Why the clearing stays permanent in obstacle costmap, when 0 (overwrite) combination method is applied?

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Hello !

I am clearing some layers markings by a layer with a detected object to be cleared and the problem is that its' clearing stay permanent even when the object disappeares.

So here more detailed information:

The goal:

• Erase any markings on the costmap under x_object when it is detected
• Continue marking with laser layers when the x_object is disappeared or was not present

The actual results:

• The laser layers mark the obstacles. Perfect ! =)
• The x_object detected and overwrites (erases) the markings. Perfect ! =)
• The x_object disappeares (goes away) Perfect ! =)
• The laser layers do not mark anymore where was detected the x_object. Bad ! =(

I tried different observation_persistence values:

• 0.0 to have only recent measurements (expected them to be available only during one cycle of observation)
• values > 0.0 to keep the observation during some time and refresh the clearings after

Question : How do you think is it possible to have the clearings present only when the object is detected ?

Code:

x_obstacle_clearing_layer:
  enabled: true
  observation_sources: x_object_clearing_cloud
  combination_method: 0

x_object_clearing_cloud:
  sensor_frame: base_footprint
  data_type: PointCloud2
  topic: /detect_x_object/object
  expected_update_rate: 2.0
  observation_persistence: 1.0
  marking: false
  clearing: true
  raytrace_range: 5.3
  obstacle_range: 4.8
  min_obstacle_height: -2.0
  max_obstacle_height: 2.0

Thank you !

P.S.: Working with ROS Kinetic