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• How do I pre-allocate the memory for libavcodec to write decoded frame data ?

  18 décembre 2018, par codemonkey

  I am trying to decode a video with libav by following the demo code : here

  I need to be able to control where the frame data in pFrame->data[0] is stored. I have tried setting pFrame->data to my own buffer as follows :

  // Determine required buffer size and allocate buffer 
  
  int numBytes = av_image_get_buffer_size(pixFmt, width, height, 1); 
  
  (uint8_t*) dataBuff = (uint8_t*) malloc (numBytes * sizeof(uint8_t)); 
  
  
  
  // Assign buffer to image planes in pFrame
  
  av_image_fill_arrays(frame->data, frame->linesize, dataBuff, pixFmt, width,
  
  height, 1);

  While this does set pFrame->data to be dataBuff (if I print their addresses, they are the same), this call ret = avcodec_receive_frame(pCodecContext, pFrame) to receive the decoded data always writes the data to a different address. It seems to manage its own memory somewhere in the underlying API and ignores the dataBuff that I assigned to pFrame right before.

  So I’m stuck—how can I tell libav to write decoded frame data to memory that I pre-allocate ? I’ve seen people ask similar questions online and in the libav forum but haven’t been able to find an answer.

  Many thanks 

• How To Extract The Duration From A Webm Using FFprobe

  8 décembre 2018, par Sixtoo

  I have the following code which stores the duration of a video file in a variable called duration :

  for /f %%i in ('ffprobe -select_streams v:0 -show_entries stream^=duration -of default^=noprint_wrappers^=1:nokey^=1 input.avi') do set duration=%%i

  However, when I try to get the duration of a .webm file I get N/A. I used the answer here How to determine webm duration using ffprobe which helped me to be able to see the duration of a webm when using ffprobe. But for some reason I can see the duration in the output of ffprobe but I cannot manage to store it in a variable.

  Please help me with this. Thank you

  ---

  Here is the command and output :

  Command :

  for /f %%i in ('ffprobe -select_streams v:0 -show_entries stream^=duration -of default^=noprint_wrappers^=1:nokey^=1 webm_copy.webm') do set duration=%%i
  
  
  
  echo %duration%

  Output :

  D:\SOFTWARE\ffmpeg\bin\test\go>for /F %i in ('ffprobe -select_streams v:0 -show_entries stream=duration -of default=noprint_wrappers=1:nokey=1 webm_copy.webm') do set duration=%i
  
  ffprobe version N-80066-g566be4f Copyright (c) 2007-2016 the FFmpeg developers
  
    built with gcc 5.3.0 (GCC)
  
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmfx --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enable-zlib
  
    libavutil      55. 24.100 / 55. 24.100
  
    libavcodec     57. 43.100 / 57. 43.100
  
    libavformat    57. 37.100 / 57. 37.100
  
    libavdevice    57.  0.101 / 57.  0.101
  
    libavfilter     6. 46.100 /  6. 46.100
  
    libswscale      4.  1.100 /  4.  1.100
  
    libswresample   2.  0.101 /  2.  0.101
  
    libpostproc    54.  0.100 / 54.  0.100
  
  Input #0, matroska,webm, from 'webm_copy.webm':
  
    Metadata:
  
      encoder         : Lavf57.37.100
  
    Duration: 00:01:31.44, start: 0.000000, bitrate: 278 kb/s
  
      Stream #0:0: Video: vp8, yuv420p, 480x360, SAR 1:1 DAR 4:3, 29.97 fps, 29.97 tbr, 1k tbn (default)
  
      Stream #0:1: Audio: vorbis, 44100 Hz, stereo, fltp (default)
  
  
  
  D:\SOFTWARE\ffmpeg\bin\test\go>set duration=N/A
  
  
  
  D:\SOFTWARE\ffmpeg\bin\test\go>echo N/A
  
  N/A

• FFmpeg dash manifest '-window_size'

  14 novembre 2018, par edwinbradford

  In the FFmpeg DASH documentation I don’t understand the purpose of -window_size which is explained as :

  Set the maximum number of segments kept in the manifest.

  If my video is 30 seconds long, the GOP size is 4 seconds and the segment length is 4 seconds, what is the meaning and purpose of a parameter to control the maximum number of segments kept in the manifest, when does this parameter need to be used and how do you determine valid values ?

  I’m guessing that the stream is being loaded into memory and the number of segments in the manifest controls how much is kept in memory at one time but it’s just a wild guess and I can’t find any further explanation.

  I am not live streaming in case it’s relevant.