Re: [help-3dldf] Re: button-hole problem

[Laurence Finston]

-------------------
> On Sun, 24 Apr 2005, Laurence Finston wrote:

> > Shearing (an affine transformation)
> > can make an ellipse non-elliptical, but since it remains a closed curve,
it is
> > neither a parabola or a hyperbola.  Is there a name for this kind of
curve?
> 
> Ellipse.

Point taken.  I'll have to try this and see what it looks like.  I imagined it
differently.

> > And is there a  way of finding out _what_ curve the projection the
projection
> > of a conic section is, i.e., finding an implicit or parametric equation to
> > describe it?
> 
> Of course: project six points and solve ax2+by2+cxy+dx+ey+f=0.

I'll have to look up how to do this.  I think there's a way to do it with
matrices.  

> But be "carefull" out there

I will.   Those stellated icosahedra have sharp points.

Thanks.

Laurence