Sub-pixel Technique Research Articles

Linking crown fire likelihood with post-fire spectral variability in Mediterranean fire-prone ecosystems

Background Fire behaviour assessments of past wildfire events have major implications for anticipating post-fire ecosystem responses and fuel treatments to mitigate extreme fire behaviour of subsequent wildfires. Aims This study evaluates for the first time the potential of remote sensing techniques to provide explicit estimates of fire type (surface fire, intermittent crown fire, and continuous crown fire) in Mediterranean ecosystems. Methods Random Forest classification was used to assess the capability of spectral indices and multiple endmember spectral mixture analysis (MESMA) image fractions (char, photosynthetic vegetation, non-photosynthetic vegetation) retrieved from Sentinel-2 data to predict fire type across four large wildfires Key results MESMA fraction images procured more accurate fire type estimates in broadleaf and conifer forests than spectral indices, without remarkable confusion among fire types. High crown fire likelihood in conifer and broadleaf forests was linked to a post-fire MESMA char fractional cover of about 0.8, providing a direct physical interpretation. Conclusions Intrinsic biophysical characteristics such as the fractional cover of char retrieved from sub-pixel techniques with physical basis are accurate to assess fire type given the direct physical interpretation. Implications MESMA may be leveraged by land managers to determine fire type across large areas, but further validation with field data is advised.

An end-to-end sea fog removal network using multiple scattering model.

An end-to-end sea fog removal network using multiple scattering model was proposed. In this network, the atmospheric multiple scattering model was re-formulated and used for sea fog removal. Compared with the atmospheric single scattering model, the atmospheric multiple scattering model could more comprehensively consider the effect of multiple scattering, which was important to the dense fog scenes, such as in ocean scene. Therefore, we used the atmospheric multiple scattering model to avoid image blurring. The model can directly generate the dehazing results, and unify the three parameters of the transmission map, the atmospheric light and the blur kernel into one formula. The latest smooth dilation and sub-pixel techniques were used in the network model. The latest techniques can avoid the gridding artifacts and the halo artifacts, the multi-scale sub-network was used to consider the features of multi-scale. In addition, multiple loss functions were used in end-to-end network. In the experimental results, the model was superior to the state-of-the-art models in terms of quantitatively and qualitatively.

A Review of Satellite Remote Sensing Techniques of River Delta Morphology Change

River deltas are important coastal depositional systems that are home to almost half a billion people worldwide. Understanding morphology changes in deltas is important in identifying vulnerabilities to natural disasters and improving sustainable planning and management. In this paper, we critically review literature on satellite remote sensing techniques that were used to study delta morphology changes. We identify and categorize these techniques into 3 major classes: (1) one-step change detection, 2) two-step change detection, and (3) ensemble classifications. In total, we offer a review of 18 techniques with example studies, and strengths and caveats of each. Synthesis of literature reveals that sub-pixel-based algorithms perform better than pixel-based ones. Machine learning techniques rank second to sub-pixel techniques, although an ensemble of techniques can be used just as effectively to achieve high feature detection accuracies. We also evaluate the performance of the 7 most commonly used techniques in literature on a sample of global deltas. Findings show the unsupervised classification significantly outperforms the others, and is recommended as a first-order delta morphological feature extraction technique in previously unknown, or, data sparse deltaic territories. We propose four pathways for future advancement delta morphological remote sensing: (1) utilizing high-resolution imagery and development of more efficient data mining techniques, (2) moving toward universal applicability of algorithms and their transferability across satellite platforms, (3) use of ancillary data in image processing algorithms, and (4) development of a global-scale repository of deltaic data for the sharing of scientific knowledge across disciplines.

Robust and High-Precision Vision System for Deflection Measurement of Crane Girder With Camera Shake Reduction

Crane girder is subject to alternating load in long time, which makes it prone to fatigue and fracture. Vision deflection measurement has become an effective method for structural health monitoring. However, crane girder does not always exhibit high contrast feature and artificial markers have to be pasted. Existing artificial markers cannot perform well in poor light conditions and the accuracy is limited. In this paper, we design an anti-interference marker by pasting a film onto a circular surface light source. The marker allows the vision system to work well under different light conditions. In addition, the reduction of camera shake and sub-pixel technique have been studied to improve the accuracy. According to the characteristics of the marker, corresponding image processing algorithms are also studied. Firstly, the designed marker can suppress environmental interference even at long distances with a wireless module and a small aperture. Secondly, the center of the marker in the image can be calculated using a sub-pixel detection technique. Finally, a camera shake reduction method is applied by using multiple reference markers and scale conversion factors are taken into account to get the physical displacement. To verify the measurement results, two experiments were carried out using a digital dial gauge sensor and laser displacement sensor for comparison. The experimental results proved the robustness and high precision of the vision system.

Spectral characteristics of talc and mineral abundance mapping in the Jahazpur Belt of Rajasthan, India using AVIRIS-NG data

ABSTRACT Spaceborne data with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Short Wave Infrared (SWIR) bands have been used worldwide in discriminating different clay minerals. However, due to wide bandwidth of ASTER sensors, it is difficult to distinguish between pure and impure variety of talc mineral. The present study highlights the efficacy of airborne hyperspectral data Airborne Visible/Infrared Imaging Spectrometer – Next Generation (AVIRIS-NG) for spectral analysis in talc mineral in the Jahazpur Belt of Bhilwara district in Rajasthan, India. Due to its better spectral and spatial resolution, AVIRIS-NG is found to be extremely useful in identifying talc mineral in SWIR region. Talc is characterized by doublet shaped spectral pattern in AVIRIS-NG image with strong absorption dips at 2289.75 nm and 2314.80 nm which could not be picked up in ASTER image. Spectral matching of talc pixels is verified with the field spectra of samples and available spectral library of United States Geological Survey, Jet Propulsion Laboratory, and International Geological Correlation Programme. Variation in reflectance percentage of different talc pixels is attributed to variation in chemical composition of talc. However, a large number of mixed pixels with spectral pattern of both talc and kaolinite minerals have been identified in AVIRIS-NG image which are found to be an impure variety of talc. Whole-pixel to sub-pixel classification techniques with Spectral Angle Mapper, Spectral Feature Fitting, Mixture Tuned Matched Filtering, and Linear Spectral Unmixing (LSU) techniques have been applied for talc abundance mapping in Jahazpur area. LSU is found to be the most useful and accurate technique in classifying spectrally pure to impure variety of talc.

MLC positioning verification for small fields: a new investigation into automatic EPID-based verification methods.

Multileaf-collimator (MLC) defined small fields in radiotherapy are used in high dose, ultra-conformal techniques such as stereotactic radiotherapy and stereotactic radiosurgery. Proximity to critical structures and irreversible damage arising from inaccurate delivery mean that correct positioning of the MLC system is of the utmost importance. Some of the existing techniques for MLC positioning quality assurance make use of electronic portal imaging device (EPID) images. However, conventional collimation verification algorithms based on the full width at half maximum (FWHM) fail when applied to small field images acquired by an EPID due to overlapping aperture penumbrae, lateral electron disequilibrium and radiation source occlusion. The objective of this study was to investigate sub-pixel edge detection and other techniques with the aim of developing an automatic and autonomous EPID-based method suitable for MLC positional verification of small static fields with arbitrary shapes. Methods investigated included derivative interpolation, Laplacian of Gaussian (LoG) and an algorithm based on the partial area effect hypothesis. None of these methods were found to be suitable for MLC positioning verification in small field conditions. A method is proposed which uses a manufacturer-specific empirically modified FWHM algorithm which shows improvement over the conventional techniques in the small field size range. With a measured mean absolute difference from planned position for Varian linacs of 0.01 ± 0.26mm, compared with the erroneous FWHM value of 0.70 ± 0.51mm. For Elekta linacs the proposed algorithm returned 0.26 ± 0.25mm, in contrast to the FWHM result of 1.79 ± 1.07mm.

MEASURING PHOTOGRAMMETRIC CONTROL TARGETS IN LOW CONTRAST IMAGES

Abstract: This paper presents an experimental assessment of photogrammetric targets and subpixel location techniques to be used with low contrast images such as images acquired by hyperspectral frame cameras. Eight different target patterns of varying shape, background, and size were tested. The aim was to identify an optimum distinctive pattern to serve as control point in aerial surveys of small areas using hyperspectral cameras when natural points are difficult to find in suitable areas. Three automatic techniques to identify the target point of interest were compared, which were weighted centroid, template matching, and line intersection. For assessment, hyperspectral images of the set of targets were collected in an outdoor 3D terrestrial calibration field. RGB images were also acquired for reference and comparison. Experiments were conducted to assess the accuracy at the sub-pixel level. Bundle adjustment with several images was used, and vertical and horizontal distances were directly measured in the field for verification. An experiment with aerial flight was also performed to validate the chosen target. The analysis of residuals and discrepancies indicated that a circular target is best suited as the ground control in aerial surveys, considering the condition in which the target appears with few pixels in the image.

Per-pixel and sub-pixel classifications of high-resolution satellite data for mangrove species mapping

High spatial resolution sensors such as IKONOS and QuickBird, are expected to classify mangrove species more accurately than coarse spatial resolution satellite images. Conventional per-pixel classification techniques could not improve the classification accuracy when such high-resolution images are applied. Such failure has encouraged the invention of more sophisticated and deterministic techniques i.e. subpixel classifications. In this study, the mangrove forest at Sungai Belungkor, Johor, Malaysia was classified using IKONOS data. Two classification approaches were applied, namely per-pixel and sub-pixel techniques. The conventional per-pixel classifiers used in this study were Maximum Likelihood (ML), Minimum Distance to Mean (MDM) and Contextual Logical Channel (CLC) while the Linear Mixture Model (LMM) was selected as the sub-pixel classification approach. The classification results revealed that the CLC classification with a contrast texture measure at window size 21 x 21 yielded the highest accuracy (82%) in comparison to the ML (68%) or MDM (64%). The spatial distribution of the classified mangrove species and classes coincided with the common mangrove zones in Malaysia. For the results of the LMM, the fraction of pixels measured from the satellite imagery and observed in the field gave a good correlation with an R2 value of 0.83 for Bakau minyak, a moderate correlation with an R2 of approximately 0.71 for Bakau kurap and an R2 of 0.75 for the ‘Others’ type of mangrove species. An error image was also created to compare the best fitting spectrum produced by the inversion of the LMM with the original observed spectrum, where the maximum RMS error was only 5%.

Assessing Land Cover Change and Anthropogenic Disturbance in Wetlands Using Vegetation Fractions Derived from Landsat 5 TM Imagery (1984–2010)

Anthropogenic disturbance of wetlands in Canada is extensive. In wetland monitoring programs disturbance assessment often relies on single date field and geo-spatial data, thereby rendering detection of the nature, timing and magnitude of disturbance events and trends difficult. The Landsat temporal archive provides potential for more comprehensive temporal change analysis. However, its 30 m pixel size may result in omission of small disturbances or lack of spatial precision near boundaries where change often occurs. Spectral mixture analysis (SMA) is a subpixel technique that has been used to assess change in a variety of land cover types, but rarely for non-coastal wetlands. This research utilized SMA vegetation, bare, and moisture fractions derived from 26 Landsat 5 Thematic Mapper (TM) scenes from 1984 to 2010 in two-date comparisons and time series analysis to assess disturbance in two wetland complexes in eastern Ontario. The 1984–2010 two-date analysis showed overall declines in vegetation and moisture fractions and increases in the bare fraction, while time series analysis over the 26 year period showed more variable inter-annual changes including years of sudden change, cyclic trends and more gradual trends. Anthropogenic disturbances that were identified included a lake created for recreational purposes, logged/cultivated areas, and wetland degradation due to encroaching development.

Three-dimensional planar object tracking with sub-pixel accuracy

Subpixel techniques are commonly used to increase the spatial resolution in tracking tasks. Object tracking with targets of known shape permits obtaining information about object position and orientation in the three-dimensional space. A proper selection of the target shape allows us to determine its position inside a plane and its angular and azimuthal orientation under certain limits. Our proposal is demonstrated both numerical and experimentally and provides an increase the accuracy of more than one order of magnitude compared to the nominal resolution of the sensor. The experiment has been performed with a high-speed camera, which simultaneously provides high spatial and temporal resolution, so it may be interesting for some applications where this kind of targets can be attached, such as vibration monitoring and structural analysis.

A LIKELY MICRO-QUASAR IN THE SHADOW OF M82 X-1

The ultra-luminous X-ray source (ULX) M82 X-1 is one of the most promising intermediate mass black hole candidates in the local universe based on its high X-ray luminosities ($10^{40}-10^{41} {\rm erg s^{-1}}$) and quasi-periodic oscillations, and is possibly associated with a radio flare source. In this work, applying the sub-pixel technique to the 120 ks Chandra observation (ID: 10543) of M82 X-1, we split M82 X-1 into two sources separated by 1.1$\arcsec$. The secondary source is not detected in other M82 observations. The radio flare source is found to associate not with M82 X-1, but with the nearby transient source S1 with an outburst luminosity of $\sim 10^{39} {\rm erg s^{-1}}$. With X-ray outburst and radio flare activities analogous to the recently discovered micro-quasar in M31, S1 is likely to be a micro-quasar hidden in the shadow of M82 X-1.

Edge Detection Technology and its Application in the Model Test of Vehicle Bridge Coupled Vibration

At present, point sensors are mainly used in the measurement of the vehicle and bridge coupled vibration system. And only limited points of the system are measured. Test results can not be compared with calculated and analyzed results in the full field of the system. In the model test of the vehicle bridge vibration, test video is captured with a camera, each frame of the video is processed by the sub-pixel edge detection techniques to identify the deformation of the beam at each time, and the time history of the vibration response at each point of the beam. The model test results are compared with the calculated results of the vehicle bridge coupled vibration system. It is shown in the comparison that the beam deformation curves of both the test and analysis at each time are agreed quite good, and the time history responses of each point of the beam are also in good agreement. It means that the application of the video detection techniques in the vehicle and bridge coupled vibration model test is successful. Taking into account the advantages of the full field high density measurement and the synchronized measurement of the moving vehicle and bridge, further research and extension on the video detection technology are necessary.

亚像素采样对LED显示质量的提升

Clear display of high-resolution pictures on LED displays is one of key problems to be solved in displaying quality improvement areas.Aiming at the features of LED displays,such as low-resolutions,high production costs and flexible in pixels arrangements,applications of sub-pixel sampling technique were analyzed.Firstly,the mathematical models of LED displays were set up from the information processing perspective.Then under theluminance and chrominance separated space such as YUV color space,by frequency analysis,it was demonstrated that aliasing in display images can be transferred from the luminance to the chrominance which was insensitive to human eyes.The Nyquist limits of them in all directions were effectively expanded.The simulation results on the real LEDdisplay further show the improvement of displaying quality by sub-pixel sampling.

Vision-Based Displacement Sensor for Monitoring Dynamic Response Using Robust Object Search Algorithm

This paper develops a vision-based displacement measurement system for remote monitoring of vibration of large-size structures such as bridges and buildings. The system consists of one or multiple video cameras and a notebook computer. With a telescopic lens, the camera placed at a stationary point away from a structure captures images of an object on the structure. The structural displacement is computed in real time through processing the captured images. A robust object search algorithm developed in this paper enables accurate measurement of the displacement by tracking existing features on the structure without requiring a conventional target panel to be installed on the structure. A sub-pixel technique is also proposed to further reduce measurement errors cost-effectively. The efficacy of the vision system in remote measurement of dynamic displacements was demonstrated through a shaking table test and a field experiment on a long-span bridge.

Comparison and Simulation of Subpixel Imaging Modes for Linear CCD

Subpixel technique of linear CCD is effective to enhance the spatial resolution without increasing the focal length of optics and reducing the pixel size. To compare image quality of two main subpixel imaging modes, quincunx sampling and four-point sampling, a method to quantitatively evaluate image quality of subpixel based on MTF was proposed. The MTF of quincunx and four-point sampling modes were derived. Analytical results shows that theoretical limiting resolution of quincunx sampling and four-point sampling is improved to 1.4 and 1.86 times respectively, and MTF values at Nyquist frequency of two modes are increased by 0.1106 and 0.1679, respectively. MTFA in (0, 0.5) of two subpixel imaging modes were calculated and results illustrates that four-point sampling offers much more improvement with image quality than quincunx sampling, at the cost of double amount of data. A model for simulating subpixel imaging using Matlab was established, and simulation results of spoke target verify the theoretical analysis.

Measurement of wide frequency range structural microvibrations with a pocket digital camera and sub-pixel techniques

Analysis of vibrations and displacements is a hot topic in structural engineering. Although there is a wide variety of methods for vibration analysis, direct measurement of displacements in the mid and high frequency range is not well solved and accurate devices tend to be very expensive. Low-cost systems can be achieved by applying adequate image processing algorithms. In this paper, we propose the use of a commercial pocket digital camera, which is able to register more than 420 frames per second (fps) at low resolution, for accurate measuring of small vibrations and displacements. The method is based on tracking elliptical targets with sub-pixel accuracy. Our proposal is demonstrated at a 10 m distance with a spatial resolution of 0.15 mm. A practical application over a simple structure is given, and the main parameters of an attenuated movement of a steel column after an impulsive impact are determined with a spatial accuracy of 4 µm.

Adjusted subpixel method enables optimisation of bronchial measurements in high-resolution CT

The purpose of this paper is to present a new technique of high-resolution computed tomography (HRCT) bronchial measurement, which was worked out in our centre and is based on a mathematically adjusted subpixel method (ASM). This study used a bronchial tree model comprising two polyvinyl chloride/rubber pipes with various densities placed in a larger plastic object. External and internal diameters of both pipes corresponded to the sizes of structures that are usually evaluated in HRCT scans (i.e. the bronchus with accompanying blood vessel). The model was subjected to repeated HRCT measurements of the external and internal diameters of model pipes using a subpixel technique and ASM. Significant differences were observed between mean values of external and internal diameters of model pipes determined by means of subpixel method or ASM, with the latter being the ones that resembled real diameters of the model pipes more closely. Mean external diameters of the larger and smaller model pipes determined by ASM differed from the real diameters by 0.23% and 0.57%, respectively. The internal diameter of the larger pipe was on average 25.96% larger, whereas the internal diameter of the smaller pipe was 48.05% smaller than the real diameter. The technique of non-linear prediction of HRCT bronchial measurements described in this paper raises attractive prospects in its clinical application owing to potential improvements in the objectivity and reproducibility of bronchial wall measurements.

An experimental technique to measure the capillary waves in electrified microjets

Backlight optical imaging is an experimental technique with an enormous potential in microfluidics to study very varied fluid configurations and phenomena. In this paper, we show the capability of this technique to precisely characterize the capillary waves growing in electrified microjets. For this purpose, images of electrified liquid jets formed by electrospray were acquired and processed using a sub-pixel resolution technique. Our results reflect the validity and usefulness of optical imaging for this type of application.

Modeling Urban Landscape Dynamics Using Subpixel Fractions and Fuzzy Cellular Automata

This study proposes a fuzzy cellular automata model based on the subpixel fractions extracted from multitemporal satellite images and discusses the relationship between sophisticated remote sensing techniques and an urban process model within the socioeconomic dimension. Accordingly, the major objectives of the present research are: (1) to incorporate the subpixel membership derived from remote sensing images into a fuzzy cellular automata model to simulate urban landscape change; (2) to standardize the quantitative method to incorporate subpixel information into a subcell cellular automata model and to find a better way to determine the parameters in the model's development, calibration, and validation. The comparison between the traditional cell-based model and subcell model suggests that the subpixel technique improves the accuracy of both urban mapping and modeling using medium resolution satellite images.

Antialiasing Filter Design for Subpixel Downsampling via Frequency-Domain Analysis

In this paper, we are concerned with image downsampling using subpixel techniques to achieve superior sharpness for small liquid crystal displays (LCDs). Such a problem exists when a high-resolution image or video is to be displayed on low-resolution display terminals. Limited by the low-resolution display, we have to shrink the image. Signal-processing theory tells us that optimal decimation requires low-pass filtering with a suitable cutoff frequency, followed by downsampling. In doing so, we need to remove many useful image details causing blurring. Subpixel-based downsampling, taking advantage of the fact that each pixel on a color LCD is actually composed of individual red, green, and blue subpixel stripes, can provide apparent higher resolution. In this paper, we use frequency-domain analysis to explain what happens in subpixel-based downsampling and why it is possible to achieve a higher apparent resolution. According to our frequency-domain analysis and observation, the cutoff frequency of the low-pass filter for subpixel-based decimation can be effectively extended beyond the Nyquist frequency using a novel antialiasing filter. Applying the proposed filters to two existing subpixel downsampling schemes called direct subpixel-based downsampling (DSD) and diagonal DSD (DDSD), we obtain two improved schemes, i.e., DSD based on frequency-domain analysis (DSD-FA) and DDSD based on frequency-domain analysis (DDSD-FA). Experimental results verify that the proposed DSD-FA and DDSD-FA can provide superior results, compared with existing subpixel or pixel-based downsampling methods.