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## Re: [ePiX-devel] possible bug in plane#plane(P const&, P const&, P const

**From**: |
Andrew D. Hwang |

**Subject**: |
Re: [ePiX-devel] possible bug in plane#plane(P const&, P const&, P const&) |

**Date**: |
Thu, 6 Dec 2007 19:07:34 -0500 (EST) |

On Thu, 6 Dec 2007, Maik Beckmann wrote:

`The local variable perp in plane#plane(P const&, P const&, P const&),
``which is later assigned to m_perp and thus returned by plane#perp(), is
``calculated like this
` P perp((p3-p1)*(p2-p1));
I expected plane#perp() to return the normal vector of the plane, but the
preceding statement calculated the opposite direction (see attached pdf).
If this is intended its ok for me, since then plane#perp() returns a real
perpendicular :). Is it?

Hi Maik,

`This definition is indeed the opposite of what I'd expect, namely, the
``points p1, p2, p3 "should" be oriented counterclockwise with respect to
``the unit normal. I'll change the normal calculation to (p2-p1)*(p3-p1) in
``the next set of changes. (The facet class suffers from the same "backward"
``convention; will fix that, too. Probably the dyslexic switch happened
``because I visualized the factors with p2-p1 spatially to the right...! :)
`
Thanks for pointing this out.
Best,
Andy
Andrew D. Hwang address@hidden
Department of Math and CS http://mathcs.holycross.edu/~ahwang
College of the Holy Cross (508) 793-2458 (Office: 320 Swords)
Worcester, MA, 01610-2395 (508) 793-3530 (fax)