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Author

Hongyu Ren

Bio: Hongyu Ren is an academic researcher from University of Huddersfield. The author has contributed to research in topics: Displacement (psychology) & Camera resectioning. The author has an hindex of 4, co-authored 5 publications receiving 119 citations. Previous affiliations of Hongyu Ren include National University of Defense Technology.

Papers
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Journal ArticleDOI
TL;DR: An accurate system calibration method is presented in this paper to calibrate stereo deflectometry and can effectively reduce reprojection error, which correspondingly improves the final reconstruction accuracy.
Abstract: An accurate system calibration method is presented in this paper to calibrate stereo deflectometry. A corresponding iterative optimization algorithm is also proposed to improve the system calibration accuracy. This merges CCD parameters and geometrical relation between CCDs and the LCD into one cost function. In this calibration technique, an optical flat acts as a reference mirror and simultaneously reflect sinusoidal fringe patterns into the two CCDs. The normal vector of the reference mirror is used as an intermediate variable to implement this iterative optimization algorithm until the root mean square of the reprojection errors converge to a minimum. The experiment demonstrates that this method can optimize all the calibration parameters and can effectively reduce reprojection error, which correspondingly improves the final reconstruction accuracy.

57 citations

Journal ArticleDOI
TL;DR: A modified easy-implementation integration method based on HFLI (EI-HFLI), which can work in arbitrary domains, and can directly and conveniently handle incomplete gradient data is proposed.
Abstract: Stereo deflectometry is defined as measurement of the local slope of specular surfaces by using two CCD cameras as detectors and one LCD screen as a light source. For obtaining 3D topography, integrating the calculated slope data is needed. Currently, a high-order finite-difference-based least-squares integration (HFLI) method is used to improve the integration accuracy. However, this method cannot be easily implemented in circular domain or when gradient data are incomplete. This paper proposes a modified easy-implementation integration method based on HFLI (EI-HFLI), which can work in arbitrary domains, and can directly and conveniently handle incomplete gradient data. To carry out the proposed algorithm in a practical stereo deflectometry measurement, gradients are calculated in both CCD frames, and then are mixed together as original data to be meshed into rectangular grids format. Simulation and experiments show this modified method is feasible and can work efficiently.

35 citations

Journal ArticleDOI
TL;DR: A new LSI method is proposed to integrate gradient data, based on an approximation that the normal vector of one point is perpendicular to the vectors connecting points at either side, which is effective and accurate at handling deflectometry measurement.
Abstract: Least-squares integration (LSI) and radial basis function integration (RBFI) methods are widely used to reconstruct specular surface shapes from gradient data in a deflectometry measurement. The traditional LSI method requires gradient data having a rectangular grid, and the RBFI method is effective at handling small size measurement data sets. Practically, the amount of gradient data is rather large, and data grids are in quadrilateral shapes. With this in mind, a new LSI method is proposed to integrate gradient data, which is based on an approximation that the normal vector of one point is perpendicular to the vectors connecting points at either side. A small measurement data set integrated by the RBFI method is employed as a supplementary constraint of the proposed method. Simulation and experimental results show that this proposed method is effective and accurate at handling deflectometry measurement.

35 citations

Journal ArticleDOI
23 May 2017-Sensors
TL;DR: This paper presents a novel camera calibration method with an iterative distortion compensation algorithm that does not rely on a distortion mathematical model, and is stable and effective in terms of complex distortion conditions.
Abstract: Camera distortion is a critical factor affecting the accuracy of camera calibration A conventional calibration approach cannot satisfy the requirement of a measurement system demanding high calibration accuracy due to the inaccurate distortion compensation This paper presents a novel camera calibration method with an iterative distortion compensation algorithm The initial parameters of the camera are calibrated by full-field camera pixels and the corresponding points on a phase target An iterative algorithm is proposed to compensate for the distortion A 2D fitting and interpolation method is also developed to enhance the accuracy of the phase target Compared to the conventional calibration method, the proposed method does not rely on a distortion mathematical model, and is stable and effective in terms of complex distortion conditions Both the simulation work and experimental results show that the proposed calibration method is more than 100% more accurate than the conventional calibration method

30 citations

Proceedings ArticleDOI
TL;DR: Simulated and experimental results show that this methodology can conveniently calculate topography information for freeform and structured specular surfaces without integration and reconstruction processes.
Abstract: Deflectometric methods have been studied for more than a decade for slope measurement of specular freeform surfaces through utilization of the deformation of a sample pattern after reflection from a tested sample surface. Usually, these approaches require two-directional fringe patterns to be projected on a LCD screen or ground glass and require slope integration, which leads to some complexity for the whole measuring process. This paper proposes a new mathematical measurement model for measuring topography information of freeform specular surfaces, which integrates a virtual reference specular surface into the method of active fringe reflection photogrammetry and presents a straight-forward relation between height of the tested surface and phase signals. This method only requires one direction of horizontal or vertical sinusoidal fringe patterns to be projected from a LCD screen, resulting in a significant reduction in capture time over established methods. Assuming the whole system has been precalibrated during the measurement process, the fringe patterns are captured separately via the virtual reference and detected freeform surfaces by a CCD camera. The reference phase can be solved according to the spatial geometric relation between the LCD screen and the CCD camera. The captured phases can be unwrapped with a heterodyne technique and optimum frequency selection method. Based on this calculated unwrapped-phase and that proposed mathematical model, absolute height of the inspected surface can be computed. Simulated and experimental results show that this methodology can conveniently calculate topography information for freeform and structured specular surfaces without integration and reconstruction processes.

4 citations


Cited by
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Journal Article
TL;DR: In this article, a generalized equation is proposed to represent a continuum of surface reconstruction solutions of a given non-integrable gradient field, where the range of solutions is related to the degree of anisotropy in applying weights to the gradient in the integration process.
Abstract: We propose a generalized equation to represent a continuum of surface reconstruction solutions of a given non-integrable gradient field. We show that common approaches such as Poisson solver and Frankot-Chellappa algorithm are special cases of this generalized equation. For a N x N pixel grid, the subspace of all integrable gradient fields is of dimension N 2 - 1. Our framework can be applied to derive a range of meaningful surface reconstructions from this high dimensional space. The key observation is that the range of solutions is related to the degree of anisotropy in applying weights to the gradients in the integration process. While common approaches use isotropic weights, we show that by using a progression of spatially varying anisotropic weights, we can achieve significant improvement in reconstructions. We propose (a) α-surfaces using binary weights, where the parameter a allows trade off between smoothness and robustness, (b) M-estimators and edge preserving regularization using continuous weights and (c) Diffusion using affine transformation of gradients. We provide results on photometric stereo, compare with previous approaches and show that anisotropic treatment discounts noise while recovering salient features in reconstructions.

313 citations

Journal ArticleDOI
TL;DR: The fundamental principle and the basic concepts of PMD technique are introduced and followed by a brief overview of its key developments since it was first proposed to provide some suggestions for potential future investigations.

133 citations

Journal ArticleDOI
TL;DR: In this work, a model based method is applied to phase measuring deflectometry, named modal phase measuring deflectionometry, and the pose of the screen relative to the camera is pre-calibrated and further optimized together with the shape coefficients of the surface under test.
Abstract: In this work, a model based method is applied to phase measuring deflectometry, named modal phase measuring deflectometry. The height and slopes of the surface under test are represented by mathematical models and updated by optimizing the model coefficients to minimize the discrepancy between the reprojection in ray tracing and the actual measurement. The pose of the screen relative to the camera is pre-calibrated and further optimized together with the shape coefficients of the surface under test. Simulations and experiments are conducted to demonstrate the feasibility of the proposed approach.

66 citations

Journal ArticleDOI
07 Dec 2017-Sensors
TL;DR: This work presents a review of the relevant techniques regarding classical and improved phase-measuring deflectometry, and discusses the challenges and future research directions to further advance PMD techniques.
Abstract: The fast development in the fields of integrated circuits, photovoltaics, the automobile industry, advanced manufacturing, and astronomy have led to the importance and necessity of quickly and accurately obtaining three-dimensional (3D) shape data of specular surfaces for quality control and function evaluation. Owing to the advantages of a large dynamic range, non-contact operation, full-field and fast acquisition, high accuracy, and automatic data processing, phase-measuring deflectometry (PMD, also called fringe reflection profilometry) has been widely studied and applied in many fields. Phase information coded in the reflected fringe patterns relates to the local slope and height of the measured specular objects. The 3D shape is obtained by integrating the local gradient data or directly calculating the depth data from the phase information. We present a review of the relevant techniques regarding classical PMD. The improved PMD technique is then used to measure specular objects having discontinuous and/or isolated surfaces. Some influential factors on the measured results are presented. The challenges and future research directions are discussed to further advance PMD techniques. Finally, the application fields of PMD are briefly introduced.

59 citations