Zewei Cai

Suppression of projector distortion in phase-measuring profilometry by projecting adaptive fringe patterns.

In phase-measuring profilometry, the lens distortion of commercial projectors may introduce additional bending carrier phase and thus lead to measurement errors. To address this problem, this paper presents an adaptive fringe projection technique in which the carrier phase in the projected fringe patterns is modified according to the projector distortion. After projecting these adaptive fringe patterns, the bending carrier phase induced by the projector distortion is eliminated. Experimental results demonstrate this method to be effective and efficient in suppressing the projector distortion for phase-measuring profilometry. More importantly, this method does not need to calibrate the projector and system parameters, such as the distortion coefficients of the projector and the angle between the optical axes of projector and camera lenses. Hence, it has low computational complexity and enables us to improve the measurement precision for an arbitrary phase-measuring profilometry system.

Flexible phase error compensation based on Hilbert transform in phase shifting profilometry

This paper makes use of Hilbert transform to analyze and compensate the phase error caused by the nonlinear effect in phase shifting profilometry (PSP). The characteristics of the phase error distribution in Hilbert transform domain was analyzed and compared with spatial domain. A simple and flexible phase error compensation method was proposed to directly process the phase-shifting fringe images without any auxiliary conditions or complicated computation. Experimental results demonstrated that the phase error can be reduced by about 80% in three-step PSP, and more than 95% in four or more step PSP, which verified the effectiveness, flexibility, robustness and automation of the proposed phase error compensation method.

Phase error compensation methods for high-accuracy profile measurement

In a phase-shifting algorithm-based fringe projection profilometry, the nonlinear intensity response, called the gamma effect, of the projector-camera setup is a major source of error in phase retrieval. This paper proposes two novel, accurate approaches to realize both active and passive phase error compensation based on a universal phase error model which is suitable for a arbitrary phase-shifting step. The experimental results on phase error compensation and profile measurement of standard components verified the validity and accuracy of the two proposed approaches which are robust when faced with changeable measurement conditions.

Structured light field 3D imaging

In this paper, we propose a method by means of light field imaging under structured illumination to deal with high dynamic range 3D imaging. Fringe patterns are projected onto a scene and modulated by the scene depth then a structured light field is detected using light field recording devices. The structured light field contains information about ray direction and phase-encoded depth, via which the scene depth can be estimated from different directions. The multidirectional depth estimation can achieve high dynamic 3D imaging effectively. We analyzed and derived the phase-depth mapping in the structured light field and then proposed a flexible ray-based calibration approach to determine the independent mapping coefficients for each ray. Experimental results demonstrated the validity of the proposed method to perform high-quality 3D imaging for highly and lowly reflective surfaces.

Phase-3D mapping method developed from back-projection stereovision model for fringe projection profilometry

Two major methods for 3D reconstruction in fringe projection profilometry, phase-height mapping and stereovision, have their respective problems: the former has low-flexibility in practical application due to system restrictions and the latter requires time-consuming homogenous points searching. Given these limitations, we propose a phase-3D mapping method developed from back-projection stereovision model to achieve flexible and high-efficient 3D reconstruction for fringe projection profilometry. We showed that all dimensional coordinates (X, Y, and Z), but not just the height coordinate (Z), of a measured point can be mapped from phase through corresponding rational functions directly and independently. To determine the phase-3D mapping coefficients, we designed a flexible two-step calibration strategy. The first step, ray reprojection calibration, is to determine the stereovision system parameters; the second step, sampling-mapping calibration, is to fit the mapping coefficients using the calibrated stereovision system parameters. Experimental results demonstrated that the proposed method was suitable for flexible and high-efficient 3D reconstruction that eliminates practical restrictions and dispenses with the time-consuming homogenous point searching.

Portable 3D scanning system based on an inertial sensor

Portable 3D scanning systems are increasingly used in many applications at present as a result of its high flexibility, portability and high efficiency. Iterative closest points method is widely used for multi-view measurement results registration. However, there are many restrictions for portable system, the alignment often depends on landmarks on object surface or object features, in some applications, it may not achieve satisfactory expectations. In this paper, we propose to conduct the registration based on pose estimation from a low cost inertial sensor, which will increase the measurement effectiveness. Test result demonstrates that the method is feasible. With attitude information inside the system, the measurement device does not need external support information and has good prospects for application.

Imaging dynamic objects hidden behind scattering medium by retrieving the point spread function

We address a non-invasive imaging method to observe dynamic objects hidden behind a turbid medium. An initial image of the objects is first recovered by speckle correlation technique (SCT) with a single shot speckle pattern. The scattered point spread function (PSF) is then extracted by taking a deconvolution process between the initial image and its corresponding speckle pattern. Consequently, the images of the dynamic objects, within the optical memory effect (OME) range, can then be reconstructed directly with the same deconvolution process between the sequential speckle patterns and the estimated PSF. In addition, a further calibration operation is employed to enhance the robustness of the PSF, ensuring sharp images can still be observed when objects are close to or even cross the edge of OME. Experimental demonstration is presented to verify the feasibility of our proposed method.

Depth-of-field characteristic analysis of the imaging system with scattering medium

The depth-of-field (DOF) characteristic of the imaging system with scattering medium is analyzed based on the analytical model of ambiguity function as a polar display of the optical transfer function (OTF) in this paper. It is indicated that the scattering medium can help re-collect more high spatial frequencies, which are normally lost with defocusing in traditional imaging systems. Therefore, the scattering medium can be considered not as an obstacle for imaging but as a useful tool to extend the DOF of the imaging system. To test the imaging properties and limitations, we performed optical experiments in a single-lens imaging system.

An accurate hand-eye calibration algorithm with global optimization

The hand-eye system calibration, aiming to achieve the relationship between the robot hand and vision sensor mounted on it, is an important technique in the robot applications, involving automatic 3D measurement, visual serving, sensor placement planning, etc. Generally, the key issue of hand-eye calibration is equivalent to solving the homogeneous transformation matrix X from the equation of the form AX=XB. In this paper, we develop an accurate hand-eye calibration method by establishing a global objective function, in which the errors of camera calibration and robot movements have been considered. It is constructed based on the minimizing the projection error from the target benchmarks to the camera retina plane at all robot motions. The experimental results prove that the proposed algorithm can accurately solve the hand-eye calibration problem. Meanwhile, we set up an automatic 3D measurement system based on a robot and a rotary table, and developed a calibration scheme for the system to achieve the multi-view and fully automatic 3D data acquisition by using a fringe projection 3D sensor.

A method for fast 3D fringe projection measurement without phase unwrapping

At present, fringe projection profilometry has also been limited with a trade-off between speed and accuracy. For achieving high accuracy measurement, phase-shifting and phase-unwarpping operations will always be used for phase correspondence, however, the phase-unwrapping processing does not contribute to improve the phase accuracy, but just to distinguish phase steps. For futher reducing the projection pattern for phase-unwarpping, we propose a novel method for phase corresponding in bi-cameras system without phase unwrapping. Phase-to-3D mapping structures are utilized to obtain the candidate correspondences and eliminate the ambiguties with wrapped phase, which is implemented efficiently without time-comsuming phase correspondence searching. The experiments on both static and dynamic scenes are perfomed to verify its capability of 120 fps 3D reconstructing speed by overlapped using 3-step phase-shifting pattern.