5

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.

Display name
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  • What is the encoder you are using? If you don't know then what is the `$extension`? – llogan Sep 22 '20 at 17:54
  • @llogan I entered `extension` at the command line and nothing happened. I guess I don't understand the question. – Display name Sep 22 '20 at 20:44
  • `-q:v`/`-qscale:v` depends on the encoder. I wanted to know what encoder you were using to give an accurate answer. I asked for the extension (`.mp4`, `.mkv`, `.avi`, `.mov`, etc) of the output file in case you were not sure which encoder you're using. – llogan Sep 22 '20 at 22:09
  • @llogan the output files are .mp4 – Display name Sep 24 '20 at 02:06

2 Answers2

13

-q:v is probably being ignored

You are outputting MP4, so it is most likely that you are using the encoder libx264 which outputs H.264 video.

-q:v / -qscale:v is ignored by libx264.

The console output even provides a warning about this: -qscale is ignored, -crf is recommended.

For more info on -crf see FFmpeg Wiki: H.264.

When can I use -q:v?

The MPEG* encoders (mpeg4, mpeg2video, mpeg1video, mjpeg, libxvid, msmpeg4) can use -q:v / -qscale:v.

See How can I extract a good quality JPEG image from a video file with ffmpeg? for more info on this option.

llogan
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3

This option is an alias for -qscale:v which might be why you didn't encounter it during your research (eventhough my resultat came first with "ffmpeg q:v" on google). This link explains how the qscale option is not a multiplier or a percentage, it's a bitrate mode (so it's to bitrate). For a given encoder, the lower this number the higher the bitrate and quality. It usually spans from 1-31 but some encoders can accept a subset of this range.

Jao
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