How bright should a virtual object be to appear opaque in optical see-through AR?

Reproduction of occlusions and opaque surfaces are the major challenges of additive optical see-through (OST) displays. This is because the user of an OST display sees a linear mixture of display and environment light, which creates an impression of transparency unless the displayed color is sufficiently bright. The primary goal of this work is to determine how bright a displayed surface needs to be in relation to environment light to be perceived as opaque. We test multiple factors that could affect the perception of opacity: background luminance, contrast, spatial frequency, and accommodation depth in foveal vision. The subjective results, collected on a high-dynamic-range multi-focal stereo display, indicate that a virtual object needs to be, on average, 60 times brighter than the background environment light to be perceived as opaque. A higher contrast of the texture of the virtual object and a background that is out of focus can reduce the required luminance ratio. We demonstrate that a model of visual perception based on Weber’s law and accounting for contrast masking and defocus blur can predict the experimental data with an averaged prediction error of 8.29%. Existing perceptual image difference metrics (PSNR, FovVideoVDP and HDR-VDP-3) can also predict the effect of major factors, but with lower accuracy (e.g. prediction error of 34% for PSNR with PU21 encoding).