Python implemention of foxglove/rosmsg-serialization

[muety]

I am looking for a ROS-free (de-)serializer for common ROS messages, that allows me to encode and decode sensor_msgs and others without having to have an actual ROS distribution installed (esp. since ROS 1.x doesn't work on recent OS). Essentially a Python implementation of https://www.npmjs.com/package/@foxglove/rosmsg-serialization. Is there anything like that already around or perhaps planned?

[jtbandes]

You can take a look at the mcap-ros1-support package which vendors some of the dynamic deserialization code from ROS 1 so it can be run outside a ROS environment: https://github.com/foxglove/mcap/blob/734c6433b4e3ab0df403a4c41b2e6b853d6586ec/python/mcap-ros1-support/mcap_ros1/decoder.py#L4-L10

[muety]

Thanks! I'm not exactly sure how to use this, though. I read through the code and came up with this hacks solution, but I think that's not the proper way to go:

schema1: Schema = Schema(id=123, name='sensor_msgs/Image, encoding=SchemaEncoding.ROS1, data=open('/tmp/Image.msg', 'r').read().encode('utf-8'))
d1: Decoder = DecoderFactory().decoder_for('ros1', schema1)

def my_callback(data: bytes):
    return d1(data)

For this to work, I created /tmp/Image.msg (from Image.msg) and manually embedded the fields from Header.msg, because I didn't know how to dynamically include that.

I'm sure there's a better way to do this. Essentially I want to be able to to sth. like

decoder = DecoderFactory().decoder_for('ros1', magically_get_me_a_ros1_schema_for_type('sensor_msgs/Image'))

I found msg_loader.py, which might be useful, but wasn't sure how to utilize that either.

Could you please point me into the right direction here?

[muety]

Any tips you could give me on this end?

[jtbandes]

I'm not sure what I can help with exactly, but you will definitely need access to the .msg files in some form. In MCAP they are embedded in the Schema records like this: https://mcap.dev/spec/registry#ros1msg

[muety]

Access to the .msg files is not a problem. I'm only wondering what the correct APIs / functions are to load them and use them for parsing ROS messages. Could you give a brief code example for my use case?