如何通过opencv cv :: calibrateCamera返回的索引访问值？

Question

我使用opencv函数cv::calibrateCamera来校准相机，并且我想像这样cameraMatrix.at<float>(2,2)那样访问按索引返回的值，但是失败了！代码和结果是这样的：

cv::Mat cameraMatrix = cv::Mat(3, 3, CV_32F);
cv::Mat distCoeffs = cv::Mat(1, 5, CV_32F);
cv::Mat rvecs;
cv::Mat tvecs;
double reprojectError;
reprojectError = cv::calibrateCamera(pointsOnBoard, pointsOnImg, imgSize, 
                                         cameraMatrix, distCoeffs, rvecs, tvecs);
std::cout << "Intrinsic matrix：" << std::endl;
std::cout << cameraMatrix << std::endl;
std::cout << "Value at [2,2]: " << cameraMatrix.at<float>(2,2) << std::endl;

输出是这样的：

我已经尝试过一些类似的代码，并且一切正常！

float tmp[] = {1,2,3,4,5,6,7,8,9};
cv Mat test = Mat(3,3,CV_32F,tmp);
std::cout << "Value at [2,2]: " << test.at<float>(2,2) << std::endl;