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

**From**: |
Peter Vanroose |

**Subject**: |
[help-3dldf] Re: button-hole problem |

**Date**: |
Tue, 26 Apr 2005 08:39:52 +0200 (CEST) |

>* 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.

**[help-3dldf] Re: button-hole problem**, *Laurence Finston*, `2005/04/22`
**Re: [help-3dldf] Re: button-hole problem**, *Laurence Finston*, `2005/04/23`
**Re: [help-3dldf] Re: button-hole problem**, *Martijn van Manen*, `2005/04/23`
**Re: [help-3dldf] Re: button-hole problem**, *L. Nobre G.*, `2005/04/24`
**[help-3dldf] Re: button-hole problem**, *Laurence Finston*, `2005/04/24`
**[help-3dldf] Re: button-hole problem**, *Laurence Finston*, `2005/04/24`
**[help-3dldf] Re: button-hole problem**,
*Peter Vanroose* **<=**
**[help-3dldf] Re: button-hole problem**, *Peter Vanroose*, `2005/04/27`
**[help-3dldf] Re: button-hole problem**, *Peter Vanroose*, `2005/04/27`