H2GIS

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ST_TriangleDirection

Signature

LINESTRING ST_TriangleDirection(GEOMETRY geom);

Description

Computes the direction of steepest ascent of the triangle geom and represents it as a LINESTRING. Returns LINESTRING EMPTY if all vertices of the triangle have the same z-value. Throws an error if geom is not a triangle.

Examples

SELECT ST_TriangleDirection('POLYGON((0 0 0, 4 0 0, 2 3 6, 0 0 0))');
-- Answer: LINESTRING(2 1 2, 2 0 0)

-- In the next two examples, all z-values are the same.
SELECT ST_TriangleDirection('POLYGON((0 0 0, 2 0 0, 1 1 0, 0 0 0))');
-- Answer: LINESTRING EMPTY
SELECT ST_TriangleDirection('POLYGON((0 0 1, 2 0 1, 1 1 1, 0 0 1))');
-- Answer: LINESTRING EMPTY

-- We try the function on a square.
SELECT ST_TriangleDirection(
    'POLYGON((0 0 1, 3 0 0, 3 3 4, 0 3 1, 0 0 1))');
-- Exception calling user-defined function:
--     "computeDirection(POLYGON ((0 0, 3 0, 3 3, 0 3, 0 0))):
--     The geometry must be a triangle"

See also