How to use magnitude and absdiff OpenCV functions to compute distances? - opencv

Question

How can I use magnitude and absdiff? I read the explanation in the documentation, but every time it gives an error because I do not understand how exactly should be input arrays and output. Should it be vector, Mat or Scalar? I tried some but I failed, same with cartToPolar. Can anyone give me a small snippet of that, since I didn't find any examples in the documentation?
More precisely, I have the vector vector<Vec4f> lines; that contains the end point and start point of 30 lines, so I want to use magnitude to find length of each line. I learned how to use norm by for loop but I would like to use magnitude so I did it like:
double x;
length=magnitude(lines[i][2]-lines[i][0],lines[i][3]-lines[i][1],x)
but it doesn't work. I tried to define x as 1 array vector, but I couldn't.

Answer 1

You already got how to use norm to compute the distance:
Point2f a = ...
Point2f b = ..
double length = norm(a - b); // NORM_L2, NORM_L1
You can also work on all points at once. You first need to convert the coordinates from vector to matrix form, then it's just simple math:
#include <opencv2\opencv.hpp>
#include <iostream>
using namespace std;
using namespace cv;
int main()
{
vector<Vec4f> lines{ { 1, 2, 4, 6 }, { 5, 7, 1, 3 }, { 11, 12, 12, 11 } };
Mat1f coordinates = Mat4f(lines).reshape(1);
Mat1f diff_x = coordinates.col(0) - coordinates.col(2);
Mat1f diff_y = coordinates.col(1) - coordinates.col(3);
cout << "coordinates: \n" << coordinates << "\n\n";
cout << "diff_x: \n" << diff_x << "\n\n";
cout << "diff_y: \n" << diff_y << "\n\n";
cout << endl;
// sqrt((x2 - x1)^2 + (y2 - y1)^2)
Mat1f euclidean_distance;
magnitude(diff_x, diff_y, euclidean_distance);
cout << "euclidean_distance: \n" << euclidean_distance << "\n\n";
// abs(x2 - x1) + abs(y2 - y1)
Mat1f manhattan_distance = abs(diff_x) + abs(diff_y);
cout << "manhattan_distance: \n" << manhattan_distance << "\n\n";
// Another way to compute L1 distance, with absdiff
// abs(x2 - x1) + abs(y2 - y1)
Mat1f points1 = coordinates(Range::all(), Range(0, 2));
Mat1f points2 = coordinates(Range::all(), Range(2, 4));
Mat1f other_manhattan_distance;
absdiff(points1, points2, other_manhattan_distance);
other_manhattan_distance = other_manhattan_distance.col(0) + other_manhattan_distance.col(1);
cout << "other_manhattan_distance: \n" << other_manhattan_distance << "\n\n";
return 0;
}