Bala Bala - 14 days ago 5
C++ Question

how to find euclidean distance between pixels within a image in opencv

int main(){
Mat cmp, Ref, Diff;
cmp = imread("image1.tif", CV_LOAD_IMAGE_UNCHANGED);
Ref = imread("image2.tif", CV_LOAD_IMAGE_UNCHANGED);
ShiftChk(cmp, Ref);
absdiff(cmp, Ref, Diff);
imshow("difference image", Diff);
waitKey(0);
double min, max;
minMaxLoc(Diff, &min, &max);
Point min_loc, max_loc;
minMaxLoc(Diff, &min, &max, &min_loc, &max_loc);
Size sz = Diff.size();
cout << "max val : " << max << endl;//5
cout << "max val: " << max_loc << endl; //[26,38]


vector<vector<double>>test;
for (int i = 0; i < Diff.cols; i++) {
for (int j = 0; j < Diff.rows; j++) {

Point difference = Diff.at<uchar>(26, 38) - Diff.at<uchar>(j, i);
double dist = sqrt(difference.x*difference.x + difference.y*difference.y);
test.push_back(dist);
}
}
}


I am trying to find the Euclidean distance between a single point in an image to all other pixels. The distance values are to be stored in vector test but its showing some error in it. And also I don't know whether the logic I have used is correct to give the right answer(Euclidean distance). Can anyone help me out. Thanks in advance

Error message is:

error C2664:
'void std::vector<std::vector<double,std::allocator<_Ty>>,std::allocator<std::vector<_Ty,std::allocator<_Ty>>>>::push_back(const std::vector<_Ty,std::allocator<_Ty>> &)' :
cannot convert argument 1 from 'double' to 'std::vector<double,std::allocator<_Ty>> &&'

Answer

There are two major issues:

  1. You're appending the values to the test vector wrong. You need either to create an intermediate vector and push_back it to test (as shown in @0X0nosugar answer), or better initialize your vectors with correct dimensions and put the value at the right place.

    vector<vector<double>> test(Diff.rows, vector<double>(Diff.cols));
    for (int i = 0; i < Diff.rows; i++) {
        for (int j = 0; j < Diff.cols; j++) {
            test[i][j] = ...       
        }
    }
    

    As shown in the snippet above, it's better (and faster) to scan by rows, becuase OpenCV stores images row-wise.

  2. You are not computing the distance between two points. You are in fact taking the difference of the values at two given points and creating a Point object out of this (which makes no sense). Also you can avoid to compute explicitly the euclidean distance. You can use cv::norm:

     test[i][j] = norm(Point(38, 26) - Point(j, i)); // Pay attention to i,j order!
    

Putting all together:

Point ref(38, 26);
vector<vector<double>> test(Diff.rows, vector<double>(Diff.cols));
for (int i = 0; i < Diff.rows; i++) {
    for (int j = 0; j < Diff.cols; j++) {
        test[i][j] = norm(ref - Point(j,i));      
    }
}