MacOS : QOpenGLWindow + QOpenGLWidget in MainWindow + QPainter causes repaint bug

Karpi

I have made a simple program that shows the following bug :

In MacOS only,
When one uses a QPainter in a QOpenOpenGLWindow + a QPainter in a QOpenOpenGLWidget,
And when both are rendered (not hidden),
The QOpenGLWindow becomes blank.

To reproduce :

Download the project
Compile and run -> the MainWindow that contains the QOpenGLWidget appears
Click the show button -> the QOpenGLWindow appears fine (because it uses OpenGL only internally)
Click the change QPainter use button -> the QOpenGLWindow goes blank (it now uses a QPainter)
Resize the QOpenGLWindow so it updates -> it flickers
Minimize the MainWindow
Resize the QOpenGLWindow so it updates -> it displays correctly again

Download link :
https://drive.google.com/file/d/1nCEjL1GiurQhULbIXI6pbIZ2b11FovBf/view?usp=sharing

Should I send an issue to Qt ?

MacOS 10.15.7
Qt 5.15.1
Xcode 12.2

Have a nice day

SGaist

Hi and welcome to devnet,

You need to add which version of Qt you are using as well as macOS and XCode version.

Karpi

@SGaist updated my post

Karpi

Bug seems to disappear if i don't explicitly request a 3.3 Core OpenGL profile : 2.1 OpenGL profile is given and things seems to work fine.
QPainter must behave in a different way whether it has the possibility to use VAOs and stuff or not

SGaist

~~QPainter does not use OpenGL.~~

As for the Profile, Apple's implementation of OpenGL is known to lag behind in comparison to other OS like Linux and Windows.

Karpi

@SGaist

QPainter does not use OpenGL.

Are you 100% sure about that ?
Documentation seems to say the oposite :

Moreover, i have observed the effective QPainter OpenGL draw calls with RenderDoc on Windows

Anyway, i have a workaround for this issue right now so this is not much of a concern for me anymore.
I still think this is a real issue and that it should be reported. It may be related to https://bugreports.qt.io/browse/QTBUG-69644

SGaist

You are correct, it's an evolution I have missed a bit.

Thanks for the link !

Can you share your workaround ?

Karpi

@SGaist

Can you share your workaround ?

I use the QPainter to create a texture instead :

void AQOpenGLWindowSubclass::paintGL()
{
#ifdef __WIN32
    QPainter painter(this);
    
    painter.beginNativePainting();
    // Use OpenGL API
    // glSomething(...)
    // [...]
    painter.endNativePainting();
    // Use QPainter to paint directly on the window surface
    // painter.drawSomething(...);
    // [...]

#elif defined (__APPLE__)
    // Use OpenGL API
    // glSomething(...)
    // [...]

    // Create a QImage to use QPainter on
    QImage paintedImage(width, height, QImage::Format_ARGB32_Premultiplied);
    QPainter painter(&paintedImage);
    // Clear image (not always done per default it seems)
    paintedImage.fill(QColor::fromRgbF(0,0,0,0));
    // Use QPainter to paint on the image
    // painter.drawSomething(...);
    // [...]

    // Convert QImage into QTexture
    QOpenGLTexture paintedTexture(QOpenGLTexture::Target2D);
    paintedTexture.setMinMagFilters(QOpenGLTexture::Filter::Linear, QOpenGLTexture::Linear);
    paintedTexture.setData(paintedImage.mirrored(), QOpenGLTexture::DontGenerateMipMaps);

    // Use OpenGL to draw the texture on the window surface
    // But set a decent blending function before if you want to use it 
    // (It was set to something troubling like glBlendFunc(GL_ONE, GL_ZERO) or something)
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_BLEND);
    // Draw the painted texture the way you want
    // glUseProgram(...)
    // glActiveTexture(GL_TEXTURE0);
    // glBindTexture(GL_TEXTURE_2D, paintedTexture.textureId());
    // ...
#endif

SGaist

Thanks for the code !

You can use the Q_OS_XXX macros to ifdef your code.