[Desktop / GLFW] Features proposal : untethered context creation and OpenGL Compatibility Profile.

Hello everyone,

For a project I needed to access the last features of libVLC 4.0 that runs its instance on a separated thread with a shared OpenGL context.

Two issues arised :

1. libVLC requiered an OpenGL Compatibility Profile where OpenFrameworks is bound to Core Profile¹.
2. Window creation links² the window to the windowApp map and make its context current on the main thread every frame³, breaking the context when used simultaneously in another thread.

References :

• 1 : Forced Core Profile on OpenGL 3.2+.
  https://github.com/openframeworks/openFrameworks/blob/ce1bbc751595881398934e82029d74fe099899c5/libs/openFrameworks/app/ofAppGLFWWindow.cpp#L200
• 2 : Tethered GLFW window / context creation.
  https://github.com/openframeworks/openFrameworks/blob/ce1bbc751595881398934e82029d74fe099899c5/libs/openFrameworks/app/ofMainLoop.cpp#L68
• 3 : Main thread preempting the window context every loop.
  https://github.com/openframeworks/openFrameworks/blob/ce1bbc751595881398934e82029d74fe099899c5/libs/openFrameworks/app/ofMainLoop.cpp#L147

Proposed implementations :

The workarounds are simple and supposedly break-free.

For the first case, we can add a boolean flag in ofGLFWWindowSettings to use GLFW_OPENGL_COMPAT_PROFILE
instead of GLFW_OPENGL_CORE_PROFILE.

For the second case, we can either create an ofCreateContext function or add a boolean in settings (eg. useCustomThread) that pass ofMainloop.cpp#L68.

Here is an example that set off every graphics presenting capabilities, unasked for in a windowless context :

shared_ptr<ofAppBaseWindow> ofCreateContext(const ofWindowSettings & _settings) {
  ofGLFWWindowSettings settings(_settings);
  settings.doubleBuffering = false;
  settings.visible = false;
  settings.setSize(1, 1);

  shared_ptr<ofAppGLFWWindow> context = std::make_shared<ofAppGLFWWindow>();
  
  context->setup(settings);
  context->setVerticalSync(false);

  return context;
}

Any thoughts on this ?

Start with libs/openFrameworks/app/ofAppGLFWWindow.cpp for the referenced OpenGL profile setup and libs/openFrameworks/app/ofMainLoop.cpp for window registration and context-current behavior. Compare the proposed settings flag and context-creation alternatives; done means compatibility-profile selection and an untethered, windowless shared context are supported without the main loop reclaiming the context.