rostopic echo: see only "parts" of a message

ROS1
rosbag

asked 2021-08-17 22:10:19 -0600

Kansai
170 ●47 ●57 ●65

updated 2021-09-09 01:37:28 -0600

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

I have recorded a rosbag with a single topic (which is a ros image topic) Unfortunately although rqt_bag can republish the messages, I cannot visualize the images (rqt_bag crashes) because of the particular format of the images (8UC4)

I can use rviz to see the images being published, so I know they are ok to an extent.

However I would like to see the timestamps of the headers of those images.

If I republish the messages of the rosbag and in another terminal I do rostopic echo the_topic I think I will get a lot of info about the image, so I don't think I will be able to see the timestamps

Is there a way to see only some parts of the topics being published? (like only the headers?)
What are other ways to inspect a rosbag (other than just writing a script myself to inspect it)

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for the first question rostopic echo the_topic/header will do

Kansai ( 2021-09-08 21:06:31 -0600 )edit

I've updated the title of your question as it seems after your edit that's the actual question you're asking.

gvdhoorn ( 2021-09-09 01:37:54 -0600 )edit

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answered 2021-09-08 21:36:00 -0600

Jaron
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Timestamp and other, detailed information can be obtained by interacting with the rosbag api. Here's an example on how to interact with it.

For ROS 1, the ways to interact with the file are through the command line or the apis.

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answered 2021-09-09 01:38:11 -0600

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2021-09-09 01:40:42 -0600

The rostopic --help output may offer some guidance here:

  --nostr               exclude string fields
  --noarr               exclude arrays

if you add --noarr to your rostopic echo command, you'll be able to see the header and other non-array fields.

Alternatively: rostopic echo can "subscribe" to individual fields.

I put "subscribe" in quotes as in reality, it will still subscribe to the topic (so not to a field), but it will only print the value of the field, instead of the entire message.

This is mentioned on wiki/rostopic - echo:

echo <topic-name/field>

Display specific fields in a message.
$ rostopic echo /my_topic/field_name

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rostopic echo: see only "parts" of a message

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