% --- start of displayed preamble in the book --- input featpost3Dplus2D % --- end of displayed preamble in the book --- defaultfont:="ptmr8r"; warningcheck:=0; beginfig(1) SphericalDistortion := true; Spread := 50; f := 0.4*(1.5,0.5,1); numeric gridstep, sidenumber, i, coord; color pa, pb, pc, pd; gridstep = 0.1; sidenumber = 5; coord = 0.5*sidenumber*gridstep; for i=0 upto sidenumber: pa := (-coord,-coord+i*gridstep,0); pb := (coord,-coord+i*gridstep,0); pc := (-coord+i*gridstep,-coord,0); pd := (-coord+i*gridstep,coord,0); draw pathofstraightline( pa, pb ); draw pathofstraightline( pc, pd ); endfor; endfig; end;