3D Measurement Method Research Articles

3D measurement method based on Gray code and single sine fringe image

3D measurement method based on Gray code and single sine fringe image

3D shape measurement method for high-reflective surface based on a color camera

Fringe projection profilometry (FPP) has been widely utilized in many fields due to its non-contact, high accuracy, high resolution and full-field measurement capabilities. However, the limited dynamic range of the camera sensor can result in overexposure of high-reflective parts in industrial production measurement. To effectively solve the above issue, this paper proposes a 3D shape measurement method for the high-reflective surface based on a color camera. Firstly, the optimal exposure time for the low-reflective region is estimated using the relationship between the standard variance of the phase error and the modulation. Twelve blue phase-shifted fringe patterns and a uniform blue image are utilized to obtain 3D shape of high-reflective surface. Secondly, captured images are separated into red, green and blue components. The fused Gaussian low-pass filtering method with different cut-off frequencies is used to filter and denoise the green or red components and the true intensity of the blue component in the saturated regions is calculated by the unsaturated intensity of the green or red components based on the calibrated crosstalk matrix. Then the image in saturated regions is fused and normalized with the unsaturated region. The absolute phase obtained from the fused normalized fringe patterns is converted into 3D data. Finally, experiments have been carried out on measuring. The experimental results show that the proposed method is capable of obtaining the 3D shape of the surface of a high-reflective object with fewer patterns and high measurement accuracy.

High-accuracy fringe projection profilometry without phase unwrapping based on multi-view geometry constraints

The number of fringes and phase unwrapping in fringe projection profilometry result in two key factors. The first is to avoid the problems of excessive fringe patterns, and the second is phase ambiguity. This paper presents a three-dimensional (3D) measurement method without phase unwrapping. This method benefits from the geometric constraints and does not require additional images. Meanwhile, epipolar rectification is performed to calibrate the rotation matrix relationship between the new plane of the dual camera and the plane of the projector. Subsequently, using depth constraints, the point pairs with incorrect 3D positions are effectively eliminated, and the initial parallax map is obtained by establishing epipolar lines of the left and right matching points in the projector domain, obtaining the intersection points, and setting up the threshold for filtering. Finally, a function combining the modulation intensity and phase is proposed to refine the parallax map such that the 3D result is insensitive to phase error. The standard step block and standard ball were used to verify the validity of the proposed method, and the experimental results showed that the root mean square error of the method was 0.052 mm.

Structured light 3D shape measurement for translucent media base on deep Bayesian inference

Traditional structured light technique faces challenges in measuring translucent media due to low fringe modulation and strong random noise caused by subsurface scattering, thereby significantly reducing phase quality. In addition, the difficulty in obtaining ground truth makes it hard to assess reliability even though obtaining measured results. Here, we proposed a 3D measurement method for translucent media base on deep Bayesian inference to achieve both fringe enhancement and phase uncertainty evaluation. Specifically, a deep network incorporated with quatuor-branch residual block is developed to significantly enhance the fringe modulation and signal-to-noise ratio (SNR) for accurate phase recovery. Subsequently, a Bayesian inference mechanism is established for probabilistic statistics, which allows for the optimization of fringe output and provides uncertainty self-evaluation based on Monte Carlo (MC) sampling. Furthermore, by incorporating both numerical and physical constraints into the supervised learning, the network can effectively mitigate phase-shifted errors in the final results. The proposed method shows high efficiency and flexibility since it requires no additional patterns or hardware setup. Experiments validate the feasibility of the proposed method.

Efficient completely polarization-encoded binocular structured light 3D measurement method for metallic objects

In practical 3D measurement applications, stereo vision assisted with phase shift patterns is intensively studied and widely used for its high precision and excellent noise resilience. While aiming to improve matching efficiency, excessive projection patterns or unreliable algorithms may be introduced as a side effect. We propose a completely polarization-encoded phase shift (CPPS) method to overcome the above challenges. In our method, the Stokes parameter S1 of the polarization patterns is encoded. Compared to the traditional fringe patterns, our method can reduce the number of projected patterns to improve the measurement efficiency. Therefore, the exact constraints can be realized without additional patterns. Experimental results show that the CPPS method reduces the matching time by 76.6% while reducing the number of fringe patterns by half.

Large-scale-adaptive fringe projection 3D measurement.

Fringe projection profilometry (FPP) faces significant challenges regarding calibration difficulty and stitching error accumulation when operating across scenes ranging from tens to hundreds of meters. This Letter presents a calibration-free 3D measurement method by integrating a binocular vision of a FPP scanner with a wide field-of-view (FoV) vision that constructs global benchmarks to unify local 3D scanning and global 3D stitching, which is adaptable to arbitrarily large-scale scenes. A posterior global optimization model is then established to determine the reconstruction parameters and stitching poses simultaneously at each scanning node with adaptively distributed benchmarks. Consequently, the integrated vision measurement system not only eliminates the large-scale pre-calibration and stitching error accumulation but also overcomes system structural instability during moving measurement. With the proposed method, we achieved 3D measurements with an accuracy of 0.25 mm and a density of 0.5 mm for over 50-m-long scenes.

Damage evolution of coal with a strong bursting liability and 3D measurement method for elastic deformation energy distribution

When coal with a strong bursting liability is disturbed by a load, catastrophic instability disasters and rockbursts can easily occur. Loaded coal with a strong bursting liability undergoes complex damage evolution and failure behavior due to the intricate interplay among loads, gangue, fractures, and the coal matrix. Intuitively quantifying and characterizing the complex discontinuous structural features, deformation fields, and energy fields within coal is the foundation for the scientific analysis and prevention of disaster mechanisms caused by rockbursts. Therefore, studying the distribution and evolutionary processes of deformation fields and energy fields caused by the evolution of discontinuous structures within coal is highly important for the prevention and control of coal mine rockburst disasters and the protection of coal mine safety during mining operations. In this study, a series of X-ray computed tomography (X-CT) tests were conducted on coal specimens with varying gangue contents under uniaxial compressive loading. The X-CT images clearly illustrated the internal damage evolution of coal with a strong bursting liability, revealing fracture propagation, shear bands, and localized failure under varying loading conditions. Image segmentation was employed to quantitatively analyze the development of damage in coal with a strong bursting liability, revealing the influence patterns of gangue content on the prepeak deformation localization and strength of coal. Additionally, using digital volume correlation, the 3D volumetric displacement and strain of loaded impact-prone coal were determined based on the changes in natural speckle images. The nonuniform distribution of strain was mainly caused by the nonuniform distribution of gangue and pore fractures at the microscopic scale, which explains the localized deformation in coal materials. Importantly, this study provides new full-field strain tensor data and a novel method for measuring the elastic deformation energy of the 3D deformation and cracking processes of loaded coal.

Efficient and robust 3D reconstruction: Absolute phase extraction algorithm based on three gray images

The structured light three-dimensional (3D) measurement technology based on optical metrics has gained increasing applications, and there is a growing demand for capturing transient trajectory changes in dynamic scenes. Conventional temporal phase unwrapping (TPU) approaches often require multi-frequency fringe patterns for application. However, the inevitable conflict arises between the number of projected fringe patterns and the measurement speed. This paper proposes a highly accurate and robust 3D measurement method using only 3-step phase-shifting fringe patterns. To precisely reconstruct the 3D shape using the fewest fringe patterns, it is essential to extract as much information as possible from the available patterns. Therefore, we calculate the wrapped phase distribution using 3-step phase-shifting fringe patterns to ensure accuracy. Simultaneously, the trapezoidal phase-shifting method is employed to calculate the intensity ratio distribution. Through the proposed region-based indexing method, the intensity ratio map is transformed into an unwrapped phase map. Additionally, a novel self-correction method is introduced to correct the non-linear errors in the unwrapped phase map, which is then utilized for the calculation of the fringe order required for phase unwrapping. This high-speed measurement method utilizes only 3-step phase-shifting fringe patterns while retaining the advantages of TPU, meeting the requirements of high-speed and full-field measurements. Experimental results on both static and dynamic scenes validate the effectiveness and versatility of the proposed method, demonstrating its flexibility and simplicity in application. In our 3D measurement system, the method achieves 3D reconstruction of the dynamic object's trajectory at a measurement speed of 107 frames per second.

Highly accurate three-dimensional measurement of large structures using multiple stereo vision with improved two-step calibration algorithm

A multiple stereo-based three-dimensional (3D) measurement method of large-scale structures is presented, which replaces time-consuming and laborious 3D measurement procedure of laser-based conventional equipment. To have 3D reconstruction accuracy with millimeter-precision, two-step calibration algorithms are improved. Specifically, we use 2D epipolar constraint, using fundamental matrix determined by matched points, to new combination of objective function for intrinsic parameter calibration. We additionally propose an onsite structure parameter calibration algorithm using different epipolar constraint of measurement points in images of large ship block as well as reprojection error of checkerboard, to bypass the limitation of the coverage of checkerboard in large-scale measurement. The actual 3D measurement of the three ship blocks is conducted, and the 3D reconstruction accuracy is validated by comparing the 3D coordinates measured by total station. The experimental results showed that the mean 3D reconstruction error ranged from 6 to 14 mm.

Line structure light calibration based on a freely placed single cylindrical object

The traditional Zhang calibration method takes multiple checkerboard images at different positions, but in practical applications, surface measurement is usually required, in which case the planar target is not suitable. Thus, a calibration method is needed, using curved surface targets. In this paper, a line-structured light calibration method is presented based on a freely placed cylindrical object. The calibration process is as follows: the cylindrical target is placed freely and irradiated with a laser; The camera captures the target image and then the light strips of the cylindrical target and its two bus bars are extracted; The translation relation between the cylinder and the images is calculated according to the imaging principle and geometric relation; The center points of the light strip are converted to 3D coordinates according to the internal parameters of the camera for fitting the light plane. Two experiments are designed to verify the advantages of the method. The needle gauge experiment shows that 0.001 mm accuracy can be achieved in a single frame measurement, and the ring gauge experiment shows that the measurement accuracy of the system is increased by 20 %. This method is more applicable and more suitable than the traditional method for high-precision industrial 3D measurement.

Three-dimensional information measurement and reconstruction for micro-hole array based on the modified shape from focus

The geometrical characteristics of the micro-holes and the consistency of spatial distribution among the micro-holes in the micro-hole array fabricated by laser beam drilling (LBD) directly influence the cooling performance, reliability, and longevity of the workpiece. It is of great significance to establish the relationship between the LBD processing parameters and the characteristics of the micro-hole array through the three-dimensional (3D) information measurement and reconstruction of the micro-hole array, as well as to guide and improve the LBD processing technology. However, due to the small size and high aspect ratio of the micro-holes in the micro-hole array, the 3D information measurement and reconstruction of the micro-hole array face challenges. The conventional micro-hole measurement methods suffer from low efficiency, high measurement costs, and potentially destructive measurement processes. Furthermore, the conventional micro-hole measurement methods face challenges in simultaneously obtaining the geometrical characteristics of the micro-holes and the spatial distribution among the micro-holes in the micro-hole array. Therefore, a non-contact and non-destructive 3D information measurement and reconstruction method for the micro-hole array based on the modified shape from focus is proposed. A novel focus measure operator based on grayscale difference and adaptive neighborhood size is proposed to accurately evaluate the focus value and obtain the initial focus volume matrix. A novel image processing operator based on the gradient information of the image sequence is proposed to enhance the initial focus volume matrix. The experimental results indicate that the proposed focus measure operator, combined with the novel image processing operator, is more suitable and reliable for detecting the micro-holes due to its better unimodality, higher contrast, and narrower peak width than the conventional focus measure operator. The optimal focus position of each image point is obtained using the modified Gaussian curve fitting method. On this basis, the 3D information measurement and reconstruction of the micro-hole array are realized. By comparing the results with the conventional micro-hole measurement method, the feasibility and accuracy of the proposed method have been confirmed to be an effective measurement method for the micro-hole array.

From 2D to 3D: automatic measurement of the Cobb angle in adolescent idiopathic scoliosis with the weight-bearing 3D imaging

Background contextAdolescent idiopathic scoliosis (AIS) necessitates accurate spinal curvature assessment for effective clinical management. Traditional two-dimensional (2D) Cobb angle measurements have been the standard, but the emergence of three-dimensional (3D) automatic measurement techniques, such as those using weight-bearing 3D imaging (WR3D), presents an opportunity to enhance the accuracy and comprehensiveness of AIS evaluation. PurposeThis study aimed to compare traditional 2D Cobb angle measurements with 3D automatic measurements utilizing the WR3D imaging technique in patients with AIS. Study design/settingA cohort of 53 AIS patients was recruited, encompassing 88 spinal curves, for comparative analysis. Patient sampleThe patient sample consisted of 53 individuals diagnosed with AIS. Outcome measuresCobb angles were calculated using the conventional 2D method and three different 3D methods: the Analytical Method (AM), the Plane Intersecting Method (PIM), and the Plane Projection Method (PPM). MethodsThe 2D cobb angle was manually measured by 3 experienced clinicians with 2D frontal whole-spine radiographs. For 3D cobb angle measurements, the spine and femoral heads were segmented from the WR3D images using a 3D-UNet deep-learning model, and the automatic calculations of the angles were performed with the 3D slicer software. ResultsAM and PIM estimates were found to be significantly larger than 2D measurements. Conversely, PPM results showed no statistical difference compared to the 2D method. These findings were consistent in a subgroup analysis based on 2D Cobb angles. ConclusionEach 3D measurement method provides a unique assessment of spinal curvature, with PPM offering values closely resembling 2D measurements, while AM and PIM yield larger estimations. The utilization of WR3D technology alongside deep learning segmentation ensures accuracy and efficiency in comparative analyses. However, additional studies, particularly involving patients with severe curves, are required to validate and expand on these results. This study emphasizes the importance of selecting an appropriate measurement method considering the imaging modality and clinical context when assessing AIS, and it also underlines the need for continuous refinement of these techniques for optimal use in clinical decision-making and patient management.

Anthropometric Analysis of the External Ear Using CT Scan Image by Age, Gender, and Side Among the Iranian Population

The purpose of this study is to provide an accessible source of anthropometric measurements from the auricle to the tympanic membrane based on age, gender, and ear side, which can be used as a reference for optimizing the fit and comfort of ear-related devices and anthropometric analysis. To do so, in this study, a 3D measurement method based on CT scan imaging was utilized. A new coordinate system was defined to measure 41 linear, angular, and circumference dimensions. The result showed that ergonomic ear-related products should be designed based on the age and gender of the users.

Physics-based supervised learning method for high dynamic range 3D measurement with high fidelity.

High dynamic range (HDR) 3D measurement is a meaningful but challenging problem. Recently, many deep-learning-based methods have been proposed for the HDR problem. However, due to learning redundant fringe intensity information, their networks are difficult to converge for data with complex surface reflectivity and various illumination conditions, resulting in non-robust performance. To address this problem, we propose a physics-based supervised learning method. By introducing the physical model for phase retrieval, we design a novel, to the best of our knowledge, sinusoidal-component-to-sinusoidal-component mapping paradigm. Consequently, the scale difference of fringe intensity in various illumination scenarios can be eliminated. Compared with conventional supervised-learning methods, our method can greatly promote the convergence of the network and the generalization ability, while compared with the recently proposed unsupervised-learning method, our method can recover complex surfaces with much more details. To better evaluate our method, we specially design the experiment by training the network merely using the metal objects and testing the performance using different diffuse sculptures, metal surfaces, and their hybrid scenes. Experiments for all the testing scenarios have high-quality phase recovery with an STD error of about 0.03 rad, which reveals the superior generalization ability for complex reflectivity and various illumination conditions. Furthermore, the zoom-in 3D plots of the sculpture verify its fidelity on recovering fine details.

Orthogonal spatial binary coding method for high-speed 3D measurement.

Temporal phase unwrapping based on single auxiliary binary coded pattern has been proven to be effective for high-speed 3D measurement. However, in traditional spatial binary coding, it often leads to an imbalance between the number of periodic divisions and codewords. To meet this challenge, a large codewords orthogonal spatial binary coding method is proposed in this paper. By expanding spatial multiplexing from 1D to 2D orthogonal direction, it goes beyond the traditional 8 codewords to 27 codewords at three-level periodic division. In addition, a novel full-period connected domain segmentation technique based on local localization is proposed to avoid the time-consuming global iterative erosion and complex anomaly detection in traditional methods. For the decoding process, a purely spatial codewords recognition and a spatial-temporal hybrid codewords recognition methods are established to better suppress the percentage offset caused by static defocusing and dynamic motion, respectively. Obviating the need for intricate symbol recognition, the decoding process in our proposed method encompasses a straightforward analysis of statistical distribution. Building upon the development of special spatial binary coding, we have achieved a well-balance between low periodic division and large codewords for the first time. The experimental results verify the feasibility and validity of our proposed whole image processing method in both static and dynamic measurements.

Comparison of 2D and 3D measurement methods for evaluating laser structured aluminum surfaces using fractal dimension

The interlaminar strength of mechanically interlocked polymer-metal hybrids is strongly determined by the surface geometry at the interface. To improve the interlaminar strength, laser beam machining is among the most applied technologies for manufacturing a predefined and complex surface structure. Previously, the fractal dimension of interfaces in polymer-metal-hybrids has been introduced as scale-independent measure. It has been correlated quantitatively with the interlaminar strength using values derived from cross-sectional images. Therefore, the quality of the prediction directly depends on the accuracy of the measured surface data. However, a holistic acquisition of surface structure for the determination of the fractal dimension including 3D methods has not been investigated so far. In this study, laser machining was used to generate five different pin-structures on samples of the aluminum alloy EN AW-6082. Surface characterization was realized by light microscopy (2D), laser scanning microscopy (3D) and X-ray microscopy (3D). Subsequently, the fractal dimension was determined using a box-counting algorithm as well as slit-island method. The results show that all methods regarded are appropriate to determine the fractal dimension, but differ in the specific results. The functional characteristics of each method are described in detail and potentials for applications are outlined.

Robust structured light 3D measurement method based on polarization-encoded projection patterns.

Fringe projection profilometry (FPP) is widely used in 3D vision measurement because of its high robustness and measurement accuracy. In the case of HDR objects, due to the problem of surface reflectivity, the obtained image will be overexposed. This will cause the sinusoidality of the fringes projected on the surface of the object in the acquired image to be interfered, resulting in a phase error in the calculated wrapped phase. Therefore, a polarization-encoded sinusoidal structured light is proposed to enhance the sinusoidality of the fringe. The phase information contained in the polarized sinusoidal structured light fringe is only related to the polarization state, not to the light intensity. A polarization coding assisted structured light measurement strategy (PASM) is proposed. This method uses polarization coding assisted polarization phase-shifting fringes for phase unwrapping. The angle of the linear polarizer is set to zero in this method, and it does not require rotating the polarizer. It only needs a single exposure to improve the fringe quality and obtain a more stable unwrapping phase. The experimental results show that the obtained polarization fringes have better sinusoidality, and the phase unwrapping can be more accurate. The reconstructed 3D point cloud also does not appear missing and has better accuracy. It is a reliable method for vision measurement of HDR objects.

Morphological variation of gubernacular tracts for permanent mandibular canines in eruption: a three-dimensional analysis.

This study aims to evaluate the morphological features of gubernacular tract (GT) for erupting permanent mandibular canines at different ages from 5 to 9 years old with a three-dimensional (3D) measurement method. The cone-beam CT images of 50 patients were divided into five age groups. The 3D models of the GT for mandibular canines were reconstructed and analysed. The characteristics of the GT, including length, diameter, ellipticity, tortuosity, superficial area, volume, and the angle between the canine and GT, were evaluated using a centreline fitting algorithm. Among the 100 GTs that were examined, the length of the GT for mandibular canines decreased between the ages of 5 and 9 years, while the diameter increased until the age of 7 years. Additionally, the ellipticity and tortuosity of the GT decreased as age advanced. The superficial area and volume exhibited a trend of initially increasing and then decreasing. The morphological variations of the GT displayed heterogeneous changes during different periods. The 3D measurement method effectively portrayed the morphological attributes of the GT for mandibular canines. The morphological characteristics of the GT during the eruption process exhibited significant variations. The variations in morphological changes may indicate different stages of mandibular canine eruption.

A Y-shaped network based single-shot absolute phase recovery method for fringe projection profilometry

Fringe projection profilometry (FPP) is a widely used non-contact 3D measurement method. Though maturing in the last decade, it remains a significant challenge when facing the phase unwrapping of measured object surfaces in a single-shot measurement setting. With the rapid development of deep learning techniques, the adoption of a data-driven approach is gaining popularity in the field of optical metrology. This study proposes a new absolute phase recovery method based on the devised single-stage deep learning network. The aim is to ensure high-quality absolute phase recovery from a single-shot fringe projection measurement. Unlike most existing approaches, where the numerators and denominators of the wrapped phases and the fringe orders are predicted in various stages, the proposed method acquires the wrapped phases and the corresponding fringe orders within a single network, i.e. it can predict both wrapped phases and the corresponding fringe orders directly and simultaneously from the single fringe pattern projected in the single-shot mode based on a unified Y-shaped network. Experiments on benchmark datasets and models have demonstrated the effectiveness and efficiency of the technique, especially in terms of high-quality recovery of absolute phase information by using the lightweight single-stage network, and enabling the FPP-based phase 3D measurements in an online manner.

Comparison of modern 3D measurement methods for special tasks of shipbuilding industry

Research presents modern technologies of 3D measurements in shipbuilding production parts. Special template of the keel detail was made for the research. The detail measurements with a laser tracker, laser scanning, industrial photogrammetry and handheld scanning are performed. Leica Absolute Tracker AT960-LR was used for Laser tracker measurements. Laser scanning was performed with Z+F Imager 5010. Nikon D2Xs photo camera was used for Industrial photogrammetry. Handheld scanner DPI-7 from DotProduct was also used for 3D measurements of the details. All collected data were imported into 3DReshaper software for comparison. The accuracy comparison for the specific equipment used in the study is performed. The recommendations for optimal equipment use and software products in this research are also included. The authors also present the assessment of the cost and time spent on the measurements.