Off.kt

src/jvmMain/kotlin/afield/geometry/polytope/Off.kt · 60 lines

@file:afield.runtime.completion.CompletionCategory("Polytopes")

package afield.geometry.polytope

The one-way bridge to OFF, the plain-text polygon interchange format that Geomview and most polyhedron tooling read.

One function and no reader. Export exists so that a solid can be opened in something that was not written here — an independent second opinion when a test disagrees with a picture — and so that a bug report can carry the actual object. Nothing else in this package goes through it, and no part of the topology is shaped by what OFF can express.

The format itself is lossy for this library's purposes. Element identity, rank structure above 2, attributes and provenance all disappear; what survives is coordinates and polygons.

fun PolyhedralObject.toOff(): String

Minimal polygonal OFF export for diagnostics and independent test inspection.

Validates first and throws on any issue, so a malformed topology cannot be laundered into a plausible-looking file; also throws without a realization, there being nothing to write. Vertices are emitted in topology.elements(0) order and faces refer to positions in that sequence rather than to ElementId values — the two coincide for the seeds and generally do not.

Written with Float.toString, so the output does not depend on the ambient locale, and at Realization3's single precision rather than any wider format.

A face with holes throws. OFF has no way to say "and this ring is subtracted", so the alternatives were a file whose face is solid where the object's is not, or a refusal; a diagnostic that disagrees with the object it is diagnosing, and does not say so, is worse than no diagnostic. Triangulate first and write the mesh if you need the geometry out.

fun PolyhedralObject.toOff(): String {
    val realization = realization ?: throw IllegalStateException("OFF export requires a 3D realization")
    topology.validate().throwIfInvalid()
    val holed = topology.elements(2).firstOrNull { topology.faceHasHoles(it.id) }
    if (holed != null) throw IllegalArgumentException(
        "face ${holed.id.value} has ${topology.faceHoles(holed.id).size} hole(s), which OFF cannot express",
    )
    val vertices = topology.elements(0)
    val vertexIndices = vertices.mapIndexed { index, vertex -> vertex.id to index }.toMap()
    val faces = topology.elements(2)
    return buildString {
        append("OFF\n")
        append(vertices.size).append(' ').append(faces.size).append(' ').append(topology.elements(1).size).append('\n')
        for (i in vertices.indices) {
            append(java.lang.Float.toString(realization.x(i))).append(' ')
                .append(java.lang.Float.toString(realization.y(i))).append(' ')
                .append(java.lang.Float.toString(realization.z(i))).append('\n')
        }
        for (face in faces) {
            val cycle = topology.faceCycle(face.id)
            append(cycle.size)
            for (vertex in cycle) append(' ').append(vertexIndices.getValue(ElementId(vertex)))
            append('\n')
        }
    }
}