Sub-pixel Accuracy Research Articles

Robust and adaptive band-to-band image transform of UAS miniature multi-lens multispectral camera

Utilizing miniature multispectral (MS) or hyperspectral (HS) cameras by mounting them on an Unmanned Aerial System (UAS) has the benefits of convenience and flexibility to collect remote sensing imagery for precision agriculture, vegetation monitoring, and environment investigation applications. Most miniature MS cameras adopt a multi-lens structure to record discrete MS bands of visible and invisible information. The differences in lens distortion, mounting positions, and viewing angles among lenses mean that the acquired original MS images have significant band misregistration errors. We have developed a Robust and Adaptive Band-to-Band Image Transform (RABBIT) method for dealing with the band co-registration of various types of miniature multi-lens multispectral cameras (Mini-MSCs) to obtain band co-registered MS imagery for remote sensing applications. The RABBIT utilizes modified projective transformation (MPT) to transfer the multiple image geometry of a multi-lens imaging system to one sensor geometry, and combines this with a robust and adaptive correction (RAC) procedure to correct several systematic errors and to obtain sub-pixel accuracy. This study applies three state-of-the-art Mini-MSCs to evaluate the RABBIT method’s performance, specifically the Tetracam Miniature Multiple Camera Array (MiniMCA), Micasense RedEdge, and Parrot Sequoia. Six MS datasets acquired at different target distances and dates, and locations are also applied to prove its reliability and applicability. Results prove that RABBIT is feasible for different types of Mini-MSCs with accurate, robust, and rapid image processing efficiency.

High-speed and high-accuracy 3D surface measurement using a mechanical projector.

This paper presents a method to achieve high-speed and high-accuracy 3D surface measurement using a custom-designed mechanical projector and two high-speed cameras. We developed a computational framework that can achieve absolute shape measurement in sub-pixel accuracy through: 1) capturing precisely phase-shifted fringe patterns by synchronizing the cameras with the projector; 2) generating a rough disparity map between two cameras by employing a standard stereo-vision method using texture images with encoded statistical patterns; and 3) utilizing the wrapped phase as a constraint to refine the disparity map. The projector can project binary patterns at a speed of up to 10,000 Hz, and the camera can capture the required number of phase-shifted fringe patterns with 1/10,000 second, and thus 3D shape measurement can be realized as high as 10,000 Hz regardless the number of phase-shifted fringe patterns required for one 3D reconstruction. Experimental results demonstrated the success of our proposed method.

Experimental methodology for the measurement of plasticity on metals at high strain-rates

An experimental methodology has been developed for the tensile characterisation of ductile isotropic metals at high strain-rate. This study includes the region beyond plastic instability or necking, which is rarely analysed for conventional applications. The research explores an imaging technique used to track the geometry of the specimen during tensile tests and calculate true local values of stress and strain by applying Bridgman theory [1]. To improve the quality of the images taken at high strain-rate an in-situ high speed shadowgraph technique has been developed, and to obtain better results from the images a sub-pixel accuracy edge detection algorithm has been implemented. The technique has been applied to an austenitic stainless steel. Its tensile behaviour has been assessed by testing round samples at strain-rates ranging from quasi-static to ~103 s-1. The results obtained with the proposed methodology have been validated by comparison with more conventional techniques such as video-extensometer and digital image correlation in the pre-necking region and good performance even at the highest strain-rate tested has been proved.

Measurement System of Circler Diameter

Sub-pixel measurement system can, to some extent, compensate the margin positioning error caused by hardware limitation. A kind of sub-pixel measurement system of circle outer diameter based on Zernike moment is established. Sub-pixel edge detection algorithm of circular workpiece based on Zernike moment is proposed and further positioning of edge points in the gradient direction by the adoption of quadratic polynomial interpolation is conducted. Finally, circle outer diameter measurements of sub-pixel accuracy are obtained by using least square method fitting the edge points. The experiment results show that the measurement accuracy of system is high, which meets the requirement of high accuracy measurement of circular workpiece.

Correlation-coefficient Based Ring-filtering Method for Centering a Non-diffracting Fringe Image

A correlation-coefficient based ring-filtering method for centering a concentric circle fringe is proposed. The image of a non-diffracting beam, whose cross section is a concentric circle fringe, is recorded by a matrix array image sensor and downloaded onto a computer. The image is then processed by ring-filtering, with a center position in the zeroth order interference area of the non-diffracting image. Next, we change the center position of the ring-filtering and calculate the coherence coefficient between the filtered before and after images. The center of the non-diffracting image can be calculated by finding the position of the peak value of the dimensionless coherence coefficient. Because all the intensity distribution information of the non-diffracting beam is used for the calculation in the process of ring-filtering, the effect of random noise or a non-circular fringe on the centering is greatly reduced, and the center position resolution can achieve subpixel accuracy. The measurement of the spatial angle is discussed.

Effect of different wall materials and thermal-barrier coatings on the flame-wall interaction of laminar premixed methane and propane flames

We investigate the flame-wall interaction of laminar, premixed methane/air and propane/air flames in a stationary, sidewall-quenching configuration over a wide range of equivalence ratios. Both, the wall material (stainless-steel, cast iron, aluminum), the type of thermal-barrier coating (soot, zirconium dioxide, titanium dioxide) and its thickness (only for zirconium dioxide) were systematically varied. The flame-wall interaction is characterized by the measurement of quenching distances, which are based on the separate analysis of chemiluminescence images of electronically excited OH* and CH* radicals. A method has been developed to determine the position of the wall in the chemiluminescence images with sub-pixel accuracy. It turns out, that while the quenching distance strongly changes with stoichiometry and fuel, it only weakly depends on the wall material and type of coating, although the quenching distance tends to decreases with increasing thickness of the ceramic coating. Overall, the data confirms present scaling laws with a quenching Peclet number of PeQ ≈ 7.5. Quenching distances determined independently for OH* and CH* differ more strongly than the distances of peak mole fractions in an undisturbed flame, without the influence of the wall.

Accuracy improvement of high resolution satellite image georeferencing using an optimized line-based rational function model

ABSTRACTThe georeferencing procedure of high resolution satellite images (HRSIs) using proper mathematical models is an important step in 3D spatial information extraction. Since the 2000s, line-based mathematical models are considered more in photogrammetric communities. This could be due to the unique characteristics of linear features such as more reliable procedure of automatic matching as well as abundance of linear features in satellite images. In addition, the irreplaceable characteristics of Rational Function Models (RFM) such as generality and its independence to the sensor model makes it a proper mathematical model for this purpose. Although, traditional selection of the best order and terms’ combination of RFM due to the deficiency of physical interpretation of terms is not easily feasible. Hence, in this article, an optimization algorithm based on a binary particle swarm optimization (PSO) is developed to determine the optimum uncorrelated terms’ combination of a line-based RFM. For this purpose, a population of particles (representative of different RFM structures) is initialized randomly with a string of binary values, indicating the presence or omission of the corresponding terms. This is followed by directly using some conjugate lines in the image and object spaces as ground control lines (GCLs) to solve the unknown parameters of the RFM for each particle. The root mean square error (RMSE) of some check points (CPs) for each particle is considered to be its cost function and used to update the velocity of particles. The procedure is repeated to reach a stopping criteria. A comprehensive evaluation on the proposed model in comparison to the traditional line-based RFM is examined. Two different HRSIs (a GeoEye and an Ikonos Images) over different areas of Iran are used for this purpose. Based on the results, a traditional line-based RFM could not reach to sub-pixel accuracy. In contrast, the results show the potential of the proposed optimized line-based RFM to increase the accuracy to better than 0.8 pixel as well as reduce the systematic errors and the number of required control information, significantly.

Analysis and correction of errors in nanoscale particle tracking using the Single-pixel interior filling function (SPIFF) algorithm

Particle tracking, which is an essential tool in many fields of scientific research, uses algorithms that retrieve the centroid of tracked particles with sub-pixel accuracy. However, images in which the particles occupy a small number of pixels on the detector, are in close proximity to other particles or suffer from background noise, show a systematic error in which the particle sub-pixel positions are biased towards the center of the pixel. This “pixel locking” effect greatly reduces particle tracking accuracy. In this report, we demonstrate the severity of these errors by tracking experimental (and simulated) imaging data of optically trapped silver nanoparticles and single fluorescent proteins. We show that errors in interparticle separation, angle and mean square displacement are significantly reduced by applying the corrective Single-Pixel Interior Filling Function (SPIFF) algorithm. Our work demonstrates the potential ubiquity of such errors and the general applicability of SPIFF correction to many experimental fields.

Calibration, Projection, and Final Image Products of MESSENGER’s Mercury Dual Imaging System

We present an overview of the operations, calibration, geodetic control, photometric standardization, and processing of images from the Mercury Dual Imaging System (MDIS) acquired during the orbital phase of the MESSENGER spacecraft’s mission at Mercury (18 March 2011–30 April 2015). We also provide a summary of all of the MDIS products that are available in NASA’s Planetary Data System (PDS). Updates to the radiometric calibration included slight modification of the frame-transfer smear correction, updates to the flat fields of some wide-angle camera (WAC) filters, a new model for the temperature dependence of narrow-angle camera (NAC) and WAC sensitivity, and an empirical correction for temporal changes in WAC responsivity. Further, efforts to characterize scattered light in the WAC system are described, along with a mosaic-dependent correction for scattered light that was derived for two regional mosaics. Updates to the geometric calibration focused on the focal lengths and distortions of the NAC and all WAC filters, NAC–WAC alignment, and calibration of the MDIS pivot angle and base. Additionally, two control networks were derived so that the majority of MDIS images can be co-registered with sub-pixel accuracy; the larger of the two control networks was also used to create a global digital elevation model. Finally, we describe the image processing and photometric standardization parameters used in the creation of the MDIS advanced products in the PDS, which include seven large-scale mosaics, numerous targeted local mosaics, and a set of digital elevation models ranging in scale from local to global.

A Sub-Pixel Edge Detector: an Implementation of the Canny/Devernay Algorithm

An image edge detector is described which produces chained edge points with sub-pixel accuracy. The method incorporates the main ideas of the classic Canny and Devernay algorithms. The analysis shows that a slight modification to the original formulation improves the accuracy of the edge points.

Subpixel phase-based image registration using Savitzky–Golay differentiators in gradient-correlation

This paper presents a new two-step method for finding two-dimensional translational shifts with subpixel accuracy. This algorithm can measure subpixel shifts, even in images with few features, or in noisy images where many existing algorithms fail. In the first step of the algorithm (the integer part), the noise-robustness of the gradient correlation methods was improved by replacing central difference differentiators with Savitzky–Golay differentiators (SGDs). In the second step of the algorithm (the subpixel part), several modifications have been proposed to increase the accuracy and noise-robustness of phase-based methods for finding subpixel shifts. Moreover, two error metrics were introduced to quantify the output accuracy of the integer and subpixel parts of the algorithm.Comprehensive tests were conducted on 2400 standard 128 pixel × 128 pixel subimages subjected to synthetic shifts and rotations. Tests showed that the accuracy of the proposed method for finding translational shifts is of the order of a few ten-thousandths of a pixel, which is a substantial improvement over other state-of-the-art methods. For the rotation tests, the method outperformed comparable techniques. Furthermore, results showed that the proposed method generally provides better performance than other competing methods when images contained Gaussian or salt and pepper noise. The proposed method can be used in applications where high accuracy, robustness to noise, and/or computation efficiency are important.

Discontinuous Digital Image Correlation to reconstruct displacement and strain fields with discontinuities: Dislocation approach

Reconstruction of strain and displacement fields from surface images of materials and structures in the presence of discontinuities is a challenging task. Digital Image Correlation (DIC) – a commonly used technique to reconstruct displacement and strain fields when deformation is continuous – fails in the presence of discontinuities, including cracks and crevices. This paper presents a novel and entirely automated technique, Discontinuous Digital Image Correlation (DDIC), to reconstruct displacement and strain fields with high accuracy from images when deformation is either continuous or discontinuous. The technique is based on introducing additional parameters that characterize the discontinuity: the direction of the tangent to the discontinuity line and the corresponding Burgers vectors which express the difference in displacements at the opposite sides of the discontinuity line. The proposed technique is validated using synthetic images as well as images obtained from laboratory experiments. The results show that DDIC is able to reconstruct the displacement fields around discontinuities with a subpixel accuracy close to 1/100th of a pixel with a suitable surface pattern. It is also able to recover the size and angle of the discontinuity.

Three-line structured light measurement system and its application in ball diameter measurement

This paper proposes a three-dimensional measurement system of three-line structured light for ball diameter. In the system, firstly, the BP neural network is used to correct the distortion model of the camera. Zhang Zhenyou’s plane calibration method is adopted to get the camera's intrinsic and extrinsic parameters. Secondly, a sub-pixel extraction method for the center of the three-line structured light stripe is put forward to achieve sub-pixel accuracy, which is integrated by the adaptive median filtering method, threshold segmentation method, mean square gray gradient method and weighted gray center of gravity method. Finally, a three-line structured light three-dimensional measurement platform is developed, and the ball diameter measurement is carried out. It shows that the proposed method is valid, flexible and with high precision by experiment, and the three-line structured light three-dimensional measurement system can meet the requirements of high-precision and speed.

Spatially and temporally resolved regression rate measurements for the combustion of paraffin wax for hybrid rocket motor applications

Spatially and temporally resolved regression rate measurements were obtained for paraffin wax combustion in a two-dimensional slab-type rocket motor configuration using an optically accessible chamber. The use of high-intensity backlighting and a one-dimensional gray moment edge detection allowed for sub-pixel accuracy for the regression rate of the fuel-grain surface. At higher oxidizer (gaseous oxygen) mass fluxes approximately 50% of the fuel grain surface experienced a relatively uniform regression rate (±15%) and an increasing trend with streamwise position. For lower mass fluxes more scatter was observed and the average regression rate decreased slightly along the length of the grain. The spatially averaged regression rate was found to be constant after approximately two seconds of an ignition transient. A novel method of removing the ignition wires at the slab's leading edge to avoid artificial flow obstruction was demonstrated. Visualization of the entire transient ignition process as well as flame propagation along the slab length was obtained. The streamwise pressure gradient of the gaseous oxidizer was controlled by modifying the combustor cross-sectional area. Favorable pressure gradients were correlated to an amplification of the regression rate. Increased convective heat transfer associated with a low shape factor typical of a favorable pressure gradient boundary layer was proposed as the mechanism responsible for the increased regression rate. Flame progression rate was measured for various flow conditions and was shown to increase with increasing axial mass flux.

Velocidades superficiales del glaciar Upsala, Andes Patagónicos Sur, mediante el uso de correlación cruzada en imágenes satelitales: periodo 2013-2014.

In this study we present surface velocities estimation for the Upsala glacier catchment, South Patagonian Ice Field (SPI) during the summer season of years 2013 (January-March) and 2014 (March-April), including the Bertacchi, Cono, and Murallón tributaries using satellite images from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). The Cross-Correlation method was applied by COSI-Corr technique with sub-pixel accuracy. In general, it should be noted that the SPI glaciers, and Upsala glacier in particular, are fast-flowing ice bodies, which makes the technique works properly. Results of surface velocities estimation ranged from 0.22 to 2.93 md-1 for January-March 2013 and 0.12 to 5.8 md-1 for March-April 2014. In summary, COSI-Corr can achieved accurate and reliable results for glacier displacements and surface velocities estimation, also contributing in the better knowledge of the velocities change processes in time, taking into account Upsala is one of the most dynamic temperate glaciers of the SPI.

Robust geometric, phase and colour structured light projection system calibration

This paper introduces a new comprehensive procedure for both geometric and colour calibration of structured light system. In order to perform both geometric and colour calibration procedure, a new calibration artifact is proposed. The intrinsic and extrinsic parameters of projector and camera are estimated by using an extended pinhole camera model with a tangential and radial distortion. Camera image plane coordinates are obtained by extracting features from images of a calibration artifact. Projector image plane coordinates are calculated on the basis of continuous phase maps obtained from a fringe pattern phase reconstruction procedure. In order to stereo calibrate camera-projector system, pairs of corresponding image plane points are calculated with subpixel accuracy. In addition, one of three pattern views is used in colour calibration. RGB values of a colour field pattern detected by camera and their reference values are compared. This comparison leads to derivation of a colour transformation matrix. The performance of the proposed method is tested by measuring plane, sphere and distance reference. Also 360 degrees complex object 3D model from a set of measurements is obtained. Residual mean errors for all tests performed are calculated.

Efficiency of Bilateral Filter Application in Problems of Optical Flow Calculation

It is proposed to use weight coefficients of the bilateral filter to calculate the measure of similarity of image regions in optical flow calculation algorithms. The efficiency of using this measure is studied by an example of a three-dimensional recursive search algorithm. Weight coefficients are used for calculating the optical flow with subpixel accuracy by the Lucas–Kanade algorithm. A possibility of reducing the optical flow calculation error by means of using the proposed approaches is demonstrated by an example of processing of various types of images. A method of choosing the parameters of the weight functions of the bilateral filter is described and analyzed.

High-Accuracy Subpixel Image Registration With Large Displacements

This paper aims to achieve computationally efficient and high-accuracy subpixel image registration with large displacements under the rotation–scale–translation model. This paper employs the classical phase correlation algorithm and the Lucas–Kanade (LK) algorithm in a two-stage coarse-to-fine framework, for which the motivation is from the observation that the two algorithms exhibit strong complementary property between convergence range and subpixel accuracy. In this framework, the LK algorithm will also become computationally efficient owing to the small residual displacement. On the other hand, this paper takes into account the residual model with respect to the compensation scheme explicitly, and deduces formulas for the final results combination, which is expected to be closer to the true displacement vector and thus further improve the estimation accuracy. Since the compensation can be applied to either the target image or the reference image, two algorithms are presented accordingly, and analysis as well as comparison are also performed. Finally, both simulations and real image experiments are performed to verify the motivation, and the results are consistent with the analysis.

Sub-pixel Measurement System of Circle Outer Diameter Based on Zernike Moment

Sub-pixel measurement system can to some extent compensate the margin positioning error caused by hardware limitation. A kind of sub-pixel measurement system of circle outer diameter based on Zernike moment is established. Sub-pixel edge detection algorithm of circular workpiece based on Zernike moment is proposed and further positioning of edge points in the gradient direction by the adoption of quadratic polynomial interpolation is conducted. Finally, circle outer diameter measurements of sub-pixel accuracy are obtained by using least square method fitting the edge points. The experiment results show that the measurement accuracy of system is high, which meets the requirement of high accuracy measurement of circular workpiece.

Evaluating Fourier Cross-Correlation Sub-Pixel Registration in Landsat Images

Multi-temporal analysis is one of the main applications of remote sensing, and Landsat imagery has been one of the main resources for many years. However, the moderate spatial resolution (30 m) restricts their use for high precision applications. In this paper, we simulate Landsat scenes to evaluate, by means of an exhaustive number of tests, a subpixel registration process based on phase correlation and the upsampling of the Fourier transform. From a high resolution image (0.5 m), two sets of 121 synthetic images of fixed translations are created to simulate Landsat scenes (30 m). In this sense, the use of the point spread function (PSF) of the Landsat TM (Thematic Mapper) sensor in the downsampling process improves the results compared to those obtained by simple averaging. In the process of obtaining sub-pixel accuracy by upsampling the cross correlation matrix by a certain factor, the limit of improvement is achieved at 0.1 pixels. We show that image size affects the cross correlation results, but for images equal or larger than 100 × 100 pixels similar accuracies are expected. The large dataset used in the tests allows us to describe the intra-pixel distribution of the errors obtained in the registration process and how they follow a waveform instead of random/stochastic behavior. The amplitude of this waveform, representing the highest expected error, is estimated at 1.88 m. Finally, a validation test is performed over a set of sub-pixel shorelines obtained from actual Landsat-5 TM, Landsat-7 ETM+ (Enhanced Thematic Mapper Plus) and Landsat-8 OLI (Operation Land Imager) scenes. The evaluation of the shoreline accuracy with respect to permanent seawalls, before and after the registration, shows the importance of the registering process and serves as a non-synthetic validation test that reinforce previous results.