Convert Mat to Array/Vector in OpenCV

Question

I am novice in OpenCV. Recently, I have troubles finding OpenCV functions to convert from Mat to Array. I researched with .ptr and .at methods available in OpenCV APIs, but I could not get proper data. I would like to have direct conversion from Mat to Array(if available, if not to Vector). I need OpenCV functions because the code has to be undergo high level synthesis in Vivado HLS. Please help.

Answer 1

If the memory of the Mat mat is continuous (all its data is continuous), you can directly get its data to a 1D array:

std::vector<uchar> array(mat.rows*mat.cols*mat.channels());
if (mat.isContinuous())
    array = mat.data;

Otherwise, you have to get its data row by row, e.g. to a 2D array:

uchar **array = new uchar*[mat.rows];
for (int i=0; i<mat.rows; ++i)
    array[i] = new uchar[mat.cols*mat.channels()];

for (int i=0; i<mat.rows; ++i)
    array[i] = mat.ptr<uchar>(i);

---

UPDATE: It will be easier if you're using std::vector, where you can do like this:

std::vector<uchar> array;
if (mat.isContinuous()) {
  // array.assign(mat.datastart, mat.dataend); // <- has problems for sub-matrix like mat = big_mat.row(i)
  array.assign(mat.data, mat.data + mat.total()*mat.channels());
} else {
  for (int i = 0; i < mat.rows; ++i) {
    array.insert(array.end(), mat.ptr<uchar>(i), mat.ptr<uchar>(i)+mat.cols*mat.channels());
  }
}

p.s.: For cv::Mats of other types, like CV_32F, you should do like this:

std::vector<float> array;
if (mat.isContinuous()) {
  // array.assign((float*)mat.datastart, (float*)mat.dataend); // <- has problems for sub-matrix like mat = big_mat.row(i)
  array.assign((float*)mat.data, (float*)mat.data + mat.total()*mat.channels());
} else {
  for (int i = 0; i < mat.rows; ++i) {
    array.insert(array.end(), mat.ptr<float>(i), mat.ptr<float>(i)+mat.cols*mat.channels());
  }
}

---

UPDATE2: For OpenCV Mat data continuity, it can be summarized as follows:

• Matrices created by imread(), clone(), or a constructor will always be continuous.
• The only time a matrix will not be continuous is when it borrows data (except the data borrowed is continuous in the big matrix, e.g. 1. single row; 2. multiple rows with full original width) from an existing matrix (i.e. created out of an ROI of a big mat).

Please check out this code snippet for demonstration.