Java Geometry Algorithm edge(Collection points)

Description

Calculate the edge of the specified linked area.

License

Open Source License

Parameter

Parameter	Description
points	the point set of the specified linked area

Return

the point set of the edge

Declaration

public static HashSet<Point> edge(Collection<Point> points) 

Method Source Code

//package com.java2s;
import java.awt.Point;

import java.util.Collection;

import java.util.HashSet;

public class Main {
    /**/* w  w  w.  j  a v a  2  s .  co  m*/
     * Calculate the edge of the specified linked area. This algorithm is
     * implemented according to Robert algorithm.
     * 
     * @param points
     *            the point set of the specified linked area
     * @return the point set of the edge
     */
    public static HashSet<Point> edge(Collection<Point> points) {
        // Clear the edge set first.
        HashSet<Point> edge = new HashSet<Point>();
        int x = 0;
        int y = 0;
        for (Point p : points) {
            if (p.x > x)
                x = p.x;
            if (p.y > y)
                y = p.y;
        }
        // x,y are used as the size of the array, therefore it should be 1
        // larger than the maximum coordinate. In the meanwhile, Robert
        // algorithm needs one more size to deal with the algorithm model,
        // therefore x,y should plus 2.
        x += 2;
        y += 2;
        boolean[][] image = new boolean[x][y];
        boolean[][] copy = image.clone();
        for (Point p : points)
            image[p.x][p.y] = true;
        for (int i = 0; i < image.length - 1; i++)
            for (int j = 0; j < image[i].length - 1; j++) {
                // Robert algorithm.
                int p5 = image[i][j] ? 1 : 0;
                int p6 = image[i][j + 1] ? 1 : 0;
                int p8 = image[i + 1][j] ? 1 : 0;
                int p9 = image[i + 1][j + 1] ? 1 : 0;
                copy[i][j] = Math.max(Math.abs(p5 - p9), Math.abs(p8 - p6)) == 1;
            }
        for (int i = 0; i < copy.length - 1; i++)
            for (int j = 0; j < copy[i].length - 1; j++)
                if (copy[i][j])
                    edge.add(new Point(i, j));
        return edge;
    }
}

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