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• avformat/matroskadec : Don't output uninitialized data for RealAudio 28.8

  22 avril 2020, par Andreas Rheinhardt

  avformat/matroskadec : Don't output uninitialized data for RealAudio 28.8
  The Matroska demuxer splits every sequence of h Matroska Blocks into
  
  h * w / cfs packets of size cfs ; here h (sub_packet_h), w (frame_size)
  
  and cfs (coded_framesize) are parameters from the track's CodecPrivate.
  It does this by splitting the Block's data in h/2 pieces of size cfs each
  
  and putting them into a buffer at offset m * 2 * w + n * cfs where
  
  m (range 0..(h/2 - 1)) indicates the index of the current piece in the
  
  current Block and n (range 0..(h - 1)) is the index of the current Block
  
  in the current sequence of Blocks. The data in this buffer is then used
  
  for the output packets.
  The problem is that there is currently no check to actually guarantee
  
  that no uninitialized data will be output. One instance where this is
  
  trivially so is if h == 1 ; another is if cfs * h is so small that the
  
  input pieces do not cover everything that is output. In order to
  
  preclude this, rmdec.c checks for h * cfs == 2 * w and h >= 2. The
  
  former requirement certainly makes much sense, as it means that for
  
  every given m the input pieces (corresponding to the h different values
  
  of n) form a nonoverlapping partition of the two adjacent frames of size w
  
  corresponding to m. But precluding h == 1 is not enough, other odd
  
  values can cause problems, too. That is because the assumption behind
  
  the code is that h frames of size w contain data to be output, although
  
  the real number is h/2 * 2. E.g. for h = 3, cfs = 2 and w = 3 the
  
  current code would output four (== h * w / cfs) packets. although only
  
  data for three (== h/2 * h) packets has been read.
  (Notice that if h * cfs == 2 * w, h being even is equivalent to
  
  cfs dividing w ; the latter condition also seems very reasonable :
  
  It means that the subframes are a partition of the frames.)
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavformat/matroskadec.c

• embed video stream with custom meta data

  15 mai 2022, par Sergey Kolesnik

  I have an optical system that provides a UDP video stream.

  


  From device specification FAQ :

  


  


  Both single metadata (KLV) stream and compressed video (H.264) with metadata (KLV) are available on Ethernet link. Compressed video and metadata are coupled in the same stream compliant with STANAG 4609 standard. Each encoded video stream is encapsulated with the associated metadata within an MPEG-TS single program stream over Ethernet UDP/IP/ The video and metadata are synchronized through the use of timestamps.

  


  


  Also there are other devices that provide data about the state of an aircraft (velocity, coords, etc). This data should be displayed on a client GUI display alongside with video. Of course it has to be synchronized with the current video frame.

  


  One of the approaches I though of is to embed this data into the video stream. But I am not sure if it is possible or should I use another (than UDP) protocol for this purpose.

  


  Is it possible/reasonable to use such approach ? Is ffmpeg library suitable in this case ?
If not, what are the other ways to synchronize data with a video frame.
Latency is crucial. Although bandwidth is limited to 2-5 Mbps.

  


  ---

  


  It seems to be possible using ffmpeg : AVPacket can be provided with additional data using function av_packet_add_side_data which takes a preallocated buffer, size and a type AVPacketSideDataType.
However, I am not sure for now, which enum value of AVPacketSideDataType can be used for custom user-provided binary data.

  


  Something similar that might be used for my needs :

  


  How do I encode KLV packets to an H.264 video using libav*

  


• How to work with data from streaming services in my Java application ?

  24 novembre 2020, par gabriel garcia

  I'm currently trying to develop an "streaming client" as a way to organize multiple stream services (twitch, yt, mitele...) in a single desktop application written in Java.

  


  It basically relies on streamlink (which relies in ffmpeg) thanks to all it's features so my project could be defined as a frontend for streamlink.

  


  Straight to the point, one of the features I'd like to add it is the option to programatically record streams in the background and showing this video stream to the user when it's requested. Since there's also the possibility that the user wants to watch the stream without recording it, I'm forced to work with all that byte-like data sent from those streaming sources.

  


  So, the problem is basically that I do not know much about video coding/decoding/muxing/demuxing nor video theory like container structure, video formats and such.

  


  But the idea is to work with all the data sent from the stream source (let's say twitch, for example), read this bytes (I'm not sure what kind of information is sent to the client nor format) from the java.lang.Process's stdout and then present it to the client.

  


  Here's another problem : I don't know how to play video streams in JavaFX and I don't think it's even supported right now. So I would have to extract each frame and sound associated from the stdout and show them to the user each time a new frame is received (oups, another problem since I don't know when does each frame starts/ends since I'm reading each stdout's line).

  


  As a summary :

  


  

  • How can I know when does each frame starts/stops ?

  


  • How can I extract the image and sound from each frame ?

  


  


  I hope I'm not asking too much and that you could shed some light upon my darkness.