Photometric Changes Research Articles

DaLI: Deformation and Light Invariant Descriptor

Recent advances in 3D shape analysis and recognition have shown that heat diffusion theory can be effectively used to describe local features of deforming and scaling surfaces. In this paper, we show how this description can be used to characterize 2D image patches, and introduce DaLI, a novel feature point descriptor with high resilience to non-rigid image transformations and illumination changes. In order to build the descriptor, 2D image patches are initially treated as 3D surfaces. Patches are then described in terms of a heat kernel signature, which captures both local and global information, and shows a high degree of invariance to non-linear image warps. In addition, by further applying a logarithmic sampling and a Fourier transform, invariance to photometric changes is achieved. Finally, the descriptor is compacted by mapping it onto a low dimensional subspace computed using Principal Component Analysis, allowing for an efficient matching. A thorough experimental validation demonstrates that DaLI is significantly more discriminative and robust to illuminations changes and image transformations than state of the art descriptors, even those specifically designed to describe non-rigid deformations.

Comparison of local descriptors for automatic remote sensing image registration

Optical remote sensing (RS) images captured in different conditions might exhibit nonlinear changes. The registration of theses image is an important process. In this paper, we evaluate the performance of the three most successful state-of-the-art descriptors in a feature-based registration process. We have separated the detector from the descriptor as their performance depends on the position of the detected features. The descriptors are compared according to their Recall and runtime efficiency and these deals with several geometric and photometric changes. We also proposed an optimization to the SURF algorithm for color images, called O-SURF, which is a combination of the MSER detector and the SURF descriptor. The results show the effectiveness of proposed improvements compared to base SURF version. Finally, based on the test results, we propose an approach to register automatically optical RS images with subpixel accuracy.

A new look at the long-period eclipsing binary V383 Scorpii

V383Sco was discovered to be an eclipsing binary at the beginning of the XX century. This system has one of the longest orbital periods known (13.5yr) and was initially classified as a zet_Aur-type variable. It was then forgotten for decades. This study provides a detailed look at the V383Sco, using new data obtained around the last eclipse in 2007/8. There was a suspicion that this system could be similar to eclipsing systems with extensive dusty disks like EECep and eps_Aur. This and other, alternative hypotheses are considered. The ASAS-3 VI light curves have been used to examine photometric changes. Low-(LRS) and high-res.(HRS) spectra have been used for spectral classification, to analyse line profiles, as well as to determine the reddening, radial velocities (RVs) and distance. The SED was analysed. Using original numerical code, we performed a simplified model of the eclipse, taking into account the pulsations of one of the components. The LRS shows traces of molecular bands, characteristic of an M-type supergiant. The presence of this star in the system is confirmed by SED, by a strong dependence of the eclipse depth on the photometric bands, and by pulsational changes. The presence of a low excitation nebula around the system has been inferred from [OI] 6300A emission. Analysis of the RVs, reddening, and P-L relation for Mira-type stars imply a distance to the V383Sco of 8.4+-0.6 kpc. The distance to the nearby V381Sco is 6.4+-0.8 kpc. The very different and oppositely directed RVs of these systems (89.8 vs -178.8 km/s) seem to be in agreement with a bulge/bar kinematic model of the Galactic centre and inconsistent with purely circular motion. We have found evidence for the presence of a pulsating M-type supergiant in the V383Sco which periodically obscures the much more luminous F0I-type star, causing the deep (possibly total) eclipses which vary in duration and shape.

A robust watermark authentication technique based on Weber’s descriptor

One of the major challenges in the field of digital image watermarking is to authenticate the presence of watermark in the watermarked image even after it has been transformed intentionally or unintentionally. Transformation can be geometric-like rotation, scaling, and translation of image or may be due to any signal processing attack like noise corruption, compression, and cropping of image. There may also be some photometric changes, for example change in the brightness of watermarked image during transmission, due to which it becomes difficult to validate whether received image is watermarked or not. Illumination invariance property of Weber’s descriptor has engrossed to use it in the proposed watermark authentication technique. Weber’s descriptor is a descriptor based on two parameters of a pixel, differential excitation and orientation. These parameters are computed using the relative intensity value of neighbor pixels and current pixel. This descriptor remains the same even after intensity changes due to the contribution of all neighbor pixel’s intensity in its computation. It is also known to be robust to scaling and rotation. Experimental results show that the proposed watermarking technique is able to authenticate the presence of watermark in the watermarked image even when it is distorted due to geometric and photometric attacks. In addition to this, it is found to be robust against noise, cropping, and compression attacks.

A study on local photometric models and their application to robust tracking

Since modeling reflections in image processing is a difficult task, most computer vision algorithms assume that objects are Lambertian and that no lighting change occurs. Some photometric models can partly answer this issue by assuming that the lighting changes are the same at each point of a small window of interest. Through a study based on specular reflection models, we explicit the assumptions on which these models are implicitly based and the situations in which they could fail.This paper proposes two photometric models, which compensate for specular highlights and lighting variations. They assume that photometric changes vary smoothly on the window of interest. Contrary to classical models, the characteristics of the object surface and the lighting changes can vary in the area being observed. First, we study the validity of these models with respect to the acquisition setup: relative locations between the light source, the sensor and the object as well as the roughness of the surface. Then, these models are used to improve feature points tracking by simultaneously estimating the photometric and geometric changes. The proposed methods are compared to well-known tracking methods robust to affine photometric changes. Experimental results on specular objects demonstrate the robustness of our approaches to specular highlights and lighting changes.

A perceptual metric for photo retouching

In recent years, advertisers and magazine editors have been widely criticized for taking digital photo retouching to an extreme. Impossibly thin, tall, and wrinkle- and blemish-free models are routinely splashed onto billboards, advertisements, and magazine covers. The ubiquity of these unrealistic and highly idealized images has been linked to eating disorders and body image dissatisfaction in men, women, and children. In response, several countries have considered legislating the labeling of retouched photos. We describe a quantitative and perceptually meaningful metric of photo retouching. Photographs are rated on the degree to which they have been digitally altered by explicitly modeling and estimating geometric and photometric changes. This metric correlates well with perceptual judgments of photo retouching and can be used to objectively judge by how much a retouched photo has strayed from reality.

New findings on the prototypical Of?p stars

In recent years several in-depth investigations of the three Galactic Of?p stars were undertaken. These multiwavelength studies revealed the peculiar properties of these objects (in the X-rays as well as in the optical): magnetic fields, periodic line profile variations, recurrent photometric changes. However, many questions remain unsolved. To clarify some of the properties of the Of?p stars, we have continued their monitoring. A new XMM observation and two new optical datasets were obtained. Additional information for the prototypical Of?p trio has been found. HD108 has now reached its quiescent, minimum-emission state, for the first time in 50--60yrs. The echelle spectra of HD148937 confirm the presence of the 7d variations in the Balmer lines and reveal similar periodic variations (though of lower amplitudes) in the HeI5876 and HeII4686 lines, underlining its similarities with the other two prototypical Of?p stars. The new XMM observation of HD191612 was taken at the same phase in the line modulation cycle but at a different orbital phase as previous data. It clearly shows that the X-ray emission of HD191612 is modulated by the 538d period and not the orbital period of 1542d - it is thus not of colliding-wind origin and the phenomenon responsible for the optical changes appears also at work in the high-energy domain. There are however problems: our MHD simulations of the wind magnetic confinement predict both a harder X-ray flux of a much larger strength than what is observed (the modeled DEM peaks at 30-40MK, whereas the observed one peaks at 2MK) and narrow lines (hot gas moving with velocities of 100--200km/s, whereras the observed FWHM is ~2000km/s).

Dynamo processes in the T Tauri star V410 Tau

We present new brightness and magnetic images of the weak-line T Tauri star V410 Tau, made by using data from the Narval spectropolarimeter at Télescope Bernard Lyot (TBL). The brightness image shows a large polar spot and significant spot coverage at lower latitudes. The magnetic maps show a field that is predominantly dipolar and non-axisymmetric with a strong azimuthal component. The field is 50 per cent poloidal and 50 per cent toroidal, and very little differential rotation is apparent from the magnetic images. A photometric monitoring campaign on this star has previously revealed V-band variability of up to 0.6 mag, but in 2009 the light curve is much flatter. The Doppler image presented here is consistent with this low variability. Calculating the flux predicted by the mapped spot distribution gives a peak-to-peak variability of 0.04 mag. The reduction in the amplitude of the light curve, compared with previous observations, appears to be related to a change in the distribution of the spots rather than the number or area. This paper is the first from a Zeeman–Doppler imaging campaign being carried out on V410 Tau between 2009 and 2012 at the TBL. During this time, it is expected that the light curve will return to a high-amplitude state, allowing us to ascertain whether the photometric changes are accompanied by a change in the magnetic field topology.

Bolometric luminosity variations in the luminous blue variable AFGL2298

We characterise the variability in the physical properties of the luminous blue variable AFGL2298 between 1989-2008. In conjunction with published data from 1989-2001, we have undertaken a long term (2001-2008) near-IR spectroscopic and photometric observational campaign for this star and utilise a non-LTE model atmosphere code to interpret these data. We find AFGL2298 to have been highly variable during the two decades covered by the observational datasets. Photometric variations of >1.6 mag have been observed in the JHK wavebands; however, these are not accompanied by correlated changes in near-IR colour. Non-LTE model atmosphere analysis of 4 epochs of K band spectroscopy obtained between 2001-7 suggests that the photometric changes were driven by expansion and contraction of the stellar photosphere accompanied by comparatively small changes in the stellar temperature. Unclumped mass loss rates throughout this period were modest and directly comparable to those of other highly luminous LBVs. However, the bolometric luminosity of AFGL2298 appears to have varied by at least a factor of ~2 between 1989-2008, with it being one of the most luminous stars in the Galaxy during maximum. Comparison to other LBVs that have undergone non bolometric luminosity conserving `eruptions' shows such events to be heterogeneous, with AFGL2298 the least extreme example. These results - and the diverse nature of both the quiescent LBVs and associated ejecta - may offer support to the suggestion that more than one physical mechanism is responsible for such behaviour. [ABRIDGED]

A PHOTOMETRICALLY AND MORPHOLOGICALLY VARIABLE INFRARED NEBULA IN L483

We present narrow and broad K-band observations of the Class 0/I source IRAS 18148-0440 that span 17 years. The infrared nebula associated with this protostar in the L483 dark cloud is both morphologically and photometrically variable on a timescale of only a few months. This nebula appears to be an infrared analog to other well known optically visible variable nebulae associated with young stars, such as Hubble's Variable Nebula. Along with Cepheus A, this is one of the first large variable nebulae to be found that is only visible in the infrared. The variability of this nebula is most likely due to changing illumination of the cloud rather than any motion of the structure in the nebula. Both morphological and photometric changes are observed on a timescale only a few times longer than the light crossing time of the nebula, suggesting very rapid intrinsic changes in the illumination of the nebula. Our narrowband observations also found that H{sub 2} knots are found nearly twice as far to the east of the source as to its west, and that H{sub 2} emission extends farther east of the source than the previously known CO outflow.

Photometric changes on Saturn's Titan: Evidence for active cryovolcanism

We report infrared spectrophotometric variability on the surface of Saturn's moon Titan detected in images returned by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Saturn Orbiter. The changes were observed at 7°S, 138°W and occurred between October 27, 2005 and January 15, 2006. After that date the surface was unchanged until the most recent observation, March 18, 2006. We previously reported spectrophotometric variability at another location (26°S, 78°W). Cassini Synthetic Aperture RADAR (SAR) images find that the surface morphology at both locations is consistent with surface flows possibly resulting from cryovolcanic activity (Wall et al., companion paper, this issue). The VIMS‐reported time variability and SAR morphology results suggest that Titan currently exhibits intermittent surface changes consistent with present ongoing surface processes. We suggest that these processes involve material from Titan's interior being extruded or effused and deposited on the surface, as might be expected from cryovolcanism.

Illumination robust interest point detection

Most interest point detection algorithms are highly sensitive to illumination variations. This paper presents a method to find interest points robustly even under large non-uniform photometric changes. The method, which we call illumination robust feature extraction transform (IRFET), determines salient interest points in an image by calculating and analyzing contrast signatures. A contrast signature shows the response of an interest point detector with respect to a set of contrast stretching functions. The IRFET is generic and can be used with most interest point detectors. In this paper, we demonstrate that the IRFET improves the repeatability rate of the Harris corner detector significantly (by around 25% on average in the experiments).

Gusev photometric variability as seen from orbit by HRSC/Mars-express

Minnaert and two-term phase function Hapke models are used to describe the photometric properties of the martian surface using HRSC (High Resolution Stereo Camera) multi-angular observations acquired along the ongoing Mars-Express mission. These observations can be pieced together to derive integrated phase functions over a wide range of phase angles. The photometric diversity at 675 nm, as seen from orbit, of the martian surface properties across Gusev is depicted with seven units. Three photometric units widespread across the flanks of Apollinaris Patera flank and the floor of Gusev Crater are identified as having high single scattering albedo with rather forward scattering properties, low to intermediate macroscopic roughness and porous or not compacted powdered surface state as indicated by the opposition parameters. Another unit has the highest single scattering albedo, the smoothest surface in terms of macroscopic roughness, associated with an extremely forward scattering behavior. The opposition parameters are consistent with the presence of transparent particles in the surface powder layer. The distribution of this unit appears quite intermittent across the crater and does not seem to indicate any relationship with a given morphological structure. It may correspond to sparse areas where the structure of the surface dust layer is the most preserved. The most pronounced photometric changes are observed in three units associated with the low-albedo features corresponding to dark wind streaks. These units have a low single scattering albedo, are the most backscattering surfaces across Gusev, have a high surface roughness and present variable surface states as shown by the opposition parameters estimates, consistent with the occurrence of large grains organized in more or less packed layers. Clear differences are seen among these units in terms of opposition effect. While one exhibits typical characteristics for the opposition effect, another appears more unusual in terms of lobe width and the third suggests the occurrence of a packed/compressed/narrow size distribution powder surface. The opposition effect thus appears to play a significant role suggesting that the surface state optical properties across Gusev are strongly influenced by the porosity and packing characteristics or grain size distribution of the upper layer of the martian soil. The mapping aspect of the investigation is quite useful to get a better sense of the meaning of the observed photometric variations. Indeed, the Hapke modeling suggests that surface organization (surface roughness, packing state) is more important than the simple physical characterization of the intrinsic optical properties of the constitutive particles. Given the overall spatial patterns derived from the photometric analysis, the variations, at least for the western and central part of Gusev Crater, are likely partly driven by the prevailing wind regimes, considered to be oriented north-northwest/south-southeast and disturbing the very upper surface layer. The present photometric results agree with independent investigations based on thermal inertia, reflectance spectroscopy, in situ photometric and microscopic imaging and support the idea of a thin layer of fine-grained dust, being stripped off in the low albedo units to reveal a dark basaltic substrate comprising coarse-grained materials.

Comparison of sensitivity to color changes in natural and phase-scrambled scenes

Traditionally, thresholds for detecting photometric changes have been measured by using stimuli such as disks or gratings and accounted for in terms of relatively low-level mechanisms in the visual pathway. Therefore one might not expect the higher-order structures that characterize natural scenes to influence thresholds for detecting uniform photometric changes. We compared thresholds for detecting uniform photometric changes for natural and phase-scrambled versions of images of natural scenes. The chromaticity and luminance of every pixel was represented as a vector in a modified version of the MacLeod-Boynton color space and was translated, rotated, or compressed within that color space. Thresholds for all types of transformation were significantly lower in the raw compared with phase-scrambled scenes, and we attribute this to the influence of higher-order structure.

Spectropolarimetry of the massive post-red supergiants IRC +10420 and HD 179821

We present medium resolution spectropolarimetry and long term photo-polarimetry of two massive post-red supergiants, IRC +10420 and HD 179821. The data provide new information on their circumstellar material as well as their evolution. In IRC +10420, the polarization of the Ha line is different to that of the continuum, which indicates that the electron-scattering region is not spherically symmetric. The observed long term changes in the polarimetry can be associated with an axi-symmetric structure, along the short axis of the extended reflection nebulosity. Long term photometry reveals that the star increased in temperature until the mid-nineties, after which the photospheric flux in the optical levelled off. As the photometric changes are mostly probed in the red, they do not trace high stellar temperatures sensitively. And so, it is not obvious whether the star has halted its increase in temperature or not. For HD 179821 we find no polarization effects across any absorption or emission lines, but observe very large polarization changes of order 5% over 15 years. During the same period, the optical photometry displayed modest variability at the 0.2 magnitude level. This is unexpected, because large polarization changes are generally accompanied by strong photometric changes. Several explanations for this puzzling fact are discussed. Most of which, involving asymmetries in the circumstellar material, seem to fail as there is no evidence for the presence of hot, dusty material close to the star. A caveat is that the sparsely available near-infrared photometry could have missed periods of strong polarization activity. Alternatively, the variations can be explained by the presence of a non-radially pulsating photosphere. Changes in the photometry hint at an increase in temperature corresponding to a change through two spectral subclasses over the past ten years.

Dramatic Spectral and Photometric Changes of Pleione (28 Tau) between 2005 November and 2007 April

Abstract Recent photometric and spectroscopic observations of Pleione are reported. Pleione showed brightness declines in both the $B$ and $V$ bands by $\sim$ 0$\rlap {.}{^{\rm m}}$25 between 2006 January and 2007 March. Significant decreases in the strengths and widths of the long-lived emission components in H$\alpha$ and H$\beta$ were observed during this period, while additional outer double-emission components appeared in these lines. Many metallic shell lines strengthened during the observation period, being accompanied by central quasi-emission components. We interpret these spectral changes in terms of a double disk: the progressive decline of a misaligned old disk in due course of precession and the formation and growth of a new disk in the star’s equatorial plane.

Invariant salient regions based image retrieval under viewpoint and illumination variations

In this paper, we present a novel image retrieval technique based on salient regions that are invariant under viewpoint and illumination variations. The salient regions are detected according to local entropy and scale selection. The detected regions have very high repeatability under various viewpoint and illumination changes. We apply the invariant region detector on content-based image retrieval applications. The detected regions are the most informative ones, hence are potentially more effective for image indexing and retrieval. Generalized colour moment invariants are used as the invariant descriptor for characterizing the selected salient regions under geometric and photometric changes. Two datasets are used for experiments: one for evaluating the invariance of the algorithm under geometric and photometric transformations; the other for testing the algorithm on object category retrieval. The experimental results show that our proposed region detector is very efficient and effective on retrieving a variety of cluttered images with partial occlusion.

Is chronic fatigue syndrome associated with platelet activation?

Chronic fatigue syndrome (CFS) is a debilitating condition that has no known aetiology or pathophysiology. Recent investigations by other workers have suggested that individuals with CFS may have a hypercoagulable state. This study investigated various aspects of platelet activation and function in 17 patients with CFS and in 16 age-matched and sex-matched healthy controls. Platelet aggregation, platelet volume and coagulation tests were performed. Platelet aggregation was investigated by means of the photometric changes using citrated platelet-rich plasma, whole blood aggregation was calculated as the percentage fall in single platelet counts and the coagulation tests were performed on an automatic microcentrifugal analyser.A trend was observed for the patients to have lower aggregation results and a reduced mean platelet volume. However, this only reached statistical significance for one result; the rate of the aggregation slope by 1.0 microg/ml collagen [CFS patients, 18 (9-28) versus controls, 32.5 (19-36); Mann-Whitney U test, P = 0.029]. No significant differences were found for any of the measurements of coagulation. These results are in contrast to previously reported findings. However, due to the heterogeneous nature of the disease, and the resulting lifestyles of the patients, caution should be taken when comparing one group of patients with another. Nevertheless, we certainly found no evidence of increased platelet activation or of a hypercoagulable state in patients with CFS and, on the basis of these results, anti-platelet or anti-coagulant therapy is not warranted.

Combining color and shape information for illumination-viewpoint invariant object recognition

In this paper, we propose a new scheme that merges color- and shape-invariant information for object recognition. To obtain robustness against photometric changes, color-invariant derivatives are computed first. Color invariance is an important aspect of any object recognition scheme, as color changes considerably with the variation in illumination, object pose, and camera viewpoint. These color invariant derivatives are then used to obtain similarity invariant shape descriptors. Shape invariance is equally important as, under a change in camera viewpoint and object pose, the shape of a rigid object undergoes a perspective projection on the image plane. Then, the color and shape invariants are combined in a multidimensional color-shape context which is subsequently used as an index. As the indexing scheme makes use of a color-shape invariant context, it provides a high-discriminative information cue robust against varying imaging conditions. The matching function of the color-shape context allows for fast recognition, even in the presence of object occlusion and cluttering. From the experimental results, it is shown that the method recognizes rigid objects with high accuracy in 3-D complex scenes and is robust against changing illumination, camera viewpoint, object pose, and noise.

Robust photometric invariant features from the color tensor

Luminance-based features are widely used as low-level input for computer vision applications, even when color data is available. The extension of feature detection to the color domain prevents information loss due to isoluminance and allows us to exploit the photometric information. To fully exploit the extra information in the color data, the vector nature of color data has to be taken into account and a sound framework is needed to combine feature and photometric invariance theory. In this paper, we focus on the structure tensor, or color tensor, which adequately handles the vector nature of color images. Further, we combine the features based on the color tensor with photometric invariant derivatives to arrive at photometric invariant features. We circumvent the drawback of unstable photometric invariants by deriving an uncertainty measure to accompany the photometric invariant derivatives. The uncertainty is incorporated in the color tensor, hereby allowing the computation of robust photometric invariant features. The combination of the photometric invariance theory and tensor-based features allows for detection of a variety of features such as photometric invariant edges, corners, optical flow, and curvature. The proposed features are tested for noise characteristics and robustness to photometric changes. Experiments show that the proposed features are robust to scene incidental events and that the proposed uncertainty measure improves the applicability of full invariants.