Here’s a little undocumented function to horizontally concatenate two cv::Mat matrices (with the same number of rows).

It is used like this:

 cv::Mat m1(10, 5, CV_32FC3);
 cv::Mat m2(10, 3, CV_32FC3);
 cv::Mat m3;
 cv::hconcat(m1, m2, m3);
 assert(10 == m3.rows && 8 == m3.cols);

 cv::hconcat(std::vector<cv::Mat>{m1, m2}, m3);
 assert(10 == m3.rows && 8 == m3.cols);

 cv::Mat matArray[] = { m1, m2 };
 cv::hconcat(matArray, 2, m3); // Accept array + size
 assert(10 == m3.rows && 8 == m3.cols);

The function cv::hconcat() has 2 additional overloads: One for accepting an array of cv::Mats and array size, and another for accepting a cv::InputArrayOfArrays which is essentially an std::vector<cv::Mat>.

Note that this function will copy all the elements of input matrices and (possibly) reallocate the output matrix.

References:

• Header declaration in OpenCV: core.hpp;
• Hat tip to SO answer.
• The image from here shows a section of rock that has being ground away by the Rock Abrasion Tool (RAT) of the Opportunity rover on Mars.

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