title: How to seek with GPUImageMovie with AVMutableComposition
body: <p>I have an <code>AVMutableComposition</code> (which is an <code>AVAsset</code>) and I'm using <code>GPUImage</code> (<a href="https://github.com/BradLarson/GPUImage" rel="nofollow noreferrer">https://github.com/BradLarson/GPUImage</a>) to filter it by using a <code>GPUImageMovie</code>. It is working, but I need to implement seeking functionality.</p>
  
  <p>I'm currently initializing <code>GPUImageMovie</code> with my mutable composition:</p>
  
  <p><code>[[GPUImageMovie alloc] initWithAsset:asset];</code></p>
  
  <p>However, according to <a href="https://stackoverflow.com/questions/21781359/gpuimagemovie-playback-controls">GPUImageMovie playback controls</a> I need to create my own player and player item, then initialize the movie with that:</p>
  
  <pre><code>videoPlayerItem = [[AVPlayerItem alloc] initWithAsset:asset];
  videoPlayer = [[AVPlayer alloc] initWithPlayerItem:videoPlayerItem];
  GPUImageMovie *video = [[GPUImageMovie alloc] initWithPlayerItem:videoPlayerItem];
  </code></pre>
  
  <p>However, when I try to play the movie this way using <code>[videoPlayer seekToTime:kCMTimeZero]; [videoPlayer play];</code>, I'm getting a blank player. Nothing is <code>nil</code> etc, but it simply doesn't show anything.</p>
  
  <p>The only question/answer I could find was this:</p>
  
  <p><a href="https://stackoverflow.com/questions/24084583/gpuimagemovie-not-playing-video-file-when-initwithplayeritem">GPUImageMovie not playing video file when initWithPlayerItem</a></p>
  
  <p>Though its solution is irrelevant to me as the problem was with the URL scheme, which I don't have (mine is not a URL asset, but a mutable composition).</p>
  
  <p>How can I control/seek an <code>AVMutableComposition</code> played as a <code>GPUImageMovie</code>? (and yes, I need to use <code>GPUImageMovie</code>, the whole processing pipeline is tightly-coupled to use that) (I am targeting iOS 9+)</p>