Qt中如何用QImage::Format_Indexed8表示灰度图

QImage *qi = new QImage(data_ptr, width, height, QImage::Format_Indexed8);
QVector<QRgb> grayTable;
for(int i = 0; i < 256; i++) 
graytable.push_back(qRgb(i,i,i));
qi->setColorTable(grayTable);