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• 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

• FFMPEG - Video micro stutters at merge point after cutting a 1 second segment, adding fade effect to it and concatenating it back

  9 septembre 2023, par john

  So i've been using Losslesscut (it uses FFMPEG) to edit videos, but because this program doesn't have any option for fade in/out effects, I came up with this method where i cut a 1 second segment before and after every cut I would do to the video, and then I run a batch FFMPEG command that adds fade in/out effect too all 1 second segments, and then I run another batch command to concat all parts.

  


  What all this does is it keeps 99% of the video lossless, only reencoding the 1 second segments for the fade effect.

  


  The problem is each 1 second segment that gets merged back creates a micro stuttering at the merge point. And because it's a lot of segments, the video ends up with lots of micro stuttering throughout. It's nothing crazy but definetly noticeable.

  


  I believe the issue is probably caused by imprecision on the cuts made on Losslesscut (I mean, it's just using FFMPEG keyframe cut, my videos have 1 keyframe per second). The 1 second segments end up with 61, 62, or even 63 frames (the video is 60fps). Although if, for testing purposes, I merge the 1 segment without reencoding it, there is no stutter.

  


  I think the reencode further messes with the time / frames of the video, might be adding an additional frame or extra miliseconds, even tho the fade duration is 1 second.

  


  Wondering if anyone can think of something that would make the merging smooth and get rid of the stutter ?

  


  These are the fade effect and the concat command I'm using :

  


  ffmpeg -i "input" -vf "fade=t=in:st=0:d=1" -c:v h264_nvenc -profile:v high -rc constqp -qp 14 -c:a copy -video_track_timescale 90000 "output"

ffmpeg -f concat -safe 0 -i "FILELIST.TXT" -c copy "output"


  


  I also tested doing the cutting via FFMPEG directly, and still got the after merging the fade clips.

  


  Guessing it won't be possible to fix this, since it's merging a reencoded clip with a lossless one. Who knows tho.

  


  Thanks in advance.