title: VLFeat HOG feature extraction
body: <p>I found a tutorial about VLFeat HOG </p>
  
  <pre><code>http://www.vlfeat.org/overview/hog.html
  </code></pre>
  
  <p>I am a little confused by the 16*16*31 matrix. Can anyone tell me how can I extract features that can be used for classification task from the matrix that the function returns?</p>
  
  <p>Thanks!</p>