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I want to declare, populate, access a Multi-Dimensional Matrix in OpenCV (C++) which is compatible with namespace cv. I found no quick and easy to learn examples on them. Can you please help me out?

karlphillip
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garak
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2 Answers2

13

Here is a short example from the NAryMatIterator documentation; it shows how to create, populate, and process a multi-dimensional matrix in OpenCV:

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;
    Mat plane;
    NAryMatIterator it(&hist, &plane, 1);
    double s = 0;
    // iterate through the matrix. on each iteration
    // it.planes[*] (of type Mat) will be set to the current plane.
    for(int p = 0; p < it.nplanes; p++, ++it)
    {
        threshold(it.planes[0], it.planes[0], minProb, 0, THRESH_TOZERO);
        s += sum(it.planes[0])[0];
    }

    s = 1./s;
    it = NAryMatIterator(&hist, &plane, 1);
    for(int p = 0; p < it.nplanes; p++, ++it)
        it.planes[0] *= s;
}

Also, check out the cv::compareHist function for another usage example of the NAryMatIterator here.

mevatron
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  • So a 3-dimensional array should be something like this right? int sz[] = {3, 3, 3}; Mat accumarray(3, sz, CV_8U, Scalar::all(0)); accumarray.at(0, 1, 2) = 20; – garak Jan 11 '12 at 00:42
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    Well, it seems that above method doesn't work for 4 dimensional Matrix. Do you know the reason? – garak Jan 16 '12 at 14:52
  • what is the parameter minProb ? – lilouch Jan 06 '15 at 15:47
  • not sure which version this is based on, but in opencv 4, ` NAryMatIterator` takes a pointer to pointer to mat, not a pointer to mat, so this code does not compile. – oarfish Aug 31 '20 at 13:57
  • the API seems to have changed a bit, updated example is here: https://docs.opencv.org/master/d5/dd2/classcv_1_1NAryMatIterator.html#acbc0100414fe60ab212553a67dd788b6 – oarfish Aug 31 '20 at 14:00
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To create a multi-dimensional matrix that is of size 100x100x3, using floats, one channel, and with all elements initialized to 10 you write like this:

int size[3] = { 100, 100, 3 };
cv::Mat M(3, size, CV_32FC1, cv::Scalar(10));

To loop over and output the elements in the matrix you can do:

for (int i = 0; i < 100; i++)
  for (int j = 0; j < 100; j++)
    for (int k = 0; k < 3; k++) 
      std::cout << M.at<cv::Vec3f>(i,j)[k] << ", ";

However, beware of the troubles with using multi-dimensional matrices as documented here: How do i get the size of a multi-dimensional cv::Mat? (Mat, or MatND)

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Reed Richards
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