NDT概念
正态分布变换（Normal Distribution Transformation , NDT）
概率密度函数（ Probability Density Function, PDF）
First proposed for two dimensional scan data registration by Biber & Strasser in 2003.
An NDT is described as a set of PDFs.
The first step of the algorithm is to subdivide the space occupied by the scan into a grid of cells (squares in the 2D case, or cubes in 3D).
A PDF is computed for each cell, based on the point distribution within the cell.
NDT及其改进
将二维空间划分为固定大小网格，每个网格至少包括3个点（一般5个）
计算网格中点集的均值