Figuring out Single-Context, Multi-window

[Alloyed]

Right now, I'm planning to use magnum in an Digital Audio Workstation (DAW) plugin, for audio synths and effects and such! The pattern that the GUIs for these usually take is via window embedding: you write your plugin as a shared object dll/so, and then the "host" program gives you a raw pointer to the underlying window you can then embed yourself into. of course, you can run as many instances of a single gui that you'd like, which feels like a good opportunity to figure out Single Context/Multiple Window as mentioned here (I'm imagining sharing textures and fonts and stuff, which are going to be identical across instances)

#321

so how should this work, given a setup that looks like this? I guess here any window with the same mDeviceContext can share an mWglContext and then also the Magnum::Platform::GLContext? and then just call makeCurrent() as before

    if (!CreateWglContext()) {
        return false;
    }

    ::wglMakeCurrent(mDeviceContext, mWglContext);
    Magnum::Platform::GLContext::makeCurrent(nullptr);
    mGl = std::make_unique<Magnum::Platform::GLContext>();
    Magnum::Platform::GLContext::makeCurrent(mGl.get());

bool ContextImpl::CreateWglContext() {
    mDeviceContext = ::GetDC(mWindow);
    if (mDeviceContext == nullptr) {
        kitgui::log::error(GetLastWinError());
        return false;
    }
    PIXELFORMATDESCRIPTOR pfd = {0};
    pfd.nSize = sizeof(pfd);
    pfd.nVersion = 1;
    pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
    pfd.iPixelType = PFD_TYPE_RGBA;
    pfd.cColorBits = 32;

    const int pf = ::ChoosePixelFormat(mDeviceContext, &pfd);
    if (pf == 0 || ::SetPixelFormat(mDeviceContext, pf, &pfd) == false) {
        kitgui::log::error(GetLastWinError());
        return false;
    }

    mWglContext = ::wglCreateContext(mDeviceContext);
    return true;
}

[mosra]

Sorry for a delayed reply, was away for most of the last week and this notification slipped through.

audio synths and effects and such!

Very cool!

I guess here any window with the same mDeviceContext can share an mWglContext and then also the Magnum::Platform::GLContext?

As long as all windows use the same OpenGL context, you can (and should) have just one Platform::GLContext for all of them. And then the trick is in calling wglMakeCurrent() with the same GL context (second argument) but different window surface (first argument) in order to render to the other window.

I'm not sure how exactly things work in the WGL land, but I suppose it'd involve having multiple mDeviceContexts, one per window. The Remarks section of wglCreateContext() documentation docs lists various requirements for this to be possible.

[fenggy]

目前，我计划在数字音频工作站 (DAW) 插件中使用 Magnum，用于音频合成器、效果器等等！这类插件的 GUI 通常采用窗口嵌入的方式：将插件编写成共享对象 DLL/SO 文件，然后“宿主”程序会提供一个指向底层窗口的原始指针，之后你可以将插件嵌入到该窗口。当然，你可以运行任意多个 GUI 实例，这正好是一个研究这里提到的单上下文/多窗口机制的好机会（我设想共享纹理、字体等资源，这些资源在所有实例中都将保持一致）。

#321

那么，在这样的设置下，它应该如何工作呢？我猜想，任何具有相同属性的窗口mDeviceContext都可以共享一个mWglContext，然后也可以共享一个问号Magnum::Platform::GLContext，然后像以前一样调用 makeCurrent() 函数。

    if (!CreateWglContext()) {
        return false;
    }

    ::wglMakeCurrent(mDeviceContext, mWglContext);
    Magnum::Platform::GLContext::makeCurrent(nullptr);
    mGl = std::make_unique<Magnum::Platform::GLContext>();
    Magnum::Platform::GLContext::makeCurrent(mGl.get());

bool ContextImpl::CreateWglContext() {
    mDeviceContext = ::GetDC(mWindow);
    if (mDeviceContext == nullptr) {
        kitgui::log::error(GetLastWinError());
        return false;
    }
    PIXELFORMATDESCRIPTOR pfd = {0};
    pfd.nSize = sizeof(pfd);
    pfd.nVersion = 1;
    pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
    pfd.iPixelType = PFD_TYPE_RGBA;
    pfd.cColorBits = 32;

    const int pf = ::ChoosePixelFormat(mDeviceContext, &pfd);
    if (pf == 0 || ::SetPixelFormat(mDeviceContext, pf, &pfd) == false) {
        kitgui::log::error(GetLastWinError());
        return false;
    }

    mWglContext = ::wglCreateContext(mDeviceContext);
    return true;
}

I'd like to ask if this issue has been resolved, as I'll also need it in the future.
I am a digital twin application with a main window that serves as a 2D graphical interface, which can be allocated multiple 3D regions to display different 3D scenes.

[mosra]

If you use an external windowing toolkit (such as in this case with raw WINAPI, or in your case with I think wxWidgets?), it's completely under your control and there's nothing left to be done from Magnum's side. So with wxWidgets you'd do an equivalent of what I said above, making the same GL context current with different surfaces/windows/drawables (or how is this concept called there).

For Magnum's own toolkit wrappers, builtin multi-window support with SDL and GLFW is planned, and is tracked in #124.

[fenggy]

Alright, we're still working on building the IDE, and I'll make further attempts once we reach this stage.