Provide info about default qualitiy for each codec

[Ulf Zibis]

Summary of the bug:
The default value for option -q is missing, at least for encoder=libx264.
How to reproduce:

• search on https://ffmpeg.org/ffmpeg-codecs.html
• additionally try:

  % ffmpeg -h encoder=libx264
  ffmpeg version 4.1.3
  built on gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
  

From the output of using this encoder, I can see, it seems to be 28.0, but this is nowhere documented.

IMHO on https://ffmpeg.org/ffmpeg-codecs.html it should be possible to easily find for each codec:

1. the default quality
2. which option to use to change it (here -q)

Additionally ffmpeg -h encoder=blafoo should give this info.

[inof]

Actually, the default value of libx264 for CRF is 23.0.

The FFmpeg documentation says: “To get a more accurate and extensive documentation of the libx264 options, invoke the command x264 --fullhelp or consult the libx264 documentation.”

Accordingly, the x264 --fullhelp command states the CRF default value (all defalts are in square brackets):

      --crf <float>           Quality-based VBR (-12-51) [23.0]

If you prefer to look it up online, all of the defaults of libx264 are listed on its project homepage ​on this web page (under the chapter “Rate control”).

I think that the FFmpeg project should not include a copy of all of the documentation of external libraries. That would be a needless duplication, and and it would pose the risk of getting outdated soon. It's better to refer to the respective project's own documentation, as is already done in this case.

I suggest that this ticket can be closed.

[Balling]

No. 10 bit x264 mode is different from 8 bit mode it has different CRF ranges. Also we remap all crf values to make them strictly positive.