shawnfu

作为生活在阳光照射世界的居民，我们通常假设光线是从上面照下来的，并用阴影的暗示来判断物体的形状。不信你可以试着将左图向左旋转90度，就会看到一个由凹面圆构成的X，如果向右转的话，就变成了一个凸面圆构成的X。
http://jifanghua.blogbus.com/logs/2004/02/84111.html


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