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This solution was pasted from the original authors solution. However, to guarantee this will work in the feature, this should be kept as code here:

#ifndef OPENCVUTILS_H
#define OPENCVUTILS_H

#include <iostream>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/imgproc/imgproc_c.h>
#include <opencv2/core/internal.hpp>

void cvEqualizeHist( const CvArr* srcarr, CvArr* dstarr, CvMat* mask )
{
    using namespace cv;

    CvMat sstub, *src = cvGetMat(srcarr, &sstub);
    CvMat dstub, *dst = cvGetMat(dstarr, &dstub);

    CV_Assert( CV_ARE_SIZES_EQ(src, dst) && CV_ARE_TYPES_EQ(src, dst) &&
               CV_MAT_TYPE(src->type) == CV_8UC1 );

    CV_Assert( CV_ARE_SIZES_EQ(src, mask) && CV_MAT_TYPE(mask->type) == CV_8UC1);

    CvSize size = cvGetMatSize(src);
    if( CV_IS_MAT_CONT(src->type & dst->type) )
    {
        size.width *= size.height;
        size.height = 1;
    }
    int x, y;
    const int hist_sz = 256;
    int hist[hist_sz];
    memset(hist, 0, sizeof(hist));

    for( y = 0; y < size.height; y++ )
    {
        const uchar* sptr = src->data.ptr + src->step*y;
        const uchar* mptr = mask->data.ptr + mask->step*y;
        for( x = 0; x < size.width; x++ )
            if (mptr[x]) hist[sptr[x]]++;
    }

    float scale = 255.f/(cvCountNonZero(mask));
    int sum = 0;
    uchar lut[hist_sz+1];

    for( int i = 0; i < hist_sz; i++ )
    {
        sum += hist[i];
        int val = cvRound(sum*scale);
        lut[i] = CV_CAST_8U(val);
    }

    lut[0] = 0;
    cvSetZero(dst);
    for( y = 0; y < size.height; y++ )
    {
        const uchar* sptr = src->data.ptr + src->step*y;
        const uchar* mptr = mask->data.ptr + mask->step*y;
        uchar* dptr = dst->data.ptr + dst->step*y;
        for( x = 0; x < size.width; x++ )
            if (mptr[x]) dptr[x] = lut[sptr[x]];
    }
}

void equalizeHist(cv::InputArray _src, cv::OutputArray _dst, cv::InputArray _mask = cv::noArray())
{
    using namespace cv;

    Mat src = _src.getMat().clone();
    _dst.create( src.size(), src.type() );
    Mat dst = _dst.getMat();
    Mat mask;
    if (_mask.empty()) mask = Mat::ones(src.size(), CV_8UC1);
    else mask = _mask.getMat();
    CvMat _csrc = src, _cdst = dst, _cmask = mask;
    cvEqualizeHist( &_csrc, &_cdst, &_cmask);
}

#endif