A quadtree does the same thing for two-dimensional space. It takes a rectangular region and divides it into four equal quadrants: northwest, northeast, southwest, southeast. If a quadrant has too many points in it, it subdivides again and again. Each subdivision creates smaller and smaller cells where points are densely packed.
从春风唤醒生命的感触中，忽然想起了贺知章的名句：“不知细叶谁裁出，二月春风似剪刀。”可再一琢磨，诗里春风固然灵巧，用词确有新奇绝妙处，但总不免失之于锋芒过露。而自己眼见的一切，或许更近于“随风潜入夜，润物细无声”的意味。这风似乎不像剪刀，没那么利落、分明的姿态，倒更像是气是水，是弥漫的、渗透的、无处不在的柔情。它不张扬自己的到来，只是默默地让柳丝自己去绿，让草芽自己去长，让蛰虫自己去醒。像个高明的导演，自己隐在幕后，只让万物去演绎生命的繁华。
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