Path: chuka.playstation.co.uk!scea!greg_labrec@interactive.sony.com
From: Tim O'Neil <goneil@chat.carleton.ca>
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: Re: Accessing screen translated vertices
Date: Sun, 02 Nov 1997 02:11:25 -0500
Organization: SCEA News Server
Lines: 37
Message-ID: <345C279D.BD5E35D4@chat.carleton.ca>
References: <345BFB81.51488BFC@ibm.net> <345C1288.E288E7A@ibm.net>
NNTP-Posting-Host: obatanga.chat.carleton.ca
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)

Well, you're right, the process (in theory) is sorta complicated, but
really its very simple to do.  To do the three transformations you talk
about, its basically a process of multiplying the points by one matrix
for each and then doing a division relative to your projection
distance.  Fun thing about matrices is that if you multiply them all
together before multiplying the point by them, you get the same
result... guess where this work is done... GsGetLs ... the matrix that
GsGetLs gives back to you is the hierarchical transformations put
together along with the viewpoint shift, so all you have to do is take
your WorldSpace coords and multiply them by this matrix and then divide
them by the Z value.  Sorta like this (this is just from memory, so its
sorta rough)

GsCOORDINATE2 WhateverCoordSystem;
GsInitCoord2 (WORLD, &WhateverCoordSystem);
GsGetLs (&WhateverCoordSystem, &tmpls);
/* this gets the proper matrix relative to pure world coordinates*/
Vertex worldPoint = {0,0,-1000};
Vertex newPoint;
multiply (newPoint, worldPoint, tmpls);
/* assuming this function multiplies worldPoint by the tmpls matrix and
puts the result in newPoint...
   I've set up my own inlined functions for this to cover over the nasty
playstation naming, but you get the point */
short screenX = (newPoint.x * projectionDistance)/newPoint.z;
short screenY = (newPoint.y * projectionDistance)/newPoint.z;

....
This will give you the x,y screen values that 0,0,-1000 would map to
given the current viewpoint etc.  To get these points relative to
another system (ie. the x,y coords of an animated figure that you have
in another coordinate space) all you would have to do is put a different
coordinate system into the GsGetLs function.

That is pretty much it.  Hope this helps.
Tim