Generalization and explainability are two key challenges in synthetic audio detection. Effective detectors should not only reliably classify unseen data from unknown synthesis algorithms, but also provide insight into their decision-making process and explain why a given input was classified as real or fake. To promote generalization we use the Person-of-Interest approach, which allows us to detect synthetic audio using a model trained only on real data, provided that some pristine audio of the putative speaker is provided. To support explainability, we instead use an encoder-decoder backbone such that the bottleneck features ensure syntactic and semantic fidelity to the input, as well as enable reliable decisions. Experiments show that our approach outperforms both state-of-the-art models based on supervised learning and methods based on speaker verification.

Hyperspectral pansharpening has received much attention in recent years due to technological and methodological advances that open the door to new application scenarios. However, research on this topic is only now gaining momentum. The most popular methods are still borrowed from the more mature field of multispectral pansharpening and often overlook the unique challenges posed by hyperspectral data fusion, such asi)the very large number of bands,ii)the overwhelming noise in selected spectral ranges,iii)the significant spectral mismatch between panchromatic and hyperspectral components,iv)a typically high resolution ratio. Imprecise data modeling especially affects spectral fidelity. Even state-of-the-art methods perform well in certain spectral ranges and much worse in others, failing to ensure consistent quality across all bands, with the risk of generating unreliable results. Here, we propose a hyperspectral pansharpening method that explicitly addresses this problem and ensures uniform spectral quality. To this end, a single lightweight neural network is used, with weights that adapt on the fly to each band. During fine-tuning, the spatial loss is turned on and off to ensure a fast convergence of the spectral loss to the desired level, according to a hysteresis-like dynamic. Furthermore, the spatial loss itself is appropriately redefined to account for nonlinear dependencies between panchromatic and spectral bands. Overall, the proposed method is fully unsupervised, with no prior training on external data, flexible, and low-complexity. Experiments on a recently published benchmarking toolbox show that it ensures excellent sharpening quality, competitive with the state-of-the-art, consistently across all bands. The software code and the full set of results are shared online on https://github.com/giu-guarino/rho-PNN.

The ability to detect manipulated visual content is becoming increasingly important in many application fields, given the rapid advances in image synthesis methods. Of particular concern is the possibility of modifying the content of medical images, altering the resulting diagnoses. Despite its relevance, this issue has received limited attention from the research community. One reason is the lack of large and curated datasets to use for development and benchmarking purposes. Here, we investigate this issue and propose M3Dsynth, a large dataset of manipulated Computed Tomography (CT) lung images. We create manipulated images by injecting or removing lung cancer nodules in real CT scans, using three different methods based on Generative Adversarial Networks (GAN) or Diffusion Models (DM), for a total of 8,577 manipulated samples. Experiments show that these images easily fool automated diagnostic tools. We also tested several state-of-the-art forensic detectors and demonstrated that, once trained on the proposed dataset, they are able to accurately detect and localize manipulated synthetic content, even when training and test sets are not aligned, showing good generalization ability. Dataset and code are publicly available at https://grip-unina.github.io/M3Dsynth/.

In the last years it has been observed a growing interest toward deep leaning techniques for the pansharpening of multiresolution images. Due to the lack of data with ground truth, most deep learning solutions exploit synthetic reduced-resolution data to carry out supervised training. Such an approach, though granting an easy way to step over the lack of labeled data, has shown its limitations due to the statistical mismatch between real full-resolution data and synthetic reduced-resolution data, which eventually impacts on the generalization capacity of the trained models. This has motivated a recent paradigm shift from supervised to unsupervised learning frameworks for pansharpening. Unsupervised schemes, however, involve the definition of more sophisticated loss functions which comprise, at least, two fundamental terms: one responsible for the spectral quality, meant as consistency between the pansharpened image and the input multispectral component; the other accounting for the spatial quality, read as consistency between the output and the panchromatic input. Despite the very good results shown by many such unsupervised solutions, a minor attention has been devoted to the investigation of the interaction between the above mentioned loss terms and to their proper balance to grant stability while pursuing accuracy. This work aims to explore to what extent unsupervised spatial and spectral consistency losses can be reliably combined without impairing quality.

This work proposes a hybrid approach to address the pansharpening of hyperspectral images, which mixes the use of a recently proposed CNN-based solution and a classical solution such as the Gram Schmidt Adaptive method (GSA). The hyperspectral datacube is split in two sets of bands, those falling in the visible range and the remaining ones. The first set is pansharpened using the GSA approach which has proven to grant very high quality results in this range. The remaining bands, whose relationship with the panchromatic band is much weaker, undergo a fusion process based on a recently proposed hyperspectral pansharpening method known as Rolling hyperspectral Pansharpening Neural Network (R-PNN). By doing so, we are able to take the best features from both solutions, getting higher quality results compared to the marginal use of any of the two methods.

This work deals with the estimation of the tropospheric vertical column density of nitrogen dioxide from Sentinel-5P radiance data using convolutional neural networks. The current processing chain to retrieve this information from Sentinel-5P data requires a complex, computationally demanding, physical modeling that involves the use of additional side information such as meteorological variables, which are not always available. Therefore, in this proof-of-concept study, we explored the feasibility of an estimation exclusively using radiance data from Sentinel-5P, leveraging on the powerful representational capacity of deep neural networks. Preliminary results are very promising encouraging further investigation.

Generalization is a main issue for current audio deepfake detectors, which struggle to provide reliable results on out-of-distribution data. Given the speed at which more and more accurate synthesis methods are developed, it is important to design techniques that work well also on data they were not trained for. In this paper we study the potential of large-scale pre-trained models for audio deepfake detection. To this end, the detection problem is reformulated as a speaker verification framework and fake audios are exposed by the mismatch between the voice sample under test and the voice of the claimed identity. With this paradigm, no fake speech sample is necessary in training, cutting off any link with the generation method at the root. Features are extracted by general-purpose large pre-trained models, with no need for training or fine-tuning on specific datasets. At detection time only a limited set of voice fragments of the identity under test is required. Experiments on several datasets show that detectors based on pre-trained models achieve excellent performance and show strong generalization ability, rivaling supervised methods on in-distribution data and largely overcoming them on out-of-distribution data.

Over the past decade, there has been tremendous progress in creating synthetic media, mainly thanks to the development of powerful methods based on generative adversarial networks (GAN). Very recently, methods based on diffusion models (DM) have been gaining the spotlight. In addition to providing an impressive level of photorealism, they enable the creation of text-based visual content, opening up new and exciting opportunities in many different application fields, from arts to video games. On the other hand, this property is an additional asset in the hands of malicious users, who can generate and distribute fake media perfectly adapted to their attacks, posing new challenges to the media forensic community. With this work, we seek to understand how difficult it is to distinguish synthetic images generated by diffusion models from pristine ones and whether current state-of-the-art detectors are suitable for the task. To this end, first we expose the forensics traces left by diffusion models, then study how current detectors, developed for GAN-generated images, perform on these new synthetic images, especially in challenging social-network scenarios involving image compression and resizing. Datasets and code are available at https:github.com/grip-unina/DMimageDetection.

The recent paradigm shift from model-based to data-driven approaches has involved a growing number of data-fusion tasks. Specifically for pansharpening, unsupervised deep learning methods have been recently explored with the goal of overcoming the generalization limits shown by early pansharpening convolutional neural networks based on supervised training schemes. Furthermore, some of these exploit the target-adaptive modality to face the scarcity of training data. On the downside, combining usupervised training and target adaptivity causes a non-negligible increase of the computational cost. This work presents a new target adaptive scheme that allows to keep limited the computational cost at inference time while preserving accuracy.

This work proposes a simple yet effective method to adapt unsupervised convolutional neural networks for pansharpening of multispectral images to the problem of hyperspectral image pansharpening, i.e., the fusion of a single high-resolution panchromatic band with a low-resolution hyperspectral data cube. This is achieved by means of a PCA transformation which allows to compact the most of the HS image energy in a few bands, which are then suitably super-resolved using a pansharpening network designed for few spectral bands. Our experiments show very encouraging results which compare favorably against the state-of-the-art methods.

Optical remote sensing images are subject to cloud phenomena that can cause information loss in Earth observation. The main alternative is represented by the synthetic aperture radar images. However, many Earth monitoring applications exploit specific spectral features defined for multispectral data only. In this work, we propose a method that aims to recover several spectral features through deep learning-based data fusion of Sentinel-1 and Sentinel-2 time-series. The proposed approach has been experimentally validated for radiometric indexes such as the normalized difference vegetation index, the normalized difference water index, the soil-adjusted vegetation index and the atmospherically resistant vegetation index. Both numerical and visual results show that the proposed solution outperforms consistently the compared methods.

This work proposes a simple yet effective method to adapt unsupervised convolutional neural networks from multispectral to hyperspectral pansharpening. Thus, it focuses on the fusion of a single high-resolution panchromatic band with a low-resolution hyperspectral data cube. This is achieved by means of a decorrelation transform, following the principal component analysis approach, which enables the compression of a significant portion of the hyperspectral image energy into a few bands. Afterwards, a suitably adapted pansharpening network designed for four spectral bands is used to super-resolve only the principal components. Experiments demonstrate high performance in both quantitative and qualitative evaluations, favorably comparing against state-of-the-art methods.

In latest years, deep learning has gained a leading role in the pansharpening of multiresolution images. Given the lack of ground truth data, most deep learning-based methods carry out supervised training in a reduced-resolution domain. However, models trained on downsized images tend to perform poorly on high-resolution target images. For this reason, several research groups are now turning to unsupervised training in the full-resolution domain, through the definition of appropriate loss functions and training paradigms. In this context, we have recently proposed a full-resolution training framework which can be applied to many existing architectures. Here, we propose a new deep learning-based pansharpening model that fully exploits the potential of this approach and provides cutting-edge performance. Besides architectural improvements with respect to previous work, such as the use of residual attention modules, the proposed model features a novel loss function that jointly promotes the spectral and spatial quality of the pansharpened data. In addition, thanks to a new fine-tuning strategy, it improves inference-time adaptation to target images. Experiments on a large variety of test images, performed in challenging scenarios, demonstrate that the proposed method compares favorably with the state of the art both in terms of numerical results and visual output. Code is available online at https://github.com/matciotola/Lambda-PNN.

In recent years, there has been a growing interest in deep learning-based pansharpening. Thus far, research has mainly focused on architectures. Nonetheless, model training is an equally important issue. A first problem is the absence of ground truths, unavoidable in pansharpening. This is often addressed by training networks in a reduced-resolution domain and using the original data as ground truth, relying on an implicit scale invariance assumption. However, on full-resolution images, results are often disappointing, suggesting such invariance not to hold. A further problem is the scarcity of training data, which causes a limited generalization ability and a poor performance on off-training-test images. In this article, we propose a full-resolution training framework for deep learning-based pansharpening. The framework is fully general and can be used for any deep learning-based pansharpening model. Training takes place in the high-resolution domain, relying only on the original data, thus avoiding any loss of information. To ensure spectral and spatial fidelity, a suitable two-component loss is defined. The spectral component enforces consistency between the pansharpened output and the low-resolution multispectral input. The spatial component, computed at high resolution, maximizes the local correlation between each pansharpened band and the panchromatic input. At testing time, the target-adaptive operating modality is adopted, achieving good generalization with a limited computational overhead. Experiments carried out on WorldView-3, WorldView-2, and GeoEye-1 images show that methods trained with the proposed framework guarantee a pretty good performance in terms of both full-resolution numerical indexes and visual quality.

The advent of deep learning has brought a significant improvement in the quality of generated media. However, with the increased level of photorealism, synthetic media are becoming hardly distinguishable from real ones, raising serious concerns about the spread of fake or manipulated information over the Internet. In this context, it is important to develop automated tools to reliably and timely detect synthetic media. In this work, we analyze the state-of-the-art methods for the detection of synthetic images, highlighting the key ingredients of the most successful approaches, and comparing their performance over existing generative architectures. We will devote special attention to realistic and challenging scenarios, like media uploaded on social networks or generated by new and unseen architectures, analyzing the impact of suitable augmentation and training strategies on the detectors’ generalization ability.

This work presents a new unsupervised framework for training deep learning models for super-resolution of Sentinel-2 images by fusion of its 10-m and 20-m bands. The proposed scheme avoids the resolution downgrade process needed to generate training data in the supervised case. On the other hand, a proper loss that accounts for cycle-consistency between the network prediction and the input components to be fused is proposed. Despite its unsupervised nature, in our preliminary experiments the proposed scheme has shown promising results in comparison to the supervised approach. Besides, by construction of the proposed loss, the resulting trained network can be ascribed to the class of multi-resolution analysis methods.

Cloud segmentation of remotely sensed multispectral images is an important topic not only for weather forecast but, more in general, for establishing when the sensed data actually relate to the soil so that can be reliably used for some monitoring purpose. In this work, leveraging on the capability of convolutional neural networks to accurately approximate complex relationships between raw data and higher-level products, we propose a U-Net-like solution conceived for Sentinel-2 images. In order to face the scarsity of training data, a proper domain adaptation strategy has been pursued, which resorts to a labeled Landsat-8 dataset. Preliminary results show a consistent improvement over standard tools.

The rise of deep learning has impacted profoundly all aspects of image processing and remote sensing. Following this trend, in the last few years, a large number of data-driven methods have been proposed also for SAR image despeckling. However, in spite of this large effort, only limited performance gains have been observed. We believe this is mostly due to the use of training sets that are only partially fit to the task, and sometimes plain wrong. In this work we assess experimentally the impact of training set design on the performance of SAR image despeckling with the goal of highlighting solid guidelines for sensible training.

The ever higher quality and wide diffusion of fake images have spawn a quest for reliable forensic tools. Many GAN image detectors have been proposed, recently. In real world scenarios, however, most of them show limited robustness and generalization ability. Moreover, they often rely on side information not available at test time, that is, they are not universal. We investigate these problems and propose a new GAN image detector based on a limited sub-sampling architecture and a suitable contrastive learning paradigm. Experiments carried out in challenging conditions prove the proposed method to be a first step towards universal GAN image detection, ensuring also good robustness to common image impairments, and good generalization to unseen architectures.

Lung ultrasound imaging is reaching growing interest from the scientific community. On one side, thanks to its harmlessness and high descriptive power, this kind of diagnostic imaging has been largely adopted in sensitive applications, like the diagnosis and follow-up of preterm newborns in neonatal intensive care units. On the other side, state-of-the-art image analysis and pattern recognition approaches have recently proven their ability to fully exploit the rich information contained in these data, making them attractive for the research community. In this work, we present a thorough analysis of recent deep learning networks and training strategies carried out on a vast and challenging multicenter dataset comprising 87 patients with different diseases and gestational ages. These approaches are employed to assess the lung respiratory status from ultrasound images and are evaluated against a reference marker. The conducted analysis sheds some light on this problem by showing the critical points that can mislead the training procedure and proposes some adaptations to the specific data and task. The achieved results sensibly outperform those obtained by a previous work, which is based on textural features, and narrow the gap with the visual score predicted by the human experts.

Abstract PRNU-based image processing is a key asset in digital multimedia forensics. It allows for reliable device identification and effective detection and localization of image forgeries, in very general conditions. However, performance impairs significantly in challenging conditions involving low quality and quantity of data. These include working on compressed and cropped images or estimating the camera PRNU pattern based on only a few images. To boost the performance of PRNU-based analyses in such conditions, we propose to leverage the image noiseprint, a recently proposed camera-model fingerprint that has proved effective for several forensic tasks. Numerical experiments on datasets widely used for source identification prove that the proposed method ensures a significant performance improvement in a wide range of challenging situations.

Nonlocal methods are state-of-the-art in SAR despeckling, thanks to their ability to exploit image self-similarity. Given sufficient training data, however, methods based on deep learning have proven highly competitive. Therefore, to take the best of both approaches, we investigate the use of deep learning to improve nonlocal despeckling. We use plain non-iterative nonlocal means despeckling, with weights provided, for each estimation window, by a suitably trained deep CNN. Experiments on synthetic and real SAR data prove this approach to outperform conventional nonlocal methods.

We propose a new algorithm for the reliable detection and localization of video copy–move forgeries. Discovering well-crafted video copy–moves may be very difficult, especially when some uniform background is copied to occlude foreground objects. To reliably detect both additive and occlusive copy–moves, we use a dense-field approach, with invariant features that guarantee robustness to several postprocessing operations. To limit complexity, a suitable video-oriented version of PatchMatch is used, with a multiresolution search strategy, and a focus on volumes of interest. Performance assessment relies on a new dataset, designedad hoc, with realistic copy–moves and a wide variety of challenging situations. Experimental results show the proposed method to detect and localize video copy–moves with good accuracy even in adverse conditions.

We propose a new method for synthetic aperture radar (SAR) image despeckling, which leverages information drawn from coregistered optical imagery. Filtering is performed by patchwise nonlocal means, working exclusively on SAR data. However, the filtering weights are computed by taking into account also the optical guide, which is much cleaner than the SAR image, and hence more discriminative. To avoid injecting optical-domain information into the filtered image, an SAR-domain statistical test is preliminarily performed to reject right away any risky predictor. Experiments on two SAR-optical data sets prove the proposed method to suppress very effectively the speckle, preserving structural details, and without introducing significant filtering artifacts. Overall, the proposed method compares favorably with all the state-of-the-art despeckling filters, and also with our own previous optical-guided filter.

In this paper, the problem of the spatial-spectral classification of very high-resolution optical images is addressed using a kernel- and region-based approach. A novel method based on integrating region-based or object-based information into a kernel machine is developed. A Gaussian process model is used to characterize each segment in a segmentation map and to define a region-based admissible kernel accordingly. This kernel is combined with a marker-controlled watershed segmentation that incorporates scale adaptivity. Spatial-spectral fusion capabilities are also ensured by combining the resulting classification method with composite kernels.

Nonlocal filters show outstanding performance in the field of interferometric phase restoration by providing strong filtering power together with high spatial features preservation. In this work we focus on the generation of Digital Elevation Models (DEM) from a pair of interferometric SAR images. In the specific, we aim at comparing the performance of state-of-the-art InSAR filtering approaches on the basis of their noise suppression and detail preservation capabilities. We exploit a dataset of TanDEM-X SAR data relative to the volcanic area of the Kamchatka region (Russia).

In this paper, we propose a sparsity-based despeckling approach. The first main contribution of this work is the elaboration of a sparse-coding algorithm adapted to the statistics of SAR images. In fact, most sparse-coding algorithms for SAR data apply a logarithmic transform to data, so as to convert the noise from multiplicative to additive. Then, a Gaussian prior is adopted. However, using a more suitable prior for SAR data avoids introducing artifacts. The second main contribution proposed is to predict the optimal sparsity degree for each patch based on local image features. Experiments show that this strategy improves upon traditional sparse coding with a low-error-rate stopping criterion.

The block-matching 3-D (BM3D) algorithm, based on the nonlocal approach, is one of the most effective methods to date for additive white Gaussian noise image denoising. Likewise, its extension to synthetic aperture radar (SAR) amplitude images, SAR-BM3D, is a state-of-the-art SAR despeckling algorithm. In this paper, we further extend BM3D to address the restoration of SAR interferometric phase images. While keeping the general structure of BM3D, its processing steps are modified to take into account the peculiarities of the SAR interferometry signal. Experiments on simulated and real-world Tandem-X SAR interferometric pairs prove the effectiveness of the proposed method.

In this paper we investigate the use of discriminative model learning through Convolutional Neural Networks (CNNs) for SAR image despeckling. The network uses a residual learning strategy, hence it does not recover the filtered image, but the speckle component, which is then subtracted from the noisy one. Training is carried out by considering a large multitemporal SAR image and its multilook version, in order to approximate a clean image. Experimental results, both on synthetic and real SAR data, show the method to achieve better performance with respect to state-of-the-art techniques.

Ship detection is a fundamental task for SAR-based maritime surveillance. Besides providing high reliability, a good detector is required to be computationally light, in order to analyze huge areas in a reasonable time. We propose a fully convolutional neural network for ship detection in SAR images. Thanks to a relatively simple architecture, complexity remains low enough to allow for a single-stage approach, thus avoiding the possible errors of CFAR pre-screening. Experiments on a Sentinel-1 dataset prove the proposed CNN to be much more reliable than CFAR detection.

This work deals with the fusion of SAR and optical data for land cover monitoring. We first propose to use co-registered optical data as a guide for nonlocal SAR image despeckling. Then, we fuse filtered Sentinel-1 SAR data with optical Sentinel-2 data for land-use classification. Experiments show that using optical-driven despeckled SAR data largely improves classification accuracy w.r.t using the original data and even multitemporal filtered data.

Local descriptors based on the image noise residual have proven extremely effective for a number of forensic applications, like forgery detection and localization. Nonetheless, motivated by promising results in computer vision, the focus of the research community is now shifting on deep learning. In this paper we show that a class of residual-based descriptors can be actually regarded as a simple constrained convolutional neural network (CNN). Then, by relaxing the constraints, and fine-tuning the net on a relatively small training set, we obtain a significant performance improvement with respect to the conventional detector.

We devised an algorithm, inspired by contrast-set mining algorithms such as STUCCO, to automatically find statistically significant properties (correlations) in crash groups. Many earlier works focused on improving the clustering of crashes but, to the best of our knowledge, the problem of automatically describing properties of a cluster of crashes is so far unexplored. This means developers currently spend a fair amount of time analyzing the groups themselves, which in turn means that a) they are not spending their time actually developing a fix for the crash; and b) they might miss something in their exploration of the crash data (there is a large number of attributes in crash reports and it is hard and error-prone to manually analyze everything). Our algorithm helps developers and release managers understand crash reports more easily and in an automated way, helping in pinpointing the root cause of the crash. The tool implementing the algorithm has been deployed on Mozilla's crash reporting service.

We address the problem of clustering a set of images, according to their source device, in the absence of any prior information. Image similarity is computed based on noise residuals, regarded as single-image estimates of the camera's photo-response non-uniformity (PRNU) pattern. First, residuals are grouped by correlation clustering, and several alternative data partitions are computed as a function of a running decision boundary. Then, these partitions are processed jointly to extract a single, more reliable, consensus clustering and, with it, more reliable PRNU estimates. Finally, both clustering and PRNU estimates are progressively refined by merging pairs of the same-PRNU clusters, selected on the basis of a maximum-likelihood ratio statistic. Extensive experiments prove the proposed method to outperform the current state of the art both on pristine images and compressed images downloaded from social networks. A remarkable feature of the method is that it does not require the user to set any parameter, nor to provide a training set to estimate them. Moreover, through a suitable choice of basic tools, and efficient implementation, complexity remains always quite limited.

We propose a new algorithm for fast approximate nearest neighbor search based on the properties of ordered vectors. Data vectors are classified based on the index and sign of their largest components, thereby partitioning the space in a number of cones centered in the origin. The query is itself classified, and the search starts from the selected cone and proceeds to neighboring ones. Overall, the proposed algorithm corresponds to locality sensitive hashing in the space of directions, with hashing based on the order of components. Thanks to the statistical features emerging through ordering, it deals very well with the challenging case of unstructured data, and is a valuable building block for more complex techniques dealing with structured data. Experiments on both simulated and real-world data prove the proposed algorithm to provide a state-of-the-art performance.

We propose a new approach to SAR despeckling, based on the combination of multiple alternative estimates of the same data. The many despeckling methods proposed in the literature possess different and often complementary strengths and weaknesses. Given a reliable pixel-wise classification of the image, one can take advantage of this diversity by selecting the more appropriate combination of estimators for each image region. We implement a simplified version of this approach, using soft classification and two state-of-the-art despeckling tools, with opposite properties, as basic estimators. Experiments on real-world high-resolution SAR images prove the effectiveness of the proposed technique and confirm the potential of the whole approach.

In this paper a new object-oriented segmentation method for high-resolution remote sensing images is proposed. To limit computational complexity, a preliminary superpixel representation of the image is obtained by means of a suitable watershed transform. Then, a region adjacency graph is associated with the superpixels, with edge weights accounting for region similarity/dissimilarity. The final segmentation is then obtained by means of a graph-cutting approach, following a correlation clustering formulation. The optimal cut can be obtained by solving a Integer Linear Programming (ILP) problem, whose complexity, however, grows rapidly with the image size. Much faster near-optimal solutions are obtained, here, with a greedy solution. Experiments on a real-world high-resolution remote sensing image prove the potential of the approach.

We propose an interactive tool for designing ground-truth maps associated with multi-resolution remote sensing images. The target image is first segmented at object level by means of an edge-preserving algorithm. Then, a pre-classification defines groups of segments that are homogeneous both in spectral response and size. Finally, suitable candidate segments are selected and shown to the supervisor for inspection and labeling or possible rejection, in an iterative process, until the desired image covering is reached. Experimental results show that the proposed solution allows one to easily and quickly obtain ground-truth maps which are both locally and globally accurate, and where all classes are represented in a balanced manner.

We propose a new synthetic aperture radar (SAR) despeckling technique based on nonlocal filtering and driven by a coregistered optical image. A preliminary homogeneous versus heterogeneous classification of the image is used to decide where the optical guide can be safely used, thus preventing any distortion of the SAR geometry. Even in regions where the use of optical data is enabled, despeckling is carried out exclusively in the SAR domain, and the optical guide is used only to improve the predictor selection in nonlocal filtering and, hence, in the estimation process. Experiments on real-world imagery confirm the potential of the proposed approach.

A new technique for the segmentation of single- and multiresolution (MR) remote sensing images is proposed. To guarantee the preservation of details at fine scales, edge-based watershed is used, with automatically generated markers that help in limiting oversegmentation. For MR images, the panchromatic and multispectral components are processed independently, extracting both the edge maps and the morphological and spectral markers that are eventually fused at the highest resolution, thus avoiding any information loss induced by pansharpening. Numerical results on object layer extraction and simple classification tasks prove the proposed techniques to provide accurate segmentation maps, which preserve fine details and, contrary to state-of-the-art products, can single out objects equally well at very different scales.

We propose a new algorithm for the accurate detection and localization of copy-move forgeries, based on rotation-invariant features computed densely on the image. Dense-field techniques proposed in the literature guarantee a superior performance with respect to their keypoint-based counterparts, at the price of a much higher processing time, mostly due to the feature matching phase. To overcome this limitation, we resort here to a fast approximate nearest-neighbor search algorithm, PatchMatch, especially suited for the computation of dense fields over images. We adapt the matching algorithm to deal efficiently with invariant features, so as to achieve higher robustness with respect to rotations and scale changes. Moreover, leveraging on the smoothness of the output field, we implement a simplified and reliable postprocessing procedure. The experimental analysis, conducted on databases available online, proves the proposed technique to be at least as accurate, generally more robust, and typically much faster than the state-of-the-art dense-field references.

Biometric authentication systems are quite vulnerable to sophisticated spoofing attacks. To keep a good level of security, reliable spoofing detection tools are necessary, preferably implemented as software modules. The research in this field is very active, with local descriptors, based on the analysis of microtextural features, gaining more and more popularity, because of their excellent performance and flexibility. This paper aims at assessing the potential of these descriptors for the liveness detection task in authentication systems based on various biometric traits: fingerprint, iris, and face. Besides compact descriptors based on the independent quantization of features, already considered for some liveness detection tasks, we will study promising descriptors based on the joint quantization of rich local features. The experimental analysis, conducted on publicly available data sets and in fully reproducible modality, confirms the potential of these tools for biometric applications, and points out possible lines of development toward further improvements.

PRNU-based techniques guarantee a good forgery detection performance irrespective of the specific type of forgery. The presence or absence of the camera PRNU pattern is detected by a correlation test. Given the very low power of the PRNU signal, however, the correlation must be averaged over a pretty large window, reducing the algorithm's ability to reveal small forgeries. To improve resolution, we estimate correlation with a spatially adaptive filtering technique, with weights computed over a suitable pilot image. Implementation efficiency is achieved by resorting to the recently proposed guided filters. Experiments prove that the proposed filtering strategy allows for a much better detection performance in the case of small forgeries.

In this work we propose a new algorithm for copy-move forgery detection and localization, based on the fast computation of a dense nearest-neighbor field. To this end, we use PatchMatch, an iterative randomized algorithm for nearest-neighbor search, which exploits the regularity of natural images to converge very rapidly to a near-optimal and smooth field. We modify the basic algorithm to gain robustness against rotations, while keeping the original computational efficiency. Experimental results show the proposed technique to outperform almost uniformly all tested reference techniques in terms of both accuracy and speed.

Reliable segmentation of SAR images requires some forms of user supervision: we resort here to the interactive version of the Tree-Structured Markov Random Field (TS-MRF) segmentation suite. The TS-MRF model, and the associated segmentation tool, provide a flexible and spatially adaptive description of the data. In the interactive version, the user can drive the process based on the inspection of the current result, deciding step-by-step which direction to take, and switching from one segmentation modality to another. Experiments with the segmentation and classification of multitemporal SAR images prove the potential of the interactive approach and of the TS-MRF tool.

We address the problem of SAR despeckling by resorting to nonlocal filtering guided by an optical image. In fact, given the increasing availability of remote-sensing optical images, it makes perfect sense trying to use them to improve the performance of despeckling. Our technique exploits the optical image to reliably estimate the statistical similarity among pixels, which is used to evaluate the weights of nonlocal filtering. Optical data are not used to estimate SAR values, but only to guide the overall process. In addition, they are discarded altogether in regions where SAR and optical images present different local geometries, identified by a preliminary classification step, avoiding thus any additional distortion. Experimental results show the proposed approach to provide images of better quality than state-of-the-art conventional filters.

Despeckling techniques based on the nonlocal approach provide an excellent performance, but exhibit also a remarkable complexity, unsuited to time-critical applications. In this letter, we propose a fast nonlocal despeckling filter. Starting from the recent SAR-BM3D algorithm, we propose to use a variable-size search area driven by the activity level of each patch, and a probabilistic early termination approach that exploits speckle statistics in order to speed up block matching. Finally, the use of look-up tables helps in further reducing the processing costs. The technique proposed conjugates excellent performance and low complexity, as demonstrated on both simulated and real-world SAR images and on a dedicated SAR despeckling benchmark.

Objective performance assessment is a key enabling factor for the development of better and better image processing algorithms. In synthetic aperture radar (SAR) despeckling, however, the lack of speckle-free images precludes the use of reliable full-reference measures, leaving the comparison among competing techniques on shaky bases. In this paper, we propose a new framework for the objective (quantitative) assessment of SAR despeckling techniques, based on simulation of SAR images relevant to canonical scenes. Each image is generated using a complete SAR simulator that includes proper physical models for the sensed surface, the scattering, and the radar operational mode. Therefore, in the limits of the simulation models, the employed simulation procedure generates reliable and meaningful SAR images with controllable parameters. Through simulating multiple SAR images as different instances relevant to the same scene we can therefore obtain, a true multilook full-resolution SAR image, with an arbitrary number of looks, thus generating (by definition) the closest object to a clean reference image. Based on this concept, we build a full performance assessment framework by choosing a suitable set of canonical scenes and corresponding objective measures on the SAR images that consider speckle suppression and feature preservation. We test our framework by studying the performance of a representative set of actual despeckling algorithms; we verify that the quantitative indications given by numerical measures are always fully consistent with the rationale specific of each despeckling technique, strongly agrees with qualitative (expert) visual inspections, and provide insight into SAR despeckling approaches.

Graphics editing programs of the last generation provide ever more powerful tools, which allow for the retouching of digital images leaving little or no traces of tampering. The reliable detection of image forgeries requires, therefore, a battery of complementary tools that exploit different image properties. Techniques based on the photo-response non-uniformity (PRNU) noise are among the most valuable such tools, since they do not detect the inserted object but rather the absence of the camera PRNU, a sort of camera fingerprint, dealing successfully with forgeries that elude most other detection strategies. In this paper, we propose a new approach to detect image forgeries using sensor pattern noise. Casting the problem in terms of Bayesian estimation, we use a suitable Markov random field prior to model the strong spatial dependences of the source, and take decisions jointly on the whole image rather than individually for each pixel. Modern convex optimization techniques are then adopted to achieve a globally optimal solution and the PRNU estimation is improved by resorting to nonlocal denoising. Large-scale experiments on simulated and real forgeries show that the proposed technique largely improves upon the current state of the art, and that it can be applied with success to a wide range of practical situations.

The estimation of the fractal dimension of a natural surface using spectral analysis of amplitude SAR data is a powerful tool for geophysical applications. However, the obtained estimates are influenced by the presence of speckle. In this paper we present first results regarding the analysis of the effects on fractal dimension maps due to the application of despeckling techniques on SAR images. The use of simulated data allows to obtain also multilook images, presenting a very high number of looks, which should represent a virtually speckle-free image. In the results section different despeckling techniques are considered, relaying on both classical spatial filtering and non-local means. The presented results allow to draw meaningful conclusions on the effects of speckle and despeckling on spectral estimation and fractal dimension retrieving.

Detection of image forgeries is an important topic for forensics applications. One of the most interesting approaches to forgery detection relies on the photo-response non uniformity noise (PRNU), that can be considered as a sort of camera fingerprint and used as such to accomplish forgery detection. In fact, while genuine parts of an image exhibit the camera PRNU, this is not present in tampered areas. In this work, we present a new method to detect forgeries by using PRNU. In particular, we propose a minimum-risk Bayesian classification, aimed at minimizing the probability of error or, more in general, a weighted average of the two types of errors (false alarm, missing detection) according to their importance for the application. Then, we introduce a regularization term in the decision process to take into account prior information on the classification map. This step weights optimally the observed data and the regularity constraints to minimize the Bayesian risk. Since the regularization term is based on spatial properties of the decision map, classification cannot work on each pixel individually but must be carried out jointly on the whole image. The ensuing problem is NP-hard but we use relaxation and convex optimization techniques, based on proximal methods, to obtain a global optimum solution in limited time. Preliminary experiments with digital forgeries of different sizes and shapes prove that the improved technique provides a significant performance gain w.r.t. the original, at the cost of a limited increase in complexity.

The segmentation of very high resolution (VHR) images portraying complex urban scenarios is a rather challenging problem. In particular, great attention must be devoted to preserve fine man-made details, of major interest for most user applications. For this reason, edge-based segmentation methods are likely preferable to region-based methods. The latter, in fact, e.g. [1], [2], succeed in taking into account long range interactions and hence perform typically well in terms of “global” accuracy, but exhibit a lower “local” accuracy with respect to former, [3].

We propose a new framework for the quantitative assessment of SAR despeckling techniques, based on physical-level simulation of SAR images corresponding to canonical scenes. Thanks to the simulator, we can generate multiple SAR images of the same scene which differ only in the speckle content, and, hence, a true multilook SAR image, with an arbitrarily large number of looks, to use as “speckle-free” reference. Based on this concept, we select a small set of canonical scenes and, for each of them, a suitable set of objective measures which account for speckle suppression and image feature preservation. We gain insight into the system reliability by comparing the indications it gives for some sample despeckling filters with those obtained by expert visual inspection of the filtered images.

Information mining systems typically do not carry out image segmentation because a single algorithm could never perform well on the wide variety of sources and user applications encountered in practice. On the other hand, a large number of tools have been proposed in the literature that handle specific segmentation tasks very well. Dynamic segmentation is a possible solution, where the image is split recursively, in a hierarchical fashion, and different tools are used at each step to address specific segmentation tasks. In this work, the segmentation of a high-resolution test image is used as a running example and as a proof of concept of the potential of this approach.

In this work we propose a modified version of the BM3D algorithm recently introduced by Dabov et al. for the denoising of images corrupted by additive white Gaussian noise. The original technique performs a multipoint filtering, where the nonlocal approach is combined with the wavelet shrinkage of a 3D cube composed by similar patches collected by means of block-matching. Our improvement concerns the thresholding of wavelet coefficients, which are subject to a different shrinkage depending on their level of sparsity. The modified algorithm is more robust with respect to block matching errors, especially when noise is high, as proved by experimental results on a large set of natural images.

The Texture Fragmentation and Reconstruction (TFR) algorithm, recently proposed for the segmentation of textured images, has been applied with promising results to high-resolution remote-sensing images. The algorithm provides a sequence of nested segmentation maps which allow the analysis at various scales of observation. However, the performance which is very good at large scales, with complex semantic areas retrieved with remarkable accuracy, becomes less satisfactory at finer scales. In this paper we propose to use the TFR in a recursive fashion, segmenting the image in just two regions, initially, with each region further segmented only if relevant subregions emerge. The recursive TFR allows one to better adapt to local statistics and to extract significant textures also at finer scales. Early experimental results validate the effectiveness of the new algorithm.

The Texture Fragmentation and Reconstruction (TFR) algorithm, recently proposed for the segmentation of textured images, has been applied with promising results to high-resolution remote-sensing images. The algorithm provides a sequence of nested segmentation maps which allow the analysis at various scales of observation. Although for most test images TFR has proven able to recognize major semantic areas, some failures have also been observed due to the presence of large background regions that span the whole image and prevent the formation of distinct local textures. In this paper we introduce a new step in the TFR processing flow which detects background regions and divides them in multiple homogeneous fragments based on their geometric level properties. To this end, connected regions are first reduced to atomic components through a watershed-like transform, and then clustered again based on the features of the associated region-adjacency graph. Early experimental results prove the effectiveness of the new processing step, and its beneficial effect on the whole algorithm.

Segmentation of very high resolution remote-sensing images cannot rely only on spectral information, quite limited here for technological reasons, but must take into account also the rich textural information available. To this end, we proposed recently the Texture Fragmentation and Reconstruction (TFR) algorithm, based on a split-and-merge paradigm, which provides a sequence of nested segmentation maps, at various scales of observation. Early experiments on several high-resolution test images confirm the potential of TFR, but there is room for further improvements under various points of view. In this paper we describe the TFR algorithm and, starting from the analysis of some critical results propose two new version that address and solve some of its weak points.

In this paper, we propose a new algorithm for the segmentation of multiresolution remote-sensing images, which fits into the general split-and-merge paradigm. The splitting phase singles out clusters of connected regions that share the same spatial and spectral characteristics. These clusters are then regarded as atomic elements of more complex structures, particularly textures, that are gradually retrieved during the merging phase. The whole process is based on a recently developed hierarchical model of the image, which accurately describes its textural properties. In order to reduce the computational burden and preserve contours at the highest spatial definition, the algorithm works on the high-resolution panchromatic data first, using low-resolution full spectral information only at a later stage to refine the segmentation. It is completely unsupervised, with just a few parameters set at the beginning, and its final product is not a single segmentation map but rather a sequence of nested maps which provide a hierarchical description of the image, at various scales of observations. The first experimental results, obtained on a remote-sensing Ikonos image, are very encouraging and confirm the algorithm potential.

The contourlet transform was recently proposed to overcome the limited ability of wavelet to represent image edges and discontinuities. Besides retaining the desirable characteristics of wavelet transform, such as multiresolution and localization, it has two additional important features: directionality and anisotropy. In this work we propose a new image coding technique based on an hybrid contourlet-wavelet decomposition. The encoder builds upon the well-known SPIHT algorithm, which is suitably modified to take into account the new hierarchical structure of the transform coefficients and the correlation between them. Both numerical results and visual quality confirm the potential of this approach, especially for images with high texture content.

Multispectral and synthetic aperture radar (SAR) images are known to exhibit complementary properties: unlike optical sensors, SAR provides information about the soil roughness and moisture, and acquires useful data despite clouds and Sun-illumination conditions. However, the analysis of the resulting images turns out to be more difficult, as compared to the use of optical imagery, due to the noise-like speckle phenomenon. In order to exploit this complementarity for classification purposes, a criticality relies in the definition of accurate joint optical-SAR statistical models, due to the different physical natures of these two data typologies and to the corresponding differences in the related parametric models. In this paper, a region-based semiparametric classification technique is proposed for multisensor optical-SAR images. The method combines the tree-structured Markov random field approach to segmentation with the dependence tree approach to probability density estimation and with case-specific bivariate models for the distributions of optical and SAR data. A Bayesian decision rule is formulated at the segment level in order to incorporate spatial-contextual information and to gain robustness against noise.

We propose here a new algorithm for the unsupervised segmentation of multiresolution remote-sensing images. After a first segmentation step on the high-resolution panchromatic data, the image is converted in a set of disjoint regions, which are then clustered and merged progressively, based on multispectral, spatial and textural properties, producing a sequence of nested segmentation maps which provide a thorough and multi-scale description of the image. The algorithm is fast, since it works mainly at a region level, and preserves fine details thanks to the initial step at the high-resolution level. Experimental results on IKONOS data confirm the algorithm potential and point out to a few problems to address in future research.

In this letter, we improve the class-based transform-coding scheme proposed by Gelli and Poggi for the compression of multispectral images. The original spatial-coding tools, 1-D discrete cosine transform and scalar quantization, are replaced by shape-adaptive wavelet transform and set partitioning in hierarchical trees. Numerical experiments show that the improved technique outperforms the original one for medium- to high-quality compression and is consistently superior to all reference techniques.

We propose a new efficient region-based scheme for the compression of multispectral remote-sensing images. The region-based description of an image comprises a segmentation map, which singles out the relevant regions and provides their main features, followed by the detailed (possibly lossless) description of each region. The map conveys information on the image structure and could even be the only item of interest for the user; moreover, it enables the user to perform a selective download of the regions of interest, or can be used for high-level data mining and retrieval applications. This approach, with the multiple pieces of information required, may seem inherently inefficient. The goal of this research is to show that, by carefully selecting the appropriate segmentation and coding tools, region-based compression of multispectral images can be also effective in a rate-distortion sense, thus providing an image description that is both insightful and efficient. To this end, we define a generic coding scheme, based on Bayesian image segmentation and on transform coding, where several key design choices, however, are left open for optimization, from the type of transform, to the rate allocation procedure, and so on. Then, through an extensive experimental phase on real-world multispectral images, we gain insight on such key choices, and finally single out an efficient and robust coding scheme, with Bayesian segmentation, class-adaptive Karhunen-Loève spectral transform, and shape-adaptive wavelet spatial transform, which outperforms state-of-the-art and carefully tuned conventional techniques, such as JPEG-2000 multicomponent or SPIHT-based coders.

Remote-sensing images are often composed by a hierarchy of nested regions, with complex regions that are regarded as homogeneous at some observation scale, but can be further segmented at finer scales. Tree-structured Markov random fields (TS-MRF) allow one to model such images, and to develop efficient segmentation algorithms for them. TS-MRF are traditionally based on binary trees of classes, but the use of generic trees, with more degrees of freedom, can likely provide a better performance, as was shown with reference to synthetic images. Here we build upon the ideas proposed to devise a segmentation algorithm that works effectively, and with a limited computational burden, on real-world remote sensing images.

In this work we implement and compare several state-of-the-art transform coding schemes for the compression of multispectral images, in order to better understand which elements have a deeper impact on the overall performance, and which tools guarantee the best results. All schemes are based on Karhunen-Loeve transform and/or wavelet transform, in various combinations, and use SPIHT as the coding engine. Moreover, besides the ordinary techniques, their object-based counterparts are also examined, so as to study the viability of such approach [M. Cagnazzo et al., Oct 2004] for these images. Whenever possible, an optimal rate allocation strategy is applied. The experiments, performed on images acquired by two different sensors, highlight the superiority of KLT as spectral transform; the rough equivalence between object-based and ordinary techniques in terms of rate-distortion performance; and the importance of the optimal allocation.

Region-based encoding techniques have been long investigated for the compression of still images and video sequences and have recently gained much popularity, as testified by the object-based nature of the MPEG-4 video coding standard. This work aims at analyzing costs and advantages of implementing such an approach by shape-adaptive wavelet transform and shape-adaptive SPIHT. The analysis of several performance measures in a number of experiments confirm the potential of wavelet-based region-based approach, and provide insight about what performance gains and losses can be expected in various operative conditions.

Most remote sensing images exhibit a clear hierarchical structure which can be taken into account by defining a suitable model for the unknown segmentation map. To this end, one can resort to the tree-structured Markov random field (MRF) model, which describes a K-ary field by means of a sequence of binary MRFs, each one corresponding to a node in the tree. Here we propose to use the tree-structured MRF model for supervised segmentation. The prior knowledge on the number of classes and their statistical features allows us to generalize the model so that the binary MRFs associated with the nodes can be adapted freely, together with their local parameters, to better fit the data. In addition, it allows us to define a suitable likelihood term to be coupled with the TS-MRF prior so as to obtain a precise global model of the image. Given the complete model, a recursive supervised segmentation algorithm is easily defined. Experiments on a test SPOT image prove the superior performance of the proposed algorithm with respect to other comparable MRF-based or variational algorithms.

We present a new technique for the compression of remote-sensing hyperspectral images based on wavelet transform and zerotree coding of coefficients. In order to improve encoding efficiency, the image is first segmented in a small number of regions with homogeneous texture. Then, a shape-adaptive wavelet transform is carried out on each region and the resulting coefficients are finally encoded by a shape-adaptive version of SPIHT. Thanks to the segmentation map (sent as a side information) region boundaries are faithfully preserved and selective encoding strategies can be easily implemented. In addition, by-now homogeneous region textures can be more efficiently encoded.

Multitemporal remote sensing images are useful tools for many applications in natural resource management. Compression of this kind of data is an issue of interest, yet, only a few paper address it specifically, while general-purpose compression algorithms are not well suited to the problem, as they do not exploit the strong correlation among images of a multitemporal set of data. Here we propose a coding architecture for multitemporal images, which takes advantage of segmentation in order to compress data. Segmentation subdivides images into homogeneous regions, which can be efficiently and independently encoded. Moreover this architecture provides the user with a great flexibility in transmitting and retrieving only data of interest

We deal with the supervised segmentation of multi-temporal remote-sensing images following a statistical Bayesian approach. To take into account prior information on the class of images, like the correlation between neighboring pixels, as well as the available knowledge about the structure of the current image, we model the image as a tree-structured Markov random field. The data collected at two different dates are jointly processed as a single multi-component image, with the classes defined a priori based on ground truth information and grouped in changed and unchanged macro-classes. Experimental results in terms of classification accuracy prove the effectiveness of the proposed technique with respect to non-contextual methods, as well as to a disjoint approach. In addition, the classification tree allows for a direct interpretation of the result

SAR interferograms are affected by a strong noise component which often prevents correct phase unwrapping and always impairs the phase reconstruction accuracy. To obtain satisfactory performance, most filtering techniques exploit prior information by means of ad hoc, empirical strategies. In this paper, we recast phase filtering as a Bayesian estimation problem in which the image prior is modeled as a suitable Markov random field, and the filtered phase field is the configuration with maximum a posteriori probability. Assuming the image to be residue free and generally smooth, a two-component MRF model is adopted, where the first component penalizes residues, while the second one penalizes discontinuities. Constrained aimulated annealing is then used to find the optimal solution. The experimental analysis shows that, by gradually adjusting the MRF parameters, the algorithm filters out most of the high-frequency noise and, in the limit, eliminates all residues, allowing for a trivial phase unwrapping. Given a limited processing time, the algorithm is still able to eliminate most residues, paving the way for the successful use of any subsequent phase unwrapping technique.

We present a fast Bayesian algorithm for the segmentation of remote-sensing images. It alternates two processing steps, the binary Bayesian segmentation of regions, and the separation of non-connected same-class regions, which both present relatively low complexity. As a result, a detailed and reliable K-region segmentation map can be obtained in limited CPU-time. In addition, the map is organized in a tree-structure (not necessarily binary) which helps gaining insight about the meaning of component regions.

The Bayesian/MRF approach guarantees high-quality image segmentation, at the price of a significant computational cost. To limit complexity, one can resort to the recently proposed tree-structured MRF-based segmentation, which converts a Kclass problem in a sequence of much simpler binary tasks. The binary-tree structure imposed to the image, however, can also reduce segmentation accuracy. Here we propose an improved tree-structured segmentation algorithm, where disjoint regions, even belonging to the same class, are immediately split, giving rise to a generic tree structure, and are grouped again in classes only at the end of the algorithm. As a result, both segmentation speed and accuracy increase.

We present a new image segmentation algorithm based on a tree-structured binary MRF model. The image is recursively segmented in smaller and smaller regions until a stopping condition, local to each region, is met. Each elementary binary segmentation is obtained as the solution of a MAP estimation problem, with the region prior modeled as an MRF. Since only binary fields are used, and thanks to the tree structure, the algorithm is quite fast, and allows one to address the cluster validation problem in a seamless way. In addition, all field parameters are estimated locally, allowing for some spatial adaptivity. To improve segmentation accuracy, a split-and-merge procedure is also developed and a spatially adaptive MRF model is used. Numerical experiments on multispectral images show that the proposed algorithm is much faster than a similar reference algorithm based on "flat" MRF models, and its performance, in terms of segmentation accuracy and map smoothness, is comparable or even superior.

SAR images are severely degraded by speckle, and filtering is therefore a common practice. Filtering is especially useful before compression, to avoid spending valuable resources to represent noise; unfortunately, it also degrades important image features, like region boundaries. To overcome this problem, one can resort to a segmentation-based compression scheme, which allows one to preserve region boundaries, carry out intense denoising, and improve overall performance. In this work we assess the potential of segmentation-based compression through controlled experiments on synthetic SAR images. Numerical results seem to confirm the validity of this approach.

Presents a new technique, based on a MAP (maximum a posteriori probability) approach, for filtering interferometric phase images. To describe prior knowledge about the unknown image, a MRF/Gibbs model is adopted, and its energy function is defined so as to impose two desirable constraints on the solution: the absence of residues (which leads to unambiguous phase unwrapping) and the smoothness of the phase field (which reduces noise). MAP estimation is carried out by simulated annealing, obtaining accurate results even for low values of the coherence.

Because of its computational complexity, vector quantization (VQ) can work only on relatively small vectors, which severely limits its encoding performance. Trellis-coded VQ (TCVQ) circumvents this problem by using VQ in combination with a trellis encoding strategy, thus treating very large blocks of data at once. TCVQ is based on a size-N VQ codebook which is recursively partitioned to form two (possibly nearly optimal) codebooks of size N/2, four of size N/4, etc. Here we show that such a tree of codebooks can be easily designed, without the need of any postprocessing, by means of the Kohonen algorithm. Numerical experiments show the effectiveness of the proposed approach.

There is an ever-growing interest in the compression of SAR data because of the huge resources required for storage and transmission. This is especially true for spaceborne sensors, given the limited capacity of the downlink channel. Unfortunately, SAR data lack the useful properties on which compression algorithms rely; indeed, these are present in the focused images, but focusing is too complex for on-board implementation at this time. Poggi et al. (2000) proposed to perform on the satellite only the low-complexity range focusing, which increases the data correlation and better concentrates their energy. These properties were then exploited by adopting a variable-rate vector quantizer, with a clear performance improvement with respect to reference techniques. However, vector quantization (VQ) is too complex for actual on-board implementation, and therefore, here we replace VQ with trellis-coded VQ. To limit complexity, only small vectors are used, which reduces VQ's ability to exploit data dependencies; on the other hand, trellis coding allows one to encode large blocks of data at once, and to obtain a better partition of the input space. Experiments on real SAR data show that the overall performance is comparable to that of Poggi et al., but the complexity is much lower, making on-board implementation possible.

The authors carry out low bit-rate compression of multispectral images by means of the Said and Pearlman's SPIHT algorithm, suitably modified to take into account the interband dependencies. Two techniques are proposed: in the first, a three-dimensional (3D) transform is taken (wavelet in the spatial domain, Karhunen-Loeve in the spectral domain) and a simple 3D SPIHT is used; in the second, after taking a spatial wavelet transform, spectral vectors of pixels are vector quantized and a gain-driven SPIHT is used. Numerous experiments on two sample multispectral images show very good performance for both algorithms.

In spaceborne SAR systems, some form of data compression is required to reduce the bandwidth of the downlink channel. Compressing the raw data is very inefficient because such data exhibit little or no correlation. Data focusing would restore the original dependencies of the image, but it is too complex to be implemented onboard. In this paper we propose to carry out a partial, low-complexity focusing onboard, and then compress the resulting data by means of a suitable algorithm based on variable-rate adaptive vector quantization. Experiments show a performance improvement of 0.5-0.7 dB over compression techniques proposed in the literature.

We propose a new algorithm, based on a tree-structured Markov random field (MRP) model, to carry out the unsupervised classification of images. It presents several appealing features; due to the MRF model, it takes into account spatial dependencies, yet is computationally light because only binary MRFs are used and a progressive refinement of information takes place. Moreover, it is adaptive to the local characteristics of the image and provides useful side information about the segmentation process.

This paper presents a new technique for the compression of multispectral images, which relies on the segmentation of the image into regions of approximately homogeneous land cover. The rationale behind this approach is that, within regions of the same land cover, the pixels have stationary statistics and are characterized by mostly linear dependency, contrary to what usually happens for unsegmented images. Therefore, by applying conventional transform coding techniques to homogeneous groups of pixels, the proposed algorithm is able to effectively exploit the statistical redundancy of the image, thereby improving the rate distortion performance. The proposed coding strategy consists of three main steps. First, each pixel is classified by vector quantizing its spectral response vector, so that both a reliable classification and a minimum distortion encoding of each vector are obtained. Then, the classification map is entropy encoded and sent as side information, Finally, the residual vectors are grouped according to their classes and undergo Karhunen-Loeve transforming in the spectral domain and discrete cosine transforming in the spatial domain. Numerical experiments on a six-band thematic mapper image show that the proposed technique outperforms the conventional transform coding technique by 1 to 2 dB at all rates of interest.

This paper proposes a new vector quantization based (VQ-based) technique for very low bit rate encoding of multispectral images. We rely on the assumption that the shape of a generic spatial block does not change significantly from band to band, as is the case for high spectral-resolution imagery. In such a hypothesis, it is possible to accurately quantize a three-dimensional (3-D) block-composed of homologous two-dimensional (2-D) blocks drawn from several bands-as the Kronecker-product of a spatial-shape codevector and a spectral-gain codevector, with significant computation saving with respect to straight VQ. An even higher complexity reduction is obtained by representing each 3-D block in its minimum-square-error Kronecker-product form and by quantizing the component shape and gain vectors. For the block sizes considered, this encoding strategy is over 100 times more computationally efficient than unconstrained VQ, and over ten times more computationally efficient than direct gain-shape VQ. The proposed technique is obviously suboptimal with respect to VQ, but the huge complexity reduction allows one to use much larger blocks than usual and to better exploit both the statistical and psychovisual redundancy of the image. Numerical experiments show fully satisfactory results whenever the shape-invariance hypothesis turns out to be accurate enough, as in the case of hyperspectral images. In particular, for a given level of complexity and image quality, the compression ratio is up to five times larger than that provided by ordinary VQ, and also larger than that provided by other techniques specifically designed for multispectral image coding.

We propose two algorithms based on the embedded zerotree wavelet approach to encode multispectral images. The Said-Pearlman algorithm is adapted to the case of multispectral images by considering spectral vectors of pixels rather than isolated pixels and by carrying out either VQ or KLT on these vectors, so as to take into account interband dependencies. Numerical experiments on a sample multispectral image show a very good performance for both algorithms.

This paper proposes a new technique based on vector quantization (VQ) for very low bit-rate encoding of multispectral images. The new algorithm relies on the observation that in high spectral-resolution imagery the shape of a generic spatial block does not change significantly from band to band. Therefore, it is reasonable to represent each 3-D spatial/spectral block as the Kronecker product of a spatial-shape vector and a spectral-gain vector, and to jointly quantize only these representative vectors in place of the original block. Even though such an encoding strategy is suboptimal with respect to full-search VQ, the huge complexity reduction allows one to use much larger blocks and to better exploit the redundancy among close pixels of the image. Numerical experiments carried out on high spectral-resolution images show fully satisfactory results, with compression ratios exceeding 100:1, good image quality and very low encoding complexity.

This paper addresses the problem of multispectral-image compression and proposes a new encoding scheme, based on the classification of the multispectral image into regions of homogeneous land cover. Separate regions resulting from classification are efficiently encoded by means of conventional transform coding techniques, whereas the classification information is compacted resorting to spatial prediction and Ziv-Lempel coding. Simulation results show that the proposed algorithm assures both a high compression ratio and a good reproduction quality, with a reasonable computational complexity.

Address-predictive vector quantization (APVQ) exploits the interblock dependency by jointly encoding the addresses of the codewords associated with spatially close blocks. It profiles the same image quality as memoryless VQ for a much lesser bit rate (BR) and the same computational complexity. In the generalized-cost-measure-based APVQ, the two steps of the encoding process, namely, VQ and predictive address encoding, are carried out jointly by minimizing a generalized cost measure, which takes into account both the BR and the distortion. Computer simulations show that a significant improvement can be obtained with respect to APVQ in terms of both BR and distortion. Compared with memoryless VQ, a bit-rate reduction of almost 60% is obtained for the same image quality.

The authors use predictive pruned tree-structured vector quantization for the compression of medical images. Their goal is to obtain a high compression ratio without impairing the image quality, at least so far as diagnostic purposes are concerned. The authors use a priori knowledge of the class of images to be encoded to help them segment the images and thereby to reserve bits for diagnostically relevant areas. Moreover, the authors improve the quality of prediction and encoding in two additional ways: by increasing the memory of the predictor itself and by using ridge regression for prediction. The improved encoding scheme was tested via computer simulations on a set of mediastinal CT scans; results are compared with those obtained using a more conventional scheme proposed recently in the literature. There were remarkable improvements in both the prediction accuracy and the encoding quality, above and beyond what comes from the segmentation. Test images were encoded at 0.5 bit per pixel and less without any visible degradation for the diagnostically relevant region.