borders

procedure TPABorders2(var TPA: TPointArray);
var
  a: T2DPointArray;
  b: TBox;
  p: TPointArray;
  h, i: Integer;
begin
  b := pp_TPABounds(TPA);
  pp_BoxExpand(b);
  pp_TPAFloodFill(TPA, Point(b.X1, b.Y1), sd4ways, b, p);
  pp_BoxShrink(b);
  pp_TPAExtractBoxes(p, b);
  pp_TPAInvert(p);
  SetLength(TPA, 0);
  pp_TPABlob(p, a);
  h := High(a);
  for i := 0 to h do
    pp_TPABorder(a[i]);
  pp_Merge(a, TPA);
end;