The available functions are MEAN (the default), POLY, SPLINE, MEDIAN, SLOPE and SMOOTH.
All modes except POLY and SPLINE use local neighbourhoods delineated using a sample size (in pixels) defined by TUNE_FFLSMP.
Note that the required maximum number of coefficients for POLY and SPLINE are different. For POLY, the maximum number of coefficients corresponds to the number of degrees plus one, so for a cubic use coeffs=4. For a SPLINE the maximum number of coefficients corresponds to 2x(knots+7), so for 3 knots use coeffs=20. If you are using SPLINE fits then you should ensure that the value of TUNE_MAXPOLY is set accordingly, as well as using an appropriate value for TUNE_FFNXPLY and/or TUNE_FFNYPLY.
Initial suggested value: MEAN.