title: How to read Y-component from H264 4:2:0 video without default YUV2RGB->RGB2GRAY conversion
body: <p>I need fast work with gray-scale frames hence I need only Y-component from YUV 4:2:0 video I have. As I can see from profiler a lot of time is wasting for useless YUV->RGB conversion.</p>
  
  <pre><code>VideoCapture::cap.set(CV_CAP_PROP_CONVERT_RGB, false) 
  </code></pre>
  
  <p>gives no effect, as I can see from splitting </p>
  
  <pre><code>Mat ch[3]; split(frame,ch); imshow("it must be U-channel", ch[1]);
  </code></pre>
  
  <p>frames is still in RGB, not YUV, i.e. VideoCapture <strong>ignores</strong> CV_CAP_PROP_CONVERT_RGB flag (I believe ffmpeg library which is used by openCV for video-decoding can understand such kind of flags).</p>
  
  <p>So, please, does someone have idea how to get gray-scaled image from video sequence faster? OpenCL methods are ok too since I convert Mat to oclMat to process this frames after reading ( including current useless ocl::cvtColor(A, B, CV_RGB2GRAY) ).</p>