Normal Schmormal: My occasionally helpful guide to parenting kids with special needs (Down syndrome, autism, ADHD, neurodivergence)

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In three-dimensional space, a surface normal, or simply normal, to a surface at point P is a vector perpendicular to the tangent plane of the surface at P. The word normal is also used as an adjective: a line normal to a plane, the normal component of a force, the normal vector, etc. The concept of normality generalizes to orthogonality ( right angles). This article is about the normal to 3D surfaces. For the normal to 3D curves, see Frenet–Serret formulas. A polygon and its two normal vectors A normal to a surface at a point is the same as a normal to the tangent plane to the surface at the same point.

Specifically, given a 3×3 transformation matrix M , {\displaystyle \mathbf {M} ,} we can determine the matrix W {\displaystyle \mathbf {W} } that transforms a vector n {\displaystyle \mathbf {n} } perpendicular to the tangent plane t {\displaystyle \mathbf {t} } into a vector n ′ {\displaystyle \mathbf {n}

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For a plane given by the equation a x + b y + c z + d = 0 , {\displaystyle ax+by+cz+d=0,} the vector n = ( a , b , c ) {\displaystyle \mathbf {n} =(a,b,c)} is a normal.A normal vector may have length one (in which case it is a unit normal vector) or its length may represent the curvature of the object (a curvature vector). The foot of a normal at a point of interest Q (analogous to the foot of a perpendicular) can be defined at the point P on the surface where the normal vector contains Q.

If the normal is constructed as the cross product of tangent vectors (as described in the text above), it is a pseudovector. The normal distance of a point Q to a curve or to a surface is the Euclidean distance between Q and its foot P. Since a surface does not have a tangent plane at a singular point, it has no well-defined normal at that point: for example, the vertex of a cone. In general, it is possible to define a normal almost everywhere for a surface that is Lipschitz continuous.r ( s , t ) = r 0 + s p + t q , {\displaystyle \mathbf {r} (s,t)=\mathbf {r} _{0}+s\mathbf {p} +t\mathbf {q} ,}