title: Detect if an image is transparent in GraphicsMagick
body: <p>I am trying to check whether an image is actually transparent , not just check alpha channel.</p>
  
  <p>To demonstrate, lets create an image <code>a.png</code> that has an alpha channel but is fully opaque, and an image <code>b.png</code> that is the same except for one translucent pixel: </p>
  
  <p><code>gm convert rose: PNG32:a.png</code></p>
  
  <p><code>gm convert rose: -fill '#0008' -draw "matte 10,10 point" PNG32:b.png</code></p>
  
  <p>With ImageMagick we can easily check the trasparency with <code>%[opaque]</code></p>
  
  <pre><code>$ identify -format '%[opaque]' a.png
  true
  $ identify -format '%[opaque]' b.png
  false
  </code></pre>
  
  <p>What is the equivalent for graphicsmagick , <code>%A</code> only check if transparency is <strong>supported</strong> not that the image is actually transparent.</p>