[postgis-devel] [PostGIS] #1698: Review polygon collapse policy in ST_Simplify

[PostGIS]

#1698: Review polygon collapse policy in ST_Simplify
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  Reporter:  strk     |       Owner:  strk         
      Type:  task     |      Status:  closed       
  Priority:  medium   |   Milestone:  PostGIS 2.0.0
 Component:  postgis  |     Version:  trunk        
Resolution:  fixed    |    Keywords:               
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Comment(by strk):

 Ok, the whole point of ST_Simplify was (as originally developed) to reduce
 vectors for sending to clients for rendering. True, a 3-points ring is
 invalid as is the single-point one.

 But they can still have an use ! For example you can _see_ something
 rendered (modulo rendering limitations as in mapnik/AGG case:
 https://github.com/mapnik/mapnik/issues/1140) and you know _where_ the
 feature is located (if you want to zoom to it, which may send more
 vectors, less simplified, etc).

 The other thing is that you can always strip non-valids later. We know we
 may be introducing non-valids so why having 1-class invalids that we drop
 lightly and 2-class invalids which we don't drop (complex self-
 intersections).

 I've to say that I'm tempted to revert this to really keep a single vertex
 rather than 3 vertices, but knowing that AGG won't render them makes me
 belive we're doing a better service returning at least 3 points than 1.
 For all AGG users (mapserver,mapnik,what else?).

 I'm attaching 2 images of mapnik rendering a set of ~8k simplified
 polygons representing italian municipalities. The version you don't see
 much is the one with 1-vertex polygons, the other is with 3-vertex
 polygons. You can see you can recognize the country by only the collapsed
 versions of polygons rendered.

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/1698#comment:6>
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