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• How to find the presentation time stamp of a given frame number for FFMPEG decoding ?

  21 août 2017, par Deepankar Arya

  I am using the C APIs of ffmpeg for some video processing. My aim is to extract the screen shot of a given frame number. I have understood that ffmpeg has an av_seek_frame function to seek to a given timestamp(expressed in appropriated base units of the video stream). I assume that I will have to goto to the most recent I frame for the given frame(using the AVSEEK_FLAG_BACKWARD flag) and read onwards untill I meet the required frame. For that I need to give a seek time stamp to the av_seek_frame function. My main issue is that given a frame number, how do I find an associated presentation time stamp to seek to ?

• PHP - 500 error every second time after using exec() on ffmpeg

  23 août 2020, par Eduard Unruh

  I get an 500 error every second time executing this code :

  


  exec('"ffmpeg/bin/ffmpeg.exe" -y -i ' . $infile . ' -movflags faststart -pix_fmt yuv420p -maxrate 500k -bufsize 250k -vf scale=trunc(iw/2)*2:trunc(ih/2)*2 ' . $outfile . ' 2>&1', $output, $return_var);


  


  I've seen this answer :

  


  PHP Exec (ffmpeg) fails on IIS every other request

  


  But before I try to find out what changing the FastCGI protocol is and how to do it : is there a way in the code to avoid this ?

  


• Get frame time in ffmpeg

  30 mai 2013, par Srv19

  I am trying to make a little video player that has seek bar (with ffmpeg, of course). For that i need function that will, using data from frame and/or packet, get me current time in the video that should be set in seek slider.

  It should work like this :

  my_time = get_cur_time()
  
  seek(my_time + 10)
  
  assert(my_time+10 == get_cur_time())
  
  seek(my_time - 10)
  
  assert(my_time-10 == get_cur_time())

  I do understand thatffmpeg does not support precise seeking, so equality here means "something reasonably cloae).

  What code have i used for this thus far :

  frame_time = frame->pts*av_q2d(video_dec_ctx->time_base) * 1000;

  where frame is AVFrame and video_dec_ctx is AVCodecContext.

  And for seeking :

     int fn = ffmpeg::av_rescale(tsms,fmt_ctx->streams[video_stream->index]->time_base.den,
  
                                 fmt_ctx->streams[video_stream->index]->time_base.num);
  
     int frame = fn/1000;
  
  
  
     printf("\t avformat_seek_file to %d\n",frame);
  
     int flags = AVSEEK_FLAG_FRAME;
  
     if (frame < this->frame->pts)
  
         flags |= AVSEEK_FLAG_BACKWARD;
  
     if(ffmpeg::av_seek_frame(fmt_ctx,video_stream->index,frame,flags))
  
     {
  
         printf("\nFailed to seek for time %d",frame);
  
        return false;
  
     }
  
  
  
     avcodec_flush_buffers(video_dec_ctx);
  
     int got_frame = 0;
  
     do
  
     if (av_read_frame(fmt_ctx, &pkt) >= 0) {
  
         decode_packet_ro(&got_frame, 0);
  
         av_free_packet(&pkt);
  
     }
  
     else
  
     {
  
         read_cache = true;
  
         pkt.data = NULL;
  
         pkt.size = 0;
  
         break;
  
     }
  
     while(!(got_frame && this->frame->pts >= frame));

  The code does forward seeking passably, but after any attempt of backward seeking my second assertion fails. After seeking to previous position, my method of getting time does not return position less that one before seeking. That causes my seek slider to work grossly incorrectly.