setDotsPerMeterX() function cannot work on my image

[RayX]

Hi,
I want to use setDotsPerMeterX() function to change the DPI for the image, but it doesn't work, who can help me? thanks!

the following is the test code:
QImage img(100,100,QImage::Format_Mono);
img.fill(Qt::color1);

img.setDotsPerMeterX(img.dotsPerMeterX() * 2);
img.setDotsPerMeterY(img.dotsPerMeterY() * 10);

QPainter *pr = new QPainter;
QRect r(10,10,80,80);
pr->begin(&img);
pr->drawRect(r);
pr->drawLine(10,10,90,90);

pr->end();

ui->LabelShowImage->setPixmap(QPixmap::fromImage(img));