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Mat thinning_select=imread("image.png",0);
ximgproc::thinning(select, thinning_select, ximgproc::THINNING_ZHANGSUEN);

vector<Vec4i> lines;
HoughLinesP(thinning_select, lines, 1, CV_PI / 180, 80, 30,3);

Mat show_line = thinning_select.clone() = 0;
for (size_t i = 0; i < lines.size(); i++)
{
    line(show_line, Point(lines[i][0], lines[i][1]),
        Point(lines[i][2], lines[i][3]), Scalar(255), 3);
}
Mat select_float;
distanceTransform(select, select_float, DIST_C, 3);
double t_min, t_max;
Point min_point, max_point;
minMaxLoc(select_float, &t_min, &t_max, &min_point, &max_point, show_line);
cout << t_max << endl;

I will get the result 4 as the above code