pro checkerboard,x,width 
;+
; NAME:                                              
;       CHECKERBOARD
; PURPOSE:
;       To create a checkerboard array (alternating bytes 0 and 255)
;       One use of CHECKERBOARD is to study resolution and distortion
;       on the TV display
; CALLING SEQUENCE:                                             
;       checkerboard,x,[width]
; OPTIONAL INPUTS:
;       width - size of (square) output array.  If not supplied, then
;       a 512 x 512 array will be output
; OUTPUTS:
;       x  - byte array containing alternating values
; REVISON HISTORY:
;       written by B. Pfarr and W. Landsman 4/87
;-
if n_params(0) eq 0 then begin
    print,string(7B),'CALLING SEQUENCE- checkerboard,x,[width]'
    return
endif
if n_params(0) lt 2 then width = 512  ;Default is 512 x 512 output array
q=indgen(width) mod 2                 ;Row of alternating values
x = (byte(q#q))*255b
x = shift(x,1,1) or x
return
end