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Reliable world locking in augmented and mixed reality (AR/MR) devices is important for achieving immersion and critical for technical applications that rely on stable anchoring of virtual objects in the real world. To achieve this, a head-mounted display (HMD) must maintain accurate knowledge of its real-world position and orientation. We describe a method for measuring the six-degrees-of-freedom (DOF) positioning accuracy of an HMD, and how the same method can be extended to quantify the accuracy of anchoring virtual objects in the real world, i.e., world locking. An HMD is placed on a 6-DOF test jig comprising a motion system with high precision encoders and a time-synchronized imaging system. The HMD is made to display a 3D grid of unique identification markers that are detected by a machine vision camera in real time, while the robot is moving. This allows us to track the position and pose of the virtual camera and compare that with the known HMD position and pose. Using similar methodology, we can display virtual objects with a suitable number of unique identification markers. Their virtual object position and pose can then be compared to the real HMD position, thereby quantifying the accuracy of world locking. For improved accuracy, markers can also be printed out and pasted onto real-world objects. Other temporal parameters can also be computed, including motion-to-photon latency, spatial jitter, pose drift and prediction over/undershoot. The obtained results can be used to improve or recalibrate the positioning software and hardware of the head-mounted device.
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