Ah ha. I was trying to rescale the sphere vertically while rotating and
keeping it lengthwise in the vertical. Then as it moved across the floor
it would grow and shrink back again as it moved upward and back down.
All wrong.
You needed an ellipse "rolling" along shorter diameter going closer then
back to starting orientation with the floor. I see now. Maybe.


Andrew Cocker wrote:
> 
> Peter Popov <pet### [at] usanet> wrote in message
> news:371b5d6a.3508620@news.povray.org...
> 
> > Now, a 180 degrees' roll of this objects makes for a x translation of
> > 3.14159265, if you want it to roll correctly. If I understand your
> > question correctly, you are trying to model the behaviour of, say, an
> > unrolling rug, right? I mean, it moves, rolls, gets smaller, and stays
> > on the ground all at the same time.
> 
> Err, no, just a sphere scaled into an ellipse rolling along. I realise I was wrong
> expecting to be able to rotate 180 degrees in 1 unit, and I should now use 3.141...,
it's
> the vertical position of the centre of the ellipse that I am still unclear
about..how it
> moves down and then back up whilst keeping in contact with the floor. Surely this
value
> will vary depending on the rotation, not the scale of the sphere. Forget that I
mentioned
> changing the scale mid-anim..if I can get this to work as is, then I'll be happy.
> 
> > The y coordinate of the center is -1 + radius.
> >
> > The x coordinate is found by integrating 2*pi*r(clock)*dclock in the
> > range from 0 to clock.
> >
> 
> I'm fairly clear on this
> 
> > Am I getting the problem right? Is this of any help?
> 
> I'm slightly less in the dark, but no-one's switched the floodlights on yet ;-)
> 
> Andy

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