diff --git a/modules/photo/src/contrast_preserve.cpp b/modules/photo/src/contrast_preserve.cpp
index ce31e0cd26..b20bba0893 100644
--- a/modules/photo/src/contrast_preserve.cpp
+++ b/modules/photo/src/contrast_preserve.cpp
@@ -42,7 +42,7 @@
 
 #include "precomp.hpp"
 #include "opencv2/photo.hpp"
-#include "math.h"
+#include <cmath>
 #include <vector>
 #include <limits>
 #include "contrast_preserve.hpp"
@@ -64,33 +64,28 @@ void cv::decolor(InputArray _src, OutputArray _dst, OutputArray _color_boost)
     CV_Assert(!I.empty() && (I.channels()==3));
 
     // Parameter Setting
-    int maxIter = 15;
+    const int maxIter = 15;
+    const double tol = .0001;
     int iterCount = 0;
-    double tol = .0001;
     double E = 0;
     double pre_E = std::numeric_limits<double>::infinity();
 
-    Decolor obj;
-
     Mat img;
-
-    img = Mat(I.size(),CV_32FC3);
-    I.convertTo(img,CV_32FC3,1.0/255.0);
+    I.convertTo(img, CV_32FC3, 1.0/255.0);
 
     // Initialization
-    obj.init();
+    Decolor obj;
 
     vector <double> Cg;
     vector < vector <double> > polyGrad;
     vector <Vec3i> comb;
-
     vector <double> alf;
 
     obj.grad_system(img,polyGrad,Cg,comb);
     obj.weak_order(img,alf);
 
     // Solver
-    Mat Mt = Mat((int)polyGrad.size(),(int)polyGrad[0].size(), CV_32FC1);
+    Mat Mt = Mat(int(polyGrad.size()),int(polyGrad[0].size()), CV_32FC1);
     obj.wei_update_matrix(polyGrad,Cg,Mt);
 
     vector <double> wei;
@@ -98,85 +93,64 @@ void cv::decolor(InputArray _src, OutputArray _dst, OutputArray _color_boost)
 
     //////////////////////////////// main loop starting ////////////////////////////////////////
 
+    vector <double> G_pos(alf.size());
+    vector <double> G_neg(alf.size());
+    vector <double> EXPsum(G_pos.size());
+    vector <double> EXPterm(G_pos.size());
+    vector <double> temp(polyGrad[0].size());
+    vector <double> temp1(polyGrad[0].size());
+    vector <double> temp2(EXPsum.size());
+    vector <double> wei1(polyGrad.size());
+
     while(sqrt(pow(E-pre_E,2)) > tol)
     {
         iterCount +=1;
         pre_E = E;
 
-        vector <double> G_pos(alf.size());
-        vector <double> G_neg(alf.size());
-
-        vector <double> temp(polyGrad[0].size());
-        vector <double> temp1(polyGrad[0].size());
-
-        double val = 0.0;
-        for(unsigned int i=0;i< polyGrad[0].size();i++)
+        for(size_t i=0; i<polyGrad[0].size(); i++)
         {
-            val = 0.0;
-            for(unsigned int j =0;j<polyGrad.size();j++)
+            double val = 0.0;
+            for(size_t j=0; j<polyGrad.size(); j++)
                 val = val + (polyGrad[j][i] * wei[j]);
             temp[i] = val - Cg[i];
             temp1[i] = val + Cg[i];
         }
 
-        double pos = 0.0;
-        double neg = 0.0;
-        for(unsigned int i =0;i<alf.size();i++)
+        for(size_t i=0; i<alf.size(); i++)
         {
-            pos = ((1 + alf[i])/2) * exp((-1.0 * 0.5 * pow(temp[i],2))/pow(obj.sigma,2));
-            neg = ((1 - alf[i])/2) * exp((-1.0 * 0.5 * pow(temp1[i],2))/pow(obj.sigma,2));
+            const double sqSigma = obj.sigma * obj.sigma;
+            const double pos = ((1 + alf[i])/2) * exp(-1.0 * 0.5 * (temp[i] * temp[i]) / sqSigma);
+            const double neg = ((1 - alf[i])/2) * exp(-1.0 * 0.5 * (temp1[i] * temp1[i]) / sqSigma);
             G_pos[i] = pos;
             G_neg[i] = neg;
         }
 
-        vector <double> EXPsum(G_pos.size());
-        vector <double> EXPterm(G_pos.size());
-
-        for(unsigned int i = 0;i<G_pos.size();i++)
+        for(size_t i=0; i<G_pos.size(); i++)
             EXPsum[i] = G_pos[i]+G_neg[i];
 
-        vector <double> temp2(EXPsum.size());
-
-        for(unsigned int i=0;i<EXPsum.size();i++)
-        {
-            if(EXPsum[i] == 0)
-                temp2[i] = 1.0;
-            else
-                temp2[i] = 0.0;
-        }
+        for(size_t i=0; i<EXPsum.size(); i++)
+            temp2[i] = (EXPsum[i] == 0) ? 1.0 : 0.0;
 
-        for(unsigned int i =0; i < G_pos.size();i++)
-            EXPterm[i] = ((G_pos[i] - G_neg[i])/(EXPsum[i] + temp2[i]));
+        for(size_t i=0; i<G_pos.size(); i++)
+            EXPterm[i] = (G_pos[i] - G_neg[i])/(EXPsum[i] + temp2[i]);
 
-        double val1 = 0.0;
-        vector <double> wei1(polyGrad.size());
-
-        for(unsigned int i=0;i< polyGrad.size();i++)
+        for(int i=0; i<int(polyGrad.size()); i++)
         {
-            val1 = 0.0;
-            for(unsigned int j =0;j<polyGrad[0].size();j++)
+            double val1 = 0.0;
+            for(int j=0; j<int(polyGrad[0].size()); j++)
             {
                 val1 = val1 + (Mt.at<float>(i,j) * EXPterm[j]);
             }
             wei1[i] = val1;
         }
 
-        for(unsigned int i =0;i<wei.size();i++)
+        for(size_t i=0; i<wei.size(); i++)
             wei[i] = wei1[i];
 
-        E = obj.energyCalcu(Cg,polyGrad,wei);
+        E = obj.energyCalcu(Cg, polyGrad, wei);
 
         if(iterCount > maxIter)
             break;
-
-        G_pos.clear();
-        G_neg.clear();
-        temp.clear();
-        temp1.clear();
-        EXPsum.clear();
-        EXPterm.clear();
-        temp2.clear();
-        wei1.clear();
     }
 
     Mat Gray = Mat::zeros(img.size(),CV_32FC1);
@@ -186,9 +160,7 @@ void cv::decolor(InputArray _src, OutputArray _dst, OutputArray _color_boost)
 
     ///////////////////////////////////       Contrast Boosting   /////////////////////////////////
 
-    Mat lab = Mat(img.size(),CV_8UC3);
-    Mat color = Mat(img.size(),CV_8UC3);
-
+    Mat lab;
     cvtColor(I,lab,COLOR_BGR2Lab);
 
     vector <Mat> lab_channel;
diff --git a/modules/photo/src/contrast_preserve.hpp b/modules/photo/src/contrast_preserve.hpp
index 2504cee4fe..ec8274e883 100644
--- a/modules/photo/src/contrast_preserve.hpp
+++ b/modules/photo/src/contrast_preserve.hpp
@@ -41,9 +41,8 @@
 
 #include "precomp.hpp"
 #include "opencv2/photo.hpp"
-#include "math.h"
+#include <cmath>
 #include <vector>
-#include <limits>
 
 using namespace std;
 using namespace cv;
@@ -57,41 +56,34 @@ class Decolor
 
     public:
         float sigma;
-        void init();
-        vector<double> product(vector <Vec3i> &comb, const double initRGB[3]);
-        double energyCalcu(vector <double> &Cg, vector < vector <double> > &polyGrad, vector <double> &wei);
-        void singleChannelGradx(const Mat &img, Mat& dest);
-        void singleChannelGrady(const Mat &img, Mat& dest);
-        void gradvector(const Mat &img, vector <double> &grad);
-        void colorGrad(Mat img, vector <double> &Cg);
-        void add_vector(vector <Vec3i> &comb, int &idx, int r,int g,int b);
-        void add_to_vector_poly(vector < vector <double> > &polyGrad, vector <double> &curGrad, int &idx1);
-        void weak_order(Mat img, vector <double> &alf);
-        void grad_system(Mat img, vector < vector < double > > &polyGrad,
-                vector < double > &Cg, vector <Vec3i>& comb);
-        void wei_update_matrix(vector < vector <double> > &poly, vector <double> &Cg, Mat &X);
-        void wei_inti(vector <Vec3i> &comb, vector <double> &wei);
-        void grayImContruct(vector <double> &wei, Mat img, Mat &Gray);
-};
-
-int round_num(double a);
 
-int round_num(double a)
-{
-    return int(a + 0.5);
-}
+        Decolor();
+        static vector<double> product(const vector <Vec3i> &comb, const double initRGB[3]);
+        double energyCalcu(const vector <double> &Cg, const vector < vector <double> > &polyGrad, const vector <double> &wei) const;
+        void singleChannelGradx(const Mat &img, Mat& dest) const;
+        void singleChannelGrady(const Mat &img, Mat& dest) const;
+        void gradvector(const Mat &img, vector <double> &grad) const;
+        void colorGrad(const Mat &img, vector <double> &Cg) const;
+        static void add_vector(vector <Vec3i> &comb, int &idx, int r,int g,int b);
+        static void add_to_vector_poly(vector < vector <double> > &polyGrad, const vector <double> &curGrad, int &idx1);
+        void weak_order(const Mat &img, vector <double> &alf) const;
+        void grad_system(const Mat &img, vector < vector < double > > &polyGrad,
+                vector < double > &Cg, vector <Vec3i>& comb) const;
+        static void wei_update_matrix(const vector < vector <double> > &poly, const vector <double> &Cg, Mat &X);
+        static void wei_inti(const vector <Vec3i> &comb, vector <double> &wei);
+        void grayImContruct(vector <double> &wei, const Mat &img, Mat &Gray) const;
+};
 
-double Decolor::energyCalcu(vector <double> &Cg, vector < vector <double> > &polyGrad, vector <double> &wei)
+double Decolor::energyCalcu(const vector <double> &Cg, const vector < vector <double> > &polyGrad, const vector <double> &wei) const
 {
     const size_t size = polyGrad[0].size();
     vector <double> energy(size);
     vector <double> temp(size);
     vector <double> temp1(size);
 
-    double val = 0.0;
     for(size_t i=0;i< polyGrad[0].size();i++)
     {
-        val = 0.0;
+        double val = 0.0;
         for(size_t j =0;j<polyGrad.size();j++)
             val = val + (polyGrad[j][i] * wei[j]);
         temp[i] = val - Cg[i];
@@ -109,7 +101,7 @@ double Decolor::energyCalcu(vector <double> &Cg, vector < vector <double> > &pol
 
 }
 
-void Decolor::init()
+Decolor::Decolor()
 {
     kernelx = Mat(1,2, CV_32FC1);
     kernely = Mat(2,1, CV_32FC1);
@@ -121,13 +113,12 @@ void Decolor::init()
     sigma = 0.02f;
 }
 
-vector<double> Decolor::product(vector <Vec3i> &comb, const double initRGB[3])
+vector<double> Decolor::product(const vector <Vec3i> &comb, const double initRGB[3])
 {
     vector <double> res(comb.size());
-    double dp;
     for (size_t i=0;i<comb.size();i++)
     {
-        dp = 0.0;
+        double dp = 0.0;
         for(int j=0;j<3;j++)
             dp = dp + (comb[i][j] * initRGB[j]);
         res[i] = dp;
@@ -135,27 +126,23 @@ vector<double> Decolor::product(vector <Vec3i> &comb, const double initRGB[3])
     return res;
 }
 
-void Decolor::singleChannelGradx(const Mat &img, Mat& dest)
+void Decolor::singleChannelGradx(const Mat &img, Mat& dest) const
 {
-    int w=img.size().width;
-    int h=img.size().height;
-    Point anchor(kernelx.cols - kernelx.cols/2 - 1, kernelx.rows - kernelx.rows/2 - 1);
+    const int w = img.size().width;
+    const Point anchor(kernelx.cols - kernelx.cols/2 - 1, kernelx.rows - kernelx.rows/2 - 1);
     filter2D(img, dest, -1, kernelx, anchor, 0.0, BORDER_CONSTANT);
-    for(int i=0;i<h;i++)
-        dest.at<float>(i,w-1)=0.0;
+    dest.col(w - 1) = 0.0;
 }
 
-void Decolor::singleChannelGrady(const Mat &img, Mat& dest)
+void Decolor::singleChannelGrady(const Mat &img, Mat& dest) const
 {
-    int w=img.size().width;
-    int h=img.size().height;
-    Point anchor(kernely.cols - kernely.cols/2 - 1, kernely.rows - kernely.rows/2 - 1);
+    const int h = img.size().height;
+    const Point anchor(kernely.cols - kernely.cols/2 - 1, kernely.rows - kernely.rows/2 - 1);
     filter2D(img, dest, -1, kernely, anchor, 0.0, BORDER_CONSTANT);
-    for(int j=0;j<w;j++)
-        dest.at<float>(h-1,j)=0.0;
+    dest.row(h - 1) = 0.0;
 }
 
-void Decolor::gradvector(const Mat &img, vector <double> &grad)
+void Decolor::gradvector(const Mat &img, vector <double> &grad) const
 {
     Mat dest;
     Mat dest1;
@@ -165,8 +152,8 @@ void Decolor::gradvector(const Mat &img, vector <double> &grad)
     Mat d_trans=dest.t();
     Mat d1_trans=dest1.t();
 
-    int height = d_trans.size().height;
-    int width = d_trans.size().width;
+    const int height = d_trans.size().height;
+    const int width = d_trans.size().width;
 
     grad.resize(width * height * 2);
 
@@ -180,9 +167,8 @@ void Decolor::gradvector(const Mat &img, vector <double> &grad)
             grad[offset + i * height + j] = d1_trans.at<float>(i, j);
 }
 
-void Decolor::colorGrad(Mat img, vector <double> &Cg)
+void Decolor::colorGrad(const Mat &img, vector <double> &Cg) const
 {
-
     Mat lab;
 
     cvtColor(img,lab,COLOR_BGR2Lab);
@@ -199,10 +185,9 @@ void Decolor::colorGrad(Mat img, vector <double> &Cg)
     gradvector(lab_channel[2],Imb);
 
     Cg.resize(ImL.size());
-    double res =0.0;
     for(size_t i=0;i<ImL.size();i++)
     {
-        res=sqrt(pow(ImL[i],2) + pow(Ima[i],2) + pow(Imb[i],2))/100;
+        const double res = sqrt(pow(ImL[i],2) + pow(Ima[i],2) + pow(Imb[i],2))/100;
         Cg[i] = res;
     }
 }
@@ -213,21 +198,20 @@ void Decolor::add_vector(vector <Vec3i> &comb, int &idx, int r,int g,int b)
     idx++;
 }
 
-void Decolor::add_to_vector_poly(vector < vector <double> > &polyGrad, vector <double> &curGrad, int &idx1)
+void Decolor::add_to_vector_poly(vector < vector <double> > &polyGrad, const vector <double> &curGrad, int &idx1)
 {
     polyGrad.push_back(curGrad);
     idx1++;
 }
 
-void Decolor::weak_order(Mat img, vector <double> &alf)
+void Decolor::weak_order(const Mat &img, vector <double> &alf) const
 {
-    int h = img.size().height;
-    int w = img.size().width;
-    double sizefactor;
+    const int h = img.size().height;
+    const int w = img.size().width;
     if((h + w) > 800)
     {
-        sizefactor = (double)800/(h+w);
-        resize(img,img,Size(round_num(h*sizefactor),round_num(w*sizefactor)));
+        const double sizefactor = double(800)/(h+w);
+        resize(img, img, Size(cvRound(h*sizefactor), cvRound(w*sizefactor)));
     }
 
     Mat curIm = Mat(img.size(),CV_32FC1);
@@ -242,39 +226,16 @@ void Decolor::weak_order(Mat img, vector <double> &alf)
     vector <double> t1(Rg.size()), t2(Rg.size()), t3(Rg.size());
     vector <double> tmp1(Rg.size()), tmp2(Rg.size()), tmp3(Rg.size());
 
-    double level = .05;
+    const double level = .05;
 
-    for(unsigned int i=0;i<Rg.size();i++)
+    for(size_t i=0;i<Rg.size();i++)
     {
-        if(Rg[i] > level)
-            t1[i] = 1.0;
-        else
-            t1[i] = 0.0;
-
-        if(Gg[i] > level)
-            t2[i] = 1.0;
-        else
-            t2[i] = 0.0;
-
-        if(Bg[i] > level)
-            t3[i] = 1.0;
-        else
-            t3[i] = 0.0;
-
-        if(Rg[i] < -1.0*level)
-            tmp1[i] = 1.0;
-        else
-            tmp1[i] = 0.0;
-
-        if(Gg[i] < -1.0*level)
-            tmp2[i] = 1.0;
-        else
-            tmp2[i] = 0.0;
-
-        if(Bg[i] < -1.0*level)
-            tmp3[i] = 1.0;
-        else
-            tmp3[i] = 0.0;
+        t1[i] = (Rg[i] > level) ? 1.0 : 0.0;
+        t2[i] = (Gg[i] > level) ? 1.0 : 0.0;
+        t3[i] = (Bg[i] > level) ? 1.0 : 0.0;
+        tmp1[i] = (Rg[i] < -1.0*level) ? 1.0 : 0.0;
+        tmp2[i] = (Gg[i] < -1.0*level) ? 1.0 : 0.0;
+        tmp3[i] = (Bg[i] < -1.0*level) ? 1.0 : 0.0;
     }
 
     alf.resize(Rg.size());
@@ -283,29 +244,18 @@ void Decolor::weak_order(Mat img, vector <double> &alf)
 
     for(size_t i =0 ;i < Rg.size();i++)
         alf[i] -= tmp1[i] * tmp2[i] * tmp3[i];
-
-    double sum =0.0;
-    for(unsigned int i=0;i<alf.size();i++)
-        sum += abs(alf[i]);
-
-    sum = (double)100*sum/alf.size();
-
-    Rg.clear(); Gg.clear(); Bg.clear();
-    t1.clear(); t2.clear(); t3.clear();
-    tmp1.clear(); tmp2.clear(); tmp3.clear();
 }
 
-void Decolor::grad_system(Mat img, vector < vector < double > > &polyGrad,
-        vector < double > &Cg, vector <Vec3i>& comb)
+void Decolor::grad_system(const Mat &img, vector < vector < double > > &polyGrad,
+        vector < double > &Cg, vector <Vec3i>& comb) const
 {
     int h = img.size().height;
     int w = img.size().width;
 
-    double sizefactor;
     if((h + w) > 800)
     {
-        sizefactor = (double)800/(h+w);
-        resize(img,img,Size(round_num(h*sizefactor),round_num(w*sizefactor)));
+        const double sizefactor = double(800)/(h+w);
+        resize(img, img, Size(cvRound(h*sizefactor), cvRound(w*sizefactor)));
     }
 
     h = img.size().height;
@@ -336,30 +286,30 @@ void Decolor::grad_system(Mat img, vector < vector < double > > &polyGrad,
             }
 }
 
-void Decolor::wei_update_matrix(vector < vector <double> > &poly, vector <double> &Cg, Mat &X)
+void Decolor::wei_update_matrix(const vector < vector <double> > &poly, const vector <double> &Cg, Mat &X)
 {
-    int size = static_cast<int>(poly.size()), size0 = static_cast<int>(poly[0].size());
+    const int size = static_cast<int>(poly.size());
+    const int size0 = static_cast<int>(poly[0].size());
     Mat P = Mat(size, size0, CV_32FC1);
-    Mat A = Mat(size, size, CV_32FC1);
 
     for (int i = 0; i < size; i++)
         for (int j = 0; j < size0;j++)
-            P.at<float>(i,j) = (float) poly[i][j];
+            P.at<float>(i,j) = static_cast<float>(poly[i][j]);
 
-    Mat P_trans = P.t();
+    const Mat P_trans = P.t();
     Mat B = Mat(size, size0, CV_32FC1);
     for(int i =0;i < size;i++)
     {
-        for(int j = 0, end = (int)Cg.size(); j < end;j++)
-            B.at<float>(i,j) = (float) (poly[i][j] * Cg[j]);
+        for(int j = 0, end = int(Cg.size()); j < end;j++)
+            B.at<float>(i,j) = static_cast<float>(poly[i][j] * Cg[j]);
     }
 
-    A = P*P_trans;
+    Mat A = P*P_trans;
     solve(A, B, X, DECOMP_NORMAL);
 
 }
 
-void Decolor::wei_inti(vector <Vec3i> &comb, vector <double> &wei)
+void Decolor::wei_inti(const vector <Vec3i> &comb, vector <double> &wei)
 {
     double initRGB[3] = { .33, .33, .33 };
 
@@ -367,10 +317,10 @@ void Decolor::wei_inti(vector <Vec3i> &comb, vector <double> &wei)
 
     vector <int> sum(comb.size());
 
-    for(unsigned int i=0;i<comb.size();i++)
+    for(size_t i=0;i<comb.size();i++)
         sum[i] = (comb[i][0] + comb[i][1] + comb[i][2]);
 
-    for(unsigned int i=0;i<sum.size();i++)
+    for(size_t i=0;i<sum.size();i++)
     {
         if(sum[i] == 1)
             wei[i] = wei[i] * double(1);
@@ -382,10 +332,10 @@ void Decolor::wei_inti(vector <Vec3i> &comb, vector <double> &wei)
 
 }
 
-void Decolor::grayImContruct(vector <double> &wei, Mat img, Mat &Gray)
+void Decolor::grayImContruct(vector <double> &wei, const Mat &img, Mat &Gray) const
 {
-    int h=img.size().height;
-    int w=img.size().width;
+    const int h = img.size().height;
+    const int w = img.size().width;
 
     vector <Mat> rgb_channel;
     split(img,rgb_channel);
@@ -400,25 +350,15 @@ void Decolor::grayImContruct(vector <double> &wei, Mat img, Mat &Gray)
                     for(int i = 0;i<h;i++)
                         for(int j=0;j<w;j++)
                             Gray.at<float>(i,j)=Gray.at<float>(i,j) +
-                                (float) wei[kk]*pow(rgb_channel[2].at<float>(i,j),r)*pow(rgb_channel[1].at<float>(i,j),g)*
+                                static_cast<float>(wei[kk])*pow(rgb_channel[2].at<float>(i,j),r)*pow(rgb_channel[1].at<float>(i,j),g)*
                                 pow(rgb_channel[0].at<float>(i,j),b);
 
                     kk=kk+1;
                 }
 
-    float minval = FLT_MAX;
-    float maxval = -FLT_MAX;
-
-    for(int i=0;i<h;i++)
-        for(int j =0;j<w;j++)
-       {
-            if(Gray.at<float>(i,j) < minval)
-                minval = Gray.at<float>(i,j);
-
-            if(Gray.at<float>(i,j) > maxval)
-                maxval = Gray.at<float>(i,j);
-        }
+    double minval, maxval;
+    minMaxLoc(Gray, &minval, &maxval);
 
     Gray -= minval;
     Gray /= maxval - minval;
-}
\ No newline at end of file
+}
diff --git a/modules/photo/test/test_decolor.cpp b/modules/photo/test/test_decolor.cpp
index a6d13c3e94..b40bc1fd04 100644
--- a/modules/photo/test/test_decolor.cpp
+++ b/modules/photo/test/test_decolor.cpp
@@ -45,24 +45,24 @@ namespace opencv_test { namespace {
 
 TEST(Photo_Decolor, regression)
 {
-        string folder = string(cvtest::TS::ptr()->get_data_path()) + "decolor/";
-        string original_path = folder + "color_image_1.png";
+    string folder = string(cvtest::TS::ptr()->get_data_path()) + "decolor/";
+    string original_path = folder + "color_image_1.png";
 
-        Mat original = imread(original_path, IMREAD_COLOR);
+    Mat original = imread(original_path, IMREAD_COLOR);
 
-        ASSERT_FALSE(original.empty()) << "Could not load input image " << original_path;
-        ASSERT_EQ(3, original.channels()) << "Load color input image " << original_path;
+    ASSERT_FALSE(original.empty()) << "Could not load input image " << original_path;
+    ASSERT_EQ(3, original.channels()) << "Load color input image " << original_path;
 
-        Mat grayscale, color_boost;
-        decolor(original, grayscale, color_boost);
+    Mat grayscale, color_boost;
+    decolor(original, grayscale, color_boost);
 
-        Mat reference_grayscale = imread(folder + "grayscale_reference.png", 0 /* == grayscale image*/);
-        double gray_psnr = cvtest::PSNR(reference_grayscale, grayscale);
-        EXPECT_GT(gray_psnr, 60.0);
+    Mat reference_grayscale = imread(folder + "grayscale_reference.png", 0 /* == grayscale image*/);
+    double gray_psnr = cvtest::PSNR(reference_grayscale, grayscale);
+    EXPECT_GT(gray_psnr, 60.0);
 
-        Mat reference_boost = imread(folder + "boost_reference.png");
-        double boost_psnr = cvtest::PSNR(reference_boost, color_boost);
-        EXPECT_GT(boost_psnr, 60.0);
+    Mat reference_boost = imread(folder + "boost_reference.png");
+    double boost_psnr = cvtest::PSNR(reference_boost, color_boost);
+    EXPECT_GT(boost_psnr, 60.0);
 }
 
 }} // namespace