MODELLING CONCEPTS
Loops.
We need loops in subdivision surfaces; lots of loops.
Ideally full loops which circle back to their original starting point. Learning to identify the various types of loops in use on a model will be very useful!
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In subdivision surface modelling, we don’t think in terms of polygons, we think in terms of areas. These areas both contain, and are surrounded by, loops. Loops are called "netted", when they cross between local geometries. There are three kinds of loops we will come across. The yellow areas on the diagram below indicate the areas of safety for our control loops to slide between to control the curvature.
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Control Loops.
These are the main event and they control our curvature around the corners and edges of an object. We create areas of safety to slide them between. In subdivision surface modelling, we don’t model the curvature which describes edges and corners; we control it with loops. I often borrow the 'mark seam' functionality from the UV system in blender to help identify my control loops as it marks them in red. Once a model's overall shape has been modelled, these are the loops we can move to control edge curvature.
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Structural Loops.
These define the silhouette of the object and normally live at the very centre of a boundary section. Moving and reshaping these loops changes the main aspects of a model's shape. These are the main skeleton of your shape and are normally bordered by one or two control loops. They define edges, corners and extreme changes of direction which would be beyond what we would otherwise call curvature.
Holding Loops.
These aren’t meant to be moved. They are meant to keep areas of your geometry in place. They describe the areas of safety for our control loops. If the are moved, they describe the overall curvature of your object or pin certain details of your mesh in place, they are not used to manipulate the curvature between areas (local geometries).
Netted Loops.
These cross through multiple local geometries (through a border region) and aren’t meant to be moved. They describe the curvature within an area of a model and are often manipulated using cubic techniques such as the "Curve" tool, "G-stretch" or smoothing and sculpting operations. They will often encircle your model several times in seemingly crazy directions making them difficult to work with as full loops.
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Terminating Loops.
These will have four N-Poles surrounding them and are used to ensure surfaces are flat. They will contain four or more N-Poles.
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Types of loops on a Subdivision Surface cylinder