[postgis-devel] Valid geometries

[Brian M Hamlin]

     (engine_id, valid_flag)  makes more sense to me than
  (engine1_valid, engine2_valid, engine3_valid)
   

On Fri, 09 Jan 2015 12:20:25 +0100, Vincent Mora 
<vincent.mora at oslandia.com> wrote:
Hi all,
  >
  > We have a discussion about geometry validity with Hugo Mercier on irc
  > that raises questions of general interest. So here's a summary:
  >
  > SFCGAL function check the validity of input geometries, this takes time,
  > but if we don't do it the algorithm behavior can be undefined (crash,
  > infinite loop). 
  >
  > We think that a valid geometry should have a different type than a
  > geometry which validity is unknown. 
  >
  > A validity flag could be used in postgis to implement that, with three
  > possible states: valid|invalid|unknown. 
  >
  > One issue is that geos and sfcgal (absolute prescision) may have
  > different oppinion concerning the validity. Even if the geometry
  > validity is well defined (SFS), checking if a point lies on a line is
  > not the same thing for geos and SFCGAL: the latter tells you "it's
  > 'absolutely' not on the line", the former tells you "it's really, really
  > close to the line". As a result, a segment of a polygon hole touching
  > the exterior ring may cause geos to consider the geometry invalid while
  > SFCGAL will consider it valid. Bottom line we need two flags, more if at
  > some point someone wants to add a boost backend. 
  >
  > It would also be nice to be able to restrict a given table to store only
  > valid geometries. 
  >
  > There are alternatives: having a flag specific for SFCGAL, allowing to
  > set the validation policy of SFCGAL dynamically with GUC or compiling
  > SFCGAL with SFCGAL_CHECK_VALIDITY turned off. But none appeals to me as
  > a specific type for valid geometries does. 
  >
  > Which do you prefer ?
  >
  > V. 
  >
  >
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  >

  --
Brian M Hamlin
  OSGeo California Chapter
  blog.light42.com