Check if Point Is Inside A Polygon

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There is a project on Github with code: https://github.com/substack/point-in-polygon (MIT license):

function inside(point, vs) {
    // ray-casting algorithm based on
    // https://wrf.ecse.rpi.edu/Research/Short_Notes/pnpoly.html/pnpoly.html
    
    var x = point[0], y = point[1];
    
    var inside = false;
    for (var i = 0, j = vs.length - 1; i < vs.length; j = i++) {
        var xi = vs[i][0], yi = vs[i][1];
        var xj = vs[j][0], yj = vs[j][1];
        
        var intersect = ((yi > y) != (yj > y))
            && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
        if (intersect) inside = !inside;
    }
    
    return inside;
};

Usage:

// array of coordinates of each vertex of the polygon
var polygon = [ [ 1, 1 ], [ 1, 2 ], [ 2, 2 ], [ 2, 1 ] ];
inside([ 1.5, 1.5 ], polygon); // true

The test function is here: https://github.com/substack/point-in-polygon/blob/master/index.js

Note: This code doesn’t work reliably when the point is a corner of the polygon or on an edge. There is an improved version here: https://github.com/mikolalysenko/robust-point-in-polygon

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