[user-the-name]

Honestly, most SDFs you use in practice will be badly formed. Creating a well-formed SDF is very, very hard, and only possible in a few special cases.

So in practice what you try to do is find an SDF that is not TOO far off from what it should be. Ideally, it should always have an absolute value that is equal to or less than (but not by too much) the real, ideal SDF.

[andybak]

Depending on your goals - the failure modes of non-well-formed SDFs are often quite interesting. ;-)