CODEWITHSUNDEEP
java2dcollisionshapesintersection
Monkeybro10
Monkeybro10

Reputation: 257

Java collision detection between two Shape objects?

I would like to know the best way to tell if a Shape object intersects another shape. Currently I have collision detection in my game sorted out as long as it involves a Shape intersecting a Rectangle or vice versa. The problem I'm having is that the intersects() method in the Shape class can only take a Rectangle or a Point as a parameter, not another Shape. Is there an efficient way to test if two Shape objects are overlapping in any way? One way I tried was using a for loop to generate an area of points to test if they were in the shape, and then building an array of Point objects to send to the other shape to test, but this significantly dropped my framerate because of all of the unnecessary comparisons.

I looked and looked for something similar on here but didn't find anything really. Sorry in advance if this is a repeat.

Upvotes: 17

Views: 38346

Answers (4)

Lance Wentz
Lance Wentz

Reputation: 99

If you think the area intersect is too expensive, you could first do a bounds check: shapeA.getBounds().intersects(shapeB.getBounds())

If this passes, then do the area intersect check.

if( myShape.getBounds().intersects(otherShape.getBounds()) ){
    Area a = new Area(myShape);
    a.intersect(new Area(otherShape));
    if(!a.isEmpty()){
        // do something
    }
}

Upvotes: 0

user3011902
user3011902

Reputation:

You can also use the bounds of the shape itself and then compare the bounds:

public boolean collidesWith(Shape other) {
    return shape.getBounds2D().intersects(other.getBounds2D());
}

This is a bit nicer on the eyes.

Upvotes: 10

Done123
Done123

Reputation: 11

Even though user2221343 already answered Monkeybro10's question, I thought it might be helpful in some cases to know, that the outline of a shape might play a role if you use his described technique:

For example, if you draw two polygons, the collision of them won't be detected if it occurs only on the exact outline of the polygons. Only, if the areas that are included inside the polygons' outlines will overlap, the collision is detected. If you fill two polygons, but don't draw them, the collision will be detected even on the outline of the visible area.

I wrote a small example to show what I mean. Either uncomment the draw or fill command, and rise the second polygon vertically by one pixel by uncommenting the given line. Run the code and watch the result in the JFrame. If the second Polygon is risen, and both polygons are only visible by the "fill" command, they intersect with their outlines and collision is detected. If the second polygon is not risen, and both polygons are visible by the "draw" command, they intersect with their outlines but collision is not detected:

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Polygon;
import java.awt.geom.Area;

import javax.swing.JFrame;

public class Test {

    private JFrame frame;
    private Polygon polygon1;
    private Polygon polygon2;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Test window = new Test();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public Test() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame(){
            private static final long serialVersionUID = 1L;

            @Override
            public void paint(Graphics g){

                super.paint(g);

                doDrawing(g);

            }
        };
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        int nShape1 = 4;
        int xPoly1[] = {30,50,50,30};
        int yPoly1[] = {30,30,50,50};
        polygon1 = new Polygon(xPoly1,yPoly1,nShape1);

        int nShape2 = 4;
        int xPoly2[] = {35,55,55,35};
        int yPoly2[] = {50,50,70,70};

        // uncomment next line to rise second polygon vertically by one pixel
        //yPoly2[] = {49,49,69,69};

        polygon2 = new Polygon(xPoly2,yPoly2,nShape2);
    }
    public synchronized void doDrawing(Graphics g){
        g.setColor(new Color(255,0,0));

        // if you draw the polygon, collision on the exact outline won't be detected.
        // uncomment draw or fill command to see what I mean.
        g.drawPolygon(polygon1);
        g.fillPolygon(polygon1);

        g.setColor(new Color(0,0,255));

        // if you draw the polygon, collision on the exact outline won't be detected.
        // uncomment draw or fill command to see what I mean.
        g.drawPolygon(polygon2);
        g.fillPolygon(polygon2);

        Area area = new Area(polygon1);
        area.intersect(new Area(polygon2));
        if(!area.isEmpty()){
            System.out.println("intersects: yes");
        }
        else{
            System.out.println("intersects: no");
        }
    }

}

Upvotes: 1

user2221343
user2221343

Reputation: 614

Not tested, but why not:

import java.awt.geom.Area;

...

public static boolean testIntersection(Shape shapeA, Shape shapeB) {
   Area areaA = new Area(shapeA);
   areaA.intersect(new Area(shapeB));
   return !areaA.isEmpty();
}

Area implements Shape, but adds some potentially useful methods

Upvotes: 22

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