Path: chuka.playstation.co.uk!scea!greg_labrec@interactive.sony.com
From: mperdue@hrtc.net (Mario Perdue)
Newsgroups: scea.yaroze.programming.3d_graphics
Subject: Re: Wireframe stuff
Date: Thu, 04 Sep 1997 04:02:51 GMT
Organization: SCEA News Server
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On Wed, 03 Sep 1997 22:31:29 -0400, Wilf LaLonde
<wilf@objectpeople.on.ca> wrote:

>Mario Perdue wrote:
>> 
>> ... To do a perspective projection. You can use the
>> following:
>> 
>> x_screen = d * x/z;
>> y_screen = d * y/z;
>> 
>> where:
>> 
>> x_screen, y_screen    is the screen location
>> x, y, z               is the 3D location
>> d                     is the viewing distance from the screen
>> 
>But this won't work if you want to draw 3D lines on top of
>existing 3D objects (e.g., to SELECT it). You need the
>correct camera to screen transformation that is buried
>somewhere inside the renderer. Perhaps you know how
>to get hold of it???

Wilf,

That's not the question I was answering, but the transformation should
still work. You would have to first transform your real world
coordinates into viewspace coordinates, then apply the perspective
transformation. However, I would think that you could use GsGetLs() to
obtain a local-to-screen xform matrix for the object in question. Then
you could apply that matrix to the points on the object that you want
to connect with lines. I havn't tried this, but it seem reasonable.

Mario