How to share OpenGL context or data?

Question

I need to shared data (textures, vertex-buffers,... ) across all OpenGL widgets in a application.

The following code isn't working:

I've found some solutions that have one main QGLWidget and other are constructed using this main widget. Unfortunately, I can't use this approach, because all my QGLWidgets are equal and almost certainly the first(main) created QGLWidget will be destroyed before others are.

Possible approach:

single shared OpenGL context between all QGLWidgets
not working: just one QGLWidget gets rendered correctly, others behave as they weren't rendered, corrupted/random data

error for each QGLWidget construction except first one:

QGLWidget::setContext: Context must refer to this widget

Another approach:

main OpenGL context and create sub-context for each QGLWidget
not working: context->isSharing() returns false

code that I use for context creation, context1 and context2 are later passed to constructors of QGLWidgets:

QGLContext *mainContext = new QGLContext(format), *context1, *context2;
mainContext->create();
context1 = new QGLContext(format);
context1->create(mainContext);
context2 = new QGLContext(format);
context2->create(mainContext);
cout << mainContext->isSharing() << " " <<  context1->isSharing() << endl;

The ... in your description of resource sharing concerns me. Not all OpenGL objects are shareable, particularly the entire class of objects referred to as containers. Container objects in OpenGL include things like Vertex Array Objects and Framebuffer Objects. To that end, the general state machine is stored per-context and this might explain why things behave as if they were "corrupted" or "random." — Andon M. Coleman
If I use single context, the second QGLWidget acts like it wasn't even rendered, like if glClear ( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT ) wasn't performed — kravemir
That is not the same thing as context sharing. That is re-using the same context. Generally render contexts are associated with some rectangular region of your window at the lowest-level (not to be confused with the viewport mapping); Qt hides the window system implementation from you, but if you want to switch between widgets then you probably have to redefine this region somehow. This assumes, of course, that you are not drawing each widget in a different thread - because that opens an entirely different can of worms. Widgets have a makeCurrent method that should address this issue. — Andon M. Coleman

Laszlo Agocs Laszlo Agocs · Accepted Answer · 2014-01-07T14:01:55

With regards to the first approach, you are not setting up sharing but trying to force the same context to be used with different QGLWidgets. As pointed out above, this is wrong and will not work.

Instead, create the QGLWidgets normally and pass the first QGLWidget in the shareWidget parameter when creating the others. This way you will get a separate context for each QGLWidget but they will all share with the context of the first one (and thus with each other). See http://qt-project.org/doc/qt-4.8/qglwidget.html#QGLWidget

Destroying the first widget before the others should not be an issue since the shared objects will be around until any of the sharing contexts are alive.

How to share OpenGL context or data?

3 Answers