Tangents between two Convex Polygons

Last Updated : 26 Apr, 2025

Given two convex polygons, we aim to identify the lower and upper tangents connecting them.

As shown in the figure below, T_RL and T_LR represent the upper and lower tangents, respectively.

Examples:

Input: First Polygon : [[2, 2], [3, 3], [5, 2], [4, 0], [3, 1]]
Second Polygon : [[-1, 0], [0, 1], [1, 0], [0, -2]].
Output: Upper Tangent - line joining (0,1) and (3,3)
Lower Tangent - line joining (0,-2) and (4,0)
Explanation: The image clearly shows the structure and the tangents connecting the two polygons

Approach:

To find the upper tangent, we begin by selecting two points: the rightmost point of polygon a and the leftmost point of polygon b. The line joining these two points is labeled as Line 1. Since this line passes through polygon b (i.e., it is not completely above it), we move to the next point in an anti-clockwise direction on b, forming Line 2. This line is now above polygon b, which is good. However, it crosses polygon a, so we move to the next point on a in a clockwise direction, creating Line 3. Line 3 still crosses polygon a, prompting another move to Line 4. Line 4, however, crosses polygon b, so we proceed to Line 5. Finally, Line 5 does not cross either polygon, making it the correct upper tangent for the given polygons.

For finding the lower tangent we need to move inversely through the polygons i.e. if the line is crossing the polygon b we move to clockwise next and to anti-clockwise next if the line is crossing the polygon a.

Algorithm for upper tangent:

L ← line joining the rightmost point of a and leftmost point of b. while (L crosses any of the polygons) { while(L crosses b) L ← L' : the point on b moves up. while(L crosses a) L ← L' : the point on a moves up. }

Algorithm for lower tangent:

L ← line joining the rightmost point of a and leftmost point of b. while (L crosses any of the polygons) { while (L crosses b) L ← L' : the point on b moves down. while (L crosses a) L ← L' : the point on a moves down. }

Note that the above code only computes the upper tangent. A similar approach can be used to find the lower tangent as well.

CPP

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

// Determines the quadrant of a point relative to origin
int quad(vector<int> p) {
    if (p[0] >= 0 && p[1] >= 0)
        return 1;
    if (p[0] <= 0 && p[1] >= 0)
        return 2;
    if (p[0] <= 0 && p[1] <= 0)
        return 3;
    return 4;
}

// Returns the orientation of ordered triplet (a, b, c)
// 0 -> Collinear, 1 -> Clockwise, -1 -> Counterclockwise
int orientation(vector<int> a, vector<int> b, vector<int> c) {
    int res = (b[1] - a[1]) * (c[0] - b[0]) -
              (c[1] - b[1]) * (b[0] - a[0]);
    if (res == 0)
        return 0;
    if (res > 0)
        return 1;
    return -1;
}

// Compare function to sort points counter-clockwise around center
bool compare(vector<int> p1, vector<int> q1, vector<int> mid) {
    vector<int> p = {p1[0] - mid[0], p1[1] - mid[1]};
    vector<int> q = {q1[0] - mid[0], q1[1] - mid[1]};

    int one = quad(p);
    int two = quad(q);

    if (one != two)
        return (one < two);
    return (p[1] * q[0] < q[1] * p[0]);
}

// Sorts the polygon points counter-clockwise
vector<vector<int>> sortPoints(vector<vector<int>> polygon) {
    vector<int> mid = {0, 0};
    int n = polygon.size();

    // Calculate center (centroid) of the polygon
    for (int i = 0; i < n; i++) {
        mid[0] += polygon[i][0];
        mid[1] += polygon[i][1];
        polygon[i][0] *= n;
        polygon[i][1] *= n;
    }

    // Sort points based on their angle from the center
    sort(polygon.begin(), polygon.end(), 
            [mid](vector<int> p1, vector<int> p2) {
        return compare(p1, p2, mid);
    });

    // Divide back to original coordinates
    for (int i = 0; i < n; i++) {
        polygon[i][0] /= n;
        polygon[i][1] /= n;
    }

    return polygon;
}

// Finds the upper tangent between two convex polygons a and b
// Returns two points forming the upper tangent
vector<vector<int>> findUpperTangent(vector<vector<int>> a,
                                        vector<vector<int>> b) {
                                            
    int n1 = a.size(), n2 = b.size();

    // Find the rightmost point of polygon a and leftmost point of polygon b
    int maxa = INT_MIN;
    for (auto& p : a)
        maxa = max(maxa, p[0]);

    int minb = INT_MAX;
    for (auto& p : b)
        minb = min(minb, p[0]);

    // Sort both polygons counter-clockwise
    a = sortPoints(a);
    b = sortPoints(b);

    // Ensure polygon a is to the left of polygon b
    if (minb < maxa)
        swap(a, b);

    n1 = a.size();
    n2 = b.size();

    // Find the rightmost point in a
    int ia = 0, ib = 0;
    for (int i = 1; i < n1; i++)
        if (a[i][0] > a[ia][0])
            ia = i;

    // Find the leftmost point in b
    for (int i = 1; i < n2; i++)
        if (b[i][0] < b[ib][0])
            ib = i;

    // Initialize starting points
    int inda = ia, indb = ib;
    bool done = false;

    // Find upper tangent using orientation checks
    while (!done) {
        done = true;
        // Move to next point in a if necessary
        while (orientation(b[indb], a[inda], a[(inda + 1) % n1]) > 0)
            inda = (inda + 1) % n1;

        // Move to previous point in b if necessary
        while (orientation(a[inda], b[indb],
               b[(n2 + indb - 1) % n2]) < 0) {
            indb = (n2 + indb - 1) % n2;
            done = false;
        }
    }

    // Return the points forming the upper tangent
    return {a[inda], b[indb]};
}

// Main driver code
int main() {
    vector<vector<int>> a = {{2, 2}, {3, 1}, {3, 3}, {5, 2}, {4, 0}};
    vector<vector<int>> b = {{0, 1}, {1, 0}, {0, -2}, {-1, 0}};

    vector<vector<int>> tangent = findUpperTangent(a, b);
    for(auto it:tangent){
        cout<<it[0]<<" "<<it[1]<<"\n";
    }
    return 0;
}

Java

import java.util.*;

class GfG {

    // Determines the quadrant of a point
    static int quad(int[] p) {
        if (p[0] >= 0 && p[1] >= 0)
            return 1;
        if (p[0] <= 0 && p[1] >= 0)
            return 2;
        if (p[0] <= 0 && p[1] <= 0)
            return 3;
        return 4;
    }

    // Checks whether the line is crossing the polygon
    static int orientation(int[] a, int[] b, int[] c) {
        int res = (b[1] - a[1]) * (c[0] - b[0]) -
                  (c[1] - b[1]) * (b[0] - a[0]);

        if (res == 0)
            return 0;
        if (res > 0)
            return 1;
        return -1;
    }

    // Compare function for sorting
    static class PointComparator implements Comparator<int[]> {
        int[] mid;

        public PointComparator(int[] mid) {
            this.mid = mid;
        }

        public int compare(int[] p1, int[] p2) {
            int[] p = { p1[0] - mid[0], p1[1] - mid[1] };
            int[] q = { p2[0] - mid[0], p2[1] - mid[1] };

            int one = quad(p);
            int two = quad(q);

            if (one != two)
                return one - two;
            return p[1] * q[0] - q[1] * p[0];
        }
    }

    // Finds upper tangent of two polygons 'a' and 'b'
    // represented as 2D arrays and stores the result
    static int[][] findUpperTangent(int[][] a, int[][] b) {
        // n1 -> number of points in polygon a
        // n2 -> number of points in polygon b
        int n1 = a.length, n2 = b.length;

        int[] mid = { 0, 0 };
        int maxa = Integer.MIN_VALUE;

        // Calculate centroid for polygon a and adjust points for scaling
        for (int i = 0; i < n1; i++) {
            maxa = Math.max(maxa, a[i][0]);
            mid[0] += a[i][0];
            mid[1] += a[i][1];
            a[i][0] *= n1;
            a[i][1] *= n1;
        }

        // Sorting the points in counter-clockwise order for polygon a
        Arrays.sort(a, new PointComparator(mid));

        for (int i = 0; i < n1; i++) {
            a[i][0] /= n1;
            a[i][1] /= n1;
        }

        mid[0] = 0;
        mid[1] = 0;
        int minb = Integer.MAX_VALUE;

        // Calculate centroid for polygon b and adjust points for scaling
        for (int i = 0; i < n2; i++) {
            mid[0] += b[i][0];
            mid[1] += b[i][1];
            minb = Math.min(minb, b[i][0]);
            b[i][0] *= n2;
            b[i][1] *= n2;
        }

        // Sorting the points in counter-clockwise order for polygon b
        Arrays.sort(b, new PointComparator(mid));

        for (int i = 0; i < n2; i++) {
            b[i][0] /= n2;
            b[i][1] /= n2;
        }

        // If a is to the right of b, swap a and b
        if (minb < maxa) {
            int[][] temp = a;
            a = b;
            b = temp;
            n1 = a.length;
            n2 = b.length;
        }

        // ia -> rightmost point of a
        int ia = 0, ib = 0;
        for (int i = 1; i < n1; i++) {
            if (a[i][0] > a[ia][0])
                ia = i;
        }

        // ib -> leftmost point of b
        for (int i = 1; i < n2; i++) {
            if (b[i][0] < b[ib][0])
                ib = i;
        }

        // Finding the upper tangent
        int inda = ia, indb = ib;
        boolean done = false;
        while (!done) {
            done = true;
            while (orientation(b[indb], a[inda], a[(inda + 1) % n1]) > 0)
                inda = (inda + 1) % n1;

            while (orientation(a[inda], b[indb], 
                    b[(n2 + indb - 1) % n2]) < 0) {
                indb = (n2 + indb - 1) % n2;
                done = false;
            }
        }

        // Returning the upper tangent as a 2D array
        return new int[][] {
            { a[inda][0], a[inda][1] },
            { b[indb][0], b[indb][1] }
        };
    }

    // Driver code
    public static void main(String[] args) {
        
        int[][] a = {{2, 2},{3, 1},{3, 3},{5, 2},{4, 0}};
        int[][] b = {{0, 1},{1, 0},{0, -2},{-1, 0}};

        // Get the upper tangent as a 2D array
        int[][] upperTangent = findUpperTangent(a, b);

        // Store or use the result
        System.out.println(upperTangent[0][0] + " " + upperTangent[0][1]);
        System.out.println(upperTangent[1][0] + " " + upperTangent[1][1]);

    }
}

Python

from functools import cmp_to_key

def quad(p):
    if p[0] >= 0 and p[1] >= 0:
        return 1
    if p[0] <= 0 and p[1] >= 0:
        return 2
    if p[0] <= 0 and p[1] <= 0:
        return 3
    return 4

def orientation(a, b, c):
    res = (b[1] - a[1]) * (c[0] - b[0]) - (c[1] - b[1]) * (b[0] - a[0])
    if res == 0:
        return 0
    if res > 0:
        return 1
    return -1

def compare(mid):
    def cmp(p1, q1):
        p = [p1[0] - mid[0], p1[1] - mid[1]]
        q = [q1[0] - mid[0], q1[1] - mid[1]]
        one = quad(p)
        two = quad(q)
        if one != two:
            return one - two
        if p[1] * q[0] < q[1] * p[0]:
            return -1
        return 1
    return cmp

def findUpperTangent(a, b):
    n1, n2 = len(a), len(b)

    mid_a = [0, 0]
    maxa = float('-inf')
    for i in range(n1):
        maxa = max(maxa, a[i][0])
        mid_a[0] += a[i][0]
        mid_a[1] += a[i][1]

    a = sorted(a, key=cmp_to_key(compare(mid_a)))

    mid_b = [0, 0]
    minb = float('inf')
    for i in range(n2):
        minb = min(minb, b[i][0])
        mid_b[0] += b[i][0]
        mid_b[1] += b[i][1]

    b = sorted(b, key=cmp_to_key(compare(mid_b)))

    if minb < maxa:
        a, b = b, a
        n1, n2 = n2, n1

    ia = 0
    for i in range(1, n1):
        if a[i][0] > a[ia][0]:
            ia = i

    ib = 0
    for i in range(1, n2):
        if b[i][0] < b[ib][0]:
            ib = i

    inda, indb = ia, ib
    done = False
    while not done:
        done = True
        while orientation(b[indb], a[inda], a[(inda + 1) % n1]) > 0:
            inda = (inda + 1) % n1
        while orientation(a[inda], b[indb], b[(n2 + indb - 1) % n2]) < 0:
            indb = (n2 + indb - 1) % n2
            done = False

    # return integer coordinates
    return [[int(a[inda][0]), int(a[inda][1])],
            [int(b[indb][0]), int(b[indb][1])]]

# Driver Code
if __name__ == '__main__':
    a = [
        [2, 2],
        [3, 1],
        [3, 3],
        [5, 2],
        [4, 0]
    ]

    b = [
        [0, 1],
        [1, 0],
        [0, -2],
        [-1, 0]
    ]

    upperTangent = findUpperTangent(a, b)

    for point in upperTangent:
        print(f"{point[0]} {point[1]}")

using System;

public class UpperTangentFinder
{
    static int Quad(int x, int y)
    {
        if (x >= 0 && y >= 0) return 1;
        if (x <= 0 && y >= 0) return 2;
        if (x <= 0 && y <= 0) return 3;
        return 4;
    }

    static int Orientation(int[] a, int[] b, int[] c)
    {
        int res = (b[1] - a[1]) * (c[0] - b[0]) - 
                  (c[1] - b[1]) * (b[0] - a[0]);
        if (res == 0) return 0;
        return res > 0 ? 1 : -1;
    }

    static bool Compare(int[] p1, int[] p2, int[] mid)
    {
        int[] p = { p1[0] - mid[0], p1[1] - mid[1] };
        int[] q = { p2[0] - mid[0], p2[1] - mid[1] };

        int quadP = Quad(p[0], p[1]);
        int quadQ = Quad(q[0], q[1]);

        if (quadP != quadQ)
            return quadP < quadQ;
        return (p[1] * q[0]) < (q[1] * p[0]);
    }

    static int[,] SortPoints(int[,] polygon)
    {
        int n = polygon.GetLength(0);
        int[] mid = { 0, 0 };

        for (int i = 0; i < n; i++)
        {
            mid[0] += polygon[i, 0];
            mid[1] += polygon[i, 1];
            polygon[i, 0] *= n;
            polygon[i, 1] *= n;
        }

        for (int i = 0; i < n - 1; i++)
        {
            for (int j = i + 1; j < n; j++)
            {
                int[] p1 = { polygon[i, 0], polygon[i, 1] };
                int[] p2 = { polygon[j, 0], polygon[j, 1] };
                if (!Compare(p1, p2, mid))
                {
                    int tempX = polygon[i, 0], tempY = polygon[i, 1];
                    polygon[i, 0] = polygon[j, 0];
                    polygon[i, 1] = polygon[j, 1];
                    polygon[j, 0] = tempX;
                    polygon[j, 1] = tempY;
                }
            }
        }

        for (int i = 0; i < n; i++)
        {
            polygon[i, 0] /= n;
            polygon[i, 1] /= n;
        }

        return polygon;
    }

    static int[,] FindUpperTangent(int[,] a, int[,] b)
    {
        int n1 = a.GetLength(0);
        int n2 = b.GetLength(0);

        int maxa = int.MinValue;
        for (int i = 0; i < n1; i++)
            maxa = Math.Max(maxa, a[i, 0]);

        int minb = int.MaxValue;
        for (int i = 0; i < n2; i++)
            minb = Math.Min(minb, b[i, 0]);

        a = SortPoints(a);
        b = SortPoints(b);

        if (minb < maxa)
        {
            int[,] temp = a;
            a = b;
            b = temp;
            n1 = a.GetLength(0);
            n2 = b.GetLength(0);
        }

        int ia = 0, ib = 0;
        for (int i = 1; i < n1; i++)
            if (a[i, 0] > a[ia, 0])
                ia = i;

        for (int i = 1; i < n2; i++)
            if (b[i, 0] < b[ib, 0])
                ib = i;

        int inda = ia, indb = ib;
        bool done = false;

        while (!done)
        {
            done = true;
            while (Orientation(
                new int[] { b[indb, 0], b[indb, 1] },
                new int[] { a[inda, 0], a[inda, 1] },
                new int[] { a[(inda + 1) % n1, 0],
                        a[(inda + 1) % n1, 1] }) > 0){
                inda = (inda + 1) % n1;
            }

            while (Orientation(
                new int[] { a[inda, 0], a[inda, 1] },
                new int[] { b[indb, 0], b[indb, 1] },
                new int[] { b[(n2 + indb - 1) % n2, 0], 
                       b[(n2 + indb - 1) % n2, 1] }) < 0){
                           
                indb = (n2 + indb - 1) % n2;
                done = false;
            }
        }

        int[,] result = new int[2, 2];
        result[0, 0] = a[inda, 0];
        result[0, 1] = a[inda, 1];
        result[1, 0] = b[indb, 0];
        result[1, 1] = b[indb, 1];

        return result;
    }

    public static void Main(string[] args)
    {
        int[,] a = new int[,] {
            {2, 2},
            {3, 1},
            {3, 3},
            {5, 2},
            {4, 0}
        };

        int[,] b = new int[,] {
            {0, 1},
            {1, 0},
            {0, -2},
            {-1, 0}
        };

        int[,] tangent = FindUpperTangent(a, b);

        for (int i = 0; i < 2; i++)
        {
            Console.WriteLine(tangent[i, 0] + " " + tangent[i, 1]);
        }
    }
}

JavaScript

// Determine the quadrant of a point
function quad(p) {
    if (p[0] >= 0 && p[1] >= 0) return 1;
    if (p[0] <= 0 && p[1] >= 0) return 2;
    if (p[0] <= 0 && p[1] <= 0) return 3;
    return 4;
}

// Find orientation of triplet (a, b, c)
function orientation(a, b, c) {
    let res = (b[1] - a[1]) * (c[0] - b[0]) - (c[1] - b[1]) * (b[0] - a[0]);
    if (res === 0) return 0;
    return res > 0 ? 1 : -1;
}

// Compare two points based on mid
function compare(p1, p2, mid) {
    let p = [p1[0] - mid[0], p1[1] - mid[1]];
    let q = [p2[0] - mid[0], p2[1] - mid[1]];

    let quadP = quad(p);
    let quadQ = quad(q);

    if (quadP !== quadQ)
        return quadP - quadQ;
    return (p[1] * q[0]) - (q[1] * p[0]);
}

// Sort polygon points counter-clockwise
function sortPoints(polygon) {
    let n = polygon.length;
    let mid = [0, 0];

    for (let i = 0; i < n; i++) {
        mid[0] += polygon[i][0];
        mid[1] += polygon[i][1];
        polygon[i][0] *= n;
        polygon[i][1] *= n;
    }

    polygon.sort((p1, p2) => compare(p1, p2, mid));

    for (let i = 0; i < n; i++) {
        polygon[i][0] = Math.floor(polygon[i][0] / n);
        polygon[i][1] = Math.floor(polygon[i][1] / n);
    }

    return polygon;
}

// Find upper tangent between two convex polygons
function findUpperTangent(a, b) {
    let n1 = a.length;
    let n2 = b.length;

    let maxa = -Infinity;
    for (let i = 0; i < n1; i++)
        maxa = Math.max(maxa, a[i][0]);

    let minb = Infinity;
    for (let i = 0; i < n2; i++)
        minb = Math.min(minb, b[i][0]);

    a = sortPoints(a);
    b = sortPoints(b);

    if (minb < maxa) {
        let temp = a;
        a = b;
        b = temp;
        n1 = a.length;
        n2 = b.length;
    }

    let ia = 0, ib = 0;
    for (let i = 1; i < n1; i++)
        if (a[i][0] > a[ia][0])
            ia = i;

    for (let i = 1; i < n2; i++)
        if (b[i][0] < b[ib][0])
            ib = i;

    let inda = ia, indb = ib;
    let done = false;

    while (!done) {
        done = true;
        while (orientation(b[indb], a[inda], a[(inda + 1) % n1]) > 0)
            inda = (inda + 1) % n1;

        while (orientation(a[inda], b[indb], b[(n2 + indb - 1) % n2]) < 0) {
            indb = (n2 + indb - 1) % n2;
            done = false;
        }
    }

    return [a[inda], b[indb]];
}

// Driver code
let a = [[2, 2],[3, 1],[3, 3],[5, 2],[4, 0]];

let b = [[0, 1],[1, 0],[0, -2],[-1, 0]];

let tangent = findUpperTangent(a, b);

for (let point of tangent) {
    console.log(point[0] + " " + point[1]);
}

Output

upper tangent (0,1) (3,3)

Time Complexity: O(n1 log (n1) + n2 log(n2))
Auxiliary Space: O(1)

Analysis of Algorithms

Amritya Vagmi

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Article Tags :

Tangents between two Convex Polygons

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