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• c++ how to use lambdas to set ffmpeg's av_log_set_callback() to a member function ?

  13 février 2018, par Blake Senftner

  I’ve been researching how to use lambdas as callbacks, but I do not seem to be getting my syntax correct. I am working in C++ via Visual Studio 2013 Community, & I am pretty sure this type of lambda use is supported. I am working on a video library which is based on the FFMPEG libav libraries.

  The basic issue is the logging callback provided to library clients of FFMPEG’s libav. It has this calling structure :

  void logging_callback( void *ptr, int level, const char *fmt, va_list vargs  );

  This is fine for C or fine for C++ when the logging callback is a static member function, but I want to install a class member function, which needs the this parameter per the C++ nature of calling method functions.

  What I have so far looks like : (unrelated portions removed)

  class my_libav
  
  { 
  
     // logging messages callback for use by library client, any log messages generated 
  
     // by the library will be sent thru this to the lib client's logic:
  
     typedef void(*STREAM_LOGGING_CALLBACK_CB) (std::string msg, void* p_object);
  
  
  
     // my lib's logger has a void* param so clients can redirect to member functs easier
  
     void SetStreamLoggingCallback(STREAM_LOGGING_CALLBACK_CB p_stream_logger, void* p_object);
  
  
  
     // member function to be installed as FFMPEG's av_log_set_callback():
  
     void RedirectLoggingOutputs( void *ptr, int level, const char *fmt, va_list vargs );
  
  
  
     STREAM_LOGGING_CALLBACK_CB  mp_stream_logging_callback; // client's callback
  
     void*                       mp_stream_logging_object;   // client's data
  
  
  
  };
  
  
  
  void my_libav::SetStreamLoggingCallback(STREAM_LOGGING_CALLBACK_CB p_stream_logger, void* p_object)
  
  {
  
    mp_stream_logging_callback = p_stream_logger;
  
    mp_stream_logging_object = p_object;
  
  }
  
  
  
  void my_libav::RedirectLoggingOutputs(void *ptr, int level, const char *fmt, va_list vargs )
  
  {
  
    // logic that resolves the message and sends through client's callback
  
    // installed int mp_stream_logging_callback
  
  }
  
  
  
  my_libav::my_libav()
  
  {
  
    // this is the old static member function I deleted:
  
    // av_log_set_callback( &my_libav::logging_callback );
  
  
  
    // I tried using std::bind & placeholders, but this syntax is wrong too:
  
    // namespace ph = std::placeholders;
  
    // auto callback = std::bind( &my_libav::RedirectLoggingOutputs, this, ph::_1, ph::_2, ph::_3, ph::_4 );
  
    // 
  
    // av_log_set_callback( callback ); // <- 'callback' is not the correct type
  
  
  
    // this is my try at a lambda. I know that the *this* cannot be 
  
    // in the capture, but I don't know the right syntax to work around 
  
    // this issue:
  
    std::function 
  
          f2 = [this](void *ptr, int level, const char *fmt, va_list vargs ){
  
                          RedirectLoggingOutputs( ptr, level, fmt, vargs ); };
  
  
  
      av_log_set_callback( f2 ); // <- 'f2' is not the correct type, how to fix? 

  The syntax I am trying to fix is in the last 4 lines. What is the correct form ?

• Metal Gear Solid VP3 Easter Egg

  4 août 2011, par Multimedia Mike — Game Hacking

  Metal Gear Solid : The Twin Snakes for the Nintendo GameCube is very heavy on the cutscenes. Most of them are animated in real-time but there are a bunch of clips — normally of a more photo-realistic nature — that the developers needed to compress using a conventional video codec. What did they decide to use for this task ? On2 VP3 (forerunner of Theora) in a custom transport format. This is only the second game I have seen in the wild that uses pure On2 VP3 (first was a horse game). Reimar and I sorted out most of the details sometime ago. I sat down today and wrote a FFmpeg / Libav demuxer for the format, mostly to prove to myself that I still could.

  Things went pretty smoothly. We suspected that there was an integer field that indicated the frame rate, but 18 fps is a bit strange. I kept fixating on a header field that read 0x41F00000. Where have I seen that number before ? Oh, of course — it’s the number 30.0 expressed as an IEEE 32-bit float. The 4XM format pulled the same trick.

  Hexadecimal Easter Egg
  I know I finished the game years ago but I really can’t recall any of the clips present in the samples directory. The file mgs1-60.vp3 contains a computer screen granting the player access and illustrates this with a hexdump. It looks something like this :

  
  
  

  Funny, there are only 22 bytes on a line when there should be 32 according to the offsets. But, leave it to me to try to figure out what the file type is, regardless. I squinted and copied the first 22 bytes into a file :

   1F 8B 08 00   85 E2 17 38   00 03 EC 3A   0D 78 54 D5
   38 00 03 EC   3A 0D
  

  And the answer to the big question :

  $ file mgsfile
  mgsfile : gzip compressed data, from Unix, last modified : Wed Oct 27 22:43:33 1999
  

  A gzip’d file from 1999. I don’t know why I find this stuff so interesting, but I do. I guess it’s no more and less strange than writing playback systems like this.

• Enabling libmp3lame for FFMPEG on elastic beanstalk

  13 octobre 2017, par Adam Sith

  I am trying to enable libmp3lame with FFMPEG in elastic beanstalk (Amazon Redhat Linux machine).

  I am able to successfully install FFMPEG in /ffmpeg.config with the following script :

  # .ebextensions/ffmpeg.config
  
  
  
  packages:
  
    yum:
  
      autoconf: []
  
      automake: []
  
      cmake: []
  
      freetype-devel: []
  
      gcc: []
  
      gcc-c++: []
  
      git: []
  
      libtool: []
  
      make: []
  
      nasm: []
  
      pkgconfig: []
  
      zlib-devel: []
  
  sources:
  
    /usr/local/src: http://ffmpeg.org/releases/ffmpeg-3.2.tar.bz2
  
  commands:
  
    ffmpeg_install:
  
        cwd: /usr/local/src/ffmpeg-3.2
  
        command: sudo ./configure --prefix=/usr && make && make install

  I need to install libmp3lame however. I’ve tried to do this with an --enable-libmp3lame flag and the directions here. The modified script :

  packages:
  
  yum:
  
      autoconf: []
  
      automake: []
  
      cmake: []
  
      freetype-devel: []
  
      gcc: []
  
      gcc-c++: []
  
      git: []
  
      libtool: []
  
      make: []
  
      nasm: []
  
      pkgconfig: []
  
      zlib-devel: []
  
  sources:
  
    /usr/local/src: http://ffmpeg.org/releases/ffmpeg-3.2.tar.bz2
  
  commands:
  
    01-install_libmp3lame:
  
        cwd: /usr/local/src/
  
        command: curl -L -O http://downloads.sourceforge.net/project/lame/lame/3.99/lame-3.99.5.tar.gz && tar xzvf lame-3.99.5.tar.gz && cd lame-3.99.5 && ./configure --prefix="$HOME/ffmpeg_build" --bindir="$HOME/bin" --disable-shared --enable-nasm && make && make install
  
    02-ffmpeg_install:
  
        cwd: /usr/local/src/ffmpeg-3.2
  
        command: sudo ./configure --enable-libmp3lame && --prefix=/usr && make && make install

  This doesn’t work. Command 01-install_libmp3lame completes. Command 02-ffmpeg_install fails because :

  [2017-10-12T20:55:19.324Z] INFO  [24606] - [Application update app-8fe3-123456_7895@111/AppDeployStage0/EbExtensionPreBuild/Infra-EmbeddedPreBuild/prebuild_3_clover_platform/Command 02-ffmpeg_install] : Activity execution failed, because: ERROR: libmp3lame >= 3.98.3 not found

  I have tried installing libmp3lame in /ffmpeg-3.2 and got the same issue.