ST_OffsetCurve-PolarDB(PolarDB)-阿里云帮助中心

Returns an offset curve at a given distance from an input line geometry. Useful for computing parallel lines about a center line, such as road corridors and river buffers.

Syntax

geometry ST_OffsetCurve(geometry line, float signedDistance, text styleParameters)

Parameters

Parameter	Type	Description
`line`	geometry	The input line geometry.
`signedDistance`	float	The offset distance, measured in the units of the spatial reference system (SRS) of the input geometry. A positive value offsets to the left; a negative value offsets to the right.
`styleParameters`	text	Style options for the offset curve. Defaults to an empty string. See Style parameters for details.

Usage notes

All points on the returned geometry are within the specified distance from the input geometry.
A positive signedDistance places the offset curve on the left side of the input geometry, in the same direction. A negative value places it on the right side, in the opposite direction.
Z coordinates are ignored. If the input is a 3D geometry object, the function returns a 2D geometry object.
Accepts GeometryCollection and MultiLineString as input.

Style parameters

Pass style options as a space-separated string in styleParameters, for example: 'quad_segs=4 join=round'.

Field	Type	Default	Description
`quad_segs`	integer	`8`	Number of segments used to approximate a quarter circle. A higher value produces a smoother curve.
`join`	string	`round`	Join style for the offset curve. Valid values: `round`, `mitre`, `bevel`.
`mitre_limit`	float	—	Mitre limit for the join. Applies only when `join` is set to `mitre`.

Examples

Effect of quad_segs

Compares offset curves generated with quad_segs values of 2, 3, and the default (8):

SELECT
  ST_CurveToLine(ST_OffsetCurve(g, 1,   'quad_segs=2')),
  ST_CurveToLine(ST_OffsetCurve(g, 1.1, 'quad_segs=3')),
  ST_CurveToLine(ST_OffsetCurve(g, 1.2))
FROM (SELECT 'LINESTRING(0 0,0 1,1 1)'::geometry AS g) AS t;

Effect of join style

Compares round, mitre, and bevel join styles:

SELECT
  ST_CurveToLine(ST_OffsetCurve(g, 1,   'join=round')),
  ST_CurveToLine(ST_OffsetCurve(g, 1.1, 'join=mitre')),
  ST_CurveToLine(ST_OffsetCurve(g, 1.2, 'join=bevel')),
  g
FROM (SELECT 'LINESTRING(0 0,0 1,1 1)'::geometry AS g) AS t;

Effect of mitre_limit

Compares mitre_limit values of 0.1, 0.5, and 1 with join=mitre:

SELECT
  ST_CurveToLine(ST_OffsetCurve(g, 1,   'join=mitre mitre_limit=0.1')),
  ST_CurveToLine(ST_OffsetCurve(g, 1.1, 'join=mitre mitre_limit=0.5')),
  ST_CurveToLine(ST_OffsetCurve(g, 1.2, 'join=mitre mitre_limit=1')),
  g
FROM (SELECT 'LINESTRING(0 0,0 1,1 1)'::geometry AS g) AS t;

Positive vs. negative signed distance

Compares left-side (positive) and right-side (negative) offsets:

SELECT
  ST_CurveToLine(ST_OffsetCurve(g,  2)),
  ST_CurveToLine(ST_OffsetCurve(g, -2)),
  g
FROM (SELECT 'LINESTRING(0 0,0 1,1 2)'::geometry AS g) AS t;