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

[Matthias Wieser]

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".

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

> -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.

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

> 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.

> 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