blacklight

  float vx = 0;

  float vy = 0;

    if ( ( this.x + vx <= 0 ) || ( this.x + vx >= _game_.width) ) {

          this.vx = -vx;

    }

    if ( ( this.y + vy <= 0 ) || ( this.y + vy >= _game_.height) ) {

          this.vy = -vy;

    }

    this.x += vx;

    this.y += vy;

  }