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Like subdivision surfaces, NURBS are inherently a smooth surface surfaces and suffer none of the faceting artifacts that are associated with polygonal approximations. In fact, RenderMan will always efficiently render a NURB as a smooth surface. Unlike subdivision surfaces, however, NURBS have a inherent topological constraint: its a NURB's control surface is a rectangular mesh of rows and columns. While this means that their a NURB's parametric space used for texturing is easy to understand (it's a rectangle!), it also means that when creating a complex organic shape like a human character, it is now the job of the modeler to divide that complex shape into a collection of rectangular pieces, each of which is a separate NURB. Moreover, if more detail is required at some section of the mesh which requires the addition of extra knots (an intersection of a row and a column), additional rows and columns must be added across the entire NURB in order to satisfy the rectangular mesh constraint. This can very quickly lead to unwieldy models.