title: How to create a transparent image with GraphicsMagick CLI?
body: <p>I'm coming up empty handed while trying to find out how to modify the opacity of an image using the GraphicsMagick CLI. I'm simply trying to input a standard image and convert it to the corresponding image with a reduced opacity. I'm running GraphicsMagick 1.3.14. Using ImageMagick, I can issue the command:</p>
  
  <pre><code>convert input.png -alpha set -channel a -evaluate set 50% +channel output.png
  </code></pre>
  
  <p>However, the <code>-alpha</code> option (among others) is unrecognized as a valid GM convert option (<a href="http://www.graphicsmagick.org/convert.html#conv-opti" rel="nofollow">convert option reference here</a>). Any help would be greatly appreciated.</p>