title: Convert BitmapImage to grayscale, and keep alpha channel
body: <p>I'm having an issue with converting a BitmapImage (WPF) to grayscale, whilst keeping the alpha channel. The source image is a PNG.</p>
  
  <p>The MSDN article <a href="http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.formatconvertedbitmap.aspx" rel="noreferrer">here</a> works fine, but it removes the alpha channel.</p>
  
  <p>Is there any quick and effective way of converting a BitmapImage to a grayscale?</p>