QWebEngineView->render() throws nullptr exception

ANorton

Hello everyone, having a Qt 5.15.5 that was built for x86, Windows, QWebEngineView and QGraphicsScene (used for off-screen rendering)
Rendering a QWebEngineView by render() method, and everything works-as-designed.
But, in some undefined scenarios, it throws some nullpointer exception within QOpenGLFunctions::glReadPixels: a pixels is nullptr.
I think that's may be a some strange bug that should be fixed, passing some screenshots of exception points
Point first: calling QGraphicsScene render() method

And then the point where exception happens
88e528e7-c9b9-40bd-a6c5-4ad41d63236d-image.png

If you need a full call stack then I can provide it, the only problem is that this exception happens not always, once in 35-40 minutes i guess

JonB

@ANorton said in QWebEngineView->render() throws nullptr exception:

If you need a full call stack then I can provide it

Certainly that will give a clue.

ANorton

@JonB

Paul Colby

Hi @ANorton,

I could be wrong (not at all familiar with Qt's painters, graphics scenes, and OpenGL) but try removing the this from the QPainter's constructor.

Typically, Qt's QObject-derived classed accept other QObject-derived classes (such as your main_window) to set a parent-child relationship for chained destruction, which is great! :) So its not at all surprising to see you using the common pattern of constructing a Qt class with this.

However, QPainter is not QObject-derived, and is not using this typical Qt "parent" pattern. According to the docs, the QPainter(QPaintDevice *device) constructor that you are using:

Constructs a painter that begins painting the paint device immediately.

In other words, its a convenient overload that automatically invokes QPainter::begin(this) (on construction) and QPainter::end() (on destruction).

Since you're using the graphics scene to paint, I think that you're meant to use the QPainter() constructor, so that the graphics scene can handle the operation completely its own way.

Anyway, it's just a hunch, but worth a try :)

Cheers.