How to Efficiently Convert OpenCV Mat to Array or Vector?

Converting OpenCV Mat to Array/Vector

Problem:
For beginners in OpenCV, finding suitable functions to convert Mat to Array can be challenging. Existing methods like .ptr and .at may not provide the desired data format.

Answer:

For continuous Mats (where all data is stored consecutively), the data can be directly retrieved as a 1D array:

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

For non-continuous Mats, the data needs to be extracted row by row, resulting in a 2D array:

<code class="cpp">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);</code>

Update:

For increased simplicity, using std::vector is recommended:

<code class="cpp">std::vector<uchar> array;
if (mat.isContinuous()) {
    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());
    }
}</code>

For data types other than CV_8UC1 (e.g., CV_32F), use the appropriate data type in the vector definition.

Update 2:

Regarding Mat data continuity:

• Matrices created by imread(), clone(), or constructors are always continuous.
• Non-continuous Mats only arise when borrowing data from existing matrices, except when the borrowed data is continuous in the original matrix (e.g., single rows or full-width rows).

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