\* \ D^2 u_0,0 = (u_1,0 + u_0,1 + u_-1,0 + u_0,-1 - 4 u_0,0)/h^2 + O(h^2) define-step 5point-transform 1 0 x,y scale1+ 0fn save-hi signed+ 0fn 0 1 x,y scale1+ fn save-hi signed+ fn -1 0 x,y scale1+ fn save-hi signed+ fn 0 -1 x,y scale1+ fn save-hi signed+ fn 0 0 x,y scale2+ fn save-hi signed+ fn lo2hi fn image norm fn end-step *\ define-step hi2lo-step kick signed+ 0fn end-step : film-step step-count @ #data-subcells mod copy-n hi2lo-step ; : Show-filmstrip ['] film-step is update-step ; press F "Show original filmstrip." : Run-transform ['] trans-step is update-step ; press T "Repeatedly run transform on filmstrip."