tf2 Concept: About tf2 page (#1951)

* init tf2 concept page

(cherry picked from commit 41149f2)

source/Concepts.rst

@@ -21,6 +21,7 @@ Conceptual overviews provide relatively high-level, general background informati
    Concepts/About-Composition
    Concepts/About-Catment
    Concepts/About-Cross-Compilation
+   Concepts/About-Tf2
 
 
 The Core Stack Developer Concepts are much more detailed conceptual articles intended for developers who plan modify or contribute to the ROS 2 core:

source/Concepts/About-Tf2.rst

@@ -0,0 +1,64 @@
+.. _AboutTf2:
+
+About tf2
+=========
+
+.. contents:: Table of Contents
+   :depth: 2
+   :local:
+
+Overview
+--------
+
+tf2 is the transform library, which lets the user keep track of multiple coordinate frames over time.
+tf2 maintains the relationship between coordinate frames in a tree structure buffered in time and lets the user transform points, vectors, etc. between any two coordinate frames at any desired point in time.
+
+.. image:: images/ros2_tf2_frames.png
+
+Properties of tf2
+-----------------
+
+A robotic system typically has many 3D coordinate frames that change over time, such as a world frame, base frame, gripper frame, head frame, etc.
+tf2 keeps track of all these frames over time, and allows you to ask questions like:
+
+* Where was the head frame relative to the world frame 5 seconds ago?
+* What is the pose of the object in my gripper relative to my base?
+* What is the current pose of the base frame in the map frame?
+
+tf2 can operate in a distributed system.
+This means all the information about the coordinate frames of a robot is available to all ROS 2 components on any computer in the system.
+tf2 can have every component in your distributed system build its own transform information database or have a central node that gathers and stores all transform information.
+
+Tutorials
+---------
+
+We created a set of :ref:`tutorials <Tf2Main>` that walks you through using tf2, step by step.
+You can get started on the :ref:`introduction to tf2 <IntroToTf2>` tutorial.
+For a complete list of all tf2 and tf2-related tutorials check out the :ref:`tutorials <Tf2Main>` page.
+
+There are essentially two main tasks that any user would use tf2 for, listening for transforms and broadcasting transforms.
+
+If you want to use tf2 to transform between coordinate frames, your nodes will need to listen for transforms.
+What you will do is to receive and buffer all coordinate frames that are broadcasted in the system, and query for specific transforms between frames.
+Check out the writing a tf2 listener tutorial :ref:`(Python) <WritingATf2ListenerPy>` :ref:`(C++) <WritingATf2ListenerCpp>` to learn more.
+
+To extend the capabilities of a robot you will need to start broadcasting transforms.
+Broadcasting transforms means to send out the relative pose of coordinate frames to the rest of the system.
+A system can have many broadcasters that each provide information about a different part of the robot.
+Check out the writing a tf2 broadcaster tutorial :ref:`(Python) <WritingATf2BroadcasterPy>` :ref:`(C++) <WritingATf2BroadcasterCpp>` to learn more.
+
+In addition to that, tf2 can broadcast static transforms that do not change over time.
+This mainly saves storage and lookup time, though the publishing overhead is also improved.
+You should note that static transforms are published once and assumed to be not changing and therefore no time history stored.
+If you want to define static transforms in your tf2 tree, take a look at the writing static tf2 broadcaster :ref:`(Python) <WritingATf2StaticBroadcasterPy>` :ref:`(C++) <WritingATf2StaticBroadcasterCpp>` tutorial.
+
+You can also learn how to add fixed and dynamic frames to your tf2 tree in adding a frame :ref:`(Python) <AddingAFramePy>` :ref:`(C++) <AddingAFrameCpp>` tutorial.
+
+Once you are finished with the basic tutorials, you can move on to learn about tf2 and time.
+The tf2 and time tutorial :ref:`(Python) <LearningAboutTf2AndTimePy>` :ref:`(C++) <LearningAboutTf2AndTimeCpp>` teaches the basic principles of tf2 and time.
+The advanced tutorial about tf2 and time :ref:`(Python) <TimeTravelWithTf2Py>` :ref:`(C++) <TimeTravelWithTf2Cpp>` teaches the principles of time traveling with tf2.
+
+Paper
+-----
+
+There is a paper on tf2 presented at TePRA 2013: `tf: The transform library <https://ieeexplore.ieee.org/abstract/document/6556373>`_.