[ros-users] api review for urdf, scene and scene_state

John Hsu johnhsu at willowgarage.com
Wed Jun 13 20:00:08 UTC 2012


Hi Thomas,

Thank you for your feedback.  I just want to clarify things a bit and
reassure you that existing URDF API will not change (with exceptions of
minor tweaks presented on the API review page).   I do understand the
gravity of changing something as low level as the URDF, and have been
working hard on keeping it consistent and simple.  The ROS URDF stack will
continue to exist as it is, our goal is that if changes happen, they will
be transparent to current users.

As we are trying to figure out if expanding the scope of URDF is the right
thing to do, the proposed extensions will be kept separate from the
existing URDF API.

The suggested extensions in this API review is motivated by needs and
wishes that's been brought up (to me) repetitively by other URDF users over
the past couple of years, and we are doing our best to push back on
anything that will violate the simpleness-principle the URDF is based on,
while staying as general as possible.

Regarding your proposal, I'll try to comment on them inline, but these are
relevant and valuable inputs to the review, I feel they could be added to
the API review page?


My proposal:
> - keep URDF as simple as possible
>

Couldn't agree more!


> - go toward making sure that URDF is a subset of SDF
>
- do not redefine what already exists in SDF, use it.


I am also trying to avoid having to maintain two formats and converters
between formats :)  This is the reason we've stripped ROS dependency from
URDF, creating urdfdom, down the road, allowing urdfdom to be an optional
system dependency and parsed directly inside of Gazebo if detected.

On the subject of looking for a way to find a common ground for URDF and
SDF in the long run; there are many overlapping concepts highly desirable
in both URDF and SDF, but the intersection and coverage between SDF and
URDF is complicated, and this review is targeted at simply extending URDF.
 Hopefully as a side benefit, we'll find ways to converge model/state
descriptions between robotics and simulation.

- write a spec file or a REP for URDF and SDF so that it can stay a
> coherent entity (i really don't want to maintain one SDF
> and one URDF for the same robot!)
>

noted, will start a rep.



> - be nice with the other parsers: if you change the format, you break
> our code. At least, it would be nice to have a DTD, an
> XML scheme or a version tag somewhere in the file so that we can
> properly refuse too recent or too old URDF versions.
> Of course, this makes only sense if there is a spec, first.


I do agree having a DTD or version tag will be great, these suggestions
should be added to the API review page :)

Thanks,
John






On Wed, Jun 13, 2012 at 3:02 AM, Thomas Moulard <thomas.moulard at gmail.com>wrote:

> On Wed, Jun 13, 2012 at 3:52 AM, John Hsu <johnhsu at willowgarage.com>
> wrote:
> > Hi All,
> >
> > A couple of extensions to the urdf have been proposed and are on the
> > horizon.  Please review and add your comments to the API Review page.
>
> Hi,
> first thanks for proposing an API review and providing efforts in
> developing urdf.
> I think this is a very crucial tool for us, ROS users, especially in
> combination with tf.
>
> However, as this is a crucial tool, I also feel quite worried about
> the turn of even that
> is suggested by this proposal.
>
> IMHO, we do not need an API review without having a _clear_ view on
> what we (users) and WG
> wants to do with the URDF format, how we can make so that it stays
> compatible with the Gazebo
> SDF format, whether it is possible, in long term, to get rid of the
> PR-2 extensions and propose something
> for a broader range of robots (read here: closed chains, sensor fields,
> etc.)
>
> Particularly, it seems to me that this particular effort is very
> redundant to what have been done when SDF
> has been defined.
>
> The URDF format is one of the core part of the ROS ecosystem, we all
> build on top of it or use it but, AFAIK, there
> has been no effort so far to try to write specs for this format. To
> use a metaphor, I feel like you're trying to build webkit
> before writing down the HTML specifications and as you do that, I see
> a fragmentation of this "unified" robot modeling
> format that worries me.
>
> My proposal:
> - keep URDF as simple as possible
> - go toward making sure that URDF is a subset of SDF
> - do not redefine what already exists in SDF, use it.
> - write a spec file or a REP for URDF and SDF so that it can stay a
> coherent entity (i really don't want to maintain one SDF
> and one URDF for the same robot!)
> - be nice with the other parsers: if you change the format, you break
> our code. At least, it would be nice to have a DTD, an
> XML scheme or a version tag somewhere in the file so that we can
> properly refuse too recent or too old URDF versions.
> Of course, this makes only sense if there is a spec, first.
>
> Again, I did not put that on the wiki because this mail is about the
> format and not about the format implementation.
>
> What do you think?
> --
> Thomas Moulard
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> ros-users mailing list
> ros-users at code.ros.org
> https://code.ros.org/mailman/listinfo/ros-users
>
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