@jscad/modeling/src/operations/extrusions/earcut/polygonHierarchy.js

Constructive Solid Geometry (CSG) Library for JSCAD

const geom2 = require('../../../geometries/geom2')
const plane = require('../../../maths/plane')
const vec2 = require('../../../maths/vec2')
const vec3 = require('../../../maths/vec3')
const calculatePlane = require('../slice/calculatePlane')
const assignHoles = require('./assignHoles')

/*
 * Constructs a polygon hierarchy which associates holes with their outer solids.
 * This class maps a 3D polygon onto a 2D space using an orthonormal basis.
 * It tracks the mapping so that points can be reversed back to 3D losslessly.
 */
class PolygonHierarchy {
  constructor (slice) {
    this.plane = calculatePlane(slice)

    // create an orthonormal basis
    // choose an arbitrary right hand vector, making sure it is somewhat orthogonal to the plane normal
    const rightvector = vec3.orthogonal(vec3.create(), this.plane)
    const perp = vec3.cross(vec3.create(), this.plane, rightvector)
    this.v = vec3.normalize(perp, perp)
    this.u = vec3.cross(vec3.create(), this.v, this.plane)

    // map from 2D to original 3D points
    this.basisMap = new Map()

    // project slice onto 2D plane
    const projected = slice.edges.map((e) => e.map((v) => this.to2D(v)))

    // compute polygon hierarchies, assign holes to solids
    const geometry = geom2.create(projected)
    this.roots = assignHoles(geometry)
  }

  /*
   * project a 3D point onto the 2D plane
   */
  to2D (vector3) {
    const vector2 = vec2.fromValues(vec3.dot(vector3, this.u), vec3.dot(vector3, this.v))
    this.basisMap.set(vector2, vector3)
    return vector2
  }

  /*
   * un-project a 2D point back into 3D
   */
  to3D (vector2) {
    // use a map to get the original 3D, no floating point error
    const original = this.basisMap.get(vector2)
    if (original) {
      return original
    } else {
      console.log('Warning: point not in original slice')
      const v1 = vec3.scale(vec3.create(), this.u, vector2[0])
      const v2 = vec3.scale(vec3.create(), this.v, vector2[1])

      const planeOrigin = vec3.scale(vec3.create(), plane, plane[3])
      const v3 = vec3.add(v1, v1, planeOrigin)
      return vec3.add(v2, v2, v3)
    }
  }
}

module.exports = PolygonHierarchy