1,929 research outputs found
Detection of Nonaligned Double JPEG Compression Based on Integer Periodicity Maps
In this paper, a simple yet reliable algorithm to detect the presence of nonaligned double JPEG compression (NA-JPEG) in compressed images is proposed. The method evaluates a single feature based on the integer periodicity of the blockwise discrete cosine transform (DCT) coefficients when the DCT is computed according to the grid of the previous JPEG compression. Even if the proposed feature is computed relying only on DC coefficient statistics, a simple threshold detector can classify NA-JPEG images with improved accuracy with respect to existing methods and on smaller image sizes, without resorting to a properly trained classifier. Moreover, the proposed scheme is able to accurately estimate the grid shift and the quantization step of the DC coefficient of the primary JPEG compression, allowing one to perform a more detailed analysis of possibly forged image
Scaling properties in the production range of shear dominated flows
Recent developments in turbulence are focused on the effect of large scale
anisotropy on the small scale statistics of velocity increments. According to
Kolmogorov, isotropy is recovered in the large Reynolds number limit as the
scale is reduced and, in the so-called inertial range, universal features
-namely the scaling exponents of structure functions - emerge clearly. However
this picture is violated in a number of cases, typically in the high shear
region of wall bounded flows. The common opinion ascribes this effect to the
contamination of the inertial range by the larger anisotropic scales, i.e. the
residual anisotropy is assumed as a weak perturbation of an otherwise isotropic
dynamics. In this case, given the rotational invariance of the Navier-Stokes
equations, the isotropic component of the structure functions keeps the same
exponents of isotropic turbulence. This kind of reasoning fails when the
anisotropic effects are strong as in the production range of shear dominated
flows. This regime is analyzed here by means of both numerical and experimental
data for a homogeneous shear flow. A well defined scaling behavior is found to
exist, with exponents which differ substantially from those of classical
isotropic turbulence. Contrary to what predicted by the perturbation approach,
such a deep alteration concerns the isotropic sector itself. The general
validity of these results is discussed in the context of turbulence near solid
walls, where more appropriate closure models for the coarse grained
Navier-Stokes equations would be advisable.Comment: 4 pages, 4 figure
Linear Chains of Styrene and Methyl-Styrene Molecules and their Heterojunctions on Silicon: Theory and Experiment
We report on the synthesis, STM imaging and theoretical studies of the
structure, electronic structure and transport properties of linear chains of
styrene and methyl-styrene molecules and their heterojunctions on
hydrogen-terminated dimerized silicon (001) surfaces. The theory presented here
accounts for the essential features of the experimental STM data including the
nature of the corrugation observed along the molecular chains and the
pronounced changes in the contrast between the styrene and methyl-styrene parts
of the molecular chains that are observed as the applied bias is varied. The
observed evolution with applied bias of the STM profiles near the ends of the
molecular chains is also explained. Calculations are also presented of electron
transport along styrene linear chains adsorbed on the silicon surface at
energies in the vicinity of the molecular HOMO and LUMO levels. For short
styrene chains this lateral transport is found to be due primarily to direct
electron transmission from molecule to molecule rather than through the silicon
substrate, especially in the molecular LUMO band. Differences between the
calculated position-dependences of the STM current around a junction of styrene
and methyl-styrene molecular chains under positive and negative tip bias are
related to the nature of lateral electron transmission along the molecular
chains and to the formation in the LUMO band of an electronic state localized
around the heterojunction.Comment: 17 pages plus 11 figures. To appear in Physical Review
Image Forgery Localization via Fine-Grained Analysis of CFA Artifacts
In this paper, a forensic tool able to discriminate between original and forged regions in an image captured by a digital camera is presented. We make the assumption that the image is acquired using a Color Filter Array, and that tampering removes the artifacts due to the demosaicking algorithm. The proposed method is based on a new feature measuring the presence of demosaicking artifacts at a local level, and on a new statistical model allowing to derive the tampering probability of each 2 × 2 image block without requiring to know a priori the position of the forged region. Experimental results on different cameras equipped with different demosaicking algorithms demonstrate both the validity of the theoretical model and the effectiveness of our schem
Preliminary studies on the effect of Oidium tuckeri on the phenolic composition of grapes and wines
Research NoteA study of the influence of Oidium tuckeri on anthocyanins and the hydroxycinnamic tartaric esters of grapes and red wines was carried out during the 1993 harvest. After infection the level of anthocyanins in the skins of berries and in wines was lower than in healthy grapes and their wines. It is of particular interest that two bisubstituted anthocyanins reacted less sensitive to the pathogen. Wines from infected grapes contained more hydroxycinnamic tartaric esters
Reverse engineering of double compressed images in the presence of contrast enhancement
Abstract-A comparison between two forensic techniques for the reverse engineering of a chain composed by a double JPEG compression interleaved by a linear contrast enhancement is presented here. The first approach is based on the well known peak-to-valley behavior of the histogram of double-quantized DCT coefficients, while the second approach is based on the distribution of the first digit of DCT coefficients. These methods have been extended to the study of the considered processing chain, for both the chain detection and the estimation of its parameters. More specifically, the proposed approaches provide an estimation of the quality factor of the previous JPEG compression and the amount of linear contrast enhancement
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