To take a break from my other POV-ray chores, I made a simple animated demo of
what an object looks like when it rotates (in POV-ray), as if free-falling under
gravity-- but discounting wind resistance or any other extra force. Its a
comparison between applying the rotations in two axes vs. three. I made the
animation my own purposes (to easily refer to later), but it might be of
interest to others as well.

There's an obvious difference in the visual appearence of the objects. My own
preference is for the 'two-rotation' scheme; it just looks more 'natural' (or
more 'expected'?) Using all three rotation axes imparts an 'extra' force to the
object-- kind of like wind resistance (which is also interesting, of course, but
otherwise kind of strange.) The 3-axis scheme *might* also depend on the order
of how those rotations are applied, to look 'more correct'-- not just straight
<x,y,z>, in other words.

Long ago, I originally thought that a free-falling object needed all three
rotations to look natural. The idea seemed logical-- but the visual result
didn't bear that out (to my eyes, anyway.) I guess I could do a thorough
analysis of the applied 'vector forces' that cause an object to rotate in the
first place -- but that's a lot of work  ;-)  For now, I'm curious as to which
scheme you prefer, from a purely visual standpoint.

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Download 'rotations_in_2 _vs_3 _ axes.mp4.mpg' (3454 KB)