How to create a simple map for simulation in stdr simulator?

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It has been a while since I have used stdr_simulator but map in only a png file. So you can create such map manually in a drawing software like GIMP.

Then, add a metadata file with the same name but with the .yaml extension. This file will contain the following elements:

image: frieburg.png
resolution: 0.05
origin: [0.0, 0.0, 0.0]
occupied_thresh: 0.6
free_thresh: 0.3
negate: 0

The above example comes from the stdr_ressouces package.