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This algorithm is used for thinning binary image. Binary image by definition, consists of only black and white pixels. Here, in the code bellow, Black is denoted as 1 and white is denoted as 0. A result of thinning using this code is as seen in the image below. bw bwThin 8 neighborhood : 9 Equations : eqnThin Step 1 conditions for selecting black points to remove step1Eqn Step 2 conditions for selecting black points to remove step2Eqn Code For Thinning in Java:

package imageProcessing.service;

import imageProcessing.model.Point;

import java.util.LinkedList;
import java.util.List;

/**
 * Created by nayef on 1/26/15.
 */
public class ThinningService {
    /**
     * @param givenImage
     * @param changeGivenImage decides whether the givenArray should be modified or a clone should be used
     * @return a 2D array of binary image after thinning using zhang-suen thinning algo.
     */
    public int[][] doZhangSuenThinning(final int[][] givenImage, boolean changeGivenImage) {
        int[][] binaryImage;
        if (changeGivenImage) {
            binaryImage = givenImage;
        } else {
            binaryImage = givenImage.clone();
        }
        int a, b;
        List<Point> pointsToChange = new LinkedList();
        boolean hasChange;
        do {
            hasChange = false;
            for (int y = 1; y + 1 < binaryImage.length; y++) {
                for (int x = 1; x + 1 < binaryImage[y].length; x++) {
                    a = getA(binaryImage, y, x);
                    b = getB(binaryImage, y, x);
                    if (binaryImage[y][x] == 1 && 2 <= b && b <= 6 && a == 1
                            && (binaryImage[y - 1][x] * binaryImage[y][x + 1] * binaryImage[y + 1][x] == 0)
                            && (binaryImage[y][x + 1] * binaryImage[y + 1][x] * binaryImage[y][x - 1] == 0)) {
                        pointsToChange.add(new Point(x, y));
//binaryImage[y][x] = 0;
                        hasChange = true;
                    }
                }
            }
            for (Point point : pointsToChange) {
                binaryImage[point.getY()][point.getX()] = 0;
            }
            pointsToChange.clear();
            for (int y = 1; y + 1 < binaryImage.length; y++) {
                for (int x = 1; x + 1 < binaryImage[y].length; x++) {
                    a = getA(binaryImage, y, x);
                    b = getB(binaryImage, y, x);
                    if (binaryImage[y][x] == 1 && 2 <= b && b <= 6 && a == 1
                            && (binaryImage[y - 1][x] * binaryImage[y][x + 1] * binaryImage[y][x - 1] == 0)
                            && (binaryImage[y - 1][x] * binaryImage[y + 1][x] * binaryImage[y][x - 1] == 0)) {
                        pointsToChange.add(new Point(x, y));
                        hasChange = true;
                    }
                }
            }
            for (Point point : pointsToChange) {
                binaryImage[point.getY()][point.getX()] = 0;
            }
            pointsToChange.clear();
        } while (hasChange);
        return binaryImage;
    }

    private int getA(int[][] binaryImage, int y, int x) {
        int count = 0;
//p2 p3
        if (y - 1 >= 0 && x + 1 < binaryImage[y].length && binaryImage[y - 1][x] == 0 && binaryImage[y - 1][x + 1] == 1) {
            count++;
        }
//p3 p4
        if (y - 1 >= 0 && x + 1 < binaryImage[y].length && binaryImage[y - 1][x + 1] == 0 && binaryImage[y][x + 1] == 1) {
            count++;
        }
//p4 p5
        if (y + 1 < binaryImage.length && x + 1 < binaryImage[y].length && binaryImage[y][x + 1] == 0 && binaryImage[y + 1][x + 1] == 1) {
            count++;
        }
//p5 p6
        if (y + 1 < binaryImage.length && x + 1 < binaryImage[y].length && binaryImage[y + 1][x + 1] == 0 && binaryImage[y + 1][x] == 1) {
            count++;
        }
//p6 p7
        if (y + 1 < binaryImage.length && x - 1 >= 0 && binaryImage[y + 1][x] == 0 && binaryImage[y + 1][x - 1] == 1) {
            count++;
        }
//p7 p8
        if (y + 1 < binaryImage.length && x - 1 >= 0 && binaryImage[y + 1][x - 1] == 0 && binaryImage[y][x - 1] == 1) {
            count++;
        }
//p8 p9
        if (y - 1 >= 0 && x - 1 >= 0 && binaryImage[y][x - 1] == 0 && binaryImage[y - 1][x - 1] == 1) {
            count++;
        }
//p9 p2
        if (y - 1 >= 0 && x - 1 >= 0 && binaryImage[y - 1][x - 1] == 0 && binaryImage[y - 1][x] == 1) {
            count++;
        }
        return count;
    }

    private int getB(int[][] binaryImage, int y, int x) {
        return binaryImage[y - 1][x] + binaryImage[y - 1][x + 1] + binaryImage[y][x + 1]
                + binaryImage[y + 1][x + 1] + binaryImage[y + 1][x] + binaryImage[y + 1][x - 1]
                + binaryImage[y][x - 1] + binaryImage[y - 1][x - 1];
    }
}
package imageProcessing.model;

/**
 * Created by nayef on 1/27/15.
 */
public class Point {
    private int x;
    private int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }
}

Results: before

1100111
1100111
1100111
1100111
1100110
1100110
1100110
1100110
1100110
1100110
1100110
1100110
1111110
0000000

after
1100111
1000001
1000001
1000111
1000100
1000100
1000100
1000100
1000100
1000100
1000100
1000100
1111100
0000000

The equations and images of equations have been taken from book Character Recognition Systems: A Guide for Students and Practitioners by by Mohamed Cheriet, Nawwaf Kharma, Cheng-Lin Liu and Ching Suen .

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