ST_Polygon — Returns a multipolygon geometry formed by the union of pixels that have a pixel value that is not no data value. If no band number is specified, band num defaults to 1.
geometry ST_Polygon(raster rast, integer band_num=1);
Changed 3.3.0, validation and fixing is disabled to improve performance. May result invalid geometries.
Availability: 0.1.6 Requires GDAL 1.7 or higher.
Enhanced: 2.1.0 Improved Speed (fully C-Based) and the returning multipolygon is ensured to be valid.
Changed: 2.1.0 In prior versions would sometimes return a polygon, changed to always return multipolygon.
Compare polygonization of every pixel with polygonization after zero is assigned as the NODATA value. The latter preserves the gap formed by zero-valued cells.
WITH source AS (
SELECT ST_SetValues(
ST_AddBand(
ST_MakeEmptyRaster(4, 4, 0, 4, 1, -1, 0, 0, 0),
1, '8BUI', 1, NULL
),
1, 1, 1,
ARRAY[
[1, 1, 1, 1],
[1, 0, 0, 1],
[1, 1, 0, 1],
[1, 1, 1, 1]
]::double precision[][]
) AS rast
), variants AS (
SELECT 'all pixels' AS title, ST_Polygon(rast) AS geom
FROM source
UNION ALL
SELECT
'zero excluded',
ST_Polygon(ST_SetBandNoDataValue(rast, 1, 0))
FROM source
)
SELECT title, geom AS geom
FROM variants
ORDER BY CASE title
WHEN 'all pixels' THEN 1
ELSE 2
END;
title | geom ---------------+---------------------------------------------------------------------- all pixels | MULTIPOLYGON(((0 4,4 4,4 0,0 0,0 4))) zero excluded | MULTIPOLYGON(((0 4,0 0,4 0,4 4,0 4),(1 3,3 3,3 1,2 1,2 2,1 2,1 3))) (2 rows)