[geos-devel] JTS - Change to semantics of overlay operations with EMPTY outputs

[Martin Davis]

As a result of some recent discussion on the JTS list, I am thinking of 
changing the semantics of overlay operations with EMPTY outputs to 
return more narrowly typed geometries, instead of the current situation 
where GEOMETRYCOLLECTION EMPTY is returned.  This will allow easier 
cascading of overlay operations (since overlay ops do not currently 
accept GEOMETRYCOLLECTIONs as input).

I'm proposing the following rules (which are extrapolated from the 
behaviour of situations with non-empty results):

* For intersection, the output will have the type of the 
lowest-dimension input
     E.g.
     intersection(POLYGON EMPTY, LINESTRING EMPTY) = LINESTRING EMPTY

* For union, the output will have the type of the highest-dimension input
      E.g.
     union(POLYGON EMPTY, LINESTRING EMPTY) = POLYGON EMPTY

* For difference, the output will have the type of the left-hand input
     E.g.
     difference(POLYGON EMPTY, LINESTRING EMPTY) = POLYGON EMPTY
     difference(LINESTRING EMPTY, POLYGON EMPTY) = LINESTRING EMPTY

* For symmetric difference, the output will have the type of the 
lowest-dimension input
     E.g.
     symDifference(POLYGON EMPTY, LINESTRING EMPTY) = LINESTRING EMPTY
     symDifference(LINESTRING EMPTY, POLYGON EMPTY) = LINESTRING EMPTY

     (This is pretty arbitrary, but no other criteria comes to mind)


Comments are welcome.