KEYWORDS: Clouds, Image registration, Digital image correlation, 3D modeling, Principal component analysis, Optical engineering, Nondestructive evaluation, Detection and tracking algorithms, 3D metrology, Reverse modeling
Three-dimensional (3-D) modeling is widely applied in the fields of measurement engineering and manufacturing. Registration of point clouds is a crucial step in 3-D model reconstruction. We present a registration method using improved digital image correlation (DIC) coefficient. First, key points are selected using principal component analysis and further extracted using the k-means clustering algorithm. Second, a grayscale image for each key point is generated as a feature descriptor using inverse distance weighted interpolation. Then, the presented method determines key point correspondences by improving the DIC coefficient. Finally, we apply the iterative closest point algorithm to fine registration. Experiment results show that the proposed method is accurate, time efficient, and it has a good antinoise performance.
In order to solve the problem of automatic foot measurement in garment customization, a new automatic footparameter- extracting system based on stereo vision, photogrammetry and heterodyne multiple frequency phase shift technology is proposed and implemented. The key technologies applied in the system are studied, including calibration of projector, alignment of point clouds, and foot measurement. Firstly, a new projector calibration algorithm based on plane model has been put forward to get the initial calibration parameters and a feature point detection scheme of calibration board image is developed. Then, an almost perfect match of two clouds is achieved by performing a first alignment using the Sampled Consensus – Initial Alignment algorithm (SAC-IA) and refining the alignment using the Iterative Closest Point algorithm (ICP). Finally, the approaches used for foot-parameterextracting and the system scheme are presented in detail. Experimental results show that the RMS error of the calibration result is 0.03 pixel and the foot parameter extracting experiment shows the feasibility of the extracting algorithm. Compared with the traditional measurement method, the system can be more portable, accurate and robust.
A digital speckle based stereo microscope strain measurement system is developed to investigate the forming limit diagram (FLD) of miniature sheet metal under hydraulic bulge testing conditions. A stochastic speckle pattern is sprayed on the surface of the tested metal before forming. A series of images are recorded by two cameras mounted on a binocular stereo microscope during the hydroforming process. The critical major and minor strains are then calculated and plotted to construct the forming limit curve. The key technologies applied in the system are discussed in detail, including stereo microscope calibration and large deformation strain filed determination. First, considering complex optical paths and high magnification of the stereo microscope, an accurate combined distortion correction model is proposed to optimize the intrinsic and extrinsic parameters of the stereo microscope. Then, to solve the problem of strain measurement of the tested metal in large deformation situation, a large deformation measurement scheme based on deformation continuity of adjacent images is proposed. And an algorithm of limit strain determination based on spline model is proposed to calculate the critical strains at the onset of plastic instability. Finally, with our self-developed stereo microscope imaging system and sheet metal hydraulic bulging setup, FLD determination tests are conducted to validate the performance of the system. And the measured FLD is compared with the simulation results that predicted by the finite element method. The simulation and experimental results confirm that the proposed system is feasible to measure the full-field strain during the whole bulging processes and provides a better solution for forming limit diagram prediction.
KEYWORDS: Clouds, Calibration, Projection systems, Cameras, 3D modeling, 3D acquisition, 3D image reconstruction, Digital image processing, Image processing, 3D printing
This paper proposes a rapid body scanning system that uses optical digital fringe projection method. Twelve cameras and four digital projectors are placed around the human body from four different directions, so that the body surface threedimensional( 3D) point cloud data can be scanned in 5~8 seconds. It can overcome many difficulties in a traditional measurement method, such as laser scanning causes damage to human eye and low splicing accuracy using structured white light scanning system. First, an accurate calibration method based on close-range photogrammetry, is proposed and verified for calibrating the twelve cameras and the four digital projectors simultaneously, where a 1m×2m plate as calibration target with feature points pasted on its two-sides is used. An experiment indicates that the proposed calibration method, with a re-projection error less than 0.05pixels, has a considerable accuracy. The whole 3D body surface color point cloud data can be measured without splice different views of point cloud, because of the high accuracy calibration results. Then, in order to measure the whole body point cloud data with high accuracy, a combination of single and stereo camera measuring method, based on digital fringe projection, has presented to calculating 3D point cloud data. At last, a novel body chromoscan system is developed and a human body 3D digital model was scanned, by which a physical body model was manufactured using 3D printing technology.
A method is presented for obtaining good images of a sprayed speckle pattern on specimen surfaces at high temperatures, suitable for strain measurement, by digital image correlation (DIC) using plasma spray for speckle preparation in which a bandpass filter, neutral density filters, and a linear polarizing filter are used to reduce intensity and noise in images. This is accomplished by speckle preparation through the use of plasma spray and suppression of black-body radiation through the use of filters. By using plasma spray for speckle preparation and the filters for image acquisition, the method was demonstrated to be capable of providing accurate DIC measurements up to 2600°C. The full-field stretching deformation of the specimen was determined using the DIC technique. Experimental results indicate that the proposed high-temperature DIC method is easy to implement and can be applied to practical, full-field, high-temperature deformation measurements with high accuracy.
2-D digital image correlation (DIC) is traditionally used with the camera's axis perpendicular to the object surface. The nonperpendicular camera alignment will inevitably introduce measurement error. An oblique-axis 2-D DIC method based on photogrammetry compensation is proposed in this paper. Not less than three reference points are pasted on the specimen surface. The photogrammetry technique is first used to get the 3-D coordinates of these reference points. A reference plane is obtained by fitting these reference points. Then the oblique-axis camera is oriented by using the space resection method. During deformation measurement, for each image points matched by DIC, its 3-D point can be acquired by simply intersecting the point ray with the reference plane. Using this method, not only is the measurement error caused by oblique-axis compensated, but also the displacement in millimeters rather than in pixels can be determined directly. Experiments results indicate that, for in-plane deformation in oblique-axis measurement situation, our method has comparable accuracy with 3-D DIC.
KEYWORDS: 3D modeling, Head, Image registration, Optical engineering, Image processing, Digital filtering, Data modeling, 3D image processing, Image filtering, 3D scanning
We present a simple and fast integration method, which can fuse registered multiple, overlapping range images effectively while preserving the detailed features. First, normal vectors of the nodes in each range image are estimated and the registered multiple overlapping range images are combined into one point set. Due to registration errors, the normal vectors of homogeneous nodes in the overlapping areas would differ from each other. Thus, the normal vectors of the nodes are corrected iteratively by weighted averaging their neighboring normal vectors. Second, the combined point set is partitioned into a number of small clusters using the k-means clustering algorithm based on user-defined space interval. The clusters subsequently are subdivided into smaller subclusters by using the standard normal vector deviation as a measure of cluster scatter. Finally, in the cluster, the local maxima model, obtained using the mean shift clustering algorithm, is employed to represent the cluster. The proposed method is fast because it does not need to detect iteratively the overlapping areas, which is usually time consuming. In addition, because of the noisy filtering property of the mean shift clustering algorithm, the novel method can produce a smooth and watertight point surface while preserving the detailed features.
KEYWORDS: Speckle, Cameras, 3D image processing, 3D metrology, Calibration, Error analysis, Imaging systems, Digital image correlation, Distortion, Analytical research
In order to obtain more comprehensive three-dimensional (3-D) deformation data, improve the flexibility and precision of 3-D deformation measurement, and describe the real-time displacement of various points in the measurement scope, we researched speckle methods in this field. First, we expounded basic theories of speckle technique and elaborated on several important aspects of 3-D measurement, including camera calibration, images matching, and 3-D coordinate and displacement calculation. Then, we established the relevant mathematical models and developed a corresponding measurement and analysis software, XJTUDIC. Aided with the appropriate hardware devices, this software recorded and analyzed the whole process of tensile deformation. When compared with the results of the simulation analysis using ANSYS, this method achieved a relative error within ±0.75%, demonstrating the reliability, feasibility, and advantage of this method and its corresponding applied software and hardware.
A novel sharp features extraction method is proposed in this paper. First, we calculate the displacement between the
point and its local weighted average position and we label the point with salient this value as the candidate sharp feature
points and we estimate the normal direction of those candidate sharp feature points by means of local PCA methods.
Then we refine the normal estimated by inferring the orientation of the points near the candidate sharp feature region and
bilateral filtering in the normal field of point clouds. At last we project the displacement between point and its local
weighted average position along the direction of normal .We use value of this projection as the criteria of whether a
point can be labeled as sharp feature. The extracted discrete sharp feature points are represented in the form of piecewised
B-Spline lines. Experiment on both real scanner point clouds and synthesized point clouds show that our method
of sharp features extraction are simple to be implemented and efficient for both space and time overhead as well as it
robust to the noise ,outlier and un even sample witch are inherent in the point clouds.
The destruction of spatial complex tubular joints may lead to failure of the whole tubular structure, thus it is necessary to analyze the mechanical properties of spatial complex tubular joint. In this paper, a novel method based on close range photogrammetry to accurately measure the three-dimensional (3D) deformation of spatial complex tubular joints during loading test is proposed. Artificial targets are pasted on the deformation area before loading. The 3D coordinates of these targets are reconstructed by analyzing the images captured at each stage, and the coordinate systems of different stages are registered together by means of global transformation points. The whole field 3D deformation under different load levels is then obtained by tracking the homonymous targets among different stages. It is helpful for further analysis of the mechanical properties. Two different precision evaluation experiments indicate that the proposed method could achieve accuracy of 0.1mm/m. Two full scale tubular joints are tested and a feasible solution for improving the load carrying capacity of the tested tubular joints is thus obtained as per the measured results. For comparison, finite element analysis is employed to predict the deformation in a traditional way. The deformation tendency measured by two methods agrees well.
KEYWORDS: Calibration, 3D image processing, Digital image correlation, Cameras, Imaging systems, Stereoscopic cameras, 3D acquisition, Detection and tracking algorithms, 3D image reconstruction, Experimental mechanics
A three-dimensional (3-D) digital image correlation system for deformation measurement in experimental mechanics has been developed. The key technologies applied in the system are discussed in detail, including stereo camera calibration, digital image correlation, 3-D reconstruction, and 3-D displacement/strain computation. A stereo camera self-calibration algorithm based on photogrammetry is proposed. In the algorithm, the interior and exterior orientation parameters of stereo cameras and the 3-D coordinates of calibration target points are estimated together, using the bundle adjustment technique, so the 3-D coordinates of calibration target points are not needed in advance to get a reliable camera calibration result. An efficient image correlation scheme with high precision is developed using the iterative least-squares nonlinear optimization algorithm, and a method based on a seed point is proposed to provide a reliable initial value for the nonlinear optimization. After the 3-D coordinates of the object points are calculated using the triangulation method, the 3-D displacement/strain field could then be obtained from them. After calibration, the system accuracy for static profile, displacement, and strain measurement is evaluated through a series of experiments. The experiment results confirm that the proposed system is accurate and reliable for deformation measurement in experimental mechanics.
A digital photogrammetry measurement system (XJTUDP) is developed in this work, based on close range industry. Studies are carried out on key technologies of a photogrammetry measurement system, such as the high accuracy measurement method of a marker point center based on a fitting subpixel edge, coded point design and coded point autodetection, calibration of a digital camera, and automatic image point matching algorithms. The 3-D coordinates of object points are reconstructed using colinear equations, image orientation based on coplanarity equations, direct linear transformation solution, outer polar-line constraints, 3-D reconstruction, and a bundle adjustment solution. Through the use of circular coded points, the newly developed measurement system first locates the positions of the camera automatically. Matching and reconstruction of the uncoded points are resolved using the outer polar-line geometry of multiple positions of the camera. The normal vector of the marker points is used to eliminate the error caused by the thickness of the marker points. XJTUDP and TRITOP systems are tested on the basis of VDI/VDE2634 guidelines, respectively. Results show that their precision is less than 0.1 mm/m. The measurement results of a large-scale waterwheel blade by XJTUDP show that this photogrammetry system can be applied to industrial measurements.
A videogrammetric system is proposed for monitoring the 3-D deformation time history of a metal sheet surface during welding, which could not be achieved by traditional displacement sensors. Two commercial-grade digital video cameras are used for image acquisition. The major algorithms of the videogrammetric process such as target recognition, stereo matching, 3-D reconstruction, and target tracking, are discussed in detail. An improved self-calibration method based on a planar pattern is proposed and verified. Precision evaluation experiments prove that the proposed system could achieve an accuracy of 0.05 mm. Three welding deformation surveying experiments using different metal sheets are conducted to validate the performance of the videogrammetric system and the obtained data are compared with results from linear variable differential transducers. The agreement between videogrammetric and conventional results confirms the availability and reliability of the proposed videogrammetric system for monitoring dynamic welding deformation of metal sheets.
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