Painter: transform coordinates but not pen width settings

[nroberts]

I want to use QPainter's transform setup to convert coordinates in my system into the device's system (including zoom) but I don't want to apply those same changes to pens. Currently I have tried:

QTransform trans = painter->transform();
    painter->resetTransform();
 
    QPen pen = painter->pen();
    pen.setWidth(new_width);
    painter->setPen(pen);
 
    painter->setTransform(trans);

QTransform trans = painter->transform();
    painter->resetTransform();

    QPen pen = painter->pen();
    pen.setWidth(new_width);
    painter->setPen(pen);

    painter->setTransform(trans);

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This isn't working. After this call the pen is this giant rectangle rather than a two pixel line.

Any ideas?