Might be an idea to compare a,b,a,c with c,a,b,a.

I would suspect the a,b,a curve should be identical in both cases.

It might have been a typo, but I am pointing a suspicious finger at the
duplicate "a" in your sequence and suggesting you introduce a new point "c"
to test this theory.  Sorry I don't have a better way of expressing this.

I hope it works.

"Quadhall" <tre### [at] ww-interlinknet> wrote in message
news:39d20e96@news.povray.org...
>
> Greg M. Johnson wrote in message <39D1E46B.CDB2D1A6@my-dejanews.com>...
> ......
> >a,a,b,a  gives very different results from a,b,a,a
> >
> >Shouldn't any a-b-a be the same?
>
>
> Not with cubic splines in general, as it requires FOUR control points to
> determine the location of any point on the curve.  a-b-a does not give
> sufficient information to determine a cubic spline curve.  As to what is
> happening to your curve, check the documentation:
>
> *Cubic splines can be calculated in the range between the second and last
> but one argument. In the remaining parts linear splines are used.
>
> This means that when you shift the control points, different types of
> splines are being used between the same point locations, and thus
different
> appearances.  Thus you are actually only losing part of your "cubicness"
> ...the "cubicness" between the second and third points...when you change
the
> order number on your points (but gaining "cubicness" on the new second and
> new third points).  I hope my explanation is clear enough to help......
>
> Quadhall
>
>

Post a reply to this message