Compare two std::vector<cv::Point> vectors and safe common points to a third std::vector<cv::Point>

Question

I'm looking for an alternative version for the std::set_intersection function but for std::vector<cv::Point> vectors.

I try to compare two std::vector<cv::Point> vectors with different sizes. Those two contain coordinate lists. The main task of the intersection-like Method should now be to detect common pairs and safe them to a third std::vector<cv::Point> via push_back()

I searched for a function like std::set_intersection(v1.begin(),v1.end(),v2.begin(),v2.end(),back_inserter(v3));

Any ideas how to solve this?

Answer 1

As already mentioned by @BoBTFish and @NaCl, you need to use a custom comparator, and apply set_intersection on sorted vectors.

Since you need to call the comparator three times, it's useful to use a function instead of a lambda expression.

#include <opencv2/opencv.hpp>
#include <vector>
#include <algorithm>

using namespace std;
using namespace cv;

// Custom less comparator
bool lessPoints(const Point& lhs, const Point& rhs) {
    return (lhs.x < rhs.x) || ((lhs.x == rhs.x) && (lhs.y < rhs.y));
}

vector<Point> intersection(vector<Point> v1, vector<Point> v2)
{
    vector<Point> v3;
    // Sort vectors
    sort(v1.begin(), v1.end(), lessPoints);
    sort(v2.begin(), v2.end(), lessPoints);
    // Intersect
    set_intersection(v1.begin(), v1.end(), v2.begin(), v2.end(), back_inserter(v3), lessPoints);
    return v3;
}

int main()
{
    // Your vectors
    vector<Point> v1{ Point(2,3), Point(1,2), Point(5,5), Point(3,4) };
    vector<Point> v2{ Point(2,1), Point(1,2), Point(3,4), Point(6,7), Point(0,3) };

    // Find intersections
    vector<Point> v3 = intersection(v1, v2);

    // Print out vector content
    std::copy(v3.begin(), v3.end(), std::ostream_iterator<Point>(std::cout, " "));

    return 0;
}