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AltRMB — What exactly does it do? After subdivisions or loop cuts, AltRMB does not seem to select some loops.

What should one do to define a loop? How can I use select edge loops efficiently?

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An Edge Loop is a chain of connected edges where each edges shares a vertex with the previous edge, and that vertex has exactly 4 edges attached to it (except on the borders of a non-manifold (not "water-tight") mesh, where there will be only 3 edges, 2 of which have just one face connected).

An edge loop is terminated where the vertex has more than 4 edges (poles) or meets a non-manifold edge (an edge with only one face) or an Ngon (face with more than 4 verts). The latitudinal loops (rings) of a UV sphere are perfect non-terminating loops, but the longitudinal loops (segments) all end at two points (the North and South poles)

Here are some examples of edge loops (see where they change direction or terminate and why)

Simple

S-shape

P-shape

Details on the algorithm from the wiki:

  • First check to see if the selected element connects to only 3 other edges.
  • If the edge in question has already been added to the list, the selection ends.
  • Of the 3 edges that connect to the current edge, the ones that share a face with the current edge are eliminated and the remaining edge is added to the list and is made the current edge.
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There is no option to tell Blender, that something is an edge loop. An edge loop is defined by the topology of your mesh. It is a loop consisting of edges.

From Wikipedia:

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.

See Blender Wiki for more details: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Meshes/Selecting/Edges

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    $\begingroup$ That link is useful, but this is currently an essentially link-only answer... $\endgroup$ – wchargin Oct 31 '13 at 23:00
  • $\begingroup$ You're right. I put more info into the answer. $\endgroup$ – Maccesch Nov 1 '13 at 10:20

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