OpenCV Template Matching for N best matches not working - c++

Question

I have been struggling with this for quite a long while and can't seem to find the problem here. Let me try to walk you through the process.
I am trying to obtain the 10 best bounding boxes for my template matching.
Not going to show the entire image here, but here's my cv::mat when I loop through the cv::mat to search for the lowest values (results)
int a,b;
for ( a = 0; a < final_image_height; a++){
for ( b = 0; b < final_image_width; b++){
if (result_scores_mat.at<float>(a, b) < 70 ){
printf("%.2f ", result_scores_mat.at<float>(a, b));
}
}
}
This gives me some values present in the cv::mat with "lower values"
68.50 68.93 54.50 68.92 64.62 57.12 62.69 65.86 63.52 68.35 68.65 61.93 69.18 67.69
I then move into a loop whereby I called minMaxLoc() to find the minVal (using TM_SQDIFF, so minVal)
for ( i = 0; i < 10; i++){
minMaxLoc(result_scores_mat, &minVal, &maxVal, &minLoc, &maxLoc, cv::Mat());
cout<<minVal<<endl;
// for match_method TM_SQDIFF we take lowest values
matchLoc = minLoc;
drawRadius = result_radius_mat.at<float>(matchLoc.x, matchLoc.y);
// display source image and result matrix , draw rectangle around highest possible matching area
cv::rectangle( img_display_mat, matchLoc, cv::Point( matchLoc.x + 2*drawRadius, matchLoc.y + 2*drawRadius), cv::Scalar::all(255), 2, 8, 0);
result_scores_mat.at<float>(minLoc.x, minLoc.y)= 255;
}
What I am doing in the last line is replacing the value of the float at the minLoc, so that the next loop I run through minMaxLoc doesn't locate that point and gives me the next minimum value in the cv::mat.
However, I keep receiving such an output with cout<<minVal<<endl;
54.5003
54.5003
54.5003
54.5003
54.5003
54.5003
54.5003
54.5003
54.5003
54.5003
It detects rightly the 54.50 that is present in the cv::mat, but I want all the minimum values in the loop.
Is there something wrong I'm doing, or any way around this? Thank you so much!

Answer 1

It seems that you are replacing minimum value with 255 in wrong way, it means that you detect minimum value correctly, but you replace 255 in wrong position, you should replace below line
result_scores_mat.at<float>(minLoc.x, minLoc.y)= 255;
by this line:
result_scores_mat.at<float>(minLoc.y, minLoc.x)= 255;
For knowing that why we replace this lines by each other, you can take a look at the source code of .at method.It is like below
template<typename _Tp> _Tp& at(int row, int col);
/** #overload
#param row Index along the dimension 0
#param col Index along the dimension 1
*/