Okay, i have a big problem, i have made two tautochrones(my dictionary tells me
that isn't a word, but it is... Google it) using the animation stuff.  They
intersect in the middle, and also have a ball on each.  When the balls roll
down the middle and meat, they blob together, then continue on their ways.  In
case you don't know what a tautochrone is, it is a curve in which it takes a
ball the same amount of time to reach the bottom, no matter where it is placed.

Problem:  I just realized my balls rolling down the curve don't move, they just
translate along the curve.  I would like them to rotate to make them appear to
roll instead of just 'sliding'.  Please don't recommend that incredibly
complicated tutorial, i would like some real advise or a nice walk through.
The tutorial has driven me nuts... Not to mention it takes forever to render,
so could someone kindly explain how to make them 'roll' or 'rotate' while still
following the curve path.  I don't know how to post the code, so i will not take
up this whole space posting it, if you have questions about certain parts,
please ask.  Thanks :)

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"Pov-Ray Dummy" <nomail@nomail> wrote:
> [...] I would like them to rotate to make them appear to
> roll instead of just 'sliding'.

A broad approximation would be to find the distance that the centre of the ball
has moved since the previous frame, and rotate the ball (in degrees) by "z *
360 * distance / radius". This would slip slightly if the curve was tight, but
the calculation is easy to understand and do on a 2D curve when you know the
animation path. If the can't create the path directly, you'd need to make an
'inset path' from the curved surface, and make that your animation path.

Then there's a physics-based approach, which would be a lot more accurate. I
don't really have the skill nor patience to describe that here :o)

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