How can access vector <cv::Mat> data by using ptr or data method?

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How can access vector <cv::mat> data by using ptr or data method?

I used at method for access vector <cv::mat> data.

This Method is so slow, so I would like to change ptr or data method.

This is my code. How can I change?

vector<Mat> conc_img;

vector<Mat> loss_img(conc_img);

for (int iter = r_start; iter < r_start + h_size; iter++) {

                for (int nframe = 0; nframe < conc_img.size(); nframe++) {

                    part_x[nframe][cnt][0] = loss_img[nframe].at<float>(iter, ref_p[0]);

                    part_x[nframe][cnt][1] = loss_img[nframe].at<float>(iter, ref_p[1]);

                    part_x[nframe][cnt][2] = loss_img[nframe].at<float>(iter, ref_p[2]);

                    part_x[nframe][cnt][3] = loss_img[nframe].at<float>(iter, ref_p[3]);

                }

            }