Andrew Cocker wrote in message <371b934a.0@news.povray.org>...
>
>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.
>


First, remember that the cirumference of an ellipse is not 2*pi*r. An
approximation is 2*pi*sqrt[(a^2+b^2)/2] where a and b are lengths of major
and minor axes.

The other value you want - I haven't got right now. But why not just do
this: use the Superpatch's trace() function; trace a line from the centre
straight down. You can now get the radius for the current angle.

Basically like this:

#declare Ellipse=sphere{0,1 scale <.25,1,.25> rotate z*clock*360}
#declare Rad=trace(Ellipse,0,-y).y;

At least I think this should work.

Margus

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