title: How to apply zca on a huge image dataset with limited memory?
body: <p>what google told me is:</p>
  
  <ol>
  <li><p>For <code>keras</code>, the <code>ImageDataGenerator</code> function seems to have a zca_whitening which can be used out of the box. But if this option been set, it requires to call the <code>ImageDataGenerator.fit</code> on the whole dataset X. So this is not an option.</p></li>
  <li><p>For <code>sklearn</code>, the <code>IncrementalPCA</code> seems to work with a huge dataset, but I don't know how to rotate PCA to ZCA in an generator style.</p></li>
  </ol>
  
  <p>Thanks for the help!</p>