procedure TForm.OnPaint(Sender: TObject);

  procedure Tile(c: TCanvas; b: TBitMap);
  var
    x, y, h, w, i, j: integer;
  begin
    with b do
    begin
      h := b.height;
      w := b.width;
    end;
    y := 0;
    with c.Cliprect do
    begin
      i := bottom - top - 1; //высота
      j := right - left - 1; //ширина
    end;
    while y < i do
    begin
      x := 0;
      while x < j do
      begin
        c.draw(x, y, b);
        inc(x, w);
      end;
      inc(y, h);
    end;
  end;

begin
  if Sender is TForm then
    Tile(TForm(Sender).Canvas, fTileWith);
end;