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• Writing subtitles on a video on different time frames

  22 mars 2023, par sarveshsta

  I am trying to write subtitles on a video. The problem is , with cv2(opencv-python), I am unable to write text on different time frames , lets say for first two seconds I want "Hello", and for another 3 seconds the video will show "this is the introduction" and so on.

  


  My question is, how can I achieve this with opencv-python. The thing why I want to use cv2 only is it creates an object for that video and do not create a new file, and that is important for my project.

  


  If you guys have any idea how we can write subtitles on a video without creating another file, please do let me know.

  


  I also tried ffmpeg it worked absolutely fine for me and I used a .srt file for that, but again it was creating a new file and same happened with moviepy library so in any case if you could show me a way how can we overwrite the file in ffmpeg or moviepy, it would be very helpful.

  


  THANKS.

  


• How can I print output from functions in libavcodec during ffmpeg decoding using decode_video.c ?

  18 mai 2023, par 黄舒心

  I use decode_video.c in ffmpeg/doc/examples to decode videos, and I want to get some information during execution.

  


  The decode procedure will use some functions in libavcodec, and the information I want to get is in these functions. So, I tried to add printf("xxx") to some functions in libavcodec.

  


  But after I recompiled ffmpeg and decoded videos, there was no more output than before, only the output of decode_video.c is printed.

  


  Is my recompilation wrong ? Or anything else I hadn't taken into consideration ?

  


  I tried to debug decode_video.c to see what happened, but it just couldn't step into the functions in libavcodec.

  


• Revert "avcodec/nvenc : fix b-frame DTS behavior with fractional framerates"

  14 juin 2023, par Timo Rothenpieler

  Revert "avcodec/nvenc : fix b-frame DTS behavior with fractional framerates"
  This reverts commit 9a245bdf5d7860b8bc5e5c21a105a075925b719a.
  This commit basically broke all samples with fractional framerates,
  
  rather than fixing them.
  
  I at this point do not understand the original issue anymore, and I'm
  
  not sure how this slipped my initial testing.
  
  All my test samples must have happened to have a simple timebase.
  The actual dts values pretty much always are just a simple chain of
  
  1,2,3,4,5,... Or maybe slightly bigger steps. Each increase by one means
  
  an advance in time by one unit of the timebase.
  
  So a fractional framerate/timebase is already not an issue.
  So with this patch applied, the calculation might end up substracting
  
  huge values (1001 is a common one) from the dts, which would be an
  
  offset of that many frames, not of that many fractions of a second.
  
  This broke at least muxing into mp4, if the sample happened to have a
  
  fractional framerate.
  I do not thing the original issue this patch tried to fix existed in the
  
  first place, so it can be reverted without further consequences.
  

  • [DH] libavcodec/nvenc.c