Reference Model Brep

Relevant source files

The following files were used as context for generating this wiki page:

• src/convert/geometry/Parasolid.py
• src/convert/geometry/Opencascade.py
• src/convert/adapters/freecad/Brep.py
• tests/interchange/compatibility/CompatFieldsBrep.py
• src/convert/adapters/freecad/Archive.py

Boundary Representation (B-Rep)

Boundary Representation (B-Rep) defines the topological and geometric structure of 3D models within the Kit project. It provides a neutral format to facilitate bidirectional interoperability between proprietary CAD formats like SOLIDWORKS (Parasolid) and open formats like FreeCAD (OpenCASCADE).

B-Rep models represent shapes by defining their boundaries as a hierarchical arrangement of topological entities (e.g., bodies, faces, edges) associated with underlying geometric carriers (e.g., surfaces, curves, points).

1. Topology Model

The B-Rep system uses a nested hierarchy to define the connectivity of a 3D solid or sheet. Every entity includes a unique identifier, provenance metadata, and optional attributes.

1.1 Topological Hierarchy

The following entities compose the topological structure:

• BrepBody: The root container for a single solid or sheet object. It manages regions, wire IDs, and vertex IDs.
• BrepRegion: Represents a continuous volume (solid) or a set of shells (voids or sheets).
• BrepShell: Defines a boundary of a region as a collection of face uses.
• BrepFace: A bounded portion of a surface. It references one or more loops.
• BrepLoop: A closed circuit of coedges that defines the boundaries of a face.
• BrepCoedge: An oriented use of an edge within a loop.
• BrepEdge: A bounded portion of a curve, defined by start and end vertices.
• BrepVertex: A point in 3D space marking the end of an edge.

graph TD
    Body[BrepBody] --> Region[BrepRegion]
    Region --> ShellUse[BrepShellUse]
    ShellUse --> Shell[BrepShell]
    Shell --> FaceUse[BrepFaceUse]
    FaceUse --> Face[BrepFace]
    Face --> Loop[BrepLoop]
    Loop --> Coedge[BrepCoedge]
    Coedge --> Edge[BrepEdge]
    Edge --> Vertex[BrepVertex]

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The diagram shows the parent-child relationships between topological entities in a B-Rep model.

2. Geometric Carriers

Geometry defines the mathematical representation of topological entities. Every edge and face references a specific curve or surface carrier.

2.1 Surfaces

Surfaces provide the mathematical definition for faces. The project supports several analytic and free-form types.

Surface Type	Description	Key Fields
PlaneSurface	A flat, infinite surface.	Origin, Normal, Ref Direction
CylinderSurface	A cylindrical surface defined by an axis.	Origin, Axis, Radius
ConeSurface	A conical surface with a specific half-angle.	Origin, Axis, Radius, Half Angle
SphereSurface	A spherical surface centered at a point.	Center, Radius, Axis
TorusSurface	A donut-shaped surface.	Center, Axis, Major/Minor Radius
NurbsSurface	A Non-Uniform Rational B-Spline surface.	Control Points, Knots, Multiplicities

2.2 Curves

Curves provide the mathematical definition for edges.

• LineCurve: A straight line defined by an origin and direction.
• CircleCurve: A circular path defined by center, axis, and radius.
• EllipseCurve: An elliptical path defined by major and minor radii.
• NurbsCurve: A B-spline curve defined by control points and knot vectors.
• IntersectionCurve: A curve derived from the intersection of two surfaces.

3. Format Implementation

The project implements specific logic to handle B-Rep data from different CAD kernels.

3.1 Parasolid (X_T / X_B)

The Parasolid module handles binary and ASCII Parasolid data used by SOLIDWORKS. It encodes BrepModel instances into "transmit files" using specific schema markers like SCH_1200000_12006.

• Encoding: The EncodeBrepModel function validates topology before serializing entities into a byte stream. It ensures body transforms are identity and handles SOLIDWORKS-specific feature ID mapping.
• Indexing: Uses an IndexAllocator to map interchange identifiers to native record indices (e.g., Bridges, Loops, Edges).
• Vendor Attributes: Writes specific SOLIDWORKS attributes such as SWEntUnchanged and SDL/TYSA_COLOUR.

3.2 OpenCASCADE / FreeCAD

The Opencascade and Brep modules handle the ASCII B-Rep format used by OpenCASCADE and FreeCAD.

• Structure: Files start with the header CASCADE Topology V1, (c) Matra-Datavision.
• Parsing: The DecodeAsciiBrep function tokenizes the ASCII stream to reconstruct the topological graph. It validates object counts (e.g., TShapes) and ensures bottom-up ordering.
• Tessellation: Converts B-Rep surfaces into mesh data (vertices and triangles) for visualization when native execution is unavailable.

flowchart TD
    A[Neutral BrepModel] --> B{Adapter Choice}
    B -- SOLIDWORKS --> C[Parasolid Encoder]
    B -- FreeCAD --> D[OpenCASCADE Encoder]
    C --> E[Binary .x_b / ASCII .x_t]
    D --> F[ASCII .brep]

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The flowchart illustrates the transformation of a neutral BrepModel into format-specific outputs.

4. Verification and Validation

Every B-Rep model undergoes structural and geometric validation.

1. Topology Validation: Detects incomplete ownership, non-manifold edges, or disconnected wires.
2. Geometric Verification: Confirms that curve endpoints match vertex locations and that sampled points on a face reside on the underlying surface carrier.
3. Round-trip Integrity: The VerifyBrepData function ensures that encoding a model and immediately decoding it results in identical topological counts and class types.

B-Rep serves as the foundational data structure for geometry within Kit. It ensures that complex topological relationships and precise analytic surfaces are preserved during translation between different CAD applications.