Why can't I use multiple graphics cards for VR?

Many people looking at an HTC Vive or an Oculus Rift raise questions about using multiple graphics cards (GPUs) with their headset. How does it work? Is it even possible?

Let's take a look at the current state of NVIDIA's SLI and AMD's CrossFire and get to the bottom of whether or not you can use more than one GPU with VR.

What is SLI and CrossFire?

SLI and CrossFire are NVIDIA's and AMD's respective brand names for dual-GPU function. SLI stands for Scalable Link Interface, a technology that allows multiple GPUs to work together to produce a single video output. Likewise, CrossFire allows multiple AMD-made GPUs to work together to accomplish the same single video output.

Why don't multiple GPUs work natively with VR?

These technologies, when used with standard, non-VR rendering, divide the workload evenly. If you have two GPUs, each one will render alternating frames, resulting in an increased frame-rate. These do not, however, do much of anything for latency, which is not exactly great for VR. To keep latency down and to boost frame-rates, both NVIDIA and AMD required something more advanced for multi-GPU setups and VR.

NVIDIA VRWorks and AMD LiquidVR

NVIDIA has a project called VRWorks that brings SLI support to virtual reality. AMD's LiquidVR has its own section devoted to multi-GPU setups; what they call Affinity Multi-GPU is designed to assign one or more GPUs to the viewpoint of each of your eyes.

AMD's Affinity Multi-GPU

If this seems like a lot more work for your processor, you're not wrong. Both of these application programming interfaces (APIs) have taken this into account and allow for simultaneous draw calls. Your processor asks for a rendering once, the message is transmitted to both GPUs, each GPU returns an image, they are combined to form a frame, and it is finally sent to your VR headset.

Each one of your eyes sees an image rendered by a separate piece of hardware. Thanks to the magic of VRWorks and LiquidVR, you should notice a positive difference in performance when using multiple GPUs.

VR experiences that currently support multiple GPUs

The list of VR experiences that currently supports multiple GPUs is quite short — remember, standard SLI or CrossFire support usually doesn't work well with VR. People who have attempted to use two NVIDIA graphics cards with games that can be played on a standard monitor, like Elite Dangerous and Project Cars, have reported poor results.

Serious Sam VR: The Last Hope supports LiquidVR multi-GPU, while NVIDIA VR Funhouse and Trials on Tatooine both support VRWorks SLI. And that's pretty much it when it comes to commercially available experiences.

Why doesn't every VR experience support multiple GPUs?

To put it simply, multi-GPU support for most VR games and other experiences is up to the developer — in other words, just because NVIDIA and AMD have these APIs does not mean your multi-GPU setups will work with VR.

A great example of the process of implementing multi-GPU support into a VR game is that of Croteam, the developer of Serious Sam VR: The Last Hope. Back in October 2016, they posted on GPUOpen.com a recap of their experience with LiquidVR.

Croteam took this route to eliminate GPU bottlenecks while they developed the game. Indeed, enabling multi-GPU support must be a huge boon to developers, especially when it comes to the low frame-rates associated with an in-progress game.

Single-GPU setup

Why don't all developers include support? There isn't really a single, definitive answer — it's more of a combination of factors. Both VRWorks and LiquidVR are still under construction and thus are not set in stone when it comes to their use, and the majority of VR users don't have multi-GPU gaming rigs. The reality of it is that the time needed to implement multi-GPU support isn't yet, in most cases, worth the developer's time.

Wrapping up

More VR experiences will undoubtedly one day support multiple GPUs, but for now the list is quite short. Are you looking forward to the days of standardized multi-GPU experiences? Do you think SLI and CrossFire are best left alone? Let us know in the comments section!

If you're in the market for a new GPU, be sure to check out Windows Central's choice for the best graphics card for gamers.