MODELLING CONCEPTS
Local Geometry.
A subdivision surface model is considered to be made up of many local geometries. This is one of the key understandings you will need to successfully create a quad based subdivision surface model.
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Consider the cylinder:
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It is actually made up from 3 pieces of local geometry connected by two boundaries. Within each side of each boundary there is a control loop to control the curvature.
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In the example below the local geometries are in red and the boundary areas connecting them are in blue.
The control Loops are marked in red.
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Efforts have been made to ensure that the connectivity between the boundaries and the local geometries have the same vertex counts in order that the best flow for this object is maintained but even this is not necessary. If the local geometries had differing vertex counts then we can use linear and radial connection reduction techniques (which I will describe in detail later) in order to connect them. These techniques are usually applied at the borders or on the flatest available part of the mesh near to a border.
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When considering the weight of geometry for a mesh, and quad spacing/size, each of these local geometries should be considered seperately for UV mapping. It is often thought that a fairly even distribution of quad size should be maintained across an entire mesh but this is not true. Instead, the quad size within each local geometry should be the roughly the same.
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With this in mind, any object can be broken down into much smaller tasks and quads become very easy to maintain while ensuring you have excellent control over the curvature of the edges between these local geometries by using simple, consistent boundaries.
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Even the most complicated shapes are simply collections of local geometries connected by boundaries. This makes modelling, UV unwrapping and texturing of objects a relatively simple process.
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It may appear complex but when broken down into its local geometries it becomes much simpler. It is this combination of local geometies and boundaries which allow us to keep meshes light.
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local geometries (red) and boundary regions (blue) for a cylinder