Comparing World and Screen Coordinate Systems in Optical See-Through Head-Mounted Displays for Text Readability while Walking

Augmented reality (AR) optical-see-through (OST) head-mounted displays (HMD) have developed to a point where browsing information on the go is possible. In this paper, we investigate the readability of text on an AR HMD while the user is walking. There are two common methods of displaying text on a HMD: anchoring the text on the screen coordinate system or the world coordinate system. We report on the results of two laboratory experiments comparing text readability when the text is displayed in these two coordinate systems, and while the participants walked on a treadmill. In the first experiment, the participants read letter strings comprising Sloane letters, whereas the second experiment used English words. In addition to evaluating the text readability and workload experienced by participants, we employed IMU sensors to compare the effects of the text display method on the participants' head movement and gait. In both experiments, the reading speed and head movement were significantly higher and mental workload significantly lower for the world coordinate system than for the screen coordinate system. These results suggest that text readability while walking is better on the world coordinate system, and displaying text with the screen coordinate system results in an unnatural gait owing to the user trying to keep their head still in an effort to stabilize the HMD screen.