[MPlayer-users] vbitrate estimation using 3 pass encoding (divx) question

[christopher j bottaro]

hello,
a few questions.  first off, i don't see any difference in quality between 
these two methods:

(2 pass audio/video)
mencoder -oac mp3lame -lameopts vbr=3 -ovc frameno -o frameno.avi -dvd 1
mencoder -oac copy -ovc lavc -lavcopts 
vcodec=mpeg4:vhq:vbitrate=$vbitrate:vpass1 -dvd 1

(3 pass audio/video/video)
mencoder -oac mp3lame -lameopts vbr=3 -ovc frameno -o frameno.avi -dvd 1
mencoder -oac copy -ovc lavc -lavcopts 
vcodec=mpeg4:vhq:vbitrate=$vbitrate:vpass1 -dvd 1
mencoder -oac copy -ovc lavc -lavcopts 
vcodec=mpeg4:vhq:vbitrate=$vbitrate:vpass2 -dvd 1

in fact, both produce files of the same size, so why even bother with the 3rd 
pass?  btw, $vbitrate is coming from the output of the first (audio) pass.

ok, now here is my real question.  suppose i do a first (audio) pass and it 
tells me i should use a vbitrate of 635.  ok thats all fine and good, but if 
i ended up cropping scaling, or reducing the fps in the other passes, that 
vbitrate of 635 can now be raised...right?  if so, by how much?  how do i 
calculate how much i can raise the vbitrate if i scale by such and such 
amount?  or if i reduce the fps by 10 or something.  this is all assuming i 
want the divx file to be as close to 700mb as possible for burning onto a cd.

thanks for the help