Finding resources

Setting up and running a simulation can involve loading various kinds of resources, such as robot models and plugins, from different locations, which can include a local filesystem and online servers. Ignition Gazebo offers a few different mechanisms for locating required resources.

Plugins

A plugin is a shared library that adheres to a specific API and is loaded at runtime. Typically, plugins are scoped to perform a narrow set of features. For example, the diff_drive plugin, provided by Ignition Gazebo, implements a differential drive controller for mobile robots.

Ignition relies on plugins for rendering, physics simulation, sensor data generation, and many of the capabilities. The following sections describe how Ignition finds and loads different types of plugins.

System Plugins

A system plugin is used by Ignition Gazebo, and provides an entry point for simulation customization and control. Refer to the Create System Plugins tutorial for information about creating your own system plugin.

System plugins may be loaded through:

• Tags in SDF files, where filename is the shared library and name is the class to be loaded. A system plugin can be attached to different entities.
  • Attached to the world: <world><plugin>
  • Attached to a model: <model><plugin>
  • Attached to a sensor: <sensor><plugin>
• Passing the shared library and class to be loaded through PluginInfo (within ServerConfig) when instantiating the Server.

Ignition will look for system plugins on the following paths, in order:

1. All paths on the IGN_GAZEBO_SYSTEM_PLUGIN_PATH environment variable
2. $HOME/.ignition/gazebo/plugins
3. Systems that are installed with Ignition Gazebo

Ignition GUI plugins

Each Ignition GUI plugin defines a widget.

GUI plugins may be loaded through:

• Tags in SDF world files, where filename is the shared library:
  • <world><gui><plugin>
• Tags in GUI config files, where filename is the shared library:
  • <plugin>
• The plugin menu on the top-right of the screen.

Ignition will look for GUI plugins on the following paths, in order:

1. All paths set on the IGN_GUI_PLUGIN_PATH environment variable
2. GUI plugins that are installed with Ignition Gazebo
3. Other paths added by calling gz::gui::App()->AddPluginPath
4. ~/.ignition/gui/plugins
5. Plugins which are installed with Gazebo GUI

Physics engines

Ignition Physics uses a plugin architecture and its physics engines are built as plugins that are loaded at run time using Ignition Plugin.

See the Physics engines tutorial for more details.

Rendering engines

Ignition Rendering uses a plugin architecture and its render engines are built as plugins that are loaded at run time using Ignition Plugin.

At the moment, Ignition Rendering will only look for render engine plugin shared libraries installed within its <install_prefix>/lib directory. Likewise, the resources used by these engines are located in Ignition Rendering's <install_prefix>/share directory.

Sensors

Each unique type of sensor in Ignition Sensors is a plugin. When a particular sensor type is requested, the relevant plugin is loaded by Ignition Plugin and a sensor object is instantiated from it.

At the moment, Ignition Sensors will only look for sensor plugin shared libraries installed within its <install_prefix>/lib directory.

Models, lights, actors

Top-level entities such as models, lights and actors may be loaded through:

• Tags in SDF world files:
  • <world><model>
  • <world><light>
  • <world><actor>
  • <include><uri> (path / URL)
• The /world/<world_name>/create service:
  • SDF file as string (<model> / <light> / <actor> root)
  • Path / URL to SDF file
  • (TODO) gz::msgs::Model, gz::msgs::Light
• Within a system, using SdfEntityCreator or directly creating components and entities.

Ignition will look for URIs (path / URL) in the following, in order:

1. All paths on the IGN_GAZEBO_RESOURCE_PATH* environment variable (if path is URI, scheme is stripped)
2. Current running path / absolute path
3. Ignition Fuel
   1. Cache (i.e. $HOME/.ignition/fuel)
   2. Web server

• The SDF_PATH environment variable also works in some scenarios, but it's not recommended when using Ignition Gazebo.

Meshes

Mesh files may be loaded through:

• Tags in SDF files:
  • <geometry><mesh><uri>
  • <actor><skin><filename>
  • <actor><animation><filename>

Ignition will look for URIs (path / URL) in the following, in order:

1. Current running path / absolute path
2. All paths on the IGN_GAZEBO_RESOURCE_PATH* environment variable (if path is URI, scheme is stripped)

• The IGN_FILE_PATH environment variable also works in some scenarios, but it's not recommended when using Ignition Gazebo.

GUI configuration

Ignition Gazebo's GUI configuration can come from the following, in order:

1. The command line option --gui-config <file path>
2. Plugins within SDF's <world><gui>
3. $HOME/.ignition/gazebo/gui.config (if that file doesn't exist, the default gui.config file that is installed with Ignition Gazebo will be copied to that location)