DEScription temPLATEs

This repository contains various examples for how to template description files in ROS 2. Each example demonstrates the use of a templating engine, with a description format, at compile time or launch time. All examples have the same final result, which makes it easy to compare them side-by-side.

Templating engines:

• 🐍 EmPy
• 💎 ERB
• 📟 Xacro

Description formats:

• 🌱 SDF
• 🌐 URDF

Is your engine or format of choice not represented? Consider opening a pull request to add it!

Quick start

1. Install ROS Humble or higher

2. Clone this repository:

   mkdir -p ~/ws/src
   cd ~/ws/src
   git clone https://github.com/chapulina/desplate
   

3. Install dependencies:

   cd ~/ws
   rosdep install --from-paths src --ignore-src -r -y
   

4. Build and install:

   cd ~/ws
   colcon build
   

5. Run one of the examples, i.e.:

   ros2 launch desplate_empy vehicle_sdf_generate.launch.py
   

6. You should see a differential drive robot (like dolly) on RViz:

   

7. Also try checking the inertia visualization:

   

Try out all examples

Templating engine	Format	Stage	Command
EmPy	SDF	launch	ros2 launch desplate_empy vehicle_sdf_generate.launch.py
EmPy	URDF	launch	ros2 launch desplate_empy vehicle_urdf_generate.launch.py
EmPy	SDF	compilation	ros2 launch desplate_empy vehicle_sdf_installed.launch.py
EmPy	URDF	compilation	ros2 launch desplate_empy vehicle_urdf_installed.launch.py
ERB	SDF	launch	ros2 launch desplate_erb vehicle_sdf_generate.launch.py
ERB	URDF	launch	ros2 launch desplate_erb vehicle_urdf_generate.launch.py
ERB	SDF	compilation	ros2 launch desplate_erb vehicle_sdf_installed.launch.py
ERB	URDF	compilation	ros2 launch desplate_erb vehicle_urdf_installed.launch.py
Xacro	SDF	launch	ros2 launch desplate_xacro vehicle_sdf_generate.launch.py
Xacro	URDF	launch	ros2 launch desplate_xacro vehicle_urdf_generate.launch.py
Xacro	SDF	compilation	ros2 launch desplate_xacro vehicle_sdf_installed.launch.py
Xacro	URDF	compilation	ros2 launch desplate_xacro vehicle_urdf_installed.launch.py

How it works

Launch time

Generating description files at launch time is convenient if you're often iterating on parameters for those files. On the other hand, if your files aren't changing much, consider generating them at compile time to save some time every time you launch.

Generate templates at launch file following these steps:

1. Install all template files, i.e.:

   desplate/desplate_empy/CMakeLists.txt

   Lines 39 to 46 in 86b7960

   	# Templates to be generated at launch time
   	install(
   	DIRECTORY
   	sdf
   	urdf
   	DESTINATION
   	share/${PROJECT_NAME}/
   	)

2. In a launch file, get the path to the installed template file, for example:

   desplate/desplate_empy/launch/vehicle_sdf_generate.launch.py

   Lines 30 to 31 in 86b7960

   	pkg = os.path.join(get_package_share_directory('desplate_empy'))
   	template_path = os.path.join(pkg, 'sdf', 'vehicle.sdf.em')

3. Use the templating engine to generate a file and store it in a string variable. See each engine below.

4. Pass that description string to another node. For robots, that's usually robot_state_publisher, i.e.:

   desplate/desplate_common/launch/visualize.launch.py

   Lines 40 to 45 in 86b7960

   	robot_state_publisher = Node(
   	package='robot_state_publisher',
   	executable='robot_state_publisher',
   	output='screen',
   	parameters=[{'robot_description': LaunchConfiguration('description_str')}]
   	)

EmPy

Add these lines to a launch file to generate a description from an EmPy template:

desplate/desplate_empy/launch/vehicle_sdf_generate.launch.py

Lines 33 to 36 in 86b7960

	# These 3 lines generate a description file from an EmPy template
	with open(template_path) as template_file:
	template = template_file.read()
	description_str = em.expand(template, {})

ERB

Add this line to a launch file to generate a description from an ERB template:

desplate/desplate_erb/launch/vehicle_sdf_generate.launch.py

Lines 32 to 33 in 86b7960

	# This line generates a description file from an ERB template
	description_str = subprocess.run(['erb', template_path], capture_output=True).stdout.decode()

Xacro

Add this line to a launch file to generate a description from a Xacro template:

desplate/desplate_xacro/launch/vehicle_sdf_generate.launch.py

Lines 33 to 34 in 86b7960

	# This line generates a description file from a Xacro template
	description_str = xacro.process_file(template_path).toxml()

Compile time

Generating description files at compile time is efficient if you're not iterating on parameters for those files, because you're not generating them over and over every time the application is launched. On the other hand, if your files often take different arguments for each run, consider generating them at launch time.

Generate templates at compile file following these steps:

1. Invoke the templating engine with CMake to generate the description into the build directory. One way to do it is creating a custom command and making a target that depends on it. The difference for each engine will be what command is invoked, see below for each engine.

   desplate/desplate_empy/CMakeLists.txt

   Lines 11 to 20 in 86b7960

   	file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/${FORMAT})
   	add_custom_command(
   	OUTPUT generate_template_${FORMAT}
   	COMMAND empy3
   	${CMAKE_CURRENT_SOURCE_DIR}/${FORMAT}/vehicle.${FORMAT}.em >
   	${CMAKE_CURRENT_BINARY_DIR}/${FORMAT}/vehicle.${FORMAT}
   	)
   	add_custom_target(generate_template_target_${FORMAT} ALL
   	DEPENDS generate_template_${FORMAT}
   	)

2. Install the generated description, i.e.:

   desplate/desplate_empy/CMakeLists.txt

   Lines 47 to 54 in 86b7960

   	# Descriptions generated at compile time
   	install(
   	DIRECTORY
   	${CMAKE_CURRENT_BINARY_DIR}/sdf
   	${CMAKE_CURRENT_BINARY_DIR}/urdf
   	DESTINATION
   	share/${PROJECT_NAME}/
   	)

3. In a launch file, find the installed description and store it in a string, i.e.:

   desplate/desplate_empy/launch/vehicle_sdf_installed.launch.py

   Lines 28 to 33 in 86b7960

   	# Load installed file
   	pkg = os.path.join(get_package_share_directory('desplate_empy'))
   	description_path = os.path.join(pkg, 'sdf', 'vehicle.sdf')
   	with open(description_path, 'r') as description_file:
   	description_str = description_file.read().replace('\n', '')

4. Pass that description string to another node. For robots, that's usually robot_state_publisher, i.e.:

   desplate/desplate_common/launch/visualize.launch.py

   Lines 40 to 45 in 86b7960

   	robot_state_publisher = Node(
   	package='robot_state_publisher',
   	executable='robot_state_publisher',
   	output='screen',
   	parameters=[{'robot_description': LaunchConfiguration('description_str')}]
   	)

EmPy

Invoke empy on .em files from the custom CMake command:

desplate/desplate_empy/CMakeLists.txt

Lines 14 to 16 in 86b7960

	COMMAND empy3
	${CMAKE_CURRENT_SOURCE_DIR}/${FORMAT}/vehicle.${FORMAT}.em >
	${CMAKE_CURRENT_BINARY_DIR}/${FORMAT}/vehicle.${FORMAT}

ERB

Invoke erb on .erb files from the custom CMake command:

desplate/desplate_erb/CMakeLists.txt

Lines 14 to 16 in 86b7960

	COMMAND erb
	${CMAKE_CURRENT_SOURCE_DIR}/${FORMAT}/vehicle.${FORMAT}.erb >
	${CMAKE_CURRENT_BINARY_DIR}/${FORMAT}/vehicle.${FORMAT}

Xacro

Invoke xacro on .xacro files from the custom CMake command:

desplate/desplate_xacro/CMakeLists.txt

Lines 14 to 16 in 86b7960

	COMMAND xacro
	${CMAKE_CURRENT_SOURCE_DIR}/${FORMAT}/vehicle.${FORMAT}.xacro >
	${CMAKE_CURRENT_BINARY_DIR}/${FORMAT}/vehicle.${FORMAT}

Compare

This package makes it possible to compare how the same result can be achieved using different templating engines and description formats. You can use a tool like Meld to see these files side-by-side.

For example, to compare Xacro and EmPy:

Or to compare URDF and SDF: