Qt - Working with Threads

Question

I have a QTimer for executing OpenCV code and changing an image in a QLabel every 20 milliseconds, but I want to run this OpenCV code more naturally and not depend on the timer.

Instead, I want to have one main thread that deals with user input and another thread that process images with OpenCV, what I can't find is a thread safe way to change the QLabel image (pixmap) in one thread from another thread, could someone describe this process, maybe give some code examples? I also want to know the pros and cons of using QThread, since it's plataform free, it sounds to be user level thread and not a system level which usually runs smoother.

Answer 1

You can only instantiate and work with QPixmap on the main (GUI) thread of your application (e.g. what is returned by QApplication::instance()->thread())

That's not to say you can't work with a QPainter and graphics objects on other threads. Most things work, with exceptions guided by constraints imposed by the OS.

Successive versions of Qt have managed to find ways to support things that previously didn't work. For instance:

• Qt 4.0 added rendering QImages from separate threads
• Qt 4.4 added the ability to render text into images
• Qt 4.8 added the ability to use QPainter in a separate thread to render to a QGLWidget, QGLPixelBuffer and QGLFrameBufferObject
• Where Qt 4.4 introduced QFontDatabase::supportsThreadedFontRendering to check to see if font rendering was supported outside the GUI thread, in Qt5 this is considered obsolete and always returns true

Note: you shouldn't hold your breath for the day that Qt adds support to work with QPixmap on non-GUI threads. The reason they exist is to engage the graphics layer directly; and finding a way to work around it just so you could use something named QPixmap wouldn't do any good, as at that point you'd just be using something equivalent to what already exists as QBitmap.

So all that has to do with the ability to instantiate graphics objects like QFont and QImage on another thread, and use them in QPainter calls to generate a graphical image. But that image can't be tied directly to an active part of the display...you'll always be drawing into some off-screen buffer. It will have to be safely passed back to the main thread...which is the gateway that all the direct-to-widget drawing and mouse events and such must pass through.

A well known example of how one might do this is Qt's Mandelbrot Sample; and I'd suggest getting started with that... making sure you understand it completely.

Note: For a different spin on technique, you might be interested to look at my Thinker-Qt re-imagining of that same sample.