[FFmpeg-user] Confusion about quality parameters

Moritz Barsnick barsnick at gmx.net
Wed Jun 26 10:58:22 EEST 2019

On Tue, Jun 25, 2019 at 20:01:49 +0200, Ulf Zibis wrote:
> Question 4.) So why is the default of the dev build set to such bad
> quality and why are the q factors such incoherent?

This has nothing to do with the dev branch, only with your particular
build. It uses different codecs, and these behave totally differently -
both by nature, and by default settings.

> > behaves totally differently to libx264, therefore the sizes (and
> > default quality settings, and more perhaps) differ.
> This is what I really didn't strike. Thanks for the hint. But I think,
> it doesn't answer the above question.

The mpeg4 encoder encodes differently than the x264 encoder. You *may*
find settings where both achieve the same quality at the same video
bitrate, but that would take a lot of experimenting. (Note: encoding is
mostly a trade-off between bitrate/stream size, quality, and encoding
time. And the actual codec used.) Usually, an MPEG4 stream at approx.
the same quality as an H.264 stream will use significantly more
size/bandwith/bitrate. (Very generally speaking.)

That your mpeg4 video turned out much smaller is due to the fact that
ffmpeg's mpeg4 setting defaults to "200kbit/s CBR with best effort",
regardless of resolution. ffmpeg's x264 default is "CRF 23, VBR", which
tends to (well, almost always will) result in much higher quality at
sane resolutions, e.g. 480p upwards.

> > (Which, again - for everyone - is the
> > reason we request to see it. ;-))
> Obviously you didn't view my post to the end. There it is.

I did. I even quoted it. I just wanted to emphasize this for everyone
else, since it proves the point.

> Please have a thought on my question 1.)
> Where in the documentation I find, which quality is used by default and
> how to change it?
> In other words, wouldn't it make sense to add this, if this information
> is missing?

IMHO it would make sense, especially as "ffmpeg -h encoder=blafoo"
cannot tell you.


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