Algorithm to split self-intersected Path2D into se

Question

I need to get rid of self-intersections in a shape. Shape is constructed from an array of points, so all segments of that shape are lines. (only lines, no curves and arcs)

Previously, I tried to create Path2D from that points, construct an Area from it, and then using its PathIterator I created several Path2Ds, which somehow were subpaths of previous path, and so self-intersetions were gone. But this isn't working for some paths - self-intersections still remain there.

So could you point me to some place where I can find good algorithm to do similar thing?

Edit: I haven't found anything useful anywhere, so I written my own algorithm. See the answers.

Answer 1

I bookmarked your question/answer in case I had to implement something similar myself, but then I found the GEOS project which has a simple way of achieving this. I'm calling GEOS from Python/Django, but the whole thing is based on JTS (Java Topology Suite) so I'd start there and treat the following Python as psuedo-code.

Basically, the UNION operation will split a line into simply connected parts if it is not simply connected (explained here), so UNIONing the line with it's first point does what we need:

line  = LineString(list_of_lines_x_y_coordinates)
# union with first point splits into MultiLineString containing segments
segments = line.union(line[0]) 

Answer 2

If Area is not working for you, you could try using a GLUtessellator to decompose your Shape into a set of triangles, or (using the GL_LINE_LOOP option) just the boundary edges.

Answer 3

So, since I was unable to find anything like this on the web, I written my own algorithm.

It may be insanely ineffective, but it works fast enough for me.

Here it goes:

/**
 * Takes a polygon, defined by a list of lines, and splits it into several
 * paths on points of intersection. If non-self-intersected path is passed in,
 * the same path is returned.
 * @param path
 * @return
 */
public static List<List<Line2D>> splitPath(List<Line2D> lines) {
    List<List<Line2D>> splitted = new ArrayList<List<Line2D>>();
    // find intersections.
    loop1:
    for (Line2D l1 : lines) {
        for (Line2D l2 : lines) {
            if (l1 == l2) continue;
            Point2D intr;
            if ((intr = linesIntersect(l1, l2)) != null) {
                // creating two splitted subpaths
                int i1 = lines.indexOf(l1);
                int i2 = lines.indexOf(l2);

                List<Line2D> subpath1 = new ArrayList<Line2D>();
                subpath1.addAll(lines.subList(0, i1));
                subpath1.add(new Line2D.Double(l1.getP1(), intr));
                subpath1.add(new Line2D.Double(intr, l2.getP2()));
                subpath1.addAll(lines.subList(i2 + 1, lines.size()));
                splitted.addAll(splitPath(subpath1));

                List<Line2D> subpath2 = new ArrayList<Line2D>();
                subpath2.add(new Line2D.Double(intr, l1.getP2()));
                subpath2.addAll(lines.subList(i1 + 1, i2));
                subpath2.add(new Line2D.Double(l2.getP1(), intr));
                splitted.addAll(splitPath(subpath2));
                break loop1;
            }
        }
    }
    if (splitted.size() > 0) {
        return splitted;
    } else {
        return Collections.singletonList(lines);
    }
}

/**
 * Returns point of intersection of this line segments.
 * @param l1
 * @param l2
 * @return
 */
public static Point2D linesIntersect(Line2D l1, Line2D l2) {
    if (l1.getP1().equals(l2.getP2()) || l1.getP2().equals(l2.getP1())) return null;
    Point2D inter = lineIntersection(l1, l2);
    if (inter == null) return null;
    double infS = HEADER.infS;
    double x = inter.getX();
    if (((l1.getX1() > l1.getX2()) ? (x + infS > l1.getX2() && x - infS < l1.getX1()) : (x - infS < l1.getX2() && x + infS > l1.getX1())) &&
           ((l2.getX1() > l2.getX2()) ? (x + infS > l2.getX2() && x - infS < l2.getX1()) : (x - infS < l2.getX2() && x + infS > l2.getX1()))) {
        return inter;
    } else {
        return null;
    }
}

/**
 * Returns point of lines intersection, or null if they are parallel.
 * @param l1
 * @param l2
 * @return
 */
public static Point2D lineIntersection(Line2D l1, Line2D l2) {
    double a1 = l1.getY2() - l1.getY1();
    double b1 = l1.getX1() - l1.getX2();
    double c1 = a1*l1.getX1() + b1*l1.getY1();
    double a2 = l2.getY2() - l2.getY1();
    double b2 = l2.getX1() - l2.getX2();
    double c2 = a2*l2.getX1() + b2*l2.getY1();
    double det = a1*b2 - a2*b1;
    if (det != 0) {
        double x = (b2*c1 - b1*c2)/det;
        double y = (a1*c2 - a2*c1)/det;
        return new Point2D.Double(x, y);
    } else {
        // lines are parallel
        return null;
    }
}