title: How to know an Android looks whether upwards or downwards?
body: <p>I'm now creating AR application for Android, but a problem was happened.
  By my implementation, a sensor show a same pitch even though the Android looks upwards or downwards.
  Following picture shows what I want to say.</p>
  
  <p><img src="https://i.stack.imgur.com/Gwjpb.png" alt="what_I_want_to_say"></p>
  
  <p>The sensor shows that pitch == -90 if I grab Android horizontally.
  The problem is that I cannot detect a != b.(The sensor says a == b.)
  Can anyone knows how to know an Android looks upwards or downwards?
  The orientation of Android must be portrait.</p>