Unexpected behaviour with QGraphicsOpacityEffect and custom widgets

TheEnigmist

I've a custom widget A with custom event filter function.
On Enter/Leave event I trigger a QGraphicsOpacityEffect on other custom widget B to fades in/out it with a QPropertyAnimation.
B has custom Enter/Leave event function that change an internal base widget (simply change a QPixmap).
If I enter in A, B correctly fades-in , but as soon as I enter in B, it disappears (it calls Paint and paints a transparent widget).
If I leave A then B comes back for duration of QPropertyAnimation.

This behaviour doesn't appear when using QGraphicsBlurEffect
Blur video
Opacity video

I really don't know why it does this with opacity, the function are the same...

SGaist

Hi,

Which version of Qt are you using ?
Can you provide a minimal compilable example reproducing this ?

TheEnigmist

@SGaist Hi, maybe I found a where the error is creating a minimal compilable example reproducing this.
I'm using Qt6.3
This is B
B.h

class B : public QWidget{
	Q_OBJECT
public:
	B(const QPixmap& icon, QWidget* parent = nullptr);

signals:
	void clicked();

protected:
	bool eventFilter(QObject* obj, QEvent* event) override;

private:
	void updateIcon();
	QLabel* labelIcon;
	QPixmap icon;
};

B.cpp

B::B(const QPixmap& icon, QWidget* parent): QWidget(parent){
    QHBoxLayout* base_layout = new QHBoxLayout;
    labelIcon = new QLabel;
    this->icon = icon;
    base_layout->addWidget(labelIcon);
    installEventFilter(this);
    setLayout(base_layout);
    updateIcon();
}
void B::updateIcon() {
    labelIcon->setPixmap(icon);
}
bool B::eventFilter(QObject* obj, QEvent* event) {
    if (event->type() == QEvent::MouseButtonRelease) emit clicked();
    //QEvent::Enter do something and than call update icon
    if (event->type() == QEvent::Enter) updateIcon();
    //QEvent::Leave do something and than call update icon
    if (event->type() == QEvent::Leave) updateIcon();
    return QObject::eventFilter(obj, event);
}

This is A
A.h

class A : public QWidget
{
	Q_OBJECT

public:
	A(QWidget *parent = nullptr);

protected:
	bool eventFilter(QObject* obj, QEvent* event) override;

private:
	QPropertyAnimation* fadeAnimation;
};

A.cpp

A::A(QWidget* parent) : QWidget(parent){
	//init layout
	QVBoxLayout* mainLayout = new QVBoxLayout;
	installEventFilter(this);
	setLayout(mainLayout);
	setCursor(Qt::PointingHandCursor);
	installEventFilter(this);

	//wrap widget to create nested graphic effect widget!
	QWidget* wrap = new QWidget(this);
	//CREATE B with a pixmap in it
	B* b = new B(QPixmap("pixmap.png")); // pixmap.png is a simple colored rectangle
	b->setParent(wrap);
	b->move(10, 10);
	b->setFixedSize(60, 40);
	wrap->show();

	QGraphicsDropShadowEffect* shadow = new QGraphicsDropShadowEffect;
	shadow->setOffset(0, 0);
	shadow->setColor(QColor(60, 60, 60, 255));
	shadow->setBlurRadius(5);

	QGraphicsOpacityEffect *opacityEffect = new QGraphicsOpacityEffect;
	opacityEffect->setOpacity(0);
	
	b->setGraphicsEffect(opacityEffect);
	wrap->setGraphicsEffect(shadow);

	fadeAnimation = new QPropertyAnimation(opacityEffect, "opacity");
	fadeAnimation->setDuration(500);

}

bool A::eventFilter(QObject* obj, QEvent* event) {
	if (event->type() == QEvent::Enter) {
		fadeAnimation->setStartValue(0);
		fadeAnimation->setEndValue(1);
		fadeAnimation->setEasingCurve(QEasingCurve::InBack);
		fadeAnimation->start();
	}
	if (event->type() == QEvent::Leave) {
		fadeAnimation->setStartValue(1);
		fadeAnimation->setEndValue(0);
		fadeAnimation->setEasingCurve(QEasingCurve::InBack);
		fadeAnimation->start();
	}
	return QObject::eventFilter(obj, event);
}

In main.cpp create A and then move your mouse over the image you loaded and you will see it will disappear.
I found out that if I swap grapfic effect (b get shadow and wrap get opacity) I get a lot of warning when B disappear. This is the output:

Warning QPainter::begin: A paint device can only be painted by one painter at a time.
Warning QPainter::translate: Painter not active
Warning QPainter::worldTransform: Painter not active
Warning QWidgetEffectSourcePrivate::pixmap: Painter not active
Warning QPainter::worldTransform: Painter not active
Warning QPainter::setWorldTransform: Painter not active
Warning QPainter::setWorldTransform: Painter not active

So my try to nest two graphic effect failed totally, indeed if I remove shadow effect it does not trigger this behaviour.

EDIT: I tried to remove second effect in my main code but there is still the behaviour, only with opacity. Maybe it overlaps Paint function and the error is triggered, but no warning in console this time

TheEnigmist

I made some step on this error.
Based on this answer if I set nullptr on graphiceffect after propertyanimation the error disappear, but I lose the fade-out effect.

Seems to be a bug since QGraphicEffect calls a paint event while the widget is painting the inner elements, triggering the warning and failing the paint at all

EDIT: seems like qgrapficeffects is propagated on all inner element, or at least goes in conflict with inner elements

SGaist

Which version of Qt 6.3 are you using ?

TheEnigmist

@SGaist I'm using 6.3.0 with msvc compiler