[MPlayer-users] mencoder: effective use of qp, crf params for x264?

[matthew.garman at gmail.com]

On Mon, Jan 29, 2007 at 09:07:56AM -0700, Loren Merritt wrote:
> On Mon, 29 Jan 2007, matthew.garman at gmail.com wrote:
> >-x264encopts crf=18:bitrate=2000:...
> crf is an alternative to bitrate. You can't specify both. Like any other 
> option, the latter took precedence, so your encode was just a 2000kbps 
> average bitrate encode.

Ahh, okay.  Makes sense, espeically taking into consideration what
Guillaume said.

> >Before learning about the crf param, I did a two-pass encode, with
> >these x264encopts for passes one and two (respectively):
> >[...]
> >That took about the same time, resulting file size was 2.1 GB.
> 
> A two pass encode took the same time as a one pass encode with the
> same settings? No way. Even if the first pass is turbo, it's not
> free. And mencoder has to filter the video twice (spp is slow).

I'm pretty sure they both took about the same time (give or take an
hour).  But I may not have had a second mencoder job running when I
did the two pass (on a dual core CPU); can't remember clearly now.
The one-pass definately took 614 minutes.

Anyway that does bring up another question.  For reference, I'll
show those x264encopts again:

        -x264encopts bitrate=1400:subq=3:frameref=5:turbo=1:threads=auto:pass=1:bframes=4 -idx -ofps 24000/1001
        -x264encopts bitrate=1400:partitions=all:8x8dct:me=umh:subq=6:frameref=5:threads=auto:pass=2:bframes=4:b_pyramid:weight_b -idx -ofps 24000/1001 -vf spp,hqdn3d=2:1:2

Note that I use lower settings on the first pass.  Also, I don't do
any post-processing (no -vf opts) on the first pass.

What kind of impact does this have on quality?  I.e., *should* I
also be running the post-processing in the first pass?  Should I use
the exact same x264encopts on the first pass?

The docs I've read all suggest that using lower quality settings on
the first pass will save time.  Since nothing is free, I'm assuming
this does have an impact on quality---but to what degree?

Thanks again!
Matt