High DPI rounding error

Qt seems to use different roundings when calculating pixel values from device independent pixels, I have not checked the Qt source code, but I have done some testing.

My screen is set to 125% scaling so I have a device pixel ratio of 1.25 in Qt.

Calling the following method

QPixmap pixmap = icon.pixmap(QSize(26, 26));

Results in a pixmap that is 33x33 pixels

26 * 1.25 = 32.5 rounded up to 33

Then calling

painter.drawPixmap(QRect(0,0,26,26), pixmap);

This renders an image that is 32x32 pixels big.

26 * 1.25 = 32.5 rounded down to 32

This causes a bitmap scaling to occur from 33x33 -> 32x32 pixels, which results in a distorted image.