Edge Loop

An edge loop, in computer graphics, can loosely be defined as a set of connected edges across a surface. Usually the last edge meets again with the first edge, thus forming a loop. The set or string of edges can for example be the outer edges of a flat surface or the edges surrounding a 'hole' in a surface.

In a stricter sense an edge loop is defined as set of edges where the loop follows the middle edge in every 'four way junction'. The loop will end when it encounters another type of junction (three or five way for example). Take an edge on a mesh surface for example, say at one end of the edge it connects with three other edges, making a four way junction. If you follow the middle 'road' each time you would either end up with a completed loop or the edge loop would end at another type of junction.

Edge loops are especially practical in organic models which need to be animated. In organic modeling edge loops play a vital role in proper deformation of the mesh. A properly modeled mesh will take into careful consideration the placement and termination of these edge loops. Generally edge loops follow the structure and contour of the muscles that they mimic. For example, in modeling a human face edge loops should follow the orbicularis oculi muscle around the eyes and the orbicularis oris muscle around the mouth. The hope is that by mimicking the way the muscles are formed they also aid in the way the muscles are deformed by way of contractions and expansions. An edge loop closely mimics how real muscles work, and if built correctly, will give you control over contour and silhouette in any position.

An important part in developing proper edge loops is by understanding poles. The E(5) Pole and the N(3) Pole are the two most important poles in developing both proper edge loops and a clean topology on your model. The E(5) Pole is derived from an extruded face. When this face is extruded, four 4-sided polygons are formed in addition to the original face. Each lower corner of these four polygons forms a five-way junction. Each one of these five-way junctions is an E-pole. An N(3) Pole is formed when 3 edges meet at one point creating a three-way junction. The N(3) Pole is important in that it redirects the direction of an edge loop.