You will get some sort of mesh grid on your surface and you will look at a little parallelogram that is an elementary piece of that mesh and figure out what is its area and normal vector.

在曲面上做一些网格，你会看到一个网上的每一小块儿，都是小的平行四边形，我们要弄清楚它的面积和法向量是什么。

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And a new method to estimate the normal vector of the surface in Direct Voltime Rendering in Image Space is introduced.

在象空间中的直接体绘制方法中，本文提出了一种新的估计表面法向量的方法。

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Also, a solution is offered to solve the problem of generating tool axis vector when the surface normal ts perpendicular to rotary plane.

同时解决了曲面法矢垂直于回转平面时刀轴矢量难以确定的关键问题。

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