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Here my code to understand BORDER_REPLICATE :

void main(void)
{
    Mat x = (Mat_<float>(3, 3) << 1, 2, 3, 4, 5, 6, 7, 8, 9);
    Mat x7 = (Mat_<float>(7, 7) << 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 3, 3, 0, 0, 1, 1, 2, 3, 3, 0, 0, 4, 4, 5, 6, 6, 0, 0, 7, 7, 8, 9, 9, 0, 0, 7, 7, 8, 9, 9, 0, 0, 0, 0, 0, 0, 0, 0);
    Mat kernel = (Mat_<float>(3, 3) << 1, 1, 1, 1, 1, 1, 1, 1, 1);
    Mat dst;

filter2D(x, dst, -1, kernel, Point(-1, -1), 0, BORDER_REPLICATE);
cout << "*************BORDER_REPLICATE *******************\n"<<dst << "\n";
cout << "\n data for BORDER_REPLICATE are \n";
cout << x7<<"\n";
cout << "*************Filter2d for x7 *******************\n" << dst << "\n";
filter2D(x7, dst, -1, kernel, Point(-1, -1), 0, BORDER_ISOLATED);
cout << dst << "\n";

}

and results :

*************BORDER_REPLICATE *******************
[21, 27, 33;
 39, 45, 51;
 57, 63, 69]

 data for BORDER_REPLICATE are
[0, 0, 0, 0, 0, 0, 0;
 0, 1, 1, 2, 3, 3, 0;
 0, 1, 1, 2, 3, 3, 0;
 0, 4, 4, 5, 6, 6, 0;
 0, 7, 7, 8, 9, 9, 0;
 0, 7, 7, 8, 9, 9, 0;
 0, 0, 0, 0, 0, 0, 0]
*************Filter2d for x7 *******************
[21, 27, 33;
 39, 45, 51;
 57, 63, 69]
[1, 2, 4, 6, 8, 6, 3;
 2, 4, 8, 12, 16, 12, 6;
 6, 12, 21, 27, 33, 24, 12;
 12, 24, 39, 45, 51, 36, 18;
 18, 36, 57, 63, 69, 48, 24;
 14, 28, 44, 48, 52, 36, 18;
 7, 14, 22, 24, 26, 18, 9]

Here my code to understand BORDER_REPLICATE :

void main(void)
{
    Mat x = (Mat_<float>(3, 3) << 1, 2, 3, 4, 5, 6, 7, 8, 9);
    Mat x7 = (Mat_<float>(7, 7) << 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 3, 3, 0, 0, 1, 1, 2, 3, 3, 0, 0, 4, 4, 5, 6, 6, 0, 0, 7, 7, 8, 9, 9, 0, 0, 7, 7, 8, 9, 9, 0, 0, 0, 0, 0, 0, 0, 0);
    Mat kernel = (Mat_<float>(3, 3) << 1, 1, 1, 1, 1, 1, 1, 1, 1);
    Mat dst;

filter2D(x, dst, -1, kernel, Point(-1, -1), 0, BORDER_REPLICATE);
cout << "*************BORDER_REPLICATE *******************\n"<<dst << "\n";
cout << "\n data for BORDER_REPLICATE are \n";
cout << x7<<"\n";
cout << "*************Filter2d for x7 *******************\n" << dst << "\n";
filter2D(x7, dst, -1, kernel, Point(-1, -1), 0, BORDER_ISOLATED);
BORDER_ISOLATED);// Don't use BORDER_REPLICATE 
cout << dst << "\n";

}

and results :

*************BORDER_REPLICATE *******************
[21, 27, 33;
 39, 45, 51;
 57, 63, 69]

 data for BORDER_REPLICATE are
[0, 0, 0, 0, 0, 0, 0;
 0, 1, 1, 2, 3, 3, 0;
 0, 1, 1, 2, 3, 3, 0;
 0, 4, 4, 5, 6, 6, 0;
 0, 7, 7, 8, 9, 9, 0;
 0, 7, 7, 8, 9, 9, 0;
 0, 0, 0, 0, 0, 0, 0]
*************Filter2d for x7 *******************
[21, 27, 33;
 39, 45, 51;
 57, 63, 69]
[1, 2, 4, 6, 8, 6, 3;
 2, 4, 8, 12, 16, 12, 6;
 6, 12, 21, 27, 33, 24, 12;
 12, 24, 39, 45, 51, 36, 18;
 18, 36, 57, 63, 69, 48, 24;
 14, 28, 44, 48, 52, 36, 18;
 7, 14, 22, 24, 26, 18, 9]