title: How to pass base64 encoded image to Tensorflow prediction?
body: <p>I have a google-cloud-ml model that I can run prediction by passing a 3 dimensional array of float32...</p>
  
  <p><code>{ 'instances' [ { 'input' : '[ [ [ 0.0 ], [ 0.5 ], [ 0.8 ] ]  ...  ] ]' } ] }</code></p>
  
  <p>However this is not an efficient format to transmit images, so I'd like to pass base64 encoded png or jpeg.  <a href="https://cloud.google.com/ml-engine/docs/v1/predict-request#data-encoding" rel="noreferrer">This document</a> talks about doing that, but what is not clear is what the entire json object looks like.  Does the <code>{ 'b64' : 'x0welkja...' }</code> go in place of the <code>'[ [ [ 0.0 ], [ 0.5 ], [ 0.8 ] ]  ...  ] ]'</code>, leaving the enclosing 'instances' and 'input' the same?  Or some other structure?  Or does the tensorflow model have to be <em>trained</em> on base64?</p>