Support loading resources from Fuel

[nkoenig]

sdformat_urdf supports the use of SDF files, and I'd like to utilize Fuel for resource hosting. With this PR, an SDF file can remain consistent when used between Gazebo and RViz. This will also give people the option of store meshes and models in a location other than a git repository.

I think it makes sense to support this capability here, rather than modify SDF files to use absolute paths. A user may run rviz on a separate machine from Gazebo, and wouldn't necessarily be able to resolve an absolute path.

It does add an additional dependency on gz-fuel_tools. This is required to resolve urls like https://fuel.gazebosim.org/1.0/openrobotics/models/backpack. If a resource is not in the local Fuel cache, then it's downloaded.

I'm new to adding dependencies like this. Please let me know what I should do to fix or improve things.

Test

Here is a simple launch file that you can use for testing

import os

from ament_index_python.packages import get_package_share_directory 
from launch import LaunchDescription
from launch.actions import (
  DeclareLaunchArgument,
  IncludeLaunchDescription,
  RegisterEventHandler,
)
from launch.event_handlers import OnProcessExit
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node

def generate_launch_description() -> LaunchDescription:
    use_sim_time = LaunchConfiguration('use_sim_time', default=True)

    # Get SDF via xacro
    robot_description_content = """<?xml version="1.0" encoding="UTF-8"?>
      <sdf version='1.9'>
        <model name="backpack">
          <static>true</static>
          <link name="base_link">
            <collision name="collision">
              <pose>0 0 2 0 0 0</pose>
              <geometry>
                <mesh>
                  <uri>https://fuel.gazebosim.org/1.0/openrobotics/models/backpack/tip/files/meshes/Backpack.dae</uri>
                </mesh>
              </geometry>
            </collision>

            <visual name="visual">
              <geometry>
                <mesh>
                  <uri>https://fuel.gazebosim.org/1.0/openrobotics/models/backpack/tip/files/meshes/Backpack.dae</uri>
                </mesh>
              </geometry>
            </visual>
          </link>
        </model>
      </sdf>"""

    robot_description = {'robot_description': robot_description_content}

   # The node to publish robot state information
    robot_state_publisher_node = Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        output='both',
        parameters=[robot_description, {'publish_frequency': 1000.0}],
    )

    rviz_node = Node(
        package='rviz2',
        executable='rviz2',
        name='rviz2',
        output='log',
    )

    gz_spawn_entity = Node(
        package='ros_gz_sim',
        executable='create',
        output='screen',
        arguments=['-string', robot_description_content],
    )

    # Return the final launch description
    return LaunchDescription(
        [
          DeclareLaunchArgument(
              'use_sim_time',
              default_value=use_sim_time,
              description="If true, use simulation clock",
          ),
          # Launch gazebo environment
          IncludeLaunchDescription(
            PythonLaunchDescriptionSource(
              [
                os.path.join(
                  get_package_share_directory('ros_gz_sim'),
                  'launch',
                  'gz_sim.launch.py',
                )
              ]
            ),
            launch_arguments=[('gz_args', [' -r -v 4 empty.sdf'])],
          ),
          # Run the joint state publisher after the robot has been spawned
          RegisterEventHandler(
            event_handler=OnProcessExit(
              target_action=gz_spawn_entity,
              on_exit=[rviz_node],
            )
          ),
          robot_state_publisher_node,
          gz_spawn_entity,
      ]
    )

[clalancette]

So the hardest bit about this is that we have to make more Gazebo dependencies core to ROS 2. In particular, gz_fuel_tools depends on:

• gz_cmake_vendor
• gz_common_vendor
• gz_math_vendor
• gz_msgs_vendor
• gz_tools_vendor
• gz_utils_vendor

Of those, we currently only depend on cmake, utils, and math in the core, so we'd have to add in common, msgs, and tools as new dependencies. Under the new OSRA rules (which aren't quite in effect yet, but will be soon), we'll have to get ROS PMC approval to add those dependencies in. I'll keep this on the list to review once we have the PMC totally up and running.

[mjcarroll]

@nkoenig I'm also thinking about improvements to rviz mesh loading. Give me a few days to get a proposal out and maybe we can merge these two ideas in such a way that doesn't introduce dependencies.

[MichaelOrlov]

On the latest waffle triage meeting decided to move this issue to the backlog.

[clalancette]

Setting this to backlog and a draft PR for now, as we will wait to see how this comes together with what @mjcarroll is working on.