[MPlayer-users] Two-passes encoding - What happens exactly?

[Alain Barthélemy]

Le mercredi 02 juin 2004, 23:27:37 ou environ Matthias Wieser <matthias-wieser at t-online.de> a écrit:
> Alain Barthélemy wrote:
> 
> > I read though on this mailing-list that two-passes encoding improve
> > quality.
> 
> Two-passes encoding reduces quality less than one-pass encoding.
> 
> 
> > Let's have an example. I do a lot of encoding of TV signal. TV signal
> > is never of optimum quality of course. Here is my instruction line:
> >
> > mencoder -tv driver=v4l:width=640:height=480:amode=0 -ovc lavc
> 
> Is that the highest resolution your tv card is capable of? I use 
> "width=720:height=576".

Normally it should be 640x480 but I made lately a quick test with resolution
720x576 and it worked.


> 
> > -lavcopts vcodec=mpeg4:vbitrate=1800:mbd=2:acodec=mp3:v4mv -oac mp3lame
> MP3 from Libavcodec (acodec=mp3) or lame (-oac mp3lame)?

True! I was wondering myself if -ovc lavc -lavcopts ...:acodec=mp3:abitrate=96
or -oac mp3lame -lameopts ...:br=96 was not saying the same thing. What
disturbed me was that acodec and abitrate were options of -ovc (v for "video").

> 
> > -vf crop=620:460,pp=tn/lb,scale -zoom -xy 640 -sws 1
> 
> "-vf crop=672:528:24:24,pp=md,scale=576:432 -sws 1" should be the straight 
> forward way to crop and scale.

Thanks. I'll try.


> 
> "-vf denoise3d=5:4:7" should give better quality than "-vf pp=tn" (But 
> needs more CPU-time).

I'm not familiar with denoise. Still learning about video compression (and still dumb).

> 
> > Now if I want to crop the picture I suppose I do:
> >
> > mencoder -oac copy -ovc lavc -lavcopts vcodec=mpeg4,vpass=1|2 -vf
> 
> Why not cropping directly when recording TV? Just create some shell scrips 
> for 4/3 and 16/9 TV shows with and without deinterlacing.

For the good reason that the TV sender does not have the good idea to send a
test signal of a few seconds before the true sending. With an original capture
of 640x480 (I'll try now 720x576) I must sometimes crop to 620x420 or
620x380. I already did what you adviced but I had to recrop and I am afraid to
crop too much because with a TV capture you don't have a second chance (it only
happens with 16/9 scaled movies).

> 
> > As I don't fully understand what happens during the two-passes
> > encoding, I don't know if I have to recopy all the parameters.
> 
> Unlike JPEG Images, Mpeg videos can't be cropped without lossy reencoding.
> 
> > And software scaler (though I read in man page
> > that software scaler had to be used with the -zoom option)?
> 
> Thats not written in the man page (at least not in the version of the man 
> page I possess)
> 
> 
>          Matthias

-- 
Alain Barthélemy
cassandre at bartydeux.be
http://bartydeux.be
Linux User #315631