Wasn't it Greg M. Johnson who wrote:
>I have an isosurface that is rolling hills based on the noise3d function.
>
>At time increment n,  I know the tangent is at pt x1,y2,z1.
>At time increment n+1, I know the tangent is at pt. x2,y2,z2.
>
>I know the distance between the two, and the time increment is so small that
>I don't think I'll be harmed by assuming it's a straight line. Even with a
>straight line on an inclined plane, I have no idea how the ball should
>roll.  The 1d & 2d solutions are trivial.
>
>What is the rotation
>

It should be rather like the 2d case, except that you would sum 
sqrt((x2-x1)^2 + (y2-y1)^2) and sqrt((z2-z1)^2 + (y2-y1)^2)
instead of using (x2-x1) and (z2-z1).


-- 
Mike Williams
Gentleman of Leisure

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