[postgis-tickets] [PostGIS] #3712: Address MVT issues reported by specification author

[PostGIS]

#3712: Address MVT issues reported by specification author
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  Reporter:  strk     |      Owner:  Björn Harrtell
      Type:  defect   |     Status:  new
  Priority:  blocker  |  Milestone:  PostGIS 2.4.0
 Component:  postgis  |    Version:  trunk
Resolution:           |   Keywords:
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Comment (by nicklas):

 I had a little wild thought from reading this discussion.

 If I understand things right, the problem is in validness that turns up
 fro simplification and scaling.

 We also want tile generation to be as fast as possible for "on the fly"
 creation.

 The idea is about the simplification issue. If we can identify vertex
 points that have to be preserved to not create an invalid geometry then:
 If using the effective area from Visvalingam-Whyatt  algorithm, and
 manipulating the effective area (stored as m-coordinate, it is possible to
 both control geometries getting invalid and at the same time get a very
 fast "on the fly" simplification.

 This require doing some preparation, and would be a "professional
 alternative" when needing fast creation and valid geometries.

 The function ST_SimplifyVW do it like this when avoiding collapsing
 polygons. It identifies the most valuable vertex-points and raises their
 effective area to very high value.

 The hard part is identifying what vertex_points that need to be preserved
 I guess. Then the m-value of that vertex-points just have to be raised to
 something insane.

 It would mean that m-values had to be stored which will take some space
 (only in PostGIS, not in the tile of course).

 Link to ST_EffectiveArea:
 http://postgis.net/docs/manual-2.3/ST_SetEffectiveArea.html

 What I am talking about, I realise, is actually ST_EffectiveArea with some
 validity-guarantee for each removed vertex. But I don't know if it is
 possible in any easy way.

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/3712#comment:15>
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