; ----------------------------------------------------------------------
;  Compute fraction of a pixel is within given boundaries.
;  pp1 is the lower real boundary position in pixel space. 
;  pp2 is the upper real boundary position in pixel space. 
;  rb1 is the real lower pixel edge (pixel-0.5).
;  rb2 is the real upper pixel edge (pixel+0.5).
;  (How much is rb1 to rb2 enclosed within pp1 to pp2 boundaries?)
;
function nsx_boundary_fraction, pp1, pp2, rb1, rb2

  if (rb1 gt pp2) then begin
     frac= 0.0                  ;    /* fully above */
  endif else begin
     if (rb2 lt pp1) then begin
        frac= 0.0               ;    /* fully below */
     endif else begin
        if ((rb1 gt pp1)&&(rb2 lt pp2)) then begin
           frac= 1.0            ;    /* fully enclosed */
        endif else begin
           if ((rb1 lt pp1)&&(pp2 gt rb2)) then begin
              frac= (rb2 - pp1) ; /* pixel straddles lower boundary */
           endif else begin
              if ((rb2 gt pp2)&&(pp1 lt rb1)) then begin
                 frac= (pp2 - rb1) ; /* pixel straddles upper boundary */
              endif else begin
                 frac= (pp2 - pp1) ; /* boundaries must be within pixel */
              endelse
           endelse
        endelse
     endelse
  endelse
  
  return,frac

end