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

[Peter Vanroose]

>  Am I right in assuming that "projectively invariant" implies
> that this characteristic of conic sections is invariant for any
> projection whatsoever, such as projection onto a sphere or a
cylinder?

No.  By "projection" I mean "projection of planar objects onto a
plane".

> It seems to me that it ought to be possible to construct a
> transformation for which this condition would not hold.

Yes, that's right.

> If this is true, does it imply that not all transformations are
> projections?  If this is so, are projections a subset
> of transformations?

Certainly. But projections are the most general transformations that
are guaranteed to transform straight lines into straight lines.



--      Peter.