costmap generation in unknown environment

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I'm not entirely sure what you're asking. Could you be more specific? Are you saying that you used a rolling_window on your costmap? Or that you are loading an empty static map?

In the case that you're using a rolling window, your laser_scan or Point_Cloud sensor source will still fill in the costmap based on any obstacles that it sees. Of course, your costmap size must also be set appropriately for the global and local costmaps.

If you're loading an empty static map, the same is true.

~ Brian