Migration from tf_prefix

[Jack O'Quin]

The tf2 migration guide for Hydro states that tf_prefix is no longer supported.

 http://www.ros.org/wiki/hydro/Migration#tf2.2BAC8-Migration.Removal_of_support_for_tf_prefix

I agree that tf_prefix is hard to use correctly and often botched. But, there were cases where it was necessary and solved problems.

What happens to code using tf_prefix in Hydro? Will it still work via the tf version one interfaces? 

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 joq

[Tully Foote]

So it should keep working in the cases where tf_prefix was previously used.  

The basic premise of the migration is that people used to refer to "my_frame" which was resolved to "/my_frame" or "/my_prefix/my_frame" if the prefix was present

In tf2 whatever string you pass in is your frame_id. So "my_frame" is simply "my_frame" and "/my_frame" would be a different frame_id.  To avoid silent failures tf2 does not accept frame_ids starting with "/" with an tf2::InvalidArgument exception.  

Anyone using the tf::Transformer interface will have the "/" stripped from the frame_id before it is passed to tf2 under the hood.  

To maintain backwards compatability with usages of tf_prefix, tf::resolve has been modified to always return frame_ids without a leading slash, but still does the prefixing.  So tf::resolve "/map" resolves to "map" or "my_prefix/map" if a tf_prefix is set.  This has the consequence that if you call tf::resolve multiple times it will get multiply prefixed.  However calling tf::resolve multiple times is not known to be used in code and is not expected practice.  

All communication on the wire is done by tf2 and the messages will be without the slash.  

The longer term solution to replace tf_prefix is to provide methods for selectively republishing tf transforms, likely with a prefix from one robot to another, probably between masters as well.  And at the same time rewriting any frame_ids transferred between the robots to have the prefixed frame_id.  The development time for these tools unfortunately has not been found yet.  

Tully

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[Jack O'Quin]

Thanks for the explanation.

So, it sounds like most valid, existing tf_prefix uses should still work correctly using tf::resolve. At least some of that should probably be added to the migration guide...

 http://www.ros.org/wiki/hydro/Migration#tf2.2BAC8-Migration.Removal_of_support_for_tf_prefix

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 joq