In addition to handheld video see-through and wearable optical see-through, there are other AR display techniques as well.

A monocular headset shows a single image in one eye, allowing the other eye to view the real world unaugmented. It tends to be lightweight and used more as a heads-up display (HUD), as if information were projected on the front of a helmet rather than registered to the 3D world. An example of this is Google Glass. While the technology can be useful in some applications, we are not considering it in this book.

Wearable video see-through uses a head-mounted display (HMD) with a camera and combines real-world video with virtual graphics on its near-eye display. This may be possible on VR headsets such as HTC Vive and Samsung GearVR, with camera passthrough enabled, but it has a few problems. First, these VR devices do not have depth sensors to scan the environment, preventing the registration of graphics with the real 3D world.

The camera on such devices is monoscopic, yet the VR display is stereoscopic. Both the eyes see the same image, or what is called bi-ocular. This will cause issues in correctly rendering the graphics and registering to the real world.

Another problem is that the device's camera is offset from your actual eyes in front by an inch or more. The viewpoint of the camera is not the same as your eyes; the graphics would need to be registered accordingly.

For these reasons, wearable video see-through AR presently can look pretty weird, feel uncomfortable, and generally not work very well. But if you have one of these devices, feel free to try the projects in this book on it and see how it works. Also, we can expect new devices to come on the market soon which will position themselves as combined VR + AR and hopefully solve these issues, perhaps with dual stereo cameras, optical correction, or other solutions.