"Absolute phase retrieval methods for digital fringe projection profilometry: A review," Opt. Laser Eng. (2018)

[112] S. Zhang, "Absolute phase retrieval methods for digital fringe projection profilometry: A review," Opt. Laser Eng. 107, 28-37 (2018); doi:10.1016/j.optlaseng.2018.03.003

Abstract

This paper provides a review for absolute phase recovery methods that are applicable for digital fringe projection (DFP) systems. Specifically, we present two conventional absolute phase unwrapping methods: multi-frequency or -wavelength phase-shifting methods, and hybrid  binary coding and phase-shifting methods; and also introduce some non-conventional methods that are specific for DFP systems: multiview geometry methods with additional camera(s) or projector(s), DFP system geometric constraint-based phase unwrapping method, and pre-knowledge (e.g., computer-aided-design, CAD, model) based phase unwrapping method. This paper also briefly overviews hybrid methods including phase coding, composite, and pre-defined markers based absolute phase unwrapping methods. This paper explains the principle behind each individual absolute phase unwrapping method; and finally offers some practical tips to handle common phase unwrapping artifact issues.

"High-speed and high-accuracy 3D surface measurement using a mechanical projector," Opt. Express (2018)

J. -S. Hyun, George T. -C. Chiu and S. Zhang, "High-speed and high-accuracy 3D surface measurement using a mechanical projector," Opt. Express, 26(2), 1474-1487 (2018); doi:10.1364/OE.26.001474

Abstract

This paper presents a method to achieve high-speed and high-accuracy 3D surface measurement using a custom-designed mechanical projector and two high-speed cameras. We developed a computational framework that can achieve absolute shape measurement in sub-pixel accuracy through: 1) capturing precisely phase-shifted fringe patterns by synchronizing the cameras with the projector; 2) generating a rough disparity map between two cameras by employing a standard stereo-vision method using texture images with encoded statistical patterns; and 3) utilizing the wrapped phase as a constraint to refine the disparity map. The projector can project binary patterns at a speed of up to 10,000 Hz, and the camera can capture the required number of phase-shifted fringe patterns with 1/10,000 second, and thus 3D shape measurement can be realized as high as 10,000 Hz regardless the number of phase-shifted fringe patterns required for one 3D reconstruction. Experimental results demonstrated the success of our proposed method.

"Novel method for measuring dense 3D strain map of robotic flapping wings," Measurement Science and Technology (2018)

[109] B. Li and S. Zhang, "Novel method for measuring dense 3D strain map of robotic flapping wings," Measurement Science and Technology, 29(4), 045402 (2018);

Abstract

Measuring dense 3D strain map of inextensible membranous flapping wings of robots is of vital importance to the field of bio-inspired engineering. Conventional high-speed 3D videography method typically reconstructs the wing geometries through measuring sparse points with fiducial markers, and thus cannot obtain full-field mechanics of the wings in details.  In this research, we propose a novel system to measure dense strain map of the  inextensible membranous flapping wings by developing a superfast 3D imaging system and a computational framework for strain analysis. Specifically, first, we developed a 5,000 Hz 3D imaging system based on the digital fringe projection technique using the defocused binary patterns to precisely measure the dynamic 3D geometries of rapidly flapping wings. Then, we developed a geometry-based algorithm to perform point tracking on the precisely measured 3D surface data. Finally, we developed a dense strain computational method using the Kirchhoff-Love shell theory. Experiments demonstrate that our method can effectively perform point tracking and measure highly dense strain map of the wings without many fiducial markers.

"Introduction to the focused section on sensing and perception for autonomous and networked robotics" (2017)

X. Chen, M. Grossard, N. Kubota, D. Wollherr, S. X. Yang, and S. Zhang, "Introduction to the focused section on sensing and perception for autonomous and networked robotics," Int. J. Intell. Robot. Applic. 1 (4), 369–371 (2017); (Focused Section Guest Editorial) doi:10.1007/s41315-017-0040-8

"Double-pattern triangular pulse width modulation technique for high-accuracy high-speed 3D shape measurement," Opt. Express (2017)

Y. Wang, C. Jiang, and S. Zhang, "Double-pattern triangular pulse width modulation technique for high-accuracy high-speed 3D shape measurement," Opt. Express 25(24), 30177-30188 (2017); doi:10.1364/OE.25.03177

Abstract

Using 1-bit binary patterns for 3D shape measurement has been demonstrated advantageous over using  8-bit sinusoidal patterns in terms of achievable speeds. However, the phase quality generated by binary pattern(s) typically  is not  high if only a small number of phase-shifted patterns is used. This paper proposes a method to improve the phase quality  by representing each pattern with the difference of two binary patterns:  the first binary pattern is generated by triangular pulse width modulation (TPWM) technique, and the second being $\pi$ shifted from the first pattern is also generated by TPWM technique. The phase is retrieved by applying a three-step phase-shifting algorithm to the difference patterns. Through optimizing the modulation frequency of the  triangular carrier signal, we demonstrate that high-quality phase can be generated for a wide range of fringe periods (e.g., from 18 to 1140 pixels) with only six binary patterns. Since only 1-bit binary patterns are required for 3D shape measurement, this paper will present a real-time 3D shape measurement system that can achieve 30 Hz.

"Three-dimensional range geometry compression via phase encoding," Appl. Opt. (2017)

[106] T. Bell, B. Vlahov, J.P. Allebach, and S. Zhang, "Three-dimensional range geometry compression via phase encoding," Appl. Opt., 56(33), 9285-9292, (2017); doi: 10.1364/AO.56.009285

Abstract

One of the state-of-the-art methods for three-dimensional (3D) range geometry compression is to encode 3D data within a regular 24-bit 2D color image. However, most existing methods use all three color channels to solely encode 3D data, leaving no room to store other information (e.g., texture) within the same image. This paper presents a novel method which utilizes geometric constraints, inherent to the structured light 3D scanning device, to reduce the amount of data which need be stored within the output image. The proposed method thus only requires two color channels to represent 3D data, leaving one channel free to store additional information (such as a texture image). Experimental results verify the overall robustness of the proposed method. For example, a compression ratio of 3038:1 can be achieved, versus the STL format, with a root-mean-square (RMS) error of 0.47% if the output image is compressed with JPEG 80%.

Technical Paper

“Superfast, high-resolution absolute 3D recovery of a stabilized flapping flight process,” Opt. Express, (2017)

B. Li and S. Zhang, “Superfast, high-resolution absolute 3D recovery of a stabilized flapping flight process,” Opt. Express, 25(22), 27270-27282 (2017); doi:10.1364/OE.25.027270

Abstract

Scientific research of a stabilized flapping flight process (e.g. hovering) has been of great interest to a variety of fields including biology, aerodynamics and bio-inspired robotics. Different from the current passive photogrammetry based methods, the digital fringe projection (DFP) technique has the capability of performing dense superfast (e.g. kHz) 3D topological reconstruction with the projection of defocused binary patterns, yet it is still a challenge to measure a flapping flight process with the presence of rapid flapping wings. This paper presents a novel absolute 3D reconstruction method for a stabilized flapping flight process. Essentially, the slow motion parts (e.g. body) and the fast-motion parts (e.g. wings) are segmented and separately reconstructed with phase shifting techniques and Fourier transform, respectively. The topological relations between the wings and the body are utilized to ensure absolute 3D reconstruction. Experiments demonstrate the success of our computational framework by testing a flapping wing robot at different flapping speeds.

“Absolute three-dimensional shape measurement with two-frequency square binary patterns,” Appl. Opt., (2017)

C. Jiang and S. Zhang, “Absolute three-dimensional shape measurement with two-frequency square binary patterns,” Appl. Opt., 56(31), 8710-8718 (2017); doi:10.1364/AO.56.008710

Abstract

This paper presents a novel method to achieve absolute three-dimensional (3D) shape measurement solely using square binary patterns. This method uses six patterns: three low-frequency phase-shifted patterns and three phase-shifted high-frequency patterns. The phase obtained from low-frequency phase temporally unwraps the phase obtained from high-frequency patterns. The projector is defocused such that the high-frequency patterns produce high-quality phase, but the phase retrieved from low-frequency patterns has large harmonic error that fails two-frequency temporal phase unwrapping process. In this paper, we develop a computational framework to address the challenge. The proposed computational framework includes four major approaches to alleviate the harmonic error problem: i) use more than one period of low-frequency patterns enabled by geometric constraint-based phase unwrapping method; ii) artificially apply a large Gaussian filter to low frequency patterns before phase computation; iii) create an error lookup table (LUT) to compensate for harmonic error; and iv) develop a boundary error correction method to alleviate problems associated with filtering. Both simulation and experimental results demonstrated the success of the proposed method.

"Three dimensional range geometry and texture data compression with space-filling curves," Opt. Express (2017)

X. Chen and S. Zhang, "Three dimensional range geometry and texture data compression with space-filling curves," Opt. Express 25(21), 26148-26159 (2017); doi:10.1364/OE.25.026103

Abstract

This paper presents a novel method to effectively store three-dimensional (3D) data and 2D texture data into a regular 24-bit image. The proposed method uses the Hilbert space-filling curve to map the normalized unwrapped phase map to two 8-bit color channels, and saves the third color channel for 2D texture storage. By further leveraging existing 2D image and video compression techniques, the proposed method can achieve high compression ratios while effectively preserving data quality.  Since the encoding and decoding processes can be applied to most of the current 2D media platforms, this proposed compression method can make 3D data storage and transmission available for many electrical devices without requiring special hardware changes. Experiments demonstrate that if a lossless 2D image/video format is used, both original 3D geometry and 2D color texture can be accurately recovered; if lossy image/video compression is used, only black-and-white or grayscale texture can be properly recovered, but much higher compression ratios (e.g., 1543:1 against the ASCII OBJ format) are achieved with slight loss of 3D geometry quality.

"Dynamic projection theory for fringe projection profilometry," Appl. Opt., (2017)

[102] H. Sheng, J. Xu, and S. Zhang, "Dynamic projection theory for fringe projection profilometry," Appl. Opt., 56(30), 8452-8460 (2017); doi: 10.1364/AO.56.008452

Abstract

 Fringe projection profilometry (FPP)  has been widely used for 3D reconstruction, surface measurement and reverse engineering. However, fringe projection profilometry is prone to overexposure if objects have a wide range of reflectance. In this paper, we propose a dynamic projection theory based on fringe projection profilometry to rapidly measure the overexposed region with an attempt to conquer this challenge. This theory modifies the projected fringe image to the next better measurement based on the feedback provided by the previously captured image intensity. Experiments demonstrated that the number of overexposed points can be drastically reduced after one or two iterations. Compared with the state-of-the-art methods, our proposed dynamic projection theory measures the overexposed region quickly and effectively, and thus broadens the applications of fringe projection profilometry.

“Absolute phase unwrapping for dual-camera system without embedding statistical features,” Opt. Eng., (2017)

C. Jiang and S. Zhang, “Absolute phase unwrapping for dual-camera system without embedding statistical features,” Opt. Eng. 56(9), 094114 (2017), doi: 10.1117/1.OE.56.9.094114.

Abstract

This paper proposes an absolute phase unwrapping method for 3D measurement that uses two cameras and one projector. On the left camera image, each pixel has one wrapped phase value which corresponds to multiple projector candidates with different absolute phase values. We use geometric relationship of the system to map projector candidates into right camera candidates. By applying a series of candidate rejection criteria, a unique correspondence pair between two camera images can be determined. Then the absolute phase is obtained by tracing the correspondence point back to projector space. Experimental results demonstrate that the proposed absolute phase unwrapping algorithm can successfully work on both complex geometry and multiple isolated objects measurement.

"Computer-aided-design (CAD) model assisted absolute three-dimensional shape measurement" Appl. Opt. (2017)

[100] B. Li, T. Bell, and S. Zhang, "Computer-aided-design (CAD) model assisted absolute three-dimensional shape measurement,"  Appl. Opt. 56(24), 6770-6776 (2017); doi: 10.1364/AO.56.006770

Abstract

Conventional  three-dimensional (3D) shape measurement methods are typically generic to all types of objects. Yet, for many measurement conditions, such level of generality is inessential when having the pre-knowledge of object geometry. This paper introduces a novel adaptive algorithm for absolute 3D shape measurement with the assistance of the object CAD model. The proposed algorithm includes the following major steps: 1) export the 3D point cloud data from the CAD model; 2) transform the CAD model into the camera perspective; 3) obtain wrapped phase map from three phase-shifted fringe images; 4) retrieve absolute phase and 3D geometry assisted by CAD model. We demonstrate that if object CAD models are available, such algorithm is efficient in recovering absolute 3D geometries of both simple and complex objects with only three phase-shifted fringe images.  

"High-speed high-accuracy three-dimensional shape measurement using digital binary defocusing method versus sinusoidal method," Opt. Eng., (2017)

[99] J. -S. Hyun, B. Li, and S. Zhang, "High-speed high-accuracy three-dimensional shape measurement using digital binary defocusing method versus sinusoidal method," Opt. Eng. 56(7), 074102 (2017).

Abstract

This paper presents our research findings on high-speed high-accuracy 3D shape measurement using digital light processing (DLP) technologies. In particular, we compare two different sinusoidal fringe generation techniques using the DLP projection devices: direct projection of 8-bit computer generated sinusoidal patterns (a.k.a., the sinusoidal method), and the creation of sinusoidal patterns by defocusing binary patterns (a.k.a., the binary defocusing method). This paper mainly examines their performance on high-accuracy measurement applications under precisely controlled settings. Two different projection systems were tested in this study: the commercially available inexpensive projector, and the DLP development kit. Experimental results demonstrated that the binary defocusing method always outperforms the sinusoidal method if a sufficient number of phase-shifted fringe patterns can be used.

"Three-dimensional absolute shape measurement by combining binary statistical pattern matching with phase-shifting methods," Appl. Opt., (2017)

Y. An and S. Zhang, "Three-dimensional absolute shape measurement by combining binary statistical pattern matching with phase-shifting methods," Appl. Opt., (2017); (accepted)

Abstract

This paper presents a novel method that leverages the stereo geometric relationship between projector and camera for absolute phase unwrapping on a standard one-projector and one-camera structured light system. Specifically, we use only one additional binary random image and the epipolar geometric constraint to generate a coarse correspondence map between projector and camera images. The coarse correspondence map is further refined by using the wrapped phase as a constraint. We then use the refined correspondence map to determine a fringe order for absolute phase unwrapping. Experimental results demonstrated the success of our proposed method.

"Absolute three-dimensional shape measurement with a known object," Optics Express (2017)

J. Dai and S. Zhang, "Absolute three-dimensional shape measurement with a known object," Opt. Express25(9), 10384-10396 (2017); doi: 10.1364/OE.25.010384

Abstract

This paper presents a novel method for absolute three-dimensional (3D) shape measurement that does not require conventional temporal phase unwrapping. Our proposed method uses a known object (i.e., a ping-pong ball) to provide cues for absolute phase unwrapping. During the measurement, the ping-pong ball is positioned to be close to the nearest point from the scene to the camera. We first segment ping-pong ball and spatially unwrap its phase, and then determine the integer multiple of $2\pi$ to be added such that the recovered shape matches its actual geometry. The nearest point of the ball provides $z_{min}$ to generate the minimum phase $\Phi_{min}$ that is then used to unwrap phase of the entire scene pixel by pixel. Experiments demonstrated that only three phase-shifted fringe patterns are required to measure absolute shapes of objects moving along depth $z$ direction.

"Optimal path planning and control of assembly robots for hard measuring easy-deformation assemblies", IEEE Trans. Mechatronics (2017)

A. Wan, J. Xu, H. Chen, S. Zhang, and K. Chen, "Optimal path planning and control of assembly robots for hard measuring easy-deformation assemblies", IEEE Trans. Mechatronics, 22(4), 1600-1609, (2017); doi:10.1109/TMECH.2017.2671342

Abstract

Assembly robots are widely used in the electronics and automotive industries. However, assembly robots still face formidable challenges for assembling large-scale heavy-weight components such as the tail of the plane. First, the largescale component is difficult to measure; thus, the optimal assembly path is difficult to obtain. To this end, a learning from demonstration-based optimal path planning method is developed and implemented. Second, the deformation caused by a heavy-weight component will lead to a large motion error and could cause damage to the component. To solve this problem, a Gaussian process regression (GPR)-based deformation prediction and compensation method is presented to improve the robot motion accuracy. The simulation results show that the proposed GPR-based deformation compensation method can achieve high accuracy. An experimental prototype was developed to evaluate the proposed methods, and the results demonstrate the effectiveness of the proposed methods. Therefore, the proposed methods provide a path toward hard-measuring easy deformation assembly task.

"Pixel-by-pixel absolute three-dimensional shape measurement with modified Fourier transform profilometry" Appl. Opt., (2017)

[95] H. Yun, B. Li, and S. Zhang, "Pixel-by-pixel absolute three-dimensional shape measurement with modified Fourier transform profilometry", Appl. Opt., 56(5), 1472-1480, (2017); doi: 10.1364/AO.56.001472

Abstract

Single-pattern Fourier transform profilometry (FTP) method and double-pattern modified FTP method have great value on high-speed three-dimensional (3D) shape measurement, yet it is difficult to retrieve absolute phase pixel by pixel. This paper presents a method that can recover absolute phase pixel by pixel for the modified FTP method. The proposed method uses two images with different frequencies, and the recovered low frequency phase is used to temporally unwrap the high-frequency phase pixel by pixel. This paper also presents the computational framework to reduce noise impact for robust phase unwrapping. Experiments demonstrate the success of the proposed absolute phase recovery method using only two fringe patterns.

"Pixel-by-pixel absolute phase retrieval using three phase-shifted fringe patterns without markers," Opt. Laser Eng., (2017)

C. Jiang,  B. Li, S. Zhang, "Pixel-by-pixel absolute phase retrieval using three phase-shifted fringe patterns without markers," Opt. Laser Eng., 91, 232-241 (2017);  doi:10.1016/j.optlaseng.2016.12.002

This paper presents a method that can recover absolute phase pixel by pixel without embedding markers on three phase-shifted fringe patterns, acquiring additional images, or introducing additional hardware component(s). The proposed
three-dimensional (3D) absolute shape measurement technique includes the following major steps: 1) segment the measured object into different regions using rough priori knowledge of surface geometry; 2) artificially create phase maps at different z planes using geometric constraints of structured light system; 3) unwrap the phase pixel by pixel for each region by properly referring to the artificially created phase map; and 4) merge unwrapped phases from all regions into a complete absolute phase map for 3D reconstruction. We demonstrate that conventional three-step phase-shifted fringe patterns can be used to create absolute phase map pixel by pixel even for large depth range objects. We have successfully implemented our proposed computational framework to achieve absolute 3D shape measurement at 40 Hz.

"Development of a mobile tool mark characterization/comparison system," J. Forensic Sci., (2017)

L. S. Chumbley,  M. Morris, R. Spotts, and C. Macziewski, "Development of a mobile tool mark characterization/comparison system," J. Forensic Sci., 62(1), 83-91 (2017), doi: 10.1111/1556-4029.13233

Since the development of the striagraph, various attempts have been made to enhance forensic investigation through the use of measuring and imaging equipment. This study describes the development of a prototype system employing an easy-to-use software interface designed to provide forensic examiners with the ability to measure topography of a toolmarked surface and then conduct various comparisons using a statistical algorithm. Acquisition of the data is carried out using a portable 3D optical profilometer, and comparison of the resulting data files is made using software named “MANTIS” (Mark and Tool Inspection Suite). The system has been tested on laboratory-produced markings that include fully striated marks (e.g., screwdriver markings), quasistriated markings produced by shear-cut pliers, impression marks left by chisels, rifling marks on bullets, and cut marks produced by knives. Using the system, an examiner has the potential to (i) visually compare two toolmarked surfaces in a manner similar to a comparison microscope and (ii) use the quantitative information embedded within the acquired data to obtain an objective statistical comparison of the data files. This study shows that, based on the results from laboratory samples, the system has great potential for aiding examiners in conducting comparisons of toolmarks.

"Evaluation of pixel-wise geometric constraints based phase unwrapping method for low signal-to-noise-ratio (SNR) phase," Advanced Optical Technologies, (2016)

[91] Y. An, Z. Liu and S. Zhang, "Evaluation of pixel-wise geometric constraints based phase unwrapping method for low signal-to-noise-ratio (SNR) phase," Advanced Optical Technologies, 5(5-6), 423–432, (2016); doi: 10.1515/aot-2016-0048

This paper evaluates the robustness of our recently proposed geometric constraints based phase unwrapping method to unwrap low signal-to-noise ratio (SNR) phase.  Instead of capturing additional images for absolute phase unwrapping, the new phase unwrapping algorithm uses geometric constraints of the digital fringe projection (DFP) system to create a virtual reference phase map to unwrap the phase pixel by pixel. Both simulation and experimental results demonstrate that this new phase unwrapping method can even successfully unwrap low SNR phase maps that brings difficulties for conventional multi-frequency phase unwrapping methods.