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• Multiple frames lost if I use av_read_frame in FFmpeg

  12 juin 2015, par Krishna

  I have an HEVC sequence with 3500 frames and I am writing a decoder for reading it (read frame by frame and dump to yuv). In my main(), I have a for loop that calls a decoder() 3500 times (I am assuming at this stage that the main() knows how many frames there are).

  So, for every call to decoder(), I need a complete frame to be returned. This is what the decoder() looks like..

  bool decode(pFormatCtx, pCodecCtx)
  
  {
  
      int gotaFrame=0;
  
  
  
      while (gotaFrame==0) {
  
  
  
          printf("1\t");
  
  
  
          if ( !av_read_frame(pFormatCtx, &packet) ) { 
  
              if(packet.stream_index==videoStreamIndex) {
  
  
  
                  // try decoding
  
                  avcodec_decode_video2(pCodecCtx, pFrame, &gotaFrame, &packet);
  
  
  
                  if (gotaFrame) {  // decode success.
  
  
  
                      printf("2\t");
  
  
  
                      // dump to yuv ... not shown here. 
  
  
  
                      // cleanup
  
                      av_frame_unref(pFrame);
  
                      av_frame_free(&pFrame);
  
                      av_free_packet(&packet);
  
  
  
                      return true;
  
                  }
  
              }
  
          }
  
      }
  
  }

  The behavior is like this : 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 2 1 2 ...... it looks like it reads several frames before decoding one ? The first frame is an I-frame, so shouldn’t that be decoded right away ?

  With this code, I end up losing the several frames (indicated by the series of 1s). Can someone help me out here ? Is there something I am doing wrong in my code ?

  Update : the test clip is video-only. No audio.

• How well does FFmpeg scale ? Any examples ?

  17 juin 2020, par Pepper

  I'm currently working on the server side of an augmented reality project. It's a lot like http://www.livingsasquatch.com/. I'm using PHP/MySQL and FFmpeg to capture the webcam video and encoding it to .flv.

  



  Basically Flash uploads the video as a long series of .jpg images. PHP then takes those images, generates a few thumbnails and passes them through FFmpeg which converts them to a single .flv file.

  



  Here is the FFmpeg code :

  



  $ffmpeg_images = $image_directory.'/image%d.jpg';

@unlink($video_directory."/$video_id.flv"); 
$ffmpeg_video = $video_directory."/$video_id.flv";
$ffmpeg_string = FFMPEG_PATH." -f image2 -i $ffmpeg_images -f flv $ffmpeg_video";

@exec($ffmpeg_string);


  



  This seems to be working nicely in my tests, but I don't know how we'll it will scale. Since this is my first time using FFmpeg, I don't know if video encoding this way will bring the server to its knees.

  



  Does anyone have experience with FFmpeg on a high traffic site ?

  



  Is there a better way of handling this type of Webcam to .flv conversion ?

  



  Are there any examples of FFmpeg being used on a high traffic site ?

  


• Can ffmpeg trim the beginning of a video as it's written ?

  3 janvier 2024, par ttshaw1

  I'm working on an Android app where I want to keep a 30s buffer of video, and at arbitrary times save it. I have a few requirements :

  


  

  • I want to make sure I don't miss or duplicate any frames. So I think periodically starting and stopping recording won't work

  


  • I found that simultaneously encoding two video streams puts my CPU under a lot of strain, so I want to avoid doing that

  


  • I don't want to store a ring buffer of 30s worth of frames in memory as that'll require too much memory. So I think I need to encode them to a video file on disk as they come in. (though I'm realizing maybe I could write the frames to disk and encode them when I have a 30s buffer I want to keep)

  


  


  This all leads me to think the best solution would be something like a typical camera app where a video is written to an mp4 file as it's recorded. But to keep the filesize reasonable, I'd like to have the output file continuously trimmed to the last 30s, plus or minus a keyframe interval.

  


  I know that ffmpeg can do that for a video that's not being currently written. But I don't know if something about the format of an mp4 would prevent doing that for a video that is being written. For example, if an mp4 was essentially a series of bitmaps with timestamps, I'd think it's easy for ffmpeg to chop off some number of frames at the beginning while the camera API is writing frames to the end. Is there anything about the mp4 format that makes it too complicated to do that in practice ? Or is this a question that depends on the camera API's implementation ?