How do I efficiently convert an OpenCV Mat object to a C array or vector?

Convert Mat to Array/Vector in OpenCV

In OpenCV, accessing and manipulating image data is crucial for various image processing tasks. One common requirement is the conversion of the Mat object, which represents an image or matrix, to an array or vector.

Conversion to Array

If the Mat object's memory is continuous, meaning all its data is stored consecutively, you can directly obtain a 1D array using the following code:

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

where mat is the Mat object, array is the converted array, .rows, .cols, and .channels() represent the image dimensions and number of channels, and .data is a pointer to the image data.

Conversion to Vector

For non-continuous Mats, data needs to be retrieved row by row. Here's an example for converting to a 2D array/vector:

<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>

However, using a std::vector can simplify the conversion:

<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>

Mat Data Continuity

Understanding Mat data continuity is essential for efficient conversion. Here are some key points:

• Mats created using imread(), clone(), or a constructor are always continuous.
• Mats will only become non-continuous when they borrow data from an existing Mat (except when borrowing single rows or multiple rows with full original width).

For example, in the following code snippet:

<code class="cpp">cv::Mat image = cv::imread("image.jpg"); // continuous Mat
cv::Mat sub_image = image(cv::Rect(100, 100, 100, 100)); // non-continuous Mat since it borrows data from `image`</code>

image is a continuous Mat because it was created using imread(), while sub_image is non-continuous because it references a portion of the image Mat.

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