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• 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 ?

  


• ffmpeg with partial motion detection

  2 mars 2021, par Silentfury

  I have video files from a static surveillance camera and I try to re-encode them using some sort of motion detection filter (keeping the movements, droping the stills) in order to save space and play time.

  


  I have successfully tried the two below filter options but it still feels not good enough. :

  


  

  • mpdecimate e.g. -vf "mpdecimate=hi=6480:lo=6420:frac=0.1,setpts=N/(15*TB)"

  


  • select (e.g. -vf "select=gt(scene,0.4)")

  


  


  The main problem is a moving bush in the camera view, that I would like to exclude from the motion detection.

  


  Is there other/better ways of applying motion detection (and stills dropping) to ffmpeg ?
Is there any way to mask areas of the video that will be ignored in the above mentioned (or any other) motion detection filter/method ?

  


  Many thanks in advance !

  


  Edit :
in the mean time I found this excellent answer from Gyan

  


  I am testing this now but would still appreciate other ideas

  


• avcodec/ac3enc : Don't presume ch_layout to be AV_CHANNEL_ORDER_NATIVE

  7 avril 2024, par Andreas Rheinhardt

  avcodec/ac3enc : Don't presume ch_layout to be AV_CHANNEL_ORDER_NATIVE
  It is perfectly legal for users to use a custom layout
  
  that is equivalent to a supported native one.
  
  In this case the union in AVChannelLayout is not an uint64_t mask,
  
  but a pointer to a custom map.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/ac3enc.c