PR2 sees ghost obstacles in tilting laser

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Note: solved, see update at the end

I've found the root of the problem. This seems to be a bug in laser_filters. I tried reproducing this in Gazebo to rule out hardware problems, and this is what I get when visualizing the /tilt_scan_shadow_filtered topic:

The blue points on the side have an average height of 0.003 m, the red ones in the middle have an average height of -0.018 m. In Gazebo, all points are still removed by sac_ground_removal, but on the real PR2 I've been working with only the red/yellow/green points in the middle are removed, and the blue ones remain. This is exactly what I am seeing.

This is clearly a bug in the scan_to_cloud_filter_chain from laser_filters; I tried removing everything from the filter chain except the LaserScanAngularBoundsFilter, and the problem is still there. The "dent" in the point clouds is not present in the original scans:

How to reproduce (Electric):

roslaunch pr2_gazebo pr2_empty_world.launch
roslaunch pr2_2dnav_local pr2_2dnav.launch

Update: It's not a bug, it's a feature. To disable it, set the parameter use_hack of the scan_to_cloud_filter_chain node to false, for example like this (in pr2_navigation_perception/lasers_and_filters.xml):

<node pkg="laser_filters" type="scan_to_cloud_filter_chain" respawn="true" machine="c2" name="tilt_shadow_filter">
  ...
  <param name="use_hack" value="false" />
</node>

Note: In Fuerte, the parameter is called incident_angle_correction.

This solved the problem on the real (not simulated) PR2.