POP.collision = {

    rect: function(a, b) {
    
    },

    circle: function(a, b) {
        var distance_squared = ( ((a.x - b.x) * (a.x - b.x)) + 
                                ((a.y - b.y) * (a.y - b.y)));

        var radii_squared = (a.r + b.r) * (a.r + b.r);

        if (distance_squared < radii_squared) {
            return true;
        } else {
            return false;
        }
   
    }

};