VR Headsets & How VR Works

Look at a coffee cup and close one eye, then the other. The cup jumps sideways, because each eye sees it from a slightly different position - your eyes sit about 6.3 centimetres apart (the interpupillary distance, or IPD). Your brain fuses those two slightly-different views into a single image and reads the difference between them as depth. This is stereoscopy, and it is the first thing a VR headset does that a monitor cannot.

Inside the headset there are two small displays (or two halves of one panel), one in front of each eye. Unreal renders the scene twice every frame - once from a virtual left eye and once from a virtual right eye, offset by your IPD - and sends each image to the correct display. Because the two renders differ exactly the way your real eyes would differ, your brain fuses them and the building gains true, felt depth: a column is genuinely in front of the wall behind it, a ceiling is genuinely up there. This is why VR reveals scale and proportion that a flat walkthrough only hints at.

Between each display and your eye sits a lens that focuses the close-up panel so it appears comfortably far away, and stretches the image across your vision. That lens distorts the picture, so the engine pre-distorts each eye image in the opposite way (barrel distortion to cancel the lens pincushion) - a step OpenXR and the headset runtime handle for you. The practical upshot for an architect: because the engine is drawing the scene twice, VR costs you roughly double the rendering work of a flat view. That single fact drives almost every performance decision in Module 9. Keep it in mind - it is the price of standing inside the room.

Depth alone is not presence. The second, and more powerful, trick is head tracking: the headset knows where your head is and which way it is pointing, and it feeds that pose to Unreal every frame so the virtual camera matches your real head. Turn to look at a window and the window is there. Crouch to check a worktop height and your eyeline drops. Your vestibular system - the balance sensors in your inner ear - expects the world to shift precisely as you move, and when the picture obeys, your body accepts the illusion completely.

Good VR tracks six degrees of freedom (6DoF). Three are rotation - pitch (nod), yaw (shake), roll (tilt) - and three are translation - moving your head left/right, up/down, and forward/back. A cheap phone-in-a-holder or an old headset offers only 3DoF: it knows where you are looking but not that you leaned in, so you cannot physically step around a column. 6DoF is what makes an architectural review real - you lean over a balustrade, duck under a beam, walk up to a joint and put your face next to it. Every headset in this course is 6DoF.

Modern standalone headsets achieve this with inside-out tracking: cameras on the headset itself watch the room and compute your position, so no external sensors are needed (this is how a Meta Quest works out of the box). Older or high-end tethered systems like the original Valve Index use outside-in tracking with base stations mounted in the room - more setup, historically a touch more precise. Either way, the headset also tracks your two hand controllers in 6DoF, which become your pointing, grabbing and teleporting tools in the design review to come.

On a monitor, 30fps is watchable and 60fps is smooth. VR is far less forgiving, and the reason is physical rather than aesthetic. When you turn your head, the time between your motion and the matching image reaching your eyes - motion-to-photon latency - must be tiny, ideally under about 20 milliseconds. If the view lags your head even slightly, your inner ear and your eyes disagree, and that sensory conflict is exactly what triggers VR sickness: the cold sweat and nausea that ends a client review early.

To keep latency low and motion smooth, VR headsets run at high, fixed refresh rates - commonly 72, 90 or 120Hz. Ninety hertz has become the comfort benchmark, and it means the engine has about 11 milliseconds to draw both eye images - half the time a 60fps desktop frame gets, for double the pixels. This is the brutal arithmetic of VR performance: you have made the rendering job twice as big and given yourself less than half the time. Miss the budget and frames drop, latency spikes, and comfort collapses.

Headset runtimes defend against the occasional missed frame with reprojection (Meta calls its versions Asynchronous Timewarp and Spacewarp): if a fresh frame is not ready, the runtime re-shifts the last frame to match your latest head pose so the world still tracks your motion. It is a genuine safety net, but a crutch, not a cure - lean on it constantly and you see warping and judder. The honest rule for architectural VR: treat 90fps as a hard floor, not a target, and build the scene so you rarely need reprojection at all. Module 9 (hardware-and-vr-performance) is where you learn to actually hit it; here, just internalise why the number is non-negotiable.

You do not need the most expensive headset to do excellent architectural VR, but it helps to know the field. Headsets split first into standalone and tethered. A standalone headset - the Meta Quest family is the dominant example - has its own processor and battery and runs untethered, which makes it superb for taking to a client meeting or a site. Its onboard chip is modest, though, so a heavy Unreal scene usually runs by streaming from a PC over a cable (Quest Link) or Wi-Fi (Air Link), using the PC's graphics card while the Quest acts as the display and tracker. A tethered headset - the Valve Index, HTC Vive family, or the high-end Varjo Aero and XR-4 - stays plugged into a powerful workstation and simply shows what that machine renders.

The differences that matter for archviz are resolution (sharper text and finer detail on the wall), field of view (how much of your vision is filled - the Index and Varjo are wider than most), refresh rate (Index reaches 120/144Hz), and comfort and lens quality. Varjo sits at the top for photoreal fidelity and near retinal-resolution clarity, at a professional price; the Index remains a benchmark for tracking and comfort; the Quest wins overwhelmingly on cost, convenience and the fact that a client can be handed one with no wires.

What frees you from betting on any one device is OpenXR - a royalty-free open standard, backed by the Khronos Group, that sits between your Unreal project and whatever headset is plugged in. You build your VR scene against OpenXR once, and it runs on Quest, Index, Vive, Varjo and future hardware without engine-specific rewrites. This is why the next lesson enables the OpenXR plugin rather than any vendor plugin: you are targeting the standard, not the gadget. For a practice, that means the VR experience you build this year survives the hardware refresh next year.