is_simple and OGC SFA compliance

[TreeHunter9]

Hi everyone! I have a question about the OGC SFA compliance of implementation of is_simple function for linestring. As OGC SFA says:

A Curve is simple if it does not pass through the same Point twice with the possible exception of the two end points.

And that's it. So, if I understand correctly, it means that empty linestring is considered simple. But in boost implementation I can see that empty linestring is not considered simple. So, it's my interpretation of standard is wrong or implementation?

[tinko92]

From my reading of https://docs.ogc.org/is/06-103r4/06-103r4.pdf 6.1.6, it is no clear that an empty set can be seen as a curve (and by extension a linestring) at all. It defines curves as an image of a real closed interval, I would see it as a convention that a real closed interval [a, b] has a<=b and therefore contains at least a point (even though the empty set is closed). And is_simple is defined for curves in that context. Afaics BG also considers an empty linestring invalid:

geometry/include/boost/geometry/algorithms/detail/is_valid/linear.hpp

Line 63 in 196d04c

return visitor.template apply<failure_few_points>();

However, JTS documentation https://locationtech.github.io/jts/javadoc/org/locationtech/jts/operation/valid/IsSimpleOp.html states that empty geometries are simple.