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• Motion Vectors and DC coefficients from MPEG4 stream

  9 octobre 2013, par Lampis

  I need to extract information about motion vectors and DC coefficients from an MPEG4 video. I've searched relevant sites and topics and I found that a good solution is to work with the code of ffmpeg codec. Especially the ff_print_debug_info function in libavcodec/mpegvideo.c calculates relevant information.

  However, I'm new on the C/C++ field and if there is any example code that describes or explains how to extract MVs and DC coefficients that would be very helpful.

  In the more recent version I use (FFmpeg 0.10.2 from http://ffmpeg.org/download.html) there is another file mpeg4videodec.c. Is there any chance to retrieve the needed information by this code ?

• Using Xuggler to detect Webcam with Java

  18 octobre 2012, par Oneiros

  I'm trying to use Xuggler library to handle webcam video stream in Java.

  My project contains these files :
  

  • Xuggler Jar (xuggle-xuggler-5.4.jar)
  • SLF4J Jars (required by Xuggler and downloadable here) :
    • jcl-over-slf4j-1.6.4.jar
    • jul-to-slf4j-1.6.4.jar
    • log4j-over-slf4j-1.6.4.jar
    • slf4j-api-1.6.4.jar
    • slf4j-ext-1.6.4.jar
    • slf4j-migrator-1.6.4.jar
    • slf4j-simple-1.6.4.jar
  • DisplayWebcamVideo.java as main class

  I run the main method using "vfwcap" and "0" as arguments.
  Application starts correctly, i can see myself from the webcam but it's just the first frame : the stream freezes and i see this output :

  5022 [Thread-3] ERROR org.ffmpeg - [vfwcap @ 000000000039A320] real-time buffer 75% full! frame dropped!

  5622 [Thread-3] ERROR org.ffmpeg - [vfwcap @ 000000000039A320] real-time buffer 85% full! frame dropped!

  6522 [Thread-3] ERROR org.ffmpeg - [vfwcap @ 000000000039A320] real-time buffer 95% full! frame dropped!

  6822 [Thread-3] ERROR org.ffmpeg - [vfwcap @ 000000000039A320] real-time buffer 101% full! frame dropped!

  6822 [Thread-3] ERROR org.ffmpeg - [vfwcap @ 000000000039A320] real-time buffer 101% full! frame dropped!

  What's the problem here ? :(

• MP4 video slightly asynchronous - but only on initial play

  3 mai 2012, par TheSHEEEP

  Here is our procedure :

  1. We create live MP4 videos using ffmpeg (H264/AAC), both as API and as command line tool.*
  2. qt-faststart is used to move the metadata to the beginning of the file to enable fast playback in our Flash client.
  3. Video is renamed to *.m4v.
  4. User uses our Flash client to download the video and starts playing immediatly due to the metadata being at the beginning.

  Edit :
  *First, a MPG video is created with the ffmpeg API, this is then copied together with an intro and an outro video (you can simply chain MPG videos). The resulting MPG is then converted to MP4 using the following command :

  ffmpeg -i inputvideo.mpg -sameq -b 3000000 -vcodec libx264 -acodec libvo_aacenc -ac 2 -g 60 -y outputvideo.mp4

  Basically, this does work.

  The problem :

  We noticed that during the initial playback (the user hasn't clicked anything yet, video still downloading or not does not make a difference), the audio is playing asychnronous, noticeable if you look out for it.

  But as soon as you click around in the video (one time is enough, no matter where you click), the audio becomes more synchronous (still not perfect !) for the rest of the video.

  This behaviour is the same, no matter if you view it via our Flash player or Windows Media Player. Is there anything we can do from our side (in ffmpeg or qt-faststart) to make it synchronous from the start ?