Print Out the Values of a (Mat) Matrix in Opencv C++

Print out the values of a (Mat) matrix in OpenCV C++

See the first answer to Accessing a matrix element in the "Mat" object (not the CvMat object) in OpenCV C++

Then just loop over all the elements in cout << M.at<double>(0,0); rather than just 0,0

Or better still with the C++ interface:

cv::Mat M;
cout << "M = " << endl << " "  << M << endl << endl;

Print openCV Mat content with certain format in C++

#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

#include <iostream>
#include <iomanip>

using namespace cv;
using namespace std;

void print(Mat mat, int prec)
{      
    for(int i=0; i<mat.size().height; i++)
    {
        cout << "[";
        for(int j=0; j<mat.size().width; j++)
        {
            cout << setprecision(prec) << mat.at<double>(i,j);
            if(j != mat.size().width-1)
                cout << ", ";
            else
                cout << "]" << endl; 
        }
    }
}

int main(int argc, char** argv)
{
    double data[2][4];
    for(int i=0; i<2; i++)
    {
        for(int j=0; j<4; j++)
        {
            data[i][j] = 0.123456789;
        }
    }
    Mat src = Mat(2, 4, CV_64F, &data);
    print(src, 3);

    return 0;
}

Printing a value of Mat object

ptr accepts as argument the row number. You can thus get the pointer to the starting element of each row by calling pointer on each i (outside inner loop).

Then you need %d (or %u) in your printf (or you can use cout). Check the code below:

#include <opencv2/opencv.hpp>
#include<iostream>
using namespace std;
using namespace cv;

int main() 
{
    int N = 10;

    // Init matrix
    Mat destination(N, N, CV_8UC1);
    randu(destination, Scalar(0), Scalar(255));

    Size s = destination.size();
    int count = 0;
    for (int i = 0; i<s.height; i++)
    {
        unsigned char* pByte = destination.ptr<unsigned char>(i);
        for (int j = 0; j<s.width; j++) {
            count++;
            printf("value %d ", pByte[j]);
            printf("\n");

            //cout << "value " << int(pByte[j]) << endl;
        }
    }

    return(0);
}

You can see in my other answer other methods to get the value at specific position in a Mat.

Printing CV_32F opencv matrix values

Close, the loop needs to start at 0!

std::cout<<O.at<float>(0,i)<<" ";

Print multidimensional Mat in OpenCV (C++)

To print n-dim matrix you could use Matrix slice. Since 2d matrices are stored row by row, 3d matrices plane by plane and so on, you could use code:

cv::Mat sliceMat(cv::Mat L,int dim,std::vector<int> _sz)
{
cv::Mat M(L.dims - 1, std::vector<int>(_sz.begin() + 1, _sz.end()).data(), CV_8UC1, L.data + L.step[0] * 0);
return M;
}

To perform mat slice.For more dimensions you should make more slices. Example shows 3 and 4 dimension matrices:

std::cout << "3 dimensions" << std::endl;

std::vector<int> sz = { 3,3,3 };

cv::Mat L;
L.create(3, sz.data(), CV_8UC1);
L = cv::Scalar(255);

std::cout<< sliceMat(L, 1, sz);

std::cout << std::endl;
std::cout <<"4 dimensions"<< std::endl;
sz = { 5,4,3,5 };
L.create(4, sz.data(), CV_8UC1);
L = cv::Scalar(255);
std::cout << sliceMat(sliceMat(L, 1, sz),2, std::vector<int>(sz.begin() + 1, sz.end()));

end result screen

Get value from OpenCV Mat

When you initialize a Mat with 32FC1 you allocate cells of 32bits, which are represented by float in C. In order to work with a matrix of double which are 64 bit floating point values, you need to use 64FC1.

So, either change the matrix to doubles, or use at<float>

Printing out Matrix values in OPENCV does not work

It is printing a value that looks bad, but it is actually correct.

The type Vec3b is a vector with 3 byte entries, where those bytes are stored as unsigned char values to represent the 0 -> 255 color range. You are seeing the byte value of 205 in the console.

The Vec3b object most likely has the << overridden to deal with printing out the values for the user to read.

Try type casting the byte to an int to see the value:

cout << (int)image.at<Vec3b>(1, 1)[0] << std::endl;

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