Accurate 3D Measurements Research Articles

SIMULATION AND ANALYSIS OF PHOTOGRAMMETRIC UAV IMAGE BLOCKS: INFLUENCE OF CAMERA CALIBRATION ERROR

Abstract. Unmanned aerial vehicles (UAV) are increasingly used for topographic mapping. The camera calibration for UAV image blocks can be performed a priori or during the bundle block adjustment (self-calibration). For an area of interest with flat, corridor configuration, the focal length of camera is highly correlated with the height of camera. Furthermore, systematic errors of camera calibration accumulate on the longer dimension and cause deformation. Therefore, special precautions must be taken when estimating camera calibration parameters. In this paper, a simulated, error-free aerial image block is generated. error is then added on camera calibration and given as initial solution to bundle block adjustment. Depending on the nature of the error and the investigation purpose, camera calibration parameters are either fixed or re-estimated during the bundle block adjustment. The objective is to investigate how certain errors in the camera calibration impact the accuracy of 3D measurement without the influence of other errors. All experiments are carried out with Fraser camera calibration model being employed. When adopting a proper flight configuration, an error on focal length for the initial camera calibration can be corrected almost entirely during bundle block adjustment. For the case where an erroneous focal length is given for pre-calibration and not re-estimated, the presence of oblique images limits the drift on camera height hence gives better camera pose estimation. Other than that, the error on focal length when neglecting its variation during the acquisition (e.g., due to camera temperature increase) is also investigated; a bowl effect is observed when one focal length is given in camera pre-calibration to the whole image block. At last, a local error is added in image space to simulate camera flaws; this type of error is more difficult to be corrected with the Fraser camera model and the accuracy of 3D measurement degrades substantially.

High-accuracy Three-dimensional Measurement by Improving the Asymmetry of Dithered Patterns

The previously proposed dithering defocusing technology performs well for threedimensional (3D) measurement when stripes are relatively wide, yet suffers if stripes are narrow. This paper finds two asymmetries in dithered patterns generated by the Sierra Lite dithering algorithm and verifies the longitudinal fringes are more advantageous for phase-shifting technique over transverse fringes. Furthermore, this paper proposes an algorithm with a meandering scan. In each pattern, the pixels of odd lines are scanned from left to right while the even lines are scanned from right to left. The proposed method avoids the quantization errors propagating in a specific direction and greatly improves the symmetry of longitudinal fringes. Both simulation and experimental results have shown this method can effectively improve accuracy of 3D measurement especially for narrow stripes.

Automatic and Accurate 3D Measurement Based on RGBD Saliency Detection

Automatic and Accurate 3D Measurement Based on RGBD Saliency Detection

High-speed 3D shape measurement using the optimized composite fringe patterns and stereo-assisted structured light system.

In this paper, we propose a high-speed 3D shape measurement technique based on the optimized composite fringe patterns and stereo-assisted structured light system. Stereo phase unwrapping, as a new-fashioned method for absolute phase retrieval based on the multi-view geometric constraints, can eliminate the phase ambiguities and obtain a continuous phase map without projecting any additional patterns. However, in order to ensure the stability of phase unwrapping, the period of fringe is generally around 20, which limits the accuracy of 3D measurement. To solve this problem, we develop an optimized method for designing the composite pattern, in which the speckle pattern is embedded into the conventional 4-step phase-shifting fringe patterns without compromising the fringe modulation, and thus the phase measurement accuracy. We also present a simple and effective evaluation criterion for the correlation quality of the designed speckle pattern in order to improve the matching accuracy significantly. When the embedded speckle pattern is demodulated, the periodic ambiguities in the wrapped phase can be eliminated by combining the adaptive window image correlation with geometry constraint. Finally, some mismatched regions are further corrected based on the proposed regional diffusion compensation technique (RDC). These proposed techniques constitute a complete computational framework that allows to effectively recover an accurate, unambiguous, and distortion-free 3D point cloud with only 4 projected patterns. Experimental results verify that our method can achieve high-speed, high-accuracy, robust 3D shape measurement with dense (64-period) fringe patterns at 5000 frames per second.

Multiple Laser Stripe Scanning Profilometry Based on Microelectromechanical Systems Scanning Mirror Projection.

In traditional laser-based 3D measurement technology, the width of the laser stripe is uncontrollable and uneven. In addition, speckle noise in the image and the noise caused by mechanical movement may reduce the accuracy of the scanning results. This work proposes a new multiple laser stripe scanning profilometry (MLSSP) based on microelectromechanical systems (MEMS) scanning mirror which can project high quality movable laser stripe. It can implement full-field scanning in a short time and does not need to move the measured object or camera. Compared with the traditional laser stripe, the brightness, width and position of the new multiple laser stripes projected by MEMS scanning mirror can be controlled by programming. In addition, the new laser strip can generate high-quality images and the noise caused by mechanical movement is completely eliminated. The experimental results show that the speckle noise is less and the light intensity distribution is more even. Furthermore, the number of pictures needed to be captured is significantly reduced to ( is the number of multiple laser stripes projected by MEMS scanning mirror) and the measurement efficiency is increased by times, improving the efficiency and accuracy of 3D measurement.

3D measurements of micro-objects based on monocular wide-field optical microscopy with extended depth of field.

Monocular rotational microscopes suffer from shallow depth of field and reduced field of view. Consequently, we propose monocular wide-field optical microscopy with extended depth of field to enable accurate 3D measurements of micro-objects. We use a liquid lens and a 4f system to extend the system's depth of field and two additional rotational reflectors to capture multi-view image sequences. In addition, we increase the number of expansion directions in the patch-based multi-view stereo algorithm and optimize the expansion radius. Our approach outperforms that based on the patch-based multi-view stereo method and achieves a high relative measurement accuracy of 1.9%. RESEARCH HIGHLIGHTS: This study proposes 3D measurements of micro-objects via monocular rotational microscopy. The depth of field is extended by the use of a liquid lens and a 4f system. Our approach outperforms that based on the patch-based multi-view stereo method and achieves a high relative measurement accuracy of 1.9%.

Evaluating the accuracy of facial models obtained from volume wrapping: 2D images on CBCT versus 3D on CBCT

Purpose: To test for difference in accuracy of 3D facial integument measurements of three indirect facial measurement techniques compared to measuring integument facial features directly. Materials & Methods: Four measurement methods were used to measure 25 patients (15 females and 10 males): 1) direct anthropometric facial (direct), 2) volume wrapping 2D images on CBCT (2D wrap), 3) volume wrapping 3D facial scan using Artec Eva on CBCT (3D wrap), and 4) direct 3D face scan (3D scan). Statistical differences were determined at the 99% probability level and clinical significance was >0.5 mm. Results: Direct technique measurements were significantly smaller than the other three techniques in 7 of 26 (26.8%) measurements. Differences between 3 of 7 measurements exceeded 0.5 mm and were deemed clinically significant, i.e. upper face height (N-Sto), lower face height (Sn-Gn) and nasal tip projection (Sn-Prn). The remaining 23 of 26 direct measurements could be substituted by 2D wrap, 3D wrap or 3D scan measurement techniques. Compared to the direct technique, 2D volume wrap on CBCT values were closer than the values obtained from the 3D volume wrap and 3D CBCT techniques. Conclusions: Compared to the “gold standard” direct measurements, the other three techniques in 23 of 26 face and ear measurements (88.5%) could be substituted one for the other, and demonstrated a fairly high level of precision.

Assessment of Seismic Vulnerability and Retrofit Strategies of a Historic Building on the Great Silk Road Based on its Current Condition Captured by Laser Scans

ABSTRACT The technology of high-definition laser scanning is an essential tool for accurate non-destructive 3D measurements of structures. The research team used this technology and conducted an extensive laser-scanning program of many historic monuments in Uzbekistan. The program started in 2013 from scanning the famous Registan ensemble in Samarkand. Later in 2015, it was expanded to more cities and monuments and the laser scanning was conducted in Tashkent, Bukhara, Samarkand, and Shakhrisyabz. The monuments are from the Timurid Dynasty era and they are on the UNESCO World Heritage List. As a representative example, the research results obtained in Shakhrisabz (Uzbekistan) are discussed in detail. The Kok Gumbaz (Blue Dome) Mosque was scanned. It was built in 1437 and underwent several restorations and reinforcement efforts. A detailed finite element model of the monument was generated from the as-found geometry captured by laser scans. The physical properties of the monument were investigated by material tests. Based on the results of numerical simulations, recommendations on further reinforcement of the historic monument were developed.

APPLICATIONS OF ACTION CAM SENSORS IN THE ARCHAEOLOGICAL YARD

Abstract. In recent years, special digital cameras called “action camera” or “action cam”, have become popular due to their low price, smallness, lightness, strength and capacity to make videos and photos even in extreme environment surrounding condition. Indeed, these particular cameras have been designed mainly to capture sport actions and work even in case of dirt, bumps, or underwater and at different external temperatures. High resolution of Digital single-lens reflex (DSLR) cameras are usually preferred to be employed in photogrammetric field. Indeed, beyond the sensor resolution, the combination of such cameras with fixed lens with low distortion are preferred to perform accurate 3D measurements; at the contrary, action cameras have small and wide-angle lens, with a lower performance in terms of sensor resolution, lens quality and distortions. However, by considering the characteristics of the action cameras to acquire under conditions that may result difficult for standard DSLR cameras and because of their lower price, these could be taken into consideration as a possible and interesting approach during archaeological excavation activities to document the state of the places. In this paper, the influence of lens radial distortion and chromatic aberration on this type of cameras in self-calibration mode and an evaluation of their application in the field of Cultural Heritage will be investigated and discussed. Using a suitable technique, it has been possible to improve the accuracy of the 3D model obtained by action cam images. Case studies show the quality and the utility of the use of this type of sensor in the survey of archaeological artefacts.

The relationship between accurate 3D measurements of cartilage thickness and radiographic joint space width: data from the osteoarthritis initiative

Purpose: Radiographic joint space width (rJSW) measurement has historically been the preferred measure of disease progression in OA structure modification trials. Magnetic resonance imaging (MRI) can visualise change in multiple tissues, and offers increased sensitivity to change, allowing for shorter trials with fewer patients. In order to compare changes measured using MRI biomarkers with previous studies using rJSW, it is important to understand (1) how well the new biomarkers agree with radiographic measures, and (2) the contribution made by different knee tissues to changes in rJSW. This is challenging, as MR images are normally obtained with the patient in a supine position, while radiographs are taken in a standing position, with the knee slightly flexed. We aimed to assess agreement between accurate 3D MRI measurements of cartilage thickness in the medial tibiofemoral joint and quantified rJSW, in a large dataset. Methods: Analyses were performed on all knees from the Osteoarthritis Initiative at baseline, which had a KL grade and a recorded quantitative measure of joint space width (5726 knees). Cartilage in the central medial regions of the femur and tibia were automatically segmented using a method based on active appearance models, a form of statistical shape model. This method produces highly accurate segmentations (mean accuracy = 0.05 mm, 95th percentile = 0.45, meaning that all errors are less than the average edge length of one voxel). Average thickness of the central medial femur and tibia regions were added to provide a measure of total cartilage thickness. Radiographic JSW measures were provided by the OAI, and were generated using the algorithm-based method of Duryea; for this comparison, we selected a position approximately in the middle of the tibial plateau, also found to be the most responsive in clinical studies (JSW 225). Agreement between the methods was assessed using Bland Altman plots. Results: For KL grades 0, 1 and 2, the systematic bias between rJSW and cartilage thickness was around 1mm, the 95% confidence limits were 1, 1.2 and 1.26 mm respectively (Figure 1). Figure 1 also shows typical examples of MR images for each KL grade within the region in which MR and radiographic measures are taken. For KL grades 0–2, the agreement between measures is close to theoretical agreement limits, which we estimate to be 0.74 mm based on the measurement errors for the methods. In KL3 knees, the bias reduces to 0.4 mm, and the 95% CL increases to 1.69. In KL4 knees, the bias is almost eliminated (0.14 mm). Conclusions: rJSW is composed of various pathologies that are not visualised in radiographs, which consolidate all changes into a single projection. These include cartilage loss, meniscal extrusion and synovial fluid. In healthy knees, and in the early stages of the disease (KL 2), agreement between cartilage thickness and rJSW is excellent – as close as realistically possible in measures of biological systems. The systematic bias of 1mm in KL grade 0–2 knees is most likely contributed by the posterior portion of the medial meniscus which is located within the field of measurement. The decrease in bias at KL3 is probably caused by the increasing damage seen in the menisci, which are less able to keep the bones separate when the patient is standing. At KL4, the meniscus is probably incapable of keeping the bones separate when loaded. The high value for the 95% confidence limits at KL4 are likely due to multiple factors within the knee, including meniscal damage and changes in bone shape, together with the inability of the rJSW method to discriminate the bone edges, once they are very close to each other. Additionally, it can be difficult to position the flexed knee in the radiograph both accurately and reproducibly when there is no clear joint space width, and bone shapes within the joint are altered. There may in fact be 2 groups within the KL4 Bland Altman plot - one group towards the right probably still maintains some joint space width, and perhaps a more competent meniscus, and retains the bias of KL0 to KL3 knees.

Low-speed-camera-array-based high-speed three-dimensional deformation measurement method: Principle, validation, and application

High-resolution imaging devices are of interest in the development of accurate 3D vision systems. However, it is challenging to achieve a balance between the image capturing speed and resolution. The image capturing speed is relatively low for high-resolution imaging devices, which restricts their applications in high-speed 3D measurements. Therefore, a low-speed-camera-array imaging method for high-speed 3D deformation measurements is proposed. Compared with existing methods using high-speed imaging devices, it has the advantages of low cost and high flexibility achieved by combining low-speed cameras into a stereo camera array high-speed imaging system. In order to achieve accurate 3D measurements, a bundle-adjustment-principle-based system calibration method is proposed. Four experiments, including an accuracy experiment, repeatability experiment, vibration measurement of a plastic board, and out-of-plane displacement measurement of rotating blades, demonstrated the accuracy and effectiveness of the proposed method.

A detailed comparison of single-camera light-field PIV and tomographic PIV

This paper conducts a comprehensive study between the single-camera light-field particle image velocimetry (LF-PIV) and the multi-camera tomographic particle image velocimetry (Tomo-PIV). Simulation studies were first performed using synthetic light-field and tomographic particle images, which extensively examine the difference between these two techniques by varying key parameters such as pixel to microlens ratio (PMR), light-field camera Tomo-camera pixel ratio (LTPR), particle seeding density and tomographic camera number. Simulation results indicate that the single LF-PIV can achieve accuracy consistent with that of multi-camera Tomo-PIV, but requires the use of overall greater number of pixels. Experimental studies were then conducted by simultaneously measuring low-speed jet flow with single-camera LF-PIV and four-camera Tomo-PIV systems. Experiments confirm that given a sufficiently high pixel resolution, a single-camera LF-PIV system can indeed deliver volumetric velocity field measurements for an equivalent field of view with a spatial resolution commensurate with those of multi-camera Tomo-PIV system, enabling accurate 3D measurements in applications where optical access is limited.

Robotic Autoscanning of Highly Skewed Ship Propeller Blades

Abstract This paper presents an approach for a complete scanning of ship propeller blades. Inline quality control for advanced manufacturing or inspection of operational propeller blades requires automated and highly accurate 3D measurement systems. By combining an industrial robot with a high accuracy 3D camera and a laser distance sensor, the task at hand can be fully automated. The integrated system is described in detail, with focus on initial estimation of the propeller blade position and avoiding collisions with the blade while scanning. Validating experiments are performed where a highly skewed ship propeller blade is scanned with a resolution of 0.1 mm.

Three-dimensional measurement of object surface by using ellipse binary defocusing projection

BackgroundThe accuracy of three-dimensional measurement of object surface is always affected by the nonlinear gamma of the projector. The defocusing binary projection can overcome the nonlinear gamma distortion of the projector and reduce the effect of high harmonics without gamma calibration. Although researches have already reduced the errors to get a clear sinusoidal curve, there still leave room for improvement, especially when wide stripes are applied during the measurement.MethodsThis paper presents a kind of ellipse binary pattern. By analyzing the property and spectrum, the binary pattern can produce high quality sinusoidal curve and produce smaller errors. It has a better effect to use an ellipse pattern to overcome the nonlinear gamma distortion of the projector and it is suit to be used as wide stripes.ResultsSimulation and comparison experiments plus three-step phase shifted method are conducted to verify feasibility and accuracy of this binary pattern. The experimental results have indicated that this binary pattern can increase the accuracy of 3D measurement and reduce phase errors caused by the nonlinear gamma of the projector. The defocusing ellipse pattern is superior to traditional methods. In addition, the defocusing binary stripe is robust and suit to measure object with large period.ConclusionsIn this paper, an binary ellipse pattern is proposed for high-accuracy 3D measurement profilometry. It is easy to generate high-quality sinusoidal fringe pattern. Experiment results have demonstrated the feasibility and accuracy of the improved binary pattern besides it proved that the period of fringe pattern has little impact on the accuracy of measurement.

Monocular Stereo Measurement Using High-Speed Catadioptric Tracking.

This paper presents a novel concept of real-time catadioptric stereo tracking using a single ultrafast mirror-drive pan-tilt active vision system that can simultaneously switch between hundreds of different views in a second. By accelerating video-shooting, computation, and actuation at the millisecond-granularity level for time-division multithreaded processing in ultrafast gaze control, the active vision system can function virtually as two or more tracking cameras with different views. It enables a single active vision system to act as virtual left and right pan-tilt cameras that can simultaneously shoot a pair of stereo images for the same object to be observed at arbitrary viewpoints by switching the direction of the mirrors of the active vision system frame by frame. We developed a monocular galvano-mirror-based stereo tracking system that can switch between 500 different views in a second, and it functions as a catadioptric active stereo with left and right pan-tilt tracking cameras that can virtually capture 8-bit color images each operating at 250 fps to mechanically track a fast-moving object with a sufficient parallax for accurate 3D measurement. Several tracking experiments for moving objects in 3D space are described to demonstrate the performance of our monocular stereo tracking system.

3D Photogrammetric Reconstruction by Drone Scanning for FE Analysis and Crack Pattern Mapping of the “Bridge of the Towers”, Spoleto

Technological advances in the digital camera industry and computing resources make the use of photogrammetry a very fast, low-cost, contactless and non-destructive technique. It can represent a good alternative to obtain 3D information for monitoring and conservation of cultural heritage assets, especially where it is not possible to use 3D laser scanners and also in situations where areas to be inspected are not easily accessible [1]. Resolution generally depends on the number of images, their quality and the level of overlap between them, as well as hardware and software capabilities. Starting from 2D aerial or terrestrial photographic images, photogrammetry allows to reconstruct a 3D model in the form of a "point cloud" and also to derive accurate 3D measurements of large architectural elements.This paper is about stereo-photogrammetric scanning by drone performed by MENCI software s.r.l. aimed at the definition of the state of conservation of the “Bridge of the Towers” in Spoleto and its long term preservation without building scaffoldings. It was performed within the RoMA (Resilience enhancement of a Metropolitan Area) project, through an agreement between the “Italian National Agency for New Technologies, Energy and Sustainable Economic Development” (ENEA) and the “Italian Ministry of Cultural Heritage and Activities” (MIBACT).Photogrammetric scanning and FE modelling were applied within the project together with many other monitoring techniques in order to assess the bridge cracks pattern and its structural health by a multidisciplinary approach that allows their mutual validation [2].As one of the most important problems in the use of photogrammetric 3D reconstruction is the considerable demand in terms of hardware and software resources for images processing and data storage, thanks to the HPC (High Performance Computing) resources provided by the CRESCO infrastructure (Research Computational Centre on Complex Systems), it was possible to analyse and process a large amount of high-resolution photos in order to detect the crack pattern and to assess the actual damage state to be monitored over time [3].

Real-time microscopic 3D shape measurement based on optimized pulse-width-modulation binary fringe projection

In recent years, tremendous progress has been made in 3D measurement techniques, contributing to the realization of faster and more accurate 3D measurement. As a representative of these techniques, fringe projection profilometry (FPP) has become a commonly used method for real-time 3D measurement, such as real-time quality control and online inspection. To date, most related research has been concerned with macroscopic 3D measurement, but microscopic 3D measurement, especially real-time microscopic 3D measurement, is rarely reported. However, microscopic 3D measurement plays an important role in 3D metrology and is indispensable in some applications in measuring micro scale objects like the accurate metrology of MEMS components of the final devices to ensure their proper performance. In this paper, we proposed a method which effectively combines optimized binary structured patterns with a number–theoretical phase unwrapping algorithm to realize real-time microscopic 3D measurement. A slight defocusing of our optimized binary patterns can considerably alleviate the measurement error based on four-step phase-shifting FPP, providing the binary patterns with a comparable performance to ideal sinusoidal patterns. The static measurement accuracy can reach 8 μm, and the experimental results of a vibrating earphone diaphragm reveal that our system can successfully realize real-time 3D measurement of 120 frames per second (FPS) with a measurement range of in lateral and 8 mm in depth.

Colour imperfections in structured light projection for freeform measurement – a rapid test

ABSTRACTThree-dimensional (3D) optical measuring systems using structured light are often limited in practice by imperfections in the light patterns projected onto the object under test. An understanding of the fine structure of the patterns enables limits to be placed on the accuracy and precision of 3D measurements that can be obtained with such systems. A simple technique has been studied that reveals imperfections in patterns of parallel lines projected in white light. Colour fringing has been revealed using a reference grid in the form of a diffusely reflective grating or a lenticular array, to obtain magnification by means of moiré effects. The moiré magnifier gives a representation of the average error over an area. It is a very simple and robust device which may be convenient to use in an industrial production environment to provide a rapid check for the presence of colour fringing in light patterns with regular structures.

Three-dimensional proximal flow convergence automatic calculation for determining mitral valve area in rheumatic mitral stenosis.

Management of patients with mitral stenosis (MS) depends heavily on the accurate quantification of mitral valve area (MVA) using echocardiography. All currently used two-dimensional (2D) methods have limitations. Estimation of MVA using the proximal isovelocity surface area (PISA) method with real time three-dimensional (3D) echocardiography may circumvent those limitations. We aimed to evaluate the accuracy of 3D direct measurement of PISA in the estimation of MVA. Twenty-seven consecutive patients (median age of 63years; 77.8% females) with rheumatic MS were prospectively studied. Transthoracic and transesophageal echocardiography with 2D and 3D acquisitions were performed on the same day. The reference method for MVA quantification was valve planimetry after 3D-volume multiplanar reconstruction. A semi-automated software was used to calculate the 3D flow convergence volume. Compared to MVA estimation using 3D planimetry, 3D PISA showed the best correlation (rho=0.78, P<.0001), followed by pressure half-time (PHT: rho=0.66, P<.001), continuity equation (CE: rho=0.61, P=.003), and 2D PISA (rho=0.26, P=.203). Bland-Altman analysis revealed a good agreement for MVA estimation with 3D PISA (mean difference -0.03cm2 ; limits of agreement (LOA) -0.40-0.35), in contrast to wider LOA for 2D methods: CE (mean difference 0.02cm2 , LOA -0.56-0.60); PHT (mean difference 0.31cm2 , LOA -0.32-0.95); 2D PISA (mean difference -0.03cm2 , LOA -0.92-0.86). MVA estimation using 3D PISA was feasible and more accurate than 2D methods. Its introduction in daily clinical practice seems possible and may overcome technical limitations of 2D methods.

Accurate and fast 3D surface measurement with temporal-spatial binary encoding structured illumination.

Balancing the accuracy and speed for 3D surface measurement of object is crucial in many important applications. Binary encoding pattern utilizing the high-speed image switching rate of digital mirror device (DMD)-based projector could be used as the candidate for fast even high-speed 3D measurement, but current most schemes only enable the measurement speed, which limit their application scopes. In this paper, we present a binary encoding method and develop an experimental system aiming to solve such a situation. Our approach encodes one computer-generated standard 8 bit sinusoidal fringe pattern into multiple binary patterns (sequence) with designed temporal-spatial binary encoding tactics. The binary pattern sequence is then high-speed and in-focus projected onto the surface of tested object, and then captured by means of temporal-integration imaging to form one sinusoidal fringe image. Further the combination of phase-shifting technique and temporal phase unwrapping algorithm leads to fast and accurate 3D measurement. The systematic accuracy better than 0.08mm is achievable. The measurement results with mask and palm are given to confirm the feasibility.