question

# 1 06-09-2010 , 07:24 PM

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Join Date: May 2010

Posts: 37

this may sound stupid but it has been bothering me for quite a while, i did a 3d animation course but it wasnt video game oriented so i don't really know, from what i've learned modelling in triangles isn't very animation friendly because they don't bend well and they create distortion and to only model in quads, but recently i've been looking at 3d models optimized for video games and i noticed that they were pretty much all made of triangles, for example this one

user added image

so i'm wondering, in video game industry do you have to model only in triangles, if so why?

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# 2 07-09-2010 , 06:49 AM

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Location: Manchester Uk

Posts: 6,300

You dont have to model in tris, model in quads, its easier then triagulate the mesh, though it does not matter if there is the odd tri in there (same as normal modeling).

Tris are needed as this is the default for a games engine, it does not need to tesalate the geo when running.

When you render a quad mesh in maya it converts it into tris as part of the rendering process, you just dont see it do it.

"No pressure, no diamonds" Thomas Carlyle

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# 3 12-12-2010 , 12:52 AM

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Join Date: Apr 2007

Posts: 73

A quad mesh LOOKS better to me visually, but I've been wondering the same thing for awhile now. In most instances I know my mesh got converted into tris by the software, sometimes like for realflow I've converted my Mesh to tris before exporting. Is there a reason quads are considered the 'Way to Go'?. Sometimes my experience modelling would be so much easier if I was allowed to turn that 5 sided poly into 3 tris.... My GUESS has always been that it produces 'Poles'??

Learn from others mistakes, it takes too long to make them all yourself.

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# 4 12-12-2010 , 11:52 AM

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Location: Michigan, USA

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The short answer is a model made up of quads will smooth, that is to say subdivide, more evenly then a mesh made up of triangles.

The reason, which is a much longer answer, is because of the mathematical algorithm used by most 3D applications to subdivide works best on an all quad mesh. The smoothing method is called Catmull-Clark, named after Ed Catmull, and Jim Clark (one of the founders of SGI) in 1978.

The algorithm is quite simple to describe. It is recursive. Lets look at two examples the quad and the tri-angle. Imaging you have a single quad polygon that has four vertices, one at each corner and four edges connecting these verts. Now add one vertex at the centroid of the quad and draw an edge from the center of each of the polygons original 4 edges to the vertex at the center. What do you get?

A: a new polygonal surface with 4 quad polygons. Now recurse, by repeating the process on the four new polygonal faces. So for iteration two you get 4x4 = 16 quads, iteration 3 you get 16x4 = 64 quads and on and on it goes. With each iteration you get a smoother and smoother surface. Furthermore, the closer the starting quad is to planar and square the better the result.

Now lets consider the triangle. The application will use the same algorithm. So we start with a triangle with 3 verts and 3 edges and place a new vert at the centroid of the triangle and connect an edge from that point to the middle of each of the three edges and what do we get?

A: New polygonal object with 3 QUAD faces! But the resulting quads are more diamond shaped, which are referred to in mathematical terms as a deltoid or a kite. So for iteration 1 we get 3 diamond shaped quads. For iteration 2 we get 3*4 = 12 quads. Iteration 3 = 12*4 = 48 quads. But the important thing is the original three quads are kite shaped and do not smooth evenly and with each iteration this uneven smoothing can become compounded.

There are algorithms that work on triangular meshes better the Catmull-Clark but they are not widely used so quads remain the preferred base mesh for any model that is intended to be smoothed.

Also, one must also consider that edge loops run more evenly through a quad mesh then a triangular mesh. I could go into the mathematical proof of that as well but it would take more text then most would care to stomach so I will have to ask that you just take me at my word on it.

ctbram

"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675

Last edited by ctbram; 12-12-2010 at 12:05 PM.

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