{==============================================================================]
 @action: Keeps only corner points in TPA.
 @note: None
 @contributors: Janilabo, slacky
[==============================================================================}
procedure TPACorners(var TPA: TPointArray); callconv
var
  x, y, h, r, i: Integer;
  C: T2DIntegerArray;
  bx: TBox;
  up, down, left, right, topLeft, topRight, bottomLeft, bottomRight: Boolean;
begin;
  h := High(TPA);
  if (h > 0) then
  begin
    r := 0;
    bx := TPABounds(TPA);
    BoxExpand(bx);
    CreateATIA(0, ((bx.X2 - bx.X1) + 1), ((bx.Y2 - bx.Y1) + 1), C);
    for i := 0 to h do
      Inc(C[(TPA[i].X - bx.X1)][(TPA[i].Y - bx.Y1)]);
    for y := 0 to (bx.Y2 - bx.Y1) do
      for x := 0 to (bx.X2 - bx.X1) do
        if (C[x][y] > 0) then
        begin
          topLeft := (C[(x - 1)][(y - 1)] > 0);
          up := (C[x][(y - 1)] > 0);
          topRight := (c[(x + 1)][(y - 1)] > 0);
          left := (c[(x - 1)][y] > 0);
          right := (c[(x + 1)][y] > 0);
          bottomLeft := (c[(x - 1)][(y + 1)] > 0);
          down := (c[x][(y + 1)] > 0);
          bottomRight := (c[(x + 1)][(y + 1)] > 0);
          if not ((topLeft and bottomRight) or (up and down) or (topRight and bottomLeft) or (left and right)) then
          begin
            for i := 0 to (C[x][y] - 1) do
              TPA[(r + i)] := Point((x + bx.X1), (y + bx.Y1));
            r := (r + C[x][y]);
          end;
        end;
    SetLength(TPA, r);
  end;
end;