ST_MapAlgebraExpr
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## Name

ST_MapAlgebraExpr — 2 raster band version: Creates a new one band raster formed by applying a valid PostgreSQL algebraic operation on the two input raster bands and of pixeltype provided. band 1 of each raster is assumed if no band numbers are specified. The resulting raster will be aligned (scale, skew and pixel corners) on the grid defined by the first raster and have its extent defined by the "extenttype" parameter. Values for "extenttype" can be: INTERSECTION, UNION, FIRST, SECOND.

## Synopsis

`raster ST_MapAlgebraExpr(`raster rast1, raster rast2, text expression, text pixeltype=same_as_rast1_band, text extenttype=INTERSECTION, text nodata1expr=NULL, text nodata2expr=NULL, double precision nodatanodataval=NULL`)`;

`raster ST_MapAlgebraExpr(`raster rast1, integer band1, raster rast2, integer band2, text expression, text pixeltype=same_as_rast1_band, text extenttype=INTERSECTION, text nodata1expr=NULL, text nodata2expr=NULL, double precision nodatanodataval=NULL`)`;

## Description

ST_MapAlgebraExpr is deprecated as of 2.1.0. Use ST_MapAlgebra (expression version) instead.

Creates a new one band raster formed by applying a valid PostgreSQL algebraic operation to the two bands defined by the `expression` on the two input raster bands `rast1`, (`rast2`). If no `band1`, `band2` is specified band 1 is assumed. The resulting raster will be aligned (scale, skew and pixel corners) on the grid defined by the first raster. The resulting raster will have the extent defined by the `extenttype` parameter.

expression

  A PostgreSQL algebraic expression involving the two rasters and PostgreSQL defined functions/operators that will define the pixel value when pixels intersect. e.g. (([rast1] + [rast2])/2.0)::integer

pixeltype

  The resulting pixel type of the output raster. Must be one listed in ST_BandPixelType, left out or set to NULL. If not passed in or set to NULL, will default to the pixeltype of the first raster.

extenttype

  Controls the extent of resulting raster

  1. `INTERSECTION` - The extent of the new raster is the intersection of the two rasters. This is the default.

  2. `UNION` - The extent of the new raster is the union of the two rasters.

  3. `FIRST` - The extent of the new raster is the same as the one of the first raster.

  4. `SECOND` - The extent of the new raster is the same as the one of the second raster.

nodata1expr

  An algebraic expression involving only `rast2` or a constant that defines what to return when pixels of `rast1` are nodata values and spatially corresponding rast2 pixels have values.

nodata2expr

  An algebraic expression involving only `rast1` or a constant that defines what to return when pixels of `rast2` are nodata values and spatially corresponding rast1 pixels have values.

nodatanodataval

  A numeric constant to return when spatially corresponding rast1 and rast2 pixels are both nodata values.

If `pixeltype` is passed in, then the new raster will have a band of that pixeltype. If pixeltype is passed NULL or no pixel type specified, then the new raster band will have the same pixeltype as the input `rast1` band.

Use the term `[rast1.val]` `[rast2.val]` to refer to the pixel value of the original raster bands and `[rast1.x]`, `[rast1.y]` etc. to refer to the column / row positions of the pixels.

Availability: 2.0.0

## Example: 2 Band Intersection and Union

Create a new 1 band raster from our original that is a function of modulo 2 of the original raster band.

    --Create a cool set of rasters --
    DROP TABLE IF EXISTS fun_shapes;
    CREATE TABLE fun_shapes(rid serial PRIMARY KEY, fun_name text, rast raster);

    -- Insert some cool shapes around Boston in Massachusetts state plane meters --
    INSERT INTO fun_shapes(fun_name, rast)
    VALUES ('ref', ST_AsRaster(ST_MakeEnvelope(235229, 899970, 237229, 901930,26986),200,200,'8BUI',0,0));

    INSERT INTO fun_shapes(fun_name,rast)
    WITH ref(rast) AS (SELECT rast FROM fun_shapes WHERE fun_name = 'ref' )
    SELECT 'area' AS fun_name, ST_AsRaster(ST_Buffer(ST_SetSRID(ST_Point(236229, 900930),26986), 1000),
                ref.rast,'8BUI', 10, 0) As rast
    FROM ref
    UNION ALL
    SELECT 'rand bubbles',
                ST_AsRaster(
                (SELECT ST_Collect(geom)
        FROM (SELECT ST_Buffer(ST_SetSRID(ST_Point(236229 + i*random()*100, 900930 + j*random()*100),26986), random()*20) As geom
                FROM generate_series(1,10) As i, generate_series(1,10) As j
                ) As foo ), ref.rast,'8BUI', 200, 0)
    FROM ref;

    --map them -
    SELECT  ST_MapAlgebraExpr(
            area.rast, bub.rast, '[rast2.val]', '8BUI', 'INTERSECTION', '[rast2.val]', '[rast1.val]') As interrast,
            ST_MapAlgebraExpr(
                area.rast, bub.rast, '[rast2.val]', '8BUI', 'UNION', '[rast2.val]', '[rast1.val]') As unionrast
    FROM
      (SELECT rast FROM fun_shapes WHERE
     fun_name = 'area') As area
    CROSS JOIN  (SELECT rast
    FROM fun_shapes WHERE
     fun_name = 'rand bubbles') As bub

mapalgebra intersection | map algebra union

## Example: Overlaying rasters on a canvas as separate bands

    -- we use ST_AsPNG to render the image so all single band ones look grey --
    WITH mygeoms
        AS ( SELECT 2 As bnum, ST_Buffer(ST_Point(1,5),10) As geom
                UNION ALL
                SELECT 3 AS bnum,
                    ST_Buffer(ST_GeomFromText('LINESTRING(50 50,150 150,150 50)'), 10,'join=bevel') As geom
                UNION ALL
                SELECT 1 As bnum,
                    ST_Buffer(ST_GeomFromText('LINESTRING(60 50,150 150,150 50)'), 5,'join=bevel') As geom
                ),
       -- define our canvas to be 1 to 1 pixel to geometry
       canvas
        AS (SELECT ST_AddBand(ST_MakeEmptyRaster(200,
            200,
            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
                ),
       rbands AS (SELECT ARRAY(SELECT ST_MapAlgebraExpr(canvas.rast, ST_AsRaster(m.geom, canvas.rast, '8BUI', 100),
                     '[rast2.val]', '8BUI', 'FIRST', '[rast2.val]', '[rast1.val]') As rast
                    FROM mygeoms AS m CROSS JOIN canvas
                    ORDER BY m.bnum) As rasts
                    )
              SELECT rasts[1] As rast1 , rasts[2] As rast2, rasts[3] As rast3, ST_AddBand(
                        ST_AddBand(rasts[1],rasts[2]), rasts[3]) As final_rast
                FROM rbands;

rast1 | rast2
rast3 | final_rast

## Example: Overlay 2 meter boundary of select parcels over an aerial imagery

    -- Create new 3 band raster composed of first 2 clipped bands, and overlay of 3rd band with our geometry
    -- This query took 3.6 seconds on PostGIS windows 64-bit install
    WITH pr AS
    -- Note the order of operation: we clip all the rasters to dimensions of our region
    (SELECT ST_Clip(rast,ST_Expand(geom,50) ) As rast, g.geom
        FROM aerials.o_2_boston AS r INNER JOIN
    -- union our parcels of interest so they form a single geometry we can later intersect with
            (SELECT ST_Union(ST_Transform(geom,26986)) AS geom
              FROM landparcels WHERE pid IN('0303890000', '0303900000')) As g
            ON ST_Intersects(rast::geometry, ST_Expand(g.geom,50))
    ),
    -- we then union the raster shards together
    -- ST_Union on raster is kinda of slow but much faster the smaller you can get the rasters
    -- therefore we want to clip first and then union
    prunion AS
    (SELECT ST_AddBand(NULL, ARRAY[ST_Union(rast,1),ST_Union(rast,2),ST_Union(rast,3)] ) As clipped,geom
    FROM pr
    GROUP BY geom)
    -- return our final raster which is the unioned shard with
    -- with the overlay of our parcel boundaries
    -- add first 2 bands, then mapalgebra of 3rd band + geometry
    SELECT ST_AddBand(ST_Band(clipped,ARRAY[1,2])
        , ST_MapAlgebraExpr(ST_Band(clipped,3), ST_AsRaster(ST_Buffer(ST_Boundary(geom),2),clipped, '8BUI',250),
         '[rast2.val]', '8BUI', 'FIRST', '[rast2.val]', '[rast1.val]') ) As rast
    FROM prunion;

The blue lines are the boundaries of select parcels

## See Also

ST_MapAlgebraExpr, ST_AddBand, ST_AsPNG, ST_AsRaster, ST_MapAlgebraFct, ST_BandPixelType, ST_GeoReference, ST_Value, ST_Union, ST_Union

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