procedure TForm1.Button4Click(Sender: TObject);
var
  MyPolySelArray: array of TPoint;
  I: Integer;
begin
  // Create an array of TPoints of the polygon selection in a TImageEnView
  SetLength( MyPolySelArray, ImageEnView1.PolySelCount );
  for I := 0 to ImageEnView1.PolySelCount - 1 do
    MyPolySelArray[ I ] := ImageEnView1.PolySel[ I ];

   //how to modify the points to enlarge the ROI on all sides.
    ???
    ???


  // Create a polygon selection from an array of TPoints
     ImageEnView1.Deselect();
     for I := Low( MyPolySelArray ) to High( MyPolySelArray ) do
     begin
        ImageEnView1.AddSelPoint( MyPolySelArray[I].x, MyPolySelArray[ I ].y );
     end;
     ImageEnView1.EndSelect();
end;

// Enlarge the selection by ten pixels on all sides
With ImageEnView1 do
begin
  SelectionBase := iesbBitmap;
  Select( SelX1 - 10, SelY1 - 10, SelX2 - 10, SelY2 - 10 );
End;