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• lavfi : add a new filtergraph parsing API

  16 janvier 2023, par Anton Khirnov

  lavfi : add a new filtergraph parsing API
  Callers currently have two ways of adding filters to a graph - they can
  
  either
  
   create, initialize, and link them manually
  
   use one of the avfilter_graph_parse*() functions, which take a
  
    (typically end-user-written) string, split it into individual filter
  
    definitions+options, then create filters, apply options, initialize
  
    filters, and finally link them - all based on information from this
  
    string.
  A major problem with the second approach is that it performs many
  
  actions as a single atomic unit, leaving the caller no space to
  
  intervene in between. Such intervention would be useful e.g. to
  
   modify filter options ;
  
   supply hardware device contexts ;
  
  both of which typically must be done before the filter is initialized.
  Callers who need such intervention are then forced to invent their own
  
  filtergraph parsing, which is clearly suboptimal.
  This commit aims to address this problem by adding a new modular
  
  filtergraph parsing API. It adds a new  avfilter_graph_segment_parse()
  
  function to parse a string filtergraph description into an intermediate
  
  tree-like representation (AVFilterGraphSegment and its children).
  This intermediate form may then be applied step by step using further
  
  new avfilter_graph_segment*() functions, with user intervention possible
  
  between each step.
  

  • [DH] doc/APIchanges
  • [DH] libavfilter/avfilter.h
  • [DH] libavfilter/graphparser.c
  • [DH] libavfilter/version.h

• lavf : Add an MPEG-DASH ISOFF segmenting muxer

  6 octobre 2014, par Martin Storsjö

  lavf : Add an MPEG-DASH ISOFF segmenting muxer
  This is mostly to serve as a reference example on how to segment
  
  the output from the mp4 muxer, capable of writing the segment
  
  list in four different ways :
  
   SegmentTemplate with SegmentTimeline
  
   SegmentTemplate with implicit segments
  
   SegmentList with individual files
  
   SegmentList with one single file per track, and byte ranges
  The muxer is able to serve live content (with optional windowing)
  
  or create a static segmented MPD.
  In advanced cases, users will probably want to do the segmenting
  
  in their own application code.
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DBH] Changelog
  • [DBH] configure
  • [DBH] libavformat/Makefile
  • [DBH] libavformat/allformats.c
  • [DBH] libavformat/dashenc.c
  • [DBH] libavformat/version.h

• MPEG-2 AAC frame-by-frame audio decoding

  3 mai 2018, par Jonathan May

  I have individual audio frames encoded in MPEG-2 AAC. Each frame consists of 1024 16 bit PCM samples.

  I notice that each AAC frame is a different size. I assume this is a result of the MPEG-2 AAC compression algorithm and perfectly normal.

  I need a way to decode a single frame and get back the original 1024 PCM samples (with error from lossy compression, that’s fine).

  I couldn’t find information about the MPEG-2 AAC algorithm ANYWHERE online. It’s kinda nuts.

  I’ve been trying a crude work around using a library called pydub, which contains a few methods which use FFMPEG’s AAC decoder. Trying to load the audio frame as an AudioSegment using AAC encoding :

  audioData = BytesIO(frame)
  
  sound = AudioSegment.from_file(audioData, format="aac")

  gives the following error :

  [aac @ 000002d444c1aa00] Estimating duration from bitrate, this may be inaccurate\r\n
  
  Input #0, aac, from 'C:\\Users\\jmk_m\\AppData\\Local\\Temp\\tmpjl3x0xao':\r\n
  
    Duration: 00:00:00.19, bitrate: 23 kb/s\r\n
  
  Stream #0:0: Audio: aac (LC), 22050 Hz, mono, fltp, 23 kb/s\r\nStream mapping:\r\n
  
  Stream #0:0 -> #0:0 (aac (native) -> pcm_s16le (native))\r\n
  
  Press [q] to stop, [?] for help\r\n
  
  Output #0, wav, to 'C:\\Users\\jmk_m\\AppData\\Local\\Temp\\tmpxmp942e4':\r\n
  
  Metadata:\r\n
  
  ISFT            : Lavf58.10.100\r\n
  
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 22050 Hz, mono, s16, 352 kb/s\r\n
  
  Metadata:\r\n      encoder         : Lavc58.13.100 pcm_s16le\r\n
  
  [aac @ 000002d444cc7480] Reserved bit set.\r\n
  
  [aac @ 000002d444cc7480] Prediction is not allowed in AAC-LC.\r\n
  
  Error while decoding stream #0:0: Invalid data found when processing input\r\n
  
  [aac @ 000002d444cc7480] Reserved bit set.\r\n
  
  [aac @ 000002d444cc7480] Prediction is not allowed in AAC-LC.\r\n
  
  Error while decoding stream #0:0: Invalid data found when processing input\r\n
  
  [aac @ 000002d444cc7480] Prediction is not allowed in AAC-LC.\r\n
  
  Error while decoding stream #0:0: Invalid data found when processing input\r\n
  
  size=       2kB time=00:00:00.04 bitrate= 366.2kbits/s speed=5.45x    \r\n
  
  video:0kB audio:2kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 3.808594%\r\n
  
  Conversion failed!\r\n"

  If anyone has any insights as to what may be causing the error, or any alternative approaches, that’d be greatly appreciated !