Question

I’m trying to implement a aabb to circle collision.

Here’s my code:

     //From another file
     radius = (sprite.width + sprite.height) / 4;

     private function BoxToCircleCollision(box1:BoundingBox, circle1:BoundingCircle):Boolean
     {
        var nBoxMinX:Number = box1.sprite.x - box1.sprite.width / 2;
        //var nBoxMinY:Number = box1.sprite.x + box1.sprite.width / 2;
        var nBoxMaxX:Number = box1.sprite.y - box1.sprite.height / 2;
        //var nBoxMaxY:Number = box1.sprite.y + box1.sprite.height / 2;

        var nCirMinX:Number = circle1.sprite.x - circle1.radius / 2;
        //var nCirMinY:Number = circle1.sprite.y - circle1.radius;
        var nCirMaxX:Number = circle1.sprite.x + circle1.radius / 2;
        //var nCirMaxY:Number = circle1.sprite.y + circle1.radius;

        if (nBoxMaxX, 2 > nCirMinX))
        {
            Main.WriteDebug("Box max: " + nBoxMaxX + " | Circle min: " + nCirMinX);
            return true;
        }
        else
        {
            Main.WriteDebug("Box max: " + nBoxMaxX + " | Circle min: " + nCirMinX);
            return false;
        }
     }

Somehow the collision does work as expected.
Either they never move at all and “collided” was traced, or they’ll continue moving and never collide when I tried swapping values around.

Is there something i’m missing in my logic???

My box-box and circle-circle collision are working fine.

---

Thanks in advance for your help.

Answer 1

This row doesn’t look at all correct:

var nBoxMaxX:Number = box1.sprite.y - box1.sprite.height / 2;

Maybe you meant this:

var nBoxMaxX:Number = box1.sprite.x + box1.sprite.width / 2;

This line won’t compile:

if (nBoxMaxX, 2 > nCirMinX))

Edit:

Here’s a function to help you get the AABB <-> Circle collision right. It’s not a complete solution but you can combine it with the calculations you have for the AABB min and max values, should be trivial:

private function collideAABBandCircle(c : Circle, aabb:AABB) : Boolean {
    var sqDist : Number = sqDist(c.centerPoint, aabb);

    return sqDist <= c.radius * c.radius:
}

private function sqDist(p : Point, aabb:AABB) : Number {
    /* CALCULATE min and max values of aabb bounds */
    var sqDist : Number = 0.0;

    if(p.x < minX) {
        sqDist += (minX - p.x) * (minX - p.x);
    }
    if(p.x > maxX) {
        sqDist += (p.x - maxX) * (p.x - maxX);
    }

    if(p.y < minY) {
        sqDist += (minY - p.y) * (minY - p.y);
    }
    if(p.y > maxY) {
        sqDist += (p.y - maxY) * (p.y - maxY);
    }

    return sqDist;
}