OpenCV无法识别Mat尺寸

Question

I'm trying to print an image using OpenCV defining a 400x400 Mat:

plot2 = cv::Mat(400,400, CV_8U, 255);

But when I try print the points, something strange happens. The y coordinate only prints to the first 100 values. That is, if I print the point (50,100), it does not print it in the 100/400th part of the columns, but at the end. Somehow, 400 columns have turned into 100.

For example, when running this:

for (int j = 0; j < 95; ++j){
    plot2.at<int>(20, j) = 0;
}
cv::imshow("segunda pared", plot2);

Shows this (the underlined part is the part corresponding to the code above):

A line that goes to 95 almost occupies all of the 400 points when it should only occupy 95/400th of the screen.

What am I doing wrong?

Answer 1

When you defined your cv::Mat , you told clearly that it is from the type CV_8U :

plot2 = cv::Mat(400,400, CV_8U, 255);

But when you are trying to print it, you are telling that its type is int which is usually a signed 32 bit not unsigned 8 bit. So the solution is:

for (int j = 0; j < 95; ++j){
    plot2.at<uchar>(20, j) = 0;
}

Important note: Be aware that OpenCV uses the standard C++ types not the fixed ones. So, there is no need to use fixed size types like uint16_t or similar. because when compiling OpenCV & your code on another platform both of them will change together.

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BTW, one of the good way to iterate through your cv::Mat is:

for (size_t row = 0; j < my_mat.rows; ++row){
    auto row_ptr=my_mat.ptr<uchar>(row);
    for(size_t col=0;col<my_mat.cols;++col){
         //do whatever you want with  row_ptr[col]  (read/write)
    }
}