Qcomp

Quantizer curve compression, or “qcomp”, is a very old and simple idea. Lower the quality in areas of the video with high complexity and raise the quality in areas with low complexity. The original implementation dates at least back to the original libavcodec ratecontrol. There are three justifications for this:

1. High-complexity scenes generally have high motion, and one is less likely to notice quality loss in a high motion scene, since fine detail is impossible to see in motion anyways.
2. High-complexity scenes are extraordinarily costly bit-wise, so even if it makes quality somewhat worse, it might be worth it to save those bits to use elsewhere.
3. Each frame in a high-complexity scene is not referenced very far into the future since there is a great deal of change between each frame, so even from a PSNR perspective, one should allocate fewer bits to those frames as opposed to frames which are nearly static.

x264 uses qcomp pretty much unchanged except that it performs a gaussian blur over the frame complexities to avoid too much local fluctuation between quantizers.

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