ST_MapAlgebra - PolarDB - Alibaba Cloud Documentation Center

Syntax

raster ST_MapAlgebra(raster[] rasters ,
        cstring algebraExpr  default NULL,
        cstring storageoption default '')

Parameters


Parameter	Description
rasters	The original raster objects that you want to transform.
algebraExpr	The JSON string that specifies the algebraic expressions that you want to use.
storageOption	The JSON string that specifies how to store the new raster object.

Note The system only requires that the lengths and widths of the original raster objects are in the same unit of measurement. It does not check their spatial reference systems or resolutions. If the lengths and widths of the original raster objects are in different units of measurement, you can use the ST_Transform, ST_Resize, and ST_Clip functions to convert them into the same unit.

Each child JSON object in the JSON string specified by the algebraExpr parameter represents a field dictated to an algebraic expression. The following table describes these fields.


Field	Description	Type	Default value	Setting notes
algebraExpr	The algebraic expression that you want to use.	String	N/A	N/A.
nodata	Specifies whether nodata values are valid.	Boolean	false	If you set this field to true, nodata values are valid and pixels with nodata values are not transformed. If you set this field to false, nodata values are invalid and pixels with nodata values are transformed.
nodataValue	The nodata value to return for the new raster object.	float8	0	N/A.

The algebraic expression specified by the algebraExpr field contains the following keywords:

[r, b]
- r: indicates the ID of the new raster object in the array specified by the rasters parameter. Format: 0-n-1.
- b: indicates the band number of the new raster object. Format: 0-n-1.
x
The sequence number of the column where the specified pixel resides.
y
The sequence number of the row where the specified pixel resides.

The following table lists the operations that you can incorporate in an algebraic expression.


Type	Operator or function	Remarks
Operators	+ - * / % (remainder) ** (power)	N/A.
Bitwise operations	<< >> & \| ^	N/A.
Logical operations	< > == != <= >= && \|\| !	N/A.
Operations on functions	abs sqrt exp log ln sin cos tan sinh cosh tanh arcsin arccos arctan ceil floor round	You can specify only one operation on a function.
Statistical functions	min max sum mean majority minority std median range variety	You must specify two or more statistical functions.

Example 1
Transform the original raster object into a new raster object that only has one band. The return result is as follows: 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

Compute the variance of three bands.

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

Example 3

Transform the original raster objects into a new raster object that has three bands. Each band is transformed by using a unique expression.

[
    {
        "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 following table describes fields in the storageOption parameter.


Field	Description	Type	Default value	Setting notes
chunking	Specifies whether to store the new raster object as chunks.	Boolean	Same as the original raster object	N/A.
chunkdim	The dimensions used to store the new raster object as chunks.	String	Same as the original raster object	This field only takes effect when the chunking field is set to true.
chunktable	The name of the chunk table.	String	Null string ('')	By default, a temporary chunk table with a random name is generated to store data. This temporary chunk table is only valid in the current session. To save the new raster object permanently, you must specify you want to create a permanent chunk table in the chunktable field.
compression	The format used for image compression.	String	Same as the original raster object	Six compression formats are supported: None, JPEG, Zlib, PNG, LZO, and LZ4.
quality	The image quality of the new raster object.	Integer	Same as the original raster object	This field only takes effect in JPEG format.
interleaving	The interleaving type of the new raster object.	String	Same as the original raster object	Valid values: bip: band interleaved by pixel (BIP) bil: band interleaved by line (BIL) bsq: band sequential (BSQ)
endian	The endian format of the new raster object.	String	Same as the original raster object	Valid values: NDR: specifies little endian format. XDR: specifies big endian format.

Examples

-- Create a permanent chunk table.
CREATE TABLE rast_mapalgebra_result(id integer, rast raster);

-- Insert data into a chunk 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"}'
);