eureka! (and I don't mean you need a bath) alter the code as follows:

#declare roff = 90;
#declare centerX = cos(radians(rotation-roff))*x_scale;
#declare centerY = sin(radians(rotation-roff))*y_scale;

This keeps the sphere on the ground. The next trick is to translate it accordingly.

I'll work on that.

Josh

Josh English wrote:

> Thank you for the circumference formula... I've been looking for it.
>
> Here's the logic I've been applying:
> 1: create the unit sphere at the origin,
> 2: scale
> 3: rotate around z axis
> 4: translate against the y axis
> 5: translate against the x axis
>
> steps 1,2, and 3 are easy. Step 4 is giving me headaches.
>
> I've been playing with this code:
>
> #declare x_scale = 0.25;
> #declare y_scale = 1;
>
> #declare rotation=60; // this can be anything, it's the control variable
> #declare roff = 0 // or 90 or radians(90)
>
> #declare centerX = cos(rotation-roff)*x_scale; // these should give us the x/y
> values
> #declare centerY = sin(rotation-roff)*y_scale; // of the edge of the sphere (z
> axis)
>
> #declare height=vlength(<centerX,centerY,0>);
>
> This should give us the length from the edge of the sphere to the center (again
> on the z axis, where the sphere
> is biggest) and since the sphere is centered around the origin, simply
> translate*height*y.
>
> Of course, there is a basic flaw in this logic... The point that I'm measuring
> to on the edge of the sphere is being rotated around the z axis.  I haven't
> found a way out of this trap yet. It looks like it should work... so what am I
> missing? Is my process making any sense to anyone but me?
>
> Josh English
> eng### [at] spiritonecom
>
> Margus Ramst wrote:
>
> > 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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