Figure 6.10: The Ames room: (a) Due to incorrect depth cues, incorrect size perception results. (b) The room is designed so that it only appears to be rectangular after perspective projection is applied. One person is actually much further away than the other. (Figure by Alex Valavanis.)
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(a) & (b) \\

In the real world, all of the depth cues work together in harmony. We are sometimes fooled by optical illusions that are designed to intentionally cause inconsistencies among cues. Sometimes a simple drawing is sufficient. Figure 6.10 shows an elaborate illusion that requires building a distorted room in the real world. It is perfectly designed so that when viewed under perspective projection from one location, it appears to be a rectangular box. Once our brains accept this, we unexpectedly perceive the size of people changing as they walk across the room! This is because all of the cues based on perspective appear to be functioning correctly. Section 6.4 may help you to understand how multiple cues are resolved, even in the case of inconsistencies.

In a VR system, it is easy to cause mismatches and in many cases they are unavoidable. Recall from Section 5.4 that vergence-accommodation mismatch occurs in VR headsets. Another source of mismatch may occur from imperfect head tracking. If there is significant latency, then the visual stimuli will not appear in the correct place at the expected time. Furthermore, many tracking systems track the head orientation only. This makes it impossible to use motion parallax as a depth cue if the user moves from side to side without any rotation. To preserve most depth cues based on motion, it is important to track head position, in addition to orientation; see Section 9.3. Optical distortions may cause even more mismatch.

Steven M LaValle 2020-01-06