Xiaosheng Cheng

Reliability-guided phase-unwrapping algorithm for the measurement of discontinuous three-dimensional objects

Accurate and robust phase unwrapping is an important procedure for three dimensional (3D) profilometry measurement. The mask cut phase-unwrapping algorithm merges the characteristics of Goldsteins branch cut and reliability-guided path-following algorithms, so that it has higher accuracy and stability. However, it cannot handle discontinuous phase regions because it completely relies on the phase quality map to guide the placement of mask cuts, thereby easily causing phase error propagation. To overcome these drawbacks, we propose a new phase-unwrapping method that merges the residue check principle and a Laplace phase derivative variance quality map that we developed. First, the entire phase was divided into several isolated regions in accordance with the quality map. Then, each pixel of the absolute phase marker line was taken as the starting point to unwrap the discontinuous phase regions based on reliability guidance. In addition, a new list-trimming algorithm was employed to guarantee a speedy phase-unwrapping procedure. The entire phase unwrapping and accurate 3D measurement of discontinuous objects was successfully completed. The simulated and experimental data both demonstrate the validity of the proposed phase-unwrapping algorithm.

Single-tooth modeling for 3D dental model

An integrated single-tooth modeling scheme is proposed for the 3D dental model acquired by optical digitizers. The cores of the modeling scheme are fusion regions extraction, single tooth shape restoration, and single tooth separation. According to the “valley” shape-like characters of the fusion regions between two adjoining teeth, the regions of the 3D dental model are analyzed and classified based on the minimum curvatures of the surface. The single tooth shape is restored according to the bioinformation along the hole boundary, which is generated after the fusion region being removed. By using the extracted boundary from the blending regions between the teeth and soft tissues as reference, the teeth can be separated from the 3D dental model one by one correctly. Experimental results show that the proposed method can achieve satisfying modeling results with high-degree approximation of the real tooth and meet the requirements of clinical oral medicine.

Calibration Algorithm for Structured Light 3D Vision Measuring System

A novel procedure is proposed to calibrate a structured light 3D vision measuring system. Firstly, robust subpixel and substripe calibration methods are proposed for the camera and the projector. Secondly, a novel approach is introduced to solve the nature restriction of the structure light system: the calibration of the camera and the project is restricted with the locations of them. Thirdly, the systemic calibration procedure based on rigidity constraint to system architecture is developed according to the stereo vision, the L-M optimization algorithm is used to adjust the internal and external parameters of each respective component in this system. Finally, this system calibration algorithm is proved to be robustness and effectiveness by reconstructing a 3-D standard cylinder. The experiment results show that the precision of single view is about 0.06mm over a measurement volume of 250mm 200mm 250mm. It roughly meets the need of the industry.

A three-step system calibration procedure with error compensation for 3D shape measurement

System calibration, which usually involves complicated and time-consuming procedures, is crucial for any three-dimensional (3D) shape measurement system based on vision. A novel improved method is proposed for accurate calibration of such a measurement system. The system accuracy is improved with considering the nonlinear measurement error created by the difference between the system model and real measurement environment. We use Levenberg-Marquardt optimization algorithm to compensate the error and get a good result. The improved method has a 50% improvement of re-projection accuracy compared with our previous method. The measurement accuracy is maintained well within 1.5% of the overall measurement depth range.

Models partition for 3D printing objects using skeleton

Purpose It is a challenge to print a model with the size that is larger than the working volume of a three-dimensional (3D) printer. The purpose of this paper is to present a feasible approach to divide a large model into small printing parts to fit the volume of a printer and then assemble these parts into the final model. Design/methodology/approach The proposed approach is based on the skeletonization and the minima rule. The skeleton of a printing model is first extracted using the mesh contraction and the principal component analysis. The 3D model is then partitioned preliminarily into many smaller parts using the space sweep method and the minima rule. The preliminary partition is finally optimized using the greedy algorithm. Findings The skeleton of a 3D model can effectively represent a simplified version of the geometry of the 3D model. Using a model’s skeleton to partition the model is an efficient way. As it is generally desirable to have segmentations at concave creases and seams, the cutting position should be located in the concave region. The proposed approach can partition large models effectively to well retain the integrity of meaningful parts. Originality/value The proposed approach is new in the rapid prototyping field using the model skeletonization and the minima rule. Based on the authors’ knowledge, there is no method that concerns the integrity of meaningful parts for partitioning. The proposed method can achieve satisfactory results by the integrity of meaningful parts and assemblability for most 3D models.

Robust tooth surface reconstruction by iterative deformation

Digital design technologies have been applied extensively in dental medicine, especially in the field of dental restoration. The all-ceramic crown is an important restoration type of dental CAD systems. This paper presents a robust tooth surface reconstruction algorithm for all-ceramic crown design. The algorithm involves three necessary steps: standard tooth initial positioning and division; salient feature point extraction using Morse theory; and standard tooth deformation using iterative Laplacian Surface Editing and mesh stitching. This algorithm can retain the morphological features of the tooth surface well. It is robust and suitable for almost all types of teeth, including incisor, canine, premolar, and molar. Moreover, it allows dental technicians to use their own preferred library teeth for reconstruction. The algorithm has been successfully integrated in our Dental CAD system, more than 1000 clinical cases have been tested to demonstrate the robustness and effectiveness of the proposed algorithm.

A new phase error compensation method of 3-D shape measurement system using DMD projector

Structured light system using a digital micro-mirror device (DMD) projector is increasingly used for a 3-D shape measurement because of its digital nature. When the DMD projector is used in phase measuring profilometer (PMP), the precision of profile measurement will increase with the precision of phase-shift increasing. But the non-sinusoidal nature of the projected fringe patterns causes significant phase measurement error and consequent shape measurement error. In the reality, we find that the non-sinusoidal effect is never caused by only one factor. A real measurement shows that it is a combination of influences by all effects, e.g., the object independent irradiance function, the nonlinear gamma curve of the projector, the spatio-temporal characteristic of DMD, etc. In view of the above factors, a comprehensive compensation method is proposed to compensate these factors for the 3D measurement. If the compensation is well accomplished, the measurement error can be reduced average 6 times. The experiment is carried out to demonstrate the validity of this technique.

Efficient Booleans algorithms for triangulated meshes of geometric modeling

ABSTRACTBoolean operation of geometric models is an essential element in computational geometry. An efficient approach is developed in this research to perform Boolean operation for triangulated meshes represented by B-rep. This approach is much fast and robust than many existing methods. The Octree technique is adapted to facilitate the division of the common space of two meshes in order to reduce the time of Octrees construction and intersection detection. Floating point arithmetic errors and singularity of intersections are then analyzed to guarantee the unique intersection between a segment and a face, and the continuity of intersections. A novel technique based on intersecting triangles is finally proposed to create required sub-meshes based on the type of Boolean operations. Some experimental results and comparisons with other methods are presented to prove that the proposed method is fast and robust.

An accurate simulation algorithm for focus variation microscopy

The focus variation microscopy is widely used and researched in both industrial and academic field. But the 3D construction quality of surface topography is affected by the noise, the double peak value, and the discontinuous surface, and so on. A simulation method for focus variation is proposed based on the physical model of optical imaging and the Point Spread Model(PSF). At first, the linear relationship between the blur factor σ of Gaussian function and the defocus distance δ is deduced which is called point spread parameter λ, then, considering the positive correlation between blur factorσ and blur degree, the difference between the real defocused image and the calculated image by Gaussian convolution operation to real captured focused image is shown. It is used to the objective to computed the accurate value σ and the spread parameter λ. At last, the difference between focus measure valued of real image sequence and simulation image sequence is used to verify the method. The simulation method is a judgement basis for focus measurement, the single peak of focus curve, and the identification of high-frequency noise.

Parallel non-dominated sorting genetic algorithm-II for optimal part deposition orientation in additive manufacturing based on functional features

Surface finish, especially the surface finish of functional features, and build time are two important concerns in additive manufacturing. A suitable part deposition orientation can enhance the surface quality of functional features and reduce the build time. This article proposes a novel method to obtain an optimum part deposition orientation for industrial-grade 3D printing based on fused deposition modeling process by considering two objective functions at a time, namely adaptive feature roughness (the weighted sum of all feature roughnesses) and build time. First, mesh segmentation and level classification of features are carried out. Then, models for evaluation of adaptive feature roughness and build time are established. Finally, a non-dominated sorting genetic algorithm-II based on Compute Unified Device Architecture is used to obtain the Pareto-optimal set. The feasible of the algorithm is evaluated on several examples. Results demonstrate that the proposed parallel algorithm obtains a limiting solution that enhances the surface quality of functional features significantly and reduces average running time by 94.8% compared with the traditional genetic algorithm.