Question 4: How is 3D information about the surface of the face obtained?
The task of generating 3D information about the facial surface is conceptually no different from that outlined above for terrestrial and x-ray photogrammetry as may be seen in Figure 10a.
Figure 10a. Ranging the location of two representative points on the face from a pair of cameras located a known distance apart. Compare to Figures 7f and 8h.
However, whereas in stereocephalometry it is still necessary to locate each point of interest manually on both images of a stereo pair, recent advances in imaging technology now make it possible to assemble 3D surface maps of the face fairly automatically. These methods utilize technologies known collectively as "structured light imaging." Briefly stated, they involve projecting a known grid or similar image on the facial surface from one perspective and then photographing the distortion of the projected image with a single camera whose spatial location is known with respect to the location of the projector. The general method of projecting a grid on a subject to be mapped has been known for many years but was not considered practical for clinical use earlier because locating large numbers of grid intersections by hand, even on a single image, was prohibitively labor-intensive. This objection is now overcome in a number of structured light systems. In general, vertically oriented series of rainbow stripes is projected on the subject by a specialized projector and then photographed by a digital camera located a known distance from the projector.
A known pattern is projected upon the subject from a light source (A) and photographed by a digital camera (B). See Figure 10b.
Figure 10b
In a more recent development, a second digital camera (C), mounted between the fixed projector (A) and digital camera (B) captures a true-color image a few milliseconds after the primary image. The 3D information from the primary-camera-light-projector assembly is synchronized with the 2D pixel map from Camera C and is stored in a kind of look-up table. In this way, the location of each pixel on the monitor-displayed image from Camera C uniquely identifies the three dimensional coordinates of a particular point on the surface map generated by Camera B and its associated projector (A).
A camera system utilizing this approach (manufactured by 3D Metrics, Petaluma, California) is shown in Figure 10c.
Figure 10c. The 3D Metrics Camera System currently in use at CRIL.
In this implementation, one of the usual cameras of the stereo pair is replaced by a Light Projector which casts a grid-like pattern or array upon the subject from a known location and orientation.
A dedicated computer chip built into the digital camera has the ability to distinguish different frequencies of light and hence can tell, from the color of each light ray, the direction in which that particular ray has traveled from the light projector.
The projected pattern falling upon the subject is photographed by Camera 1 which is mounted a known distance from the Projector. When viewed from the perspective of the Projector, the shape of the projected pattern remains unaltered regardless of the shape of the object it falls upon (see Figure 10d).
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Figure 10d
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Figure 10e
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When viewed from the perspective of Camera 1, the projected pattern is distorted as a function of the height of the various features of the subject’s face. See Figure 10e. Since the view of the pattern from the vantage point of the projector is always an unaltered representation of the pattern itself, it need not be digitized at each exposure. Instead, the heights of the various facial features can be calculated solely from the distortion of the pattern when viewed from the vantage point of Camera 1.
The reconstructed representation of the face can be rotated on a standard computer monitor and the 3D coordinates of any visible point can be captured by pointing and clicking with a standard mouse or other similar device (see Figure 10f).
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Figure 10f
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