How to render text in QQuickFramebufferObject?



  • Motivation: I currently have 1000 simple items in my QML scene, and one of them animates at 60fps, so the entire scene repaints at 60fps. Cpu usage is on average 15% on each of my 4 virtual cores on my pc. On the target hardware the situation is even worse - 60% on each of the 4 physical cores, leading to overheating, leading to freeze. Note that I have implemented an optimization: via Loaders, unload all items that are outside the (scrolling) viewport (so, only ~18 items are loaded at any given time). The perf stats I report are with this optimization - without it it's worse.

    My solution is to start drawing all the 1000 items in a single QQuickFramebufferObject, and stop having them as actual QML Item's. That way I'll avoid Qt punishing me for just having 1000 (unloaded!) items.

    Where I'm stuck though: How to draw the text parts of the items in OpenGL?

    Approach 1: I know QPainter can be used to directly render text into a QOpenGLWidget, but that option seems to be absent in QQFBO.

    Approach 2: Have a single, parentless Text item in QML with layer.enabled: true, set its text property, wait 1 frame (for it to render) then fetch the texture. Somewhat ugly and roundabout; also may be slow-ish.

    Approach 3: Look at the source of QQuickText to see what magic it does and copy it. Might be difficult, and I'll have to comply with license restrictions.

    Approach 4: Do software rendering with a QPainter to a QImage, then upload that image to a texture. Elegant, but may be too slow.

    Any suggestions on a way to do it that doesn't have the problems in these approaches?



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