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A CAD geometry layer is the computational core that represents precise shape, connects its parts into a model, and provides algorithms to construct and modify that model. Its editable data is not the triangle mesh shown in a viewport: triangles are a derived display approximation, while a boundary-representation (B-Rep) model retains mathematical geometry and topology.
What a CAD geometry layer does
A geometry kernel sits beneath the application’s feature commands and viewport. It represents mathematical entities such as points, curves, and surfaces, evaluates them, and provides operations for assembling and changing shapes. A curve or surface can exist as geometry on its own; it becomes part of a model boundary when the topology layer connects it to other entities.
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This distinction is central to B-Rep modeling. Autodesk’s Fusion API documentation puts it directly: “A B-Rep model is defined by two things; topology and geometry.” Autodesk Fusion API: B-Rep and geometry
Geometry and topology are different kinds of information
Geometry defines shape mathematically: for example, a plane, a circle, or a curved surface. Topology defines how model elements connect and which portions of geometric entities bound the shape. Geometry answers what the supporting shape is; topology answers how the model is assembled and bounded.
A planar face with a hole makes the distinction concrete. The plane is the supporting geometry, but it does not by itself specify the face’s outer boundary or opening. Topological loops define those boundaries. A solid B-Rep is a closed volume bounded by connected surfaces, enabling operations that depend on a well-formed solid, including mass-property calculations. Autodesk Fusion API: B-Rep and geometry
The usual B-Rep hierarchy
Topology is commonly organized from smaller boundary elements into larger structures:
- Vertex: a topological point that connects edges.
- Edge: a bounded portion of a curve, usually connecting vertices.
- Wire: an ordered set of connected edges that forms a boundary loop.
- Face: a bounded region on a surface, defined by its boundary loops.
- Shell: a connected collection of faces.
- Solid: a closed volume bounded by faces.
The hierarchy describes connectivity, not a substitute for the mathematical geometry underlying edges and faces. Open CASCADE’s topology material and Autodesk’s B-Rep documentation describe this separation and structure. Open CASCADE modeling algorithms guide Autodesk Fusion API: B-Rep and geometry
What belongs in the kernel—and what belongs in the application
The kernel’s role is to represent and compute on shape. The application’s role is to make that capability useful as an editable document and user experience. The exact division varies with the product and chosen kernel; the following is a practical design split, not a universal standard.
Rank #3
| Layer | Typical responsibilities |
|---|---|
| Geometry representation and evaluation | Points, curves, surfaces, parameter evaluation, intersections, and geometric transformations. |
| Topology and shape construction | Vertices, edges, wires, faces, shells, solids, and their connectivity. |
| Modeling algorithms | Primitives, shape building, Boolean operations, fillets, offsets, features, and validity checks, according to the application’s scope. |
| Application and document model | Feature history, parameter dependencies, semantic references to model elements, persistence, undo and redo, and recovery when an operation fails. |
| Visualization | Deriving renderable meshes from the model and refreshing them when the model changes. |
Open CASCADE’s modeling-algorithm guide documents separate geometric and topological tools, with APIs for shape building, primitives, features, fillets, and offsets. Its guide is for version 6.8.0, so use it to understand the breadth of kernel responsibilities rather than as current, version-specific API instructions. Open CASCADE modeling algorithms guide
How modeling commands use lower-level algorithms
A user-facing command such as “make a fillet” or “subtract this body” is not itself the geometric operation. The application gathers the relevant parameters and model references, then invokes kernel algorithms to construct or modify the shape. The result may be a new valid shape, or the requested operation may fail because the input geometry or topology does not support it.
Design commands to handle both outcomes. The application should receive a result or an error state it can present and recover from; it should not assume that every Boolean, offset, fillet, or feature succeeds. The kernel can supply the construction and validation tools, while the application decides how to report a failure, preserve the previous document state, and let the user revise the operation.
Why viewport triangles should not be the editable model
Renderers commonly use triangles because they are convenient for display. A tessellated mesh approximates the model for the viewport; it does not, by itself, preserve the underlying curves, surfaces, boundaries, or feature semantics needed for exact or parametric editing. Treat the mesh as a derived representation, generate it from the model, and refresh it after geometry changes.
The appropriate tessellation tolerance and performance target depend on the application and are not universal. They should be chosen and tested for the product’s needs rather than inferred from the geometry/topology distinction.
Parametric edits make face and edge references fragile
In a feature-based document, changing an earlier parameter can force downstream geometry to recompute. That recomputation may split, replace, or remove faces and edges. An application feature that stores only “face 7” can then point to a different face—or to nothing—after the model changes.
This is a reference-management problem at the boundary between the kernel and the application’s document model. Open CASCADE’s OCAF documentation describes topology changes under parameter edits and presents reference keys as one approach for identifying attached data. It is not a guarantee of stable identity through every possible modeling operation, nor evidence that one naming strategy works universally. An application needs an explicit policy for rebuilding, resolving, or invalidating references when recomputation changes topology. Open CASCADE OCAF white paper
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What to assess when choosing a kernel
There is no meaningful universal ranking without matching kernels to an application’s requirements. Evaluate candidate kernels against representative models and workflows, including:
- Which geometric representations and surface types they support.
- Whether their construction and modification algorithms cover the required operations.
- How they handle difficult, degenerate, or near-degenerate inputs.
- What topology, document-history, or persistent-reference capabilities are available.
- Import and export requirements for the formats the application must support.
- License terms, deployment constraints, platform support, API language, and integration cost.
- Performance on representative application models, not assumptions based on a generic benchmark.
Verify current versions, licenses, and platform support with each kernel’s official documentation before committing to an implementation.
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