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• How can I learn the effective quality of the h264_nvenc encoder ?

  24 septembre 2020, par pfdint2

  I seek to ascertain the end-product quality of various cq ("constant quality") values for the h264_nvenc video encoder.

  


  To use the h264_nvenc encoder through ffmpeg and with a target quality, I follow the instructions in Nvidia's documentation, last paragraph of the linked anchor.

  


  I specify a Variable Bit Rate rate control mode (rc=vbr_hq) and a target quality (cq=). This should be enough to test various values of cq. The story should end there.

  


  HOWEVER, that only outputs the same perfect quality video of extremely large size for any cq value. (Using cq=1, cq=20, and cq=51 all output a file with the same hash.)

  


  Per documentation, I can also specify a maxBitRate (maxrate=). If I don't specify a maxrate, the documentation indicates :

  


  


  If maxBitRate is not specified, the encoder will use as many bits as needed to achieve the target quality.

  


  


  But that's not the behavior I'm observing. I'm seeing the encoder use as many bits as possible, not as many bits as needed. The resultant video has an overall bitrate of 100Mb/s. Double the source video.

  


  If I do set a maxrate, it is observed by the encoder. But if I have to set the bitrate correctly for every cq value, what is the point of the cq value ? I would just set cq=1 and test various bitrates, which would then only be applicable to a specific video.

  


  Is there some warning or output that notes that the bitrate was insufficient to reach the target quality that I'm not seeing ? That would at least allow me to brute force this.

  


  Or am I misunderstanding the relationship between these settings ?

  


• ffmpeg build on mac with videotoolbox enabled becomes unportable

  22 janvier 2020, par Alexander Novikov

  If i configure ffmpeg this way :

  ./configure --disable-everything --enable-static --disable-shared \
  
  --enable-gpl --enable-nonfree --enable-encoder=h264_videotoolbox,aac \ 
  
  --enable-muxer=mp4 --enable-protocol=file --enable-libfdk-aac 
  
  --enable-videotoolbox --disable-autodetect

  it works for my purposes (allows to encode h264 video with aac audio on Mac’s videotoolbox - an Apple QSV toolkit), but if i send it to any other computer except the one it was built on, it fails with something like this :

  dyld: Symbol not found: _kCVImageBufferTransferFunction_ITU_R_2100_HLG
  
    Referenced from: /Users/admin/Downloads/./ffmpeg
  
    Expected in: /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
  
   in /Users/admin/Downloads/./ffmpeg
  
  Abort trap: 6

  if i rebuild it this way :

  ./configure --disable-everything --enable-static --disable-shared
  
  --enable-gpl --enable-nonfree --enable-encoder=aac 
  
  --enable-muxer=mp4 --enable-protocol=file --enable-libfdk-aac 
  
  --disable-autodetect

  so with everything else but videotoolbox removed, it runs successfully on any other computer, so apparently ffmpeg needs to carry along something it doesn’t, for videotoolbox to work...

  i am actually building a C++ app with ffmpeg’s static libraries, but explaining what i do there will be a very long story and error message produced is exactly the same if i run it on different machines, so i better illustrate it on example of ffmpeg console utility itself.

  what are the configure switches i need to do to make the ffmpeg build portable please ?

• avformat/asfenc : write group_mutual_exclusion_objects for audio on multiple languages

  5 février 2016, par Marton Balint

  avformat/asfenc : write group_mutual_exclusion_objects for audio on multiple languages
  Improves streaming compatibility with Windows Media Services. Also tested for
  
  compatilbility in Windows Media Player, Windows Media ASF Viewer and VLC.
  This version of the patch only writes exclusion among audio streams, therefore
  
  choosing a subtitle language should be possible independently of audio language.
  Signed-off-by : Marton Balint <cus@passwd.hu>
  

  • [DH] libavformat/asf.c
  • [DH] libavformat/asf.h
  • [DH] libavformat/asfenc.c