The line of sight must be approximately perpendicular to the horizontal axis. 视线必须近似地垂直于水平轴。
Because the horizontal drag was the predominant user action, and vertical drags were rare, we biased the drag threshold towards the horizontal axis. 因为水平拖动是用户的主要动作,而垂直拖动较少,所以我们将拖动阈值向水平轴倾斜。