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• Python : Passing complex (ffmpeg) arguments to Popen

  25 juin 2016, par xaccrocheur

  This ffmpeg Popen invocation works :

  command = ['ffmpeg', '-y',
  
             '-i', filename,
  
             '-filter_complex', 'showwavespic',
  
             '-colorkey', 'red',
  
             '-frames:v', '1',
  
             '-s', '800:30',
  
             '-vsync', '2',
  
             '/tmp/waveform.png']
  
  process = sp.Popen( command, stdin=sp.PIPE, stderr=sp.PIPE)
  
  process.wait()

  But I need to use ’compand, showwavespic’ and this comma seems to be blocking the execution. I also need to pass all sorts of strange characters, like columns and, well, all that you can find in a CLI invocation.

  How can I pass complex arguments ?

• Transcode HLS Segments individually using FFMPEG

  27 mai 2013, par rayh

  I am recording a continuous, live stream to a high-bitrate HLS stream. I then want to asynchronously transcode this to different formats/bitrates. I have this working, mostly, except audio artefacts are appearing between each segment (gaps and pops).

  Here is an example ffmpeg command line :

  ffmpeg -threads 1 -nostdin -loglevel verbose \
  
     -nostdin -y -i input.ts -c:a libfdk_aac \
  
     -ac 2 -b:a 64k -y -metadata -vn output.ts

  Inspecting an example sound file shows that there is a gap at the end of the audio :

  

  And the start of the file looks suspiciously attenuated (although this may not be an issue) :

  

  My suspicion is that these artefacts are happening because transcoding are occurring without the context of the stream as a whole.

  Any ideas on how to convince FFMPEG to produce audio that will fit back into a HLS stream ?

  ** UPDATE 1 **

  Here are the start/end of the original segment. As you can see, the start still appears the same, but the end is cleanly ended at 30s. I expect some degree of padding with lossy encoding, but I there is some way that HLS manages to do gapless playback (is this related to iTunes method with custom metadata ?)

  
  

  ** UPDATED 2 **

  So, I converted both the original (128k aac in MPEG2 TS) and the transcoded (64k aac in aac/adts container) to WAV and put the two side-by-side. This is the result :

  
  

  I'm not sure if this is representative of how a client will play it back, but it seems a bit odd that decoding the transcoded one introduces a gap at the start and makes the segment longer. Given they are both lossy encoding, I would have expected padding to be equally present in both (if at all).

  ** UPDATE 3 **

  According to http://en.wikipedia.org/wiki/Gapless_playback - Only a handful of encoders support gapless - for MP3, I've switched to lame in ffmpeg, and the problem, so far, appears to have gone.

  For AAC (see http://en.wikipedia.org/wiki/FAAC), I have tried libfaac (as opposed to libfdk_aac) and it also seems to produce gapless audio. However, the quality of the latter isn't that great and I'd rather use libfdk_aac is possible.

• How to manipulate YUV frames obtained from an ffmpeg video decoder ?

  23 juillet 2014, par user3665376

  I wanted to apply effects on video while rendering on android . I decoded the mpeg4 video using ffmpeg to raw YUV(YUV420P) frames .

  I modified the Y(luminescence) bytes using the below snippet and rendered it to screen after converting it to RGB frames .

  AVFrame *frame = [YUV420P frame recieved from ffmpeg decoder]
  
  uint8_t *tempPtr = NULL;
  
  tempPtr = frame->data[0];
  
  int j;
  
  for(j = 0 ; j < frame->linesize[0] ; j++){
  
      *(tempPtr++) = 1;
  
  }

  I applied a Y value of 1 to all pixels of all frames . I expected an output video of low luminescence ie.much darker video . But I was not able to notice any change in luma of the rendered video .

  Note : The YCbCr data planes at AVFRame.data[0] , AVFrame.data[1] , Avframe.data[2] of the decoded frame has expected values . The AVFrame.buf pointers were all pointing to NULL (Don’t know whether it has anything to do with data).

  Wanted to know whether anybody has encountered this situation before ??Links to related opics also will be very much helpful.

  Thanks.