 # Fast C# code to determine 2D point containment

I have been asked a few times and I would like to share with you this piece of code. It tests if a point is inside/on/outside a polygon. It’s quite fast since no API call is involved. I am using it in the IsoVist2D & IsoVist3D calculation process. The code can be directly used in a C# component.

``````using Rhino;
using Rhino.Geometry;
using System;

namespace PancakeAlgo.Solver
{
public static class PointInsidePolygon
{
public static double Tolerance = RhinoMath.ZeroTolerance;
public static PointContainment Contains(Point2d[] polygon, Point2d ptr)
{
var crossing = 0;
var len = polygon.Length;

for (var i = 0; i < len; i++)
{
var j = i + 1;
if (j == len) j = 0;

var p1 = polygon[i];
var p2 = polygon[j];

var y1 = p1.Y;
var y2 = p2.Y;

var x1 = p1.X;
var x2 = p2.X;

if (Math.Abs(x1 - x2) < Tolerance && Math.Abs(y1 - y2) < Tolerance)
continue;

var minY = Math.Min(y1, y2);
var maxY = Math.Max(y1, y2);

if (ptr.Y < minY || ptr.Y > maxY)
continue;

if (Math.Abs(minY - maxY) < Tolerance)
{
var minX = Math.Min(x1, x2);
var maxX = Math.Max(x1, x2);

if (ptr.X >= minX && ptr.X <= maxX)
{
return PointContainment.Coincident;
}
else
{
if (ptr.X < minX)
++crossing;
}
}
else
{
var x = (x2 - x1) * (ptr.Y - y1) / (y2 - y1) + x1;
if (Math.Abs(x - ptr.X) <= Tolerance)
return PointContainment.Coincident;

if (ptr.X < x)
{
++crossing;
}
}
}

return ((crossing & 1) == 0) ? PointContainment.Outside : PointContainment.Inside;
}
}
}
``````
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