Hai, 
The tf library is not robust to time jumping backwards.  The first time you playback it will work, but the second time time the incoming data will be older than the tf buffer and be tossed on on reciept. http://www.ros.org/wiki/tf/Errors%20explained#Error:_TF_OLD_DATA

Tully

On Tue, Jul 27, 2010 at 12:35 PM, Hai Nguyen <haidai@gmail.com> wrote:
Hi,
Hi,

I'm trying to assemble some laser scans into a point cloud using
laser_assembler tutorial [1] but the periodic_snapshotter keeps
reporting that it is only publishing empty messages with zero points:
   [ INFO] [1280258736.674914761, 1275001489.217034967]: Published
Cloud with 0 points

On closer investigation this seems to be caused by a TF_OLD_DATA error
generated by scan_assembler.  Is there a recent change to how the time
mechanism works in ROS?  I'm running all these nodes on the same
computer, using --clock with "rosbag play" (on the provided bag file
laser.bag), and tried to run with /use_sim_time=True but the error
still persist.

TF error:
Node: /scan_assembler
Time: 1275001492.698066055
Severity: Warn
Location: /opt/ros/cturtle/stacks/geometry/tf/src/tf.cpp:Transformer::setTransform:179
Published Topics: /rosout

TF_OLD_DATA ignoring data from the past for frame /sensor_mount_link
at time 1.275e+09 according to authority /play_1280258731455287347
Possible reasons are listed at

My launch file for laser_assembler:
<launch>
<node pkg="laser_assembler" type="point_cloud_assembler" name="scan_assembler">
   <remap from="scan" to="tilt_scan"/>
   <param name="fixed_frame" value="base_footprint"/>
</node>
</launch>



[1] http://www.ros.org/wiki/laser_assembler/Tutorials/HowToAssembleLaserScans
--
Hai Nguyen
CS, Robotics
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--
Tully Foote
Systems Engineer
Willow Garage, Inc.
tfoote@willowgarage.com
(650) 475-2827