Why move a surfel's center by 3 sigma?

[JzHuai0108]

Dear authors,

I appreciate your great work.

This question arises from the code line

center += 3 * sqrt(eva0) * direct;

To me, this line shifts the center along the minor axis by 3 sigma.
The following lines split the voxel into 8 cubes, and check their planarity based on the minimum eigenvalue.
All sounds reasonable except for the above line.
So why the shift?

[Zale-Liu]

It is our new plane determination method which is not used in this project. More details can be found in "https://arxiv.org/abs/2305.00287".