Using JSTS buffer to identify a self-intersecting polygon

Question

I was expecting to be able to test for self intersecting polygons either by JSTS failing to construct them or by adding a buffer and testing whether they were MultiPolygons after buffering but for a certain shape this isn't working and that's well past my geometry ability to grok

//a self-intersecting shape
var poly = [[0, 3], [1, 5], [3, 1], [5, 5], [6, 3], [0, 3]];

var geomFactory = new jsts.geom.GeometryFactory();

var jstsCoordinates = poly.map(function(pt) {
  return new jsts.geom.Coordinate(pt[0], pt[1]);
});

var linearRing = geomFactory.createLinearRing(jstsCoordinates);

var jstsPolygon = geomFactory.createPolygon(linearRing).buffer(1);

console.log(jstsPolygon.getGeometryType()); //this will be polygon but I thought should be MultiPolygon

var bufferedPoly = (jstsPolygon.shell.points.coordinates.map(function(pr) {
  return [pr.x, pr.y]
}))

var svg = d3.select('svg');

//add the first shape (maginified for display)
svg.selectAll('.original').data([poly]).enter().append("polygon")
    .attr("points",function(d) { 
        return d.map(function(d) {
            return [d[0] * 10 + 10, d[1]*10].join(",");
        }).join(" ");
    })
    .attr("fill", "yellow");

//add the buffered shape below it
svg.selectAll('.buffered').data([bufferedPoly]).enter().append("polygon")
    .attr("points",function(d) { 
        return d.map(function(d) {
            return [d[0] * 10 + 10, d[1]*10 + 40].join(",");
        }).join(" ");
    })
    .attr("fill", "yellow");

svg {background-color:blue}

Using JSTS buffer to identify a self-intersecting polygon

2 Answers