Windows OpenGL and choosing from multiple video cards

There are vendor- and platform-specific extensions that can do it for you. They're usually only available in work-station drivers, and unfortunately not in the drivers for desktop-versions of the cards. For example WGL_NV_GPU_AFFINITY. I'm not sure if that actually works for multiple cards from different vendors though..

Normally, it usually(always?) uses the adapter that you have set as your primary monitor with the start-menu in the monitor control panel. As far as I know there's no reliable way to override that. It's rather annoying though, as you can actually select one monitor as primary, create one OpenGL context which will use that monitor, then go into the control panel and switch which monitor is primary while that OpenGL context is running, and afterward create one more OpenGL context, and you will have one context on each card.

I've actually tested that on my system with Windows 7 64 and one AMD and one NVidia card, and it works, but it's a pain and not really a viable solution.. though I guess you could change the primary monitor programmatically if you just want it for yourself. Since it's that simple there's _probably_ a way to hack it without changing the primary monitor too, but I don't know how.
Also, it's definitely not guaranteed to always work on other configurations. For example I don't know what happens if the same driver controls both cards.. in my case it's two completely different drivers for the two cards.

Aren't you supposed to do this by calling EnumDisplayDevices to get the list of video cards and then using CreateDC to create HDCs on those devices? Then you can use wglChoosePixelFormat (which takes the HDC) to get a render context of the right type on the right device?

Aren't you supposed to do this by calling EnumDisplayDevices to get the list of video cards and then using CreateDC to create HDCs on those devices? Then you can use wglChoosePixelFormat (which takes the HDC) to get a render context of the right type on the right device?

That doesn't work, it still creates the OpenGL context for the primary adapter on every system I've tried it (some drivers won't even allow a GL context to be created on such a DC). I've read that if you have two separate ATI cards in the same machine then it works by simply creating the window on the correct monitor, but I never had two ATI cards so I haven't tried it. I have tried with two NVidia cards and with 1 Nvidia + 1 ATI in almost every way I could think of and it never worked.

I think that is because opengl32.dll send calls to the real driver, which is written by the vendor. So if your primary monitor is nvidia, it is the nvidia GL driver that execute commands and it certainly isn't made for your secondary card if it is from another vendor.

If both cards are from the same vendor, then it should work (AMD or nvidia).They have some "special" coding in their driver to handle multi card situations.

If they are different, then I think for the second card, it just runs the default ms driver (opengl32.dll). Am I correct?

Let me bump this thread, because it looks very similar to the issue that I'm struggling with at the moment (see this gamedev thread).

In the context of switchable graphics, I end up with the problem that opengl32.dll uses the wrong device if an accelerated window is created with CreateWindow. Although the display adapter (in my problematic case a Mobile Intel 4 Express Chipset) seems to be available with support for OpenGL 2.1 (I've checked this with OpenGL Extension Viewer 4.0) the GL_RENDERER that is used for my application is Microsoft's Generic GDI renderer.

First idea, of course: Driver problem. However, at a closer look, one may come up with the idea that the system simply chooses the wrong candidate out of two (Intel, GDI). Therefore, an appropriate solution to that issue could be to choose the display device instead of using the default one. Well, as can be read in this thread, this is not that easy...

I stumbled across this thread that describes a (hacky) solution how to choose the device manually. Unfortunately, it did not work for me, and I'm not really convinced that (if it would work) this solution is robust, but this is the best I've found so far.

Another post on stackoverflow sums up ideas also expressed in this thread.

It seems that there is a stable solution for this issue: the OpenGL Extension Viewer 4.0 I've mentioned above is able to switch between different display adapters, to show its OpenGL capabilities and(!) to run rendering tests. I wonder how this is possible!

Any ideas?