[MPlayer-dev-eng] 3 pass encoding syntax poll

[Loren Merritt]

When I came up with the current syntax for N-pass encoding with lavc or 
x264 (pass=1 on the first pass, pass=3 on all subsequent), I though it was 
the most logic method: N-pass simply enables both 1st pass stats file and 
2nd pass ratecontrol, so I used 3 == 1|2.
However, I have seen many confused users, so apparently this isn't so 
intuitive, and nobody reads the section in the manpage describing its 
usage.
So I have decided to change the syntax and/or behavior of multipass 
encoding. But since I was wrong the first time, I will now ask opinions 
before choosing a replacement.

Currently:
   pass=2 performs 2-pass encoding, and can't be part of N-pass.
   pass=3 is used on all passes beyond the 1st of a N-pass encode.

Possibilities:
* Specify both pass number and total number of passes: "pass=1/3".
   (Of course, default to 2-pass encoding if denominator isn't specified.)
   Benefit: allows 3-pass-aware turbo.

* Change "3" to "N". Disadvantage: I fear people will still try to use
   "pass=2" in the 2nd of 3.

* Separate boolean option "npass".

* Always perform N-pass encoding. (It is, after all, identical to 2-pass
   if you run only one Nth pass.)

Additionally (especially in the last option), I think it would be 
beneficial to use separate statsfiles for each pass, so that cancelling an 
encode doesn't erase the results of previous passes. This would require 
changing the -passlogfile option somehow, since there would be multiple 
files to specify. How best to go about this?

--Loren Merritt