Tutorial introduction to Ignition Physics (#104)

* Initial introduction draft
* Change Ignition Robotics -> Ignition
* Fix diagram
* Reduce lines to 80

Co-authored-by: anindex <[email protected]>
Co-authored-by: Claire Wang <[email protected]>
Co-authored-by: Louise Poubel <[email protected]>

tutorials/01_intro.md

@@ -2,18 +2,85 @@
 
 Next Tutorial: \ref installation
 
-Ignition Physics is a component in Ignition, a set of libraries
-designed to rapidly develop robot and simulation applications. The main
-goal of the library is to provide an abstraction layer to various physics
-engines, which gives end users the ability to use multiple physics engines
-with minimal change to their application code. Ignition Physics uses
-a plugin architecture where each physics engine is implemented as a plugin
-that can be loaded at runtime. To enable users the ability to choose
-between physics engines, Ignition Physics introduces [\"Features\"](@ref ignition::physics::Feature), a design concept used to encode the
-capabilities of a physics engine. Users can request for physics engines
-that support a set of features and the plugin loading mechanism loads only
-the engines that implement the requested features.
-
-The following is a list of currently supported physics engines:
-
-  * [DART](http://dartsim.github.io/)
+## Motivation
+
+Ignition Physics (`ign-physics`) is a library component in Ignition, a set of
+libraries designed to rapidly develop robot and simulation applications.
+The main goal of the library is to provide an abstraction layer to various
+physics engines, which gives end-users the ability to use multiple
+physics engines with minimal change to their application code.
+Users can request for physics engines that support a set of features and the plugin
+loading mechanism loads only the engines that implement the requested features.
+Besides, a user-selected physics engine can be integrated into the existing
+code-base by writing a compatible plugin interface, which enables
+Ignition Physics extensibility and modularity.  
+
+## Overview
+
+For a big picture of the Ignition Physics operation in Ignition ecosystem, see
+the abstract diagram below:
+
+<img src="https://user-images.githubusercontent.com/18066876/94801505-6bbf6980-03e6-11eb-97e5-e5f0dc68229f.png"/>
+
+In general, `ign-gazebo` is the main simulation library, in which its
+functionalities are powered by many component libraries.
+For example, its graphical drawing is supported by `ign-rendering` or simulated
+sensors that are defined and implemented in `ign-sensors`.
+In particular, this library `ign-physics` provides an abstract interface to
+physics engines, which simulates dynamic transformations and interactions of
+objects in `ign-gazebo`.
+The communication between these libraries at runtime is provided by
+`ign-transport` and `ign-msgs`.
+
+Ignition Physics uses a plugin architecture where each physics engine is
+implemented as a plugin that can be loaded at runtime.
+To enable users the ability to choose between physics engines, Ignition Physics
+introduces four keys elements:
+
+- \ref ignition::physics::Entity "Entity": the base class, which abstracts all
+proxy objects, which contains minimal descriptive data
+(e.g. a unique identifier, a reference-counter, etc.) pointing the corresponding
+implemented objects.
+- \ref ignition::physics::FeaturePolicy "FeaturePolicy": the policy class,
+which provides metadata to features about their simulation engine specifications.
+FeaturePolicy supports customizing Ignition Physics' APIs by the number of
+dimensions (2D or 3D) and also the floating-point scalar type (float or double).
+- \ref ignition::physics::Feature "Feature": defines the concept used to encode
+the capabilities of a physics engine.
+- \ref ignition::physics::FeatureList "FeatureList": aggregates a list of features.
+
+Depending on which external physics engine is used (e.g. DART, TPE, Bullet, etc.),
+the internal interface to the physics engine might be different.
+However, the API provided by Ignition Physics will be unchanged between
+different physics engines.
+
+## Supported physics engines
+
+For a list of supported physics engines and their descriptions, please refer
+to \ref physicsplugin
+
+## Development timeline
+
+### Features logs
+
+**Ignition Physics 1.x**
+- Initial release
+- Define base concepts: Entity, FeaturePolicy, Feature and FeatureList.
+- Add features for `dartsim` physics engines (more detail in \ref physicsplugin).
+- Add RequestFeatures API for casting the features of an entity to a new feature set when possible.
+- Enforce joint effort limit in `dartsim-plugin`.
+
+**Ignition Physics 2.x**
+- Support sdformat 1.7 frame semantics.
+- Support compiling against dart 6.9.
+- Trivial Physics Engine (TPE)- partial implementation
+- Add features for TPE physics engines (more detail in \ref physicsplugin).
+- Extend contact data with force, normal, and penetration depth.
+- Add Base and EntityManagement to `tpeplugin`
+
+**Ignition Physics 3.x**
+
+### Future roadmap
+
+Ignition Physics evolves closely with the Ignition ecosystem.
+For a broader overview, please visit [Ignition's roadmap](https://ignitionrobotics.org/about).