[FFmpeg-user] Confusion about quality parameters

[Ulf Zibis]

Hi,

my input is a 1.5 minute mp4 video with 19.5 MB

When I use (with build from dev):
    ./ffmpeg in.mp4 -c:a copy out.mp4
    ffmpeg version N-93873-g6276b4db97 Copyright (c) 2000-2019 the
FFmpeg developers
      built with gcc 7 (Ubuntu 7.4.0-1ubuntu1~18.04)
in the output lines e.g.
   frame=   51 fps=0.0 q=31.0 size=     256kB time=00:00:02.05
bitrate=1018.9kbits/
I see q=31.0. (the output file is only 5.9 MB)
In the first view this seems to correspond to:
    2 Codec Options
    qmax integer (encoding,video)
        Set max video quantizer scale (VBR). Must be included between -1
and 1024, default value is 31.
But this seems not to be the option to change, to have a better quality.

1. question -> Where in the documentation is stated, which quality is
used by default and how to change it?

When I use (with static release build 4.1.3 from johnvansickle):
    ffmpeg in.mp4 -c:a copy out.mp4
in the output lines e.g.
   frame=   54 fps=0.0 q=28.0 size=       0kB time=00:00:02.20
bitrate=   0.2kbits/
I see q=28.0. (the output file is 16.3 MB)

When using my system's standard version 3.4.6 I get the same q value, so
this seems corresponding to 4.1.3.
Also the visual quality of the output seems similar and has 16.4 MB.
In the heading info (which is missing in the dev build) I also see crf=23.0

2. question -> How does crf correspond to q?

When I use -q 28 (with build from dev):
    ./ffmpeg in.mp4 -q 28 -c:a copy out.mp4
in the output lines e.g.
   frame=   70 fps=0.0 q=28.0 size=       0kB time=00:00:02.84
bitrate=   0.1kbits/
I also see q=28.0, but the output file has only 6.1 MB and really bad
quality, similar to the default with q=31.0

3. question -> What is the reason for this big difference with the same
q=28.0?

Thanks,
-Ulf

--
Von meinem Seibert gesendet