[ah-]

From what I've seen so far it copies every frame via the CPU and that gets quite slow if you want to do it 60 times a second at high resolution.

Optimus etc. use DMA, so you can copy directly between the two devices.

Hopefully looking glass will support that as well in the future. Last time I looked at least nvidias DMA support was officially restricted to quadro only, so not sure it'll actually happen.

[zeusk]

Microsoft has support for cross-adapter rendering for (SurfaceBook dGPU is hotpluggable) - where textures are resident in the dGPU memory (when in use) but the render target surface lies in iGPU's memory which directly scans out that surface every vsync.

Technically, it is slightly more complex because of how vsync works but zero copy cross adapter rendering is possible.

I was an intern at the display kernel team this past summer and worked on a similar project for VR displays (which adds further complexity due to late stage reprojection and sensitivity to pipeline latency).