How to trigger a QQuickFramebufferObject repaint?

kaixoo

I am subclassing QQuickFramebufferObject and QQuickFramebufferObject::Renderer to embed an OpenGL canvas in my application. I am calling render() to queue a repaint, but that isn't working.
Instead, the canvas is only being painted briefly when it is resized, and then goes back to white. Calling render() does nothing.

Here is my QQuickFramebufferObject implementation:

ApplicationCanvas::Renderer::Renderer(const ApplicationCanvas *app_canvas) : m_app_canvas(app_canvas) {
	QObject::connect(m_app_canvas, &ApplicationCanvas::onDrawQueued, [=]() { render(); });
    // render() internally calls draw_to_screen()
};

void ApplicationCanvas::Renderer::draw_to_screen(SkCanvas* canvas) {
	canvas->drawColor(SK_ColorBLACK); // does not work
        for (auto& viewlayer : m_viewlayer_list) {
                viewlayer->draw_to_screen(canvas);
        };

	// 'Flushing' is what actually renders this frame and shows it to screen
	canvas->flush();
};

void ApplicationCanvas::keyPressEvent(QKeyEvent *event) {
        ...

	emit onDrawQueued();
};

GrecKo

update() should be called on your QQuickFBO, not its renderer.

Change all your emit onDrawQueued(); to just update();

SGaist

Hi and welcome to devnet,

Did you already took a look at the QQuickWidget - QQuickView Comparison Example ?

It shows a usage example of this class.

Hope it helps

kaixoo

Thanks! I didn't know an example existed. I read how it works.

Still, the example uses update() to trigger a repaint. I am using that too. And in my case it does not work and I don't know why.

I am subclassing QQuickFramebufferObject::Renderer twice, I assume it has something to do with this?

QQuickFramebufferObject::Renderer -> SkiaRenderer -> ApplicationCanvas::Renderer

Full source code: https://gitlab.com/advanced-effects/Advanced-Effects/-/blob/implement-canvas-objects/src/projectscene/view/canvas/application_canvas.cpp?ref_type=heads

Here is the full file:

kaixoo

Here's a video demonstration of the issue: https://youtu.be/ojqcShBNmcI
When it's WHITE, it's NOT rendering. When it's BLACK, it IS rendering.

GrecKo

update() should be called on your QQuickFBO, not its renderer.

Change all your emit onDrawQueued(); to just update();

kaixoo

I don't know why I though update() was a method of the FBO::Renderer. I changed it now, and I'm having the same issue.

Image gets temporarily rendered when resizing the widget, and then goes back to blank.

EDIT: Oh, actually, it was my own fault. Was calling canvas->flush() which deletes the canvas after drawing. :facepalm: