异步渲染到Qt OpenGL窗口

Question

I am writing a Windows Visual Studio 2017 C++ program that will render a series of frames from a capture card onto a Qt 5.9 OpenGL Window. Loading and processing the images is done asynchronously in a thread until it is stopped:

void readFile(std::string filename)
{
  auto reader = openreader(file);

   while(!done())
   {
      QImage frame = reader.read();
      processFrame(frame);
      displayFrame(frame); /// <--- here
}

and the thread might be kicked off by

std::thread workit(readfile,"c:\funtimes.dat");

What is the proper pattern to follow to get the QImage frame containing pixels onto the screen? In .Net some care must be taken to only modify the gui from the main thread. How does that work with Qt?

The current way I have is to set a local variable to equal a copy of the frame, then call Update() on the OpenGL QWidget. That is, the code code displayFrame looks about like:

void displayFrame(QImage frame)
{
   // Mutex here
   oglWindow.copyOfFrame= frame.copy();
   oglWindow.update(); 
}

This triggers a paintEvent that draws to the window with

painter.drawImage(QPoint(width()/2 - bi.width()/2, height()/2 - bi.height()/2), copyOfFrame);

Is this safe and appropriate when a thread outside the main window message pump is triggering updates or is another mechanism required?

Answer 1

If your asynchronous image loader is a subclass of QThread, you can signal the main thread from the image loader and QT will ensure that the handling code slot is executed in the main thread. See the Qt Threads and QObjects doc

If the image loader isn't a QThread, just a regular Win32 thread, you should use QCoreApplication::PostEvent for thread safe delivery