function modpat1d,NX,F1D,theta,aspp,xoffset,yoffset,dx,dy,xpixel,ypixel


; CREATE THE XPIXEL, YPIXEL ARRAYS ONCE IF THEY HAVE NOT BEEN MADE:
n=nx^2
if (n_elements(xpixel) LE 1) then begin
  xpixel=(findgen(n) mod nx)*aspp + xoffset-aspp*nx/2.  ; in arcsec
  ypixel=fix(findgen(n)/nx)*aspp + yoffset-aspp*nx/2.
endif
f1d_size=N_elements(F1D)
if (f1d_size LT 2048) then begin
  print,'F1D too small: size=',f1d_size
  print,'Stopping.'
  stop
endif
s0=f1d_size/2.-0.5

;  FIND THE INDEX OF F1D BY PROJECTING A PERPENDICULAR FROM THE PIXEL:
 k1=cos(theta)
 k2=sin(theta)
 s=k1*xpixel+k2*ypixel+(s0+k1*dx+k2*dy)
 z=f1d(s)

return,z
end