Learning OpenCV

Accessing Histograms

There are several ways to access a histogram's data. The most straightforward method is to use OpenCV's accessor functions.

double cvQueryHistValue_1D(
  CvHistogram* hist,
  int          idx0
);
double cvQueryHistValue_2D(
  CvHistogram* hist,
  int          idx0,
  int          idx1
);
double cvQueryHistValue_3D(
  CvHistogram* hist,
  int          idx0,
  int          idx1,
  int          idx2
);
double cvQueryHistValue_nD(
  CvHistogram* hist,
  int*         idxN
);

Each of these functions returns a floating-point number for the value in the appropriate bin. Similarly, you can set (or get) histogram bin values with the functions that return a pointer to a bin (not to a bin's value):

float* cvGetHistValue_1D(
  CvHistogram* hist,
  int          idx0
);
float* cvGetHistValue_2D(
  CvHistogram* hist,
  int          idx0,
  int          idx1
);
float* cvGetHistValue_3D(
  CvHistogram* hist,
  int          idx0,
  int          idx1,
  int          idx2
);
float* cvGetHistValue_nD(
  CvHistogram* hist,
  int*         idxN
);

These functions look a lot like the cvGetReal*D and cvPtr*D families of functions, and in fact they are pretty much the same thing. Inside of these calls are essentially those same matrix accessors called with the matrix hist->bins passed on to them. Similarly, the functions for sparse histograms inherit the behavior of the corresponding sparse matrix functions. If you attempt to access a nonexistent bin using a GetHist*() function in a sparse histogram, then that bin is automatically created and its value set to 0. Note that QueryHist*() functions do not create missing bins.

This leads us to the more general ...