Fallback to OpenGL 3 if other rendering drivers are not supported

[SheepYhangCN]

Add an option to allow renderer fall back to OpenGL 3 if Vulkan (and D3D12 on Windows, Metal on Apple devices) are not supported
Free players from using --rendering-driver opengl3 manually
Free developers from recieving "Why my game crashed with a message that I can't understand"

[akien-mga]

This might solve godotengine/godot-proposals#8006, but it should support all platforms, not just Windows.

I support the idea in general, I think this is indeed needed to better handle users who don't support modern graphics APIs.

[SheepYhangCN]

but it should support all platforms, not just Windows.

I will try to do this for other platforms later
Since Metal is native now so MacOS/iOS shouldn't need for OpenGL fallback?

[Calinou]

Since Metal is native now so MacOS/iOS shouldn't need for OpenGL fallback?

Metal support is only available on Apple Silicon hardware, but most Intel Macs have Vulkan support through MoltenVK. That said, old Intel Macs can't run Vulkan through MoltenVK due to missing hardware features, so the Vulkan -> OpenGL fallback would still be needed on macOS.

The same goes for iOS - old devices can't run Vulkan through MoltenVK either: #74227

[SheepYhangCN]

Since Metal is native now so MacOS/iOS shouldn't need for OpenGL fallback?

Metal support is only available on Apple Silicon hardware, but most Intel Macs have Vulkan support through MoltenVK. That said, old Intel Macs can't run Vulkan through MoltenVK due to missing hardware features, so the Vulkan -> OpenGL fallback would still be needed on macOS.

Understood

[clayjohn]

The code looks good, but when I test locally (with modifications to make rendering_context->initialize() fail), I get a deadlock. But only when opening the project through the project manager. If the project is opened on its own, it falls back correctly.

I'm investigating further now.

[clayjohn]

I figured out the issue, the fallback code wasn't setting the rendering_method, therefore, any calls to OS::get_singleton()->get_current_rendering_method() returned the wrong value.

We don't use the current rendering method often, but we do check it when rendering to ensure that the opengl context is bound to the correct window:

godot/servers/rendering/renderer_viewport.cpp

Lines 261 to 264 in 694d3c2

	if (OS::get_singleton()->get_current_rendering_method() == "gl_compatibility") {
	// This is currently needed for GLES to keep the current window being rendered to up to date
	DisplayServer::get_singleton()->gl_window_make_current(p_viewport->viewport_to_screen);
	}

In the editor, this code wouldn't get called. So the "loading" popup would take the openGL context and then not return it leading to the main screen never updating. So it wasn't a deadlock after all!

The solution is to set the rendering_method at the same time that we set the driver:

OS::get_singleton()->set_current_rendering_driver_name(rendering_driver);
OS::get_singleton()->set_current_rendering_method("gl_compatibility");

[SheepYhangCN]

Fixed

[clayjohn]

Looks great! The final step before merging is to squash all the commits together so that the whole PR only contains 1 big commit with all your changes. We like to merge one commit at a time to keep the git history clean and navigable.

If you don't know how to do that, we have a helpful tutorial in the official documentation https://docs.godotengine.org/en/latest/community/contributing/pr_workflow.html#the-interactive-rebase

[SheepYhangCN]

Everything should be okay now I think

[clayjohn]

Yep, looks great! Thank you