Question

While I was coding an algorithm of collision detection I have come up at this problem. It is something weird which is beyond my understanding.

The problem here is that if in my algorithm, presented in function tryMove(), I add potentialArea to moveLineArea and I am doing detection of change in the spaceTestArea (which is created from the moveLineArea) after subtracting areas taken by all units, I have a collision with a unit that is not even close x=280,y=120, where moving unit is at x=1880,y=120, and it is moving to x=1914,y=126.

I would like to know what might be the reason of this issue and what to do in order to avoid it in future.

I must say I have a temporary solution (tryMove2()) but please do not let it influence your thinking, i.e. I do not like this solution and I strongly believe that the first solution (tryMove()) should work and it must be me who forgot about something.

Please see below the code presenting the problem.

import java.awt.Point;
import java.awt.Polygon;
import java.awt.geom.Area;
import java.awt.geom.Ellipse2D;
import java.util.ArrayList;
import java.util.List;

/**
 * Test showing some unexpected and weird behaviour of area subtraction.
 * @author Konrad Borowiecki
 */
public class TestTryMove {
    private static final List<Point> unitCenterPoints = new ArrayList<Point>();
    static{
        unitCenterPoints.add(new Point(1720, 120));
        unitCenterPoints.add(new Point(1880, 120));
        unitCenterPoints.add(new Point(1800, 200));
        unitCenterPoints.add(new Point(1720, 280));
        unitCenterPoints.add(new Point(1880, 280));
        unitCenterPoints.add(new Point(120, 120));
        unitCenterPoints.add(new Point(280, 120));
        unitCenterPoints.add(new Point(200, 200));
        unitCenterPoints.add(new Point(120, 280));
        unitCenterPoints.add(new Point(280, 280));
        unitCenterPoints.add(new Point(120, 1720));
        unitCenterPoints.add(new Point(280, 1720));
        unitCenterPoints.add(new Point(200, 1800));
        unitCenterPoints.add(new Point(120, 1880));
        unitCenterPoints.add(new Point(280, 1880));
    }
    public static void main(String[] args) {
        int[] xpointsOK = new int[]{1876, 1884, 1918, 1910};//for Move OK
        int[] ypointsOK = new int[]{139, 101, 108, 146};//for Move OK
        Polygon lineOK = new Polygon(xpointsOK, ypointsOK, xpointsOK.length);

        int[] xpointsFAIL = new int[]{1877, 1883, 1917, 1911};//for problem no move
        int[] ypointsFAIL = new int[]{139, 101, 107, 145};//for problem no move
        Polygon lineFAIL = new Polygon(xpointsFAIL, ypointsFAIL, xpointsFAIL.length);

        Point endPointCPOK = new Point(1914, 127);//Move OK
        Point endPointCPFAIL = new Point(1914, 126);//problem no move
        //where in both cases it should be move OK              
        System.out.println("******TEST for method tryMove()******");            
        System.out.println("TEST 1: this will FAIL");
        System.out.println("Result="+tryMove(endPointCPFAIL, lineFAIL)); 
        System.out.println("\nTEST 2: this will be OK");
        System.out.println("Result="+tryMove(endPointCPOK, lineOK));


        System.out.println("******TEST for method tryMove2()******");   
        System.out.println("TEST 1: this will be OK");
        System.out.println("Result="+tryMove2(endPointCPFAIL, lineFAIL)); 
        System.out.println("\nTEST 2: this will be OK");
        System.out.println("Result="+tryMove2(endPointCPOK, lineOK));
    }
    /**
     * Tests if a unit represented by point of index 1 in the list of 
     * unitCenterPoints (i.e. [1880, 120]) can make a move to the given endPointCP.
     * (Please notice we are ignoring this unit in the algorithm 
     * i.e. if(i != movingUnitIndexInTheArray)).
     * @param endPointCP the point where the unit moves to.
     * @param line the line of the move of the thickness equal to units width (mod=40), 
     * drawn between the current unit's center point and the endPointCP, 
     * represented as a polygon object.
     * @return true if move possible; false otherwise. 
     */
    private static boolean tryMove(Point endPointCP, Polygon line){
        Area potentialArea = getArea(endPointCP);
        Area moveLineArea = new Area(line);
        moveLineArea.add(potentialArea);
        //this area is used for testing if nothing stays on the way of the move
        Area spaceTestArea = new Area(moveLineArea);
        //the index of the unit making the move in the unitCenterPoints list
        int movingUnitIndexInTheArray = 1;
        //we are subtracting from spaceTestArea all areas of units
        for(int i = 0; i < unitCenterPoints.size(); i++)
            if(i != movingUnitIndexInTheArray) {
                Point p = unitCenterPoints.get(i);
                Area uArea = getArea(p);
                spaceTestArea.subtract(uArea);
                //we have intersection then return false, we cannot make this move  
                if(spaceTestArea.isEmpty() || !spaceTestArea.equals(moveLineArea)) {
                    System.out.println("No move --- a unit is on the way. "
                            + "Conflicting point is="+p +"; for i="+i);
                    return false;
                }
            }
        System.out.println("Move OK.");
        return true;
    }

    private static boolean tryMove2(Point endPointCP, Polygon line){
        Area potentialArea = getArea(endPointCP);
        Area moveLineArea = new Area(line);
        //test if unit can move to the new position
        Area potentialTestArea = new Area(potentialArea);
        //this area is used for testing if nothing stays on the way of the move
        Area spaceTestArea = new Area(moveLineArea);
        //the index of the unit making the move in the unitCenterPoints list
        int movingUnitIndexInTheArray = 1;
        //we are subtracting from spaceTestArea all areas of units
        for(int i = 0; i < unitCenterPoints.size(); i++)
            if(i != movingUnitIndexInTheArray) {
                Point p = unitCenterPoints.get(i);
                Area uArea = getArea(p);
                spaceTestArea.subtract(uArea);
                potentialTestArea.subtract(uArea);
                //we have intersection then return false, we cannot make this move  
                if(spaceTestArea.isEmpty() || !spaceTestArea.equals(moveLineArea)
                        || potentialTestArea.isEmpty() || !potentialTestArea.equals(potentialArea)) {
                    System.out.println("No move --- a unit is on the way. "
                            + "Conflicting point is="+p +"; for i="+i);
                    return false;
                }
            }
        System.out.println("Move OK.");
        return true;
    }
    /**
     * Gets the area taken by a unit given the unit's center point.
     * @param p the center point of a unit.
     * @return circle area.
     */ 
    private static Area getArea(Point p) {
        int mod = 40;//this is width and height of a unit
        Ellipse2D circle = new Ellipse2D.Double(p.x - mod / 2, p.y - mod / 2, mod, mod);
        return new Area(circle);
    }
}

And this is the output it is producing:

******TEST for method tryMove()******
TEST 1: this will FAIL
No move --- a unit is on the way. Conflicting point is=java.awt.Point[x=280,y=120]; for i=6; where moving unit point is=java.awt.Point[x=1880,y=120]; the unit is moving to=java.awt.Point[x=1914,y=126]
Result=false

TEST 2: this will be OK
Move OK.
Result=true
******TEST for method tryMove2()******
TEST 1: this will be OK
Move OK.
Result=true

TEST 2: this will be OK
Move OK.
Result=true

In order for you to see the problem better I have two images presenting it for two endPoints, first 1914, 126 when the method fails, and second 1914, 127 when it is OK.

If more description is needed I will answer ASAP. Thank you all in advance.

EDIT1:
As suggested by @trashgod I did tried and implemented a solution which uses intersect() method. I do not like that for every test you must create a new object. Can you suggest some optimization for this algorithm.

private static boolean tryMove3(Point endPointCP, Polygon line){
    Area potentialArea = getArea(endPointCP);
    Area moveLineArea = new Area(line);
    moveLineArea.add(potentialArea);
    //this area is used for testing if nothing stays on the way of the move
    //the index of the unit making the move in the unitCenterPoints list
    int movingUnitIndexInTheArray = 1;
    //we are subtracting from spaceTestArea all areas of units
    for(int i = 0; i < unitCenterPoints.size(); i++)
        if(i != movingUnitIndexInTheArray) {
            Point p = unitCenterPoints.get(i);
            Area uArea = getArea(p);
            Area spaceTestArea = new Area(moveLineArea);
            spaceTestArea.intersect(uArea);
            //we have intersection then return false, we cannot make this move  
            if(!spaceTestArea.isEmpty()) {
                System.out.println("No move --- a unit is on the way. "
                        + "Conflicting point is="+p +"; for i="+i
                        + "; where moving unit point is="
                        +unitCenterPoints.get(movingUnitIndexInTheArray)
                        +"; the unit is moving to="+endPointCP
                        +"; spaceTestArea.isEmpty()="+spaceTestArea.isEmpty());
                return false;
            }
        }
    System.out.println("Move OK.");
    return true;
}

Answer 1

I’m guessing you’re running afoul of one of the ways java.awt.geom.Area can become empty. If it helps, I took a picture, below. Alternatively, could you use createTransformedShape() and contains()?

Addendum: tryMove3() appears correct. If it really profiles worse, I see several possibilities:

• Cache any static Area, perhaps those associated with each center point.

• Do a rough vicinity check based on getBounds() or a quadtree and skip remote pairs.

JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.add(new JPanel(){

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.scale(10, 10);
        g2d.translate(-1875, -100);
        g2d.setColor(Color.green);
        g2d.draw(lineOK);
        g2d.setColor(Color.green.darker());
        g2d.drawRect(endPointCPOK.x, endPointCPOK.y, 1, 1);
        g2d.setColor(Color.red);
        g2d.draw(lineFAIL);
        g2d.setColor(Color.red.darker());
        g2d.drawRect(endPointCPFAIL.x, endPointCPFAIL.y, 1, 1);
    }
});
f.pack();
f.setSize(450, 500);
f.setLocationRelativeTo(null);
f.setVisible(true);