surface normal

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surface normal (plural surface normals)

  1. (differential geometry) A unit vector at any given point P of a surface M which is perpendicular to the tangent plane TP(M) of M at P. For a surface with parametrization  \vec x(u,v) , it is given by
 \vec n = \pm {\vec x_u \times \vec x_v \over \lVert \vec x_u \times \vec x_v \rVert}
where the sign can be chosen at will.