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#include <iostream>
#include <Eigen/Core>
#include <unsupported/Eigen/CXX11/Tensor>
#include <opencv2/core/eigen.hpp>
#include <opencv2/opencv.hpp>
using namespace Eigen;
using namespace std;
using namespace cv;
int main()
{
    Mat img = imread("test.jpg");
    Mat img2;
    Eigen::Tensor<uint8_t, 3> a_tensor;
    cv2eigen(img, a_tensor);
    a_tensor.slice(Eigen::array<size_t, 3> {500, 500, 0}, Eigen::array<size_t, 3> {200, 400, 3}).setZero();
    eigen2cv(a_tensor, img2);
}

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Eigen::array<int, 3> three_dims{ {H, W, 1} };
Tensor<float, 3> result_map_3d = result_map_tmp_2d.reshape(three_dims);
Mat res;
eigen2cv(result_map_3d, res);