Java 2D Polygon - Polygon Collision Detection

Question

Recently I have been using the Polygon class to create asteroids as well as bullets and a spaceship. I am currently trying to create the collision detection for the program however it appears that the collision detection only works around 1/5 of the time (no pattern appears as to why it works).

Here's the code.. Creating the Polygon:

void renderPoly() {   
    int j;
    int s = sides;
    double r, angle;
    int x, y;

    for (j = 0; j < s; j++) {
        angle = 2 * Math.PI / s * j;
        r = MIN_ROCK_SIZE + (int) (Math.random() * (MAX_ROCK_SIZE - MIN_ROCK_SIZE));
        x = (int) (r * Math.cos(angle));
        y = (int) (r * -Math.sin(angle));

        cOM[0] += x;
        cOM[1] += y;
        pointData[j][0] = x;
        pointData[j][1] = y;
    }

    cOM[0] /= asteroidShape.npoints;
    cOM[1] /= asteroidShape.npoints;

    for (int i = 0; i < asteroidShape.npoints; i++) {
        pointData[i][0] += cOM[0];
        pointData[i][1] += cOM[1];
    }    
}

rotating and moving the polygon:

void move() {    
    int x, y, i;
    //change rotation
    theta += rotVel;

    //change x
    asteroidData[0] += deltaX;

    //change y
    asteroidData[1] += deltaY;

    for (i = 0; i < asteroidShape.npoints; i++) {

        x = (int) (pointData[i][0] * Math.cos(theta) - pointData[i][1] * Math.sin(theta) );
        y = (int) (pointData[i][0] * Math.sin(theta) + pointData[i][1] * Math.cos(theta) );

        asteroidShape.xpoints[i] = x + asteroidData[0];
        asteroidShape.ypoints[i] = y + asteroidData[1];

        asteroidShape.invalidate();
    }
}

check if touching bullet:

    boolean hitBullet(Bullet b) {
    this.asteroidShape.invalidate();
    for (int i = 0; i < b.bulletShape.npoints; i++) 
        if (this.asteroidShape.contains(b.bulletShape.xpoints[i], b.bulletShape.ypoints[i]) )
            return true;

    for (int j = 0; j < this.asteroidShape.npoints; j++)
        if (b.bulletShape.contains(this.asteroidShape.xpoints[j], this.asteroidShape.ypoints[j]) )
            return true;

    return false;

}

(the ship method is the same except the constructor requires a ship object)

as well as the loop that calls it in the 'game' class:

    for (int i = 0; i < aArray.length-1; i++) {
    if (aArray[i] != null) {

        for (int j = 0; j < bArray.length-1; j++) {
            if (bArray[j] != null) {

                if (aArray[i].hitBullet(bArray[j])) {
                    aArray[i] = null;
                    bArray[j] = null;
                    i = aArray.length-1;
                    j = bArray.length-1;
                }

            }
            else {
                i = aArray.length-1;
                j = bArray.length-1;
            }
        }

    }
    else {
        i = aArray.length-1;
    }
}

I have been looking around at alternative solutions such as the Separating Axis Theorem however I do have convex polygons at times and since this method (.contains()) already exists I would like to use it.

Any help would be appreciated, thanks!

Answer 1

The easy way to solve this that I've found is to convert Shapes (in your case Polygon(2D?)) into Areas. You can use Area.intersect(Area) to see if two Areas have collided