtr = IntPtr.Zero
Try
hBitmap = bitmap.GetHbitmap(Color.FromArgb(0)) 'grab a GDI handle from this GDI+ bitmap
oldBitmap = Win32.SelectObject(memDC, hBitmap)
Dim size As Win32.Size = New Win32.Size(bitmap.Width, bitmap.Height)
Dim pointSource As Win32.Point = New Win32.Point(0, 0)
Dim topPos As Win32.Point = New Win32.Point(f.Left, f.Top)
Dim blend As Win32.BLENDFUNCTION = New Win32.BLENDFUNCTION()
blend.BlendOp = Win32.AC_SRC_OVER
blend.BlendFlags = 0
blend.SourceConstantAlpha = opacity
blend.AlphaFormat = Win32.AC_SRC_ALPHA
Win32.UpdateLayeredWindow(f.Handle, screenDC, topPos, size, memDC, pointSource, 0, blend, Win32.ULW_ALPHA)
Catch ex As Exception
Finally
Win32.ReleaseDC(IntPtr.Zero, screenDC)
If hBitmap <> IntPtr.Zero Then
Win32.SelectObject(memDC, oldBitmap)
Win32.DeleteObject(hBitmap)
End If
Win32.DeleteDC(memDC)
End Try
Return True
End Function
Nice and easy but if I put some control (button, text box ...) on the form, they will disappear. I guest that UpdateLayeredWindow will paint over the form hDC so we can't see anything behind it. So how to draw some form control on the form? I've try to loop through all control and render to png bitmap before calling api but that will be static image.

Answer 1

So I guest I have to create two from, a background form alpha transparent and a foreground form with control on it. The control form will only display the area of control.