# Fitting a plane to noisy points in 3D

In March 2015 I wrote an article for a simple way to fit a plane to many points in 3D. This article will introduce an improvement that handle noisy input better.

The original method can be summarized as follows:

1. Calculate the centroid of the points
2. Calculate the covariance matrix of the points relative to the centroid
3. Find the main axis (the component of the plane normal which will have the largest absolute value) and do simple linear regression along that axis

This works well when the points lie in a plane. What if they do not? What if they are noisy?

The problem with this solution is that is doing simple linear regression when we really want is Orthogonal regression. The most accurate way to fit a plane to noisy points is to modify the last step:

1. Calculate the centroid of the points
2. Calculate the covariance matrix of the points relative to the centroid
3. Calculate the smallest Eigenvector of the covariance matrix. This is the plane normal.

If you have a library to calculate the Eigenvectors of a 3x3 matrix, and performance is not important to you, this is what you should do. You can stop reading now!

However, if performance matters or don't have the patience to implement an Eigenvector solver, keep reading.

To illustrate the problem with the solution in the previous article, I created a small program which generates 50 000 random points on the surface sphere. For each point, I find the closest 32 neighbors to that point and fit a plane to these 32 points. I then color-code the central point based on the normal of the plane.

Here is the result using the 2015 method:

When I compare the computed normal to the exact one using the Eigenvector solution the mean error is just 0.004°. So when the noise is low, the old method works extremely well.

And here is the same data with the 2015 method:

The average error is now around 16.5° on average – not great! And you can immediately see the problem: the solution is non-smooth when we switch from one main axis to another.

Luckily, a small tweak can give us a much better result. The idea is to measure how well suited each axis is and then do a weighted sum of the normals based on this suitability weight. I did some experimentation, and by weighing the results of each axis by the square of the normal we get this:

The error is now only 4.8°, less than a third of the old method, and with no visible seams. This improvement holds up even if you change the number of points or the size of the noise.

So the new method is:

1. Calculate the centroid of the points
2. Calculate the covariance matrix of the points relative to the centroid
3. Do linear regression along the X, Y and Z axis
4. Weigh the result of the linear regressions based on their numeric stability

The resulting code is still fast and simple:

// Fit a plane to a collection of points.
// Fast, and accurate to within a few degrees.
// Returns None if the points do not span a plane.
fn plane_from_points(points: &[Vec3]) -> Option<Plane> {
let n = points.len();
if n < 3 {
return None;
}

let mut sum = Vec3{x:0.0, y:0.0, z:0.0};
for p in points {
sum = &sum + &p;
}
let centroid = &sum * (1.0 / (n as f64));

// Calculate full 3x3 covariance matrix, excluding symmetries:
let mut xx = 0.0; let mut xy = 0.0; let mut xz = 0.0;
let mut yy = 0.0; let mut yz = 0.0; let mut zz = 0.0;

for p in points {
let r = p - &centroid;
xx += r.x * r.x;
xy += r.x * r.y;
xz += r.x * r.z;
yy += r.y * r.y;
yz += r.y * r.z;
zz += r.z * r.z;
}

xx /= n as f64;
xy /= n as f64;
xz /= n as f64;
yy /= n as f64;
yz /= n as f64;
zz /= n as f64;

let mut weighted_dir = Vec3{x: 0.0, y: 0.0, z: 0.0};

{
let det_x = yy*zz - yz*yz;
let axis_dir = Vec3{
x: det_x,
y: xz*yz - xy*zz,
z: xy*yz - xz*yy,
};
let mut weight = det_x * det_x;
if weighted_dir.dot(&axis_dir) < 0.0 { weight = -weight; }
weighted_dir += &axis_dir * weight;
}

{
let det_y = xx*zz - xz*xz;
let axis_dir = Vec3{
x: xz*yz - xy*zz,
y: det_y,
z: xy*xz - yz*xx,
};
let mut weight = det_y * det_y;
if weighted_dir.dot(&axis_dir) < 0.0 { weight = -weight; }
weighted_dir += &axis_dir * weight;
}

{
let det_z = xx*yy - xy*xy;
let axis_dir = Vec3{
x: xy*yz - xz*yy,
y: xy*xz - yz*xx,
z: det_z,
};
let mut weight = det_z * det_z;
if weighted_dir.dot(&axis_dir) < 0.0 { weight = -weight; }
weighted_dir += &axis_dir * weight;
}

let normal = normalize(&weighted_dir);
if normal.is_finite() {
Some(plane_from_point_and_normal(&centroid, &normal))
} else {
None
}
}