add ARROW_LIST to visualization_msgs/Marker

[christian-rauch]

The visualization_msgs/Marker should add a type for lists of arrows.
The message definition already has types for lists for spheres, cubes and lines. The types appear inconsistent since spheres and cubes exist as single and list version, but the line only exist as list version and the arrow and the cylinder only exist as single version. Having all basic shapes as single and list version would be more consistent.

The closest to ARROW_LIST is LINE_LIST, but the line does not visualise the direction, i.e. the start and end point. The line is also just visualised as 2D rectangle with the normal oriented towards the camera view.

[tfoote]

An arrow field support does sound like it would be valuable. To work toward this I'd suggest tat you put together a coordinated PR both here and to the rviz Marker display for a coordinated review.

[LemonPi]

Any luck with this?

[rr-tom-noble]

Similar PR implemented for ROS1 here (adds ARROW_STRIP, more similar to LINE_STRIP in that the arrows are defined tip to tail):

#190
ros-visualization/rviz#1786

The suggestion was to implement for ROS2 first, then backport, so there's a corresponding set of PRs currently open:

ros2/common_interfaces#218
ros2/rviz#972

Latter PR is in progress as I'm waiting for answers from someone with more experience with rviz2 for help porting over some of the non-obvious ROS1->ROS2 changes. The repo seems less active compared to rviz1 in terms of getting a response. Slighlty frustrating as I could finish off the PRs in a day or so once I have those answers. Any help getting them resolved would be much appreciated.