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• Libav (ffmpeg) What is the most robust way to set the output stream time base ?

  29 octobre 2016, par Jason C

  This is a follow up to the solution to this question. My question is : When creating a new output stream, what is the most robust way to ensure that the output stream time base is set to a valid value for arbitrary formats ?

  Another way to phrase this question is : If I leave the output stream time base set to 0/0, will avformat_write_header always initialize it to something appropriate ?

  Consider the following snippet (assume, unlike the above linked question, that I’m just encoding video and I do not have any input video timing info to refer to or copy from) :

  AVFormatContext *formatx;
  
  AVCodec *codec;
  
  AVStream *stream;
  
  ...
  
  stream = avformat_new_stream(formatx, codec);
  
  stream->time_base = { 1, 10000 };
  
  ...
  
  avformat_write_header(formatx, NULL);

  Here, my rationale is as follows :

  • I observed, for MOV output formats, that if the stream time base is 0/0 when avformat_write_header is called, it is changed to 1/90000. Conclusion : At least one format (MOV) has a preferred time base that is set here, so others may be the same.
  • I do not know if avformat_write_header can be relied on to do this first, so I figure I’ll give it an initial reasonable value (1/10000) just in case.

  So this covers cases where avformat_write_header doesn’t set the time base. However, now I’ve observed two worrisome things :

  • If I do initialize the time base (to 1/10000 in this case), avformat_write_header does not modify it. No worries yet, except...
  • As an experiment I set it to 1/1000000. The MOV muxer issued a warning that the time base was too high. This means avformat_write_header seems to obey the time base that was set even if it’s not necessarily appropriate for the muxer.

  So my conflict is as follows :

  • If I don’t set the time base before writing the header, then I know that in at least some cases avformat_write_header will initialize it to something appropriate. However, I don’t know if this is true in all cases, so I run the risk of this failing (or do I ? that’s the question here).
  • If I do set the time base before writing the header, then I’m safe in situations where avformat_write_header doesn’t, but I run the risk of breaking the muxer, since I can’t know what time bases are valid for arbitrary muxers (or can I ?)
  • The time base can’t be changed after writing the header, of course. So I can’t initialize it to 0/0 then check it for validity and set it to something afterwards. That is, if I set it to 0/0, and avformat_write_header does not fill it in, then I’ve missed an opportunity to set it myself and the program unnecessarily fails.

  So what do I do ? How do I ensure that an output stream time base is both a) always set, and b) always set to something appropriate for the muxer ?

• Cutting a video accurately at a specific time without losing quality

  6 février 2018, par mdasari

  I am using ffmpeg to cut a video accurately at a given time to another time. I know, we can simply re-encode it, but will lose the quality. Also, I do not want to use copy option because of key frames issue (i.e, the video won’t be cutt accurately). These are the commands that I used.

  ffmpeg -ss 55 -i input.mp4 -t 00:03:06 -vcodec copy -acodec copy out.mp4 
  
  ffmpeg -ss 55 -i input.mp4 -t 00:03:06 out.avi

• Softwares for adding real time text to a video [on hold]

  30 juillet 2013, par user763410

  I am trying to add real time text (like weather information, stock quotes) to a video and broadcast it. My videos are going to be 2 hours long. I have been searching for decent softwares which can do the work for me. FFmpeg can do overlays only if text to be added is available fully before running ffmpeg code, so its not real time. I have not been able to able to get gstreamer to work on my hardware.

  So, My question is :
  Please suggest some softwares which can add (text)data to a video in real time.
  Please don't post comparisons as that will trigger moderator's anger ! I don't want to start a flame war. Purpose of the question is to make a list. Btw, this should work from command line rather than a GUI.

  (Ps : If the question is closed because of being "not cnonstructive, please answer it on my google docs page here.
  http://goo.gl/14dR2H)

  Thanks very much in advance.