散乱点云近离群点识别算法

针对原始曲面变化度的局部离群系数(SVLOF)无法有效滤除三维实体的棱边或棱角处的离群点问题,提出了一种散乱点云近离群点的滤除算法。该算法首先将SVLOF定义在类k邻域上,并将SVLOF的定义内容进行了扩展,使其既能滤除平滑曲面上的离群点,又能滤除三维实体的棱边或棱角点处的离群点,同时仍然保留SVLOF原有的足够宽泛的阈值选取空间。仿真数据和实际数据的实验结果均表明,在效率基本保持不变的情况下,所提算法能比原始SVLOF算法更有效地检测出距离主体点云近的离群点。     

Meshing genus-1 point clouds using discrete one-forms

We present an algorithm to mesh point clouds sampled from a closed manifold surface of genus 1. The method relies on a doubly periodic global parameterization of the point cloud to the plane, so no segmentation of the point cloud is required. Based on some recent techniques for parameterizing higher genus meshes, when some mild conditions on the sampling density are satisfied, the algorithm generates a closed toroidal manifold which interpolates the input and is geometrically similar to the sampled surface.     

Study and development of morphological analysis guidelines for point cloud management: The “decisional cube”

When talking about reverse engineering, it is necessary to focus on the management of point clouds. Generally speaking, every 3D scanner device codifies simple and complex geometries providing different point cloud densities as an output. Point cloud density is usually more correlated with the technical specifications of the device employed rather than with the morphology of the object acquired. This situation is due to the frequent use of structured grids by a large quantity of devices. In order to solve this problem, we therefore need to integrate the classical structured grid acquisition with a smart selective one, which is able to identify different point cloud densities in accordance with the morphological complexity of the object regions acquired.Currently, we can reach the destination in many different ways. Each of them is able to provide different performances depending on the object morphology and the performances of 3D scanner devices. Unfortunately, there does not yet exist one universal approach able to be employed in all cases. For this reason, the present paper aims to propose a first analysis of the available methodologies and parameters, in order to provide final users with some guidelines for supporting their decisions according to the specific application they are facing. Moreover, the developed guidelines have been illustrated and validated by a series of case studies of the proposed method.     

Multiresolution model based extraction of product feature lines in reverse engineering

In this paper, multiresolution models are employed in the context of reverse engineering for feature line extraction. Starting with a proper triangulation of the cloud point data as a priori, our feature line extraction algorithm has three steps: (1) establishing a Gauss normal sphere and creating multiresolution models for the Gauss sphere based on different levels of subdivisions for the sphere regions; (2) mapping the unit normal vectors of triangular faces in the multiresolution Gauss sphere and merging those connected triangular faces whose unit normal vectors fallen on the same Gauss sphere region with a given resolution to form super-faces (the collection of triangular facets with similar normal); and (3) extracting the boundaries from the super-faces and generating the feature lines from the extracted boundaries. We then use these feature lines as a base for tracing boundary curves for B-spline surface construction. Since feature lines maintain the characteristics of the original product models, in this way, we have a good chance to reconstruct B-spline surfaces with high quality. Examples are given in the paper to show the feature line extraction, the influence of the feature lines extracted under different resolutions, and the final reconstructed B-spline surfaces based on these feature lines.     

Extension of surface reconstruction algorithm to the global stitching and repairing of STL models

In the design of complex parts involving free-form or sculptured surfaces, the design is usually represented by a B-rep model. But in production involving rapid prototyping (RP) or solid machining, the B-rep model is often converted to the popular STL model. Due to defects such as topological and geometric errors in the B-rep model, the resulting STL model may contain gaps, overlaps, and inconsistent orientations. This paper presents the extension of a surface reconstruction algorithm to the global stitching of STL models for RP and solid machining applications. The model to be stitched may come from the digitization of physical objects by 3D laser scanners, or the triangulation of trimmed surfaces of a B-rep model. Systematic procedures have been developed for each of these two different but equally important cases. The result shows that the proposed method can robustly and effectively solve the global stitching problem for very complex STL models.     

Optimal shape error analysis of the matching image for a free-form surface

When using an optical non-contact scanning system to measure an object that has a large surface, large curvature, or a full 360° profile, one can acquire only one set of sectional measurement points each time. For reconstructing the entire object, every set of sectional measurement points acquired at different positions must match. Therefore, the optimal shape error analysis for the matching image of two or more sets of sectional measurement points is desired. This paper presents a measurement system that combines two CCD cameras, one line laser and a three-axis motion stage. It forms an optical non-contact scanning system in association with the mathematical method of direct shape error analysis for the use in reverse engineering. This analysis and measurement system can be used for the profile measurements of free-form objects. It analyzes the matching image of a free-form surface with high efficiency and accuracy. The validity and applicability of this system are demonstrated by two practical examples.     

An application of digital point cloud to historic architecture in digital archives

Currently, the process of conservation by digitizing historic buildings is a key technology in cultural heritage preservation that needs further improvement. However, a comprehensive survey of projects of digital archives shows that traditional, time-consuming construction methods were adopted most, as in taking pictures or cartographing physical buildings through AutoCAD to construct digital data. This study adopted reverse engineering to explore digitalizing constructions and employed a fast and precise 3-D laser scanner to retrieve point cloud data and then import the data into Rapidform XOR2, an application used in reverse engineering, to process point cloud data and construct 3-D models. The results were then converted from 3-D models into 2-D pictures by parameterization software Pro/ENGINEER to provide designers precise and realistic dimensional data, which effectively reduced time in cartography and increased historic building dimension precision during digitalization. The foregoing data was documented and compared, and a digital value-added effect could be possible in the future.     

Comparative study of different digitization techniques and their accuracy

The various manufacturers of digitization systems speak of the effectiveness and accuracy of their tools under optimal conditions, but actual experimentation with simple or complex objects and different materials yields results that on occasions refute the effectiveness of those systems. In order to help choose a digitization system on the basis of its accuracy and the quality of the distribution of points and triangular meshes, in the field of reverse engineering, we compared five digitization techniques (three versions of the laser scanner, a fringe projection version and an X-ray version): (1) an ordered point cloud obtained with a laser incorporated in a CMM, (2) a disordered point cloud obtained with a manual laser the position of which is determined with a Krypton Camera, (3) an Exascan manual laser with targets, (4) an ordered point cloud obtained by high precision Computerized Tomography (CT) and (5) an Atos fringe projection scanner with targets. Each of the three calibrated pieces (a sphere, a cylinder and a gauge block) was measured five times by the five digitization systems to confirm the accuracy of the measurement. A comparison was also made of the meshes generated by the five software packages (Focus-Inspection, Metris, VxScan, Mimics and Atos) of the five digitization systems for the three calibrated pieces and two more complex pieces (a bone and an automobile window winder pulley) to determine meshing quality. Finally, all the pieces were meshed by triangulation in the Catia V5 DSE (Digitized Shape Editor) module in order to test the quality of the points distribution.