How Opacity and Background Affect Surface Contact Perception in Optical See-Through AR

For virtual objects to appear spatially anchored in Augmented Reality (AR), users must be able to tell whether they rest on a real surface. Humans judge this primarily through cast shadows and contact edges (the visible boundaries where a virtual object meets the physical surface). However, additive blending in Optical See-Through (OST) displays weakens cast shadows, leaving contact edges as the main available cue. How rendering parameters and environmental factors jointly shape this edge-based judgment remains poorly understood. We investigated this on an OST display with segmented dimming (Magic Leap 2) by manipulating virtual object opacity (20%-100%), background spatial frequency (0.5, 1.5, 4.5 cpd), and ambient lighting (500 lx, 1500 lx). A within-subjects study (N = 39) assessed contact judgment accuracy, signal detection sensitivity ($d$'), confidence ratings, and eye-tracking behaviour. Results show that 60%-80% opacity yields the highest accuracy and efficiency, as this range keeps the virtual object visible without occluding the surface texture needed to judge contact. This advantage was most pronounced over mid-frequency backgrounds (1.5 cpd). Notably, users feel most confident at 100% opacity despite lower objective performance, revealing a disconnect between perceived and actual judgment quality. Eye tracking reveals two distinct failure modes: at 100% opacity, relatively minimal gaze effort at the contact edge suggests overconfidence-driven underinspection; at 20% opacity, prolonged fixations indicate that increased effort cannot compensate for insufficient surface texture. Together, these findings show that the most visually solid rendering is not the most perceptually effective, pointing toward contact-state-driven opacity adaptation as a design strategy for OST-AR.