Name
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。
Synopsis
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
-- Simplify a polygon with a threshold of 0.5
SELECT ST_AsText(CG_Simplify(ST_GeomFromText('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));
-- Simplify a 3D terrain geometry while preserving topology and Z values
SELECT ST_AsText(CG_Simplify(ST_GeomFromText('LINESTRING Z(0 0 0, 0 1 1, 0.1 1 1, 0.2 1 1, 0.3 1 1, 1 1 2)'), 0.2, true));
-- Simplify a geometry with both Z and M values
SELECT ST_AsText(CG_Simplify(ST_GeomFromText('LINESTRING ZM(0 0 0 1, 0 1 1 2, 0.1 1 1 3, 0.2 1 1 4, 0.3 1 1 5, 1 1 2 6)'), 0.2));
-- Simplify two geometry together preserving Z and M values, without topology
SELECT ST_AsText(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));
-- Simplify two geometry together preserving Z and M values, with topology
SELECT ST_AsText(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));
WITH depts_pds 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 FROM depts_pds;
原始几何
WITH depts_pds 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 (ST_Dump(CG_Simplify(geom, 0.5, true))).geom FROM depts_pds;
以 0.5 进行简化，并保持拓扑
WITH depts_pds 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 (ST_Dump(CG_Simplify(geom, 0.5, false))).geom FROM depts_pds;
以 0.5 进行简化