title: Wand turns transparent background to black
body: <p>I am trying to grayscale with python using Wand, but when I do</p>
  <pre><code>from wand.image import Image
  with Image(filename='image.png') as img:
      img.type = 'grayscale'
      img.save(filename='image_gray.png')
  </code></pre>
  <p>it turns the transparent background into black. If I use one with white background it works. What do I do wrong. And also as grayscaling is</p>
  <pre><code>Y = 0.2126 * RED + 0.7152 * GREEN + 0.0722 * BLUE
  </code></pre>
  <p>Where can I do that manually in Wand, say if I want to change the values a bit. I looked in the documentation and in various forums but I couldn't find any answer, only stuff for photoshop.</p>
  <p>Thanks!</p>