When three or more tags are used on a rigid object, and the relative positions of the tags are known, 3D information about the object can be recovered using techniques developed in [Azarbayejani and Pentland, 1995]. Registered 3D graphics can be then be overlaid on the real object. Such registered graphics can be very useful in the maintenance of machinery. Extending a concept by [Feiner et al., 1993], Figure 10 shows 3D animated images demonstrating repair instructions for a laser printer. The registration method becomes increasingly stable with additional known feature points. Since the tags have known dimensions, two feature points can be recovered: the right and left-hand sides of the tag. More recently, 2D visual tags have been developed which encode 32 to 128 bits of information in a 7 by 5 grid of color squares. Due to the inherent planar structure of these tags, only one tag is needed to a align a 3D graphics overlay.
Figure:
A maintenance task using 3D animated graphics. The left
side shows the laser printer to be repaired. The right side
shows the same printer with the overlying transparent instructions
showing how to reach the toner cartridge.
Note that the visual tags shown in Figure 10 consist of small LED alphanumeric displays. For expensive machinery such as an aircraft, a manufacturer may want to embed such tags to aid in repair diagnostics. Such displays may indicate error codes (similar to some of today's printers and copiers) that the technician's wearable computer can sense. Thus, appropriate graphical instructions can automatically overlay the user's visual field. In addition, active tags may blink in spatial-temporal patterns to communicate with the wearable computer or to aid tracking in visually complex environments. Adding infrared or radio communications between the repair object and the wearable computer may allow more complicated cooperative diagnostics or repair instructions tailored to the user's level of expertise. Of course, the features of these more advanced systems must be weighed against the low cost of the passive tags discussed earlier.