Incorrect free-space clearing with multiple lidars when 'tracking_frame = base_link'

[aminyaa]

I am using Cartographer for mapping with two laser scanners mounted on my robot:

• One on the front-left
• One on the rear-right

I set tracking_frame = base_link in the configuration.
When the robot moves during mapping, the map starts marking certain cells as free (white) incorrectly. Specifically:

• Cells seen by the front laser are correctly marked as free relative to that sensor.
• However, because the map is being updated relative to base_link, some points that are not actually visible from base_link are still being cleared.
• As a result, free cells are created behind obstacles, in regions that should remain unknown.
  This creates "white streaks" or cleared zones in the map that should not exist.

Expected Behavior:

• Free space should only be marked in the actual FOV of each lidar, considering its position and orientation relative to base_link.
• Obstacles should not be "over-cleared" just because they are not visible in the base_link frame.

Steps to Reproduce:

• Add two lidars to the robot (front-left and rear-right).
• Configure Cartographer with tracking_frame = base_link.

Move the robot around obstacles.

• Observe that some areas behind obstacles are cleared in the map incorrectly.

Environment:

• Cartographer version: 2.0.9003
• ROS 2 version: humble

Additional Notes:

• The issue seems related to how free cells are raytraced relative to the tracking_frame.
• If each sensor’s frame could be respected when marking free space, this problem might be avoided.