Translucent background not rendering correctly in Qt6 with hardware rendering but works with Software rendering

[Muhammad Hassan Shahid]

This is the code to reproduce it

#include <QQuickWidget>
#include <QQuickItem>
#include <QQmlError>
#include <QtWidgets>

class MainWindow : public QMainWindow {
Q_OBJECT
public:
MainWindow();
private:
QQuickWidget *m_quickWidget;
};

MainWindow::MainWindow()
: m_quickWidget(new QQuickWidget)
{
setFixedSize(QSize(800,600));

  m_quickWidget->setAttribute(Qt::WA_TranslucentBackground);
  m_quickWidget->setClearColor(Qt::transparent);

  QUrl source("qrc:transparent.qml");

  m_quickWidget->resize(300,300);
  m_quickWidget->setWindowFlags(Qt::SplashScreen);
  m_quickWidget->setAttribute(Qt::WA_AlwaysStackOnTop);
  m_quickWidget->setAttribute(Qt::WA_TranslucentBackground);
  m_quickWidget->setClearColor(Qt::transparent);
  m_quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
  m_quickWidget->setSource(source);

  QWidget *centralWidget = new QWidget();
  centralWidget->setStyleSheet("background-color: blue");
  QHBoxLayout *layout = new QHBoxLayout(centralWidget);
  layout->addWidget(m_quickWidget);
  setCentralWidget(centralWidget);

}

int main(int argc, char **argv)
{
QApplication app(argc, argv);

  MainWindow mainWindow;
  mainWindow.show();

  return app.exec();

}

#include "main.moc"

[JoeCFD]

@Muhammad-Hassan-Shahid
Can you try:
QQuickWindow::setSceneGraphBackend( QSGRendererInterface::OpenGL );

The other options are here:
https://doc.qt.io/qt-6/qsgrendererinterface.html

[Muhammad Hassan Shahid]

@JoeCFD When I use the QSGRendererInterface enums, compiler gives me an error that it can't convert it to Qstring and indeed there is no overload of QQuickWindow::setSceneGraphBackend for parameter of type QSGRendererInterface::GraphicsApi

[JoeCFD]

@Muhammad-Hassan-Shahid Sorry you call this one
void QQuickWindow::setGraphicsApi(QSGRendererInterface::GraphicsApi api)

The source code is here
https://codebrowser.dev/qt6/qtdeclarative/src/quick/items/qquickwindow.cpp.html
check line 3773
there are only two options: OpenVG may be the one you need.

    // Special cases: these are different scenegraph backends.
    switch (api) {
    case QSGRendererInterface::Software:
        setSceneGraphBackend(QStringLiteral("software"));
        break;
    case QSGRendererInterface::OpenVG:
        setSceneGraphBackend(QStringLiteral("openvg"));
        break;
    default:
        break;
    }

https://doc.qt.io/qt-6/qtquick-visualcanvas-adaptations-openvg.html

[Muhammad Hassan Shahid]

@JoeCFD I tried OpenVG but the result the same as the hardware rendering one. It only works with the software rendering.

[JoeCFD]

@Muhammad-Hassan-Shahid
check if the backend setting is correct.
from here:
QString QQuickWindow::sceneGraphBackend()

https://www.qt.io/blog/2017/03/31/qt-quick-openvg

[Muhammad Hassan Shahid]

@JoeCFD The function returns OpenVG but the Qt complains "Could not create scene graph context for backend 'openvg' - check that plugins are installed correctly"

[JoeCFD]

@Muhammad-Hassan-Shahid said in Translucent background not rendering correctly in Qt6 with hardware rendering but works with Software rendering:

Could not create scene graph context for backend 'openvg' - check that plugins are installed correctly

The messages are not very helpful. Which plugins are not installed properly? You have to figure that out. I do not know it either. But you know the reasons now.

[Muhammad Hassan Shahid]

@JoeCFD The error is from Qt itself and it doesn't say which plugins exactly. Is there another way this functionality can be achieved in Qt which works with hardware rendering?

[JoeCFD]

@Muhammad-Hassan-Shahid message in running time or compiling time?
Linux or Windows?

[Muhammad Hassan Shahid]

@JoeCFD runtime error on windows

[Muhammad Hassan Shahid]

@Muhammad-Hassan-Shahid The solution is to set ui->quickWidget->quickWindow()->setColor(QColor(0,0,0,0)); after the main window has been rendered to bypass this issue with hardware rendering but this problem should be solved by Qt in the future. There shouldn't be a need to do something like this in the first place.

This is the code to reproduce it

#include <QQuickWidget>
#include <QQuickItem>
#include <QQmlError>
#include <QtWidgets>

class MainWindow : public QMainWindow {
Q_OBJECT
public:
MainWindow();
QQuickWidget *m_quickWidget;
};

MainWindow::MainWindow()
: m_quickWidget(new QQuickWidget)
{
setFixedSize(QSize(800,600));

m_quickWidget->setAttribute(Qt::WA_TranslucentBackground);
m_quickWidget->setClearColor(Qt::transparent);

QUrl source("qrc:transparent.qml");

m_quickWidget->resize(300,300);
m_quickWidget->setWindowFlags(Qt::SplashScreen);
m_quickWidget->setAttribute(Qt::WA_AlwaysStackOnTop);
m_quickWidget->setAttribute(Qt::WA_TranslucentBackground);
m_quickWidget->setClearColor(Qt::transparent);
m_quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
m_quickWidget->setSource(source);

QWidget *centralWidget = new QWidget();
centralWidget->setStyleSheet("background-color: blue");
QHBoxLayout *layout = new QHBoxLayout(centralWidget);
layout->addWidget(m_quickWidget);
setCentralWidget(centralWidget);
}

int main(int argc, char **argv)
{
QApplication app(argc, argv);

MainWindow mainWindow;
mainWindow.show();

mainWindow.m_quickWidget->quickWindow()->setColor(QColor(0,0,0,0));

return app.exec();
}

#include "main.moc"

[SGaist]

You should open a ticket on the bug report system to make it known to Qt developers.