Pixels visibly shuffling on keyframe

[klinika]

Just noticed this problem on a low bitrate mostly static scene, everytime a keyframe kicks in the pixels visibly shuffle positions until the next keyframe, creating a pumping-like effect. Probably wouldn't have noticed it if it weren't for the low keyframe interval. Here's a sample: http://www.mediafire.com/?2zmnj9mdllc . Observe the picture slightly up from the middle where it's most easily noticeable. Increasing the bitrate considerably removes the problem. Is it possible to reduce or eliminate the effect at this bitrate? Or is this common knowledge I'm yet again clueless about? :p

The command line is the same for 1st and 2nd pass, but no matter what the options are, as long as keyint is suitably small the effect is visible
--bitrate 800 --stats test.stats --keyint 14 --min-keyint 2 --bframes 2 --ref 4 --mixed-refs --b-pyramid --bime --weightb --subme 6 --direct auto --filter -1,-1 --me hex --no-fast-pskip --no-dct-decimate --threads auto --thread-input --progress --no-psnr

[akupenguin]

The only way I theoretically know how to solve it is: open-gop + multi-level B-pyramid. That would spread the change evenly over all frames.
There is also a certain amount of reduction possible with psy quantization, but that can never completely remove the problem.

None of the above is implemented in x264.

For existing solutions, the only thing I can recommend is a temporal denoise filter.

[klinika]

Thank you for the explanation akupenguin. Compromise between noise reduction and bitrate it is then.