private
{ Private declarations }
fmx, fmy: integer;
flx, fly: integer;

// helper function
function ContrastingColor(AColor: TColor): TColor;
// Return s contracting color of the passed color
begin

  if ((AColor and $FF) * 77 + ((AColor shr 8) and $FF) * 150 + ((AColor shr 16) and $FF) * 29)
    > 127 * 256 then
    Result := clBlack
  else
    Result := clWhite;

end;

procedure TForm1.ImageEnView1DrawBackBuffer(Sender: TObject);
var
  bx, by: Integer;
begin

   bx := ImageEnView1.XScr2Bmp ( fmx );
   by := ImageEnView1.YScr2Bmp ( fmy );

  if (bx >= 0) and (bx <= ImageEnView1.IEBitmap.Width - 1) and (by >= 0) and
    (by <= ImageEnView1.IEBitmap.Height - 1) then
  begin

    with ImageEnView1.BackBuffer.Canvas do
    begin

      Pen.Mode := pmCopy;
      Pen.Width := 3;
      { Make sure brush is visible regardless of the color under the  cursor by setting pen color to a contrasting with the color under the cursor }
      Pen.Color := ContrastingColor(ImageEnView1.IEBitmap.Canvas.Pixels[bx, by]);
      Brush.Style := bsClear;
      MoveTo(0, by);
      LineTo(ImageEnView1.IEBitmap.Width, by);
      MoveTo(bx, 0);
      LineTo(bx, ImageEnView1.IEBitmap.Height);

    end;

  end;

end;

procedure TForm1.ImageEnView1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
begin
  fmx := X;
  fmy := Y;
  flx := X;
  fly := Y;
  ImageEnView1.Update;
end;

procedure TForm1.ImageEnView1DrawBackBuffer(Sender: TObject);
begin
  with ImageEnView1.BackBuffer.Canvas do
  begin
    Pen.Mode := pmCopy;
    Pen.Width := 3;
    Pen.Color := clRed;
    Brush.Style := bsClear;
    MoveTo(0, fmy);
    LineTo(ImageEnView1.Width, fmy);
    MoveTo(fmx, 0);
    LineTo(fmx, ImageEnView1.Height);
  end;
end;