Drawing specified part of QPainterPath on QImage [zoom and pan]

Question

i.e. if I specify some cubic lines from example specified in Qt5 tutorial:

QPainterPath path;
path.addRect(20, 20, 60, 60);

path.moveTo(0, 0);
path.cubicTo(99, 0,  50, 50,  99, 99);
path.cubicTo(0, 99,  50, 50,  0, 0);

QPainter painter(this);
painter.fillRect(0, 0, 100, 100, Qt::white);
painter.setPen(QPen(QColor(79, 106, 25), 1, Qt::SolidLine,
                    Qt::FlatCap, Qt::MiterJoin));
painter.setBrush(QColor(122, 163, 39));

painter.drawPath(path);

which constructs this set of curves

Now I'd like to render only a part of those curves on QImage specified by some region with starting point=[20px,50px] with width=80px and height=50px so resulting would look like this:

Or if it is possible, to render with 3x zoom, so resulting QImage would look the same but had size=[240px,150px]

I am new to Qt, so could someone please showed me a working code example?

Answer 1

You can transform the painter coordinate system:

    QPainter painter(this);
    painter.scale(3, 3); // zoom 3 times
    painter.translate(-20, -50); // offset origin to 20x50
    // ... render stuff

This has an advantage over the other answer, because it will be rendered as if you provided larger coordinates, instead of rendering it small and then enlarging the raster image, which will degrade image quality. Also, it is possible that Qt will optimize it to not render outside of the image, so it will render less, and you don't need to crop and throw results away.

Result:

Compare that to an upscaled raster: