Virtual reality rendering is very demanding on hardware for two reasons. First, the refresh rate of most VR headsets is 90 hz instead of the standard 60 hz refresh rate most computer monitors operate at. This means that rendering must complete in about 11 milliseconds instead of 16. Second, we have to render the scene twice with two different views, one for each eye. Without any special optimizations, this roughly means that we have to pack 16 milliseconds worth of rendering code into five milliseconds.
There are a few optimizations we can make to improve efficiency over a naive implementation. Although VR rendering requires two different views to be rendered, the two views are only a few centimeters apart.
We can perform culling for both views at once by simply widening the camera frustum a little bit so that both eyes are included the camera volume.
For rendering, Nvidia provides an extension for single-pass stereo rendering. This doubles up the geometry and renders to two different viewports at once, ensuring that the engine makes the same number of draw calls when rendering in stereo or normal mode. (The same can be done using a geometry shader with Intel graphics, although it has yet to be determined if VR will be possible on this hardware.) Here is the result:
Combined with all the other optimizations I've packed into Turbo, like clustered forward rendering and a multithreaded rendering architecture designed specifically for VR, this makes VR rendering blazingly fast. How fast is it? Well, it's so fast that SteamVR diagnostics thinks that it is going backwards in time. Take a look at the timer in the lower left corner here:
The obvious conclusion is that we have successfully broken the speed of light barrier and time travel will be possible shortly. Start thinking about what items or information you want to gift your past self with now, so that you can be prepared when I start offering temporal tourism packages on this site.