## Boundary Topology

Topology is a mapping of logical connections that unifies a collection of subsets of geometric entities into a whole, often called a solid model or just a solid.

The bounding (limiting) of surfaces in a B-Rep topology is accomplished through an operation called trimming. This operation imprints curve(s) into a surface’s parametric space and limits the surface to the portion of the parametric space bounded by that curve and others. The resulting trimmed surface has a non-rectangular parametric space which affords it a great deal of flexibility in modeling complex shapes.

### Volumetric Representation

Volumetric representation is the method of describing a solid object explicitly from a series of solid, space-filling primitives.

Constructive Solid Geometry (CSG) is the method of taking solid primitives and combining them hierarchically using the standard Boolean operations: union, intersection, and difference. The individual solid primitives are typically trivial shapes such as spheres and blocks, but can be arbitrarily complex.

Spatial occupancy modeling involves “digitizing” the region of interest into pixels (2D) or voxels (3D). The relative properties of adjacent pixels are used to define boundaries within the region. X-ray, MRI, and CT scan are examples of spatial occupancy models.

Implicit geometry modeling defines a shape by an implicit function that evaluates to zero on the shape’s surface, a negative value on its interior, and a positive value elsewhere.

### Originating Intent & Software

A geometry model may be created specifically for the purpose of simulation. Whether or not this model is derived from a master model in MCAD software, it will typically include simplifications and abstractions that vary based on the type of simulation to be performed (e.g., solid mechanics, fluid dynamics, electromagnetics).

Simulation models are often created using software other than MCAD, such as software developed in-house or commercial off-the-shelf (COTS) software. This is especially true during the conceptual design phase when software tools like these may be the predominant tool for generating the outer mold line (OML).