[postgis-devel] [PostGIS] #272: ST_LineCrossingDirection should be negatively symmetric

[PostGIS]

#272: ST_LineCrossingDirection should be negatively symmetric
----------------------+-----------------------------------------------------
  Reporter:  robe     |       Owner:  pramsey      
      Type:  defect   |      Status:  new          
  Priority:  medium   |   Milestone:  postgis 1.4.1
 Component:  postgis  |     Version:  1.4.X        
Resolution:           |    Keywords:               
----------------------+-----------------------------------------------------
Comment (by robe):

 Hopefully I want embarass myself too much by my obvious lack of direction.
 ---

 Now getting to the manual -- observe that in the manual there are 2 cases
 where our function is not negatively symmetric

 http://www.postgis.org/documentation/manual-
 svn/ST_LineCrossingDirection.html

 {{{
 ELECT ST_LineCrossingDirection(foo.line1, foo.line2) As l1_cross_l2 ,
           ST_LineCrossingDirection(foo.line2, foo.line1) As l2_cross_l1
 FROM (
  SELECT
   ST_GeomFromText('LINESTRING(25 169,89 114,40 70,86 43)') As line1,
   ST_GeomFromText('LINESTRING (171 154, 20 140, 71 74, 2.99 90.16)') As
 line2
 ) As foo;

  l1_cross_l2 | l2_cross_l1
 -------------+-------------
            2 |          -3

 }}}


 {{{
 SELECT ST_LineCrossingDirection(foo.line1, foo.line2) As l1_cross_l2 ,
           ST_LineCrossingDirection(foo.line2, foo.line1) As l2_cross_l1
 FROM (SELECT
         ST_GeomFromText('LINESTRING(25 169,89 114,40 70,86 43)') As line1,
         ST_GeomFromText('LINESTRING(2.99 90.16,71 74,20 140,171 154)') As
 line2
         ) As foo;

  l1_cross_l2 | l2_cross_l1
 -------------+-------------
           -2 |          0

 }}}

 Now if you look at the diagram of these
 In case 1 -- the lines cross at 3 points, however from the perspective of
 the first line (green) -- the blue line crosses it on the left side first
 time, the second time right, and the 3rd time left.  (you have to consider
 the directionality from the perspective of an observer walking from the
 start to end point of the first line -- and which side of him/her is the
 second line passing  ).

 The blue line (line 2)  finally ends its journey on the left side relative
 to the green line. (2 : LINE MULTICROSS END RIGHT).

 Now from the perspective of line 2 (the blue line) -- the green line
 crosses it in 3 places - at point one it is crossing to the right of it,
 at pt 2 it is crossing to the left of it and the third to the left of it.
 But they end up in different directions and their first crosses don't even
 seem in the same direction to me.  Yet this is (-3) -- I guess I'm not
 quite sure what is meant by
 END SAME FIRST LEFT (because it would seem if one ends in same first left,
 the other should end same first right).

 The second example is clearly wrong because -

 how can one line cross another and the other doesn't cross it back except
 in the Twilight Zone :).  This I think is clearly a number counting issue.
 Maybe we should just get rid of that 3,-3 option.

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/272#comment:1>
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