Motion capture involves sampling motion at regular intervals so as to recreate the motion as best as possible - the higher the sampling rate, the better the quality.  This is practically the opposite of regular keyframing techniques where keyframes represent landmarks of a motion with interpolation providing the motion flow between.  This is why sparse keyframing to get motion that looks organic and real is so difficult and motion capture is used so often instead.

With respect to IPP, Poser and Cinema 4D use completely different animation spline interpolation algorithms.  R9 and R10 interpolation are also different.  The Poser spline interpolation is rectified on C4D keyframes using the key tangents in an attempt to match more closely but it is definitely not possible for an exact match.  Maybe a 'smooth' option or variable setting could help to remove the choppiness?

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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