Join simple polygons and draw the resulting contours in Eyeshot

Question

What is the simplest way to join a collection of polygons and draw the resulting contour using Eyeshot? So far I was able only to get filled regions but I am interested in the merging of the contours.

Answer 1

Since you have the region it is very easy to get the contour from it.

// if you know the region is a simple region (not containing full circles) get the curves
List<ICurve> curves = (region.ContourList.FirstOrDefault() as CompositeCurve).CurveList;

ICurve are line and arc only as far as i know. So you can test:

bool isLine = curves[0] is Line
bool isArc = curves[0] is Arc

All curves in the list are ordered so you can reform the region easily. Also, if the region.ContourList contain more than 1 contour then it mean you have holes in your region. First element will always be the main contour and all the following elements are also contour but of the holes.

The contour curve is given counter clockwise and the inner are clockwise.

Answer 2

I came up to this solution. To display the polygons simply iterate through the joined regions, iterate through the ContourList and create the LinearPaths.

private List<PolyRegion2D> Joiner(IEnumerable<Polygon2D> polygons) {
        // The resulting polygons are unconnected
        List<PolyRegion2D> res = new List<PolyRegion2D>();

        // Put every polygon in a region to do the unions.
        LinkedList<PolyRegion2D> polygonRegions = new LinkedList<PolyRegion2D>();
        foreach (Polygon2D polygon in polygons) {
            polygonRegions.AddLast(new PolyRegion2D(new Polygon2D[]{polygon}));
        }

        while (polygonRegions.Count > 0) {
            PolyRegion2D first = polygonRegions.First.Value;
            polygonRegions.RemoveFirst();
            PolyRegion2D union;
            LinkedListNode<PolyRegion2D> connected = FindConnected(first, polygonRegions, out union);
            if (connected == null) {
                // Unconnected polygon
                res.Add(first);
            } else {
                // Intersection found
                polygonRegions.Remove(connected);
                polygonRegions.AddFirst(union);
            }
        }

        return res;
    }

    private LinkedListNode<PolyRegion2D> FindConnected(PolyRegion2D poly, LinkedList<PolyRegion2D> polys, out PolyRegion2D union) {
        LinkedListNode<PolyRegion2D> node = polys.First;
        while(node != null){
            PolyRegion2D[] union_ = PolyRegion2D.Union(poly, node.Value);
            if (union_.Length == 1) {
                union = union_[0];
                return node;
            }
            node = node.Next;
        }
        union = null;
        return null;
    }