Description

Create a raster with one 8BUI band given three input bands (from one or more rasters). Any input band whose pixel type is not 8BUI will be reclassified using ST_Reclass.

	This function is not like ST_ColorMap with the grayscale keyword as ST_ColorMap operates on only one band while this function expects three bands for RGB. This function applies the following equation for converting RGB to Grayscale: 0.2989 * RED + 0.5870 * GREEN + 0.1140 * BLUE

Availability: 2.5.0

Examples: Variant 1

SET postgis.gdal_enabled_drivers = 'ENABLE_ALL';
SET postgis.enable_outdb_rasters = True;

WITH apple AS (
	SELECT ST_AddBand(
		ST_MakeEmptyRaster(350, 246, 0, 0, 1, -1, 0, 0, 0),
		'/tmp/apple.png'::text,
		NULL::int[]
	) AS rast
)
SELECT
	ST_AsPNG(rast) AS original_png,
	ST_AsPNG(ST_Grayscale(rast)) AS grayscale_png
FROM apple;
					

original_png

grayscale_png

Examples: Variant 2

SET postgis.gdal_enabled_drivers = 'ENABLE_ALL';
SET postgis.enable_outdb_rasters = True;

WITH apple AS (
	SELECT ST_AddBand(
		ST_MakeEmptyRaster(350, 246, 0, 0, 1, -1, 0, 0, 0),
		'/tmp/apple.png'::text,
		NULL::int[]
	) AS rast
)
SELECT
	ST_AsPNG(rast) AS original_png,
	ST_AsPNG(ST_Grayscale(
		ARRAY[
			ROW(rast, 1)::rastbandarg, -- red
			ROW(rast, 2)::rastbandarg, -- green
			ROW(rast, 3)::rastbandarg, -- blue
		]::rastbandarg[]
	)) AS grayscale_png
FROM apple;
					

See Also

ST_AsPNG, ST_Reclass, ST_ColorMap