Intially, I thought the sensor bar is a sophisticated piece of hardware that acts as the receiver of signals emitted from the Wii-mote and transmitts these signals into the Wii-console for processing. In that case, one would need to hack the Wii-console in order to pull motion tracking information from the device. Later, I realized that this is far from the truth. In fact, it’s the exact opposite – the sensor bar is actually the emitter of infra-red signals, and it is the wii-mote that is doing all the magic!
The Wii-mote itself is a high-resolution IR camera that is capable of tracking 4 infra-red blobs simultaneously. If you know something about blob-tracking from Computer Vision, this is exactly the same concept. Except one crucial difference – no other distractors in the environment! Unlike vision techniques for doing blob tracking, finding the blob from a naturally cluttered environment is extremely challenging. No matter what feature you use, almost always there will be something in the background that is going to cause confusion to the tracking algorithm. To make matters worse, different lighting conditions will royalty screw up tracking as well.
The infra-red technology the Wii uses is different - for most home-environments, there are no other IR light sources other than the ones on the sensor bar. With such clean signal, tracking is almost trivial. That is why the Wii works regardless of how cluttered you home is, or which time of day you are playing – it makes no difference whether it is broad day-light or at night in the dark.
Other piece of useful information is that IR lights are not visible to the human eye. However, a camera (such as a web-cam) can pick it up. Similarly, so will the IR camera on the wii-mote. Try placing your web-cam toward the sensor bar and you will see. It’s quite fun.