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• Trouble with frame accuracy applying subcaps using .ass files to 23.976fps video [closed]

  25 août 2023, par WhatsYourFunction

  Currently testing .ass subcaps in a VFX workflow.
The goal is to drop specfic text over specific shots and the in/out points have to be frame accurate
We're working in a 23.976 project.

  


  Currently having no trouble using FFmpeg to generate frame-accurate subclips of individual shots from a full-show export by converting hh:mm:ss:ff to seconds and then handling the 24 to 23.976 offset, using the following alorithm :

  


  InPoint_Seconds = ConvertToSeconds(InPoint_Hmsf_FullShow) - ConvertToSeconds(Start_Hmsf_FullShow) // Convert from SMTPE Time Code to seconds.
InPoint_Seconds = InPoint_Seconds * (1001 / 1000) //Handle 24 to 23.976 offset
OutPoint_Seconds = [Same idea as above]
Duration_Seconds = Output_Seconds - InPoint_Seconds

> ffmpeg -ss InPoint_Seconds -t Duration_Seconds -i SourcePath -c copy DestPath


  


  So generating frame-accurate copies of portions of a larger file works with perfect accuracy

  


  BUT when applying the same logic to subcaps using .ass files, sometimes they land with frame accuracy, and sometimes they don't (They'll be 1 frame late at most, and it does not increase over the span of the source clip).

  


  Curious if anyone has any ideas.

  


• MP4Box Dash mpd not working

  7 décembre 2017, par Durlabh Sharma

  I’m trying to make a DASH client with Exoplayer as client on Android side. I am able to create individual Dash streams for audio or video. But I am unable to create a stream that includes both audio and video in single mpd file.

  For video, command used is :

  MP4Box -dash 4000 -frag 4000 -rap -segment-name segment_ ../video_enc.mp4#video

  For audio, command is :

  MP4Box -dash 4000 -frag 4000 -rap -segment-name segment_ ../video_enc.mp4#audio

  They both work perfectly fine. But when I go to create a single mpd, it creates mpd but it just doesn’t work. Even after creating all segments and fragments, it still fails to playCommand used is :

  MP4Box -dash 4000 -frag 4000 -rap -bs-switching no -profile dashavc264:live -segment-name segment_ ../video_enc.mp4#audio ../video_enc.mp4#video

• H264 decompression/decomposition into JPG - quality issues

  5 juin 2013, par Respectech

  I'm trying to decode a 1920x1080 30fps h264 stream. Individual frames look outstanding in VLC media player (pausing the playback), but when I decode using avconv (ffmpeg has been deprecated and replaced by avconv), the frame quality is really poor by comparison (my primary complaint is blockiness).

  Here's how I am calling avconv :

  avconv -i video.h264 -s 1920x1080 -f image2 temp/images/video-%03d.jpg

  Is there a jpg output setting for avconv ? I read what I suspected were the salient parts of the avconv documentation (namely, http://libav.org/avconv.html#image2-1), and couldn't find any way to specify the output jpg quality.

  In addition, it appears avconv reads the entire stream before it starts decoding it, so if the stream is in progress, it only decodes to where the stream was when avconv started the decoding process. Is there any way around this ? In other words, if a 10-second-long stream is started at t seconds and avconv is started at t+1 seconds, avconv will only decode 1 second of the stream.