OpenCV - Confusion using calcHist

Question

I\'ve read the documentation for calcHist() many times, but I think my inexperience with OpenCV and rusty programming skills are completely precluding me from understanding

Answer 1

I modified the code from here

You might also want to take a look at the documentation of cvtColor here

Note that I have not tried to compile or run this code so I do not guarantee that it will work. But nonetheless, it might be useful as a reference.

Mat hist;
int nimages = 1; // Only 1 image, that is the Mat scene.
int channels[] = {0} // Index for hue channel
int dims = 1 // Only 1 channel, the hue channel
int histSize[] = {9} // 9 bins, 1 each for Red, RY, Yellow, YG etc.
float hranges[] = { 0, 180 }; // hue varies from 0 to 179, see cvtColor
const float *ranges[] = {hranges};

// Compute the histogram.
calcHist(&scene, 
nimages, 
channels, 
Mat(), // No mask
hist, dims, histSize, ranges, uniform=true)

// Now hist will contain the counts in each bin.
// Lets just print out the values. Note that you can output Mat using std::cout
cout << "Histogram: " << endl << hist << endl;

// To access the individual bins, you can either iterate over them
// or use hist.at<uchar>(i, j); Note that one of the index should be 0
// because hist is 1D histogram. Print out hist.rows and hist.cols to see if hist is a N x 1 or 1 x N matrix.
/*
MatIterator_<uchar> it, end;
int binIndex = 0;
for( it = hist.begin<uchar>(), end = hist.end<uchar>(); it != end; ++it)
{
  printf("Count in %d bin: %d\n", binIndex, *it);
  ++binIndex;
}
*/