Adding simple positional tracking to VR goggles like Cardboard inexpensively with paper tracking markers.

Cardboard, using front camera : Place marker in front of you, or just face a monitor with this page on-screen
Rift DK1 tracking : Put a marker on front of Rift
Debug : Debug camera and orientation data

• Have about an inch or more or whitespace surrounding the marker for better tracking
• For testing, a phone with the marker on-screen works well
• Tracking is contrast dependent, so the marker being well lit is important

Possibilities:

• increased robustness, via better algorithms and multiple marker tracking
• native Cardboard Android app for better performance
• combining inside-out and outside-in tracking to produce middle-out tracking, for optimal tip-to-tip efficiency
• abandon project once Google Tango-like positional tracking supported natively and ubiquitiously

The heavy lifting done by these pieces glued together:

cardboard : google cardboard
three.js : awesome 3d library for the web
js-aruco : js port of the aruco library for tracking