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• How do I properly enable ffmpeg for matplotlib.animation ?

  7 mars 2017, par spanishgum

  I have covered a lot of ground on stack so far trying to get ffmpeg going so I can make a timelapse video.

  I am on a CentOS 7 machine, running python3.7.0a0.

  python3
  
  >>> import numpy as np
  
  >>> np.__version__
  
  '1.12.0'
  
  >>> import matplotlib as mpl
  
  >>> mpl.__version__
  
  '2.0.0'
  
  >>> import mpl_toolkits.basemap as base
  
  >>> base.__version__
  
  '1.0.7'

  I found this github gist on installing ffmpeg. I used the chromium source, and installed without a prefix option (using the default).

  I have confirmed that ffmpeg is installed, although I don’t know anything about testing whether it works.

  which ffmpeg
  
  /usr/local/bin/ffmpeg
  
  
  
  ffmpeg -version
  
  ffmpeg version N-83533-gada281d Copyright (c) 2000-2017 the FFmpeg dev elopers
  
  built with gcc 4.8.5 (GCC) 20150623 (Red Hat 4.8.5-11
  
  configuration:
  
  libavutil      55. 47.100 / 55. 47.100
  
  libavcodec     57. 80.100 / 57. 80.100
  
  libavformat    57. 66.102 / 57. 66.102
  
  libavdevice    57.  2.100 / 57.  2.100
  
  libavfilter     6. 73.100 /  6. 73.100
  
  libswscale      4.  3.101 /  4.  3.101
  
  libswresample   2.  4.100 /  2.  4.100

  I tried to run a few sample examples I found online :

  [1] http://matplotlib.org/examples/animation/basic_example_writer.html

  [2] http://stackoverflow.com/a/23098090/3454650

  Everything works fine up until I try to save the animation file.

  [1]

   anim.save('basic_animation.mp4', writer = FFwriter, fps=30, extra_args=['-vcodec', 'libx264'])

  [2]

  im_ani.save('im.mp4', writer=writer)

  I found here that explictly setting the path to ffmpeg might be necessary so I added this to the top of the test scripts :

  plt.rcParams['animation.ffmpeg_path'] = '/usr/local/bin/ffmpeg'

  I tried a few more tweaks in the code but always get the same response, which I do not know how to begin deciphering :

  Traceback (most recent call last):
  
    File "testanim.py", line 27, in <module>
  
      writer.grab_frame()
  
    File "/usr/local/lib/python3.7/contextlib.py", line 100, in __exit__
  
      self.gen.throw(type, value, traceback)
  
    File "/usr/local/lib/python3.7/site-packages/matplotlib/animation.py", line 256, in saving
  
      self.finish()
  
    File "/usr/local/lib/python3.7/site-packages/matplotlib/animation.py", line 276, in finish
  
      self.cleanup()
  
    File "/usr/local/lib/python3.7/site-packages/matplotlib/animation.py", line 311, in cleanup
  
      out, err = self._proc.communicate()
  
    File "/usr/local/lib/python3.7/subprocess.py", line 836, in communicate
  
      stdout, stderr = self._communicate(input, endtime, timeout)
  
    File "/usr/local/lib/python3.7/subprocess.py", line 1474, in _communicate
  
      selector.register(self.stdout, selectors.EVENT_READ)
  
    File "/usr/local/lib/python3.7/selectors.py", line 351, in register
  
      key = super().register(fileobj, events, data)
  
    File "/usr/local/lib/python3.7/selectors.py", line 237, in register
  
      key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data)
  
    File "/usr/local/lib/python3.7/selectors.py", line 224, in _fileobj_lookup
  
      return _fileobj_to_fd(fileobj)
  
    File "/usr/local/lib/python3.7/selectors.py", line 39, in _fileobj_to_fd
  
      "{!r}".format(fileobj)) from None
  
  ValueError: Invalid file object: &lt;_io.BufferedReader name=6>
  
  </module>

  Is there something with my configuration that is malformed ? I searched google for this error for some time but never found anything relevant to animations / ffmpeg. Any help would be greatly appreciated.

  ---

  UPDATE :

  @LordNeckBeard pointed me here : https://trac.ffmpeg.org/wiki/CompilationGuide/Centos

  I ran into problems with installing the x264 encoding dependency. Some files in libavcodec/*.c (in the make output) were reporting undefined references to several functions. After a wild goose chase found this : https://mailman.videolan.org/pipermail/x264-devel/2015-February/010971.html

  To fix the x264 installation, I simply added some configure flags :

  ./configure --enable-static --enable-shared --extra-ldflags="-lswresample -llzma"

  ---

  UPDATE :

  So everything installed fine after fixing the libx264 problems. I went ahead and copied the ffmpeg binary from the ffmpeg_build folder into /usr/local/bin/ffmpeg.

  After running the script I was getting problems where ffmpeg could not find the libx264 shared object. I think I will have to recompile everything using different prefixes. My intuition tells me there are old files laying around after I have messed with everything, using some configuration that is broken.

  So I decided maybe I should just try to use NUX : http://linoxide.com/linux-how-to/install-ffmpeg-centos-7/
  I installed ffmpeg using the new rpm, but to no avail. I still was not able to run ffmpeg because of a missing shared object.

  Finally, instead of usiong files copied into my /usr/local/bin folder, I ran ffmpeg directly from the build bin directory. Turns out that this does work properly !

  So in essence, if I want to install ffmpeg system wide, I need to manually compile from sources again but using a nonlocal prefix.

• Write audio packet to file using ffmpeg

  27 février 2017, par iamyz

  I am trying to write audio packet to file using ffmpeg. The source device sending the packet after some interval. e.g.

  First packet has a time stamp 00:00:00 
  
  Second packet has a time stamp 00:00:00.5000000
  
  Third packet has a time stamp 00:00:01
  
  And so on...

  Means two packet per second.

  I want to encode those packets and write to a file.

  I am referring the Ffmpeg example from link Muxing.c

  While encoding and writing there is no error. But output file has only 2 sec audio duration and speed is also super fast.

  The video frames are proper according the settings.

  I think the problem is related to calculation of pts, dts and duration of packet.

  How should I calculate proper values for pts, dts and duration. Or is this problem related to other thing ?

  Code :

  void AudioWriter::WriteAudioChunk(IntPtr chunk, int lenght, TimeSpan timestamp)
  
  {
  
      int buffer_size = av_samples_get_buffer_size(NULL, outputStream->tmp_frame->channels, outputStream->tmp_frame->nb_samples,  outputStream->AudioStream->codec->sample_fmt, 0);
  
  
  
      uint8_t *audioData = reinterpret_cast(static_cast(chunk));
  
      int ret = avcodec_fill_audio_frame(outputStream->tmp_frame,outputStream->Channels, outputStream->AudioStream->codec->sample_fmt, audioData, buffer_size, 1);
  
  
  
      if (!ret)
  
         throw gcnew System::IO::IOException("A audio file was not opened yet.");
  
  
  
      write_audio_frame(outputStream->FormatContext, outputStream, audioData);
  
  }
  
  
  
  
  
  static int write_audio_frame(AVFormatContext *oc, AudioWriterData^ ost, uint8_t *audioData)
  
  {
  
         AVCodecContext *c;
  
         AVPacket pkt = { 0 };
  
         int ret;
  
         int got_packet;
  
         int dst_nb_samples;
  
  
  
         av_init_packet(&pkt);
  
         c = ost->AudioStream->codec;
  
  
  
         AVFrame *frame = ost->tmp_frame;
  
  
  
        if (frame)
  
        {
  
            dst_nb_samples = av_rescale_rnd(swr_get_delay(ost->swr_ctx, c->sample_rate) + frame->nb_samples, c->sample_rate, c->sample_rate, AV_ROUND_UP);
  
            if (dst_nb_samples != frame->nb_samples)
  
              throw gcnew Exception("dst_nb_samples != frame->nb_samples");
  
  
  
            ret = av_frame_make_writable(ost->AudioFrame);
  
            if (ret < 0)
  
               throw gcnew Exception("Unable to make writable.");
  
  
  
            ret = swr_convert(ost->swr_ctx, ost->AudioFrame->data, dst_nb_samples, (const uint8_t **)frame->data, frame->nb_samples);
  
            if (ret < 0) 
  
              throw gcnew Exception("Unable to convert to destination format.");
  
  
  
            frame = ost->AudioFrame;
  
  
  
            AVRational timebase = { 1, c->sample_rate };
  
            frame->pts = av_rescale_q(ost->samples_count, timebase, c->time_base);
  
            ost->samples_count += dst_nb_samples;
  
        }
  
  
  
        ret = avcodec_encode_audio2(c, &pkt, frame, &got_packet);
  
        if (ret < 0) 
  
          throw gcnew Exception("Error encoding audio frame.");
  
  
  
        if (got_packet)
  
        {
  
          ret = write_frame(oc, &c->time_base, ost->AudioStream, &pkt);
  
          if (ret < 0)
  
              throw gcnew Exception("Audio is not written.");
  
        }
  
        else
  
           throw gcnew Exception("Audio packet encode failed.");
  
  
  
        return (ost->AudioFrame || got_packet) ? 0 : 1;
  
  }
  
  
  
  static int write_frame(AVFormatContext *fmt_ctx, const AVRational *time_base, AVStream *st, AVPacket *pkt)
  
  {
  
      av_packet_rescale_ts(pkt, *time_base, st->time_base);
  
      pkt->stream_index = st->index;
  
      return av_interleaved_write_frame(fmt_ctx, pkt);
  
  }

• What codec/format to use for fastest possible decoding ?

  2 mars 2017, par JT J

  I’m using an ffmpeg script (in Windows) that extracts all the keyframes from a video and pastes them into a folder. I’ve made sure that my drive speed, CPU, and RAM are not causing a bottleneck.

  The quality of the video is actually not important at all in this case. I need to encode the video that the script extracts frames from so that it has the fastest possible decoding speed. File size and quality are not important, only read speed. The video does not have audio. What would work best for me ?

  If it matters, here’s the script I’m working with :

  ffmpeg -i input.mp4 -vf "select=eq(pict_type\,I)" -vsync 1 %%3d.bmp

  Sorry if sound like I don’t know what I’m talking about, this is not a topic I am super familiar with. I appreciate your help !