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• How can I detect ffmpeg vs libav in CMake ?

  19 décembre 2015, par Scott Lamb

  My project uses libavformat to connect to rtsp:// URLs. It’s important that it set a socket timeout and reconnect on error. Unfortunately, the stimeout open option for this only exists in ffmpeg (and in particular, its libavformat versions >= 55.1.100), not the competing project libav (any version). And some systems I’d like to support (such as Raspbian Jessie) are still bundled with libav.

  So, I think my best option is to detect whether I have a suitable version using cmake, and install ffmpeg in-tree if not. I think I should be able to do this via something like :

  pkg_check_modules(FFMPEG libavutil libavcodec libavformat)
  
  
  
  if(not FFMPEG_FOUND or FFMPEG_VERSION VERSION_LESS 55.1.101)
  
    ExternalProject_Add(
  
      FfmpegProject
  
      URL "http://ffmpeg.org/releases/ffmpeg-2.8.3.tar.xz"
  
      URL_HASH "SHA1=a6f39efe1bea9a9b271c903d3c1dcb940a510c87"
  
      INSTALL_COMMAND "")
  
    ...set up flags and such to use this in-tree version...
  
  endif()

  except that I don’t know how to detect libav vs ffmpeg. I don’t see anything in the pkgconfig stuff or libavformat/version.h to distinguish them. The version numbers they use seem to overlap. It’s not obvious to me at all how to tell the difference programmatically, much less do so with a not-weird cmake rule. Any ideas ?

• Cannot get first frames using avformat_seek_file

  14 octobre 2015, par JonesV

  I want to seek for an arbitrary frame in a video using libav. More precisely, using the function avformat_seek_file, which apparently uses av_seek_frame internally.

  I want to make a backward search (i.e. to get the closest possible frame before the one I seek), so that I can then go forward until I find precisely the one I want. For this, I use the function as follows :

  avformat_seek_file(..., ...,
  
                     std::numeric_limits::min(),
  
                     target_pts,
  
                     target_pts,
  
                     ...);

  Which means that I don’t have any tolerance about finding a frame that comes after my target_pts, but I am happy with any frame coming before.

  I am using the Big Buck Bunny videos for testing. Using the 480p H.264 video, I can seek any pts without problems. But using the 480p OGG video, I can’t. Actually, I can seek for any frame after pts = 73, but not before. Seeking for pts = 0 sets the video to pts = 73.

  One might think that the stream actually begins at pts = 73, but this is not what <stream>.start_time</stream> returns. Moreover, if I only load the video and read the frames in order, I can get the first 73 frames without any problem. The issue is that I can never come back to one of those frames by using avformat_seek_file.

  Last point : if I use the flag AVSEEK_FLAG_ANY, then it works. But that might result in me decoding only a part of the frame I want, which is not a solution for me.

  Can anybody explain this weird behavior ?

• Cannot get first frames using av_seek_file

  31 janvier 2014, par JonesV

  I want to seek for an arbitrary frame in a video using libav. More precisely, using the function avformat_seek_file, which apparently uses av_seek_frame internally.

  I want to make a backward search (i.e. to get the closest possible frame before the one I seek), so that I can then go forward until I find precisely the one I want. For this, I use the function as follows :

  avformat_seek_file(..., ...,
  
                     std::numeric_limits::min(),
  
                     target_pts,
  
                     target_pts,
  
                     ...);

  Which means that I don't have any tolerance about finding a frame that comes after my target_pts, but I am happy with any frame coming before.

  I am using the Big Buck Bunny videos for testing. Using the 480p H.264 video, I can seek any pts without problems. But using the 480p OGG video, I can't. Actually, I can seek for any frame after pts = 73, but not before. Seeking for pts = 0 sets the video to pts = 73.

  One might think that the stream actually begins at pts = 73, but this is not what <stream>.start_time</stream> returns. Moreover, if I only load the video and read the frames in order, I can get the first 73 frames without any problem. The issue is that I can never come back to one of those frames by using avformat_seek_file.

  Last point : if I use the flag AVSEEK_FLAG_ANY, then it works. But that might result in me decoding only a part of the frame I want, which is not a solution for me.

  Can anybody explain this weird behavior ?