ST_AsTile function for cropping raster objects into tiles - PolarDB

Crops a 256 or 512 pixel tile from a raster object within the specified geometry bounds and returns it as a binary stream. Outputs PNG format with all bands included by default.

Syntax

record ST_AsTile(raster raster_obj, geometry geom, cstring export_options default '');

Return values

Returns a record with two fields:

Field	Type	Description
`ext`	—	The image format of the output tile
`data`	bytea	The binary tile data (256 or 512 pixels)

Parameters

Parameter	Type	Description
`raster_obj`	raster	The input grid object
`geom`	geometry	The geometry object that defines the tile bounds
`export_options`	cstring	A JSON string of export options. Defaults to `''` (empty string).

Export options

Pass export_options as a JSON string. All fields are optional.

Option	Type	Default	Description
`bands`	cstring	`''`	The bands to include in the output, separated by commas. By default, all bands are included.
`format`	cstring	`PNG`	The output image format. Valid values: `PNG`, `JPEG`, `GTiff`.
`pyramid_level`	integer	`-1`	The pyramid level to use. Defaults to `-1`, which selects the optimal level automatically.
`dim`	integer	`256`	The pixel size of the output tile. Valid values: `256`, `512`.
`nodata`	boolean	`true`	Specifies whether to apply the nodata value. If you do not specify this option, the nodata value of the object is used.
`nodatavalue`	f8	`0`	The nodata value to apply. Takes effect only when `nodata` is `true`.
`strength`	string	`none`	The display enhancement method. Valid values: `none` (no enhancement), `stats` (stretch based on statistical values), `ratio` (stretch based on ratios).
`ratio_offset`	integer	`2`	The percentage offset for ratio-based enhancement. Stretches the output from the nth to the (100-n)th percentile. Takes effect only when `strength` is `ratio`.
`alpha`	boolean	`false`	Specifies whether to add an alpha channel for transparency.
`quality`	integer	`75`	The compression quality. Valid values: `0`-`100`. The value `0` indicates the poorest compression quality, and the value `100` indicates the optimal compression quality.

Examples

Export bands 0, 1, and 2 as PNG

SELECT ST_AsTile(rast,
    ST_GeomFromText('POLYGON((-80 30,-100 30, -100 100, -80 100, -80 30))',4326),
    '{"format":"PNG","bands":"0,1,2"}')
FROM raster_table;

Export bands 0, 1, and 2 as GTiff at pyramid level 7

SELECT ST_AsTile(rast,
    ST_GeomFromText('POLYGON((-80 30,-100 30, -100 100, -80 100, -80 30))',4326),
    '{"format":"GTiff","bands":"0,1,2","pyramid_level":7}')
FROM raster_table;

Export a PNG tile with statistical enhancement and transparency

SELECT ST_AsTile(rast,
    ST_GeomFromText('POLYGON((-80 30,-100 30, -100 100, -80 100, -80 30))',4326),
    '{"format":"PNG","bands":"0,1,2","strength":"stats","alpha":true}')
FROM raster_table;