Does BGFX only support one Window/Swapchain under Vulkan?

[superhac]

We have a an app that uses SDL3 for window creation and then uses BGFX as the render for the surfaces that are provided by SDL3. So the screens look like this:

First is the table, then backglass, and the DMD respectively. The table renders like it should and the game plays fine, but both the backglass and DMD appear to only get the first frame and then never update again.

The first window, (the table) gets setup like this: https://github.com/vpinball/vpinball/blob/4082ab00bb5aabea09f045c58424a6a89034ede3/src/renderer/RenderDevice.cpp#L803

The other two windows get setup here with a frame buffer: https://github.com/vpinball/vpinball/blob/4082ab00bb5aabea09f045c58424a6a89034ede3/src/renderer/RenderDevice.cpp#L1471-L1493

Is there some sort of trick, a limitation of BGFX or is this a problem with SDL3? The application runs fine under X11/Vulkan but as linux moves to Wayland/Vulkan this is going to be an issue. Any help or insight would be great.

I turned on debug and these are my logs:
bgfx_logs.txt

I see only one instance of vkCreateSwapchainKHR:

/home/superhac/repos/new/vpinball/external/linux-x64/Debug/bgfx/bgfx.cmake/bgfx/src/renderer_vk.cpp (1898): BGFX        0xde802a70 vkCreateSwapchainKHR
2025-11-05 07:50:20.731 INFO  [186566] [RenderDevice::tBGFXCallback::traceVargs@88] /home/superhac/repos/new/vpinball/external/linux-x64/Debug/bgfx/bgfx.cmake/bgfx/src/renderer_vk.cpp (1898): BGFX        0xde802a70 vkCreateSwapchainKHR

Thanks

[bkaradzic]

Check example-22-widnows to get idea how multiple windows work in bgfx:
https://bkaradzic.github.io/bgfx/examples.html#windows

[superhac]

Thanks, that does indeed work as it is. Not sure how I missed that example. ;) I also then tested moving the core BGFX rending logic from the example into my test SDL3 app and came to the same result. So now my original question is answered. In the main vpx code though, I think I traced it down to this event in SDL3: SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED. I'm not sure how that is generated, but i think it has to do with changing the DPI. Is that another case where the framebuffer needs to be destroyed and recreated? Thanks again for your help!

[bkaradzic]

I don't think you need to destroy swap chain when DPI is changed.

[superhac]

Interesting on SDL3 migration there are two notes in the doc::

The following symbols have been removed:

    SDL_WINDOWEVENT_SIZE_CHANGED - handle the SDL_EVENT_WINDOW_RESIZED and SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED events instead
    SDL_WINDOWEVENT_TAKE_FOCUS

The SDL_EVENT_WINDOW_SIZE_CHANGED event has been removed, and you can use SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED to detect window backbuffer size changes.

It appears that the second I receive the SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED the auxiliary windows stop rendering. I will have to dig into that and see if that's the issue.