ST_Rescale-PolarDB(PolarDB)-阿里云帮助中心

Rescales the pixel dimensions of a raster object while keeping its geospatial extent unchanged. Returns the rescaled raster object.

Syntax

raster ST_Rescale(raster rast,
           f8 scale_x,
           f8 scale_y,
             cstring processexpr default '',
             cstring storageOption default '');

raster ST_Rescale(raster raster,
           f8 scale_xy,
             cstring processexpr default '',
             cstring storageOption default '')

Parameters

Parameter	Description
rast	The raster object to rescale.
scale_x	The scale ratio in the x direction.
scale_y	The scale ratio in the y direction.
scale_xy	The scale ratio applied equally to both the x and y directions.
processExpr	A JSON string that controls how pixels are resampled and how nodata values are handled.
storageOption	A JSON string that controls how the output raster object is stored.

processExpr fields

Field	Type	Default	Description
resample	text	Near	The resampling algorithm. Valid values: `Near`, `Average`, `Cubic`, `Bilinear`.
nodata	Boolean	false	Controls whether nodata values in the source raster are treated as valid. If `true`, pixels with nodata values are preserved and not resampled. If `false`, pixels with nodata values are resampled like normal pixels.
nodataValue	float8 or float8[]	NULL	Assigns new nodata values to the output raster's bands. Specify one value to apply it to all bands, or specify one value per band (the count must match the band count).

Exercise caution when setting nodata and nodataValue. If the source raster has no pixels with nodata values, set nodata to false and omit nodataValue. Otherwise, image artifacts may occur.

storageOption fields

Field	Type	Default	Description
chunking	Boolean	Same as source	Whether to store the output raster as chunks.
chunkdim	String	Same as source	The chunk dimensions. Takes effect only when `chunking` is `true`.
chunktable	String	Empty string (`''`)	The chunk table name. If set to NULL or an empty string (`''`), a temporary chunk table with a randomly generated name is created for the current session and deleted when the session ends. To persist the output raster beyond the current session, specify a permanent chunk table name.
compression	String	Same as source	The compression format. Valid values: `None`, `JPEG`, `Zlib`, `PNG`, `LZO`, `LZ4`.
quality	Integer	Same as source	The image quality level. Takes effect only in JPEG format.
interleaving	String	Same as source	The interleaving format. Valid values: `bip` (band interleaved by pixel, BIP), `bil` (band interleaved by line, BIL), `bsq` (band sequential, BSQ).
endian	String	Same as source	The byte order. Valid values: `NDR` (little endian), `XDR` (big endian).

Examples

The examples below use datasource_table as the source table, which is loaded from a GeoTIFF file. Method 1 writes to a permanent chunk table; method 2 writes to a temporary chunk table valid only in the current session.

CREATE TABLE IF NOT EXISTS datasource_table(id integer, rast raster);
INSERT INTO datasource_table VALUES(1, ST_ImportFrom('rbt','$(RAST_DATA_DIR)/512_512_1_bsq_8u_geo.tif', '{}'));

Method 1: Write to a permanent chunk table

Specify chunktable to persist the output raster beyond the current session.

CREATE TABLE rat_rescale_result(id integer, rast raster);

-- Rescale without specifying a nodata value
INSERT INTO rat_rescale_result(id, rast)
SELECT 10, ST_Rescale(rast, 1024, 1024,
    '{"resample":"Near","nodata":false}',
    '{"chunking":true,"chunkdim":"(256,256,1)","compression":"none","interleaving":"bsq","chunktable":"result_rbt"}')
FROM datasource_table
WHERE id = 1;

-- Rescale with a single nodata value; pixels with nodata values are preserved
INSERT INTO rat_rescale_result(id, rast)
SELECT 11, ST_Rescale(rast, 1024, 1024,
    '{"resample":"Near","nodata":true,"nodataValue":255}',
    '{"chunking":true,"chunkdim":"(256,256,1)","compression":"none","interleaving":"bsq","chunktable":"result_rbt"}')
FROM datasource_table
WHERE id = 1;

-- Rescale with per-band nodata values
INSERT INTO rat_rescale_result(id, rast)
SELECT 12, ST_Rescale(rast, 1024, 1024,
    '{"resample":"Near","nodata":false,"nodataValue":[255,255,255]}',
    '{"chunking":true,"chunkdim":"(256,256,1)","compression":"none","interleaving":"bsq","chunktable":"result_rbt"}')
FROM datasource_table
WHERE id = 1;

Method 2: Write to a temporary chunk table

Omit chunktable to write to a session-scoped temporary chunk table. Use this approach for intermediate computations within the same session.

CREATE TEMP TABLE rat_rescale_result_temp(id integer, rast raster);

INSERT INTO rat_rescale_result_temp(id, rast)
SELECT 1, ST_Rescale(rast, 1024, 1024,
    '{"resample":"Near","nodata":false,"nodataValue":[255,255,255]}',
    '{"chunking":true,"chunkdim":"(256,256,1)","compression":"none","interleaving":"bsq"}')
FROM datasource_table
WHERE id = 1;