With the rapid advance of real-time computer graphics, head-mounted displays (HMDs) have become popular tools for 3D visualization. One of the most promising and challenging future uses of HMDs, however, is in applications where virtual environments enhance rather than replace real environments. In such applications, a virtual image is superimposed on a real image. The unique problem raised by this superimposition is the difficulty that the human visual system may have in integrating information from these two environments. As a starting point to studying the problem of information integration in see-through environments, we investigate the quantification of depth and size perception of virtual objects relative to real objects in combined real and virtual environments. This starting point leads directly to the important issue of system calibration, which must be completed before perceived depth and sizes are measured. Finally, preliminary experimental results on the perceived depth of spatially nonoverlapping real and virtual objects are presented.

We studied whether the blur/sharpness of an occlusion boundary between a sharply focused surface and a blurred surface is used as a relative depth cue. Observers judged relative depth in pairs of images that differed only in the blurriness of the common boundary between two adjoining texture regions, one blurred and one sharply focused. Two experiments were conducted; in both, observers consistently used the blur of the boundary as a cue to relative depth. However, the strength of the cue, relative to other cues, varied across observers. The occlusion edge blur cue can resolve the near/far ambiguity inherent in depth-from-focus computations.

This study of form vision explores the relationships between orientation and spatial frequency in suprathreshold discrimination tasks. Orientation discrimination thresholds for sine-wave gratings were 0.3-0.5 deg, much less than the roughly 10-24-deg orientational bandwidth of channels; spatial-frequency discrimination thresholds were 3-7%, much less than the roughly 1.2-octave spatial-frequency bandwidth of channels. We find that spatial-frequency discrimination between two gratings was as acute when the two gratings were orthogonal as when they were parallel. Orientation discrimination between two gratings was as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies. Thus orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing.

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