[FFmpeg-user] Somewhat confused by these results

[Moritz Barsnick]

Hi Phil,

On Tue, Jul 21, 2015 at 09:27:13 +1000, Philip Rhoades wrote:

>  From these results it appears to me that:
> 
> - selecting the speed has very little effect on file size (except for 
> Midway_NuclearFree_9_ultrafast.mkv)

As they say: YMMV. In other words, it depends. Basically, it depends on
your material and its type of scenes (and various other details). I
have run my own experiments on SD and HD TV material, and come to my
own conclusions, with larger variations than yours. (It also depends on
the compute optimizations my processor allows for. So these results are
basically guidelines for one self.)

That said: The size effect you are seeing, i.e. squeezing another 5 %
or so from the size, is important for some people. I for myself try to
take advantage of that if I have the time.

> - selecting qp = 0 has a dramatic effect on file size - compared to 
> leaving it out - but again, the speed option had very little additional 
> effect on file size
> 
> - ALL of these outputs eyeball fine to me compared to the original 
> Mencoder avi!

That's what defines quality: What the target audience's eyes make of
it! So if it's fine for you, then excellent! The "-qp 0" is apparently
one way of making a truly lossless libx264 encoding. I don't know why
you would want that, except for "archiving" purposes. But that requires
a lot of disk space, and you often can't tell the difference anyway.
You may instead want to experiment with different settings of "-crf" to
adjust the quality of your encoding, a value of 23 is default (and
quite good for most SD [and HD] recordings in my experience).

If you want to know a bit more about your H.264 possibilities, do read
this:
https://trac.ffmpeg.org/wiki/Encode/H.264

Cheers,
Moritz