java - 这个奇怪的区域减法问题的原因是什么？

Question

当我编写碰撞检测算法时，我遇到了这个问题。这是一件奇怪的事情，超出了我的理解。

这里的问题是，如果在我的算法中，在函数 tryMove() 中出现，我将 potentialArea 添加到 moveLineArea 并且我正在检测 spaceTestArea(从 moveLineArea 创建)在减去所有单元占用的区域后，我与一个单元发生碰撞这甚至不接近 x=280,y=120，其中移动单元位于 x=1880,y=120，并且正在移动到 x= 1914 年，y=126。

我想知道这个问题的可能原因是什么以及如何避免将来发生。

我必须说我有一个临时解决方案 (tryMove2()) 但请不要让它影响你的想法，即我不喜欢这个解决方案并且我坚信第一个解决方案 (tryMove()) 应该可以工作，一定是我忘记了什么。

请参阅下面显示问题的代码。

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);
    }
}

这是它产生的输出:

******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

为了让您更好地看到问题，我用两张图片展示了两个端点，第一个 1914, 126 当方法失败时，第二个 1914, 127什么时候可以。

如果需要更多描述，我会尽快答复。提前谢谢大家。

编辑 1:正如@trashgod 所建议的，我确实尝试并实现了一个使用 intersect() 方法的解决方案。我不喜欢每次测试都必须创建一个新对象。你能对这个算法提出一些优化建议吗。

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

我猜你正在与其中一种方式发生冲突 java.awt.geom.Area可以变空。如果有帮助，我在下面拍了张照片。或者，您可以使用 createTransformedShape() 和 contains() 吗？

附录:tryMove3() 看起来是正确的。如果情况真的更糟，我看到了几种可能性:

• Cache任何静态区域，可能是与每个中心点相关联的区域。

• 根据 getBounds() 或 quadtree 进行粗略的邻域检查并跳过远程对。

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);