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

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:

Answer 2

I've got a code example for you. But it isn't that hard to find out how. You just have to read the documentation, all is easy to find. Qt's documentation is really great.
QApplication a(argc, argv);
QImage img(100, 100, QImage::Format_ARGB32);
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(&img);
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);
painter.end();
QPixmap pixmap( QPixmap::fromImage(img).copy(20, 50, 80, 50).scaled(240,150) );
// option 1, use a QLabel ( only for simple cases )
QLabel label;
label.setPixmap( pixmap );
label.show();
// option 2, use a QGraphicsScene ( far more flexible )
QGraphicsView view;
QGraphicsScene scene;
scene.addPixmap( pixmap );
scene.setSceneRect( img.rect() );
view.setScene(&scene);
view.show();
return a.exec();