Virtual reality headsets: fooling your eyes

A VR headset has to fool you twice:

• that a flat screen centimetres from your face is a 3D world with depth;
• that the world stays put when you turn your head, like the real one does.

Trick one is optics. Trick two is sensors plus a very fast processor.

• Two screens (or one screen split in half), one in front of each eye.
• Lenses between each eye and its screen. They focus the image so your eye can see a screen that close, and stretch it to fill your field of view.
• Sensors: a gyroscope (measures rotation), an accelerometer (measures movement), often a magnetometer, and cameras for tracking position.
• A processor (in the headset or a connected computer) that redraws the scene.
• Headphones for sound that matches where you're looking.

"Describe how a VR headset gives the user the experience of being in a 3D world." Usually 3 to 4 marks.

The marks are for two images, lenses, sensors tracking the head, and updating the images in real time.

1. Two screens show two slightly different images of the scene, one for each eye (like the two views your eyes get in real life).
2. Lenses focus each image onto the eye and widen it to fill the field of view.
3. The brain combines the two images into one scene with depth: a stereoscopic 3D effect.
4. Sensors (gyroscope, accelerometer) detect the user's head movement.
5. The processor uses the sensor data to redraw both images in real time, so the view changes to match where the user is looking.
6. Headphones play sound that also changes with head direction.

Step 5 is the forgotten one. Sensors alone don't change anything; the processor has to re-render the view, many times a second.