[Ffmpeg-devel] qp offsets (i,b,p)

[Tuukka Toivonen]

ffmpeg has options -b_qfactor, -i_qoffset, etc.
They modify the QP (quantization parameter) value between different types 
of frames. Obviously this sometimes help, or the arguments wouldn't be 
there.

Now the actual question is, why?

Or actually: is anyone aware of any research papers, webpages, books, or 
anything (keywords for google, I found nothing useful), that would
1) explain why changing the QP helps
2) give algorithm for selecting QP offsets automatically

This is not specifically about ffmpeg, but I'd be grateful
of any references.