title: When does VideoToolbox' VTCompressionSession benefit from hardware acceleration?
body: <p>I've been working on the <a href="http://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/sys/applemedia" rel="nofollow">gstreamer applemedia encoder plugins</a> and improved the VideoToolbox based video encoding. Running a gstreamer pipeline like:</p>
  
  <pre><code>$ gst-launch-1.0 filesrc location=source.avi ! decodebin  ! vtenc_h264 ! h264parse ! qtmux name=mux ! filesink location=sink.mp4
  </code></pre>
  
  <p>I was expecting to see a very low CPU usage when encoding h264 video using <code>VTCompressionSession</code> on Mac OS systems. However, on the systems I've tested: Mid 2009 Macbook Pro with GeForce 9600M and Mid 2011 Mac mini with Radeon HD 6630M the encoding still consumes between 80% and 130% CPU - which indicates it's not hardware accelerated.</p>
  
  <p>On which hardware configurations, or given which compression parameters (for example for which <code>kVTCompressionPropertyKey_ProfileLevel</code>) does <code>VTCompressionSession</code> use hardware accelerated encoding?</p>