IIRC there is some quality downgrade for higher fps using CRF based encoding, also aq-motion should consider frame duration too
edit:if they ported crf rate control straight from x264, it should be still somewhere in the code
Code:
commit c583687fab832ba7eaf8626048f05ad1f861a855 [revision 1867]
Author: Fiona Glaser <fiona@x264.com>
Date: Thu Dec 23 19:33:01 2010 -0500
VFR/framerate-aware ratecontrol, part 2
MB-tree and qcomp complexity estimation now consider the duration of a frame in their calculations.
This is very important for visual optimizations, as frames that last longer are inherently more important quality-wise.
Improves VFR-aware PSNR as much as 1-2db on extreme test cases, ~0.5db on more ordinary VFR clips (e.g. deduped anime episodes).
WARNING: This change redefines x264's internal quality measurement.
x264 will now scale its quality based on the framerate of the video due to the aforementioned frame duration logic.
That is, --crf X will give lower quality per frame for a 60fps video than for a 30fps one.
This will make --crf closer to constant perceptual quality than previously.
The "center" for this change is 25fps: that is, videos lower than 25fps will go up in quality at the same CRF and videos above will go down.
This choice is completely arbitrary.
Note that to take full advantage of this, x264 must encode your video at the correct framerate, with the correct timestamps.