Question

I have a QMainWindow with three widgets inside that are promoted to a class containing a subclassed QThread. They each draw on a local QImage in their rexpective qthread which is sent with a signal once its drawn and then rendered by calling update (mandlebrot example) from the slot. Is this safe or dangerous? They do not share any data, at least none that I am generating and am wondering what data they could be sharing that is outside of my coding range ie that is generated by Qt automatically.

Answer 1

From the documentation:

QPainter can be used to paint onto QImage, QPrinter, and QPicture paint devices. Painting onto QPixmaps and QWidgets is not supported. On Mac OS X the automatic progress dialog will not be displayed if you are printing from outside the GUI thread.

Any number of threads can paint at any given time, however only one thread at a time can paint on a given paint device. In other words, two threads can paint at the same time if each paints onto separate QImages, but the two threads cannot paint onto the same QImage at the same time.

Note that on X11 systems without FontConfig support, Qt cannot render text outside of the GUI thread. You can use the QFontDatabase::supportsThreadedFontRendering() function to detect whether or not font rendering can be used outside the GUI thread.

So as long as you’re careful about the font issue on X11, don’t use any Pixmaps or Cursors (which are implemented as Pixmaps), and don’t try to paint onto the same image at the same time from multiple threads… it should work.

Note that in the Mandelbrot example, the QImage is passed by value over the signal (implicitly shared), meaning that if either thread attempts a write it will get its own copy.