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    PolarDB:ST_CurveRecognize

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    PolarDB:ST_CurveRecognize

    Last Updated:Mar 28, 2026

    Identifies curves in a trajectory and returns the curvature radius and boundary positions of each detected curve.

    Syntax

    • Syntax 1

      SETOF trpeak ST_CurveRecognize (trajectory traj, float8 radius_threshold, float8 angle_threshold, float8 angle_compress_threshold default 0);

    • Syntax 2

      SETOF trpeak ST_CurveRecognize (trajectory traj, float8 radius_threshold, float8 angle_threshold, float8 expansion_radius1, float8 expansion_radius2, float8 angle_compress_threshold default 0);

    Parameters

    ParameterDescription
    trajThe source trajectory.
    radius_thresholdThe curvature radius threshold for detecting curve center points. The function groups consecutive points whose curvature radius is below this threshold and selects the point with the smallest curvature radius as the center point of the curve.
    angle_thresholdThe minimum rotation angle for a curve boundary point. Boundary points with a rotation angle below this value are removed, splitting the curve at those points. After removal, every point on each resulting curve segment has a rotation angle greater than this threshold.
    expansion_radius1The maximum curvature radius allowed at each point adjacent to a curve center. Used to expand the curve boundary outward from the center point. Available in Syntax 2 only. Default value when using Syntax 1: radius_threshold * 2.
    expansion_radius2The maximum average curvature radius from an adjacent point to the curve center. Used together with expansion_radius1 to control boundary expansion. Available in Syntax 2 only. Default value when using Syntax 1: radius_threshold * 4.
    angle_compress_thresholdThe minimum rotation angle required to sample a point. Points with a rotation angle below this value are skipped, reducing sampling density and preventing over-sampling from obscuring curve boundaries. Default value: 0.

    Response parameters:

    ParameterDescription
    locThe serial number of the turning point on the trajectory that serves as the curve center. If loc equals n, the center point is the (n+1)th turning point of the trajectory.
    heightThe curvature radius at the curve center point. A positive value indicates a clockwise curve; a negative value indicates a counterclockwise curve.
    startlocThe start point of the curve.
    endlocThe end point of the curve.

    Description

    ST_CurveRecognize detects curves in a trajectory through three steps:

    1. Center point selection: The function finds all consecutive trajectory points whose curvature radius is smaller than radius_threshold, then selects the point with the smallest curvature radius from that group as the curve center point.

    2. Boundary expansion: Starting from each center point, the function expands the curve boundary to include adjacent points that meet both of the following conditions:

      • The curvature radius at the point is smaller than expansion_radius1.

      • The average curvature radius from that point to the center point is smaller than expansion_radius2.

      Note

      When using Syntax 1, expansion_radius1 and expansion_radius2 default to radius_threshold * 2 and radius_threshold * 4, respectively.

    3. Boundary adjustment: The function adjusts each curve boundary based on angle_threshold:

      • If the rotation angle at the original boundary point exceeds angle_threshold, the boundary expands outward.

      • If the rotation angle at the original boundary point is below angle_threshold, the boundary point is removed and the boundary shrinks inward.

    Examples

    SELECT (ST_CurveRecognize('{"trajectory":{"version":1,"type":"STPOINT","leafcount":16,"start_time":"2000-01-01 00:00:00","end_time":"2000-01-16 00:00:00","spatial":"LINESTRING(0 0,1 1,2 2,4 3,3 4,5 7,8 8,7 4,0 0,1 1,2 2,4 3,3 4,5 7,8 8,7 4)","timeline":["2000-01-01 00:00:00","2000-01-02 00:00:00","2000-01-03 00:00:00","2000-01-04 00:00:00","2000-01-05 00:00:00","2000-01-06 00:00:00","2000-01-07 00:00:00","2000-01-08 00:00:00","2000-01-09 00:00:00","2000-01-10 00:00:00","2000-01-11 00:00:00","2000-01-12 00:00:00","2000-01-13 00:00:00","2000-01-14 00:00:00","2000-01-15 00:00:00","2000-01-16 00:00:00"]}}', 15, 1)).*;

    The query uses radius_threshold=15 and angle_threshold=1. The expected output is:

     loc |      height       | startloc | endloc
-----+-------------------+----------+--------
   2 |  5.70087712549569 |        2 |      2
   4 |  2.10237960416286 |        4 |      8
  10 |  5.70087712549569 |       10 |     10
  12 |  2.10237960416286 |       12 |     15
   3 | -1.17851130197758 |        3 |      3
  11 | -1.17851130197758 |       11 |     11
(6 rows)