Two-dimensional Digital Image Research Articles

Turbo-coded 16-ary OAM shift keying FSO communication system combining the CNN-based adaptive demodulator.

In this paper, a novel turbo-coded 16-ary orbital angular momentum - shift keying-free space optical (OAM-SK-FSO) communication system combining a convolutional neural network (CNN) based adaptive demodulator under strong atmospheric turbulence is proposed for the first time. The feasibility of the scheme is verified by transmitting a 256-grayscale two-dimensional digital image. The bit error ratio (BER) performance of the system is investigated and the effect of different factors such as turbulence strength, propagation distance, code rate, length of random interleaver and length of bit interleaver is also taken into account. An advanced encoder/decoder structure and mapping scheme are applied to diminish the influence of CNN misclassification and reduce the BER effectively. With the optimal encoder/decoder structure and CNN model settings, the BER varies from 0 to 4.89×10-4 when the propagation distance increases from 200m to 1000m for a given turbulence strength Cn2 equals 5×10-14m-2/3. For a determined propagation distance equals 400m, the BER ranges from 0 to 4.01×10-4 when Cn2increases from 1×10-15m-2/3 to 4×10-13m-2/3. Our numerical simulations demonstrate that the proposed system can provide better BER performance under strong atmospheric turbulence and conditions when the classification ability of CNN is limited.

Digital image correlation through image registration in the frequency domain

The present work analyses displacement field computations for several digital image correlation test cases obtained through the application of image registration in the Fourier domain. The applied method relies on the properties of the discrete Fourier transform with regard to the cross-correlation function to find the rigid displacement between two subsets. The computed displacement fields were compared to their analogous values obtained through a commercial digital image correlation code. The test cases cover both two-dimensional and three-dimensional digital image correlation and make use of the ability of the developed method to calculate the translation and rotation between subsets from images at different loading states, as well as its capacity to match subsets from two cameras of a stereo rig. The presented analysis aims to contribute to the investigation of the use of correlation in the frequency domain as the basis for an alternative to conventional digital image correlation which could provide advantages in terms of robustness and efficiency, particularly in field measurements, where the lack of control over lighting and other factors impacting image quality requires a system which is optimised towards low dependence on the optimal quality of these conditions.

Utility of three-dimensional models in resident education on simple and complex intracardiac congenital heart defects.

Applications of three-dimensional (3D) printed models in medicine include preprocedure planning, patient education, and clinical training. Reproducing complex anatomy as a 3D printed model can be useful for understanding congenital heart defects (CHD). We hypothesized that using 3D printed models during didactic sessions with resident physicians will improve trainees' understanding of CHD. We performed a prospective, randomized educational intervention for teaching pediatric and pediatric/emergency medicine residents about simple (ventricular septal defect [VSD]) and moderately complex (tetralogy of Fallot [ToF]) CHD. Residents were divided into two groups: intervention and control. Each group completed a subjective survey about their comfort with the anatomy, evaluation, and treatment of VSD and ToF and took an objective test on VSD and ToF. They separately received the same 20 min lecture, including projected two-dimensional digital images of VSD and ToF; the intervention group was given 3D printed models created using the same imaging data. After the lecture, the groups repeated the survey and test questions. Twenty-six residents participated in the VSD session, 34 in the ToF. There were no differences in demographics between control and intervention groups. All residents had higher subjective comfort with VSD and ToF after the lectures. There was no difference in baseline test scores for VSD or ToF groups. The control group scored higher on the VSD postlecture test. The intervention group scored higher on the ToF postlecture test. Incorporation of 3D printed models into lectures about CHD imparts a greater acute level of understanding, both subjective and objective, for pediatric and combined pediatric/emergency medicine residents. There does not seem to be an added benefit for understanding ventricular septal defects, but there is for tetralogy of Fallot, likely due to increased complexity of the lesion and difficulty visualizing spatial relationships in CHD with multiple components.

RGB Colour Encoding Improvement for Three-Dimensional Shapes and Displacement Measurement Using the Integration of Fringe Projection and Digital Image Correlation.

Three-dimensional digital image correlation (3D-DIC) has become the most popular full-field optical technique for measuring 3D shapes and displacements in experimental mechanics. The integration of fringe projection (FP) and two-dimensional digital image correlation (FP + DIC) has been recently established as an intelligent low-cost alternative to 3D-DIC, overcoming the drawbacks of a stereoscopic system. Its experimentation is based on the colour encoding of the characterized fringe and speckle patterns required for FP and DIC implementation, respectively. In the present work, innovations in experimentation using FP + DIC for more accurate results are presented. Specifically, they are based on the improvement of the colour pattern encoding. To achieve this, in this work, a multisensor camera and/or laser structural illumination were employed. Both alternatives are analysed and evaluated. Results show that improvements both in three-dimensional and in-plane displacement are obtained with the proposed alternatives. Nonetheless, multisensor high-speed cameras are uncommon, and laser structural illumination is established as an important improvement when low uncertainty is required for 2D-displacement measurement. Hence, the uncertainty has been demonstrated to be reduced by up to 50% compared with results obtained in previous experimental approaches of FP + DIC.

Deformation Monitoring System Based on 2D-DIC for Cultural Relics Protection in Museum Environment with Low and Varying Illumination

Quantifying the condition of large cultural relics, such as marine archaeological shipwreck, is important to verify stability and reliability. Deformation monitoring system plays a key role in the preservation and long-term conservation of cultural relics. Two-dimensional digital image correlation (2D-DIC) method has proven its efficiency in being able to provide accurate quantitative information of structural deformations. In this study, a deformation monitoring system with four cameras based on 2D-DIC is developed to perform noncontact, optically based measurement to monitor the deformation of shipwreck in museum environment with low and varying illumination. Because the consistency of the accuracy of 2D-DIC measurements for different locations is the most basic requirement in the application of structural deformation monitoring, selecting the appropriate exposure time and quantifying the bias errors on 2D-DIC measurements should be helpful to the optimal use of this optical nondestructive testing technique. A theoretical criterion is deduced to quantitatively characterize the dependence of interpolation bias upon natural patterns and illuminations. Then, an exposure adjustment scheme is built based on the aforementioned criterion. Numerical experiments reveal that the exposure adjustment scheme is able to provide consistency interpolation error for different natural patterns even though the environmental illumination is different as well. The deformation monitoring system with the proposed exposure adjustment scheme is promising for developing flexible and robust in situ structural health monitoring for use in museum environment with low and varying illumination, making 2D-DIC technique a really useful tool for in situ long-term monitoring of large cultural relics.

Full-field measurement with nanometric accuracy of 3D superficial displacements by digital profile correlation: A powerful tool for mechanics of materials

In recent decades, technological innovations have prompted the development of pioneering materials that attempt to satisfy new and forthcoming needs. The innovation in these materials is usually from their peculiar properties, which in many cases make them uniquely appropriate for a specific application. In this context, the development of innovative measurement techniques can provide more comprehensive understanding and knowledge of these materials' properties, often non-conventional and not easily retrieved by standard procedures. In order to obtain a full-field 3D displacement vector of a surface under investigation, the author proposes the combination of two well-known measurement techniques: the Confocal Microscopy (CM) and the two-dimensional Digital Image Correlation (2D-DIC). Specifically, CM has demonstrated its ability to successfully attain microscopic topography on a highly finished surface with sub-micrometric roughness, and such a technique could be used as a carrier to successfully apply the 2D-DIC algorithm. By this approach, it is not difficult to reach an accuracy of a few nanometers on the displacement measurement. The feasibility of the procedure proposed herein was demonstrated by two case studies: a tensile test of a Ni-alloy edge crack specimen, and a hardness test carried out on a thick AISI 1040 disk.

On the identification of earlywood and latewood radial elastic modulus ofPinus pinasterby digital image correlation: A parametric analysis

This work addresses the reconstruction of strain gradient fields at the wood growth ring scale from full-field deformation measurements provided by digital image correlation. Moreover, the spatial distribution of the earlywood and latewood radial modulus of elasticity is assessed. Meso-scale tensile tests are carried out on Pinus pinaster Ait. wooden specimens oriented in the radial–tangential plane under quasi-static loading conditions. A parametric analysis of the two-dimensional digital image correlation extrinsic and intrinsic setting parameters is performed, in a balance between spatial resolution and resolution. It is shown that the parametric module is an effective way to quantitatively support the choice of digital image correlation parameters in the presence of the high deformation gradient fields generated by the structure–property relationships at the scale of observation. Under the assumption of a uniaxial tensile stress state, the spatial distribution of the radial elastic modulus across the growth rings is obtained. It is observed that the ratio of the radial modulus of elasticity between latewood and earlywood tissues can vary significantly as a function of the digital image correlation parameters. It is pointed out, however, that a convergence value can be systematically established. Effectively, earlywood and latewood stress–strain curves are obtained and elastic properties are determined assuming the converged digital image correlation setting parameters.

Determination of leg cross-sectional curvatures and application in pressure prediction for lower body compression garments

It has been recognized that the cross-sectional curvatures of lower extremities directly influence pressure magnitudes and distributions exerted by compression garments. In the practice of compression therapy, higher peak pressures produced by compression shells occurred at anatomic sites with smaller radius of curvatures and led to side effects and discomfort perception. An effective and operational method to determine leg curvature properties in order to predict pressure performances is desirable to improve comfort and mechanical function of compression garment. By employing three-dimensional (3D) digital anthropometry and two-dimensional (2D) digital image simulation, the curvatures and radius of curvatures of a total of 300 cross-sectional slices involving 1200 anatomic sites along the lower limbs were determined onto the ten healthy female subjects when they were and were not wearing compression stockings. Based on the determined cross-sectional characteristics, the skin pressures were calculated using the circumference-based and the radius of curvature-based Laplace’s equations, respectively, which were further validated against the experimental skin pressures measured by a PicoPress transducer. This study provided quantitative evidence in the exploration of the working mechanisms of uneven pressures produced by compression garments, and established a standardized method to determine cross-section-related curvature characteristics for pressure assessment and prediction, which will contribute to improving user compliance of compression garments in long-term wear.

Effects of superhydrophobic surfaces on the flow around an NACA0012 hydrofoil at low Reynolds numbers

In the present study, the effects of superhydrophobic surface on the flow around an NACA0012 hydrofoil are experimentally investigated at low Reynolds number range of 0.2–1.0 $$\times 10^{4}$$ . The velocity fields were measured using two-dimensional digital particle image velocimetry in a water tunnel while varying the angle of attack from $$0^\circ$$ to $$20^\circ$$ . The spray-coating of hydrophobic nanoparticles was used to create superhydrophobic surfaces. Depending on the Reynolds number and angle of attack, we found that the effects of superhydrophobic surface show up differently, which is determined by the relative strength of turbulence caused by both the surface slip (influence of trapped air pockets) and roughness, compared to that of background (i.e., uncontrolled) flow. In general, the superhydrophobic surface imposes a little influence on the wake behind a hydrofoil when the angle of attack is very low (attached flow) or high (fully separated flow). At intermediate angles of attack (flow separates between the leading and trailing edges), however, it is measured that the flow over superhydrophobic surface has a stronger turbulence, and thus, the enhanced shear-layer instability forces the early vortex rollup in the wake and reduction of vortex formation length. Interestingly, there is a transitional range of angle of attack, in which this effect is reversed, and thus, the vortex rollup is slightly delayed. This trend can be explained based on the changes in the uncontrolled flow structures, and finally, we classify the effects of superhydrophobic surface in terms of Reynolds number and angle of attack, in the considered ranges of both.

Measurement of strain on concrete using an ordinary digital camera

Two-Dimensional Digital Image Correlation (2D-DIC) technique was applied in this experimental study, to measure the steel–concrete bond strain at concrete surface. The bond strain was caused by tensile force when it was subjected to embed reinforced bar of concrete block in the pullout test. A series of digital images from surface of concrete were taken by a semi-professional digital camera (Nikon D80) during the loading. Acquired images were used as input data for two DIC software to calculate deformation. After verifying the displacement results by measured LVDT results, full-field strain components on concrete were calculated at various levels of forces. The results show the measurement of displacement and strain by this method are acceptable to a great extent.

Adaptive image interpolation technique based on cubic trigonometric B‐spline representation

A cubic trigonometric B-spline representation with two parameters is constructed in this work. A soft computing technique, genetic algorithm, is used to find the optimal value of the parameters in the description of B-spline so that the sum square error is minimised. The newly constructed B-spline is then utilised to interpolate two-dimensional digital images. The image quality metrics peak signal-to-noise ratio, structure SIMilarity index, multi-scale structure SIMilarity index and feature SIMilarity index are used to investigate the quality of interpolated digital images. Comparison with already existing image interpolation schemes leads to the conclusion that the proposed image interpolation technique is found to be a valuable scheme for the problems related to digital image interpolation.

Experimental and numerical investigation on the bond strength of self-sensing composite joints

Laboratory experiments demonstrate that a novel carbon nanotube (CNT)-based sensing layer embedded in the bondline of an adhesively bonded structural joint can detect and monitor deformation and damage progression of the adhesive layer. In this study, experimental and numerical investigations were performed to identify any effect of an embedded CNT-based sensing layer on the bond strength of that joint. To evaluate the mechanical behavior of such a bondline configuration, two sets of single-lap specimens, with and without sensing layer, were prepared and tested to determine the bond strengths of the respective types. Two-dimensional digital image correlation (2D DIC) was utilized to estimate the load-displacement response of the test specimens. Three-dimensional cohesive surface finite element models of the test specimens, with and without the sensing layer, were created and validated using the experimental measurements. It is shown that the embedded CNT-based sensing layer does not influence the bond strength of the single-lap joint.

Characterizing the tensile behavior of additively manufactured Ti-6Al-4V using multiscale digital image correlation

The tensile behavior of an additively manufactured (AM) Ti-6Al-4V alloy is examined at multiple length scales using two-dimensional digital image correlation (DIC) techniques. The work focuses on characterizing the relationship between microstructure and bulk mechanical properties of the AM Ti-6Al-4V specimens and understanding how unique microstructural features pertaining to AM metals, such as columnar grain morphology and nonequilibrium phase structures, influence deformation behavior at the microscale (i.e., the length scales of those features). Lower magnification in situ DIC experiments were performed at the macroscale during tension experiments to assess the bulk properties, and a higher magnification ex situ DIC study was performed at the microscale using a microscope to observe heterogeneous strain accumulation at that scale. At the macroscale, the AM Ti-6Al-4V exhibited greater strength than conventionally processed Ti-6Al-4V, due to its acicular, or needle-like, grain structure. At the microscale, alternating regions of high and low strains were observed to accumulate along these needles, or laths. The effects of build orientations (related to microstructure anisotropy), powder bed layering thicknesses (30 and 60 µm), and heat treatment, on the properties were also examined. A decrease in strength was observed in samples subjected to annealing heat treatment compared to as-built specimens with no heat treatment. Also, the AM specimens exhibited higher tensile strength when the direction of applied load was perpendicular to the AM build orientation. This observed anisotropy is thought to be related to interfaces of consecutively deposited layers because regions of high strain at the microscale were observed to accumulate at changes in structure associated with the deposition of layers.

METHODS OF DISTANCE MEASUREMENT’S ACCURACY INCREASING BASED ON THE CORRELATION ANALYSIS OF STEREO IMAGES

To solve the problem of increasing the accuracy of restoring a three-dimensional picture of space using two-dimensional digital images, it is necessary to use new effective techniques and algorithms for processing and correlation analysis of digital images. Actively developed tools that allow you to reduce the time costs for processing stereo images, improve the quality of the depth maps construction and automate their construction. The aim of the work is to investigate the possibilities of using various techniques for processing digital images to improve the measurements accuracy of the rangefinder based on the correlation analysis of the stereo image. The results of studies of the influence of color channel mixing techniques on the distance measurements accuracy for various functions realizing correlation processing of images are presented. Studies on the analysis of the possibility of using integral representation of images to reduce the time cost in constructing a depth map areproposed. The results of studies of the possibility of using images prefiltration before correlation processing when distance measuring by stereo imaging areproposed.It is obtained that using of uniform mixing of channels leads to minimization of the total number of measurement errors, and using of brightness extraction according to the sRGB standard leads to an increase of errors number for all of the considered correlation processing techniques. Integral representation of the image makes it possible to accelerate the correlation processing, but this method is useful for depth map calculating in images no more than 0.5 megapixels. Using of image filtration before correlation processing can provide, depending on the filter parameters, either an increasing of the correlation function value, which is useful for analyzing noisy images, or compression of the correlation function.

Evaluating the Potential of Three-Dimensional Laser Surface Scanning as an Alternative Method of Obtaining Morphometric Data

External anatomical measurements have a long tradition in morphometric studies. Recently, three-dimensional laser surface scanners have revolutionized the way such measurements can be obtained. The aim of this study was to quantify measurement error (hereafter ME) and reliability of several linear skull measurements of the red fox obtained with three different techniques: using calipers, and by analyzing two- and three-dimensional digital images. Differences between observers had the highest ME values for most traits, followed by method and replicate. Inter-observer variability contributed the most to total ME. Measurements obtained from calipers and two-dimensional digital images showed high agreement, validating their use in traditional morphometric studies. The laser surface scanner was unreliable when compared with the reference in almost half the traits. To achieve greater accuracy, in future studies full advantage of the device's capabilities should be taken. Objects should be scanned in high defini...

Accurate measurement of elastic modulus of specimen with initial bending using two-dimensional DIC and dual-reflector imaging technique

Common optical methods are not suitable for accurate measurement of elastic modulus of specimens with initial bending because it is rather difficult to separate the axial strain caused by the axial load from the surface strain which consists of the axial strain and the bending strain caused by additional bending moment. Inspired by the strain-gauge technique, averaging the two strains on the opposite surfaces can eliminate the effect of bending strain of the specimen with initial bending, which is easily implemented by dual-reflector imaging and two-dimensional digital image correlation (2D-DIC). With dual-reflector imaging, the front and rear surfaces of the specimen are recorded by using a single digital camera. Consequently, the strains of two optical extensometers constructed on these two surfaces are obtained with common 2D-DIC, and averaging these strains can eliminate the effect of bending moment and out-of-plane motion of the specimen. Therefore, the elastic modulus can be determined with high measurement accuracy. The effectiveness of the proposed method was first verified using self-manufactured tensile equipment which leads to large out-of-plane motion during the test. Furthermore, uniaxial tensile tests of stainless-steel specimens, including static and continuous tensile tests, were conducted to evaluate the accuracy of the proposed method. The experimental results show good agreement between elastic modulus obtained using the proposed method and a strain gauge and the relative error between them is less than 0.5%, which shows excellent performance of the proposed method on accurate measurement of elastic modulus of specimens with initial bending.

3D representation and CNC machining of 2D digital images

In this paper a new paradigm for CNC carving of digital images in the form of lookalike three-dimensional (3D) surfaces on wooden or metallic plaques is discussed. This work is focused on development of a single page windows based console application for conversion of a two-dimensional (2D) digital image into a 3D freeform surface representation in the form of a point cloud data and STL format. Subsequently, the 3D surface data is then used for generating an efficient toolpath data for 3-axis CNC finish machining using a ball end mill tool. The results from the developed algorithm are validated using a machining simulation in the virtual environment of an Open-GL based 3D graphical simulator ToolSim [1].

Post-Processing of High-Speed Video-Laryngoscopic Images to Two-Dimensional Scanning Digital Kymographic Images

Post-Processing of High-Speed Video-Laryngoscopic Images to Two-Dimensional Scanning Digital Kymographic Images

Accuracy-enhanced constitutive parameter identification using virtual fields method and special stereo-digital image correlation

The virtual fields method (VFM) is generally used with two-dimensional digital image correlation (2D-DIC) or grid method (GM) for identifying constitutive parameters. However, when small out-of-plane translation/rotation occurs to the test specimen, 2D-DIC and GM are prone to yield inaccurate measurements, which further lessen the accuracy of the parameter identification using VFM. In this work, an easy-to-implement but effective “special” stereo-DIC (SS-DIC) method is proposed for accuracy-enhanced VFM identification. The SS-DIC can not only deliver accurate deformation measurement without being affected by unavoidable out-of-plane movement/rotation of a test specimen, but can also ensure evenly distributed calculation data in space, which leads to simple data processing. Based on the accurate kinematics fields with evenly distributed measured points determined by SS-DIC method, constitutive parameters can be identified by VFM with enhanced accuracy. Uniaxial tensile tests of a perforated aluminum plate and pure shear tests of a prismatic aluminum specimen verified the effectiveness and accuracy of the proposed method. Experimental results show that the constitutive parameters identified by VFM using SS-DIC are more accurate and stable than those identified by VFM using 2D-DIC. It is suggested that the proposed SS-DIC can be used as a standard measuring tool for mechanical identification using VFM.

Finite element inversion method for interfacial stress analysis of composite single-lap adhesively bonded joint based on full-field deformation

A hybrid inversion method combining the finite element method and full-field displacement data was proposed to analyze the adhesive interface stress at a composite single-lap bonded joint. The displacement field distribution for the outer surface of the overlapped composite plate was measured through the two-dimensional digital image correlation method at different times by performing a tensile test. At the same time, the finite element analysis of the single-lap joint was carried out, which provided the data on structural deformation characteristics. Then, a polynomial interpolation method was used to establish the relation between the internal and external surface displacements. The displacements of the inner surface of the plate can be calculated from the measured displacement data of the outer surface of the plate by using the polynomial function. The displacements of the inner surface of the plate are regarded as the displacement load conditions for the outer and inner surfaces of the plate to establish the finite element model of the adhesive layer. The validity of the hybrid inversion method was proved by FEM analysis and experiments.