Add support for quaternion in URDF about urdfdom

[traversaro]

Thanks for proposing this! Given what was discussed in #123 (comment), I think that it is useful to clarify what version number this is new URDF+quaternion will use (as requested in #123 (comment)).

It is going to be URDF spec 2.0 or 1.1? I am not an expert of versioning for file formats, and the usually used semantic versioning refers to software APIs (with corresponding ABIs), not file formats. The only example that I know of strictly defined versioning rules for a file format are FMI's versioning rules, defined in https://fmi-standard.org/docs/3.0/#VersioningLayered .

However, philosophy aside, the version support was added in #133, but unless I am missing something no explicit semantics was defined for it. However, there is an implicit semantic defined by the check in

urdfdom/urdf_parser/src/model.cpp

Line 128 in 29426ec

if (!version.equal(1, 0))

: all existing urdfdom version will refuse to load any document that URDF that have a version different from "1.0" . From that point of view, this means that we can safely call the URDF with quaternion both 1.1 or any other number, as old urdfdom that do not read quaternions will always do the right thing: refuse to load the document.

Regardless of weather we use "1.1" or "2.0" as a version, I guess we need to modify the following point in the code:

• urdfdom/urdf_parser/src/model.cpp

  Line 128 in 29426ec

  if (!version.equal(1, 0))

  : urdfdom should now accept the new version as well

• urdfdom/urdf_parser/src/pose.cpp

  Line 110 in 14f6db1

  const char* quat_str = xml->Attribute("quat_xyzw");

  : Not sure about this, but perhaps quaternion should be parsed only if the urdf version is explicitly specified as 1.1/2.0, and print a clear error message otherwise? In this way we make sure that people that use quaternion properly set the version in their urdf
• https://github.com/ros/urdfdom/pull/194/files#diff-f6e0c5c27bb1c6f9c99cab653d4849b0fe93a493cf60b5f0fcf2bf408588d7c6R4 : the updated spec is now the one of 1.1/2.0 URDF spec

[traversaro]

https://github.com/ros/sdformat_urdf ?

I guess no modification is required in this? This code creates a urdf::ModelInterfaceSharedPtr from a SDF model, without passing via the textual representation of the URDF, that is modified by this proposal. In memory, the urdf::Rotation struct already uses quaternion.

Whatever gazebo is using when doing gz sdf -p robot.urdf > robot.sdf or when spawning robot from an urdf robot description ? @ahcorde

This code is the URDF parser in https://github.com/gazebosim/sdformat/blob/sdf14/src/parser_urdf.cc . However, I do not think it needs any modification as it already uses urdfdom and so it reads the urdf pose from urdfdom's structures, see https://github.com/gazebosim/sdformat/blob/sdf14/src/parser_urdf.cc#L2802 . However, I think the problem here is of deployment. If I recall correctly (but things may have changed, I do not have a Humble install to quickly check) sdformat and gazebo packages use the urdfdom that ships with official Debian/Ubuntu repos (apt package: liburdfdom-dev, see for example https://packages.ubuntu.com/jammy/liburdfdom-dev), while ROS2 installs its own version of urdfdom package (ros-humble-urdfdom or something similar, see http://repo.ros2.org/ubuntu/main/pool/main/r/ros-humble-urdfdom/). So, to ensure that any change is available and supported in Gazebo for ROS Jazzy, we would need to make sure that urdfdom with this modifications is available in Ubuntu 24.04 repos. I am not sure if we are still in time for that, probably @j-rivero knows more. A possible alternative is that ROS installs its own ABI compatible sdformat package, to avoid any relation with the system's urdfdom.

[traversaro]

Another related question: unfortunatly we still do not have the spec in a github repo (mainly my fault, see ros/urdfdom_headers#62). However, could you propose here or somewhere how you would edit the spec in http://wiki.ros.org/urdf/XML to document quaternions?