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• opencv installation ffmpeg error

  22 avril 2016, par tGeek

  I have been trying to install opencv for a long time in my fedora 20 32 bit system.I followed the instructions from http://docs.opencv.org/trunk/doc/py_tutorials/py_setup/py_setup_in_fedora/py_setup_in_fedora.html#install-opencv-python-in-fedora
  But after all cmake commands when I m trying to make it I am getting an error again and again.

      scanning dependencies of target opencv_createsamples
  
       [100%] Building CXX object    apps/haartraining/CMakeFiles/opencv_createsamples.dir/createsamples.cpp.o
  
      Linking CXX executable ../../bin/opencv_createsamples
  
     /lib/libavcodec.so.55: undefined reference to `vpx_codec_vp9_dx_algo'
  
      /lib/libavcodec.so.55: undefined reference to `vpx_codec_vp9_cx_algo'
  
      collect2: error: ld returned 1 exit status
  
      make[2]: *** [bin/opencv_createsamples] Error 1
  
     make[1]: *** [apps/haartraining/CMakeFiles/opencv_createsamples.dir/all] Error 2
  
      make: *** [all] Error 2

  I have followed so many articles about the ffmpeg versions and updating stuff but did understand a little. but when I tried

  $ sudo yum update ffmpeg

  I got the error as

  >Package(s) ffmpeg available, but not installed.
  
  No packages marked for update

  What should I do ?

• 24kHz audio file problem : unsupported bitrate 64000

  28 novembre 2018, par R. Vait

  I use alexa audio tags a lot. I know that now audio tags support 24kHz audio files so tried converting my audio files from 16kHz. I used the provided command in the docs to do so :

  ffmpeg -i  -ac 2 -codec:a libmp3lame -b:a 48k -ar 24000 

  But when I try to play this file, I get an invalid response error, saying : Error: The audio is of an unsupported bitrate 64000. By looking into file details I clearly see, that bitrate is 48kbps and sample rate is 24kHz. I don’t see any value where it would say 64 or anything close to it.

  If I encode my file back to 16kHz it plays fine again.

  It seems that there is a problem with this command, because if I encode my files using audacity, they work with 24kHz. I still would prefer to use ffmpeg, because I need to encode a lot of files.

  I am asking, not about file format, format is correct. I need files in 24kHz sample rate and that is what causes issues. I saw another question about similar problem and others having the discussion there about sample rates, but no one was able to encode file to be 24kHz using ffmpeg.

  Did anyone had any luck on encoding files to 24kHz using ffmpeg ?

• Finding duration/number of frames of WebM using FFMPEG/libavformat

  12 septembre 2015, par gcgrant

  I’m trying to retrieve the duration and number of frames from a WebM file using FFMPEG and libavformat. I’m adapting some code that was initially written to work with MP4s, and when passing an MP4 it seems to work fine.

  I have FFMPEG 2.1 compiled with the vp8 parser and decoder and the matroska demuxer.

  I have an AVStream pointer to the video stream, *video_st. The code looks for the number of frames in video_st->nb_frames, and for the duration in video_st->duration. These both appear to be 0.

  The API docs for for video_st->duration (https://ffmpeg.org/doxygen/trunk/structAVStream.html#a4e04af7a5a4d8298649850df798dd0b) say that "If a source file does not specify a duration, but does specify a bitrate, this value will be estimated from bitrate and file size." I can confirm that that this file does specify a duration and a bitrate (as I tested it on my desktop with ffmpeg -i). So it seems odd that this is returning 0. Perhaps my FFMPEG build configuration is incorrect ?

  As for nb_frames, the docs say "number of frames in this stream if known or 0". Is there anything I can do if this returns 0 ? Again, ffmpeg -i does return an estimate for fps ; that would be good enough, as I could estimate the number of frames with that and the duration.

  Thanks !