Thanks for Dick to pull this topic up so that i can get into more detail.
Softimage copied the function of subpatch from Lightwave since XSI version 1.5 name as subdivision. The idea is to smooth a poly, multyply 4 times of its own. In many cases can hide alot of errors: including double edges and points.
I quote myself "The corner of the mouth "Must" split into 2 edges". Artist better be keeping it as a habbit of making 2 edges rather than 1. It is alright to hide this error by smoothing the surface however, if you are making a low polygon character, this may not be hidden.
I've given 2 different way so of modeling of the corner of the mouth. 1 with single edge and another with double. You can see the wired corner with a visible line expending from the mouth with a single edge. You can tell right away that this person is using a sigle edge. Even with subpatch, it does hide alot of problems because it draws alot more edges in between the polys to smooth it out. But in some face shapes if the corner of the mouth is very extreme such as a big smile. It still have the wierd line expending from the corner.
Both polygons(over 5 sided poly) and triangles will damage your surface. However, a triangles is still better than a polygon.
In a low polygon surface. Triangles will not effect the render in most cases. Because when the software doing render, it is going to divide all the polys into triangles anyway. So if your triangle isn't in a chaotic position it probably won't cost any problem.
Same idea in High polygon surface. Subpatch or smooth or subdiv will try to multiply it by 4 (or 3 if you have a tri) so it works fine.
However a poly will cost much problems. When the software is triangulating the poly it will change its way to draw triangles due to different angles. Yes, diffreren angles of your camera. Which means you'll get a different shapes in different angles.
High poly in subpatch will cost even worse problems. off alot of time it doens't know how to multiply. Some buggy software will cost the result of crashing. Otherwise it will give you wierd shapes. Aspecially when you are making faceshapes blending them using morph targets. Which will cost the poly crashing each others.
