Yes, based on my test 😀
The "main" is the one connected to the desktop you choose in "Show only on 1" / Show only on 2" box (I got the 2 VGAs connected to the same monitor, DVI for the NVidia GeForce and HDMI for the Intel HD Graphics).
Re-tested with Unreal (227j) and the updated Win10 1809 (build 253) using the DDRAW (DX6/7) renderer: only the main (choosen) GPU manages the rendering process, power saving config is simply not considered.
So that just leaves one question unanswered; is this the limitation in Windows itself or the drivers?
wrote:A man can be himself only so long as he is alone; and if he does not love solitude, he will not love freedom; for it is only when he is alone that he is really free.
Good question!
I guess Windows.
OpenGL has the same behaviour of DX<DX9 (main GPU only).
I don't really know how this was handled in the first Optimus "on the fly switch" iterations, but at least starting from 8.1 (more precisely build 9321, according to leaked source code) Windows calls to the new function Direct3D9ForceHybridEnumeration from inside d3d9 dll.
Newer direct3d versions presumably never needed such extra mechanism because of saner design assumptions about the number of gpus in a system (and it's a easy thing even with vulkan and opengl.. it's just that you have to use the gpu driver control panel and not the DX-only windows settings), so I think the deal here is that nobody ever went back to add it to dx7 or dx8. Somewhat out of laziness maybe, or perhaps because somebody reasoned that at the end of the day (Intel and AMD drivers legacy apis bugs aside) "modern" integrated graphics should be well up to the task of running 15 years old games.
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