Effects of shading model and opacity on depth perception in optical see-through augmented reality

Augmented reality (AR) technologies create an immersive environment by augmenting the real world with rendered virtual objects. One of the key requirements of an AR system is to understand how users perceive the depth of an AR object. Perceived distances to AR objects can be based on various depth cues such as rendering method used for the virtual object. The existing researches on the shading model and opacity mainly focus on the shape perception or the position relationship of several virtual objects in the virtual environment. We predicted that shading models and opacity would impact a user's estimation of a virtual object's depth in AR. We conducted two experiments to investigate the impact of color, size (Experiment 1), shading model, and opacity (Experiment 2) on the depth perception in an optical see-through head-mounted display (HMD). We found that the virtual object's color and size impact estimation efficacy. An interaction effect between color and size was also found. The results showed that shading models with specular highlights could help to improve depth perception in AR. Additionally, users had the lowest matching error when the opacity of a virtual object was 0.8.