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Index › Programming Questions & Help › OpenGL and 3D Libraries › rendering quad_strip (or quads)
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rendering quad_strip (or quads) (Read 1107 times)
endasf
rendering quad_strip (or quads)
Jul 7th, 2008, 7:05am  
hi;
i'm trying to render an octohedron with a QUAD_STRIP shape...
i know that there is something obvious i am missing here, so i thought i would post it;
my length is defined in another area of my code, but you get the idea; the problem is that it's not "wrapped" correctly...
could someone please either expound upon the standards used to do this type of thing with a given number of vertices or point me to a good web resource?

fwiw, my cd3 function just maps signed, normalized floats to processing coords.

Code:
beginShape(QUAD_STRIP);
c3d(len,0.0,0.0,'v');  
c3d(-len,0.0,0.0,'v');
c3d(0.0,len,0.0,'v');
c3d(0.0,-len,0.0,'v');
c3d(0.0,0.0,len,'v');
c3d(len,0.0,0.0,'v');  
endShape();


thanks
e
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...endasf...
 
HackerLastPip
Re: rendering quad_strip (or quads)
Reply #1 - Jul 7th, 2008, 7:52am  
I think you're going to have a hard time making an octahedron out of quadrilaterals, as the constituent part is a triangle.  Here's some code that does it with TRIANGLE_STRIP:

 beginShape(TRIANGLE_STRIP);
 vertex(len, 0.0, 0.0);  
 vertex(0.0, len, 0.0);
 vertex(0.0, 0.0, len);
 vertex(-len, 0.0, 0.0);
 vertex(0.0, -len, 0.0);
 vertex(0.0, 0.0, len);  
 vertex(len, 0.0, 0.0);
 endShape();
 
 beginShape(TRIANGLE_STRIP);
 vertex(len, 0.0, 0.0);  
 vertex(0.0, len, 0.0);
 vertex(0.0, 0.0, -len);
 vertex(-len, 0.0, 0.0);
 vertex(0.0, -len, 0.0);
 vertex(0.0, 0.0, -len);  
 vertex(len, 0.0, 0.0);
 endShape();

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koogy
Re: rendering quad_strip (or quads)
Reply #2 - Jul 7th, 2008, 9:44am  
how are you getting an octahedron from 6 points (2 of which are the same)?

with openGL quad strips are specified as follows with the points zigzagging, if you see what i mean.

Code:

1--3--5--7
|  |  |  |
0--2--4--6


search for quad strip here for better, non-ascii diagram
http://web.cs.wpi.edu/~matt/courses/cs563/talks/OpenGL_Presentation/OpenGL_Presentation.html

but processing may be different
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koogy
Re: rendering quad_strip (or quads)
Reply #3 - Jul 7th, 2008, 1:19pm  
koogy wrote on Jul 7th, 2008, 9:44am:
how are you getting an octahedron from 6 points (2 of which are the same)


ah, i see, now that i have woken up.

i think you need two TRIANGLE_FAN shapes and not TRIANGLE_STRIPs. first vertex should be the centre.
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endasf
Re: rendering quad_strip (or quads)
Reply #4 - Jul 7th, 2008, 6:06pm  
@Hacker-- good point.

Sorry that I missed that-- I never understood the principle behind the different drawing modes in the first place.

I suppose if I really wanted an all-in-one solution, I could use POLYGONs all the time, but I know from a bit of reading that this is way more expensive...

Interesting.

@Hacker/Koogs
Do either of you have a solid reference for this type of thing outside of the OGL redbook/guide?  I suppose I'm looking for info on computational geometry, eh?

Thanks,
e
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...endasf...
 
HackerLastPip
Re: rendering quad_strip (or quads)
Reply #5 - Jul 7th, 2008, 6:37pm  
This page is absolutely fantastic, though it gets quite dense at times --   http://www.euclideanspace.com/    

I have also consulted this page -- http://www.lighthouse3d.com/opengl/tutorials.shtml

You can also try reading the forums of various 3D game engines ( Irrlicht, CrystalSpace, JavaMonkey Engine) which have been known to contain nuggets of wisdom.

Borrowing a book on 3D programming wouldn't be a bad idea if you have no clue how 3D math works, though in general I find the 3D programming doorstops overpriced and light on information.

On another note, while I consulted earlier code to write that (my first 3D form was an octahedron), I drew a simple diagram to get everything sorted out.  Without fail, making simple drawings or diagrams is the best help -- you can wing 2D but 3D is just too complicated!
                                                                                               
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endasf
Re: rendering quad_strip (or quads)
Reply #6 - Jul 7th, 2008, 6:49pm  
awesome.
that first link contains exactly what i was looking for (i don't really understand 4d points yet, so i don't want to even look at them! Smiley )...

i really appreciate the references; this will get me going.
btw: i agree with you that many times 3d gaming books can be overwhelming for the wrong reasons.

thanks
e
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...endasf...
 
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