Unexpected result when resizing Vector of Vector of OpenCV Mat - c++

Question

I am on Ubuntu 16.04, GCC 5.4, latest OpenCV. Suppose I have a vector of double
std::vector<std::vector<double>> vecvecdouble;
vecvecdouble.resize(3, std::vector<double>(3, 0));
for (int j = 0; j < 3; j++){
for (int i = 0; i < 3; i++){
if (i == 0){
vecvecdouble[i][j] = 1;
vecvecdouble[i][j] = 1;
}
if (i == 1){
vecvecdouble[i][j] = 2;
vecvecdouble[i][j] = 2;
}
if (i == 1 && j == 0){
std::cout << vecvecdouble[i - 1][j] << std::endl;
std::cout << vecvecdouble[i][j] << std::endl;
std::cout << vecvecdouble[i + 1][j] << std::endl;
}
}
}
It prints
1
2
0
as expected. However, if I do the same thing with OpenCV cv::mat
std::vector<std::vector<cv::Mat>> vecvecmat;
vecvecmat.resize(
3, std::vector<cv::Mat>(3, cv::Mat(4, 4, CV_64FC1, cv::Scalar(0.0))));
for (int j = 0; j < 3; j++){
for (int i = 0; i < 3; i++){
if (i == 0){
vecvecmat[i][j].at<double>(0, 0) = 1;
vecvecmat[i][j].at<double>(0, 1) = 1;
}
if (i == 1){
vecvecmat[i][j].at<double>(0, 0) = 2;
vecvecmat[i][j].at<double>(0, 1) = 2;
}
if (i == 1 && j == 0){
std::cout << vecvecmat[i - 1][j] << std::endl;
std::cout << vecvecmat[i][j] << std::endl;
std::cout << vecvecmat[i + 1][j] << std::endl;
}
}
}
It prints
[2, 2, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0]
[2, 2, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0]
[2, 2, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0]
which is completely unexpected because I am expecting it to print
[1, 1, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0]
[2, 2, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0]
[0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0;
0, 0, 0, 0]
However, if I don't try to resize the vector in one single line and go through two for loops it actually returns the expected result
std::vector<std::vector<cv::Mat>> vecvecmat;
vecvecmat.resize(3);
for (int i = 0; i < 3; i++){
for (int j = 0; j < 3; j++){
cv::Mat mymat = cv::Mat(4, 4, CV_64FC1, cv::Scalar(0.0));
vecvecmat[i].push_back(mymat);
}
}
for (int j = 0; j < 3; j++){
for (int i = 0; i < 3; i++){
if (i == 0){
vecvecmat[i][j].at<double>(0, 0) = 1;
vecvecmat[i][j].at<double>(0, 1) = 1;
}
if (i == 1){
vecvecmat[i][j].at<double>(0, 0) = 2;
vecvecmat[i][j].at<double>(0, 1) = 2;
}
if (i == 1 && j == 0){
std::cout << vecvecmat[i - 1][j] << std::endl;
std::cout << vecvecmat[i][j] << std::endl;
std::cout << vecvecmat[i + 1][j] << std::endl;
}
}
}
What is wrong with the line?
vecvecmat.resize(3, std::vector<cv::Mat>(3, cv::Mat(4, 4, CV_64FC1, cv::Scalar(0.0))));

Answer 1

The documentation for the copy constructor for cv::Mat says (emphasis mine):
Parameters
m Array that (as a whole or partly) is assigned to the constructed matrix. No data is copied by these constructors. Instead, the header pointing to m data or its sub-array is constructed and associated with it. The reference counter, if any, is incremented. So, when you modify the matrix formed using such a constructor, you also modify the corresponding elements of m. If you want to have an independent copy of the sub-array, use Mat::clone().
The initial matrix that you construct in your call to std::vector::resize is being copied into each element of the vector with this copy constructor*, so they are all pointing to the same data. When you modify one matrix, you modify all of them.
* or maybe with operator=, which does the same thing (I'm not sure which, but it doesn't affect the result)
I suggest initializing your vector like this:
std::vector<std::vector<cv::Mat>> vecvecmat;
vecvecmat.resize(3, std::vector<cv::Mat>());
for(auto& v : vecvecmat)
{
for(std::size_t i = 0; i < 3; ++i)
{
v.emplace_back(4, 4, CV_64FC1, cv::Scalar(0.0));
}
}