This is a generic implementation of TPI based on the current PostGIS TPI implementation, with the extra ability of allowing a custom focal inner and outter mean radius (different from one). This implementation of TPI was based on the current implementation of TPI by PostGIS (https://postgis.net/docs/RT_ST_TPI.html) and the folowing document http://www.jennessent.com/downloads/tpi-poster-tnc_18x22.pdf

In order to create the new TPI functions in your database, execute the folowing queries:

1. _st_tpicustom4ma.sql
2. st_tpicustom.sql

After that you should have _st_tpicustom4ma and st_tpicustom functions in your database.

_st_tpicustom4ma is the callback function used by map algebra. The main function is:

st_tpicustom(raster rast, integer nband, raster customextent, text pixeltype="32BF", integer inner radius in pixels, integer outter radius in pixels, boolean interpolate_nodata=FALSE)

This is an example with a inner annnulus of 5 pixels and outter annulus of 10 pixels:

select st_tpicustom(r.rast,5,10) as rast
from srtm as r,parcels as foo
where foo.id=111 and ST_Intersects(foo.geometry,rast)

This query will generate a tpi 300 meters if the input is a SRTM of 30 meters of pixel size.