...
ST_MapAlgebra(
    b1.rast, -- raster to operate on
    1,
    'generate_random_raster(double precision[][][], integer[][],  ''3'', ''1'')'::regprocedure
  ),
...

...
ST_MapAlgebra(
    b1.rast, -- raster to operate on
    1,
    'generate_random_raster(double precision[][][], integer[][],  VARIADIC ARRAY[3,5]::text')'::regprocedure
  ),
...