当点乘的数量积为零时,这两个向量垂直。
These two vectors are perpendicular exactly when their dot product is zero.
或许你可以画出每点都垂直于向量场的曲线。
You might be able to draw curves that are perpendicular to it.
总的来说，您需要一个与照射平面相垂直的方向向量。
The general idea is that you need a direction vector that is orthogonal to the lit surface.
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