[ros-users] About tf and synchronization

[Tully Foote]

On Tue, Dec 21, 2010 at 4:16 PM, Rodrigo Baravalle <rbaravalle at gmail.com>wrote:

> Hi!
>
> Here are the lines of code of the callback, when an amcl pose arrives..
>
> I used the same frame_id, but, is the timestamp correct? I used the
> same as in the tutorial:
> http://www.ros.org/wiki/tf/Tutorials/tf%20and%20Time%20(C%2B%2B)<http://www.ros.org/wiki/tf/Tutorials/tf%20and%20Time%20%28C%2B%2B%29>
>
> void MergePC::callback(const geometry_msgs::PoseWithCovarianceStampedPtr&
> o) {
> ...
>    // Read data from the Camera
>    pc_current.header.frame_id = "swissranger";
>    dev_->readData(pc_current,pc2_current,image_d_, image_i_,
> image_c_, image_d16_);
>
>     // I put 100.0, but is the same if I put 20.0, 3.0, or whatever
>     tlistener.waitForTransform("/map", "/swissranger",
>                               ros::Time(0), ros::Duration(100.0));
>
>     // pc_current is the point cloud arriving from the camera, so it
> has the "swissranger" frame_id
>     // here is where it fails..
>     tlistener.transformPointCloud("/map", pc_current, pc_tf);
> ...
> }
>
> Thanks in advance!
>

Hi Rodrigo,

Your call to wait for transform is going to wait until any transform between
"/map" and "/swissranger".  Time(0) is special and will match anything.

you want to waitForTransform betwen "/map" and pc_current.header.frame_id at
time pc_current.header.stamp, as that is what you are about to call with
transformPointCloud.

Tully



> 2010/12/21 Wim Meeussen <meeussen at willowgarage.com>:
> >> The problem is that I always get an error message like this:
> >>
> >> "You requested a transform that is 12.571 miliseconds in the past,
> >> but the most recent transform in the tf buffer is 49.194 miliseconds
> old.
> >>  When trying to transform between /swissranger and /map."
> >
> >> I have already tried with "waitForTransform" between the two frames,
> >> but nothing seems to happen. I can see by using "rosrun tf
> >> view_frames" that amcl is publishing things in the future: "
> >> Broadcaster: /amcl ... Most recent transform: -0.086 sec old ... ".
> >
> > Based on this information your tf tree looks fine. You are just
> > requesting a transformation before that information is in the tf
> > buffer.  Calling waitForTransform before you request the actual
> > transform should work.  Did you call waitForTransform with the same
> > frame id's and timestamp as the actual transform? Could you show the
> > few lines of code where you do the tf calls?
> >
> > Wim
> >
> >
> > --
> > --
> > Wim Meeussen
> > Willow Garage Inc.
> > <http://www.willowgarage.com)
> > _______________________________________________
> > ros-users mailing list
> > ros-users at code.ros.org
> > https://code.ros.org/mailman/listinfo/ros-users
> >
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-- 
Tully Foote
Systems Engineer
Willow Garage, Inc.
tfoote at willowgarage.com
(650) 475-2827
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