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做几何变换时,法线比较特殊,需要保证法线经过变换之后仍然垂直于原先的切平面,如果使用和顶点变换相同的变换,无法保证这一点

如图,线段ab经过放缩变换之后成为a'b',但法线经过同样的放缩矩阵变换之后发生错误,实际法线需要另外的矩阵计算
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