How to remove a vertex from a MultiPolygon and return the bounding polygon in JTS

Question

I'm trying to complete a code implementing the Kirkpatrick algorithm in Java using JTS and it's literally the first time doing working with such tools. There's a part that I need to remove a known point P (in the independent set) from a region which is consisted of multiple polygons all intersecting in P and return the remaining hole to be triangulated. The question is how to do remove a point P from a MultiPolygon?

 /**
 * Find a bounding polygon if point p is removed from affected regions
 *
 * @param p
 * @param affectedRegions
 * @return
 */
public Polygon findBoundingPolygon(Coordinate p, List<Geometry> affectedRegions) {
    // TODO: write a function that finds a bounding polygon
    GeometryFactory fact = new GeometryFactory();
    Geometry regions = fact.buildGeometry(affectedRegions);
    
    // TODO: bound <- a concave hull of regions
    
    LinearRing shell = fact.createLinearRing(coordsToLinearRingCoords(bound));
    return new Polygon(shell, null, fact);
}

Answer 1

Removing a vertex from a polygon is relatively easy:

  static final double tol = 1e-4;
  static final GeometryFactory gf = new GeometryFactory();
  public Polygon removePoint(Polygon p, Point pt) {
    if (p.getNumPoints() < 4) {
      throw new RuntimeException("Can't reduce polygon below 4 points");
    }
    ArrayList<Coordinate> points = new ArrayList<>();
    for (Coordinate c : p.getCoordinates()) {
      if (!(Math.abs(c.x - pt.getX()) < tol && Math.abs(c.y - pt.getY()) < tol)) {
        points.add(c);
      }
    }
    return gf.createPolygon(points.toArray(new Coordinate[points.size()]));
  }

If you are dealing with a MultiPolygon then simply iterate through the Polygons one at a time:

ArrayList<Polygon> polys = new ArrayList<>();
for (int i=0; i<mp.getNumGeometries();i++) {
  polys.add(me.removePoint(mp.getGeometryN(i), p));
}

Calculating the ConvexHull is also simple:

ConvexHull huller = new ConvexHull(poly.getCoordinates(), gf);
Polygon hull = (Polygon) huller.getConvexHull();