Initialize 3D matrix with OpenCV with classes

Question

I'm new to c++, and I'm trying to implement A vessel segmentation technique using "Separate Header and Implementation Files" like:

http://www.cppforschool.com/tutorial/separate-header-and-implementation-files.html

My question is, in the main file how to initialize with all 0's the empty 3D matrix.

Main.cpp

#include <opencv2/core/core.hpp>
#include "filter.h"
#include <iostream>
using namespace std;
    using namespace cv;


int main(){
    filter match_kernel;
    match_kernel.kernel(& mascara);

return 0;
}

Match.cpp

#include <iostream>
#include <string>
#include <math.h>
#include <opencv2/core/core.hpp>
#include <fstream>
#include "filter.h"
#define PI 3.14159265359
using namespace std;

void filter::kernel(float *mascara){

    using namespace cv;

    int center=7;
    int sizes[] = {15, 15, 12};
    double angi=(PI/12.0); 
    double mi=0.0;
    int counter=0;
    int x=0;
    int y=0;

    double u=0.0;
    double v=0.0;
    double cof=0.0;
    Mat matriz(3, sizes,CV_32FC1, mascara);

    counter=0;
    mi=0;

    for (int z = 0; z < 12; z++){
        Mat masc = Mat::zeros(15,15, CV_32FC1);
        counter = 0;
        mi = 0.0;
        for (int i = 0; i < 15; i++){
            for (int j = 0; j < 15; j++){
                x=i-center;
                y=j-center;
                u=x*cos(z*angi)+y*sin(z*angi);
                v=-x*sin(z*angi)+y*cos(z*angi);
            if(abs(u)<=6.0 && abs(v)<=4.5){
                cof=-exp(-pow(u,2.0)/(8.0));
                matriz.at<float>(i, j, z)=cof;                  
                masc.at<float>(i,  j)=1.0;
                mi += cof;
                counter += 1;
            }           
        }
    }
    mi=mi/counter;
    cout<<"mi"<<mi <<endl;
    for(int i = 0; i < 15; i++){
        for(int j = 0; j < 15; j++){
            matriz.at<float>(i, j, z)=(masc.at<float>(i, j))*round(10.0*(matriz.at<float>(i, j, z)-mi));///
        }
    }

}
}

filter.h

#include <opencv2/core/core.hpp>
#ifndef _FILTER_H_
#define _FILTER_H_

using namespace std;
class filter{
public:
    filter(){}
    void kernel(float *mascara);

private:
    cv::mascara=Scalar::all(0);

};

#endif

In the main.cpp file, we don't know how to initialize the 3D matrix with all 0's

int main(){
        filter match_kernel;
        match_kernel.kernel(& mascara);