mjhjmtu

I have a 4-band raster dataset that I have imported into a Postgres/PostGIS table using raster2pgsql. I am Using ST_MapAlgebra() with a callback function to perform some band math on the dataset. I have set the return type as double precision. However, I am finding that anytime I return a decimal value (e.g., .25 or -.25) it only returns zero. However, if I return a whole number (e.g., 25 or -15), I can receive the values back from the call. I played around with this, but can’t figure out why this is occurring. Any help would be appreciated. Here is the syntax of my callback function:

CREATE OR REPLACE FUNCTION urban_veg.ndvi_callback (
 value double precision, 
 pos integer, 
 VARIADIC userargs text
) 
RETURNS double precision AS 
$$
 DECLARE
 r1_val double precision;
 r4_val double precision;
 result double precision;
 BEGIN
 r1_val := value[1][1][1]::double precision;
 r4_val := value[2][1][1]::double precision;
 result := (r4_val-r1_val)/(r4_val+r1_val);
 RETURN result;
 END;
$$ 
LANGUAGE 'plpgsql' IMMUTABLE COST 1000;

The call to the function is:

ST_MapAlgebra(
 ARRAY[
 ROW(rast, 1), 
 ROW(rast, 4) ]::rastbandarg, 
 'urban_veg.ndvi_callback(
 double precision,integer,text
 )'::regprocedure, '16BSI'::text)

You specify the return pixeltype to be '16BSI' (16bit signed integer); given this, ST_MapAlgebra won't return a float, not even with a gun to it's head...

Try with '32BF' (32bit float).