FUNCTION zshift,A,dx,dy
;+
;B=zshift(A,dx,dy) is the same as shift(A,dx,dy) except that the
;vacated pixels are filled with zeros. Works only with 2D.
;-

d=size(A,/dim)
AS=shift(A,dx,dy)
IF dx GT 0 THEN AS[0:dx-1,*]=0
IF dx LT 0 THEN AS[d[0]+dx:*,*]=0
IF dy GT 0 THEN AS[*,0:dy-1]=0
IF dy LT 0 THEN AS[*,d[1]+dy:*]=0

RETURN,AS
END