Add ImageMarkerArray message

[amacneil]

Proposal to add ImageMarkerArray, which is to ImageMarker as MarkerArray is to Marker.

Can also port this to ros2 fairly easily if needed.

[tfoote]

What use case does this serve and what semantic meaning does it provide? This does not appear to add any semantic meaning which means that it's hard to reuse effectively as a standalone. And being just an array it can be embedded as a submessage just as easily as ImageMarker [] versus ImageMarkerArray.

To make this message valuable it should have non-trivial documenatation which defines how to interpret the data. In particular what is the expected semantic meaning of the grouping.

[elephantum]

+1 for adding ImageMarkerArray

My use case is to send results of ML-detection model as a bunch of markers to topic instead of rendering results to image.
Foxglove Studio offers ability to render marker topic on image topic, hence much more effective client side rendering.

Array is needed in order to render more than one marker at once.

Pre-defined (vs custom msg) is needed because Foxglove does not know anything about my custom msg type, so it can't use it for rendering.

[amacneil]

Exactly. ImageMarker provides a way to publish image annotations on a separate topic (so that they can be rendered separately by viewing software). All the semantic meaning exists within ImageMarker.

ImageMarkerArray is simply a way for users to publish multiple annotations in a single frame (for example detected objects). It is useful for the same reason MarkerArray is useful for rviz.

It would be helpful to include this as a standard message definition in ROS so that libraries and tools can be interoperable.

[jhurliman]

Until this is merged, I've published this as foxglove_msgs/ImageMarkerArray - ros/rosdistro#29682

[amacneil]

Closing PR because:

1. No further major releases of ROS 1 are planned after noetic. Any new messages should go in ros2/common_interfaces.
2. No "official" ROS visualization tools (rviz, rqt) even use the ImageMarker message - as far as I know Foxglove Studio and Webviz are the only tools that can render it (see: ros answers). So I don't see much point in adding this to ROS 2 unless other tools are going to use it as well.

Since image markers are a Foxglove-specific feature for now, we have chosen to publish foxglove_msgs/ImageAnnotations under a separate package for ROS 1 and 2.