How to convert opencv mat into string c++

Question

float matrix_data[] = {0.9867, 0.02454, -0.1603,
                       0.01921, 0.9638, 0.2657,
                       0.16112, -0.2652, 0.9506};

cv::Mat res_mat = cv::Mat(3,3,CV_32F,matrix_data);
cout << "res_mat :" << res_mat<<endl;

I know I can see the output of the mat by this but I want to add this mat to a string, something like this:

std::string my_str = "my mat :";
my_str += to_String(res_mat);

so the desired result will be like :

my mat : 0.9867, 0.02454, -0.1603, 0.01921, 0.9638, 0.2657,0.16112, -0.2652, 0.9506

rafix07 · Accepted Answer · 2020-04-17 04:32:55Z

2

Use std::ostringstream. Before that, create new header for your matrix to treat it as vector:

cv::Mat oneRow = res_mat.reshape(0,1);    // Treat as vector 
std::ostringstream os;
os << oneRow;                             // Put to the stream
std::string asStr = os.str();             // Get string 
asStr.pop_back();                         // Remove brackets
asStr.erase(0,1);
cout << "res_mat :" << asStr <<endl;

answered Apr 17, 2020 at 4:32

rafix07

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2 Comments

oarfish Aug 27 at 10:07

which opencv header defines operator<<() for ostream and Mat? Without including anything, this does not compile (in current opencv versions).

rafix07 Aug 27 at 10:30

In my version it is contained by cvstd.inl.hpp

Yunus Temurlenk · Accepted Answer · 2020-04-17 05:33:18Z

1

You may also reach every element of the Mat and adding the elements to a string type. This can be also used.

Here is the code:

#include <opencv2/opencv.hpp>
#include <string>

using namespace cv;
using namespace std;

int main()
{

    float matrix_data[] = {0.9867, 0.02454, -0.1603,
                           0.01921, 0.9638, 0.2657,
                           0.16112, -0.2652, 0.9506};

    cv::Mat res_mat = cv::Mat(3,3,CV_32F,matrix_data);
    cout << "res_mat :" << res_mat<<endl;

    std::string my_str = "my mat :";

    for(int i=0; i<res_mat.rows; i++)
    {
       for(int j=0; j<res_mat.cols; j++)
       {
           my_str += to_string(res_mat.at<float>(i,j)) + ", ";
       }
    }

    cout<<my_str<<endl;

}

answered Apr 17, 2020 at 5:33

Yunus Temurlenk

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1 Comment

유 길재 Over a year ago

perfect this was what I was looking for

John · Accepted Answer · 2021-07-09 23:23:46Z

1

You can get a string representation by using String& operator << (String& out, const Mat& mtx)

float matrix_data[] = {0.9867, 0.02454, -0.1603,
                       0.01921, 0.9638, 0.2657,
                       0.16112, -0.2652, 0.9506};

cv::Mat res_mat = cv::Mat(3, 3, CV_32F, matrix_data);

std::string my_str = "my mat :";
my_str << res_mat;  // ← it's that simple

answered Jul 9, 2021 at 23:23

John

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