Fortunately, all of these tricks can be moved to the infrared (IR) part of the spectrum so that features are visible to cameras, but not to humans. Patterns can be painted onto objects that highly reflect IR energy. Alternatively, IR LEDs can be mounted onto devices. This is the case for the Oculus Rift headset, and the IR LEDs are even hidden behind plastic that is transparent for IR energy, but appears black to humans; see Figure 9.13.
In some settings, it might be difficult to mount LEDs on the objects, as in the case of tracking the subtle motions of an entire human body. This is called MOCAP or motion capture, which is described in Section 9.4. In MOCAP systems, powerful IR LEDs are positioned around the camera so that they illuminate retroreflective markers that are placed in the scene. Each marker can be imagined as a spherical mirror in the IR part of the spectrum. One unfortunate drawback is that the range is limited because IR energy must travel from the camera location to the target and back again. Since energy dissipates quadratically as a function of distance, doubling the distance results on one-fourth of the energy level arriving at the camera.
At this point, it is natural to wonder why an entire image is being captured if the resulting image processing problem is trivial. The main reason is the proliferation of low-cost digital cameras and image processing software. Why not simply design an emitter-detector pair that produces a binary reading, indicating whether the visibility beam is occluded? This is precisely how the detection beam works in an automatic garage door system to ensure the door does not close on someone: An IR LED emits energy to a detection photodiode, which is essentially a switch that activates when it receives a sufficient level of energy for its target wavelength (in this case IR). To reduce the amount of energy dissipation, mirrors or lenses could be used to focus the energy.
Even better, an IR laser can be aimed directly at the detector. The next task is to use lenses and moving mirrors so that every detector that is visible from a fixed location will become illuminated at some point. The beam can be spread from a dot to a line using a lens, and then the line is moved through space using a spinning mirror. This is the basis of the lighthouse tracking system for the HTC Vive headset, which is covered later in this section.
Steven M LaValle 2016-12-31