In determining the compressibility of the original video, I assume you want to know how small you can make the final transcoded video file. I believe you’d have to use something like xvid4conf. (It’s available as a Debian package.) I think they’re often referred to as “bitrate calculators”.

What you’re describing in terms of “output” actually sounds like the Xvid passlog file. My understanding is that, the first pass is used to determine how much the file can be compressed, amongst other things of course. All the data is then dumped into the passlog file. The info. in the passlog file is then used in the second pass.

As an experiment, edit your script so that the transcoding from the first pass is not dumped to /dev/null :

mencoder “$1″ -profile pass1 -o pass1.avi

Run the script in near full screen and take a quick look at mencoder’s stdout. Hit cntrl + c once it actually starts the transcoding. You should notice a line where it says that it ignores bitrate settings in the FIRST pass encoded video file.

Anyway, if you’re goal is to shrink the file, but still make it look AND sound okay, you’ll need to investigate all the other Mencoder options in the man page. There’s a lot.

For starters, you can scale down the resolution, say from 720 pixels wide to 640 pixels wide or less. I think standard def TV is 480 px wide. Also, if the audio part of the video file is in AC3 or PCM, encode with libmp3lame.

However, I find using the fixed quantizer option the easiest and fastest.