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• created .webm from .mp4 with ffmpeg is async

  25 février 2015, par chuckice

  since a couple of days i’m trying to figure out a solution for syncronization issue, using ffmpeg as a transcoding application.

  i create a 1-fps .webm-preview of my .mp4-Video with the following ffmpeg command line :
  ffmpeg -i input.mp4 -vcodec libvpx -b:v 500k -ss 0 -qmin 10 -qmax 42 -maxrate 500k -bufsize 1000k -vf scale=768:576 -an -r 1 -g 0 -y output.webm

  after my .webm-format is transcoded, i realize that f.e. position 00:00:10.000 in both videos does not have the same image.

  after reading parts of the ffmpeg documentation, i found the parameter -vsync

  -vsync parameter

  0, passthrough : Each frame is passed with its timestamp from the demuxer to the muxer.

  1, cfr : Frames will be duplicated and dropped to achieve exactly the requested constant frame rate.

  2, vfr : Frames are passed through with their timestamp or dropped so as to prevent 2 frames from having the same timestamp.

  drop : As passthrough but destroys all timestamps, making the muxer generate fresh timestamps based on frame-rate.

  -1, auto : Chooses between 1 and 2 depending on muxer capabilities. This is the default method.

  As i’m kind of new to ffmpeg, i’d like to ask for help to spare my troubles.

  Anybody ?

  Update : Digging deeper i found out that my mp4-Source had an offset in the beginning.

  So i should solve this problem first.

• created .webm from .mp4 with ffmpeg is async due to start_time>0 in video stream

  25 février 2015, par chuckice

  since a couple of days i’m trying to figure out a solution for syncronization issue, using ffmpeg as a transcoding application.

  i create a 1-fps .webm-preview of my .mp4-Video with the following ffmpeg command line :

  ffmpeg -i input.mp4 -vcodec libvpx -b:v 500k -ss 0 -qmin 10 -qmax 42 -maxrate 500k -bufsize 1000k -vf scale=768:576 -an -r 1 -g 0 -y output.webm

  after my .webm-format is transcoded, i realize that f.e. position 00:00:10.000 in both videos does not have the same image.

  after reading parts of the ffmpeg documentation, i found the parameter -vsync

  -vsync parameter

  0, passthrough : Each frame is passed with its timestamp from the demuxer to the muxer.

  1, cfr : Frames will be duplicated and dropped to achieve exactly the requested constant frame rate.

  2, vfr : Frames are passed through with their timestamp or dropped so as to prevent 2 frames from having the same timestamp.

  drop : As passthrough but destroys all timestamps, making the muxer generate fresh timestamps based on frame-rate.

  -1, auto : Chooses between 1 and 2 depending on muxer capabilities. This is the default method.

  As i’m kind of new to ffmpeg, i’d like to ask for help to spare my troubles.

  Anybody ?

  Update : Digging deeper i found out that my mp4-Source had an offset in the beginning.

  So i should solve this problem first.

  Documenting my work, i found out, that the video-Stream in my .mp4-Format has the following offset, defined as start_time.

  ffprobe -v quiet -show_streams -print_format json -i archiv.mp4 
  
  {
  
      "streams": [
  
          {
  
              "index": 0,
  
              "codec_name": "h264",
  
              "codec_long_name": "H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10",
  
              "profile": "High",
  
              "codec_type": "video",
  
              "codec_time_base": "1/100",
  
              "codec_tag_string": "avc1",
  
              [..]
  
              "start_time": "1.080000",
  
              [..]
  
      ]

  Is there a way, resetting this start_time of the stream by ffmpeg ? Right now, im using the following command-line :

  ffmpeg -i input.ts -vf yadif -c:v libx264 -crf 24 -c:a libmp3lame -y -strict -2 -b:v 4000k -copyts -start_at_zero archiv.mp4

• Install PyAV on Windows

  16 juin 2015, par m.barz

  I got a problem building and thus setting up PyAV 32-bit on Windows 8 (x64). I tried the workflow for Windows as indicated in their documentation, but did not succeed :

  • I cross-compiled ffmpeg on Ubuntu 14.04 with the aid of a script.
  • I copied the *shared-install folder to Windows and set the PKG_CONFIG_PATH environment variable to the corresponding libs/pkgconfig path.
  • I copied the ffmpeg DLLs and its dependencies from the MinGW bin-folder to PyAV’s av folder as mentioned in PyAV’s docs.
  • I ran make build-mingw32 (where build-mingw32 is a recipe triggering python distutils like that : CFLAGS=$(CFLAGS) LDFLAGS=$(LDFLAGS) python setup.py build_ext --inplace -c mingw32)

  Finally I run into trouble with that last command. When the build process tries to create codec.pyd there is a bunch of undefined reference to _head_C_build27_cpython_PCBuild_libpython27_a errors leading gcc to fail. See cmd output below :

  

  I tried this on a fresh virtual machine running Windows 8.1 Pro having installed the MinGW toolchain including msys (I used the installer from mingw.org). I also tried the MinGW installer provided here.
  I hope someone already experienced similar issues using gcc and can help me with that !

  Thanks in advance and best regards,
  Michael