Architecture

Brep is a thin public interface. It holds a reference to a native object — the backend's own Brep representation — and owns no geometry logic itself. Every method delegates to whichever backend is active.

graph TD
    U["your code"] --> B["Brep<br/>(public interface, brep.py)"]
    B --> P["operations.py<br/>@pluggable functions"]
    P -->|"OCP importable"| OP["backend/occ/plugins.py<br/>@plugin"]
    P -->|"Rhino importable"| RP["backend/rhino/plugins.py<br/>@plugin"]
    OP --> OI["backend/occ/*.py"]
    RP --> RI["backend/rhino/*.py"]
    OI --> ON["TopoDS_Shape"]
    RI --> RN["Rhino.Geometry.Brep"]

Backend dispatch

Dispatch uses COMPAS's plugin system. Every operation is declared once as a @pluggable function in operations.py — just a signature, no implementation. Each backend registers a @plugin implementation, gated by what's importable:

• OCC activates when OCP (cadquery-ocp-novtk) is importable.
• Rhino activates when Rhino is importable (rhinoinside, or running inside Rhino/Grasshopper).

Calling brep.fillet(...) calls the brep_fillet pluggable, and COMPAS routes it to whichever @plugin is active. Nothing in Brep, and nothing in your code, knows or cares which one that is.

Topology objects

BrepVertex, BrepEdge, BrepLoop, BrepFace, BrepTrim (see What is a Brep?) are thin wrappers around a native handle, not data containers. A property like BrepFace.surface calls into the native kernel on first access and caches the result — it doesn't copy geometry out eagerly.

NurbsCurve and NurbsSurface are the exception: plain Python value types (control points, knots, weights) with no backend dependency, used as the return type for curve/surface properties.

Exchange format

A Brep is only ever alive inside one backend process. Moving one across a process boundary — Grasshopper to CI, OCC to Rhino — happens through a STEP-inspired JSON exchange document, produced by Brep.__data__ / Brep.__from_data__. It encodes the same entities STEP does (vertices, edges with curves, faces with surfaces, loops with trims) but as COMPAS JSON. See ADR-0001 for the reasoning.

Brep.to_step / Brep.from_step are separate — for interop with third-party CAD tools, not for moving between the two backends.