Can't get ST_MapAlgebra to return fractional values

Question

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.

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)

Answer 1

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).