opencv 通过指针访问图像像素值，输出为空的问题

for (int i = 0; i < img_roi_gray_at.rows; ++i)
{
uchar* datatemp = img_roi_gray_at.ptr<uchar>(i);
for (int j = 0; j < img_roi_gray_at.cols; ++j)
{
//cout << (int)datatemp[j] << endl;
cout << datatemp[j] << endl;
}
}

for (int i = 0; i < img_roi_gray_at.rows; ++i)
{
uchar* datatemp = img_roi_gray_at.ptr<uchar>(i);
for (int j = 0; j < img_roi_gray_at.cols; ++j)
{
cout << (int)datatemp[j] << endl;
//cout << datatemp[j] << endl;
}
}