[postgis-devel] [PostGIS] #1909: intersection problem

[PostGIS]

#1909: intersection problem
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  Reporter:  debe     |       Owner:  pramsey      
      Type:  defect   |      Status:  closed       
  Priority:  medium   |   Milestone:  PostGIS 1.5.4
 Component:  postgis  |     Version:  1.5.X        
Resolution:  invalid  |    Keywords:               
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Changes (by pramsey):

 * cc: mdavis@… (added)
  * status:  new => closed
  * resolution:  => invalid


Comment:

 It's difficult to understand, but doing computer geometry we live in a
 quantized world. Imagine a piece of graph paper. Draw a line from one
 intersection to another. Now, look at all the points on that line which do
 **not** touch an intersection. Those are all the valid geometric points on
 the line that you cannot represent using integers. The same thing is true
 of lines between two double-precision points. Most of the space on that
 line will be points that cannot be exactly represented in double
 precision. Your substring includes the 0.0 proportion (the start point)
 and that's representable. The 0.6 proportion is not. The result is a new
 line that is **very slightly** offset from the original. Enough that the
 exact attempt at an intersection only picks up the original point.

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Ticket URL: <http://trac.osgeo.org/postgis/ticket/1909#comment:1>
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