# Maximum distance between two points in coordinate plane using Rotating Caliper’s Method

Prerequisites: Graham Scan’s Convex Hull, Orientation.

Given a set of N points in a coordinates plane, the task is to find the maximum distance between any two points in the given set of planes.

Examples:

Input: n = 4, Points: (0, 3), (3, 0), (0, 0), (1, 1)
Output: Maximum Distance = 4.24264
Explanation:
Points having maximum distance between them are (0, 3) and (3, 0)

Input: n = 5, Points: (4, 0), (0, 2), (-1, -7), (1, 10), (2, -3)
Output: Maximum Distance = 17.11724
Explanation:
Points having maximum distance between them are (-1, 7) and (1, 10)

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Naive Approach: The naive idea is to try every possible pair of points from the given set and calculate the distances between each of them and print the maximum distance among all the pairs.

Below is the implementation of the above approach:

## Java

 import java.awt.*;  import java.util.ArrayList;     public class Main {         // Function calculates distance      // between two points      static long dist(Point p1, Point p2)      {          long x0 = p1.x - p2.x;          long y0 = p1.y - p2.y;          return x0 * x0 + y0 * y0;      }         // Function to find the maximum      // distance between any two points      static double maxDist(Point p[])      {          int n = p.length;          double max = 0;             // Iterate over all possible pairs          for (int i = 0; i < n; i++) {                 for (int j = i + 1; j < n; j++) {                     // Update max                  max = Math.max(max,                                 dist(p[i],                                      p[j]));              }          }             // Return actual distance          return Math.sqrt(max);      }         // Driver Code      public static void main(String[] args)      {          // Number of points          int n = 5;             Point p[] = new Point[n];             // Given points          p = new Point(4, 0);          p = new Point(0, 2);          p = new Point(-1, -7);          p = new Point(1, 10);          p = new Point(2, -3);             // Function Call          System.out.println(maxDist(p));      }  }

## Python3

 from math import sqrt     # Function calculates distance  # between two points  def dist(p1, p2):             x0 = p1 - p2      y0 = p1 - p2      return x0 * x0 + y0 * y0     # Function to find the maximum  # distance between any two points  def maxDist(p):         n = len(p)      maxm = 0        # Iterate over all possible pairs      for i in range(n):          for j in range(i + 1, n):                             # Update maxm              maxm = max(maxm, dist(p[i], p[j]))         # Return actual distance      return sqrt(maxm)         # Driver Code  if __name__ == '__main__':             # Number of points      n = 5        p = []             # Given points      p.append([4, 0])      p.append([0, 2])      p.append([-1, -7])      p.append([1, 10])      p.append([2, -3])         # Function Call      print(maxDist(p))     # This code is contributed by mohit kumar 29

Output:

17.11724276862369


Time Complexity: O(N2), where N is the total number of points.
Auxiliary Space: O(1)

Efficient Approach: The above naive approach can be optimized using Rotating Caliper’s Method.

Rotating Calipers is a method for solving a number of problems from the field of computational geometry. It resembles the idea of rotating an adjustable caliper around the outside of a polygon’s convex hull. Originally, this method was invented to compute the diameter of convex polygons. It can also be used to compute the minimum and maximum distance between two convex polygons, the intersection of convex polygons, the maximum distance between two points in a polygon, and many things more.

To implement the above method we will use the concept of the Convex Hull. Before we begin a further discussion about the optimal approach, we need to know about the following:

• Unsigned Area Of Triangle: If we are given three points P1(x1, y1), P2(x2, y2) and P3(x3, y3) then is the signed area of triangle. If the area is positive then three points are in the clockwise order, Else they are in anti-clockwise order and if the area equals to zero then, the points are co-linear. If we take absolute value, then this will represent the unsigned area of the triangle. Here, unsigned basically means area without direction i.e., we just need the relative absolute value of the area. Therefore, we can remove (1/2) from the formula. Hence,

Relative Area of Triangle = abs((x2-x1)*(y3-y2)-(x3-x2)*(y2-y1))

• Antipodal Points: It is those points which are diametrically opposite to each other. But for us, antipodal points are those which are farthest from each other in the convex polygon. If we choose one point from the given set, then this point can only achieve it’s maximum distance if and only if we can find it’s antipodal point from the given set.

Below are the steps:

1. Two points having maximum distance must lie on the boundary of the convex polygon formed from the given set. Therefore, use Graham Scan’s convex hull method to arrange points in counter-clockwise order.
2. We have N points, Initially start from point P1 and include those points from set of given points such that area of region always increases by including any points from the set.
3. Starting from point P1, Choose K = 2 and increment K while area(PN, P1, PK) is increasing and stop before it starts decreasing. Now the current point PK might be the antipodal point for P1. Similarly, find antipodal point for p2 by finding area(P1, P2, PK) and incrementing K form where we previously stopped and so on.
4. Keep updating the maximum distance for each antipodal points occurs in the above steps as the distance between intial point and point by including area was maximum.

Below is the implementation of the above approach:

## Java

 // Java Program for the above approach  import java.awt.*;  import java.util.*;  import java.util.Map.Entry;     public class Main {         // Function to detect the orientation      static int orientation(Point p,                             Point q,                             Point r)      {          int x = area(p, q, r);             // If area > 0 then          // points are clockwise          if (x > 0) {              return 1;          }             // If area<0 then          // points are counterclockwise          if (x < 0) {              return -1;          }             // If area is 0 then p, q          // and r are co-linear          return 0;      }         // Function to find the area      static int area(Point p, Point q, Point r)      {          // 2*(area of triangle)          return ((p.y - q.y) * (q.x - r.x)                  - (q.y - r.y) * (p.x - q.x));      }         // Function to find the absolute Area      static int absArea(Point p,                         Point q, Point r)      {          // Unsigned area          // 2*(area of triangle)          return Math.abs(area(p, q, r));      }         // Function to find the distance      static int dist(Point p1, Point p2)      {          // squared-distance b/w          // p1 and p2 for precision          return ((p1.x - p2.x) * (p1.x - p2.x)                  + (p1.y - p2.y) * (p1.y - p2.y));      }         // Function to implement Convex Hull      // Approach      static ArrayList      convexHull(Point points[])      {          int n = points.length;             Point min = new Point(Integer.MAX_VALUE,                                Integer.MAX_VALUE);             // Choose point having min.          // y-coordinate and if two points          // have same y-coordinate choose          // the one with minimum x-coordinate          int ind = -1;             // Iterate Points[]          for (int i = 0; i < n; i++) {              if (min.y > points[i].y) {                  min.y = points[i].y;                  min.x = points[i].x;                  ind = i;              }              else if (min.y == points[i].y                       && min.x > points[i].x) {                  min.x = points[i].x;                  ind = i;              }          }          points[ind] = points;          points = min;             // Sort points which have          // minimum polar angle wrt          // Point min          Arrays.sort(points, 1, n,                      new Comparator() {                             @Override                         public int compare(Point o1,                                             Point o2)                          {                                 int o = orientation(min, o1, o2);                                 // If points are co-linear                              // choose the one having smaller                              // distance with min first.                              if (o == 0) {                                  return dist(o1, min)                                      - dist(o2, min);                              }                                 // If clockwise then swap                              if (o == 1) {                                  return 1;                              }                                 // If anticlockwise then                              // don't swap                              return -1;                          }                      });             Stack st = new Stack<>();             // First hull point          st.push(points);             int k;          for (k = 1; k < n - 1; k++) {              if (orientation(points,                              points[k],                              points[k + 1])                  != 0)                  break;          }             // Second hull point          st.push(points[k]);             for (int i = k + 1; i < n; i++) {              Point top = st.pop();                 while (orientation(st.peek(),                                 top,                                 points[i])                     >= 0) {                  top = st.pop();              }                 st.push(top);              st.push(points[i]);          }             ArrayList hull              = new ArrayList<>();             // Iterate stack and add node to hull          for (int i = 0; i < st.size(); i++) {              hull.add(st.get(i));          }          return hull;      }         // Function to find the maximum      // distance between any two points      // from a set of given points      static double     rotatingCaliper(Point points[])      {          // Takes O(n)          ArrayList convexHull              = convexHull(points);          int n = convexHull.size();             // Convex hull point in counter-          // clockwise order          Point hull[] = new Point[n];          n = 0;             while (n < convexHull.size()) {              hull[n] = convexHull.get(n++);          }             // Base Cases          if (n == 1)              return 0;          if (n == 2)              return Math.sqrt(dist(hull, hull));          int k = 1;             // Find the farthest vertex          // from hull and hull[n-1]          while (absArea(hull[n - 1],                         hull,                         hull[(k + 1) % n])                 > absArea(hull[n - 1],                           hull,                           hull[k])) {              k++;          }             double res = 0;             // Check points from 0 to k          for (int i = 0, j = k;               i <= k && j < n; i++) {              res = Math.max(res,                             Math.sqrt((double)dist(hull[i],                                                    hull[j])));                 while (j < n                     && absArea(hull[i],                                hull[(i + 1) % n],                                hull[(j + 1) % n])                            > absArea(hull[i],                                      hull[(i + 1) % n],                                      hull[j])) {                     // Update res                  res = Math.max(                      res,                      Math.sqrt(dist(hull[i],                                     hull[(j + 1) % n])));                  j++;              }          }             // Return the result distance          return res;      }         // Driver Code      public static void main(String[] args)      {          // Total points          int n = 5;          Point p[] = new Point[n];             // Given Points          p = new Point(4, 0);          p = new Point(0, 2);          p = new Point(-1, -7);          p = new Point(1, 10);          p = new Point(2, -3);             // Function Call          System.out.println(rotatingCaliper(p));      }  }

Output:

17.11724276862369


Time Complexity: O(N*log N)
Auxiliary Space: O(N)

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