How to Efficiently Convert OpenCV Mat to an Array or Vector?

Convert Mat to Array or Vector in OpenCV

In OpenCV matrix operations, occasionally there's a need to convert Mat to an Array or a Vector. This article provides a comprehensive guide to achieve such conversions, utilizing OpenCV's robust functionality.

Direct Conversion to Array

If the Mat object mat possesses continuous memory, its 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>

Conversion to Row-Based Array

In cases where the memory is not continuous, the data can be obtained 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>

Alternative Vector Conversion

Using std::vector offers an alternative approach for converting Mat to an array:

<code class="cpp">std::vector<uchar> array;
if (mat.isContinuous()) {
    // array.assign(mat.datastart, mat.dataend); // May cause issues for sub-matrices
    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>

Converting to Vectors of Other Types

For OpenCV Mats with other data types, such as CV_32F, the conversion process is similar:

<code class="cpp">std::vector<float> array;
if (mat.isContinuous()) {
    // array.assign((float*)mat.datastart, (float*)mat.dataend); // May cause issues for sub-matrices
    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());
    }
}</code>

Understanding Mat Data Continuity

It's crucial to comprehend Mat data continuity to effectively perform conversions. Key points to note are:

• Mats created through imread(), clone(), or a constructor are inherently continuous.
• Data discontinuities only occur when a Mat borrows data from an existing Mat (except for certain scenarios involving full original width).

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