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Geometry refers to the physical items represented by the model (such as points, curves, and surfaces), independent of their spatial--or topological--relationships. The
ACIS free-form geometry routines are based on non-uniform rational B-splines (NURBS).
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In addition to manifold geometry,
ACIS can represent nonmanifold geometry. Geometry can be bounded, unbounded, or semi-bounded, allowing for complete and incomplete bodies. For example, a solid can have faces missing and existing faces can have missing edges. Solids can also have internal faces that divide the solid into cells.
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The
ACIS philosophy related to geometric definitions is that the representation be essentially coordinate system independent, numerically stable, and easily transformed. To this end every definition uses classes to represent positions in space, displacements between positions, and directions. In addition, some require scalar values to represent distances or dimensionless quantities. These are simply floating point numbers, and their specific meaning is determined within the transformation algorithm.
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Chapter 2,
How ACIS Uses C++, discusses the underlying math foundation classes needed to define geometry, which help define a model.
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