How to check if a given point lies inside or outside a polygon?

Given a polygon and a point ‘p‘, find if ‘p‘ lies inside the polygon or not. The points lying on the border are considered inside.

Examples:

Approach: The idea to solve this problem is based on How to check if two given line segments intersect, and to be used as follows:

1. Draw a horizontal line to the right of each point and extend it to infinity
2. Count the number of times the line intersects with polygon edges.
3. A point is inside the polygon if either count of intersections is odd or point lies on an edge of polygon.  If none of the conditions is true, then point lies outside.

How to handle point ‘g’ in the above figure?

Note that we should return true if the point lies on the line or the same as one of the vertices of the given polygon. To handle this, after checking if the line from ‘p’ to extreme intersects, we check whether ‘p’ is collinear with vertices of the current line of polygon. If it is collinear, then we check if the point ‘p’ lies on current side of polygon, if it lies, we return true, else false.

Following is the implementation of the above approach: 

C++

#include <bits/stdc++.h>
using namespace std;

struct Point {
    int x, y;
};

struct line {
    Point p1, p2;
};

bool onLine(line l1, Point p)
{
    // Check whether p is on the line or not
    if (p.x <= max(l1.p1.x, l1.p2.x)
        && p.x >= min(l1.p1.x, l1.p2.x)
        && (p.y <= max(l1.p1.y, l1.p2.y)
            && p.y >= min(l1.p1.y, l1.p2.y)))
        return true;

    return false;
}

int direction(Point a, Point b, Point c)
{
    int val = (b.y - a.y) * (c.x - b.x)
              - (b.x - a.x) * (c.y - b.y);

    if (val == 0)

        // Collinear
        return 0;

    else if (val < 0)

        // Anti-clockwise direction
        return 2;

    // Clockwise direction
    return 1;
}

bool isIntersect(line l1, line l2)
{
    // Four direction for two lines and points of other line
    int dir1 = direction(l1.p1, l1.p2, l2.p1);
    int dir2 = direction(l1.p1, l1.p2, l2.p2);
    int dir3 = direction(l2.p1, l2.p2, l1.p1);
    int dir4 = direction(l2.p1, l2.p2, l1.p2);

    // When intersecting
    if (dir1 != dir2 && dir3 != dir4)
        return true;

    // When p2 of line2 are on the line1
    if (dir1 == 0 && onLine(l1, l2.p1))
        return true;

    // When p1 of line2 are on the line1
    if (dir2 == 0 && onLine(l1, l2.p2))
        return true;

    // When p2 of line1 are on the line2
    if (dir3 == 0 && onLine(l2, l1.p1))
        return true;

    // When p1 of line1 are on the line2
    if (dir4 == 0 && onLine(l2, l1.p2))
        return true;

    return false;
}

bool checkInside(Point poly[], int n, Point p)
{

    // When polygon has less than 3 edge, it is not polygon
    if (n < 3)
        return false;

    // Create a point at infinity, y is same as point p
    line exline = { p, { 9999, p.y } };
    int count = 0;
    int i = 0;
    do {

        // Forming a line from two consecutive points of
        // poly
        line side = { poly[i], poly[(i + 1) % n] };
        if (isIntersect(side, exline)) {

            // If side is intersects exline
            if (direction(side.p1, p, side.p2) == 0)
                return onLine(side, p);
            count++;
        }
        i = (i + 1) % n;
    } while (i != 0);

    // When count is odd
    return count & 1;
}

// Driver code
int main()
{
    Point polygon[]
        = { { 0, 0 }, { 10, 0 }, { 10, 10 }, { 0, 10 } };
    Point p = { 5, 3 };
    int n = 4;

    // Function call
    if (checkInside(polygon, n, p))
        cout << "Point is inside.";
    else
        cout << "Point is outside.";

    return 0;
}

Java

// Java code for the above approach
import java.util.*;

public class Gfg {
  public static class Point {
    public int x, y;
    public Point(int x, int y)
    {
      this.x = x;
      this.y = y;
    }
  }

  public static class Line {
    public Point p1, p2;
    public Line(Point p1, Point p2)
    {
      this.p1 = p1;
      this.p2 = p2;
    }
  }

  static int onLine(Line l1, Point p)
  {
    // Check whether p is on the line or not
    if (p.x <= Math.max(l1.p1.x, l1.p2.x)
        && p.x >= Math.min(l1.p1.x, l1.p2.x)
        && (p.y <= Math.max(l1.p1.y, l1.p2.y)
            && p.y >= Math.min(l1.p1.y, l1.p2.y)))
      return 1;

    return 0;
  }

  static int direction(Point a, Point b, Point c)
  {
    int val = (b.y - a.y) * (c.x - b.x)
      - (b.x - a.x) * (c.y - b.y);

    if (val == 0)

      // Collinear
      return 0;

    else if (val < 0)

      // Anti-clockwise direction
      return 2;

    // Clockwise direction
    return 1;
  }

  static int isIntersect(Line l1, Line l2)
  {
    // Four direction for two lines and points of other
    // line
    int dir1 = direction(l1.p1, l1.p2, l2.p1);
    int dir2 = direction(l1.p1, l1.p2, l2.p2);
    int dir3 = direction(l2.p1, l2.p2, l1.p1);
    int dir4 = direction(l2.p1, l2.p2, l1.p2);

    // When intersecting
    if (dir1 != dir2 && dir3 != dir4)
      return 1;

    // When p2 of line2 are on the line1
    if (dir1 == 0 && onLine(l1, l2.p1) == 1)
      return 1;

    // When p1 of line2 are on the line1
    if (dir2 == 0 && onLine(l1, l2.p2) == 1)
      return 1;

    // When p2 of line1 are on the line2
    if (dir3 == 0 && onLine(l2, l1.p1) == 1)
      return 1;

    // When p1 of line1 are on the line2
    if (dir4 == 0 && onLine(l2, l1.p2) == 1)
      return 1;

    return 0;
  }

  static int checkInside(Point[] poly, int n, Point p)
  {

    // When polygon has less than 3 edge, it is not
    // polygon

    if (n < 3)
      return 0;

    // Create a point at infinity, y is same as point p
    Point pt = new Point(9999, p.y);
    Line exline = new Line(p, pt);
    int count = 0;
    int i = 0;
    do {

      // Forming a line from two consecutive points of
      // poly
      Line side
        = new Line(poly[i], poly[(i + 1) % n]);
      if (isIntersect(side, exline) == 1) {

        // If side is intersects exline
        if (direction(side.p1, p, side.p2) == 0)
          return onLine(side, p);
        count++;
      }
      i = (i + 1) % n;
    } while (i != 0);

    // When count is odd
    return count & 1;
  }

  // Driver code
  public static void main(String[] args)
  {
    Point[] polygon
      = { new Point(1, 0), new Point(8, 3),
         new Point(8, 8), new Point(1, 5) };
    Point p = new Point(3, 5);
    int n = polygon.length;
    if (checkInside(polygon, n, p) == 1)
      System.out.println("Point is Inside");
    else
      System.out.println("Point is Outside");
  }
}

// This code is contributed by Potta Lokesh

Python3

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

class line:
    def __init__(self, p1, p2):
        self.p1 = p1
        self.p2 = p2

def onLine(l1, p):
    # Check whether p is on the line or not
    if (
        p.x <= max(l1.p1.x, l1.p2.x)
        and p.x >= min(l1.p1.x, l1.p2.x)
        and (p.y <= max(l1.p1.y, l1.p2.y) and p.y >= min(l1.p1.y, l1.p2.y))
    ):
        return True
    return False

def direction(a, b, c):
    val = (b.y - a.y) * (c.x - b.x) - (b.x - a.x) * (c.y - b.y)
    if val == 0:
        # Collinear
        return 0
    elif val < 0:
        # Anti-clockwise direction
        return 2
    # Clockwise direction
    return 1

def isIntersect(l1, l2):
    # Four direction for two lines and points of other line
    dir1 = direction(l1.p1, l1.p2, l2.p1)
    dir2 = direction(l1.p1, l1.p2, l2.p2)
    dir3 = direction(l2.p1, l2.p2, l1.p1)
    dir4 = direction(l2.p1, l2.p2, l1.p2)

    # When intersecting
    if dir1 != dir2 and dir3 != dir4:
        return True

    # When p2 of line2 are on the line1
    if dir1 == 0 and onLine(l1, l2.p1):
        return True

    # When p1 of line2 are on the line1
    if dir2 == 0 and onLine(l1, l2.p2):
        return True

    # When p2 of line1 are on the line2
    if dir3 == 0 and onLine(l2, l1.p1):
        return True

    # When p1 of line1 are on the line2
    if dir4 == 0 and onLine(l2, l1.p2):
        return True

    return False

def checkInside(poly, n, p):
    # When polygon has less than 3 edge, it is not polygon
    if n < 3:
        return False

    # Create a point at infinity, y is same as point p
    exline = line(p, Point(9999, p.y))
    count = 0
    i = 0
    while True:
        # Forming a line from two consecutive points of poly
        side = line(poly[i], poly[(i + 1) % n])
        if isIntersect(side, exline):
            # If side is intersects ex
            if (direction(side.p1, p, side.p2) == 0):
                return onLine(side, p);
            count += 1

        i = (i + 1) % n;
        if i == 0:
            break

    # When count is odd
    return count & 1;


# Driver code
polygon = [  Point( 0, 0 ), Point( 10, 0 ), Point( 10, 10 ), Point( 0, 10 ) ];
p = Point( 5, 3 );
n = 4;

# Function call
if (checkInside(polygon, n, p)):
    print("Point is inside.")
else:
    print("Point is outside.")

C#

using System;
using System.Collections.Generic;

public class Gfg
{
  public class Point {
    public int x, y;
    public Point(int x, int y)
    {
      this.x=x;
      this.y=y;
    }
  }

  public class line {
    public Point p1, p2;
    public line(Point p1, Point p2)
    {
      this.p1=p1;
      this.p2=p2;
    }
  }

  static int onLine(line l1, Point p)
  {
    // Check whether p is on the line or not
    if (p.x <= Math.Max(l1.p1.x, l1.p2.x)
        && p.x >= Math.Min(l1.p1.x, l1.p2.x)
        && (p.y <= Math.Max(l1.p1.y, l1.p2.y)
            && p.y >= Math.Min(l1.p1.y, l1.p2.y)))
      return 1;

    return 0;
  }

  static int direction(Point a, Point b, Point c)
  {
    int val = (b.y - a.y) * (c.x - b.x)
      - (b.x - a.x) * (c.y - b.y);

    if (val == 0)

      // Collinear
      return 0;

    else if (val < 0)

      // Anti-clockwise direction
      return 2;

    // Clockwise direction
    return 1;
  }

  static int isIntersect(line l1, line l2)
  {
    // Four direction for two lines and points of other line
    int dir1 = direction(l1.p1, l1.p2, l2.p1);
    int dir2 = direction(l1.p1, l1.p2, l2.p2);
    int dir3 = direction(l2.p1, l2.p2, l1.p1);
    int dir4 = direction(l2.p1, l2.p2, l1.p2);

    // When intersecting
    if (dir1 != dir2 && dir3 != dir4)
      return 1;

    // When p2 of line2 are on the line1
    if (dir1 == 0 && onLine(l1, l2.p1)==1)
      return 1;

    // When p1 of line2 are on the line1
    if (dir2 == 0 && onLine(l1, l2.p2)==1)
      return 1;

    // When p2 of line1 are on the line2
    if (dir3 == 0 && onLine(l2, l1.p1)==1)
      return 1;

    // When p1 of line1 are on the line2
    if (dir4 == 0 && onLine(l2, l1.p2)==1)
      return 1;

    return 0;
  }

  static int checkInside(Point[] poly, int n, Point p)
  {

    // When polygon has less than 3 edge, it is not polygon
    if (n < 3)
      return 0;

    // Create a point at infinity, y is same as point p
    Point pt=new Point(9999, p.y);
    line exline = new line(p,pt);
    int count = 0;
    int i = 0;
    do {

      // Forming a line from two consecutive points of
      // poly
      line side = new line( poly[i], poly[(i + 1) % n] );
      if (isIntersect(side, exline)==1) {

        // If side is intersects exline
        if (direction(side.p1, p, side.p2) == 0)
          return onLine(side, p);
        count++;
      }
      i = (i + 1) % n;
    } while (i != 0);

    // When count is odd
    return count & 1;
  }

  // Driver code
  public static void Main(string[] args)
  {
    Point[] polygon
      = { new Point(0, 0 ), new Point(10, 0 ), new Point(10, 10 ), new Point(0, 10 ) };
    Point p = new Point(5, 3 );
    int n = 4;

    // Function call
    if (checkInside(polygon, n, p)==1)
      Console.WriteLine("Point is inside.");
    else
      Console.WriteLine("Point is outside.");
  }

}

// This code is contributed by ratiagarawal.

Javascript

class Point {
    //int x, y;
    constructor(x,y)
    {
        this.x=x;
        this.y=y;
    }
}

class line {
    //Point p1, p2;
    constructor(p1,p2)
    {
        this.p1=p1;
        this.p2=p2;
    }

};

function onLine(l1, p)
{
    // Check whether p is on the line or not
    if (p.x <= Math.max(l1.p1.x, l1.p2.x)
        && p.x >= Math.min(l1.p1.x, l1.p2.x)
        && (p.y <= Math.max(l1.p1.y, l1.p2.y)
            && p.y >= Math.min(l1.p1.y, l1.p2.y)))
        return true;

    return false;
}

function direction(a, b, c)
{
    let val = (b.y - a.y) * (c.x - b.x)
              - (b.x - a.x) * (c.y - b.y);

    if (val == 0)

        // Collinear
        return 0;

    else if (val < 0)

        // Anti-clockwise direction
        return 2;

    // Clockwise direction
    return 1;
}

function isIntersect(l1, l2)
{
    // Four direction for two lines and points of other line
    let dir1 = direction(l1.p1, l1.p2, l2.p1);
    let dir2 = direction(l1.p1, l1.p2, l2.p2);
    let dir3 = direction(l2.p1, l2.p2, l1.p1);
    let dir4 = direction(l2.p1, l2.p2, l1.p2);

    // When intersecting
    if (dir1 != dir2 && dir3 != dir4)
        return true;

    // When p2 of line2 are on the line1
    if (dir1 == 0 && onLine(l1, l2.p1))
        return true;

    // When p1 of line2 are on the line1
    if (dir2 == 0 && onLine(l1, l2.p2))
        return true;

    // When p2 of line1 are on the line2
    if (dir3 == 0 && onLine(l2, l1.p1))
        return true;

    // When p1 of line1 are on the line2
    if (dir4 == 0 && onLine(l2, l1.p2))
        return true;

    return false;
}

function checkInside(poly, n, p)
{

    // When polygon has less than 3 edge, it is not polygon
    if (n < 3)
        return false;

    // Create a point at infinity, y is same as point p
    let tmp=new Point(9999, p.y);
    let exline = new line( p, tmp );
    let count = 0;
    let i = 0;
    do {

        // Forming a line from two consecutive points of
        // poly
        let side = new line( poly[i], poly[(i + 1) % n] );
        if (isIntersect(side, exline)) {

            // If side is intersects exline
            if (direction(side.p1, p, side.p2) == 0)
                return onLine(side, p);
            count++;
        }
        i = (i + 1) % n;
    } while (i != 0);

    // When count is odd
    return count & 1;
}

// Driver code
    let polygon= [ new Point(0, 0 ), new Point(10, 0 ), new Point(10, 10 ),  new Point(0, 10) ];
    let p = new Point( 5, 3 );
    let n = 4;

    // Function call
    if (checkInside(polygon, n, p))
        console.log("Point is inside.");
    else
        console.log("Point is outside.");

 // This code is contributed by poojaagarwal2.

Output

Point is inside.

Time Complexity: O(n) where n is the number of vertices in the given polygon.
Auxiliary Space: O(1), since no extra space has been taken.

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