drawballs(context) {
        if (this.nw != null) {
            this.nw.drawballs(context);
            this.ne.drawballs(context);
            this.sw.drawballs(context);
            this.se.drawballs(context);
        }
        for (let c of this.balls) {
            context.beginPath();
            context.arc(c.x, c.y, c.r, 0, 2 * Math.PI, false);
            if (c.isColided) {
                context.fillStyle = 'blue';
            } else {
                context.fillStyle = 'green';
            }
            context.fill();
            context.lineWidth = 0.1;
            if (c.isColided) {
                context.strokeStyle = 'blue';
            } else {
                context.strokeStyle = 'green';
            }
            context.closePath();
            context.stroke();
        }
    }
}