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Compiling libpointmatcher with Python

This tutorial presents the different steps of compiling pypointmatcher, the libpointmatcher's Python module, on Ubuntu and Mac OS X.

Prerequisites

To get started, you will need the same prerequisites as libpointmatcher, but also some additional dependencies as listed here:

Name Version
(Tested August 2020 on Ubuntu 18.04)
pybind11 2.5.0
Python3 3.6.9
Dependencies
python3-dev 3.6.7
catch 1.10.0
pytest 5.4.3

pytest needs to be installed with pip:

pip3 install pytest

But catch and python3-dev need to be installed with a package manager:

Ubuntu users:

sudo apt install catch python3-dev

Mac OS users:

brew install catch2

The rest of this tutorial will guide you through the necessary steps to compile pypointmatcher.

pybind11

pybind11 is a library used to create Python bindings of existing C++ code and vice versa. So, in order to be able to compile pypointmatcher, you must either install pybind11 on your system or add it as a git submodule in the libpointmatcher's contrib/ directory. You must then create a symbolic link to this git submodule in the python/ directory. Go here for the installation steps or here for the git sudmodule steps.

The very first step is to clone pybind11 into a directory of your choice.

At the moment, pypointmatcher can only be compiled with version 2.5.0 of pybind11. To install the right version, run the following commands:

cd pybind11
git checkout v2.5.0

Once this is done, run the following commands:

mkdir build && cd build
cmake ..
make check -j 4

This will both compile and run pybind11 tests. Next, you can install pybind11 by running this command:

sudo make install

Once this is done, return to libpointmatcher's build/ directory.

You're now ready to proceed to the configuration step.

Adding pybind11 as a git submodule

An alternative to installing pybind11 on your system is to add its repository as a git submodule and create a symbolic link into the python/ directory. To do this, you will first need to clone the repository as a git submodule by running the following commands in your terminal from the contrib/ directory.

cd contrib
git submodule add https://github.com/pybind/pybind11.git

This will add pybind11 as a git submodule of libpointmatcher into the contrib/ directory. Then, still from the contrib/ directory, run this command to create a symbolic link to pybind11 in the python/ directory:

ln -sr pybind11 ../python/pybind11

At the moment, pypointmatcher can only be compiled with version 2.5.0 of pybind11. To install the right version, run the following commands:

cd pybind11
git checkout v2.5.0

Finally, tell CMake that you want to use pybind11 as a git submodule by setting the USE_SYSTEM_PYBIND11 variable to OFF:

cmake -D USE_SYSTEM_PYBIND11=OFF ..

IMPORTANT: When this method is used, it is very important to checkout the version 2.5.0 of pybind11 or it will be impossible to generate the build files.

Once this is done, return to libpointmatcher's build/ directory.

You're now ready to proceed to the configuration step.

Configuring the variables

Note: It is recommended to create a virtual environment before proceeding with the next steps. For this, you can use the virtualenv tool. If you are not familiar with Python virtual environments, you can read this tutorial, which explains very well the reasons for using a virtual environment, or watch this video tutorial

Specifying the path

First, you need to specify where you want the module to be installed. To do so, you must provide the path by setting the CMake variable PYTHON_INSTALL_TARGET with an absolute path to your Python environment site-packages location. This can be achieve manually or automatically.

The manual way:

Launch the Python interpreter and run the following commands to find the path to the site-packages/ directory of your current Python environment:

>>> import site
>>> site.getsitepackages()

Note: If you are using the system's Python environment, replace the getsitepackages() function call by getusersitepackages().

This will output a list of installation paths for your current Python environment. Now, choose the one that is located in the python_env_path/lib/python3.x/ directory. The command to run should look like this:

cmake -D PYTHON_INSTALL_TARGET=python_env_path/lib/python3.x/site-packages ..

NOTE: Replace the x with your Python minor version number.

The automatic way:

If you don't want to set the path manually, here's a command that should automatically pick the right one for you:

cmake -D PYTHON_INSTALL_TARGET=$(python3 -c "import site; print(site.getsitepackages()[0])") ..

Note: If you are using the system's Python environment, replace the site.getsitepackages()[0] by site.getusersitepackages().

IMPORTANT: This last example is the default behavior if no path has been set before compiling the module. Please, make sure that this corresponds to a valid location or the module will be installed in a wrong location and this will lead to an import error.

Enabling the compilation

By default, pypointmatcher compilation is disabled. In order to compile it, you must set the CMake variable BUILD_PYTHON_MODULE to ON:

cmake -D BUILD_PYTHON_MODULE=ON ..

Everything is now set up to proceed to the compilation and the installation.

Compilation

Now, to compile pypointmatcher into the build/ directory, run the following command:

make pypointmatcher -j N

where N is the number of jobs (or threads) you allow at once on your computer for the compilation. If no argument is passed after -j, there will be no limit to the number of jobs.

Note: Depending on your system, the compilation can take quite some time, so consider leaving the -j command with no argument in order to speed up this step.

Installation

And finally, to install the module on your system, run the following command:

sudo make install