thinning.cpp

#include "precomp.hpp"

using namespace std;

namespace cv {
namespace ximgproc {

// Applies a thinning iteration to a binary image
static void thinningIteration(Mat img, int iter){
    Mat marker = Mat::zeros(img.size(), CV_8UC1);
    for (int i = 1; i < img.rows-1; i++)
    {
        for (int j = 1; j < img.cols-1; j++)
        {
            uchar p2 = img.at<uchar>(i-1, j);
            uchar p3 = img.at<uchar>(i-1, j+1);
            uchar p4 = img.at<uchar>(i, j+1);
            uchar p5 = img.at<uchar>(i+1, j+1);
            uchar p6 = img.at<uchar>(i+1, j);
            uchar p7 = img.at<uchar>(i+1, j-1);
            uchar p8 = img.at<uchar>(i, j-1);
            uchar p9 = img.at<uchar>(i-1, j-1);

            int A  = (p2 == 0 && p3 == 1) + (p3 == 0 && p4 == 1) +
                     (p4 == 0 && p5 == 1) + (p5 == 0 && p6 == 1) +
                     (p6 == 0 && p7 == 1) + (p7 == 0 && p8 == 1) +
                     (p8 == 0 && p9 == 1) + (p9 == 0 && p2 == 1);
            int B  = p2 + p3 + p4 + p5 + p6 + p7 + p8 + p9;
            int m1 = iter == 0 ? (p2 * p4 * p6) : (p2 * p4 * p8);
            int m2 = iter == 0 ? (p4 * p6 * p8) : (p2 * p6 * p8);

            if (A == 1 && (B >= 2 && B <= 6) && m1 == 0 && m2 == 0)
                marker.at<uchar>(i,j) = 1;
        }
    }

    img &= ~marker;
}

// Apply the thinning procedure to a given image
void thinning(InputArray input, OutputArray output){
    Mat processed = input.getMat().clone();
    // Enforce the range of the input image to be in between 0 - 255
    processed /= 255;

    Mat prev = Mat::zeros(processed.size(), CV_8UC1);
    Mat diff;

    do {
        thinningIteration(processed, 0);
        thinningIteration(processed, 1);
        absdiff(processed, prev, diff);
        processed.copyTo(prev);
    }
    while (countNonZero(diff) > 0);

    processed *= 255;

    output.assign(processed);
}

} //namespace ximgproc
} //namespace cv