title: bit_pattern_31_[256*4] in opencv orb feature detector
body: <p>I'm learning orb-slam and  opencv source code, and inside the <code>orb.cpp</code>  which lies on modules/features2d/src/ directory I see a bit pattern named as </p>
  
  <pre><code>bit_pattern_31_[256*4]
  </code></pre>
  
  <p>But I really don't know what's its usage. I search the google and bing long time without any answer given.<br>
  So any one know the usage or reference of this majic bit pattern?</p>