性能优化function updateBalls(){

    for( var i = 0 ; i < balls.length ; i ++ ){

        balls[i].x += balls[i].vx;
        balls[i].y += balls[i].vy;
        balls[i].vy += balls[i].g;

        if( balls[i].y >= WINDOW_HEIGHT-RADIUS ){
            balls[i].y = WINDOW_HEIGHT-RADIUS;
            balls[i].vy = - balls[i].vy*0.75;
        }
    }

    var cnt = 0
    for( var i = 0 ; i < balls.length ; i ++ )
        if( balls[i].x + RADIUS > 0 && balls[i].x -RADIUS < WINDOW_WIDTH )
            balls[cnt++] = balls[i]

    while( balls.length > Math.min(300,cnt) ){
        balls.pop();
    }
}