Question

I’m working with Areas in Java.

My test program draws three random triangles and combines them to form one or more polygons. After the Areas are .add()ed together, I use PathIterator to trace the edges.

Sometimes, however, the Area objects will not combine as they should… and as you can see in the last image I posted, extra edges will be drawn.

I think the problem is caused by rounding inaccuracies in Java’s Area class (when I debug the test program, the Area shows the gaps before the PathIterator is used), but I don’t think Java provides any other way to combine shapes.

Any solutions?

Example code and images:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Area;
import java.awt.geom.Line2D;
import java.awt.geom.Path2D;
import java.awt.geom.PathIterator;
import java.util.ArrayList;
import java.util.Random;

import javax.swing.JFrame;

public class AreaTest extends JFrame{
    private static final long serialVersionUID = -2221432546854106311L;


    Area area = new Area();
    ArrayList<Line2D.Double> areaSegments = new ArrayList<Line2D.Double>();

    AreaTest() {
        Path2D.Double triangle = new Path2D.Double();
        Random random = new Random();

        // Draw three random triangles
        for (int i = 0; i < 3; i++) {
            triangle.moveTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
            triangle.lineTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
            triangle.lineTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
            triangle.closePath();
            area.add(new Area(triangle));
        }       

        // Note: we're storing double[] and not Point2D.Double
        ArrayList<double[]> areaPoints = new ArrayList<double[]>();
        double[] coords = new double[6];

        for (PathIterator pi = area.getPathIterator(null); !pi.isDone(); pi.next()) {

            // Because the Area is composed of straight lines
            int type = pi.currentSegment(coords);
            // We record a double array of {segment type, x coord, y coord}
            double[] pathIteratorCoords = {type, coords[0], coords[1]};
            areaPoints.add(pathIteratorCoords);
        }

        double[] start = new double[3]; // To record where each polygon starts
        for (int i = 0; i < areaPoints.size(); i++) {
            // If we're not on the last point, return a line from this point to the next
            double[] currentElement = areaPoints.get(i);

            // We need a default value in case we've reached the end of the ArrayList
            double[] nextElement = {-1, -1, -1};
            if (i < areaPoints.size() - 1) {
                nextElement = areaPoints.get(i + 1);
            }

            // Make the lines
            if (currentElement[0] == PathIterator.SEG_MOVETO) {
                start = currentElement; // Record where the polygon started to close it later
            } 

            if (nextElement[0] == PathIterator.SEG_LINETO) {
                areaSegments.add(
                        new Line2D.Double(
                            currentElement[1], currentElement[2],
                            nextElement[1], nextElement[2]
                        )
                    );
            } else if (nextElement[0] == PathIterator.SEG_CLOSE) {
                areaSegments.add(
                        new Line2D.Double(
                            currentElement[1], currentElement[2],
                            start[1], start[2]
                        )
                    );
            }
        }

        setSize(new Dimension(500, 500));
        setLocationRelativeTo(null); // To center the JFrame on screen
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setResizable(false);
        setVisible(true);
    }

    public void paint(Graphics g) {
        // Fill the area
        Graphics2D g2d = (Graphics2D) g;
        g.setColor(Color.lightGray);
        g2d.fill(area);

        // Draw the border line by line
        g.setColor(Color.black);
        for (Line2D.Double line : areaSegments) {
            g2d.draw(line);
        }
    }

    public static void main(String[] args) {
        new AreaTest();
    }
}

A successful case:

A failing case:

Answer 1

Here:

    for (int i = 0; i < 3; i++) {
        triangle.moveTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
        triangle.lineTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
        triangle.lineTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
        triangle.closePath();
        area.add(new Area(triangle));
    }       

you are adding in fact
1 triangle in the first loop
2 triangles in the second loop
3 triangles in the third loop

This is where your inaccuracies come from. Try this and see if your problem still persists.

    for (int i = 0; i < 3; i++) {
        triangle.moveTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
        triangle.lineTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
        triangle.lineTo(random.nextInt(400) + 50, random.nextInt(400) + 50);
        triangle.closePath();
        area.add(new Area(triangle));
        triangle.reset();
    }    

Note the path reset after each loop.

EDIT: to explain more where the inaccuracies come from here the three paths you try to combine. Which makes it obvious where errors might arise.