Shape acquisition of three-dimensional (3D) objects is of significant importance for various real-world applications including machine vision, reverse engineering, industrial inspection, and medical imaging. An economic reliable real-time technique that delivers such information is fringe projection imaging. The imaging system comprises a projector-camera pair in which successive phase-shifted fringe patterns are projected onto objects, become distorted, and are then captured by the camera. These captured distorted fringes carry valuable information about the object's depth, which can be retrieved through phase shifting algorithms.However, conventional fringe projection imagers fail to recover depth data from objects of high dynamic range (HDR) where fringe visibility is greatly reduced in dark regions, bright areas, or over surfaces having large reflectivity variation. For instance, shiny metal objects reflect illuminating light specularly and saturate the camera without carrying any depth content.
In this research, we developed a single-shot multi-polarization fringe projection (MPFP) algorithm that is capable of processing HDR dynamic scenes. The projected fringes in the imaging system, shown in Fig. 1, are linearly polarized prior to incidence on the object and are captured after reflection through a pixelated micropolarizer camera leading to dissimilar measurements in the multi-polarized channels.
Figure 1: Multi-polarization fringe projection (MPFP) imaging system.
The MPFP algorithm eliminates any saturation in the raw measurements and forms a decision map that determines the selected polarized channel across all fringe patterns on a pixel-by-pixel basis. The results, shown in Fig. 2, are enhanced fringe images that deliver a complete phase calculations and depth rendering of the targeted objects.
Figure 2: Traditional and multi-polarization fringe projection imaging of simple object; (a) simple three-surfaces object captured by unpolarized camera, (b) raw polarized data of first distorted fringes, (c) shape rendering of five fringe images at separate polarizations, (d) multi-polarization decision map, (e) merging results of first fringe images, and (f) shape rendering of five enhanced fringes.
In summary, the algorithm eliminates saturated or low-contrast fringe regions by selecting different polarization measurements (or the right combination of polarization angle and exposure time if further enhancement is desired), in order to maintain good fringe visibility. This leads to greater coverage of the object in the shape rendering, better measurement of object topography, and thus a more accurate rendering of the object shape.
This work was a collaborative effort with Prof. Rongguang Liang in the
College of Optical Sciences, University of Arizona.
Publications:
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Basel Salahieh, Zhenyue Chen, Jeffrey J. Rodriguez, and Rongguang Liang "Multi-polarization fringe projection imaging for high dynamic range objects," Optics Express, vol. 22, no. 8, Apr 2014, pp. 10064-10071. [ PDF ]
