Browsing by Subject "Filtering"

Now showing 1 - 12 of 12

Open Access
An adaptive speckle suppression filter for medical ultrasound imaging
(Institute of Electrical and Electronics Engineers, 1995-06) Karaman, M.; Kutay, M. A.; Bozdagi, G.
An adaptive smoothing technique for speckle suppression in medical B-scan ultrasonic imaging is presented. The technique is based on filtering with appropriately shaped and sized local kernels. For each image pixel, a filtering kernel, which fits to the local homogeneous region containing the processed pixel, is obtained through a local statistics based region growing technique. The performance of the proposed filter has been tested on the phantom and tissue images. The results show that the filter effectively reduces the speckle while preserving the resolvable details. The simulation results are presented in a comparative way with two existing speckle suppression methods.
Open Access
Chirp filtering in the fractional Fourier domain
(Optical Society of America, 1994-11-10) Dorsch, R. G.; Lohmann, A. W.; Bitran, Y.; Mendlovic, D.; Özaktaş, Haldun M.
In the Wigner domain of a one-dimensional function, a certain chirp term represents a rotated line delta function. On the other hand, a fractional Fourier transform (FRT) can be associated with a rotation of the Wigner-distribution function by an angle connected with the FRT order. Thus with the FRT tool a chirp and a delta function can be transformed one into the other. Taking the chirp as additive noise, the FRT is used for filtering the line delta function in the appropriate fractional Fourier domain. Experimental filtering results for a Gaussian input function, which is modulated by an additive chirp noise, are shown. Excellent agreement between experiments and computer simulations is achieved.
Open Access
A comparative performance analysis for the commonly used time series filters in economics : Hodrick-Prescott versus Baxter-King
(2001) Yüksel, Ebru
This thesis compares the performance of the Hodrick-Prescott filter commonly employed in economic analysis to separate the trend of a given non-stationary time series from its cyclical components, to that of the Band-Pass filter developed by Baxter and King. The performances of detrending techniques under consideration are evaluated by constructing special time series that mimic the pattern of actually observed series of interest using synthesized cyclical and trend components. As an illustration of the use of this approach, the behavior of the ISE-100 index of Istanbul Stock Exchange and the Jasdaq index of Japanese Stock Market are analyzed.
Open Access
Extensions to common laplace and fourier transforms
(Institute of Electrical and Electronics Engineers, 1997-11) Onural, L.; Erden, M. F.; Özaktaş, Haldun M.
The extended versions of common Laplace and Fourier transforms are given. This is achieved by defining a new function fe(p), p 2 C related to the function to be transformed f(t), t 2 R. Then fe(p) is transformed by an integral whose path is defined on an inclined line on the complex plane. The slope of the path is the parameter of the extended definitions which reduce to common transforms with zero slope. Inverse transforms of the extended versions are also defined. These proposed definitions, when applied to filtering in complex ordered fractional Fourier stages, significantly reduce the required computation.
Open Access
H∞ filter design for vehicle tracking under delayed and noisy measurements
(IEEE, 2007-06) Ezercan, Sami; Özbay, Hitay
In many intelligent vehicles applications tracking plays an important role. This paper considers tracking of a vehicle under delayed and noisy measurements. For this purpose we design an H∞ optimal filter for linear systems with time delays in the state and output variables. By using the duality between filtering and control, the problem at hand is transformed to a robust controller design for systems with time delays. The skew Toeplitz method developed earlier for the robust control of infinite dimensional systems is used to solve the H∞ filtering problem. The results are illustrated with simulations and effects of the time delay on the tracking performance are demonstrated. ©2007 IEEE.
Open Access
On stability of interval matrices
(Institute of Electrical and Electronics Engineers, 1994-02) Sezer, M. E.; Siljak, D. D.
New sufficient, and sometimes necessary and sufficient conditions, are obtained for Schur- and Hurwitz-stability of interval matrices by relying on the concept of connective stability and M-matrices. The necessity part is broadened to include interval matrices with mixed signs of the off-diagonal elements, provided the sign patterns follow that of the Morishima matrix. The obtained results are extended to cover convex combinations of interval matrices.
Open Access
Optimal filtering in fractional Fourier domains
(Institute of Electrical and Electronics Engineers, 1997-05) Kutay, M. A.; Özaktaş, Haldun M.; Arıkan, Orhan
For time-invariant degradation models and stationary signals and noise, the classical Fourier domain Wiener filter, which can be implemented in O(N log N) time, gives the minimum mean-square-error estimate of the original undistorted signal. For time-varying degradations and nonstationary processes, however, the optimal linear estimate requires O(N/sup 2/) time for implementation. We consider filtering in fractional Fourier domains, which enables significant reduction of the error compared with ordinary Fourier domain filtering for certain types of degradation and noise (especially of chirped nature), while requiring only O(N log N) implementation time. Thus, improved performance is achieved at no additional cost. Expressions for the optimal filter functions in fractional domains are derived, and several illustrative examples are given in which significant reduction of the error (by a factor of 50) is obtained.
Open Access
Past, present, and future on news streams: discovering story chains, selecting public front-pages, and filtering microblogs for predicting public reactions to news
(2017-09) Toraman, Çağrı
News streams have several research opportunities for the past, present, and future of events. The past hides relations among events and actors; the present re ects needs of news readers; and the future waits to be predicted. The thesis has three studies regarding these time periods: We discover news chains using zigzagged search in the past, select front-page of current news for the public, and lter microblogs for predicting future public reactions to events. In the rst part, given an input document, we develop a framework for discovering story chains in a text collection. A story chain is a set of related news articles that reveal how different events are connected. The framework has three complementary parts that i) scan the collection, ii) measure the similarity between chain-member candidates and the chain, and iii) measure similarity among news articles. For scan- ning, we apply a novel text-mining method that uses a zigzagged search that reinves- tigates past documents based on the updated chain. We also utilize social networks of news actors to reveal connections among news articles. We conduct two user studies in terms of four effectiveness measures: relevance, coverage, coherence, and ability to disclose relations. The rst user study compares several versions of the framework, by varying parameters, to set a guideline for use. The second compares the framework with 3 baselines. The results show that our method provides sta- tistically signi cant improvement in effectiveness in 61% of pairwise comparisons, with medium or large effect size; in the remainder, none of the baselines signi cantly outperforms our method. In the second part, we select news articles for public front pages using raw text, without any meta-attributes such as click counts. Front-page news selection is the task of nding important news articles in news aggregators. A novel algorithm is introduced by jointly considering the importance and diversity of selected news articles and the length of front pages. We estimate the importance of news, based on topic modelling, to provide the required diversity. Then, we select important documents from important topics using a priority-based method that helps in tting news content into the length of the front page. A user study is conducted to measure effectiveness and diversity. Annotation results show that up to 7 of 10 news articles are important, and up to 9 of them are from different topics. Challenges in selecting public front-page news are addressed with an emphasis on future research. In the third part, we lter microblog texts, speci cally tweets, to news events for predicting future public reactions. Microblog environments like Twitter are increas- ingly becoming more important to leverage people's opinion on news events. We create a new collection, called BilPredict-2017 that includes events including terror- ist attacks in Turkey from 2015 to 2017, and also Turkish tweets that are published during these events. We lter tweets by using important keywords, analyze them in terms of several features. Results show that there is a high correlation between time and frequency of tweets. Sentiment and spatial features also re ect the nature of events, thus all of these features can be utilized in predicting the future.
Open Access
Right-triangular subdivision for texture mapping ray-traced objects
(Springer, 1998) Akdemir, U.; Özgüç, B.; Güdükbay, Uğur; Selçuk, A.
The introduction of global illumination and texture mapping enabled the generation of high-quality, realistic looking images of computer graphics models. We describe a fast and efficient 2D texture mapping algorithm that uses triangle-to-triangle mapping, taking advantage of mapping an arbitrary triangle to a right triangle. This requires fewer floating point operations for finding the 2D texture coordinates and little preprocessing and storage. Texture mapping is combined with ray tracing for better visual effects. A filtering technique alternative to area sampling is developed to avoid aliasing artifacts. This technique uses linear eye rays, and only one eye ray per pixel is fired. A uniform supersampling filtering technique eliminates aliasing artifacts at the object edges.
Open Access
Strong light-matter interaction in lithography-free perfect absorbers for photoconversion, photodetection, light emission, sensing, and filtering applications
(2022-01) Ghobadi, Amir
The efficient harvesting of electromagnetic (EM) waves by subwavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. These metamaterial based perfect light absorbers are of particular interest in many applications, including thermal photovoltaics, photovoltaics, emission, sensing, filtering, and photodetection applications. Although advances in nanofabrication have provided the opportunity to observe strong light-matter interaction in various optical nanostructures, the repeatability and upscaling of these nano units have remained a challenge for their use in large-scale applications. Thus, in recent years, the concept of lithography-free metamaterial absorbers (LFMAs) has attracted much attention in different parts of the EM spectrum, owing to their ease of fabrication and high functionality. In this thesis, the unprecedented potential of these LFMAs will be explored. This thesis explores the material and architecture requirements for the realization of a LFMA from ultraviolet (UV) to far-infrared (FIR) wavelength regimes. For this aim, we theoretically investigate the required conditions to realize an ideal perfect absorber. Then, based on the operation wavelength and application, the proper material and design architecture is defined. Later, to experimentally realize these ideal LFMAs, lithography-free large-scale compatible routes are developed to generate nanostructures in centimeter scales. Finally, the application of these LFMAs has been demonstrated in various fields including filtering, sensing, emission, photodetection, and photoelectrochemical water splitting. This thesis study demonstrates that, by the use of proper material and design configuration, it is possible to realize these LFMAs in every portion of the EM spectrum with a vast variety of potential applications. This, in turn, opens up the opportunity of the practical application of these perfect absorbers in large-scale dimensions. In the last section of the thesis, we discuss the progress, challenges, and outlook of this field to outline its future direction.
Open Access
Texture mapping with ray tracing
(1993-10) Akdemir, Uğur
By using texture generation and global illumination techniques, it is possible to produce realistic computer images. Currently, ray tracing is one of the most popular global illumination techniques due to its simplicity, elegance, and easy implementation. In this thesis, texture mapping techniques are used with ray tracing to generate high quality visual effects. The implementation of the mapping process is presented and an approach for combining prefiltering techniques with ray tracing is introduced. General sweep surfaces produced by Topologybook are used for modeling.
Open Access
A two-stage stochastic programming approach for reliability constrained power system expansion planning
(Elsevier, 2018) Peker, Meltem; Kocaman, Ayşe Selin; Kara, Bahar Yetiş
Probabilistic realizations of outages and their effects on the operational costs are highly overlooked aspects in power system expansion planning. Since the effect of randomness in contingencies can be more prominent especially when transmission switching is considered, in this paper we introduce contingency-dependent transmission switching concept to ensure N-1 criterion. To include randomness of outages and the outputs (i.e. flow on the lines/generation amounts) during the outages, we represent each contingency by a single scenario. Status of transmission lines, generation amounts and power flow decisions are defined as recourse actions of our two-stage stochastic model, therefore, expected operational cost during the contingencies are taken into account in a more accurate manner. A solution methodology with a filtering technique is also proposed to overcome the computational burden. The model and the solution methodology are tested on the IEEE Reliability Test System and IEEE 118-bus power system and the results show that the solution method finds the solutions for these power systems in significantly shorter solution times. The solution method is also tested on a new data set for the 380-kV Turkish transmission network. Suggestions for possible extensions of the problem and the modifications of the solution approach to handle these extensions are also discussed.