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Copy histogram of an image to another

asked 2016-03-17 04:03:32 -0500

bjorn89 gravatar image

updated 2016-06-01 18:11:48 -0500

Hi all! Here's my problem: I have two images of the same thing acquired with different illumination condition. I found that is possible to copy the histogram of a reference image to a destination image, and it's called histogram specification or histogram matching. As shown here https://studentathome.wordpress.com/2..., in matlab it's pretty simple. There is a way to do the same thing on OpenCV?

EDIT The code linked on the page link text gives me error on the lines:

do1ChnHist(chns[i], src_mask, src_hist, src_cdf);
do1ChnHist(chns1[i], dst_mask, dst_hist, dst_cdf);

it says that I can't pass a matrix (src_hist etc) because the function accepts double*. How can I make it work with Opencv 3.1?

EDIT 2 Now it compiles and run, but i obtain as a matched image a black image. Since I need to match the histogram of the whole image, I created as mask a Mat on ones. Am I doing right? Any suggestion?

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answered 2016-03-18 06:21:03 -0500

berak gravatar image

imho, if you use "typed" Mat's it all gets much easier, since you can skip all the at<type> and ptr<type> calls:

#include <opencv2/opencv.hpp>
using namespace cv;

#include <vector>
using namespace std;

#define HISTMATCH_EPSILON 0.000001
#define BTM_DEBUG
// Compute histogram and CDF for an image with mask
void do1ChnHist(const Mat_<uchar> &img, const Mat_<uchar> &mask, Mat_<double> &h, Mat_<double> &cdf) 
{
    for(size_t p=0; p<img.total(); p++)
    {
        if(mask(p) > 0)
        {
            uchar c = img(p);
            h(c) += 1.0;
        }
    }

    normalize(h,h,1,0,NORM_MINMAX);

    cdf(0) = h(0);
    for(int j=1; j<256; j++)
    {
        cdf(j) = cdf(j-1) + h(j);
    }

    normalize(cdf,cdf,1,0,NORM_MINMAX);
}

// match histograms of 'src' to that of 'dst', according to both masks
void histMatchRGB(Mat &src, const Mat &src_mask, const Mat &dst, const Mat &dst_mask)
{
#ifdef BTM_DEBUG
    namedWindow("original source",CV_WINDOW_AUTOSIZE);
    imshow("original source",src);
    namedWindow("original query",CV_WINDOW_AUTOSIZE);
    imshow("original query",dst);
#endif

    vector<Mat_<uchar>> chns, chns1;
    split(src,chns);
    split(dst,chns1);

    for (int i=0; i<3; i++)
    {
        Mat_<double> src_hist = Mat_<double>::zeros(1,256);
        Mat_<double> dst_hist = Mat_<double>::zeros(1,256);
        Mat_<double> src_cdf  = Mat_<double>::zeros(1,256);
        Mat_<double> dst_cdf  = Mat_<double>::zeros(1,256);

        do1ChnHist(chns[i],  src_mask, src_hist, src_cdf);
        do1ChnHist(chns1[i], dst_mask, dst_hist, dst_cdf);

        uchar last = 0;

        Mat_<uchar> lut(1,256);
        for (int j=0;j<src_cdf.cols;j++)
        {
            double F1j = src_cdf(j);

            for (uchar k = last; k<dst_cdf.cols; k++)
            {
                double F2k = dst_cdf(k);
                if(abs(F2k - F1j) < HISTMATCH_EPSILON || F2k > F1j)
                {
                    lut(j) = k;
                    last = k;
                    break;
                }
            }
        }

        LUT(chns[i],lut,chns[i]);
    }

    Mat res;
    merge(chns,res);

#ifdef BTM_DEBUG
    namedWindow("matched",CV_WINDOW_AUTOSIZE);
    imshow("matched",res);

    waitKey(0);
#endif

    res.copyTo(src);
}

int main(int argc, char **argv)
{
    Mat dst = imread("im/bb_beach.png");
    Mat src = imread("im/bb_cam.png");
    Mat mask = Mat(src.size(), CV_8U, Scalar(255));
    //Mat src_mask = imread("image008_mask.bmp",0);
    //Mat dst_mask = imread("image003_mask.bmp",0);

    histMatchRGB(dst,mask,src,mask);
    return 0;
}
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Comments

Hi! I've tried your solution and it works, but it doesn't match perfectly the histogram. it is normal? I've seen that the matlab module matches perfectly!

bjorn89 gravatar imagebjorn89 ( 2016-03-18 08:16:35 -0500 )edit

idk, all i tried is to make the code run. (and there's no matlab here)

berak gravatar imageberak ( 2016-03-18 08:25:09 -0500 )edit
0

answered 2016-03-17 04:33:06 -0500

I think this source is a sample you may want.

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Comments

Hi! If found by myself the same website, but I can't make it work with opencv 3.1. In particular, it gives me error on vector (which I correct by using vector <intZ), on double* _src_cdf = src_cdf.ptr(); (which I can't resolve), on LUT(chns[i], lut, chns[i]);(it undelines chns1, also this unresolved) and on every h[c][/ c] (it underlines the slash). How can I proceed?

bjorn89 gravatar imagebjorn89 ( 2016-03-18 03:35:39 -0500 )edit
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Asked: 2016-03-17 04:03:32 -0500

Seen: 692 times

Last updated: Mar 18 '16