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FaceIdentityTable.md

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title FaceIdentityTable
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FaceIdentityTable

Maps a render-path face ordinal, the value stored in ViewportBody.faceIndices and mirrored in CADBodyMetadata.faceIndices, back to the Shape it was tessellated from, and, when a BRepGraph was supplied, to the durable GraphUID minted from that graph.

Why it exists

The mesher assigns Mesh.Triangle.faceIndex by walking faces with a raw, non-deduplicating TopExp_Explorer traversal, the same one Shape.faces() uses. Shape.subShapes(ofType: .face) and a BRepGraph's node ordering both deduplicate instead: a face shared between two shells collapses to one entry there. The two enumerations agree on a clean single solid and silently diverge, shifting every later index, once a face is shared between shells. Resolving a face ordinal via shape.subShapes(ofType: .face)[ordinal] assumes they agree and can silently name the wrong face. FaceIdentityTable captures the correspondence directly at tessellation time instead of asking a consumer to reconstruct it from a mismatched enumeration.

See also EdgeIdentityTable and VertexIdentityTable, which mirror this table for ViewportBody.edgeIndices / vertexIndices.

API

public struct FaceIdentityTable: Sendable {
    public let shapes: [Shape]
    public let uids: [BRepGraph.GraphUID?]?

    public init(shapes: [Shape], uids: [BRepGraph.GraphUID?]? = nil)
    public func shape(forOrdinal ordinal: Int) -> Shape?
    public func uid(forOrdinal ordinal: Int) -> BRepGraph.GraphUID?
}
  • shapes is indexed by the ordinal stored in ViewportBody.faceIndices / CADBodyMetadata.faceIndices, built from Shape.faces() so it always names the exact face tessellated into the triangles carrying that ordinal.
  • uids is populated only when a BRepGraph was supplied to the entry point that produced this table. Each element is nil if that ordinal's face could not be resolved in the graph.
  • shape(forOrdinal:) / uid(forOrdinal:) return nil for an out-of-range ordinal (or, for uid(forOrdinal:), when no graph was supplied at all).

Obtained from CADFileLoader.shapeToBodyMetadataAndIdentity or shapeToBodyMetadataAndIdentities.

Example

let box = Shape.box(width: 10, height: 5, depth: 3)!
let graph = BRepGraph(shape: box)!
let (body, meta, faceTable) = CADFileLoader.shapeToBodyMetadataAndIdentity(
    box, id: "box", color: SIMD4<Float>(0.6, 0.6, 0.65, 1), graph: graph
)
guard let body, let faceTable else { return }

// Resolve a GPU face pick back to its Shape and durable GraphUID.
let pickedOrdinal = Int(body.faceIndices[0])
let pickedFace = faceTable.shape(forOrdinal: pickedOrdinal)
let pickedUID = faceTable.uid(forOrdinal: pickedOrdinal)