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• Adding Subtitles to a VAAPI/QSV 10bit Accelerated Transcode

  17 juin 2023, par Enverex

  I've been converting some of my BluRays to watch on a streaming machine elsewhere in the house but I'm having trouble when it comes to burning subtitles, or more specifically, I'm having trouble figuring out what FFMPEG wants from me to make the process actually work.

  


  It's easy enough to do this in software, but I'm using hardware decoding and encoding and that's where the complexity seems to come from (VAAPI/QSV for decoding, QSV AV1 for encoding).

  


  I can use the following when dealing with SDR content :

  


  -vf "scale_vaapi=w='min(1920,iw)':h=-8:mode=nl_anamorphic:format=p010le:extra_hw_frames=120,hwmap=derive_device=qsv,format=qsv"


  


  And the following when dealing with HDR content :

  


  -vf "scale_vaapi=w='min(1920,iw)':h=-8:mode=nl_anamorphic:format=p010le,tonemap_vaapi=format=p010le:t=bt709:m=bt709:p=bt709:extra_hw_frames=120,hwmap=derive_device=qsv,format=qsv"


  


  But I cannot, in the hundreds of permutations I've tried now, find a single way to shoehorn subtitle baking into the process. I'm using SRT files for simplicity, so I just need to add subtitles=blah.srt somewhere to get it to work, but the crux of the issue is knowing where in the chain it needs to go and more importantly, what supporting arguments it needs with it (e.g. hwupload, hwdownload, and their associated switches, etc).

  


  Pretty much every single attempt just results in :

  


  


  Impossible to convert between the formats supported by the filter 'graph 0 input from stream 0:0' and the filter 'auto_scale_0'
Error reinitializing filters !
Failed to inject frame into filter network : Function not implemented

  


  


  So, what am I missing ?

  


• ffmpeg quality conversion options (video compression)

  25 septembre 2020, par Jason Hunter

  Can you provide a link, or an explanation, to the -q:v 1 argument that deals with video/image quality, and compression, in ffmpeg.

  


  Let me explain...

  


  for f in *
   do 
     extension="${f##*.}"
     filename="${f%.*}"
     ffmpeg -i "$f" -q:v 1 "$filename"_lq."$extension"
     rm -f "$f"
   done


  


  The ffmpeg for loop above compresses all images and videos in your working directory, it basically lowers the quality which results in smaller file sizes (the desired outcome).

  


  I'm most interested in the -q:v 1 argument of this for loop. The 1 in the -q:v 1 argument is what controls the amount of compression. But I can't find any documentation describing how to change this value of 1, and describing what it does. Is it a percentage ? Multiplier ? How do I adjust this knob ? Can/should I use negative values ? Integers only ? Min/max values ? etc.

  


  I started with the official documentation but the best I could find was a section on video quality, and the -q flag description is sparse.

  


  


  -frames[:stream_specifier] framecount (output,per-stream)
  
Stop writing to the stream after framecount frames.
  
.
  
-q[:stream_specifier] q (output,per-stream)
  

  
-qscale[:stream_specifier] q (output,per-stream)
  
Use fixed quality scale (VBR). The meaning of q/qscale is codec-dependent. If qscale is used without a stream_specifier then it applies only to the video stream, this is to maintain compatibility with previous behavior and as specifying the same codec specific value to 2 different codecs that is audio and video generally is not what is intended when no stream_specifier is used.

  


  


• `FFmpeg` `segment_time` inserts blank frames at the beginning of segment

  27 août 2017, par asdf

  I am using the following command to cut videos in parts.
  I need parts to be less than passed max_length param, I do not need precise cuts.

  ffmpeg -i input.mov \
  
  -segment_time 20 -f segment -reset_timestamps 1 input_%d.mov

  Every first part video file cut by ffmpeg is not 20s, but 20.5s long. And it has several black frames at the beginning inserted by ffmpeg.

  I don’t mind having 19.5s long parts if that is needed because of keyframes. But I need to get rid of those black frames at the beginning so that videos cut in parts could be concatenated in a single video later without any troubles.

  Could you give me an advice ?

  ---

  Upd :

  There were no black frames in an original video which was 23 seconds long.
  I made a cut setting 12 as a segment length.

  I also had to add -c copy because I would like to keep original quality if that’s possible.

  Leading black frames were added after a cut to part 1 which is 12 seconds long.

  Trailing black frames were added after a cut to part 2 which is 10 seconds long.

  https://www.dropbox.com/sh/zo8evta3w1xho3o/AABpTB3w3CuPa1WcghkSIVfna?dl=0

  The command used was

  ffmpeg -y -i /Users/nt/Desktop/cut/AAL_CrSlowMo_041413_1.mov -c copy -segment_time 12 -f segment -reset_timestamps 1 /Users/nt/Desktop/cut/AAL_CrSlowMo_041413_1_%d.mov

  Here’s output of ffmpeg :
  https://gist.github.com/nmtitov/ad102539f95fa03b95f10a46f8d99663