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• Can I know which byte range to read from a remote mp4 file for FFMpeg to decode a keyframe ?

  12 octobre 2023, par db9117

  I need to decode a of keyframe of a video file (mp4, h264 encoded). I know the timestamp of the keyframe I want to extract/decode. I want to minimize amount of data being read in memory. For this, I need to know beforehand exactly the minimal byte range I would require that encompasses this keyframe. How do I know what is the minimal byte range in the whole mp4 byte stream I need to read in order to be able to decode the keyframe ?

  


  I currently find the appropriate keyframe in the index_entries contained in the header. I get its byte position (pos attribute) and timestamp (timestamp attribute). I calculate the range as follows :

  


  startBytes : minimum of :

  


  

  1. the pos of the keyframe

  


  2. the pos of the nearest index entry in the audio stream happening at or before the keyframe's timestamp.

  


  


  This way when it's decoding the frame, if it also needs the audio content for demuxing, it would have it.

  


  endBytes : maximum of :

  


  

  1. the pos of the next frame in the video stream's index, after the keyframe

  


  2. the pos of the next frame in the audio stream's index after the timestamp of the wished keyframe.

  


  


  This way I know that I have everything up until the next frame in the index, which theoretically should be enough to decode the keyframe only.

  


  I then read the appropriate byte range.

  


  When I try to decode the frame, I run in a loop until I succeed :

  


  

  • avcodec_read_frame

  


  • avcodec_send_packet

  


  • avcodec_receive_frame

  


  


  I ignore AVERROR(EAGAIN) errors.

  


  avcodec_receive_frame fails multiple times with error AVERROR(EAGAIN) which I ignore, until it fails saying that the memory it wants to read isn't available (wants to read after endBytes). I explicitly tell it to fail if it wants to read more than it has already read.

  


  Note : for other keyframes at other positions in other videos, it sometimes succeeds (probably because the range is big enough by chance), but it fails more often than not.

  


  My question is : Why is the end of the range not enough to be able to decode only the one keyframe ? Is there any way to more precisely calculate the exact range in bytes I would need in order to decode a particular keyframe ?

  


• Close and reopen stream

  7 janvier 2020, par Expressingx

  I’m streaming video from a camera into a file. Using mp4 but if there is unexpected power off the video gets corrupted. So I’m going to store the video in a backup file where ffmpeg will write frames to temporary file and in some interval the content of that file will be copied to the backup file. So far so good, but in order backup file to be not corrupted I have to write the trailer first (I guess) to the temp file and eventually close the stream and then copy the content. After that reopen the stream to the temp file. But I’m getting Attempted to read or write protected memory if I try to call av_write_trailer() after its already called once.

  Is there even a way to achieve that or if I expect power off I should use other format like flv ?

• doc/decoders,decoders : add various missing final dots

  21 juin 2013, par Stefano Sabatini

  doc/decoders,decoders : add various missing final dots
  Enhance consistency.
  

  • [DH] doc/decoders.texi
  • [DH] doc/encoders.texi