title: Mean value of each channel of several images
body: <p>I want to compute mean value for every RGB channel through all dataset stored in a numpy array. I know it's done with <code>np.mean</code> and I know its basic usage.</p>
  
  <pre><code>np.mean(arr, axis=(??))
  </code></pre>
  
  <p>But as the array has 4 dimensions, I'm a bit lost in setting the correct axis. All examples I found were dealing with just 1-D or 2-D arrays. So how should the function call look like having an array e.g. <code>(1000, 512, 512, 3)</code>?</p>