  Stereoscopic, head-tracked display systems can show users realistic,
world-locked virtual objects and environments. However, discrepancies between
the rendering pipeline and physical viewing conditions can lead to perceived
instability in the rendered content resulting in reduced immersion and,
potentially, visually-induced motion sickness. Precise requirements to achieve
perceptually stable world-locked rendering (WLR) are unknown due to the
challenge of constructing a wide field of view, distortion-free display with
highly accurate head and eye tracking. We present a system capable of rendering
virtual objects over real-world references without perceivable drift under such
constraints. This platform is used to study acceptable errors in render camera
position for WLR in augmented and virtual reality scenarios, where we find an
order of magnitude difference in perceptual sensitivity. We conclude with an
analytic model which examines changes to apparent depth and visual direction in
response to camera displacement errors.
