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

Converting OpenCV Mat to Array/Vector

In OpenCV, obtaining data from a Mat object can be challenging for beginners. This article explores the process of converting a Mat into an array or vector.

Direct Conversion

If the Mat's memory is continuous, direct conversion to a 1D array is possible:

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

Row-by-Row Conversion

For non-continuous Mats, row-by-row access is necessary for creating 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>

Simplified Approach with std::vector

For std::vector, the conversion becomes simpler:

<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>

Data Continuity Considerations

Mat data continuity ensures that all data is contiguous in memory.

• Imread(), clone(), and constructor-created Mats are always continuous.
• Non-continuous Mats result from borrowing data from existing Mats (e.g., row/column selections).

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