cv :: Mat使用数组初始化

Question

I experienced a strange behavior after for example initializing through

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

int main() {
    cv::Mat h = cv::Mat(2, 2, CV_32F, {1.0, 2.0, 1.0, 0.0});
    std::cout << h << std::endl;
    return 0;
}

cout prints out [1, 1; 1, 1]. WTF just happened? I'm using eclipse on ubuntu, gcc version 5.4, OpenCV 3.2

Answer 1

You're not using a valid Mat constructor. You have a few options:

1. From an array:

    float pf[] = { 1.f, 2.f, 3.f, 4.f }; Mat1f m1(2, 2, pf); 

   or

    std::vector<float> vf = { 1.f, 2.f, 3.f, 4.f }; Mat1f m2(2, 2, vf.data()); 

2. With comma initializers:

    Mat1f m3 = (Mat1f(2, 2) << 1.f, 2.f, 3.f, 4.f); 

3. If the matrix is small, you can use Matx :

    Matx22f m4(1.f, 2.f, 3.f, 4.f); 

---

Note that a Mat1f is a typedef for Mat_<float> , which is a Mat of type CV_32FC1 .

---

Using your method doesn't work because {1.0, 2.0, 1.0, 0.0} constructs cv::Scalar , so you call the constructor Mat(int rows, int cols, int type, cv::Scalar) . Since you have only 1 channel, the matrix is initialized with the first value of this scalar, which is the first value in your initializer list.

Note that this is just a coincidence since your matrix has 4 elements (the maximum number supported by Scalar s). If you use a higher number of elements:

cv::Mat h(2, 3, CV_32F, {3.f, 2.f, 1.f, 0.f, 2.f, 5.f});

the code should not compile.