在Lazarus中将图像转换为灰度的问题

Image1.Picture.LoadFromFile('image.jpg');

var
  x, y: Integer;
  GrayValue: Byte;
begin
  for y := 0 to Image1.Picture.Height - 1 do
  begin
    for x := 0 to Image1.Picture.Width - 1 do
    begin
      GrayValue := (GetRValue(Image1.Canvas.Pixels[x, y]) +
                    GetGValue(Image1.Canvas.Pixels[x, y]) +
                    GetBValue(Image1.Canvas.Pixels[x, y])) div 3;
      Image1.Canvas.Pixels[x, y] := RGB(GrayValue, GrayValue, GrayValue);
    end;
  end;
end;

GrayImage.Canvas.Draw(0, 0, Image1.Picture.Graphic);