Re: [NTG-context] How to "unfill" a picture?

[Zhichu Chen]

I guess I should use as few variables as possible, I suppose
no one would name their paths "Helper" ;)

This macro assumes the argument is clockwise, I've written
a macro to find out whether a path is clockwise or not, but
it took me forever to run the code. Maybe there's already
some primitive could do this.

\startMPinclusions
def getridof(expr p) =
  save Helper, Oringin, Mode, MagicPoint ;

  pair Oringin ; Oringin := center p ;

  pair MagicPoint ;

  numeric Mode ; Mode := 0 ;

  path Helper[] ;

  Helper0 := p ;
  Helper1 := llcorner currentpicture -- lrcorner currentpicture ;
  Helper2 := lrcorner currentpicture -- urcorner currentpicture ;
  Helper3 := urcorner currentpicture -- ulcorner currentpicture ;
  Helper4 := ulcorner currentpicture -- llcorner currentpicture ;
  for i=1 upto 4 :
    Helper[i+4] := Helper[i] ;
  endfor
  Helper11 := llcorner currentpicture -- Oringin ;
  Helper12 := lrcorner currentpicture -- Oringin ;
  Helper13 := urcorner currentpicture -- Oringin ;
  Helper14 := ulcorner currentpicture -- Oringin ;


  if (((llcorner currentpicture -- Oringin) intersectiontimes p) = (-1,-1)) :
    if (((lrcorner currentpicture -- Oringin) intersectiontimes p) = (-1,-1)) :
      if (((urcorner currentpicture -- Oringin) intersectiontimes p) =
(-1,-1)) :
        Mode := 4 ;
      else :
        Mode := 3 ;
      fi
    else :
      Mode := 2 ;
    fi
  else :
    Mode := 1 ;
  fi

  Helper100 = Helper[10+Mode] cutafter p ;
  MagicPoint = point length(Helper100) of Helper100 ;

  Helper0 := Helper[Mode] for i=1 upto 3 : -- Helper[Mode+i] endfor
          -- Helper100
          -- (Helper0 cutbefore MagicPoint)
          -- (Helper0 cutafter MagicPoint)
          -- (reverse Helper100)
          -- cycle ;

  clip currentpicture to Helper0 ;

  draw Helper0 withcolor cyan ;
enddef ;
\stopMPinclusions
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