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• Prepending generated audio silence when merging audio w/ non-zero starting PTS and video with zero-based PTS for equal duration, aligned streams

  20 juillet 2021, par hedgehog90

  When extracting segments from a media file with video and audio streams without re-encoding (-c copy), it is likely that the requested seek & end time specified will not land precisely on a keyframe in the source.

  


  In this case, ffmpeg will grab the nearest keyframe of each track and position them with differing starting PTS values so that they remain in sync.

  


  Video keyframes tend to be a lot more spaced apart, so you can often end up with something like this :

  


  

  


  Viewing the clip in VLC, the audio will start at 5 seconds in.

  


  However, in other video players or video editors I've noticed this can lead to some playback issues or a/v desync.

  


  A solution would be to re-encode both streams when extracting the clip, allowing ffmpeg to precisely seek to the specified seek time and generating equal length & synced audio and video tracks.

  


  However, in my case I do not want to re-encode the video, it is costly and produces lower quality video and/or greater file sizes. I would prefer to only re-encode the audio, filling the initial gap with generated silence.

  


  This should be simple, but everything I've tried has failed to generate silence before the audio stream begins.

  


  I've tried apad, aresample=sync=1, and using amerge to combine the audio with anullsrc. None of it works.

  


  All I can think to possibly get around this is to use ffprobe on the misaligned source to retrieve the first audio PTS, and in a second ffmpeg process apply this value as a negative -itoffset, then concatting the audio track with generated silence lasting the duration of silence... But surely there's a better way, with just one instance of ffmpeg ?

  


  Any ideas ?

  


• How to convert a single photo into video with some filters without losing performance ?

  11 avril 2019, par Zedd W

  I have a picture in jpeg format : cat.

  I converted this picture into a video through some ffmpeg filter : scale and zoompan. The video is that the picture is slowly enlarged

  Then, I encountered some problems when using the following command.

  command 1

  ffmpeg -loop 1 -i cat.jpeg -filter_complex "scale=iw*2:-1,zoompan=z='zoom+0.002':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=720x960" -pix_fmt yuv420p -vcodec libx264 -preset ultrafast -crf 28 -y -t 5 out.mp4

  command 2 (Just adjust the scale value of command 1)

  ffmpeg -loop 1 -i cat.jpeg -filter_complex "scale=iw*8:-1,zoompan=z='zoom+0.002':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=720x960" -pix_fmt yuv420p -vcodec libx264 -preset ultrafast -crf 28 -y -t 5 out.mp4

  Problems :

  1. The picture in the video will zoom in slowly, which is true, but until about the fourth second, the picture will return to the minimum and re-enlarge.Is this the filter parameter I used wrong ?

  2. There is a warning after running the command : [swscaler @ 0x113c66000] Warning: data is not aligned! This can lead to a speed loss. The picture size is 720 x 960 and the video is the same. 720/16 is an integer and 960/16 also an integer. So what not aligned ?

  3. There is another warning : deprecated pixel format used, make sure you did set range correctly. Did I set the pix_fmtparameter wrong ?

  4. command 2 and command 1 are different scale value, But the time they run will vary greatly (If you try to run it). Using a larger scale will have better results. So how to choose a scale value or have better way ?

• How to convert a single picture into video with some filters without losing performance ?

  11 avril 2019, par Zedd W

  I have a picture in jpeg format : cat.

  I converted this picture into a video through some ffmpeg filter : scale and zoompan. The video is that the picture is slowly enlarged

  Then, I encountered some problems when using the following command.

  command 1

  ffmpeg -loop 1 -i cat.jpeg -filter_complex "scale=iw*2:-1,zoompan=z='zoom+0.002':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=720x960" -pix_fmt yuv420p -vcodec libx264 -preset ultrafast -crf 28 -y -t 5 out.mp4

  command 2 (Just adjust the scale value of command 1)

  ffmpeg -loop 1 -i cat.jpeg -filter_complex "scale=iw*8:-1,zoompan=z='zoom+0.002':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=720x960" -pix_fmt yuv420p -vcodec libx264 -preset ultrafast -crf 28 -y -t 5 out.mp4

  Problems :

  1. The picture in the video will zoom in slowly, which is true, but until about the fourth second, the picture will return to the minimum and re-enlarge.Is this the filter parameter I used wrong ?

  2. There is a warning after running the command : [swscaler @ 0x113c66000] Warning: data is not aligned! This can lead to a speed loss. The picture size is 720 x 960 and the video is the same. 720/16 is an integer and 960/16 also an integer. So what not aligned ?

  3. There is another warning : deprecated pixel format used, make sure you did set range correctly. Did I set the pix_fmtparameter wrong ?

  4. command 2 and command 1 are different scale value, But the time they run will vary greatly (If you try to run it). Using a larger scale will have better results. So how to choose a scale value or have better way ?