基于UG平台的截面特征曲线全局约束优化研究

截面曲线是曲面建模过程中的重要特征,将截面特征曲线重建技术与正向设计技术相结合是实现复杂特征曲面模型重建的重要方法。针对这一问题提出了一种基于UG草图的截面特征曲线全局约束优化技术,并对实现该方法的约束表达、全局约束优化模型的建立及求解、基于UG平台的算法实现等关键技术进行了深入研究。应用实例表明,基于UG的截面特征曲线全局约束优化方法可以得到满足约束条件下的与数据点逼近的最优曲线,基于该曲线可以快速、准确地重建出具有复杂特征的曲面模型。     

基于激光点云数据的叶片型面三维重构

在四坐标叶片型面检测基础上,提出基于激光点云数据的叶片型面三维重构方法。激光位移传感器对叶片型面进行多视角扫描采样,快速采集叶片型面海量点云数据。运用由点到线到面的数学建模原理,先基于端点一阶导矢连续法拟合出光顺NURBS曲线,再依据分片能量法构建辅助曲面拟合出光顺NURBS曲面,最后对分片NURBS曲面进行统一描述,构造出精确光滑的叶片型面,实现叶片型面的三维重构。实验结果表明：采用该方法实现对各类叶片复杂型面的三维重构,重构误差均<0.015 mm,能够满足精密零件的测量需求。     

Research and implementation of NURBS interpolator with continuous feedrate for high-speed machining

Using non-uniform rational B-splines (NURBS) rather than piecewise lines and arc segments as a toolpath makes it much easier to achieve high-speed control in die and mould machining. When NURBS is used as a toolpath, two issues must be solved. The first is how to convert the discrete cutter location (CL) data offset from a CAD model into a NURBS curve. The second is how to maintain low velocity fluctuation while interpolating the NURBS curve with limited calculation time. This study proposes a NURBS toolpath generation method from line segments and a real-time NURBS interpolator for CNC machines to achieve the goal of high-speed machining. A CL data simplification method with shape holding is presented based on identifying and correcting the defect point from the original CL data, and removing redundant CL data within shape-maintaining requirements. Then the conversion of all simplified CL data to NURBS curves is done by applying NURBS fitting or interpolation with optional directional constraints. A precise method based on nonlinear mapping between the NURBS's parameter and arc length is proposed and implemented to obtain a continuous feedrate and machining quality. The simulation performed on CL points for a wave surface demonstrates that the proposed approach reduces the NC blocks to 6% while staying within the tolerance of deviation. The machining experiment shows that the proposed approach can increase machining quality while reducing machining time by about 22%.     

A mortar formulation for 3D large deformation contact using NURBS-based isogeometric analysis and the augmented Lagrangian method

NURBS-based isogeometric analysis is applied to 3D frictionless large deformation contact problems. The contact constraints are treated with a mortar-based approach combined with a simplified integration method avoiding segmentation of the contact surfaces, and the discretization of the continuum is performed with arbitrary order NURBS and Lagrange polynomial elements. The contact constraints are satisfied exactly with the augmented Lagrangian formulation proposed by Alart and Curnier, whereby a Newton-like solution scheme is applied to solve the saddle point problem simultaneously for displacements and Lagrange multipliers. The numerical examples show that the proposed contact formulation in conjunction with the NURBS discretization delivers accurate and robust predictions. In both small and large deformation cases, the quality of the contact pressures is shown to improve significantly over that achieved with Lagrange discretizations. In large deformation and large sliding examples, the NURBS discretization provides an improved smoothness of the traction history curves. In both cases, increasing the order of the discretization is either beneficial or not influential when using isogeometric analysis, whereas it affects results negatively for Lagrange discretizations.     

Unification scheme for 3D surface reconstruction using physically based models

We propose a unification framework for three-dimensional shape reconstruction using physically based models. A variety of 3D shape reconstruction techniques have been developed in the past two decades, such as shape from stereopsis, from shading, from texture gradient, and from structured lighting. However, the lack of a general theory that unifies these shape reconstruction techniques into one framework hinders the effort of a synergistical image interpretation scheme using multiple sensors/information sources. Most shape-from-X techniques use an “observable” (e.g., the stereo disparity, intensity, or texture gradient) and a model, which is based on specific domain knowledge (e.g., the triangulation principle, reflectance function, or texture distortion equation) to predict the observable, in 3D shape reconstruction. We show that all these “observable–prediction-model” types of techniques can be incorporated into our framework of energy constraint on a flexible, deformable image frame. In our algorithm, if the observable does not confirm to the predictions obtained using the corresponding model, a large “error” potential results. The error potential gradient forces the flexible image frame to deform in space. The deformation brings the flexible image frame to “wrap” onto the surface of the imaged 3D object. Surface reconstruction is thus achieved through a “package wrapping” or a “shape deformation” process by minimizing the discrepancy in the observable and the model prediction. The dynamics of such a wrapping process are governed by the least action principle which is physically correct. A physically based model is essential in this general shape reconstruction framework because of its capability to recover the desired 3D shape, to provide an animation sequence of the reconstruction, and to include the regularization principle into the theory of surface reconstruction.     

主动多基线立体视觉及其在机器人焊接技术中的应用研究

介绍了多基线立体视觉的基本原理针对焊接工件等一类表面缺纹纹理物体的三维深度恢复这一难题,提出采用基于条纹光照明方式的主动多基立体视觉方法解决这类物体的三维视觉建模问题,并通过验证明方法的有效性和可靠性。     

Reconstruction of a one-dimensional surface from inverse transmission

An optimization algorithm for the reconstruction of a surface profile is presented. This algorithm uses the far-field transmitted intensity to retrieve the surface profile of a thin film. We approach the inverse transmission problem as a constrained optimization problem. A mathematical representation of the surface based on B-spline curves and an optimum profile search with the self-adaptation evolutionary strategy (ES) are adopted. As the input data for the ES algorithm for surface inversion, the transmitted intensity has been measured by both a laser bidirectional reflectometer instrument and an in-line digital speckle camera system. The reconstructed 1D surfaces are compared with the profile measured by an atomic force microscope.     

Cooperative matching paradigm for the analysis of stereo image sequences

A general framework for the analysis of sequences of stereo images is presented in this article. The proposed paradigm integrates spatial and temporal information to achieve robustness in establishing stereo correspondence from images that possess a limited number of salient features. Specifically, the recovery of dynamic stereo information, and thus, the reconstruction of a three-dimensional object structure, is accomplished using a feature-based approach. The multiset feature correspondence problem is cast into a four-dimensional problem which is subsequently solved using a greedy-type local search algorithm. No assumption is made about the type of motion. The proposed algorithm may be applied to process image sequences containing both rigid and nonrigid shapes. The efficacy of the proposed approach is tested using both synthetic data and real images of a live frog's ventricle acquired by a stereo light microscope. We also consider the incorporation of domain-specific knowledge to enhance the performance of the cooperative matching scheme. © 1998 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 9, 192–200, 1998     

A 2D topology optimisation algorithm in NURBS framework with geometric constraints

In this paper, the solid isotropic material with penalisation (SIMP) method for topology optimisation of 2D problems is reformulated in the non-uniform rational BSpline (NURBS) framework. This choice implies several advantages, such as the definition of an implicit filter zone and the possibility for the designer to get a geometric entity at the end of the optimisation process. Therefore, important facilities are provided in CAD postprocessing phases in order to retrieve a consistent and well connected final topology. The effect of the main NURBS parameters (degrees, control points, weights and knot-vector components) on the final optimum topology is investigated. Classic geometric constraints, as the minimum and maximum member size, have been integrated and reformulated according to the NURBS formalism. Furthermore, a new constraint on the local curvature radius has been developed thanks to the NURBS formalism and properties. The effectiveness and the robustness of the proposed method are tested and proven through some benchmarks taken from literature and the results are compared with those provided by the classical SIMP approach.     

高速凸轮NURBS廓线改进人工鱼群多目标动力学优化

为改善高速凸轮机构动力学性能,首先采用非均匀有理B样条（NURBS）重构凸轮廓线,建立高速凸轮单自由度弹性动力学模型,进行高速凸轮机构多目标动力学优化设计。然后利用改进人工鱼群算法求解该模型,获取系统Pareto最优解从而得到优化后的NURBS廓线。最后,分析和比较高速凸轮机构在NURBS和修正正弦两种廓线下的动力学响应。结果表明：改进人工鱼群算法可以有效解决多目标优化问题,优化后NURBS廓线运动学特性与修正正弦廓线相差不大,在一定程度上降低了高速凸轮机构的残余振动幅值,提高了高速凸轮机构的定位精度和动力性能,减少了振动和冲击的噪声。     

Estimating three-dimensional vehicle motion in an outdoor scene using stereo image sequences

We propose a motion estimation system that uses stereo image pairs as the input data. To perform experimental work, we also obtain a sequence of outdoor stereo images taken by two metric cameras. The system consists of four main stages, which are (1) determination of point correspondences on the stereo images, (2) correction of distortions in image coordinates, (3) derivation of 3D point coordinates from 2D correspondences, and (4) estimation of motion parameters based on 3D point correspondences. For the first stage of the system, we use a four-way matching algorithm to obtain matched point on two stereo image pairs at two consecutive time instants (t_i and t_{i + 1}). Since the input data are stereo images taken by cameras, it has two types of distortions, which are (i) film distortion and (ii) lens distortion. These two distortions must be corrected before any process can be applied on the matched points. To accomplish this goal, we use (i) bilinear transform for film distortion correction and (ii) lens formulas for lens distortion correction. After correcting the distortions, the results are 2D coordinates of each matched point that can be used to derive 3D coordinates. However, due to data noise, the calculated 3D coordinates to not usually represent a consistent rigid structure that is suitable for motion estimation; therefore, we suggest a procedure to select good 3D point sets as the input for motion estimation. The procedure exploits two constraints, rigidity between different time instants and uniform point distribution across the object on the image. For the last stage, we use an algorithm to estimate the motion parameters. We also wish to know what is the effect of quantization error on the estimated results; therefore an error analysis based on quantization error is performed on the estimated motion parameters. In order to test our system, eight sets of stereo image pairs are extracted from an outdoor stereo image sequence and used as the input data. The experimental results indicate that the proposed system does provide reasonable estimated motion parameters.     

Stereo calibration of binocular ultra-wide angle long-wave infrared camera based on an equivalent small field of view camera

ABSTRACT

In this paper, a stereo calibration method for binocular ultra-wide angle long-wave infrared camera is proposed on the basis of an equivalent small field of view camera. Extrinsic parameters are calibrated through the corrected images from the left and right cameras. They can be viewed as images taken by a small field of view camera. The calibration procedure consists of three steps: monocular calibration, distortion correction and extrinsic parameters calibration. In order to evaluate the accuracy of the method, stereo vision of the camera is modelled and a 3D reconstruction approach is presented. A series of experiments, including intrinsic parameters, extrinsic parameters and 3D reconstruction, are conducted to validate the proposed method. The results show that the baseline length error decreases to 0.67%, and the relative error for the 3D reconstruction of corners is smaller than 8.11%. In contrast to the common stereo calibration method, it improves calibration accuracy.     

基于视差空间运动估计的高精度立体视觉定位

立体视觉定位算法的运动估计通常在3D欧式空间中进行,但由于特征点3D坐标的噪声各向异性且分布不均匀,3D重建在深度方向上比另两个方向上的准确性差,从而导致3D欧式空间运动估计精确不高.本文提出了一种新的基于视差空间运动估计的高精度立体视觉定位算法.算法首先采用视差空间4点闭环线性解法和RANSAC算法得到初始鲁棒运动估计和匹配内点.接着,利用新的视差空间再投影误差函数提出了基于LM算法的视差空间运动参数非线性优化方法,对初始运动参数进一步优化.视差空间噪声分布均匀且各向同性,本文的初始运动参数线性估计和非线性优化都在视差空间中进行且能达到全局最小.仿真实验和真实实验结果表明,本文算法能得到高精度的立体视觉定位结果,优于传统的3D欧式空间运动估计方法.     

基于复杂曲线的NURBS蒙皮曲面

本文提出了一种用ＮＵＲＢＳ曲线统一描述由不同阶次的曲线段构成的复合曲线，再进一步用此复合曲线构造ＮＵＲＢＳ蒙皮曲面的方法，用于进行复杂外形产品的设计，对于实际问题中可能出现的情况，作者对ＮＵＲＢＳ曲线曲面的节点处理采用了不同的技巧，并用实例加以验证。     

Skeleton body pose tracking from efficient three-dimensional motion estimation and volumetric reconstruction

We address the problem of body pose tracking in a scenario of multiple camera setup with the aim of recovering body motion robustly and accurately. The tracking is performed on three-dimensional (3D) space using 3D data, including colored volume and 3D optical flow, which are reconstructed at each time step. We introduce strategies to compute multiple camera-based 3D optical flow and have attained efficient and robust 3D motion estimation. Body pose estimation starts with a prediction using 3D optical flow and then is changed to a lower-dimensional global optimization problem. Our method utilizes a voxel subject-specific body model, exploits multiple 3D image cues, and incorporates physical constraints into a stochastic particle-based search initialized from the deterministic prediction and stochastic sampling. It leads to a robust 3D pose tracker. Experiments on publicly available sequences show the robustness and accuracy of our approach.     

A novel minimum weight formulation of topology optimization implemented with reanalysis approach

In this paper, we develop an efficient diagonal quadratic optimization formulation for minimum weight design problem subject to multiple constraints. A high-efficiency computational approach of topology optimization is implemented within the framework of approximate reanalysis. The key point of the formulation is the introduction of the reciprocal-type variables. The topology optimization seeking for minimum weight can be transformed as a sequence of quadratic program with separable and strictly positive definite Hessian matrix, thus can be solved by a sequential quadratic programming approach. A modified sensitivity filtering scheme is suggested to remove undesirable checkerboard patterns and mesh dependence. Several typical examples are provided to validate the presented approach. It is observed that the optimized structure can achieve lighter weight than those from the established method by the demonstrative numerical test. Considerable computational savings can be achieved without loss of accuracy of the final design for 3D structure. Moreover, the effects of multiple constraints and upper bound of the allowable compliance upon the optimized designs are investigated by numerical examples.     

A generalized approach to the reconstruction of a restricted class of digitized planar curves

Reconstruction of an original continuous curve and the estimation of its parameters from the digitized version of the curve is a challenging problem, as quantization always causes some loss of information. In this paper, we have developed a scheme for reconstruction which is applicable to a class of curves having at the most two parameters. The class of curves for which the scheme works has also been characterized. We have shown that for one-parameter curves the exact domain of values of the parameter can be obtained. But in the two-parameter case, only the smallest rectangle containing the domain can be realised. The distinctive feature of our scheme is that it provides a unified approach to solve the reconstruction and the domain-finding problem for a class of curves.     

Computational imaging systems: joint design and end-to-end optimality

A framework is proposed for optimal joint design of the optical and reconstruction filters in a computational imaging system. First, a technique for the design of a physically unconstrained system is proposed whose performance serves as a universal bound on any realistic computational imaging system. Increasing levels of constraints are then imposed to emulate a physically realizable optical filter. The proposed design employs a generalized Benders' decomposition method to yield multiple globally optimal solutions to the nonconvex optimization problem. Structured, closed-form solutions for the design of observation and reconstruction filters, in terms of the system input and noise autocorrelation matrices, are presented. Numerical comparison with a state-of-the-art optical system shows the advantage of joint optimization and concurrent design.