; ----------------------------------------------------------------------
;  Centroid to dip in the ppsum[] array to find more accurate offset value.
;
function nsxng_wpcent, pp, npp, ppsum

; Limits. 
  ii1=pp-3                      
  if (ii1 lt 0    ) then ii1=0
  if (ii1 gt npp-1) then ii1=npp-1
  ii2=pp+3
  if (ii2 lt 0    ) then ii2=0
  if (ii2 gt npp-1) then ii2=npp-1

; Find base level (lower of two sides. 
  hi1=0.
  for ii=long(ii1),pp do begin
     if (ppsum[ii] gt hi1) then hi1=ppsum[ii]
  endfor
  hi2=0.
  for ii=long(pp),ii2 do begin
     if (ppsum[ii] gt hi2) then hi2=ppsum[ii]
  endfor
  if (hi1 lt hi2) then base=hi1 else base=hi2

; Limits.
  ii1=pp-1   
  if (ii1 lt 0    ) then ii1=0
  if (ii1 gt npp-1) then ii1=npp-1
  ii2=pp+1                      
  if (ii2 lt 0    ) then ii2=0
  if (ii2 gt npp-1) then ii2=npp-1

; Centroid. */
  sum=0.
  wsum=0.                     
  for ii=long(ii1),ii2 do begin
     wgt = base - ppsum[ii]
     if (wgt ge 0.) then begin
        sum = sum + ( wgt * double(ii) ) 
        wsum= wsum+ wgt
     endif
  endfor
  if (wsum gt 0.) then cent = sum / wsum else  cent=double(pp)

  return,cent
end