PointNearLinestring

PointInRangeOfLinestring.cs

// This snippet shows how to check whether a point is near a 
// polyline. For a given polyline you can check if the 
// distance of a point to the line string is not greater 
// a certain value.
// You can use this for geo-fencing or UI hit-testing.
using System;
using System.Collections.Generic;
using JSIL;
using JSIL.Meta;

public class Program
{
    public static void Main()
    {
        Console.WriteLine("The red points are are more than 10 pixels away from the lines string.");

        dynamic document = Builtins.Global["document"];
        dynamic window = Builtins.Global["window"];

        var canvas = document.createElement("canvas");
        var ctx = canvas.getContext("2d");
        var body = document.getElementsByTagName("body")[0];
        body.appendChild(canvas);

        var linestring = new List<Point> { 
            new Point(20,30), new Point(50, 50), 
            new Point(60, 45), new Point(100, 50), new Point(50, 100), new Point(20,30) };

        DrawLinestring(ctx, linestring, "black", 2);

        var pointsToCheck = new List<Point> { 
            new Point(60, 70), new Point(80, 60), new Point(60, 20), new Point(80, 80) };
        foreach (var point in pointsToCheck)
        {
            var color = PointInRangeOfLinestring(linestring, point, 10)? "blue" : "red";
            DrawPoint(ctx, point, color);
        }
    }

    public static void DrawPoint(dynamic ctx, Point p, string color)
    {
        ctx.fillStyle = color;
        ctx.fillRect(p.X - 2, p.Y - 2, 4, 4);
    }

    public static void DrawLinestring(dynamic ctx, IList<Point> lineString, string color, double width)
    {
        ctx.lineWidth = width;
        ctx.strokeStyle = color;
        ctx.beginPath();
        ctx.moveTo(lineString[0].X, lineString[0].Y);
        for (int i = 1; i < lineString.Count; i++)
            ctx.lineTo(lineString[i].X, lineString[i].Y);
        ctx.stroke();
    }


    public static bool PointInRangeOfLinestring(List<Point> lineString, Point p, double range)
    {
        double range2 = range * range; // square of range

        for (int i = 0; i < lineString.Count - 1; i++)
        {
            Point c = ClosestPointOnSegment(lineString[i], lineString[i + 1], p);
            Point d = new Point(c.X - p.X, c.Y - p.Y); // distance vector

            if (d.X * d.X + d.Y * d.Y <= range2)
                return true;
        }

        return false;
    }

    public static Point ClosestPointOnSegment(Point a, Point b, Point p)
    {
        var d = new Point(b.X - a.X, b.Y - a.Y);
        double number = (p.X - a.X) * d.X + (p.Y - a.Y) * d.Y;

        if (number <= 0.0)
            return a;

        double denom = d.X * d.X + d.Y * d.Y;

        if (number >= denom)
            return b;

        return new Point(a.X + (number / denom) * d.X, a.Y + (number / denom) * d.Y);
    }
}

public class Point
{
    public Point(double x, double y)
    {
        this.X = x;
        this.Y = y;
    }

    public double X { get; set; }
    public double Y { get; set; }
}