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How to calculate the median of pixel values in opencv (python)

asked 2017-10-17 04:57:47 -0500

Asad gravatar image

I want to get an image matrix of a still image and then calculate the median of that matrix for some purpose. Can you help me how to do so?

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LBerger gravatar imageLBerger ( 2017-10-17 05:27:06 -0500 )edit

^^ yes, for blurring.

what exactly do you need this for ? opencv has no direct way for this.

(you could do something with histograms, but we'd need to know more..)

berak gravatar imageberak ( 2017-10-17 06:44:32 -0500 )edit

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answered 2017-10-17 07:05:05 -0500

VxW gravatar image

you can try this:

double medianMat(cv::Mat Input, int nVals){
    // COMPUTE HISTOGRAM OF SINGLE CHANNEL MATRIX
    float range[] = { 0, nVals };
    const float* histRange = { range };
    bool uniform = true; bool accumulate = false;
    cv::Mat hist;
    calcHist(&Input, 1, 0, cv::Mat(), hist, 1, &nVals, &histRange, uniform, accumulate);

    // COMPUTE CUMULATIVE DISTRIBUTION FUNCTION (CDF)
    cv::Mat cdf;
    hist.copyTo(cdf);
    for (int i = 1; i <= nVals-1; i++){
        cdf.at<float>(i) += cdf.at<float>(i - 1);
    }
    cdf /= Input.total();

     // COMPUTE MEDIAN
     double medianVal;
     for (int i = 0; i <= nVals-1; i++){
         if (cdf.at<float>(i) >= 0.5) { medianVal = i;  break; }
     }
     return medianVal/nVals; 
}

this great and fast solution was found at (see last answer): https://stackoverflow.com/questions/3...

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Asked: 2017-10-17 04:57:47 -0500

Seen: 11,878 times

Last updated: Oct 17 '17