Multi-context/window rendering.

[Tryum]

Is it possible to use magnum to render to different contexts ?

I'm using multiple-windows, doing the all the complex rendering from the main context through render to texture. I'm wondering if I can do the quad rendering for the other windows through Magnum or if I should do direct openGL calls.

[mosra]

Hi!

(apologies if you know this already) not widely known, but it's possible (and encouraged) to use a single GL context to render to multiple windows. For rendering to a different window you need to make it current on a different surface. Having one GL context per window is also possible, but the overhead from context switching and the limitations of sharing data between different GL contexts is not worth the pain.

As far as Magnum::GL is concerned, it should "just work", no need to use direct OpenGL calls. The window switching is done by the platform-/toolkit-specific "make context current" call.

You only need to do extra work on the "application" side, but from the above I assume you're doing that already. There's a (quite stale) PR #168 that is adding multi-window single-context support to Platform::Sdl2Application, so you can look there to see how's that done for SDL. However, with the current state of the Sdl2Application API it should be possible to have multi-window support without needing to patch the Sdl2Application itself -- by calling SDL window functions directly. Plus for each event there's direct access to the underlying SDL_Event to know what window it is attached to.

[Tryum]

Ok, I'll give it a shot on my glfw app !
Thanks.

[Tryum]

Hmm,
So, at first I thought it was not working. But then messing around, I discovered that Imgui integration messes things around. If I disable imgui it works.
What's more interesting is that if I create a 3rd window, the rendering in the 3rd one is ok !

[Tryum]

GL::Context::resetState() to the rescue ;)

[mosra]

Can you share what exactly was the problem? We recently discovered issues with the ImGui rendering which were also "fixable" by calling resetState(), but ultimately it was a state tracker bug, fixed in ee16c25 / d6e0186.

So maybe this is yet another case of a state tracker bug that should be fixed on the engine side?

[Tryum]

I wrote a small sample at home, I'll push it tonight.

To test I used the red-square-one-liner.
First try I had nothing (just the clear screen)
Then I disabled the clearing, and the square was flickering.
I also noticed that if I call two draws, then I had a solid square.

[Tryum]

Here it is : https://github.com/Tryum/magnum-imgui-state-bug (press a key to toggle imgui rendering).

I did try to first draw a square in the primary window, it seems to solve the state tracker issue, as everything was rendered fine.

I didn't realise that the commit you referenced were recent, I'll try to setup a master build of magnum to try out.

[mosra]

Not seeing any flickering with current master ... so I guess that recent fix solved this as well?

[Tryum]

I just discovered the --head option of vcpkg trying in a few minutes ;)

[Tryum]

Nop problem not solved in my case with head builds.

What can I do to help tracking the state inconsistency ?

[mosra]

Apologies if you said this elsewhere -- what's the system and GPU you're running on? Can you paste the engine startup log here?

Some ideas to try:

• The log mentions what all optional extensions is it using. Running the app with e.g. --magnum-disable-extensions "GL_ARB_direct_state_access" will make it use different code paths and maybe that could narrow the issue down. The usual suspects responsible for problems (or having driver bugs) are GL_ARB_direct_state_access and GL_ARB_vertex_array_object.
• Try running the app with --magnum-gpu-validation on. Maybe it'll show some GL errors or other debug info that could be helpful.
• I am not sure if the framebuffer-related state is stored per-window or dragged along when making the context current on a different window, so maybe setting a clear color / viewport / scissor on one window affect the others as well, causing some flicker etc?

[Tryum]

I'm running on a windows 10 PC with a 1080ti. On special thing is I'm running in a VM with GPU pass-through, but I'll dual-boot to a native win10 to clear that out.

Here's the engine startup log :

Renderer: GeForce GTX 1080 Ti/PCIe/SSE2 by NVIDIA Corporation
OpenGL version: 3.3.0 NVIDIA 419.35
Using optional features:
    GL_ARB_ES2_compatibility
    GL_ARB_direct_state_access
    GL_ARB_get_texture_sub_image
    GL_ARB_invalidate_subdata
    GL_ARB_multi_bind
    GL_ARB_robustness
    GL_ARB_separate_shader_objects
    GL_ARB_texture_filter_anisotropic
    GL_ARB_texture_storage
    GL_ARB_texture_storage_multisample
    GL_ARB_vertex_array_object
    GL_KHR_debug
Using driver workarounds:
    no-layout-qualifiers-on-old-glsl
    nv-zero-context-profile-mask
    nv-windows-dangling-transform-feedback-varying-names
    nv-cubemap-inconsistent-compressed-image-size
    nv-cubemap-broken-full-compressed-image-query
    nv-compressed-block-size-in-bits

I'll try your suggestions.