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From: ads@canon.co.uk (Adam Billyard)
Newsgroups: comp.graphics
Subject: Re: 3D Viewing parameters from Foley & VD, etc.
Message-ID: <1991May22.095848.728@canon.co.uk>
Date: 22 May 91 09:58:48 GMT
Article-I.D.: canon.1991May22.095848.728
References: <1991May21.095505.25679@engin.umich.edu>
Reply-To: ads@canon.co.uk
Organization: Canon Research Europe, Guildford, UK
Lines: 28

> I'm a computer programmer of a few years trying to absorb some of the material
> from Foley & VD, etc. re: 3D Viewing. I don't have the book in front of me
> just now, but one problem I need help with is the parameter VPN (View Plane
> Normal). Early on, it seems that VPN points AWAY from whatever you are viewing,
> but in a later diagram, they show it pointing TOWARD what's being viewed.
> 
> So, which is it?

The confusion may lie in the switch of coordinate systems that occurs
during the progression along the viewing pipeline.  The VPN points away
from the COP, toward the VRP.  It is essentially synonymous with the
'LookAt' vectors used by some viewing specifications. 

In World Coordinates the positive Z can be thought of as coming out of
the XY plane toward you, in device or View coordinates, the positive Z
increases as it moves away from you. The VPN however, always points away
from the COP (which is where you are).

	Adam.

PS I found Computer Graphics by Salmon and Slater much more accessible in
their dealings with 3D viewing models than Foley & VD; you might take a
look at it.