Decimating a Mesh

Mesh.simplified(_:) runs a quadric-error-metric (QEM) edge-collapse decimation pass over the mesh and returns a SimplifiedMesh — the reduced mesh plus the metadata describing the operation, including the achieved Hausdorff distance from input to output. It is fallible: it returns nil when the options are invalid or the input mesh is empty.

Reduce to a target triangle count

Pass a SimplifyOptions (nested as Mesh.SimplifyOptions) with targetTriangleCount set:

import OCCTSwift
import OCCTSwiftMesh

let mesh: Mesh = shape.mesh()                       // a tessellated Shape from OCCTSwift

guard let result = mesh.simplified(.init(targetTriangleCount: 5_000)) else {
    // nil → empty input, or invalid options (neither/both targets set, out of range)
    return
}

print("\(result.beforeTriangleCount) → \(result.afterTriangleCount) triangles")
print("Hausdorff error: \(result.hausdorffDistance)")   // absolute, in input mesh units
let reduced: Mesh = result.mesh

afterTriangleCount is a soft target: the algorithm may return more triangles than requested when topology preservation or the Hausdorff cap stops it early (a tetrahedron can’t go below 4 triangles, for example). Always read afterTriangleCount for the actual count and hausdorffDistance to understand the quality.

Read and cap the Hausdorff error

hausdorffDistance is reported on every successful call, whether or not you set a cap. To make deviation the controlling constraint, set maxHausdorffDistance (absolute, in input units) — decimation halts as soon as the measured deviation would exceed it:

var options = Mesh.SimplifyOptions(targetReduction: 0.9)   // ask for an aggressive 90% cut...
options.maxHausdorffDistance = 0.05                        // ...but never deviate more than 0.05

guard let result = mesh.simplified(options) else { return }

if result.hausdorffDistance >= 0.05 {
    print("Hit the deviation cap at \(result.afterTriangleCount) triangles")
}

maxHausdorffDistance must be >= 0; a negative value makes simplified(_:) return nil.

Preserve boundary edges

When decimating an open shell — or a mesh you’ll later sew back into a B-Rep — keep the free edges intact so the boundary doesn’t pull inward. preserveBoundary defaults to true:

var options = Mesh.SimplifyOptions(targetReduction: 0.5)
options.preserveBoundary = true     // free (boundary) edges are not collapsed
options.preserveTopology = true     // collapses that change genus are rejected (always on today)

guard let result = mesh.simplified(options) else { return }

preserveTopology is a forward-compatibility flag — the current backend always preserves topology, so setting it to false does not (yet) produce more aggressive decimation. See the decimation algorithm notes for the full backend semantics, vendored meshoptimizer pin, and Hausdorff-unit details.