Abstract
Global or indirect illumination effects such as interreflections and subsurface scattering severely degrade the performance of structured light-based 3D scanning. In this paper, we analyze the errors in structured light, caused by both long-range (interreflections) and short-range (subsurface scattering) indirect illumination. The errors depend on the frequency of the projected patterns, and the nature of indirect illumination. In particular, we show that long-range effects cause decoding errors for low-frequency patterns, whereas short-range effects affect high-frequency patterns.
Based on this analysis, we present a practical 3D scanning system which works in the presence of a broad range of indirect illumination. First, we design binary structured light patterns that are resilient to individual indirect illumination effects using simple logical operations and tools from combinatorial mathematics. Scenes exhibiting multiple phenomena are handled by combining results from a small ensemble of such patterns. This combination also allows detecting any residual errors that are corrected by acquiring a few additional images. Our methods can be readily incorporated into existing scanning systems without significant overhead in terms of capture time or hardware. We show results for several scenes with complex shape and material properties.

























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Global illumination should not be confused with the oft-used “ambient illumination” that is subtracted by capturing image with the structured light source turned off.
Strictly speaking, since all binary patterns have step edges, all of them have high spatial frequencies. For the analysis and discussion in this paper, low-frequency patterns implies patterns with thick stripes. Similarly, high-frequency patterns mean patterns with only thin stripes.
The inverse pattern can be generated by subtracting the image from image of the fully lit scene.
For example, pico-projectors are increasingly getting popular for structured light applications in industrial assembly lines. However, due to imperfect optics, they can not resolve patterns with thin stripes, for example, a striped pattern of 2-pixel width.
The color of the incident illumination can be decoded from the image of the illuminated scenes on a per-pixel basis, even for non-white scenes (Caspi et al. 1998). It is not required to assume spatial smoothness or color neutrality of the scene.
Two additional images of the scene, one under all white illumination, and one under all black illumination were acquired to establish the per-pixel intensity thresholds for binarization.
It is relatively easy to generate codes with small maximum stripe-width. For example, we could find 10-bit codes with a maximum stripe-width of 9 pixels by performing a brute-force search. In comparison, conventional Gray codes have a maximum stripe-width of 512 pixels.
Due to imperfect projector optics, insufficient camera/projector resolution or misalignment between projector and camera pixels, the depth results from individual codes might suffer from spatial aliasing. This problem is more pronounced for the high-frequency XOR codes. To prevent aliasing from affecting the final depth estimate, we apply a median filter (typically 3×3 or 5×5) to the individual correspondence maps before performing the consistency check.
We projected only the logical codes in subsequent iterations, thus requiring 82 images in total.
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Acknowledgements
We thank Jay Thornton, Joseph Katz, John Barnwell and Haruhisa Okuda (Mitsubishi Electric Japan) for their help and support. Mohit Gupta was partially supported by ONR grant N00014-11-1-0295. Srinivasa Narasimhan was partially supported by NSF grants IIS-0964562 and CAREER IIS-0643628 and a Samsung SAIT GRO grant. Ashok Veeraraghavan was partially supported by NSF Grants IIS-1116718 and CCF-1117939. The authors thank Vincent Chapdelaine-Couture for sharing their data-sets.
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A preliminary version of this paper appeared in Gupta et al. (2011).
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Gupta, M., Agrawal, A., Veeraraghavan, A. et al. A Practical Approach to 3D Scanning in the Presence of Interreflections, Subsurface Scattering and Defocus. Int J Comput Vis 102, 33–55 (2013). https://doi.org/10.1007/s11263-012-0554-3
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DOI: https://doi.org/10.1007/s11263-012-0554-3