CostMap2D no longer showing unknown space

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Hello,

I am running ROS Hydro on Ubuntu 12.04.

I have written an application that runs alongside gmapping_demo.launch but has its own Costmap2DROS object keeping track of the global costmap. While developing my app, I have been able to use the getCostmap() function to get a copy of the character map. Relevant code is shown:

costmap_2d::Costmap2D* mymap = globalmap_->getCostmap();
boost::unique_lock< boost::shared_mutex > lock(*(mymap->getLock()));
mymap->saveMap("/tmp/map");

I then go through the "mymap" pointer and copy its cost values to a character array.

This used to work perfectly, but today I noticed that the saved map no longer keeps track of unknown space (cost = 255), instead just saving it as zeros, as though all unknown space was freespace. To illustrate this, here is the map generated by saveMap():

I have searched the file, and 255 does not appear anywhere.

I am quite stumped as to why this is happening. The only thing I can think of is that I allowed Ubuntu to install updates yesterday, and the navigation packages were included. Has something changed in the way they work? I am wondering if some bug was introduced.

Saving the map via rosrun map_server map_saver still runs the way I expect it to.

If there is any more information I can provide, please let me know. Thanks in advance.