描述

创建一个新的单波段栅格，该栅格通过对输入栅格 rast1、rast2 应用 tworastuserfunc 指定的有效的 PostgreSQL 函数而形成。如果未指定 band1 或 band2，则假定为波段 1。新栅格将具有与原始栅格相同的地理参考、宽度和高度，但仅有一个波段。

如果传入了 pixeltype，则新栅格将具有该像素类型的波段。如果传入的像素类型为 NULL 或省略，则新栅格波段将具有与输入 rast1 波段相同的像素类型。

tworastuserfunc 参数必须是 SQL 或 PL/pgSQL 函数的名称和签名，并强制转换为 regprocedure。一个 PL/pgSQL 函数示例是

CREATE OR REPLACE FUNCTION simple_function_for_two_rasters(pixel1 FLOAT, pixel2 FLOAT, pos INTEGER[], VARIADIC args TEXT[])
    RETURNS FLOAT
    AS $$ BEGIN
        RETURN 0.0;
    END; $$
    LANGUAGE 'plpgsql' IMMUTABLE;

tworastuserfunc 可以接受三个或四个参数：一个双精度值、一个双精度值、一个可选的整数数组和一个可变文本数组。第一个参数是 rast1 中单个栅格单元的值（无论栅格数据类型如何）。第二个参数是 rast2 中单个栅格单元的值。第三个参数是以“{x,y}”形式表示的当前处理单元的位置。第四个参数表示 ST_MapAlgebraFct 的所有剩余参数都应传递给 tworastuserfunc。

将 regprodedure 参数传递给 SQL 函数需要传递完整的函数签名，然后将其强制转换为 regprocedure 类型。要将上述示例 PL/pgSQL 函数作为参数传递，则参数的 SQL 为

'simple_function(double precision, double precision, integer[], text[])'::regprocedure

请注意，该参数包含函数名称、函数参数的类型、名称和参数类型周围的引号，以及强制转换为 regprocedure。

tworastuserfunc 的第四个参数是一个 variadic text 数组。 对任何 ST_MapAlgebraFct 调用中所有尾随文本参数都将传递给指定的 tworastuserfunc，并包含在 userargs 参数中。

可用性：2.0.0

示例：在画布上将栅格作为单独的波段进行叠加

-- define our user defined function --
CREATE OR REPLACE FUNCTION raster_mapalgebra_union(
    rast1 double precision,
    rast2 double precision,
    pos integer[],
    VARIADIC userargs text[]
)
    RETURNS double precision
    AS $$
    DECLARE
    BEGIN
        CASE
            WHEN rast1 IS NOT NULL AND rast2 IS NOT NULL THEN
                RETURN ((rast1 + rast2)/2.);
            WHEN rast1 IS NULL AND rast2 IS NULL THEN
                RETURN NULL;
            WHEN rast1 IS NULL THEN
                RETURN rast2;
            ELSE
                RETURN rast1;
        END CASE;

        RETURN NULL;
    END;
    $$ LANGUAGE 'plpgsql' IMMUTABLE COST 1000;

-- prep our test table of rasters
DROP TABLE IF EXISTS map_shapes;
CREATE TABLE map_shapes(rid serial PRIMARY KEY, rast raster, bnum integer, descrip text);
INSERT INTO map_shapes(rast,bnum, descrip)
WITH mygeoms
    AS ( SELECT 2 As bnum, ST_Buffer(ST_Point(90,90),30) As geom, 'circle' As descrip
            UNION ALL
            SELECT 3 AS bnum,
                ST_Buffer(ST_GeomFromText('LINESTRING(50 50,150 150,150 50)'), 15) As geom, 'big road' As descrip
            UNION ALL
            SELECT 1 As bnum,
                ST_Translate(ST_Buffer(ST_GeomFromText('LINESTRING(60 50,150 150,150 50)'), 8,'join=bevel'), 10,-6) As geom, 'small road' As descrip
            ),
   -- define our canvas to be 1 to 1 pixel to geometry
   canvas
    AS ( SELECT ST_AddBand(ST_MakeEmptyRaster(250,
        250,
        ST_XMin(e)::integer, ST_YMax(e)::integer, 1, -1, 0, 0 ) , '8BUI'::text,0) As rast
        FROM (SELECT ST_Extent(geom) As e,
                    Max(ST_SRID(geom)) As srid
                    from mygeoms
                    ) As foo
            )
-- return our rasters aligned with our canvas
SELECT ST_AsRaster(m.geom, canvas.rast, '8BUI', 240) As rast, bnum, descrip
                FROM mygeoms AS m CROSS JOIN canvas
UNION ALL
SELECT canvas.rast, 4, 'canvas'
FROM canvas;

-- Map algebra on single band rasters and then collect with ST_AddBand
INSERT INTO map_shapes(rast,bnum,descrip)
SELECT ST_AddBand(ST_AddBand(rasts[1], rasts[2]),rasts[3]), 4, 'map bands overlay fct union (canvas)'
    FROM (SELECT ARRAY(SELECT ST_MapAlgebraFct(m1.rast, m2.rast,
            'raster_mapalgebra_union(double precision, double precision, integer[], text[])'::regprocedure, '8BUI', 'FIRST')
                FROM map_shapes As m1 CROSS JOIN map_shapes As m2
    WHERE m1.descrip = 'canvas' AND m2.descrip <> 'canvas' ORDER BY m2.bnum) As rasts) As foo;

接受额外参数的用户定义函数

CREATE OR REPLACE FUNCTION raster_mapalgebra_userargs(
    rast1 double precision,
    rast2 double precision,
    pos integer[],
    VARIADIC userargs text[]
)
    RETURNS double precision
    AS $$
    DECLARE
    BEGIN
        CASE
            WHEN rast1 IS NOT NULL AND rast2 IS NOT NULL THEN
                RETURN least(userargs[1]::integer,(rast1 + rast2)/2.);
            WHEN rast1 IS NULL AND rast2 IS NULL THEN
                RETURN userargs[2]::integer;
            WHEN rast1 IS NULL THEN
                RETURN greatest(rast2,random()*userargs[3]::integer)::integer;
            ELSE
                RETURN greatest(rast1, random()*userargs[4]::integer)::integer;
        END CASE;

        RETURN NULL;
    END;
    $$ LANGUAGE 'plpgsql' VOLATILE COST 1000;

SELECT ST_MapAlgebraFct(m1.rast, 1, m1.rast, 3,
            'raster_mapalgebra_userargs(double precision, double precision, integer[], text[])'::regprocedure,
                '8BUI', 'INTERSECT', '100','200','200','0')
                FROM map_shapes As m1
    WHERE m1.descrip = 'map bands overlay fct union (canvas)';
                    

另请参阅

ST_MapAlgebraExpr, ST_BandPixelType, ST_GeoReference, ST_SetValue