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From: ciemo@bananapc.wpd.sgi.com (Dave Ciemiewicz)
Newsgroups: comp.sys.sgi
Subject: Re: quad strips bugs ?
Message-ID: <1991Apr29.164513.8056@dragon.wpd.sgi.com>
Date: 29 Apr 91 16:45:13 GMT
References: <9104280445.AA10669@karron.med.nyu.edu>
Sender: news@dragon.wpd.sgi.com (CNews Account)
Reply-To: ciemo@bananapc.wpd.sgi.com (Dave Ciemiewicz)
Organization: Silicon Graphics, CASE Group
Lines: 67

In article <9104280445.AA10669@karron.med.nyu.edu>, Dan Karron@UCBVAX.BERKELEY.EDU writes:
|> 
|> I am trying to use the bgnqstrip and endqstrip commands to draw
|> filled squares.
|> 
|> I keep getting a figure that looks like it is starting at the
|> local origin, then drawing my square, then returns to the origin.
|> 
|> I have worked around it by using the polygon functions, but
|> I was wondering if this was a known problem. My data is good because
|> I get the expected results from the poly calls. 
|> 
|> Cheers!
|> 

Quad-strips are a bit tricky for the uninitiated.  The following example
may clarify things enough for you to be successful.

EXAMPLE
        To create a quad strip for the following diagram:

                    v0       v2       v4
                  (0,1,0)  (1,1,0)  (2,1,0)
                        o-----o-----o
                        |     |     |
                        |     |     |
                        |     |     |
                        o-----o-----o
                  (0,0,0)  (1,0,0)  (2,0,0)
                    v1       v3       v5

        thus drawing the quadrilaterals (v0, v1, v3, v2) and (v2, v3, v5, v4),
        you should specify the vertices in the order:

                bgnqstrip();
                v3f(v0);
                v3f(v1);
                v3f(v2);
                v3f(v3);
                v3f(v4);
                v3f(v5);
                endqstrip();

        Notice that this zigzag traversal is special:

                    v0       v2       v4
                  (0,1,0)  (1,1,0)  (2,1,0)
                        o   _ o   _ o
                        |   / |   / |
                        |  /  |  /  |
                        v /   v /   v
                        o     o     o
                  (0,0,0)  (1,0,0)  (2,0,0)
                    v1       v3       v5

        v0, v1, and v2 (in this example) specify a normal for backface removal
        which is pointing out of the screen or page toward you.

Hope this helps.

-- 

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