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

[Alain Barthélemy]

When I started using MPlayer/Mencoder I had been told to use two-pass encoding
to reduce video bitrate and file size. Thus I used two-passes encoding.

Now I burn my files on DVD's and I don't care if the file is more than 700Mb.

I still have a better quality (subjective?) with a video bitrate of
1800kbit/s. Size of file thus about 1400Mb.

I read though on this mailing-list that two-passes encoding improve quality. I
don't disagree of course but I would like to know how it happens. On the man
page, the only thing I read is "vpass activates two pass mode etc ...".

--------------

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 -lavcopts
vcodec=mpeg4:vbitrate=1800:mbd=2:acodec=mp3:v4mv -oac mp3lame -lameopts
cbr:br=96:mode=3 -vf crop=620:460,pp=tn/lb,scale -zoom -xy 640 -sws 1 tv:// -o
movie.avi -endpos <sec>

I am not original. I just try what I read on the list.

Now if I want to crop the picture I suppose I do:

mencoder -oac copy -ovc lavc -lavcopts vcodec=mpeg4,vpass=1|2 -vf
crop=620:380,pp=tn/lb,scale -zoom -xy 640 -sws 1 -o /dev/null|movie2.avi

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. I explain: -vf is for filtering
thus once filtering is done it should not be filtered a second time except, I
suppose, if I recrop and rescale. But post-processing? And software scaler
(though I read in man page that software scaler had to be used with the -zoom
option)?

My question is. Do I have to recopy all the options? Sorry if it is
stupid. Thus if I don't make any change in the resolution, video bitrate and
size of the movie, I suppose I must type the following line and it should be
enough:

mencoder -oac copy -ovc lavc -lavcopts vcodec=mpeg4,vpass=1|2 -o
/dev/null|movie2.avi.

Normally I should have a better (re-)encoding.

Thanks for any enlightement.

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