Finding max and min locations of a SparseMat in OpenCV

I want to find the location of max in a SparseMat matrix, using OpenCV libraries in C++. I tried the following code that does not work:

  int Tdim=1;
  int Tsize[]={2,3};
  SparseMat samplesT(Tdim,Tsize,CV_32F);
  int el[]={1,2};                   // max location
  samplesT.ref<float>(el)=5;        // setting max value
  double TmaxVal, TminVal;
  int TmaxIdx, TminIdx;
  minMaxLoc(samplesT, &TminVal, &TmaxVal, &TminIdx, &TmaxIdx); 

  cout<<"TmaxIdx= "<<(TmaxIdx)<<endl;    // returns: 1
  cout<<"TmaxVal= "<<(samplesT.ref<float>(TmaxIdx))<<endl; // returns: 5

It is really strange that

 samplesT.ref<float>(TmaxIdx)

correctly returns the max value of the matrix, however, TmaxIdx only contains the row of the max value in the matrix!

I would appreciate any methods could be used to find x and y of the max value in an OpenCV SparseMat matrix.

Answers


There are two problems. First, since Tdim equals 1, you're working with a 1D matrix instead of a 2D one, so you need to set Tdim to 2. The other problem is that TmaxIdx and TminIdx should be int arrays of length 2, not single ints (since you're looking for a 2D coordinate). With that change, you get the following:

int TmaxIdx[2], TminIdx[2];
minMaxLoc(samplesT, &TminVal, &TmaxVal, TminIdx, TmaxIdx);

Then you can verify the solution below:

fprintf(stdout, "TmaxIdx= (%d,%d)\n", TmaxIdx[0], TmaxIdx[1]); // TmaxIdx= (1, 2)

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