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.
Well, the question we have now is what is the area of this little piece of surface and what is its normal vector?
现在的问题是，曲面上这一小块儿的面积是什么，及其法向量是什么？
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The area of image is calculated by using edge tracing and vector analysis in this paper. The result is available in analysing practical biomedical and metallurgical images.