procedure TForm1.FormPaint(Sender: TObject);
var bmp: TBitmap;
    i, j: Integer;
    b: Byte;
    pCol: PByte;
begin
  bmp := TBitmap.Create;
  bmp.PixelFormat := pf24bit;
  bmp.Width := ClientWidth;
  bmp.Height := ClientHeight;
  try
    bmp.BeginUpdate();
    for j := 0 to bmp.Height - 1 do
    begin
      pCol := bmp.ScanLine[j];
      for i := 0 to bmp.Width - 1 do
      begin
        if (((i div 100) mod 2) xor ((j div 100) mod 2)) = 0 then
          b := 255
        else
          b := 0;
        pCol^ := b; Inc(pCol);
        pCol^ := b; Inc(pCol);
        pCol^ := b; Inc(pCol);
      end;
    end;
    bmp.EndUpdate();
    Canvas.Draw(0,0,bmp);
  finally
    bmp.Free;
  end;
end;