title: GDI+, using DrawImage to draw a transperancy mask of the source image
body: <p>Is it possible to draw a transperancy mask of an image (that is, paint all visible pixels with a constant color) using <code>Graphics::DrawImage</code>? I am not looking for manually scanning the image pixel-by-pixel and creating a seperate mask image, I wonder if it's possible to draw one directly from the original image.</p>
  
  <p>My guessing is that it should be done with certain manipulations to <code>ImageAttributes</code>, if possible at all.</p>
  
  <p>The color of the mask is arbitrary and should be accurate, and it would be a plus if there can be a threshold value for the transparency.</p>