Qmaster useful for multi-pass encoding?

Question

The Compressor manual cautions several times against multi-pass submissions:
But with two-pass VBR and Multi-pass encoding, each segment is treated individually so the bit-rate allocation generated in the first pass for any one segment does not include information from the segments processed on other computers.
This would imply that Qmaster is not worthwhile on multi-pass processing. Exactly what does this warning mean?
G5 Dual 2GHz   Mac OS X (10.4.8)   2.5GB RAM

Answer 1

Multi Pass encoding means it looks at the media once before it encodes, and looks for the best place to put key frames, VBR is Variable Bit Rate and means that complex frames (like tree leaves blowing in the wind) get to use a higher bit rate then less complex frames (like talking heads in front of solid colors).
In the realm of Qmaster, if you allow Job Segmenting in your Multi-pass VBR setting, then each node will get it's own chunk (or segment) of the movie to work on, and will not be aware of information about other segments. Thus complex frames in a given segment can only "borrow" bitd from less complex frames in that same segment, and each node will do 2 passes only on it's segment, so in theory this misses optimizations that it could find if it was looking at the whole file.
If the segment breaks happen to fall during complex schenes, you will not get as good of an encode as you would in an unsegmented job.
That said, you can un-check 'allow job segmenting' in the compressor setting, and that eliminates this issue