results compareHist in openCV

Question

I'm trying to compare two histograms which I stored as an array. I'm new with the c++ interface (cv::Mat) and calculating histograms in OpenCV.

My code:

int testArr1[4] = {12, 10, 11, 11};
int testArr2[4] = {12, 0, 11, 0};
cv::Mat M1 = cv::Mat(1,4,CV_8UC1, testArr1);
cv::Mat M2 = cv::Mat(1,4,CV_8UC1, testArr2);

int histSize = 4;
float range[] = {0, 20};
const float* histRange = {range};
bool uniform = true;
bool accumulate = false;
cv::Mat a1_hist, a2_hist;

cv::calcHist(&M1, 1, 0, cv::Mat(), a1_hist, 1, &histSize, &histRange, uniform, accumulate );
cv::calcHist(&M2, 1, 0, cv::Mat(), a2_hist, 1, &histSize, &histRange, uniform, accumulate );

double compar_c = cv::compareHist(a1_hist, a2_hist, CV_COMP_CORREL);
double compar_chi = cv::compareHist(a1_hist, a2_hist, CV_COMP_CHISQR);
double compar_bh = cv::compareHist(a1_hist, a2_hist, CV_COMP_BHATTACHARYYA);
double compar_i = cv::compareHist(a1_hist, a2_hist, CV_COMP_INTERSECT);
cout << "compare(CV_COMP_CORREL): " << compar_c << "\n";
cout << "compare(CV_COMP_CHISQR): " << compar_chi << "\n";
cout << "compare(CV_COMP_BHATTACHARYYA): " << compar_bh << "\n";
cout << "compare(CV_COMP_INTERSECT): " << compar_i << "\n";

The results are a bit unexpected:

compare(CV_COMP_CORREL): 1
compare(CV_COMP_CHISQR): 0
compare(CV_COMP_BHATTACHARYYA): 0
compare(CV_COMP_INTERSECT): 4

For intersection, for example, I expected something like 0.5. What am I doing wrong? Can I not put arrays in a cv::mat? Or did I choose the wrong histogram "settings"?

score 1 · Accepted Answer · answered Mar 13 '12 at 16:53

The problem are your first 4 lines where you are converting the c array of integers to a matrix of chars. The constructor assumes a char array and therefore can't read the values properly. Your matrices M1 and M2 don't contain the correct values.

But if you change the following lines, so that the type of the array matches the type of the matrix:

char testArr1[4] = {12, 10, 11, 11};
char testArr2[4] = {12, 0, 11, 0};

I get the following output from your program:

compare(CV_COMP_CORREL): 0.57735
compare(CV_COMP_CHISQR): 2.66667
compare(CV_COMP_BHATTACHARYYA): 0.541196
compare(CV_COMP_INTERSECT): 2

results compareHist in openCV

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