CQP/Bitrate Variability in New x264 Guide

[NWDaniels91]

In the new x264 Gordian Knot guide, in the area about rate control, it says the following:

Bitrate variability indicates how much the bitrate can fluctuate over the whole movie. Setting this to 0 would result in a pure CBR stream, 100% would result in perfect CQP. So, you can consider it to be a "quantizer curve compression".

I searched the forums here for CQP and found absolutely nothing (comparitively, there are 2747 threads that mention CBR). I also searched Wikipedia and found nothing. From the context, I think I understand about what the bitrate variability does, I just need a good definition for CQP. Also, without getting too technical or starting a big argument about it, could someone explain the disadvantages of setting this bitrate variability pretty high, even 100%?

[akupenguin]

Constant Quantization Parameter. Also known as "CQ" or "constant quant". I might have made up the abbreviation "CQP", but it was the obvious dual to "CBR".

As for why it's suboptimal: You get more quality per bit if you spend those bits in low-motion scenes. Also, you need lower QPs in low-motion scenes for a given percieved quality (IMO).

[Didée]

Originally posted by akupenguin
You get more quality per bit if you spend those bits in low-motion scenes. Also, you need lower QPs in low-motion scenes for a given percieved quality (IMO).
Ah. Balm on my worn soul. Lately I was getting the impression many guys feel the need to lower quants in hi-mo and upper the quants in lo-mo, for the importance to get the same level of blocky-(-or-not-)-ness throughout the flic. "Single frame examination dysfunction" :)

[Teegedeck]

And usually such curve-compression doesn't act on motion but on framesize, to make things even more complex.