Path: chuka.playstation.co.uk!news
From: James Russell <jruss@ibm.net>
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: Clipping using fog param
Date: Thu, 16 Oct 1997 14:13:54 +1300
Organization: PlayStation Net Yaroze (SCEE)
Lines: 33
Message-ID: <34456A52.461D08FB@ibm.net>
NNTP-Posting-Host: slip202-135-86-126.ak.nz.ibm.net
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Mailer: Mozilla 4.01 [en] (Win95; I)
X-Priority: 3 (Normal)

Hi,

I've just started doign 3D stuff so forgive me if this is stoopid
question:

I want to have a 3D object cut through a 2D background. You see the
background, and then the 3D object slowly rises though the background
and becomes visible. The obvious problem with this is that because the
PSX is a polygon drawer, not a renderer, when the object cuts the
background plane it's polygons will "pop" out (suddenly appear) as their
average Z value becomes nearer than the Z of the background plane.

What I *want* is for the polygon to cut through the background as a
proper renderer (eg Lightwave) would do it. Mathematically, this would
involve calculating the intersection of the background plane and each
polygon, and basically involve recreating the object on the fly. This is
HEINOUS and don't want to have to do it.

However, please correct me if I'm wrong, but isn't there some built in
fogging? Would it be possible to configure the fog to "cut out" at a
particular Z value, instead of gradually fading out? This would give me
the desired effect.

This won't work if all the fogging does is look at the average Z of the
polygon and adjust the transparency for it. If this is the case, the
whole polygon will fade out and the I'm back where I started.

Are there any documents describing fogging details? Once again the docs
prove sparse...

Ta 

J