I found a way to easyly approximate very accurately the cubic spline used
in the lathe with a regular bezier spline (I can explain it if someone is
interested).

  But now I need the quadratic spline. Can someone explain the idea behind
it? I know that for each set of 3 points the curve between the 2nd and 3rd
points is calculated using the 1st point as control point (in the cubic
spline also the 4th point was used as control point but not with the
quadratic spline). But how does the 1st point affect the orientation of
the curve in the 2nd and 3rd point? (In the cubic spline the tangent at
point n is parallel to the line that connects points n-1 and n+1)
  I can't see any rule of this kind looking at the curve generated by
povray. Does anyone know it?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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