private void JPEGDefaultTileSize(ref int tw, ref int th)
{
base.DefTileSize(ref tw, ref th);
tw = Tiff.roundUp(tw, m_tif.m_dir.td_ycbcrsubsampling[0] * JpegConstants.DCTSIZE);
th = Tiff.roundUp(th, m_tif.m_dir.td_ycbcrsubsampling[1] * JpegConstants.DCTSIZE);
}