postgresql - Postgis - 如何简化多边形的起点

Question

我正在使用 ST_Simplify 来简化一些多边形。它适用于其中的大多数，但我有一些带有不必要起点的多边形，我想将其删除。

这是一个可以简化的多边形示例:

SELECT ST_AsText(ST_Simplify(ST_GeomFromText(
    'POLYGON((0 0, 5 0, 10 0, 10 10, 0 10, 0 0))'
    ), 1))

这是一个无法简化的等效多边形(相同点，不同起点)的示例:

SELECT ST_AsText(ST_Simplify(ST_GeomFromText(
    'POLYGON((5 0, 10 0, 10 10, 0 10, 0 0, 5 0))'
    ), 1))

我不希望 Douglas-Peucker 算法 (ST_Simplify) 改变多边形的起点或终点。我应该怎么做才能简化第二个多边形？

编辑我可能有一些凹多边形:

SELECT ST_AsText(ST_Simplify(ST_GeomFromText(
    'POLYGON((5 0, 10 0, 8 8, 10 10, 0 10, 0 0, 5 0))'
    ), 1))

Answer 1

按照 John Barça 的示例，添加距离测试以简化关闭点，并使其适用于所有几何形状，我得到了这个解决方案:

CREATE OR REPLACE FUNCTION simplify_geometry(geom geometry, tolerance float)
  RETURNS geometry AS
$BODY$

DECLARE geomType text;
DECLARE simplified geometry;
DECLARE testSegment geometry;
DECLARE simplifiedElements geometry[];

BEGIN

  geomType := GeometryType(geom);

  IF ST_IsEmpty(geom) THEN
    RETURN geom;
  ELSIF geomType ~ '(MULTI|COLLECTION)' THEN
    simplifiedElements := array(SELECT simplify_geometry((ST_Dump(geom)).geom, tolerance));
    RETURN ST_Collect(simplifiedElements);
  ELSIF geomType = 'LINESTRING' THEN
    simplified := ST_Simplify(geom, tolerance);

    IF ST_IsClosed(simplified) THEN
      testSegment := ST_MakeLine(ST_PointN(simplified, 2), ST_PointN(simplified, ST_NPoints(simplified)-1));
      IF ST_Distance(ST_StartPoint(simplified), testSegment) < tolerance THEN
        simplified := ST_RemovePoint(simplified, 0);
        simplified := ST_RemovePoint(simplified, ST_NPoints(simplified)-1);
        simplified := ST_AddPoint(simplified, ST_StartPoint(simplified));
      END IF;
    END IF;

    RETURN simplified;
  ELSIF geomType = 'POLYGON' THEN
    simplifiedElements := array(SELECT simplify_geometry(ST_ExteriorRing((ST_DumpRings(geom)).geom), tolerance));
    RETURN ST_MakePolygon(simplifiedElements[1], simplifiedElements[2:array_length(simplifiedElements,1)]);
  ELSE
    RETURN ST_Simplify(geom, tolerance);
  END IF;

END
$BODY$  LANGUAGE plpgsql IMMUTABLE