From:	Shawn Hargreaves <shawn AT talula DOT demon DOT co DOT uk>
To:	djgpp AT delorie DOT com
MMDF-Warning:	Parse error in original version of preceding line at punt-2.mail.demon.net
Subject:	Re: cross_product() - Allegro
Date:	Wed, 14 May 1997 09:06:13 +0100
MIME-Version:	1.0
Message-ID:	<863609349.097651.0@[194.129.18.166]>

KaRNaGE writes:
> i was wondering how i could use cross_product() to get the normal of a
> vector (x, y, z)...and the normal of a face (triangle)...thankz...

A vector doesn't have a normal, or rather it has an infinite number of
them. Think about it: a normal is just a line at right angles to something
else, and in 3d there are a lot of different ways to arrange two lines so
they are perpendicular!

The cross product returns a vector that is perpendicular to both of two
input vectors. If you have a triangle, use two of its edges as the input
vectors, eg. for the triangle x1,y1,z1, x2,y2,z2, x3,y3,z3, the normal is:

    fixed xnormal, ynormal, znormal;

    cross_product(x2-x1, y2-y1, z2-z1,
                          x3-x2, y3-y2, z3-z2,
		  &xnormal, &ynormal, &znormal);

And there you have your normal. There are obviously two possible results
for any single polygon: which one you get will be determined by whether
your vertices were stored in a clockwise or anticlockwise direction (to get
the other normal, just negate the output vector).


--
Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk
Beauty is a French phonetic corruption of a short cloth neck ornament

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