The current generation of VR headsets have significant optical aberration issues; recall from Section 4.3 that these become worse as the distance from the optical axis increases. It is important to distinguish between two cases: 1) Looking through the center of the lens while detecting distortion at the periphery, and 2) rotating the eyes to look directly through the edge of the lens. Distortion might be less noticeable in the first case because of lower photoreceptor density at the periphery; however, mismatches could nevertheless have an impact on comfort and sickness. Optical flow signals are strong at the periphery, and mismatched values may be perceived as incorrect motions.
In the second case, looking directly through the lens might reveal lack of focus at the periphery, caused by spherical aberration. Also, chromatic aberration may become visible, especially for sharp white lines against a black background. Furthermore, errors in pincushion distortion correction may become evident as a straight line appears to become curved. These problems cannot be fixed by a single distortion correction function (as covered in Section 7.3) because the pupil translates away from the optical axis when the eye rotates. A different, asymmetric correction function would be needed for each eye orientation, which would require eye tracking to determine which correction function to use at each time instant.
To observe pincushion or barrel distortion the evaluator should apply a canonical yaw motion over as large of an amplitude as possible, while fixating on an object. In this case, the VOR will cause the eye to rotate over a large range while sweeping its view across the lens from side to side, as shown in Figure 12.2. If the virtual world contains a large, flat wall with significant texture or spatial frequency, then distortions could become clearly visible as the wall appears to be ``breathing'' during the motion. The effect may be more noticeable if the wall has a regular grid pattern painted on it.
Finally, many users do not even notice the limited field of view of the lens. Recall from Section 5.4 that any flat screen placed in front of the eye will only cover some of the eye's field of view. Therefore, photoreceptors at the periphery will not receive any direct light rays from the display. In most cases, it is dark inside of the headset, which results in the perception of a black band around the visible portion of the display. Once this is pointed out to users, it becomes difficult for them to ignore it.
Steven M LaValle 2020-01-06