Wings 3D/User Manual/The Nature of Subdivision Modeling/The Nature of Edge Loops

The Nature of Edge Loops
Edge Loop is a term you will see used often in polygon modeling circles. It is often confusing to new users just what edge loops are and what their significance to modeling is. Edge loops can be thought of in two senses:  A series of edges connecting vertices that are shared by four faces. In the aesthetic sense, the controlling elements that define the overall form of the object you are trying to model. 

In the first sense they are important to Wings because if you maintain them well you can breeze up and down the edge loop chain with the F3 and F4 keys to do all sorts of tweaks and adjustments in no time. Let them fall apart and you'll be selecting things manually by hand constantly to make such adjustments.

But it's deeper than that. It has to do with how the overall form of the model will eventually turn out; whether or not it flows. I am told that Bay Raitt first coined the term as a way to create and maintain overall aesthetic control over a model, to use them to mimic the structure of the object, such as muscles.

So you can see that there are edge loops and there are edge loops. What I mean by that is that there are the actual loops themselves, composed of a series of edges all joined together at non-pole intersections, and there are the edge loops that define the form and nature of your model. Learning about the former is relatively easy and is necessary in order to develop the latter. But the real power and beauty of edge loops is in the latter, in the way they control and define the form and detail of the model. Here is where the true art occurs.

As stated before, technically, edge loops are a series of edges joined together at non-pole intersections. A non-pole intersection is an intersection of four edges at a given vertex. This is why quads usually help maintain edge loops, they help minimize poles. The edges that encircle an arm are edge loops.

In Wings, there are tools that let you take advantage of edge loops in your modeling. You can use Loop Cut to cut off parts of the model when needed. You can select by edge loops and by edge rings (series of opposing edges of adjacent quads), and then work with that selection. You can use the F3 and F4 keys to march up and down a chain of edge loops to do rapid model adjustments. And of course there is the all-important central edge loop.

The central edge loop is important if you want to use Mirror or Virtual Mirror to speed up your modeling. If you maintain your central edge loop, the loop that runs entirely around the long axis of your model, then you will be able to model on one half of the model, Loop Cut off the other half, and then Mirror, which will cut your modeling time in half. By maintain, I mean to keep the edges straight and not have it interrupted by poles. The first point is an absolute requirement, or very strange things will happen when you mirror. You can work around the second point. If when you go to select the loop it only partially encircles your model because of pole interruptions, you can manually select other parts to get a full loop for the Loop Cut operation.

Poles are vertices where more or less than four edges converge. An edge loop can only extend past a vertex at which four edges come together. If you think of a polygonal mesh as a city map with edges for streets, faces for city blocks and vertices for junctions, then drawing an edge loop means going straight on at four-way junctions and stopping at any others.

Keep in mind that quads (faces with four edges) do not ensure the absence of poles. To demonstrate this, create a cube and smooth it once. Now select an edge one level down from the top vertex. Do a Select | Edge Loop. The selected loop will not run all the way around the object, despite the fact that it is made entirely of quads. Examine where it stopped and you will see that there is indeed a three edged pole present where it stopped. Connect that vertex to the top most vertex (thus creating two triangles) and try the experiment again. The edge loops now runs on longer, despite the fact that there are now triangles in the model. Why? Because there are now four edges	at 	that 	intersection; you have eliminated one pole, which was not necessarily a good thing to do.