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• How to use ffmpeg to calculator output video file size before convert ?

  6 juillet 2016, par flashvnn

  I am starting with developing a video converter application, it has a feature allowing users to choose a video.

  Then they can preview the output file size before they decide to convert the video.

  Can ffmpeg calculator output the file size before converting it ?

• NSOperationQueue's threads just don't die

  8 novembre 2013, par u2Fan

  Sorry, it's a bit wordy, but I wanted to make sure I was clear ! ;-)

  I have an iOS app that uses FFMPEG for streaming RTSP. I've multi-threaded FFMPEG using NSOperationQueue such that most its work, other than painting the image to the screen, of course, happens in background threads.
  Works great ! ...except for the fact that threads the NSOperationQueue creates never die !

  I init the Queue in the class' init method with :

  self->opQ = [[NSOperationQueue alloc] init];
  
  [self->opQ setMaxConcurrentOperationCount:1];

  I add methods to the Queue using blocks :

  [self->opQ addOperationWithBlock:^{
  
          [self haveConnectedSuccessfullyOperation];
  
  }];

  Or

  [self->opQ addOperationWithBlock:^{
  
          if (SOME_CONDITION) {
  
              [self performSelectorOnMainThread:@selector(DO_SOME_CRAP) withObject:nil waitUntilDone:NO];
  
          }
  
      }];

  Later, when I need to tear down the RTSP stream, in addition to telling FFMPEG to shut down, I call :

  [self->opQ cancelAllOperations];

  Which does indeed stop the threads from doing any work , but never actually destroys them. Below, you'll see a screen shot of threads that are doing nothing at all. This is what my threads look like after starting/stoping FFMPEG several times.

  

  I seem to remember reading in Apple's documentation that NSOperations and the threads they are run on are destroyed once they are done executing, unless otherwise referenced. This doesn't appear to be the case.

  Do I just need to destroy the NSOperationQueue, then re-init it when I need to start up FFMPEG again (I just realized I haven't tried this) ? Anyone know how I need to kill these extra threads ?

  THANKS !

• Extracting metadata from incomplete video files

  17 juillet 2013, par npgall

  Can anyone tell me where metadata is stored in common video file formats ? And if it would be located towards the start of the file, or scattered throughout.

  I'm working with a remote object store containing a lot of video files and I want to extract metadata, in particular video duration and video dimensions from those files, without streaming the entire file contents to the local machine.

  I'm hoping that this metadata will be stored in the first X bytes of files, and so I can just fetch a byte range starting at the beginning instead of the whole file, passing this partial file data to ffprobe.

  For testing purposes I created a 22MB MP4 file, and used the following command to supply only the first 1MB of data to ffprobe :

  head -c1024K '2013-07-04 12.20.07.mp4' | ffprobe -

  It prints :

  avprobe version 0.8.6-4:0.8.6-0ubuntu0.12.04.1, Copyright (c) 2007-2013 the Libav developers
  
    built on Apr  2 2013 17:02:36 with gcc 4.6.3
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x1a6b7a0] stream 0, offset 0x10beab: partial file
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'pipe:':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 0
  
      compatible_brands: isom3gp4
  
      creation_time   : 1947-07-04 11:20:07
  
    Duration: 00:00:09.84, start: 0.000000, bitrate: N/A
  
      Stream #0.0(eng): Video: h264 (High), yuv420p, 1920x1080, 20028 kb/s, PAR 65536:65536 DAR 16:9, 29.99 fps, 30 tbr, 90k tbn, 180k tbc
  
      Metadata:
  
        creation_time   : 1947-07-04 11:20:07
  
      Stream #0.1(eng): Audio: aac, 48000 Hz, stereo, s16, 189 kb/s
  
      Metadata:
  
        creation_time   : 1947-07-04 11:20:07

  So I see the first 1MB was enough to extract video duration 9.84 seconds and video dimensions 1920x1080, even though ffprobe printed the warning about detecting a partial file. If I supply less than 1MB, it fails completely.

  Would this approach work for other common video file formats to reliably extract metadata, or do any common formats scatter metadata throughout the file ?

  I'm aware of the concept of container formats and that various codecs may be used represent the audio/video data inside those containers. I'm not familiar with the details though. So I guess the question may apply to common combinations of containers + codecs ? Thanks in advance.