ST_MapAlgebra for algebraic and analytical raster operations-ApsaraDB RDS(RDS)-阿里云帮助中心

The ST_MapAlgebra function generates a new raster object by computing pixel values from multiple source objects based on an algebraic expression.

Syntax

raster ST_MapAlgebra(raster[] rasters ,
      cstringalgebraExpr  default NULL,
     cstringstorageoptiondefault '')

Parameters

Parameter Name	Description
rasters	An array of raster objects to use in the algebraic operation.
algebraExpr	A JSON string that represents the algebraic expression.
storageOption	A JSON string that represents the storage options for the result.

Note

The input raster objects must have the same height and width. The function does not check for spatial reference or resolution. If these properties do not match, you must first process the objects using ST_Transform, ST_Resize, and ST_Clip.

The algebraExpr parameter is a JSON array of objects. Each object specifies an algebraic expression. The parameters are as follows.

Parameter	Description	Type	Default	Notes
algebraExpr	The algebraic expression.	string	-	-
nodata	Specifies whether to use nodata.	boolean	false	If true, and a pixel value is nodata, the corresponding pixel in the output raster is also nodata. If the value is false, it is treated as a number.
nodataValue	The nodata value.	float8	0	-

The algebraic expression for algebraExpr supports the following keywords.

[r, b]
- r: The ID of the raster in the array. The value can be an integer from 0 to n-1.
- b: The band number of the corresponding raster. The value can be an integer from 0 to n-1.
x
The column number of the pixel.
y
The row number of the pixel.

The expression supports the following operations:

Category	Operator/Function	Notes
Operators	+ - * / % (remainder) ** (power)	-
Bitwise operations	<< >> & \| ^	-
Logical operations	< > == != <= >= && \|\| !	-
Mathematical functions	abs sqrt exp log ln sin cos tan sinh cosh tanh arcsin arccos arctan ceil floor round	Takes one parameter.
Statistical functions	min max sum mean majority minority std median range variety	Takes at least two parameters.

Example 1
This example generates a raster object with one band. The value of each pixel is calculated as raster[0]band[0] + raster[1]band[0] * raster[1]band[1].
```
[
    {
        "expr":"([0,0] + [1,0] * [1,1]) ",
        "nodata": true,
        "nodataValue":999
    }
]
```

Example 2

This example calculates the variance of three bands.

[
    {
        "expr":"(std([0,0],[0,1],[0,2]))",
        "nodata": true,
        "nodataValue":999
    }
]

Example 3

This example generates a raster with three bands. A different expression is used to calculate each band.

[
    {
        "expr":"(min([0,0],[0,1],[0,2]))",
        "nodata": true,
        "nodataValue":999
    },
    {
        "expr":"(max([0,0],[0,1],[0,2]))",
        "nodata": true,
        "nodataValue":999
    },
    {
        "expr":"(mean([0,0],[0,1],[0,2]))",
        "nodata": true,
        "nodataValue":999
    }
]

The storageOption parameter is described as follows.

Parameter Name	Description	Type	Default	Notes
chunking	Specifies whether to use chunk storage.	boolean	Same as the source raster	-
chunkdim	The dimension information for the chunks.	string	Same as the source raster	This parameter is valid only when chunking is set to true.
chunktable	The name of the chunk table.	string	''	If you pass an empty string (''), a temporary chunk table with a random name is created to store the data. This temporary table is valid only for the current session. To keep an accessible raster object, specify a chunk table name.
compression	The compression algorithm.	string	Same as the source raster	Supported algorithms: none, jpeg, zlib, png, lzo, and lz4.
quality	The compression quality.	integer	Same as the source raster	This parameter applies only to the jpeg compression algorithm.
interleaving	The interleaving method.	string	Same as the source raster	Must be one of the following: bip: Band interleaved by pixel BIL: Band Interleaved by Pixel bsq: Band sequential
endian	The byte order.	string	Same as the source raster	Must be one of the following: NDR: Little-endian XDR: Big-endian

Examples

-- A permanent table
CREATE TABLE rast_mapalgebra_result(id integer, rast raster);

-- Insert data into the table
WITH foo AS (
  SELECT 1 AS rid, rast AS rast from t1 WHERE id = 1
UNION ALL
  SELECT 2 AS rid, rast AS rast from t2 WHERE id = 2
)
INSERT INTO rast_mapalgebra_result
SELECT 1, ST_MapAlgebra(
    ARRAY(SELECT rast FROM foo ORDER BY rid),
    '[{"expr":"([0,0] + 0.5 * [1,0] - ([1,1])","nodata": true, "nodataValue":999}]',
    '{"chunktable":"algebra_rbt"}'
);