Path: chuka.playstation.co.uk!news From: Alex Herbert Newsgroups: scee.yaroze.programming.3d_graphics Subject: Re: What is a normal? Date: Fri, 14 Aug 1998 13:37:54 +0000 Organization: PlayStation Net Yaroze (SCEE) Lines: 27 Message-ID: <35D42FA2.8E653C98@ndirect.co.uk> References: <35D3F965.D5D79825@nospam.easynet.co.uk> <35D42D6C.CEC81F6D@scee.sony.co.uk> Reply-To: aherbert@ndirect.co.uk NNTP-Posting-Host: dialin2-22.ndirect.co.uk Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 4.04 [en] (Win95; I) James Russell wrote: > A normal is a vector (of any length, except zero) that is perpendicular ("at right angles to") a > plane. They are useful for many reasons. They are most commonly used to calculate the amount of > light that falls on an object (the angle that the normal subtends with the vector from the polygon > to the light source is used to attenuate the colour of the polygon) and polygon visibility (if you > have a standard that states that a polygon's normal is positive on the 'viewable' side, then > transform the polygon, you can easily tell if you can see that side of the polygon, because the > normal will be wither facing towards you (visible) or away from you (invisible)). > Although the Gs libs don't use normals for backface culling. (I guess this is so you can use primitives without normals, and still get culling). The libs actually look at the first 3 vetices of each polygon, and if they are in clockwise order the poly is drawn. If anti-clockwise then they are not. > A 'normalised' normal is a normal vector with length 1. > Yeah. Unlike the lighting vectors, TMD normals should be normalised, although interesting effects can be acieved with un-normalised normals. Herbs