George MacKerron: code blog

I’ve been playing with the much-enhanced raster support in PostGIS 2.1 in the last few days. Amongst other things I’ve been producing maps with ST_MapAlgebra, which has changed a little since 2.0.

The example callback function in the documentation is somewhat unhelpfully bare-bones: it always returns 0, touching none of its arguments. So here’s an equally useless — but rather more informative — example of a callback function for the simple one-raster, one-band, one-pixel-at-a-time case:

create or replace function 
callback_fn(pixel float[], pos integer[], variadic userargs text[]) 
  returns float 
  language plpgsql 
  immutable  -- careful: this function is immutable, yours may not be
as $$
  declare
    pixval float;
    inputx integer;
    inputy integer;
  begin
    pixval := pixel[1][1][1];  -- pixel indices: [raster #][xdistance][ydistance]
    inputx := pos[1][1];       -- pos indices:   [raster #][x = 1, y = 2]
    inputy := pos[1][2];       --                (raster #0 is the output raster)

    return pixval + inputx + inputy;
  end;
$$;

-- example call:

select st_mapalgebra(rast, 1, 'callback_fn(float[], integer[], text[])'::regprocedure) 
from raster_table;

And here’s something that caught me out: unless you’re passing some userargs, don’t declare your callback function to be strict. If you do, your callback function will never be called because its userargs argument is always null.