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

[PostGIS]

#272: ST_LineCrossingDirection should be negatively symmetric
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 Reporter:  robe     |       Owner:  pramsey      
     Type:  defect   |      Status:  new          
 Priority:  medium   |   Milestone:  postgis 1.4.1
Component:  postgis  |     Version:  1.4.X        
 Keywords:           |  
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 I propose that to minimize on confusion

 It should hold true that:

  ST_LineCrossingDirection(a,b) = - ST_LineCrossingDirection(b,a)

 --- below is a relevant excerpt from the previous crashing ticket  -

 Regina

 {{{
 But I think there is still a problem.

 In our documentation -- We still have this example that doesn't behave
 right

 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;

 (gives me -2, 0)

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

 Mark's Response

 {{{
 Yeah; I see exactly the same answers. Unfortunately though, I can't make
 much sense of the integer constants in the manual to work out what the
 function *should* actually be doing. Can anyone explain this in more
 detail? For example, do the LEFT/RIGHT parts refer to spatial position or
 the order of the input parameters?

 ATB,

 Mark.

 }}}

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