title: Cropping two transparent png images using Imagick gives different results
body: <p>I am having different result when cropping two png files.</p>
  
  <p>Imagick Version using <code>convert -version</code>:</p>
  
  <ul>
  <li>Version: ImageMagick 6.6.5-10 2010-11-26 Q16</li>
  <li>Copyright: Copyright (C) 1999-2010 ImageMagick Studio LLC</li>
  <li>Features: OpenMP</li>
  </ul>
  
  <p>The two files using <code>identify</code>:</p>
  
  <ul>
  <li><code>works.png PNG 1218x610 1218x610+0+0 8-bit DirectClass 755KB 0.000u 0:00.000</code>  </li>
  <li><code>doesntwork.png PNG 70x70 70x70+0+0 8-bit DirectClass 1.64KB 0.000u 0:00.000</code></li>
  </ul>
  
  <p>I am using this command:</p>
  
  <pre><code>convert &lt;original&gt;.png -crop 50x50+0+0 &lt;target&gt;.png
  </code></pre>
  
  <p>The problem is that <code>works.png</code> is 100% correct, while <code>doesntwork.png</code> is a black square.</p>
  
  <p>Both original images seem to be the same - and do display correctly on <code>windows 7</code>.</p>
  
  <p>The images:</p>
  
  <p><code>works.png</code>:</p>
  
  <p><img src="https://i.stack.imgur.com/qFfCj.png" alt="enter image description here"></p>
  
  <p><code>doesntwork.png</code>:</p>
  
  <p><img src="https://i.stack.imgur.com/Nb1H0.png" alt="enter image description here"></p>