Opencv中用at<格式> 与用ptr<格式> 的不同

Mat M(200, 200, CV_64F);for(int i = 0; i < M.rows; i++){        for(int j = 0; j < M.cols; j++)        M.at<double>(i,j)=CV_PI;}/////笔记Note that the at<> method is not very efficient as it has to calculate the exactmemory position from the pixel row and column. This can be very time consumingwhen we process the whole image pixel by pixel. The second method uses the ptrfunction, which returns a pointer to a specific image row. The following snippetobtains the pixel value of each pixel in a color image://///节省处理时间的方式，用ptr函数，返回一个指向图像行的指针。uchar R, G, B;for (int i = 0; i < src2.rows; i++){    Vec3b* pixrow = src2.ptr<Vec3b>(i);    for (int j = 0; j < src2.cols; j++)    {        B = pixrow[j][0];        G = pixrow[j][1];        R = pixrow[j][2];    }}////笔记In the example above, ptr is used to get a pointer to the first pixel in each row.Using this pointer, we can now access each column in the innermost loop.

例子来自《Opencv computer vision application programming cookbook》