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• ffmpeg C API - creating queue of frames

  25 mai 2016, par lupod

  I have created using the C API of ffmpeg a C++ application that reads frames from a file and writes them to a new file. Everything works fine, as long as I write immediately the frames to the output. In other words, the following structure of the program outputs the correct result (I put only the pseudocode for now, if needed I can also post some real snippets but the classes that I have created for handling the ffmpeg functionalities are quite large) :

  AVFrame* frame = av_frame_alloc();
  
  int got_frame;
  
  
  
  // readFrame returns 0 if file is ended, got frame = 1 if
  
  // a complete frame has been extracted
  
  while(readFrame(inputfile,frame, &got_frame)) {
  
    if (got_frame) {
  
      // I actually do some processing here
  
      writeFrame(outputfile,frame);
  
    }
  
  }
  
  av_frame_free(&frame);

  The next step has been to parallelize the application and, as a consequence, frames are not written immediately after they are read (I do not want to go into the details of the parallelization). In this case problems arise : there is some flickering in the output, as if some frames get repeated randomly. However, the number of frames and the duration of the output video remains correct.

  What I am trying to do now is to separate completely the reading from writing in the serial implementation in order to understand what is going on. I am creating a queue of pointers to frames :

  std::queue queue;
  
  int ret = 1, got_frame;
  
  while (ret) {
  
    AVFrame* frame = av_frame_alloc();
  
    ret = readFrame(inputfile,frame,&got_frame);
  
    if (got_frame) 
  
      queue.push(frame);
  
  }

  To write frames to the output file I do :

  while (!queue.empty()) {
  
    frame = queue.front();
  
    queue.pop();
  
    writeFrame(outputFile,frame);
  
    av_frame_free(&frame);
  
  }

  The result in this case is an output video with the correct duration and number of frames that is only a repetition of the last 3 (I think) frames of the video.

  My guess is that something might go wrong because of the fact that in the first case I use always the same memory location for reading frames, while in the second case I allocate many different frames.

  Any suggestions on what could be the problem ?

• Capturing PCM audio data stream into file, and playing stream via ffmpeg, how ?

  11 avril 2015, par icarus74

  Would like to do following four things (separately), and need a bit of help understanding how to approach this,

  1. Dump audio data (from a serial-over-USB port), encoded as PCM, 16-bit, 8kHz, little-endian, into a file (plain binary data dump, not into any container format). Can this approach be used :

     $ cat /dev/ttyUSB0 > somefile.dat

  Can I do a ^C to close the file writing, while the dumping is in progress, as per the above command ?

  2. Stream audio data (same as above described kind), directly into ffmpeg for it to play out ? Like this :

     $ cat /dev/ttyUSB0 | ffmpeg

  or, do I have to specify the device port as a "-source" ? If so, I couldn’t figure out the format.

  Note that, I’ve tried this,

  $ cat /dev/urandom | aplay

  which works as expected, by playing out white-noise..., but trying the following doesn’t help :

  $ cat /dev/ttyUSB1 | aplay -f S16_LE

  Even though, opening /dev/ttyUSB1 using picocom @ 115200bps, 8-bit, no parity, I do see gibbrish, indicating presence of audio data, exactly when I expect.

  3. Use the audio data dumped into the file, use as a source in ffmpeg ? If so how, because so far I get the impression that ffmpeg can read a file in standard containers.

  4. Use pre-recorded audio captured in any format (perhaps .mp3 or .wav) to be streamed by ffmpeg, into /dev/ttyUSB0 device. Should I be using this as a "-sink" parameter, or pipe into it or redirect into it ? Also, is it possible that in 2 terminal windows, I use ffmpeg to capture and transmit audio data from/into same device /dev/ttyUSB0, simultaneously ?

  My knowledge of digital audio recording/processing formats, codecs is somewhat limited, so not sure if what I am trying to do qualifies as working with ’raw’ audio or not ?

  If ffmpeg is unable to do what I am hoping to achieve, could gstreamer be the solution ?

  PS> If anyone thinks that the answer could be improved, please feel free to suggest specific points. Would be happy to add any detail requested, provided I have the information.

• Specify timestamp in ffmpeg video segment command

  12 mai 2015, par Soumya

  I have a continuous RTSP stream coming from a camera over the network.
  I want to dump the stream but in video files of length 1 min each.

  I an using the following command

  ffmpeg -i "rtsp://user:pass@example.com"  -f mp4 -r 12 -s 640x480 -ar 44100 \
  
  -ac 1 -segment_time 60 -segment_format mp4 "out%03d.mp4"

  The name of the files being created are of the form out001.mp4, out002.mp4, etc.

  I want to include the timestamp (hour and minute) in the name of the file segments eg. 09-30.mp4, 09-31.mp4, etc.

  If it is mandatory to provide a serial number for the segment, is it possible to get something like 09-30-001.mp4, 09-31-002.mp4 ?