opencvConvertNCHW

#include <opencv2/opencv.hpp>

std::vector<float> images2nchw(std::vector<cv::Mat> images, cv::Size size, double factor=1.0, bool flip_rb = true) {
    std::vector<float> data;
    data.reserve(size.area() * 3 * images.size());
    size_t base = 0;
    for(auto&& image : images)
    {
        assert(image.type() == CV_8UC3);
        cv::resize(image, image, size);

        int frame_size = size.area();
        int iter_rows = image.rows;
        int iter_cols = image.cols;

        if(image.isContinuous())
        {
            iter_cols = image.total();
            iter_rows = 1;
        }

        constexpr std::array<int, 3> no_swap {0, 1, 2};
        constexpr std::array<int, 3> swap_rb {2, 1, 0};
        const auto& index_ref = flip_rb ? swap_rb : no_swap;
        for(auto c : index_ref)
            for (int i = 0; i < iter_rows; ++i)
            {
                const auto* line = image.ptr<cv::Vec3b>(i);
                for(int j = 0; j < iter_cols; ++j)
                    data.push_back((*line++)[c]);
            }
    }
    return data;
}

int main()
{
    cv::Mat ones = cv::Mat::ones({10, 10}, CV_8UC3);

    std::cout << ones << '\n';
    auto v = images2nchw({ones, ones}, ones.size(), 2);

    for (int i = 0; i < ones.channels() * 2; ++i) {
        for (int j = 0; j < ones.size().area(); ++j) {
            std::cout << v[i * ones.size().area() + j] << ' ';
        }
        std::cout << '\n';
    }

}