How to Convert a Mat Object to an Array or Vector in OpenCV?
Oct 26, 2024 pm 01:48 PM
Converting Mat to Array in OpenCV
OpenCV provides various methods for converting between different data structures. In this article, we will focus on the conversion from a Mat object to an array or vector.
Direct Conversion from Mat to Array
For a continuous Mat, where the data is stored contiguously in memory, you can directly access its data as a 1D array:
<code class="cpp">cv::Mat mat;
// ...
std::vector<uchar> array(mat.rows * mat.cols * mat.channels());
if (mat.isContinuous()) {
array = mat.data;
}</code>
Conversion to 2D Array
For a non-continuous Mat, you can retrieve its data row by row and store it in a 2D array:
<code class="cpp">cv::Mat mat;
// ...
std::vector<std::vector<uchar>> array(mat.rows);
for (int i = 0; i < mat.rows; ++i) {
array[i] = std::vector<uchar>(mat.cols * mat.channels());
array[i] = mat.ptr<uchar>(i);
}</code>
Conversion to Vector
If using the C Standard Library's vector, you can employ the following approach:
<code class="cpp">cv::Mat mat;
// ...
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>
Mat Data Continuity
It's worth noting that the continuity of a Mat affects the efficiency of data access. Matrices created by imread(), clone(), or a constructor are typically continuous. Conversely, matrices created from an ROI (region of interest) of a larger Mat may not be continuous.
This code snippet demonstrates the differences in data continuity:
<code class="cpp">cv::Mat big_mat = cv::Mat::zeros(1000, 1000, CV_8UC3); cv::Mat sub_mat = big_mat(cv::Rect(10, 10, 100, 100)); std::cout << "big_mat is continuous: " << big_mat.isContinuous() << std::endl; // true std::cout << "sub_mat is continuous: " << sub_mat.isContinuous() << std::endl; // false</code>
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