Eigen

Eigen is a C++ template library for linear algebra.

install from source

According to How to install eigen 3.3 in Ubuntu 14.04?, Eigen is a header-only library, so we don't have to "install" it.

First download eigen-3.3.4.tar.gz(or other newer versions) from its gitlab repo, and then untar it.

Symlink the Eigen folder into /usr/local/include:

ln -s /xxx/eigen-3.3.4/Eigen /usr/local/include/Eigen

Also:

ln -s /xxx/eigen-3.3.4/unsupported/ /usr/local/include/unsupported

otherwise when we make pcl, it will shows fatal error: unsupported/Eigen/NonLinearOptimization: No such file or directory.

Note that the arguments of ln -s must be full links, otherwise the target will be invalid.

install through apt

apt install libeigen3-dev #get 3.3.7-2 in Ubuntu 20.04

use it in another project

cmake_minimum_required(VERSION 3.0)
project(MY_GRAND_PROJECT)

include_directories("/usr/include/eigen3")

add_executable(...)
target_link_libraries(...) 

check version

How to check the version number of Eigen C++ template library?

Open $(eigen_installation_dir)/eigen3/Eigen/src/Core/util/Macros.h and find EIGEN_WORLD_VERSION, EIGEN_MAJOR_VERSION, EIGEN_MINOR_VERSION like:

#define EIGEN_WORLD_VERSION 3
#define EIGEN_MAJOR_VERSION 3
#define EIGEN_MINOR_VERSION 7

Note: PCL 1.11.1 uses Eigen 3.3.7, PCL 1.12.1 uses Eigen 3.4.0