func test_pattern(n)
 ;square test pattern of size n
 ;concentric sinusoidal rings with wavelength as 1/r
 r = fltarr(n,n)
 n2 = n/2
 r = (indgen(r,0)-n2)^2 + (indgen(r,1)-n2)^2
 r = r * (r lt .895*n2*n2)
 y = sin(r/n2)
 y = scale256(y)
 return, y
 endfunc