So martin this does not really sound like an API change? The contract
still says I will return a non null geometry; and you can call isEmpty()
on the result? Or did you say in the javadocs that you were going to
return an empty multi geometry?
In general I like the idea of a change that lets me chain the methods
together in a useful manner. The practice reminds of perl - which should
not be taken as encouraging the practice :-)
Comments; this sounds fine - you have a clear motivation. If you can
post the javadoc associated with this change I could give you better
feedback.
Jody
Martin Davis wrote:
JTS'ers:
I'm considering making a change to the return value of overlay
operations (intersection, union, difference, symdifference) which
result in an empty geometry. I'm soliciting feedback on whether this
seems like a good idea, and whether there are likely to be any
compatibility problems.
The SFS spec doesn't specify any behaviour in this situation.
Consequently I had to choose some semantics for this situation. At
the time I chose to have an overlay operation which results in an
empty geometry return a GEOMETRYCOLLECTION EMPTY. This was done with
the idea that all operations returning empty would return the same
value (a value representing a "null geometry").
However, I now realize that the problem with this approach is that
this then prevents overlay operations from being cascaded, since GC's
are not supported as input. It seems it might be nicer to return a
geometry which is cascadable, and also which provides a bit more
information about the situation.
I propose the following rules for determining the geometry type of an
empty result value:
intersection(g1, g2) => empty geom with dim = min(dim(g1, g2)
union(g1, g2) => empty geom with dim = max(dim(g1, g2)
difference(g1, g2) => empty geom with dim = dim(g1)
symdifference(g1, g2) => empty geom with dim = max(dim(g1, g2)
Examples:
intersection(POLYGON EMPTY, LINESTRING EMPTY) => LINESTRING EMPTY
union(POLYGON EMPTY, LINESTRING EMPTY) => POLYGON EMPTY
difference(POLYGON EMPTY, LINESTRING EMPTY) => POLYGON EMPTY
symdifference(POLYGON EMPTY, LINESTRING EMPTY) => POLYGON EMPTY
Comments?
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