Gazebo Gazebo

API Reference

6.16.0

Migration from Gazebo Classic: Plugins

Gazebo Classic supports 6 different C++ plugin types, each providing access to different parts of the API, like physics, rendering, sensors, GUI, etc. Due to Ignition Gazebo's architecture based on an ECS , plugin interfaces are somewhat different, but more varied and in many cases much more powerful. Some plugins in Ignition are systems within Ignition Gazebo, while others are specific plugin types from other Ignition libraries.

Note
Plugin types other than systems may be added to Ignition Gazebo in the future.

For example, plugins which get and set properties of simulation entities would be Ignition Gazebo systems. On the other hand, there are now plugin interfaces which didn't exist in Gazebo Classic, such as integrating a new physics or rendering engine, and these can be also used in projects outside of Ignition Gazebo.

Take a look at the comparison below:


Classic plugin Features Ignition equivalent Differences
World Get/set properties of the world and its children Gazebo system Get/set components.
Model Get/set properties of the model and its children Gazebo system Get/set components.
Visual Get/set properties of the visual and its children Gazebo system Get/set components.
Sensor Get/set sensor properties and readings Gazebo system Get/set components.
World / Model Access physics-engine-specific features Physics plugin Loaded by ign-physics
Visual Access rendering-engine-specific features Rendering plugin Loaded by ign-rendering
Sensor Connect to callbacks Standalone program Subscribe to Ignition Transport messages.
All Connect to simulation events Gazebo system Use PreUpdate, Update and PostUpdate callbacks for the update loop, and the event manager for other events.
GUI Add an overlay UI GUI plugin Support for overlays and docked widgets
GUI / System Change the default UI GUI plugin / SDF All GUI elements can be removed or configured through SDF
System Access command line arguments TBD -

Another key difference is that systems will be able to access all entity properties at once, despite the entity type. So while in Gazebo Classic you may need 3 different plugins to interact with physics, rendering and sensors, on Ignition you could, for example, set a camera's visual color, its velocity and frame rate, all from a single plugin.

Plugin interfaces

Let's take a look at a typical Gazebo Classic plugin which accesses a property from an entity and does something with it. In this case, it will print the current pose of a link.

In general, the plugin will need to:

  1. Get the link and store it.
  2. Register a callback that is called at every simulation update.
  3. At the callback, query the link's pose and print it.

On classic Gazebo, that would look something like this:

#include <gazebo/common/Plugin.hh>
// Inherit from ModelPlugin
class MyPlugin : public ModelPlugin
{
// Implement Load callback, provided by ModelPlugin
// and called once at startup.
virtual void MyPlugin::Load(physics::ModelPtr _model, sdf::ElementPtr _sdf)
{
// Read property from SDF
auto linkName = _sdf->Get<std::string>("link_name");
// Store link pointer
this->link = _model->GetLink(linkName);
// Register callback to be called at every iteration
this->updateConnection = event::Events::ConnectWorldUpdateBegin(
std::bind(&MyPlugin::OnUpdate, this));
}
// Custom callback called at every iteration
void MyPlugin::OnUpdate()
{
// Get link pose and print it
std::cout << this->link->WorldPose() << std::endl;
}
// Must keep the connection
private: event::ConnectionPtr updateConnection;
// Keep pointer to link
private: physics::LinkPtr link;
};
// Register plugin
GZ_REGISTER_MODEL_PLUGIN(MyPlugin)

On Ignition Gazebo, that would be implemented as follows:

#include <gz/sim/Model.hh>
#include <gz/sim/Util.hh>
#include <gz/sim/System.hh>
#include <gz/plugin/Register.hh>
using namespace gz;
using namespace sim;
using namespace systems;
// Inherit from System and 2 extra interfaces:
// ISystemConfigure and ISystemPostUpdate
class MyPlugin
: public System,
public ISystemConfigure,
public ISystemPostUpdate
{
// Implement Configure callback, provided by ISystemConfigure
// and called once at startup.
virtual void Configure(const Entity &_entity,
EntityComponentManager &_ecm,
EventManager &/*_eventMgr*/) override
{
// Read property from SDF
auto linkName = _sdf->Get<std::string>("link_name");
// Create model object to access convenient functions
auto model = Model(_entity);
// Get link entity
this->linkEntity = model.LinkByName(_ecm, linkName);
}
// Implement PostUpdate callback, provided by ISystemPostUpdate
// and called at every iteration, after physics is done
virtual void PostUpdate(const UpdateInfo &/*_info*/,
const EntityComponentManager &_ecm) override
{
// Get link pose and print it
std::cout << worldPose(this->linkEntity, _ecm) << std::endl;
}
// ID of link entity
private: Entity linkEntity;
};
// Register plugin
MyPlugin::ISystemConfigure,
MyPlugin::ISystemPostUpdate)
// Add plugin alias so that we can refer to the plugin without the version
// namespace
IGNITION_ADD_PLUGIN_ALIAS(MyPlugin, "gz::sim::systems::MyPlugin")

The example above uses headers like Model.hh and Util.hh, which offer convenient APIs for some tasks that are common during simulation. However, the real power of the ECS architecture is the direct access to all entities and components. Let's take a look at how to do the same just using the ECM's API:

#include <gz/sim/System.hh>
#include <gz/plugin/Register.hh>
using namespace gz;
using namespace sim;
using namespace systems;
// Inherit from System and 2 extra interfaces:
// ISystemConfigure and ISystemPostUpdate
class MyPlugin
: public System,
public ISystemConfigure,
public ISystemPostUpdate
{
virtual void Configure(const Entity &_entity,
EntityComponentManager &_ecm,
EventManager &/*_eventMgr*/) override
{
// Read property from SDF
auto linkName = _sdf->Get<std::string>("link_name");
// Get link entity by querying for an entity that has a specific set of
// components
this->linkEntity = _ecm.EntityByComponents(
}
virtual void PostUpdate(const UpdateInfo &/*_info*/,
const EntityComponentManager &_ecm) override
{
// Get link's local pose
auto pose = _ecm.Component<components::Pose>(this->linkEntity)->Data();
// Get link's parent entity
auto parent = _ecm.Component<components::ParentEntity>(this->linkEntity);
// Iterate over parents until world is reached
while (parent)
{
// Get parent entity's pose
auto parentPose = _ecm.Component<components::Pose>(parent->Data());
if (!parentPose)
break;
// Add pose
pose = parentPose->Data() * pose;
// keep going up the tree
parent = _ecm.Component<components::ParentEntity>(parent->Data());
}
std::cout << pose << std::endl;
}
// ID of link entity
private: Entity linkEntity;
};
MyPlugin::ISystemConfigure,
MyPlugin::ISystemPostUpdate)
IGNITION_ADD_PLUGIN_ALIAS(MyPlugin, "gz::sim::systems::MyPlugin")

In summary, the key differences between Gazebo Classic and Ignition Gazebo are:

  • Plugins must inherit from the ISystemConfigure class to be able to override the Configure callback that gives access to many things, such as the SDF pointer. This used to be done through the Load callback.
  • Plugins no longer need to manage connections to loop-related events. Instead, they implement an interface such as ISystemPostUpdate.
  • Plugins don't have direct access to physics objects such as physics::Model. Instead, they can either deal directly with entities and their components by calling functions in the ECM, or using convenient objects such as gz::sim::Model which wrap the ECM interface.

All these changes are meant to give plugin developers more flexibility to only use the features they need, and several layers of abstraction which can be chosen according to the developer's experience and specific use-case.

#define IGNITION_ADD_PLUGIN(PluginClass,...)
uint64_t Entity
An Entity identifies a single object in simulation such as a model, link, or light....
Definition: gz/sim/Entity.hh:58
T bind(T... args)
STL class.
STL class.
Component< Entity, class ParentEntityTag > ParentEntity
This component holds an entity's parent entity.
Definition: gz/sim/components/ParentEntity.hh:41
Component< gz::math::Pose3d, class PoseTag > Pose
A component type that contains pose, gz::math::Pose3d, information.
Definition: gz/sim/components/Pose.hh:35
#define IGNITION_ADD_PLUGIN_ALIAS(PluginClass,...)
math::Pose3d worldPose(const Entity &_entity, const EntityComponentManager &_ecm)
Helper function to compute world pose of an entity.
Component< NoData, class ModelTag > Model
A component that identifies an entity as being a model.
Definition: gz/sim/components/Model.hh:111
Component< NoData, class LinkTag > Link
A component that identifies an entity as being a link.
Definition: gz/sim/components/Link.hh:33
T endl(T... args)
Base class for a System.
Definition: gz/sim/System.hh:75
Component< std::string, class NameTag, serializers::StringSerializer > Name
This component holds an entity's name. The component has no concept of scoped names nor does it care ...
Definition: gz/sim/components/Name.hh:37