We are trying to use Qt 4.8.5 for some Linux-based embedded devices in our company. I use Qt embedded without X server. I need to plot measured data and update them very often (20-30fps, but only a small portion of the widget). The system is ARM based, 400Mhz, have no GPU and no FPU. I subclassed QWidget and overridden the paintEvent(). I have WA_OpaquePaintEvent and WA_StaticContents set. For testing, my paint event is empty, and I call the update() function of the widget form a timer set to 50ms. My problem is that the empty update is eating up 30% of the CPU. The amount varies with the area of the update, so I think QT may redraw something in the background. I have read many posts but I cannot find the solution for my problem. If I comment out the update call, the CPU usage drops to ~1% (even if I generate a sine in the timer for testing the widget, which should be much more complex than an empty function call). My widget is rectangular, is not transparent and I want to handle the full drawing procedure from the paint event.
Is it possible to reduce this overhead, and handle the whole painting process by my own?
The "empty update" is not empty - it repaints the whole window :)
Have you read the below?
To rapidly update custom widgets with simple background colors, such as real-time plotting or graphing widgets, it is better to define a suitable background color (using setBackgroundRole() with the QPalette::Window role), set the autoFillBackground property, and only implement the necessary drawing functionality in the widget's paintEvent().
You should also be using
QWidget::scroll(), since internally it does scroll the backing store of the window, and that's much more efficient than repainting the entire thing if only a tiny slice is added to it.