c++ - How to fill Matrix with zeros in OpenCV?

Question

The code below causes an exception. Why?

#include <opencv2/core/core.hpp>
#include <iostream>

using namespace cv;
using namespace std;

void main() {

    try {
        Mat m1 = Mat(1,1, CV_64F, 0);
        m1.at<double>(0,0) = 0;
    }
    catch(cv::Exception &e) {
        cerr << e.what() << endl;
    }

}

Error is follows:

OpenCV Error: Assertion failed (dims <= 2 && data && (unsigned)i0 < (unsigned)size.p[0] && (unsigned)(i1*DataType<_Tp>::channels) < (unsigned)(size.p[1]*channels()) && ((((sizeof(size_t)<<28)|0x8442211) >> ((DataType<_Tp>::depth) & ((1 << 3
) - 1))*4) & 15) == elemSize1()) in unknown function, file %OPENCV_DIR%uildincludeopencv2coremat.hpp, line 537

UPDATE

If tracing this code, I see that constructor line calls the constructor

inline Mat::Mat(int _rows, int _cols, int _type, void* _data, size_t _step)

Why? This prototype has 5 parameters, while I am providing 4 arguments.

Answer 1

How to fill Matrix with zeros in OpenCV?

To fill a pre-existing Mat object with zeros, you can use Mat::zeros()

Mat m1 = ...;
m1 = Mat::zeros(1, 1, CV_64F);

To intialize a Mat so that it contains only zeros, you can pass a scalar with value 0 to the constructor:

Mat m1 = Mat(1,1, CV_64F, 0.0);
//                        ^^^^double literal

The reason your version failed is that passing 0 as fourth argument matches the overload taking a void* better than the one taking a scalar.