For anyone used to the limited 3D effects created with diffusing glasses and standard flat displays, the sense of depth provided by a stereoscopic VR headset is an impressive improvement. But trying to switch your focus between objects at different virtual distances can still be an eye-crossing, headache-inducing experience with these flat-panel displays. A team of researchers at Stanford University recently published an interesting potential solution to this problem, using a couple of layered LCD screens to provide real-world depth cues in a small VR headset form factor.

The main problem with depth focusing in traditional stereoscopic HMDs is that, despite fancy lensing and left/right eye image separation, you're still looking at a flat panel placed very close to your face. Trying to focus on "far away" objects on that stereoscopic screen means keeping a fixed focal distance but changing the "vergence" angle of your eyes—in essence, going a little cross-eyed for a moment. That can lead to "visual discomfort and fatigue, eyestrain, diplopic vision, headaches, nausea, compromised image quality, and it may even lead to pathologies in the developing visual system of children," the Stanford researchers write.

In addition, a traditional display doesn't know where you're looking at any point. That means the system cannot determine which parts of the image should be rendered with the slight "retinal blur" that applies to out-of-focus objects in our real-world vision. Paradoxically, having the entire virtual world be perfectly "in focus" on that LCD display makes things seems a little less real.

To fix these problems, the Stanford researchers created a prototype headset that includes a translucent LCD panel sitting about 1cm in front of a standard, opaque LCD. With some GPU pre-processing, this "light field stereoscope" headset can display nearby objects on the front LCD and farther-away objects on the rear, creating what the researchers call a "4D" image that layers a basic virtual light field on top of the usual stereoscopic left/right eye 3D separation.

That layered image incorporates crucial, unconscious depth cues as your eyes transition between near and far objects, "result[ing] in correct or nearly correct retinal blur and the ability to freely focus the eyes within the scene," the researchers write. That reduces the need to cross your eyes to try to focus and makes everything look more appropriately "deep" without the need for any fancy eye-tracking or a bulky volumetric display solution.

The prototype headset isn't a perfect solution to these depth problems. You can't comfortably focus out to "visual infinity" with the two-panel solution—users still run into vergence problems with objects farther than about 1.2 meters away in the virtual world. Previous research suggests it would take a five-layered LCD display to allow for easy focus out to any distance. That would add significantly to a headset's bulk and overhead, though—even updating two displays in the prototype led to increased computational load and latency.

Problems aside, it's still a compelling, relatively feasible solution to one of the major remaining problems with the sense of visual accuracy in current VR headsets. This is the kind of research that makes us excited about just how much better virtual reality displays are going to be in the near future.