Path: chuka.playstation.co.uk!news
From: SCEE TECH <ps_yaroze@interactive.sony.com>
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: Re: TMD Normals
Date: Mon, 23 Jun 1997 18:43:52 +0100
Organization: Sony Computer Entertainment Europe
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Jon Prestidge (alias Moose) wrote:
> 
> Can someone explain how normals are defined for the simple TMD primitives
> F_3 and F_4?     I understand the concept of normals OK, but what
> measurements are used e.g.:
>     ....is the normal defined as an imaginary x,y,z  point that the normal
> line is projected to from the centre of the polygon's surface?
>     ...or is it an angle that one of the vertices of the polygon is rotated
> by, to form a normal line?  If so, which vertex does it relate to?
>     ....or other??
> 
> ... please also include typical values so that I'll know if I'm thinking
> along the same lines.
> 
> Cheers in advance if anyone can help,
> 
> Jon

The normal is the cross, or outer, product of 2 vectors in the plane of
the polygon. Cross product is:
c.vx = v1.vy * v2.vz - v1.vz * v2.vx
c.vy = v1.vz * v2.vx - v1.vx * v2.vz
c.vz = v1.vx * v2.vy - v1.vy * v2.vx

Just take 3 of the vertices in your polygon (v0,v1,v2) calculate v0 -
v1, v2 - v0 and slam it in the cross product, and thats your normal.

It may be worth you getting a 3d graphics book.

Stuart