Gazebo Physics

API Reference

2.6.2
Understandng the Physics Plugin

This is an introduction to different physics engines and how they are integrated into the Ignition Physics library.

Ignition physics

The Ignition Physics library integrates external physics engines into the Ignition Simulation eco-system. It allows users to select from multiple supported physics engines based on their simulation needs. Its plugin interface loads physics engines with requested features at runtime. It is also possible to integrate your own selected physics engine by writing a compatible plugin interface.

To get a more in-depth understanding of how the physics plugin works in Ignition, we will start with some high level concepts and definitions.

High level concept

Conceptually, the physics plugin can be viewed from two sides of its interface: user vs. implementation.

Each physics engine provides access to different features implemented by the Ignition Physics engine. The interface is made possible through the Ignition Plugin library, which instantiates Features in FeatureLists and supplies pointers to the selected engine. This "user side interface" makes the Ignition Physics library "callable" from other Ignition libraries.

The implementation side interface handles specific implementations of each Feature. Depending on what external physics engine we are using (DART, TPE etc.), the interface might be different. This interface is more internal facing, i.e. used mostly inside the Ignition Physics library.

The implementation of the physics plugin revolves around four key elements.

  1. Entity

    This is the base class of all "proxy objects". The "proxy objects" are essentially interfaces into the actual objects which exist inside of the various physics engine implementations. The proxy objects contain the minimal amount of data (e.g. a unique identifier, a reference-counter for the implementation object, and a reference to the implementation interface that it needs) necessary to interface with the object inside of the implementation that it refers to.

  2. FeaturePolicy

    FeaturePolicy is a "policy class" used to provide metadata to features about what kind of simulation engine they are going to be used in. Many physics simulations software libraries model 3-dimensional systems, though some (like Box2d) only consider 2-dimensional systems. A FeaturePolicy is used to customize Ignition Physics' APIs by the number of dimensions (2 or 3) and also the floating point scalar type (float or double). Dartsim and TPE reference implementations both use FeaturePolicy3d (3 dimensions, double).

  3. Feature

    This class defines the concept of a Feature, examples like GetWorldFromEngine, GetEngineInfo etc. There is a pre-defined list of features in Ignition Physics. They are implemented by using external physics engines' APIs to fulfill simulation needs requested by Ignition.

  4. FeatureList

    This is the class that aggregates a list of features. FeatureLists can be constructed in hierarchies, e.g. a FeatureList can be passed into another FeatureList, and the set of all features in the new list will be the sum.

FeatureList definitions

This list of FeatureLists is specific to the implementation of Dartsim and TPE-plugin. Users do not need to organize their own plugin implementations this way.

Name Definition
Base contains data structures and functions that define and use "proxy objects"
CustomFeatures retrieves World entity from physics engine
EntityManagementFeatures provides features to get, remove and construct entities
FreeGroupFeatures finds free group entities and sets world pose, linear and angular velocities
JointFeatures defines types of joints used and sets joint properties
KinematicsFeatures computes frame relative to world
LinkFeatures applies external force and torque to link
SDFFeatures constructs entities from SDF file
ShapeFeatures retrieves Shape related properties like BoundingBox, ShapeSize etc.
SimulationFeatures updates World and everything within by defined stepsize

Dart vs. TPE

Dart (Dynamic Animation and Robotics Toolkit) is an open source library that provides data structures and algorithms for kinematic and dynamic applications in robotics and computer animation. It is the default physics engine used in Ignition Simulation. The source code for Dartsim plugin can be found in Ignition Physics repository under dartsim directory.

TPE (Trivial Physics Engine is an open source library created by Open Robotics that enables fast, inexpensive kinematics simulation for entities at large scale. It supports higher-order fleet dynamics without real physics (eg. gravity, force, constraint etc.) and multi-machine synchronization. Ignition support for TPE targets Citadel and onward releases. The source code for TPE plugin can be found in Ignition Physics repository under tpe/plugin directory

The following is a list of features supported by each physics engine to help users select one that fits their needs.

Entity comparison

The following is a table of Entity names used in Ignition Physics plugin interface, Dart and TPE. Entities are arranged in top-down hierarchical order.

Physics Plugin Dart TPE
Engine Engine Engine
World World World
Frame Frame N/A
Model Skeleton Model
Joint Joint N/A
Link BodyNode Link
Shape Shape Collision
Box/Sphere/Cylinder etc. Box/Sphere/Cylinder etc. Box/Sphere/Cylinder/Mesh etc.

Feature comparison

The following is a table of implemented Features of Dartsim and TPE-Plugin.

Features Dartsim TPE-Plugin
GetEntities ✓ (no joint in TPE)
RemoveEntities
ConstructEmptyWorldFeature
ConstructEmptyModelFeature
ConstructEmptyLinkFeature
CollisionFilterMaskFeature
FindFreeGroupFeature
SetFreeGroupWorldPose
SetFreeGroupWorldVelocity
GetBasicJointState
SetBasicJointState
GetBasicJointProperties
SetJointTransformFromParentFeature
SetJointTransformToChildFeature
DetachJointFeature
SetFreeJointRelativeTransformFeature
AttachFixedJointFeature
SetRevoluteJointProperties
GetRevoluteJointProperties
AttachRevoluteJointFeature
SetPrismaticJointProperties
GetPrismaticJointProperties
AttachPrismaticJointFeature
SetJointVelocityCommandFeature
LinkFrameSemantics
ShapeFrameSemantics
FreeGroupFrameSemantics
AddLinkExternalForceTorque
sdf::ConstructSdfWorld
sdf::ConstructSdfModel
sdf::ConstructSdfLink
sdf::ConstructSdfJoint
sdf::ConstructSdfCollision
sdf::ConstructSdfVisual
GetShapeKinematicProperties
SetShapeKinematicProperties
GetShapeBoundingBox
GetBoxShapeProperties
AttachBoxShapeFeature
GetCylinderShapeProperties
AttachCylinderShapeFeature
GetSphereShapeProperties
AttachSphereShapeFeature
mesh::GetMeshShapeProperties
mesh::AttachMeshShapeFeature
ForwardStep
GetContactsFromLastStepFeature