For the historical record, I finally got it working. My code:

`

QRect screenGeometry = QGuiApplication::primaryScreen( )->geometry( ); int x13; int x23; int y13; int y23; if( screenGeometry.width( ) < width( )) { x13 = m_image->getScrollX( ) + screenGeometry.width( ) / 3.0; x23 = m_image->getScrollX( ) + 2 * screenGeometry.width( ) / 3.0; } else { x13 = m_image->getScrollX( ) + width( ) / 3.0; x23 = m_image->getScrollX( ) + 2 * width( ) / 3.0; } if( screenGeometry.height( ) < height( )) { y13 = m_image->getScrollY( ) + screenGeometry.height( ) / 3.0; y23 = m_image->getScrollY( ) + 2 * screenGeometry.height( ) / 3.0; } else { y13 = m_image->getScrollY( ) + height( ) / 3.0; y23 = m_image->getScrollY( ) + 2 * height( ) / 3.0; } QPainter painter( this ); painter.setPen( QPen( Qt::red, 4, Qt::SolidLine, Qt::FlatCap )); painter.drawLine( 0, y13, width( ), y13 ); painter.drawLine( 0, y23, width( ), y23 ); painter.drawLine( x13, 0, x13, height( )); painter.drawLine( x23, 0, x23, height( ));

}

`

Solved.

For further readers:
I've added a check to the child widget whether a user interaction happens within its drawn borders (a closed QPainterPath).
If so, I consume the event and continue from there, if not, I ignore the event and pass it to the parent widget, i.e. the actual MyWidget, so that the overlaying child widget has some transparency for my defined events (some key and mouse interaction).

Pretty easy if I think about it now.
The idea of having some kind of a donut shaped widget (yellow part in my sketch), which does not occupy the center of its parent widget was stuck in my head way too long :))

@Joe-von-Habsburg My example recast with a changing background:

// widget.h #ifndef WIDGET_H #define WIDGET_H #include <QWidget> class Widget : public QWidget { Q_OBJECT public: Widget(QWidget *parent = nullptr); ~Widget(); // QWidget interface protected: void mousePressEvent(QMouseEvent *event); void mouseReleaseEvent(QMouseEvent *event); void paintEvent(QPaintEvent *event); private slots: void setBackground(); private: QImage mBackground; QPointF mFrom; QPointF mTo; }; #endif // WIDGET_H // widget.cpp #include "widget.h" #include <QPaintEvent> #include <QPainter> #include <QLinearGradient> #include <QRandomGenerator> #include <QTimer> #include <QDebug> Widget::Widget(QWidget *parent) : QWidget(parent) , mBackground(500, 500, QImage::Format_RGB32) { setBackground(); QTimer *t = new QTimer(this); connect(t, &QTimer::timeout, this, &Widget::setBackground); t->start(1000); } Widget::~Widget() { } void Widget::mousePressEvent(QMouseEvent *event) { mFrom = event->position(); mTo = mFrom; } void Widget::mouseReleaseEvent(QMouseEvent *event) { mTo = event->position(); update(); } void Widget::paintEvent(QPaintEvent *event) { QPainter p(this); p.drawImage(event->rect(), mBackground); if (mFrom != mTo) { QPen pen(Qt::red); pen.setWidth(3); p.setPen(pen); p.drawLine(mFrom, mTo); } p.end(); } void Widget::setBackground() { QPainter p(&mBackground); const QRectF rectf(mBackground.rect()); QLinearGradient grad(rectf.topLeft(), rectf.bottomRight()); grad.setColorAt(0, QColor::fromRgb(QRandomGenerator::global()->generate())); grad.setColorAt(1, QColor::fromRgb(QRandomGenerator::global()->generate())); p.fillRect(mBackground.rect(), QBrush(grad)); p.end(); update(); }

@JonB thanks for googling :) . I have seen some more example. In case of GraphicsView fill transparent on scene did not help. I am pretty sure there is some way to do it, need to google more

@Pl45m4 In this function I am painting on axis->labelPix which is a QPixmap. The axis->labelPix is then put in its correct position using QPainter in the paintEvent() of PlotWidget.

thank you for the direction.

The constructors are still wrong. Should be

QSliderButton::QSliderButton(QWidget* parent) : QWidget(parent) {}

and the empty one is just redundant. You don't need it at all.

Better yet don't manually implement any constructors, just do

class QSliderButton : public QWidget { Q_OBJECT public: using QWidget::QWidget; ...

The using statement will inherit all the constructors from the base class so you don't have to implement any.

I'm afraid that doesn't help for my use case, since I need the images to appear as clear and clean as possible when being scaled down.

@timob256 said in Is stroke the text (Circle text ):

cannot align the bypass line with the inner line

Sorry, i don't understand.

If you want the background to be green, add a fillPath before stroke path:

painter.fillPath(path,Qt::green); painter.strokePath(path,pen);

Simply two QPainter::drawArc() calls with a different color.

@Vijay-R-Gawade said in Qt app freeze when remove usb drive while qpainter is drwaing:

@SGaist Timer is used to add some delay to Update Progress bar on UI by one step when each record from DB is drawn.

That's a fishy procedure to update your progress bar.

One of the issue here is that the conversion must be done each time the draw method is called. If you know that your images will not change or not much then you should do the conversion upfront so that the painting can be optimized.

@Christian-Ehrlicher Thanks for tracking that down. Hope Qt gets it fixed soon.

Thank you so much nagesh.

@TenG
Be aware that not all of Qt modules are supported in WASM. Before anything, check the list at https://doc.qt.io/qt-5/wasm.html#supported-qt-modules against what is in your app. For example, it doesn't do databases, you can't access the local file system, and so on. Also I noticed this time that it requires a Qt static build, but maybe you have to have that for iOS anyway, I don't know.

If it works for you it's a really simple solution compared to rewriting anything. Just check out the gotchas.

Thanks for the code !

You can use the Q_OS_XXX macros to ifdef your code.

so write a test program, meter the performance of canvas writes vs out-of-bounds writes, and report back.