3D Modeling Question - Why only quads?

I've been trying to find out why using quads is the best method for modeling.

My questions are: Why is having a triangle in a 3d model not acceptable?

Does it affect any physics Engines it may be loaded in to?

Does it slow down rendering or make for unsafe geomatry?

e.g. I'm modeling the leg portion of a character. In order for the knee to bend and deform properly I need to create edges at the front and connect them to the one edge at the back of the knee. This of course creates a triangle instead of a quad around the side portions of the leg.

Any info would be appreciated.

My advice is go with whatever works best for your game. Aren't all quads translated into triangles anyway in Blitz3d? I might be wrong about that...

It's purely for artistic reasons, it has nothing to do with physics engines, rendering, unsafe geometry or anything else. I'm not an artist, so probably an artist will explain the artistic reasons, but I would guess that it's about creating good edgeloops, geometry that will deform well when animated, etc.

And yes, there's no such thing as quads to a 3D card. It's all triangles by that point.

When I posted a model on here some time ago there was some dissaproval because the model wasn't entirely modelled with quads and there were some stray triangles near the hip portion.

Yes all quads ar made basically of 2 triangles but there appears to be dissaproval of modeling that in't pure quads.

3impact for instance I think doesn't allow stray traingles or vertices. It's difficult trying to model as it is whithout having to try and make everything a quad.

[edit] Thanks for the replies. Yeah Edgel loops are another thing that's difficult to conform to. But it's nice to know that it doesn't matter if you have a stray triangle or not.

Well I haven't used 3Impact in a while, it's true, and it is a pretty ghastly little engine for its peculiarities and arbitrary rules ( not to mention some out and out lies ) but the only issue I'm aware of with it is that you can't have unwelded objects if you want to cast shadows, because their engine makes no attempt to fix stencil volumes built from open geometry. That wouldn't have anything to do with quads though. You could just as easily have an unwelded quad as an unwelded triangle. If that's not the issue you're referring to, you'd have to link me to more details on it for me to be able to say anything further.

quads are very handy for modeling just simplify your model a bit makes it easyer to work with rather than a dence mass of lines. Ive got to be honist though when Cinema4d triangulates a model, most of the time you have to turn alot of edges to get it to deform well.

I found this tutorial helpful when i studied subdivision modeling :

http://maxrovat.sns.hu/subdiv/subdivmodeling.htm

Quads deform better, it all depends how low poly you're going.

With quads you get nice edge loops, and quad model is easy to subdivide/reduce and all that. It is just a neat and pro way of doing things because it has it's advantages. But I don't consider few tri's to be a terrible thing.

You can make a high-poly mesh and reduce it to a bumpmap plus low-poly if you use quads. If you are not doing that, then it doesn't matter what you use.

I always thought a quad was always 2 triangles anyway (under the hood) even though a modelling prog will treat a "quad" as if it is strictly a different type of geometry, it isn't really, not as far as the hardware/renderer is concerned.
At least that used to be the case, unless i missed something :/

I'm guessing most engines triangulate quad-based models anyway (except for .obj maybe)?

all quads are 2 triangles, a mesh, by definition, is composed of triangles. I think they use quads to simplify texturing, plus, for me, they're easiest to work with.

they are just cleaner and faster to work with, more predictable. So if your not trying to make super low poly meshes where every single tri counts you should try and work with quads.

Mustang mentioned many of the modeling reasons, but edges chamfer easier too without leaving nasty pinching in your meshes when you meshsmooth.

Its not such a big deal with static meshes, or if your doing low poly work without normal maps. Just normal maps are pretty mainstream now and not considered next gen anymore unless your using blitz3D.

a quad by definition sadly isn't 2 triangles. thats the main point to use them.
You can use, at least in OpenGL, don't know about DX, other surface types to make them smooth curved.
Ever played Q3, then you know how it looks like

Why Quads its simple for good Topology and model refinement and a must for subdivisional modelling. The Quad model is easily converted into bezier patch/curves the fact that the quad surface can be divided into Four Smaller Quads of a similar form is what gives it its edge over Triangles.
When Dividing A triangle it is difficult to maimtain a similar form so the polycounts soon exceed the visual gain.
Edge and RingLoops these are excellent modelling methods which are simplified by the Quads when used on triangles the Loops become jagged and hard to refine causing apps to heavily use vertex pull/push methods which are almost archaic in technology terms
As for textureMapping more vertexes = more detail Quad/Tri make no actual difference the only improvement is the number of vertexes used There is the Fact that a Quad should only have 4 and 2 Tris can have 4 or 6.

For low-poly games, do what works for you.
For movie-quality meshes, make it quad faces to avoid nasty lighting anomalies when rendering a sub-divided mesh.

Some places you just can't avoid doing triangles, especially if you wish to make a light model polycount wise.

if you are doing a really low poly model then tris is the way to go, but these days quads are used because its just easier to visualise when modelling if you work in quads. its alll about flow and edge loops now.
think about it, the less visual clutter on the screen the easier it is to produce a clean looking model.

also most people doing next gen export the model to zbrush or mudbox and tris don't work very well for sculpting in these packages.

Triangle Strips are supposedly the most efficient and quads are just an abstraction of that - ie quad strips.

Thanks for all the replies and info. :)

Here's another question regarding modelling.

IF I were to combine objects would this affect any animation I were to set up?

e.g. I have a cube for the body. I cylinder for the arm. I don't weld the cylinder vertex by vertex to the cube instead I just use the option in Truespace to combine objects together. Then export as single mesh.

The two objects are seperate and not welded or joined together they are just combined in to a single mesh.