Class cv::NAryMatIterator#

n-ary multi-dimensional array iterator. View details

#include <opencv2/core/mat.hpp>

Collaboration diagram for cv::NAryMatIterator:

Detailed Description#

n-ary multi-dimensional array iterator.

Use the class to implement unary, binary, and, generally, n-ary element-wise operations on multi-dimensional arrays. Some of the arguments of an n-ary function may be continuous arrays, some may be not. It is possible to use conventional MatIterator ‘s for each array but incrementing all of the iterators after each small operations may be a big overhead. In this case consider using NAryMatIterator to iterate through several matrices simultaneously as long as they have the same geometry (dimensionality and all the dimension sizes are the same). On each iteration it.planes[0], it.planes[1],… will be the slices of the corresponding matrices.

The example below illustrates how you can compute a normalized and threshold 3D color histogram:

void computeNormalizedColorHist(const Mat& image, Mat& hist, int N, double minProb)
{
    const int histSize[] = {N, N, N};

    // make sure that the histogram has a proper size and type
    hist.create(3, histSize, CV_32F);

    // and clear it
    hist = Scalar(0);

    // the loop below assumes that the image
    // is a 8-bit 3-channel. check it.
    CV_Assert(image.type() == CV_8UC3);
    MatConstIterator_<Vec3b> it = image.begin<Vec3b>(),
                             it_end = image.end<Vec3b>();
    for( ; it != it_end; ++it )
    {
        const Vec3b& pix = *it;
        hist.at<float>(pix[0]*N/256, pix[1]*N/256, pix[2]*N/256) += 1.f;
    }

    minProb *= image.rows*image.cols;

    // initialize iterator (the style is different from STL).
    // after initialization the iterator will contain
    // the number of slices or planes the iterator will go through.
    // it simultaneously increments iterators for several matrices
    // supplied as a null terminated list of pointers
    const Mat* arrays[] = {&hist, 0};
    Mat planes[1];
    NAryMatIterator itNAry(arrays, planes, 1);
    double s = 0;
    // iterate through the matrix. on each iteration
    // itNAry.planes[i] (of type Mat) will be set to the current plane
    // of the i-th n-dim matrix passed to the iterator constructor.
    for(int p = 0; p < itNAry.nplanes; p++, ++itNAry)
    {
        threshold(itNAry.planes[0], itNAry.planes[0], minProb, 0, THRESH_TOZERO);
        s += sum(itNAry.planes[0])[0];
    }

    s = 1./s;
    itNAry = NAryMatIterator(arrays, planes, 1);
    for(int p = 0; p < itNAry.nplanes; p++, ++itNAry)
        itNAry.planes[0] *= s;
}