[postgis-users] Semantics of difference() function

[Obe, Regina]

Martin,
Thanks for the explanations.  Regarding the below see my comments below
 
>Note - touches() implies intersects().  As I mentioned above, you need
>to be thinking in terms of testing Interior Intersection.  There are no
>named predicates which do this - you have to work with the DE-9IM
>pattern directly.  (I have added an interiorsIntersect predicate to JTS
>to do just this, since it's useful in some situations.
>> More disturbing is why when I cut a polygon with a line I get (shall we
>> say bizarre results).  A slightly larger polygon.
>>  
>This is a weird one. Can you post the actual geometries?  In JTS when I
>try differencing a line from a circular point buffer, I get a single
>polygon (with extra vertices where the line cut it).  This is the
>expected behaviour.  I don't know why PostGIS/GEOS is doing something
>different.
 
Actually the PostGIS/GEOS behavior is consistent with your JTS I think.  What I was getting was a single Polygon too with extra vertices
where the line cut throw.  It was when I was trying to figure out what the difference was between my original
polygon and this new polgon difference line.  Simply overlaying them on a screen I couldn't tell the difference so I differenced them and was shocked when my old polygon was completely contained by new polygon and new polygon was bigger by 2 super small polygons.  I thought it was more funny than anything.
 
new polygon = old polygon - line
old polygon - new polygon = empty collection
new polygon - old polygon = multipolygon (with 2 polygons)
 
If you look at the attached image of these blobs.  It is caused by those extra vertices added on to the polygon as you mentioned I think.   I was suspecting something like this but nice to get verification.
 
For those interested - the test case is
SELECT 1 As gid, g.geom1, g.geom2, ST_Difference(g.geom1, g.geom2) As diff_geom12, 
ST_Difference(g.geom2, g.geom1) As diff_geom21, ST_SymDifference(g.geom1,g.geom2) As diff_symgeom12
INTO diftest2
FROM (SELECT ST_Buffer(ST_MakePoint(9,3),2) As geom1, 
    ST_MakeLine(ST_MakePoint(8,0), ST_MakePoint(10,9)) As geom2) As g;

SELECT ST_Area(geom1) = ST_Area(diff_geom12) As is_areasame, 
    St_AsBinary(geom1) = St_AsBinary(diff_geom12) As binary_equal, 
        ST_Equals(geom1,diff_geom12)  As spatially_equal,
    ST_GeometryType(geom1) As geom1_type, 
    ST_GeometryType(diff_geom12) As geom12_type
FROM diftest2;
 
yields
is_areasame | binary_equal | spatially_equal | geom1_type | geom12_type
-------------+--------------+-----------------+------------+-------------
 t           | f            | f               | ST_Polygon | ST_Polygon
 
 
---Comparing them with difference which probably adds 
SELECT 
    ST_AsText(ST_Difference(geom1,diff_geom12)) As diff_geom1_geom12,
   ST_GeometryType(ST_Difference(diff_geom12,geom1)) As diff_geom12_geom1
FROM diftest2;

    diff_geom1_geom12     | diff_geom12_geom1
--------------------------+-------------------
 GEOMETRYCOLLECTION EMPTY | ST_MultiPolygon
 
thanks,
Regina
 
 
 


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