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• Cleaning up after av_frame_get_buffer

  30 juillet 2022, par Jason C

  There are two aspects of my question. I'm using libav, ffmpeg, 3.1.

  



  ---

  



  First, how do you appropriately dispose of a frame whose buffer has been allocated with av_frame_get_buffer ? E.g. :

  



  AVFrame *frame = av_frame_alloc();
frame->width = ...;
frame->height = ...;
frame->format = ...;
av_frame_get_buffer(frame, ...);


  



  Do any buffers have to be freed manually, beyond the call to av_frame_free(frame) ? The documentation doesn't mention anything special, but in my experience the ffmpeg documentation often leaves out important details, or at least hides them in places far away from the obvious spots. I took a look at the code for av_frame_free and av_frame_unref but it branched out quite a bit and I couldn't quite determine if it covered everything.

  



  ---

  



  Second, if something beyond av_frame_free needs to be done, then is there any catch-all way to clean up a frame if you don't know how its data has been allocated ? For example, assuming someBuffer is already allocated with the appropriate size :

  



  AVFrame *frame2 = av_frame_alloc();
frame2->width = ...;
frame2->height = ...;
frame2->format = ...;
av_image_fill_arrays(frame2->data, frame2->linesize, someBuffer, 
                     frame2->format, frame2->width, frame2->height, 1);


  



  Is there a way to free both frame and frame2 in the above examples using the exact same code ? That is frame and its data should be freed, and frame2 should be freed, but not someBuffer, since libav did not allocate it.

  


• Play H.264 camera streams using OpenCV

  26 octobre 2018, par peter bence

  How to play H.264 camera streams using OpenCV ? I was searching for a while but i didn’t find an answer. I think OpenCV can encode and decode h.264 videos since it uses ffmpeg and it is the documentation of the class VideoWriter ensures that as shown in this example :

  #include <iostream> // for standard I/O
  
  #include <string>   // for strings
  
  
  
  #include <opencv2></opencv2>core/core.hpp>        // Basic OpenCV structures (cv::Mat)
  
  #include <opencv2></opencv2>highgui/highgui.hpp>  // Video write
  
  
  
  using namespace std;
  
  using namespace cv;
  
  
  
  int main()
  
  {
  
      VideoWriter outputVideo; // For writing the video
  
  
  
      int width = ...; // Declare width here
  
      int height = ...; // Declare height here
  
      Size S = Size(width, height); // Declare Size structure
  
  
  
      // Open up the video for writing
  
      const string filename = ...; // Declare name of file here
  
  
  
      // Declare FourCC code
  
      int fourcc = CV_FOURCC('H','2','6','4');
  
  
  
      // Declare FPS here
  
      int fps = ...;
  
      outputVideo.open(filename, fourcc, fps, S);
  
  
  
      // Put your processing code here
  
      // ...
  
  
  
      // Logic to write frames here... see below for more details
  
      // ...
  
  
  
      return 0;
  
  }
  
  </string></iostream>

  So can OpenCV encode-decode h.264 stream as well ? if yes, please let me know how. thanks !!

• MPEG-4 and alpha transparency - the ongoing saga

  16 janvier 2016, par OrangeDog

  I’d like to reach some sort of definitive answer for the following questions :

  1. Is alpha transparency supported in MPEG-4-based codecs ?
  2. Is there any way to be reasonably certain that there is no alpha-channel from the output of ffprobe ?

  Some links I’ve found on the subject :

  • http://stackoverflow.com/a/5057631/476716

  Also, transparent alpha layers, an optional part of the H.264 specification, are now supported in H.264-based QuickTime movies.

  • http://software.intel.com/en-us/articles/alpha-channel-support-in-the-h264-decoder-in-ipp/

  The support for alpha channel in the H.264 decoder is now available in Intel® Integrated Performance Primitives 6.0 version.

  • http://forums.creativecow.net/readpost/335/30475

  The MPEG4 Simple Studio Profile supports auxiliary channels, which could be used for alpha or depth — and presumably even displacement, motion vectors, normals, etc.

  However, I’ve never seen a concrete example, or indeed any details on how you use these announced features.