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• FFMPEG video encoding time increases over time

  8 août 2018, par NStof

  I am creating a fairly basic DVR in C# using the FFMPEG wrapper in Accord.net. Essentially it works by getting the raw frames from the on-board frame grabber, displays the frame on screen and places it in a buffer. In a Separate thread in 20 second intervals, the frames are taken from the buffer and using Accord.Video.FFMPEG.VideoFileWriter.WriteVideoFrame(Bitmap bmp) the frame is encoded and saved to disk. Video is saved in two minute file chunks.
  This works happily for about 24-30 hours, however after that it looks like the average time it takes to encode/save each frame increases to the point where it gets new frames faster than they can be saved. This causes buffer to grow and ends in tears.

  What I would like to know is why does the time it takes to complete the WriteVideoFrame(Bitmap bmp) function increase over time.

  What I think I know so far :
  I do not know if this problem is caused by something in FFMPEG or in the Accord.net wrapper.
  I am reasonably sure it is not caused by hardware. Neither CPU usage nor HDD are working particularly hard. In fact, when I monitor the CPU usage, it does not work any harder or less hard when the encoding time increases.
  When I stop recording (close and dispose all Accord.net objects) and start again, it does not reset the encoding speed. Only when I close the software and start it again does it fix the problem.
  Any thoughts and help with this will be greatly appreciated. If more information is required, please let me know.

• Is it possible to increase horizontal and vertical padding on ffmpeg burned subtitles - using .srt format ?

  6 mai 2022, par Dr. House

  Using command :

  


  ffmpeg -i source_video_path.mp4 -vf "subtitles=srt_source.srt:force_style='OutlineColour=&H80000000,BorderStyle=4,BackColour=&H80000000,Outline=0,Shadow=0,MarginV=25,Fontname=Arial,Fontsize=10,Alignment=2'" video_destination.mp4


  


  I get

  


  

  


  Ideally I would like to increase the subtitles background padding - especially horizontally.

  


• How to specify text-only subtitles in ffmpeg

  20 juin 2019, par Daniel Engel

  I’m trying to script some basic DVD-to-home media server processing. One of the things I’m trying to do, which seems like it should be pretty basic, is extract the text (srt) subtitles from the .mp4 file from the initial rip.

  I can see the stream, and if I select the stream explicitly (by stream number), I can extract it. However, I want to script the ffmpeg command. If I try to just specify the subtitle stream with a -map option, it maps the first subtitle stream, which is often a bitmap stream instead of text stream.

  So, after ripping some of my DVD’s, I’d like to process everything in a batch script, something like :

  for video in *.mp4; do
  
    ...
  
    (special code to get $subtitle filename with .srt instead of .mp4)
  
    ffmpeg -i "$video" -map 0:s "$subtitle"
  
    ...
  
  done

  I’d like the "-map" option to cause ffmpeg to select only text-based subtitles (like mov_text), and not, for example, dvd_subtitle, irrespective of which is first in the list of streams.

  Does anybody know of a way to do this ?