确定点是否在多边形中

Question

As per my requirement, I am drawing polygons on google map shown in the image below.(using maps v2)

Now I need to show an alert when user enters that particular polygons.

How to identify if my current location is with in the polygon. (Need optimized way without draining battery)

Thanks in advance.

Answer 1

Just tried Ray Casting algorithm which identifies point in polygon. This works perfect.

Refer http://en.wikipedia.org/wiki/Point_in_polygon for thesis of Ray-Casting

private boolean isPointInPolygon(LatLng tap, ArrayList<LatLng> vertices) {
        int intersectCount = 0;
        for (int j = 0; j < vertices.size() - 1; j++) {
            if (rayCastIntersect(tap, vertices.get(j), vertices.get(j + 1))) {
                intersectCount++;
            }
        }

        return ((intersectCount % 2) == 1); // odd = inside, even = outside;
    }

    private boolean rayCastIntersect(LatLng tap, LatLng vertA, LatLng vertB) {

        double aY = vertA.latitude;
        double bY = vertB.latitude;
        double aX = vertA.longitude;
        double bX = vertB.longitude;
        double pY = tap.latitude;
        double pX = tap.longitude;

        if ((aY > pY && bY > pY) || (aY < pY && bY < pY)
                || (aX < pX && bX < pX)) {
            return false; // a and b can't both be above or below pt.y, and a or
                            // b must be east of pt.x
        }

        double m = (aY - bY) / (aX - bX); // Rise over run
        double bee = (-aX) * m + aY; // y = mx + b
        double x = (pY - bee) / m; // algebra is neat!

        return x > pX;
    }

Answer 2

I found ray-casting method unreliable but I ended up using the PolyUtil from google maps.

You need the dependency compile 'com.google.maps.android:android-maps-utils:0.5'

And then the method looks like this

PolyUtil.containsLocation(userLocation, polyPointsList, false);

EDIT

This is the description of this method found in source code

Computes whether the given point lies inside the specified polygon. The polygon is always considered closed, regardless of whether the last point equals the first or not. Inside is defined as not containing the South Pole -- the South Pole is always outside. The polygon is formed of great circle segments if geodesic is true, and of rhumb (loxodromic) segments otherwise.

Answer 3

Refer this link

Polygon Touch detection Google Map API V2

Its RayCasting algorithm, it may help you :)

A brief description about the algorithm:

A horizontal line is drawn from your point to the right, if it intersects the sides of polygon at odd number of times then the point is inside the polygon else outside :)

These wiki links will give you complete idea:

http://en.wikipedia.org/wiki/Point_in_polygon

http://rosettacode.org/wiki/Ray-casting_algorithm

Answer 4

Here is the code in Dart, taken from: https://github.com/KohlsAdrian/google_maps_utils/blob/master/lib/poly_utils.dart

  /// Checks if [point] is inside [polygon]
  static bool containsLocationPoly(Point point, List<Point> polygon) {
    num ax = 0;
    num ay = 0;
    num bx = polygon[polygon.length - 1].x - point.x;
    num by = polygon[polygon.length - 1].y - point.y;
    int depth = 0;

    for (int i = 0; i < polygon.length; i++) {
      ax = bx;
      ay = by;
      bx = polygon[i].x - point.x;
      by = polygon[i].y - point.y;

      if (ay < 0 && by < 0) continue; // both "up" or both "down"
      if (ay > 0 && by > 0) continue; // both "up" or both "down"
      if (ax < 0 && bx < 0) continue; // both points on left

      num lx = ax - ay * (bx - ax) / (by - ay);

      if (lx == 0) return true; // point on edge
      if (lx > 0) depth++;
    }

    return (depth & 1) == 1;
  }