基于数据重建的三角网格模型简化优化方法

由CT切片数据重建得到的三角网格模型常存在数据量大、狭长三角形多等问题,针对这些问题,研究了一种保持特征的高质量三角网格模型的简化优化方法。该方法分为网格简化和网格优化两个阶段。首先,采用基于曲面变化的二次误差度量计算边折叠代价,并按代价值的大小进行迭代的折叠简化,可较好地保持模型表面的特征;其次,通过二阶加权伞算子对简化模型中局部存在的狭长三角形进行优化处理,改善三角网格模型的质量。实验结果表明,该方法能够较好地保持特征区域的细节信息,并可靠地生成高质量、低几何误差的简化模型。     

基于质心Voronoi图重构的UDSM边折叠简化

城市数字表面模型网格(UDSM)的相邻网格常常出现曲率剧变，而这些位置是UDSM的细节部分，简化过程中应当尽量保持。针对该情况，引入了质心Voronoi图重划分网格，将曲率较小的表面的点云密度大大降低。重划分的网格表面细节与周围的平滑表面的三角网格尺寸悬殊，在该基础上使用二次误差矩阵边折叠进行LOD构建时网格发生明显变化，范围大大减少。算法在时间性能与网格误差与直接边折叠相近的前提下，更多地保存简化后的网格细节。     

基于工业CT图像的自适应三维网格模型重建

针对X射线扫描得到的工业CT图像重建三角网格模型存在尖锐特征丢失、狭长三角形和三角面片数量多等问题，提出一种自适应三维网格模型重建算法。首先对图像进行预处理；其次采用八叉树结构确定体元；然后利用二次误差函数（QEF）构建自适应八叉树；最后剖分四边形生成三角网格。使用立方体数据和两组实际扫描的CT数据对所提算法的性能进行验证分析，实验结果表明：所提算法在简化网格的同时仍能保持物体的尖锐特征，减少了狭长三角形的数量。利用所提算法生成三角网格模型的简化率可达90%，简化后网格质量大于0.3的三角网格平均占比为99%，有效地提高了由工业CT图像重建三角网格模型的质量。     

结合前沿推进的Delaunay三角化网格生成及应用

采用一种新的混合网格生成方法,生成复杂区域的非结构化网格.结合前沿推进法和Delaunay三角化两种非结构网格生成方法的特点,在边界处采用前沿推进法进行三角形初始网格的生成,在边界区域内部采用Delaunay三角化方法自动生成内部节点.分析表明,该算法简化网格生成过程,能够快速有效地生成非结构化网格.在计算时间以及网格的均匀性方面与其他方法相比具有一定的优势.最后,用混合网格生成方法生成方柱绕流的计算域网格,并运用基于特征线方程的分离算法进行流场计算.     

基于三端口网络模型的折叠波导行波管注波互作用理论研究

将周期性慢波结构中的单元结构等效为一个三端口网络,利用高频结构模拟软件确定等效模型的参数,由此建立了一种通用的慢波结构等效模型.该方法不需要进行复杂的电路等效,直接由高频软件得到通道内场分布,因此较解析方法简单,又不像等效线路模型那么烦琐.基于该三端口网络模型,建立了适用于折叠波导行波管的一维注波互作用非线性理论模型,编写了数值计算程序,对一支折叠波导行波管进行了模拟.该理论模型模拟的结果与实验结果误差小于10%.该理论模型可以用于指导新型折叠波导行波管的设计及非线性模拟研究.     

基于工业CT图像重构的网格模型的全局优化方法

针对用工业CT切片图像直接重构得到的网格模型质量不高的问题,提出一种不受拓扑结构限制的隐式曲面重构全局优化方法。该方法将三维表面模型用隐式函数来表示,通过模型提供的点云信息计算出隐式函数,提取等值面,实现曲面重构。针对隐式曲面重构数据处理量大的问题,引入FFTW快速傅里叶变换来提高效率。实验结果表明,该方法能够同时实现三角网格模型的去噪、网格平滑、简化以及孔洞修补,与保特征的均匀化网格平滑算法相比,去噪效果更好,效率更高。     

确定延迟时间互信息法的一种算法

介绍了相空间重构中确定重构延迟时间的互信息法理论及其具体的计算方法.针对互信息法应用中计算复杂、程序难以编写问题,提出了一种基于网格层数的简便实用程序算法.对网格层数参数进行了结果分析,表明在利用互信息法确定重构延迟时间时,只需要计算到某一个网格层数即可,不需要计算出精确的互信息值,简化了计算的复杂程度.最后通过对Lorenz和Rossler两个吸引子的Lyapunov指数计算,证实了该算法的合理性.     

采用相位半角的两步相移法

针对经典两步相移算法对光强不均匀分布和物体不均匀反射率处理能力不强的问题,分析相移量为90°的情况,采用光强模型,直接对光强进行数据操作,利用三角关系,给出了求解折叠相位相位半角的计算方法,通过高度-相位差公式得到物体的三维形貌数据,避免了因归一化操作过程中取最值方法引起的误差.实验对比分析了经典四步相移法和两步移相法,表明在环境光强可以忽略不计或足够小的情况下,该方法误差范围为±0.2mm,结果优于经典两步相移法,接近于测量误差最小的四步相移法.     

采用相位半角的两步相移法EI北大核心CSCD

针对经典两步相移算法对光强不均匀分布和物体不均匀反射率处理能力不强的问题,分析相移量为90°的情况,采用光强模型,直接对光强进行数据操作,利用三角关系,给出了求解折叠相位相位半角的计算方法,通过高度-相位差公式得到物体的三维形貌数据,避免了因归一化操作过程中取最值方法引起的误差.实验对比分析了经典四步相移法和两步移相法,表明在环境光强可以忽略不计或足够小的情况下,该方法误差范围为±0.2mm,结果优于经典两步相移法,接近于测量误差最小的四步相移法.     

垂直直线特征的双目视觉位姿测量方法

针对直线特征,提出了基于两条垂直直线特征双目视觉位姿测量解析算法。该算法根据直线特征和其在摄像机图像上投影直线共面性质建立模型,利用单位四元素法简化了投影模型变量,进而在仅知被测物体上两条直线相互垂直的信息,利用两条垂线投影在两个摄像机图像上四条直线图像,求解出被测物体的位姿参数。同时分析了退化情况下位姿求解方法,即当直线向量平行于两个摄像机基线向量时,直线向量在左右摄像机投影到同一条直线上时的位姿求解方法。基于Matlab仿真实验表明该算法对于噪声具有一定的稳健性,即使两条直线不垂直而有一个角度误差时,仍能保证一定的精度。实物实验表明该算法可以满足一般测量精度要求。     

保特征的自适应三角网格规范化算法

为了提高基于工业CT图像重构的三角网格质量，提出了顶点调整和特征因子相结合的网格规范化算法。引入模型特征因子和局部网格质量提高程度作为网格调整的控制条件，保留原始模型的局部细节特征；采用法矢量阈值按规律递增的方式自适应控制网格调整，实现不同曲率特征的自动识别。实验结果表明，与现有方法相比，该方法能更好地规范模型的三角网格，同时保留了原始模型的细节特征。     

Adaptive model simplification in real-time rendering for visualization

In this paper, we propose four different geometric measures to identify appropriate triangles to be simplified in 3D complex model. Each measure yields different weight on the same surface and produces a unique simplified model that worth to be analyzed. The proposed measures involve consideration on the resulting of the surfaces collapse, the high peak and low peak of the triangles mesh, the irregular triangle shape, the capacity and boundary view on the triangles mesh. The chosen triangle is to be collapsed based criterion on Half-edge Collapse Transformation method. From the empirical results, one of the proposed measures presents almost excellence in all the criteria mentioned above. The empirical results include the quality of the surface models (visualization purpose), the efficiency of the measures and the overall appearance preservation of the simplified models. The proposed measures are then to be compared to three existing measures. From the analyzed results, we combine the measures to adapt to the user’s response for generating the user-desired simplified models.     

近对称谐振腔的变换圆图解分析

利用传播圆-变换圆图解分析方法，通过对不同结构谐振腔的分析和对比，得出一种宜于推广的自锁模激光谐振腔——近对称谐振腔，其锁模运转易于启动且工作性能比较稳定；通过实验对近对称腔的工作性能进行了验证，并对一种五镜锁模腔进行了分析。五镜腔实质上等价于近对称四镜腔。     

角锥棱镜谐振腔激光模式模拟研究

为了探究角锥棱镜谐振腔激光模式,以角锥-平面镜腔为例,将角锥棱镜等效为衍射光栅,考虑角锥镜棱宽在谐振腔中的衍射效应以及二面角误差引起的附加相位分布对谐振腔激光模式的影响,在光学谐振腔理论的基础上,建立了求解本征模式的理论分析模型.采用快速傅里叶法数值模拟不同腔长、角锥镜棱宽和二面角误差情况下该无源谐振腔激光输出模式分布情况.结果表明,在腔长30 cm、角锥镜棱宽小于75μm、二面角误差在-10′~5′之间时,可实现光斑完整的圆形分布输出模式,且有较好的光束质量;棱宽不小于0.4 mm,二面角误差在-40′~10′之间时,光斑为TEM 03阶横模,光场呈六瓣分布;当角锥镜棱宽为0.4 mm、二面角误差为3′,腔长从30~90 cm范围内增加时,该谐振腔输出的激光模式从TEM 03转换成TEM 10.     

OPTIMIZATION OF SIMPLIFIED MODELS MESHED WITH FINITE TRIANGULAR PLATE ELEMENTS

Designers often want to analyze more and more sophisticated structures, thus leading to very large finite element models (typically 10 00 000 degrees of freedom for a body car, for example). These models being too costly for the early stages of design and optimization can be reduced by a substructure analysis or a mesh simplification of the components. A methodology is proposed in this paper for simplifying finite triangular plate element models leading to a dramatic reduction in the number of degrees of freedom while preserving the dynamical properties of the initial system. In particular, the proposed method is developed for models composed of the plate element STIFF63 generated by the software ANSYS. The principle consists in determining the parameters (thickness, Young's modulus, density) of the triangular elements of a coarse model which replaces a large set of elements of the refined model. The simplified mesh must satisfy one of two criteria. The first requires that the mass and stiffness matrices of the simplified model be as close as possible to the Guyan condensed matrices of the refined model on the reduced node set, whilst the second requires that the dynamical properties of the global structure be preserved. The application of these approaches is illustrated on two test structures using the gradient method to solve the resulting optimization problem. The second approach is shown to give the best results. Typically, the size of the models can be reduced by a factor of 20 whilst preserving the dynamical properties of the structure at low frequencies.     

Motion deblurring using edge map with blurred/noisy image pairs

Motion deblurring methods using blurred/noisy image pairs usually include denoising process of the noisy image. Because both remaining noise and distorted fine details in the denoised image cause an error on deblurring, we propose an algorithm using an edge map of the noisy image to retain sharp edge information while neglecting noise in any smooth region that does not contain information about the motion that occurred during the exposure. In addition, the blur kernel is efficiently estimated by employing the fast total variation regularization method for the gradients of blurred and noisy images only on edge regions. For latent image restoration, another fidelity term is added, which compares the gradients of the noisy and estimated latent images on edge regions to preserve the fine details of the noisy image. To model a sparse distribution of real-world image gradients, a deconvolution method imposing hyper-Laplacian priors based on an alternating minimization scheme is also derived to restore a latent image efficiently. Experimental results show that the peak signal-to-noise ratios of the restored images against the original latent images have been increased by 11.1% on average, when compared to the existing algorithms using an image pair.     

Error analysis on dihedral angle of corner-cube reflectors in Fourier transform spectrometer北大核心CSCD

Fourier transform spectrometer is an important spectral analysis instrument. To research the influence of the dihedral angle error of corner-cube reflectors on interference quality of oscillating Fourier transform spectrometer, the theoretical analysis and computer simulation were carried out. According to the oscillating Fourier transform spectrometer model, the mathematical expressions of the interference intensity and modulation depth with dihedral angle error were deduced. The tolerance of dihedral angle was determined by the modulation depth criterion. The optical design software Zemax was used to establish the model for simulation and verify the results of theoretical derivation. According to the modulation depth criterion, the tolerance of dihedral angle obtained by the theoretical model is 2.52″, and that obtained by the Zemax simulation is 2.38″. The error is 0.14″, which is acceptable. The analysis results show that the established theoretical model is reasonable, which has certain reference value for the design and installation of oscillating Fourier transform spectrometers. © 2022 Editorial office of Journal of Applied Optics. All rights reserved.     

Color constancy enhancement under poor illumination

In this Letter, the color constancy and its realization were studied and a novel color constancy image enhancement algorithm under poor illumination was presented. The purpose of this algorithm is to maintain the hue of an image during the processing so that the change of saturation can be minimized. The original image was first multiplied by a scale parameter obtained by the adaptive quadratic function to enhance the luminance, and then the edge details were restored by a shifting parameter. Numerical results of the Simon Fraser University (SFU) image database indicated that the proposed algorithm performed much better in preserving the hue and saturation and avoiding color distortion compared with the existing image enhancement algorithms.