# calculate surface normal with random sampling of points

Given a surface in $R^3$ and a point P on the surface, I want to calculate the surface normal in this point, the vector that is perpendicular to the surface.

However, I do not know the whole surface, but merely a random sampling of points on the surface.

How can I calculate a good approximation of the surface normal?

The surface is non-intersecting, smooth and roughly planar, if that matters

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## 1 Answer

1. Select some number of close points of P. Within some radius, or the K closest neighbors.

2. Fit a plane to those points using Linear least squares. Possibly weigh points closer to P higher, using Weighted least squares. $$a \cdot \left( x - x_0 \right) + b \cdot \left( y - y_0 \right) + c \cdot \left( z - z_0 \right) = 1$$ $$A X = B$$ $$\hat X = \left( A^\intercal A \right)^{-1} A^\intercal B \\ \textbf{or} \\ \hat X = \left( A^\intercal W A \right)^{-1} A^\intercal W B$$

A has one row for each point, minus P.
B is column vector filled with ones.
X is the vector $\begin{pmatrix} a & b & c \end{pmatrix}^\intercal$.
$\hat X$ is the estimation of X.
W is a diagonal matrix with the weight of each point.

3. Calculate the normal of the plane. This is just the norm of X in the above equations.

Here are some papers on the subject:

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Thanks! This is essentially what I am trying to do, but simply fitting a plane seems like a very rough approximation. Is there some way I can use the relationships between the points, such as weighting points that are closer to P higher, or weighting points that are close to each other lower? I an make up some weighting scheme, but ideally I am looking for a method that is based on geometric principles – HugoRune Jun 28 '12 at 22:04