title: openpose extract only the skeleton
body: <p>I'm using <a href="https://github.com/CMU-Perceptual-Computing-Lab/openpose" rel="nofollow noreferrer">openpose</a> in a <a href="https://github.com/seancook/docker-openpose-cpu" rel="nofollow noreferrer">docker container</a>. 
  The following command create images, where the detected keypoints are printed on the original images. </p>
  
  <pre><code>openpose -display=0 -image_dir=/data -write_images=/data -face=true -hand=true
  </code></pre>
  
  <p>I would like to have the keypoints without the original image on a black background. Like shown here:
  <a href="https://i.stack.imgur.com/lEaX4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lEaX4.png" alt="wanted result"></a></p>
  
  <p>How can I do this?</p>