Can GRASS GIS create signed distance maps?

Can GRASS GIS create signed distance maps? For example, if I had coastal data, could GRASS GIS create something like this:

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Signed coastal distance map

Creating regular distance maps (unsigned) is easy using `r.buffer`, but `r.buffer` won't take negative values. Is there another way to create signed distance maps (the map above was created from other data, not via GRASS)