FUNCTION fwhm_g2dmodel, a, xp, yp 

; Description: This program is for use within the function "fwhm_g2dfit.pro".
;              No parameter checking is carried out to minimise overheads.
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
;   24/04/2009 - Further module optimisation performed (dmb).
;   06/05/2008 - Module created (dmb)

;Use the input parameters to calculate the function values

;Return the image model