CG_Simplify
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名称

CG_Simplify — 在保留几何对象关键特征以及 Z/M 值的前提下，降低其复杂度。

大纲

geometry CG_Simplify(geometry geom, double precision threshold, boolean preserveTopology = false);

描述

使用 SFCGAL 的简化算法对几何进行简化，在保留几何关键特征的前提下减少点或顶点数量，并在简化过程中保留 Z 和 M 值。

该算法基于约束三角剖分，使用 CGAL Polyline Simplification 2 库，并额外处理以保留 Z、M 坐标。当要求保持拓扑且几何间存在相交时，会在相交点对 Z 和 M 值进行插值处理。

此函数适用于 类地形的 3D（2.5D）几何，但并不适合处理如墙体等垂直表面。

可用性: 3.6.0 - 需要 SFCGAL >= 2.1.0

该方法需要SFCGAL后端。

该函数支持 3d 并且不会丢失 z-index。

该功能支持M坐标。

参数

geom

输入几何

threshold

最大距离阈值（以几何单位计），用于控制简化程度。值越大，输出几何被简化得就越多。

preserveTopology

如果设为 true，函数会确保几何的拓扑结构被保留；在此模式下，当几何发生相交时，相交点的 Z 和 M 值会进行插值处理。默认值为 false。

返回值

返回一个保留了 Z 和 M 值的简化几何对象。

示例

This example simplifies a polygon with a threshold of 0.5.

Code
SELECT CG_Simplify(
    'POLYGON((0 0,0 1,0.1 1,0.2 1,0.3 1,0.4 1,0.5 1,1 1,1 0,0 0))',
    0.5
  );
栅格输出
POLYGON((0 0,0 1,1 1,1 0,0 0))
Figure
Geometry figure for visual-cg-simplify-01

Preserving topology can keep additional support points in a mixed ZM collection.

Code
SELECT CG_Simplify(
    'GEOMETRYCOLLECTION ZM(
      LINESTRING ZM(-1 -1 3 4,0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150),
      POLYGON ZM((0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150,0 0 10 100))
    )',
    2,
    false
  );
栅格输出
GEOMETRYCOLLECTION ZM (LINESTRING ZM (-1 -1 3 4,2 19 25 250,-4 20 15 150),POLYGON ZM ((0 0 10 100,2 19 25 250,-4 20 15 150,0 0 10 100)))
Figure
Geometry figure for visual-cg-simplify-02

The topology-preserving variant keeps the shared anchor point at the collection intersection.

Code
SELECT CG_Simplify(
    'GEOMETRYCOLLECTION ZM(
      LINESTRING ZM(-1 -1 3 4,0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150),
      POLYGON ZM((0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150,0 0 10 100))
    )',
    2,
    true
  );
栅格输出
GEOMETRYCOLLECTION ZM (LINESTRING ZM (-1 -1 3 4,0 0 10 100,2 19 25 250,-4 20 15 150),POLYGON ZM ((0 0 10 100,2 19 25 250,-4 20 15 150,0 0 10 100)))
Figure
Geometry figure for visual-cg-simplify-03

Compare simplification with and without topology preservation, using a threshold of 50.

Code
WITH data AS (
  SELECT ST_GeomFromText('GEOMETRYCOLLECTION(
    POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)),
    POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46)))
  ') AS geom
)
SELECT geom AS input_geometry,
       CG_Simplify(geom, 50, true) AS topology_preserved,
       CG_Simplify(geom, 50, false) AS topology_not_preserved
FROM data;
栅格输出
GEOMETRYCOLLECTION(POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)), POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46))) | GEOMETRYCOLLECTION(POLYGON((88.46 158.85,147.31 173.85,146.15 103.46,112.69 118.46,34.23 121.92,88.46 158.85)), POLYGON((112.69 118.46,146.15 103.46,185.38 43.46,10 83.85,34.23 121.92,112.69 118.46))) | GEOMETRYCOLLECTION(POLYGON((88.46 158.85,173.85 119.62,34.23 121.92,88.46 158.85)), POLYGON((112.69 118.46,190 60.77,10 83.85,112.69 118.46)))
Figure
Geometry figure for visual-cg-simplify-04