The support library ``nottf2`` ------------------------------ .. include:: ../the_topic_is_under_heavy_construction.rst .. danger:: You do not need this support library if you're just creating basic rotations. You can create easy rotations directly using ``math.cos``, ``math.sin``, and the class ``Quaternion`` part of ``geometry_msgs``. The support library becomes useful if you need to, for instance, multiply quaternions to obtain relative rotations with ``quaternion_multiply``. In ``rclpy``, ``tf2`` does not (currently?) have quaternion operations. But I got you covered. Kinda. It's more use at your own risk type of arrangement. For the library to be properly found in our environment, it needs to be installed in the system's Python. We can do so as follows. .. code-block:: console python3 -m pip install nottf2 --break-system-packages .. caution:: The ``nottf2`` library is provided as-is, with no warranty. Use at your own risk (See ``MIT`` license for more details). The creator of ``nottf2``, me, will not accept any blame or liability for any issues, including but limited to broken robots or broken code in your assignments. If you feel it's insufficient or unreliable, please create and use your own implementation, or someone else's that fits your needs. Create :program:`ros2` package that uses ``nottf2`` --------------------------------------------------- Let's first create our sample package, as follows, that will depend on ``geometry_msgs``. .. code-block:: console cd ~/ros2_tutorial_workspace/src ros2 pkg create python_package_that_uses_nottf2 \ --build-type ament_python \ --dependencies geometry_msgs We will be presented with the usual output. .. dropdown:: ros2 pkg create output .. code-block:: console going to create a new package package name: python_package_that_uses_nottf2 destination directory: ~/ros2_tutorial_workspace/src package format: 3 version: 0.0.0 description: TODO: Package description maintainer: ['root '] licenses: ['TODO: License declaration'] build type: ament_python dependencies: [] creating folder ./python_package_that_uses_nottf2 creating ./python_package_that_uses_nottf2/package.xml creating source folder creating folder ./python_package_that_uses_nottf2/python_package_that_uses_nottf2 creating ./python_package_that_uses_nottf2/setup.py creating ./python_package_that_uses_nottf2/setup.cfg creating folder ./python_package_that_uses_nottf2/resource creating ./python_package_that_uses_nottf2/resource/python_package_that_uses_nottf2 creating ./python_package_that_uses_nottf2/python_package_that_uses_nottf2/__init__.py creating folder ./python_package_that_uses_nottf2/test creating ./python_package_that_uses_nottf2/test/test_copyright.py creating ./python_package_that_uses_nottf2/test/test_flake8.py creating ./python_package_that_uses_nottf2/test/test_pep257.py [WARNING]: Unknown license 'TODO: License declaration'. This has been set in the package.xml, but no LICENSE file has been created. It is recommended to use one of the ament license identifiers: Apache-2.0 BSL-1.0 BSD-2.0 BSD-2-Clause BSD-3-Clause GPL-3.0-only LGPL-3.0-only MIT MIT-0 Package structure ----------------- Highlighted below are the files that we will create or modify. .. code-block:: console :emphasize-lines: 5, 9 python_package_that_uses_nottf2/ |-- package.xml |-- python_package_that_uses_nottf2 | |-- __init__.py | `-- operations_showcase.py |-- resource | `-- python_package_that_uses_nottf2 |-- setup.cfg |-- setup.py `-- test |-- test_copyright.py |-- test_flake8.py `-- test_pep257.py Add sample code for ``nottf2`` ------------------------------ Create the following sample Python script. It will serve to show the operations we did earlier mathematically. :download:`operations_showcase.py <../../../ros2_tutorial_workspace/src/python_package_that_uses_nottf2/python_package_that_uses_nottf2/operations_showcase.py>` .. literalinclude:: ../../../ros2_tutorial_workspace/src/python_package_that_uses_nottf2/python_package_that_uses_nottf2/operations_showcase.py :language: python :lines: 24- :linenos: Update the :file:`setup.py` --------------------------- As usual, we add the necessary entry point in :file:`setup.py`. We also add ``nottf2`` as an usual Python dependency, although, in :program:`colcon`, this currently seems to be mostly cosmetic. :download:`setup.py <../../../ros2_tutorial_workspace/src/python_package_that_uses_nottf2/setup.py>` .. literalinclude:: ../../../ros2_tutorial_workspace/src/python_package_that_uses_nottf2/setup.py :language: python :emphasize-lines: 15,24 Build and source ---------------- Before we proceed, let us build and source once. .. include:: ../the_canonical_build_command.rst Run Example ----------- We run our newly created program as follows. .. code-block:: console ros2 run python_package_that_uses_nottf2 operations_showcase The result will be as follows. .. code-block:: console The rotation of 3.141592653589793 radians about the x-axis is r1=geometry_msgs.msg.Quaternion(x=1.0, y=0, z=0, w=6.123233995736766e-17). The inverse rotation of r1 is r1_conj=geometry_msgs.msg.Quaternion(x=-1.0, y=0, z=0, w=6.123233995736766e-17). The rotation of 3.141592653589793 radians about the z-axis is r2=geometry_msgs.msg.Quaternion(x=0, y=0, z=1.0, w=6.123233995736766e-17). The quaternion multiplication of r12=r1r2 is r12=geometry_msgs.msg.Quaternion(x=6.123233995736766e-17, y=-1.0, z=6.123233995736766e-17, w=3.749399456654644e-33). Note that the results are reasonably close to the ones we calculated mathematically. However, given the limitations on current computers related to floating point accuracy, you can **always** expect a level of inaccuracy. This is **not** limited or affected by the use of quaternions, this is an inherent limitation of our computers.