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NeHe OpenGL 3d-Triangles Vs. Quads?

Hi,

I'm a little confused.

Was looking at the NeHe SDL1.2+OpenGL 3d tutorial lesson 23 sources.
It displays a 3d world with floor, ceiling, & walls + movement with [ARROW] keys & aim with mouse.
This lesson loads in a map file, but the information loaded to display 3d world is triangles?

Why use triangles? Was thinking quads would be a better choice but perhaps I am wrong?
(the 3d demo draws squares for floor, ceiling, and walls - so was thinking quads would be better)

Can someone explain why triangles are used in this demo and not quads?

Also, how was the map file created? Is there some program I can install on Ubuntu 10.04
that would allow me to make a similar map file? (can't see typing out a larger map)

Thanks!

I don't know the exact reason for triangles, but I believe it's because triangles are more versatile - I have a friend that does 3d modelling and that's all using triangles.  I'm sure there's a better reason, but for most complex shapes, it's just easier.  If you're just doing boxes with no textures, quads is fine.  If you're doing a complex 3d shape, triangles are the way to go.

On Wed, Jun 30, 2010 at 9:02 PM, JeZ-l-Lee wrote:

Quads are usually converted to triangles before rendering, and quads
don't exist in OpenGL 3.x.

-Casey

On Wed, Jun 30, 2010 at 9:02 PM, JeZ-l-Lee wrote:
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Hi,

Thanks for the response.

I wish to make a simple 3d game with square tiles for floor, ceiling, and walls.
The squares would have square textures.

Would be OK to use quads for this?

Jesse


On 06/30/2010 09:06 PM, Alex Barry wrote:

As Casey pointed out, quads get converted into two triangles, so it's probably better practice to just use triangles.

On Wed, Jun 30, 2010 at 9:11 PM, Jesse Palser wrote:

Hello !



Put two triangles together and you have a quad.

In 3D programming triangles are the most used things, because
a single triangle is always flat, no matter how you set the three points.

This is important for algorithms like, is the triangle viewable on the screen
or only parts of it and so on.

It would be a lot harder with things like real quads, that can be in a non flat form.

So for a wall you just glue two triangles together and using the correct
texture UV coords, you get what you want.


CU
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Triangles are the basic building block of any polygon. All polygons
can be built from triangles. This is the reason graphics hardware and
a lot of graphics software use triangles as their graphical primitives
(with some notable exceptions like line segments and points.)

On Wed, Jun 30, 2010 at 9:02 PM, JeZ-l-Lee wrote:



--
http://codebad.com/
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Hi,

So I must use triangles...I accept that.
But am still confused...

My textures would be squares - for example 256x256.
How do I draw one half into one triangle and the other half in another
triangle?

Please forgive my ignorance, I've never done 3d game programming before.
Thanks!

Jesse


On 06/30/2010 09:25 PM, Torsten Giebl wrote:

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Just to chime in, I've had success with just using a DrawQuad function
that internally renders as triangles, but I remember when I rewrote my
program to use triangles instead of quads and found no appreciable
speed up. If you ever work on high-class stuff, you may run into a
situation where the difference is noticeable.

As a further point, though, it is pretty common to crunch things to
display lists, vertex buffer objects or whatever if you need more
speed. Turning triangles into quads naively also means extra vertices
(previous techniques solve this problem).

Although this sounds hard, OpenGL will do the hard part for you--just
pretend like the triangles are quads and list the texture points
beside the appropriate vertices.

Say you want a quad using the points [x1, y1], [x2, y1] to [x2, y2],
[x1,y2]. Just render triangle one at [x1, y1], [x2, y1], [x1, y2]
using tex coordinates: [0,0], [1, 0], [0, 1]. Then the second triangle
is at [x2,y1], [x2, y2], [x1,y2] with tex coordinates [1,0], [1, 1],
[0,1].

To reiterate, though, write it as quads to get something up and
running, maybe it will be fast enough for your needs. If this is your
first project, save the high-performance tuning for your second
project

I think someone mentioned the Redbook not too long, this is an
excellent resource to accompany online tutorials:
http://www.glprogramming.com/red/

John
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Hello !



The UV texture coordinates you assign, basically cut out a triangle form of the texture.

Look at Lesson 6 of the NeHe Demos, he explains on the site how the cut out is done.

If you want to be successfull, it is important that you start from the first lesson
and then go on from there.

Back in 2001 bought a book about "OpenGL Programming" :

http://www.amazon.com/OpenGL-Programming-Prima-Techs-Development/dp/0761533303/ref=sr_1_11?ie=UTF8&s=books&qid=1277949252&sr=8-11

As from this year, it tolds nothing about shaders, but it explains nearly everything from the beginning to
generating 3D landscapes and how to build a simple engine, loading player models and so on.


CU
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This is the book I learned from, years back (1st or 2nd ed, I
recommend you get the 4th ed as it's much updated):
http://www.opengl.org/sdk/docs/books/SuperBible/

On Wed, Jun 30, 2010 at 9:30 PM, Jesse Palser wrote:
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Using a triangle strip with 4 coplanar vertices is the same as drawing a quad.  Something like this:
glBegin(GL_QUADS);glTexCoord2f(0, 0);
glVertex3f(0, 0);
glTexCoord2f(1, 0);
glVertex3f(1, 0);
glTexCoord2f(1, 1);
glVertex3f(1, 1);
glTexCoord2f(0, 1);
glVertex3f(0, 1);
glEnd();


glBegin(GL_TRIANGLE_STRIP);glTexCoord2f(0, 0);
glVertex3f(0, 0);
glTexCoord2f(1, 0);
glVertex3f(1, 0);
glTexCoord2f(1, 1);
glVertex3f(1, 1);
glTexCoord2f(0, 1);
glVertex3f(0, 1);
glEnd();



Jonny D



On Wed, Jun 30, 2010 at 7:01 PM, Justin Coleman wrote: