557 research outputs found
Algoritmos colaborativos para sistemas de recomendação
Tese de mestrado integrado. Engenharia Informática e Computação. Faculdade de Engenharia. Universidade do Porto. 201
Estimating parameters of a multipartite loglinear graph model via the EM algorithm
We will amalgamate the Rash model (for rectangular binary tables) and the
newly introduced - models (for random undirected graphs) in the
framework of a semiparametric probabilistic graph model. Our purpose is to give
a partition of the vertices of an observed graph so that the generated
subgraphs and bipartite graphs obey these models, where their strongly
connected parameters give multiscale evaluation of the vertices at the same
time. In this way, a heterogeneous version of the stochastic block model is
built via mixtures of loglinear models and the parameters are estimated with a
special EM iteration. In the context of social networks, the clusters can be
identified with social groups and the parameters with attitudes of people of
one group towards people of the other, which attitudes depend on the cluster
memberships. The algorithm is applied to randomly generated and real-word data
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Inspection and evaluation of artifacts in digital video sources
Streaming digital video content providers such as YouTube, Amazon, Hulu, and Netflix collaborate with production teams to obtain new and old video content. These collaborations lead to an accumulation of video sources, some of which might contain unacceptable visual artifacts. Artifacts may inadvertently enter the video master at any point in the production pipeline, due to any of a number of equipment and user failures. Unfortunately, these artifacts are difficult to detect since no pristine reference exists for comparison. As of now, few automated tools exist that can effectively capture the most common forms of these artifacts. This work studies no-reference video source inspection for generalized artifact detection and subjective quality prediction, which will ultimate inform decisions related to acquisition of new content.
Automatically identifying the locations and severities of video artifacts is a difficult problem. We have developed a general method for detecting local artifacts by learning differences in the statistics between distorted and pristine video frames. Our model, which we call the Video Impairment Mapper (VID-MAP), produces a full resolution map of artifact detection probabilities based on comparisons of excitatory and inhibatory convolutional responses. Validation on a large database shows that our method outperforms the previous state-of-the-art of even distortion-specific detectors.
A variety of powerful picture quality predictors are available that rely on neuro-statistical models of distortion perception. We extend these principles to video source inspection, by coupling spatial divisive normalization with a series of filterbanks tuned for artifact detection, implemented using a common convolutional framework. We developed the Video Impairment Detection by SParse Error CapTure (VIDSPECT) model, which leverages discriminative sparse dictionaries that are tuned to detect specific artifacts. VIDSPECT is simple, highly generalizable, and yields better accuracy than competing methods.
To evaluate the perceived quality of video sources containing artifacts, we built a new digital video database, called the LIVE Video Masters Database, which contains 384 videos affected by the types of artifacts encountered in otherwise pristine digital video sources. We find that VIDSPECT delivers top performance on this database for most artifacts tested, and competitive performance otherwise, using the same basic architecture in all cases.Electrical and Computer Engineerin
On the Sensor Pattern Noise Estimation in Image Forensics: A Systematic Empirical Evaluation
Extracting a fingerprint of a digital camera has fertile applications in image forensics, such as source camera identification and image authentication. In the last decade, Photo Response Non_Uniformity (PRNU) has been well established as a reliable unique fingerprint of digital imaging devices. The PRNU noise appears in every image as a very weak signal, and its reliable estimation is crucial for the success rate of the forensic application. In this paper, we present a novel methodical evaluation of 21 state-of-the-art PRNU estimation/enhancement techniques that have been proposed in the literature in various frameworks. The techniques are classified and systematically compared based on their role/stage in the PRNU estimation procedure, manifesting their intrinsic impacts. The performance of each technique is extensively demonstrated over a large-scale experiment to conclude this case-sensitive study. The experiments have been conducted on our created database and a public image database, the 'Dresden image databas
Inversion of perceived direction of motion caused by spatial undersampling in two children with periventricular leukomalacia
We report here two cases of two young diplegic patients with cystic periventricular leukomalacia who systematically, and with high sensitivity, perceive translational motion of a random-dot display in the opposite direction. The apparent inversion was specific for translation motion: Rotation and expansion motion were perceived correctly, with normal sensitivity. It was also specific for random-dot patterns, not occurring with gratings. For the one patient that we were able to test extensively, contrast sensitivity for static stimuli was normal, but was very low for direction discrimination at high spatial frequencies and all temporal frequencies. His optokinetic nystagmus movements were normal but he was unable to track a single translating target, indicating a perceptual origin of the tracking deficit. The severe deficit for motion perception was also evident in the seminatural situation of a driving simulation video game. The perceptual deficit for translational motion was reinforced by functional magnetic resonance imaging studies. Translational motion elicited no response in the MT complex, although it did produce a strong response in many visual areas when contrasted with blank stimuli. However, radial and rotational motion produced a normal pattern of activation in a subregion of the MT complex. These data reinforce the existent evidence for independent cortical processing for translational, and circular or radial flow motion, and further suggest that the two systems have different vulnerability and plasticity to prenatal damage. They also highlight the complexity of visual motion perception, and how the delicate balance of neural activity can lead to paradoxical effects such as consistent misperception of the direction of motion. We advance a possible explanation of a reduced spatial sampling of the motion stimuli and report a simple model that simulates well the experimental results
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