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由重构做开运算
普通的开运算可以去掉较小的对象，操作是先腐蚀，再膨胀。但是开运算在膨胀中会使形状变形。
可以用重构代替膨胀来做开运算，这个运算定义为<<latex: 执行失败 [Missing parentheses in call to 'print'. Did you mean print(...)?] (see also the log)>>
例子：
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