Converting a row of cv::Mat to std::vector - c++

Question

I have a fairly simple question: how to take one row of cv::Mat and get all the data in std::vector? The cv::Mat contains doubles (it can be any simple datatype for the purpose of the question).
Going through OpenCV documentation is just very confusing, unless I bookmark the page I can not find a documentation page twice by Googling, there's just to much of it and not easy to navigate.
I have found the cv::Mat::at(..) to access the Matrix element, but I remember from C OpenCV that there were at least 3 different ways to access elements, all of them used for different purposes... Can't remember what was used for which :/
So, while copying the Matrix element-by-element will surely work, I am looking for a way that is more efficient and, if possible, a bit more elegant than a for loop for each row.

Answer 1

It should be as simple as:
m.row(row_idx).copyTo(v);
Where m is cv::Mat having CV_64F depth and v is std::vector<double>

Answer 2

Data in OpenCV matrices is laid out in row-major order, so that each row is guaranteed to be contiguous. That means that you can interpret the data in a row as a plain C array. The following example comes directly from the documentation:
// compute sum of positive matrix elements
// (assuming that M is double-precision matrix)
double sum=0;
for(int i = 0; i < M.rows; i++)
{
const double* Mi = M.ptr<double>(i);
for(int j = 0; j < M.cols; j++)
sum += std::max(Mi[j], 0.);
}
Therefore the most efficient way is to pass the plain pointer to std::vector:
// Pointer to the i-th row
const double* p = mat.ptr<double>(i);
// Copy data to a vector. Note that (p + mat.cols) points to the
// end of the row.
std::vector<double> vec(p, p + mat.cols);
This is certainly faster than using the iterators returned by begin() and end(), since those involve extra computation to support gaps between rows.

Answer 3

From the documentation at here, you can get a specific row through cv::Mat::row, which will return a new cv::Mat, over which you can iterator with cv::Mat::begin and cv::Mat::end. As such, the following should work:
cv::Mat m/*= initialize */;
// ... do whatever...
cv::Mat first_row(m.row(0));
std::vector<double> v(first_row.begin<double>(), first_row.end<double>());
Note that I don't know any OpenCV, but googling "OpenCV mat" led directly to the basic types documentation and according to that, this should work fine.
The matrix iterators are random-access iterators, so they can be passed to any STL algorithm, including std::sort() .
This is also from the documentiation, so you could actually do this without a copy:
cv::Mat m/*= initialize */;
// ... do whatever...
// first row begin end
std::vector<double> v(m.begin<double>(), m.begin<double>() + m.size().width);
To access more than the first row, I'd recommend the first snippet, since it will be a lot cleaner that way and there doesn't seem to be any heavy copying since the data types seem to be reference-counted.

Answer 4

You can also use cv::Rect
m(cv::Rect(0, 0, 1, m.cols))
will give you first row.
matrix(cv::Rect(x0, y0, len_x, len_y);
means that you will get sub_matrix from matrix whose upper left corner is (x0,y0) and size is (len_x, len_y). (row,col)

Answer 5

I think this works,
an example :
Mat Input(480, 720, CV_64F, Scalar(100));
cropping the 1st row of the matrix:
Rect roi(Point(0, 0), Size(720, 1));
then:
std::vector<std::vector<double> > vector_of_rows;
vector_of_rows.push_back(Input(roi));