Generalized XML security views We investigate a generalization of the notion of XML security view introduced by Stoica and Farkas (Proceedings of the 16th International Conference on Data and Applications Security (IFIP'02). IFIP Conference Proceedings, vol. 256, pp. 133-146. Kluwer, Dordrecht, 2002) The model consists of access control policies specified over DTDs with XPath expressions for data-dependent access control. We provide the notion of security views characterizing information accessible to authorized users. This is a transformed DTD schema that can be used by users for query formulation. We develop an algorithm to materialize an authorized version of the document from the view and an algorithm to construct the view from an access control specification. We show that our view construction combined with materialization produces the same result as the direct application of the DTD access specification on the document. We also propose a number of generalizations of possible security policies and show how they affect view construction algorithm. Finally, we provide an evaluation of our system. XML [12] has become the prime standard for data representation and exchange on the Web. In light of the sensitive nature of many business data applications, this also raises the important issue of security in XML and the selective exposure of information to different classes of users based on their access privileges.. To address such security issues, there is a need for a generic, flexible security model that can effectively support multiple policies for controlling access to XML content at various levels of granularity. Perhaps even more importantly, enforcing such access-control policies should not imply any drastic degradation in either performance or functionality for the underlying XML query-execution engine. In addition, access-control enforcement should not complicate the maintenance of the consistency and integrity of the data when either the XML data or the access policies are updated.. An XML security model must support 1. a simple and powerful fine grained authorization mechanism that can control access to both content and structure (e.g., restricting access to entire subtrees or specific elements in the document tree based on their content or location); 2. efficient mechanisms for the enforcement of security policy without fully annotating the underlying document to decouple data management from policy management; 3. schema information, characterizing all and only those elements accessible to each authorized user, in the same way that a relational database offers security views to their users.. An access control should not inhibit schema availability, i.e., the availability of necessary schema information (e.g., DTD [12] or an XML Schema [21] ) specifying the structure of the accessible data. Furthermore, XML documents are typically accompanied by a schema that specifies the internal structure of the data. For the same reasons that a database schema is needed for query formulation and processing for traditional databases, XML schemas are also important for XML query formulation and optimization; and schemas are critical for XML data exchange and integration [4] .. While specifications and enforcement of access control are well understood for traditional databases [20, 35, 41, 43] , the study of security for XML is less established. Early security models that have been proposed for XML do not meet criteria 3 mentioned above and, to a lesser extent, criterion 1 and 2. In particular, cryptographically enforced access control to XML documents [10, 37] considers only protection of XML data but not a schema; different parts of the XML tree are encrypted by different keys (typically one key for a particular combination of access control rules applicable to an XML element); finally, these kinds of protection make querying difficult since a large amount of decryption is needed. Run-time policy evaluation scenarios [10, 32] , requires policy propagation from the root to the requested node. This may result in calculation of accessibility decisions for every node test of the user query. Such a time-consuming query answering can be improved only partly in the presence of DTD [14, 39] , but it happens not for all queries. Other optimization techniques rely on compressed accessibility map [30, 51, 52] , or combining access control policy with user queries [36] . However, the user still does not have the schema of accessible data. This problem was slightly resolved in Bertino and Ferrari [9] , Damiani et al. [17] , where a "loosened" variant (i.e., DTD where every forbidden element is "optional") of the schema is delivered. The problem with the loosened version is that it still reveals all element names, including sensitive ones, of the initial DTD. Moreover, the overall security policy enforcement is performed at the document level by fully annotating the entire XML document/database; this requires possibly expensive view materialization (see Sect. 10), and complicates consistency and integrity maintenance.. In addition, fixing the access control policies at the instance level without providing or computing a schema makes it difficult for the security officer to understand how the authorized view of a document for a user or a class of users actually looks like. Hence, such a solution is hardly practical for large and complex documents. Revelation of excessive schema information might lead to security breaches: an unauthorized user can deduce or infer confidential information via multiple queries (essentially if the authorization specifications are not closed under intersection) and analyze the schema even if only accessible nodes are queried.. To overcome the limitations outlined above, the notion of an XML security views was initially proposed by Stoica and Farkas [44] and later refined by Fan et al. [22] . The basic idea is to provide a schema that describes the data that can be seen by the user, as well as a (hidden) set of XPath expressions that describes how to compute the data in the view from the original data. This paper elaborates on certain issues left open in Fan et al. [22] . In particular, we studied access control and security specifications defined over general DTDs in terms of regular expressions rather than normalized DTDs of Fan et al. [22] . Furthermore, we developed a new algorithm for deriving a security view definition from more intuitive access control specification (w.r.t. a non-recursive DTD) without introducing dummy element types, and thus preventing inference of sensitive information from the XML structure revealed by dummies. In addition, we provided an extension of a view derivation algorithm in a bottom-up direction. Other our contributions include the study of different access control policies and experimental evaluation of DTD view construction for Y|N labels and view-based versus naïve materialization.. The former shows that our method is scalable and has an unessential degradation tending to become constant with the growth of the number of annotations. The latter demonstrates advantages of view-based approach over a naïve one from the view point of both performance and space consumption.. Several extensions to the security model are targeted for the future work. First, we plan to extend the definitions of security views and authorization specifications by supporting more complex XML Schema [21] instead of DTDs. Second, we are also studying extensions of our algorithm for deriving security-view definitions with respect to recursive DTDs/schemas. In addition, our next step toward enforcing inference control will be to investigate reasoning techniques in the presence of integrity constraints and ID/IDREF attributes. The observation concerning leakage of information in the case where the security specifications are not closed under intersection is left for the future work. Finally, we are studying a query evaluation in the presence of a query rewriting use case. Two main drawbacks can be easily observed in the first group of run-time policy evaluation scenarios. The first one is that for every user request, accessibility of an XML node is calculated via a propagation of security annotations which may involve the whole tree. The second drawback is that the user is supplied with neither a schema of available data nor an XML view.. The second group of proposals slightly alleviates the aforementioned problems: an XML view is calculated only once and may be queried by the user locally. However, a meaningful DTD schema is still missing. The same is related to crypto-proposals; the difference is that the user may extract the view locally as well if he has a set of corresponding keys.. The group of schema-based view proposals seems to provide the most promising solutions. Namely, the proposal by Stoica and Farkas [44] can construct views that preserve semantic meaning among element types of the original DTD which is missing in more recent proposals [22, 33] . However, this proposal still needs to be evolved in the sense that it is not clear how the constructed view can be used for XML querying. Some intuiting on a possible evolution can be seen in Fan et al. [22] (query rewriting) and Kuper et al. [33] (view materialization). Finally, the latest proposal in schema-based views was published in Mohan et al. [38] . Their querying language is based on XQuery rather than on XPath. However, we have some doubts about the practical applicability of the method due to its error-prone nature while policy designing and modification. A typical approach to specifying and enforcing access control for traditional databases is to define views on which security permissions should be applied (e.g., multi-level security view for a relational database [35, 41, 43, 48] , discretionary access control over relational databases [6] and object-oriented databases [11] ). For instance, Lunt et al. [34] showed how to use standard SQL queries to implement the SeaView multilevel secure database. It is simple to define a relational view via an SQL query, and derive the relational schema of the view. In contrast, the hierarchical structure and the dependency (e.g., ancestor and descendants) of XML data as well as the presence of disjunction and recursion in DTDs make it impossible to define a security view via a single query, and moreover, they introduce new challenges in how to generate a view that conforms to a view DTD. The challenges were observed in [4] , which showed that even for XML views of relational data, it is highly nontrivial to ensure that the views typecheck. This observation has been further confirmed in [2] which showed that type checking of XML views of relational data, even for simple DTDs, is computationally intractable: co-NEXPTIME for extremely restricted view definitions, and undecidable for realistic views.. The insight to construct XML security views was found in [41] where it was demonstrated how to compute a full database labelling from a partial one implied by security views. From the XML viewpoint, a partial assignment of security labels to XML document nodes can be also extended to a full assignment. From the latter, it is easy to compute an XML analogue of relational view by means of "sanitization" operation which hides (e.g., deletes or encrypts) nodes with negative authorizations, but reveals (e.g., moves up to a permitted ancestor in the case of deletion of a forbidden parent) their permitted children [7, 17, 23, 32] . In the following sections, we will give an overview of these and other security models of access control to XML documents.

O B O O O O O O O O O O B B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O


The ERICA Switch Algorithm for ABR Traffic Management in ATM Networks We propose an explicit rate indication scheme for congestion avoidance in ATM networks. In this scheme, the network switches monitor their load on each link, determining a load factor, the available capacity, and the number of currently active virtual channels. This information is used to advise the sources about the rates at which they should transmit. The algorithm is designed to achieve e ciency, fairness, controlled queueing delays, and fast transient response. The algorithm is also robust to measurement errors caused due to variation in ABR demand and capacity. We present performance analysis of the scheme using both analytical arguments and simulation results. The scheme is being implemented by several ATM switch manufacturers. The ATM Forum Tra c Management Speci cation provides in precise details the rules for the source and destination end system behaviors for the available bit rate ABR service for asynchronous transfer mode ATM networks 1 . The switch behavior, however, is only coarsely speci ed. This provides the exibility for various vendors to implement their own switch rate allocation algorithms. Several switch algorithms have been developed 2 -9 . This paper describes one of the earliest switch algorithms.. The Explicit Rate Indication for Congestion Avoidance ERICA algorithm was presented at the ATM Forum in February 1995. Since then, its performance has been independently studied in many papers 4, 5, 7 , and we h a v e incorporated several modi cations into the algorithm 9, 10 . This paper provides a consolidated description and a performance analysis of the scheme. The paper is organized as follows.. returning from the destination to the source are called backward RM cells BRMs. When a source receives a BRM, it computes its allowed cell rate ACR using its current ACR, CI and NI ags, and the ER eld of the RM cell 13 . In this paper, we have described the design and evaluation of the ERICA switch algorithm for ATM ABR congestion control. As a basis for the design process, we presented a simple switch model and formulated design goals. The key design goals are max-min fair steady state operation with controlled queueing delays, stability, and robustness to variation in ABR load and capacity. We then presented the ERICA algorithm, showing how the goals and simplicity determine every step in the algorithm.. The scheme entails that the switches periodically monitor their load on each link and determine a load factor, the available capacity, the queue length, and the number of currently active virtual channels. This information is used to calculate a fair and e cient allocation of the available bandwidth to all contending sources. The measurement aspects which determine the robustness of the algorithm are treated in depth. An analytical proof of convergence to steady state max-min fairness is given in appendix A. In addition, we present simulation results illustrating how the scheme meets the desired goals such as goodsteady state performance high utilization, controlled queuing delay, max-min fairness, quick convergence from network transients, and robustness to load capacity v ariation.. The ERICA scheme has considerably in uenced the design of contemporary switch schemes. Notably, the ATM Forum tra c management speci cation 4.

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A mean-value performance analysis of a new multiprocessor architecture This paper presents a preliminary performance analysis of a new large-scale multiprocessor: the Wisconsin Multicube. A key characteristic of the machine is that it is based on shared buses and a snooping cache coherence protocol. The organization of the shared buses and shared memory is unique and non-hierarchical. The two-dimensional version of the architecture is envisioned as scaling to 1024 processors.We develop an approximate mean-value analysis of bus interference for the proposed cache coherence protocol. The model includes FCFS scheduling at the bus queues with deterministic bus access times, and asynchronous memory write-backs and invalidation requests. We use our model to investigate the feasibility of the multiprocessor, and to study some initial system design issues. Our results indicate that a 1024-processor system can operate at 75 -95% of its peak processing power, if the mean time between cache misses is larger than 1000 bus cycles (i.e. 50 microseconds for 20 MHz buses; 25 microseconds for 40 MHz buses). This miss rate is not unreasonable for the cache sizes specified in the design, which are comparable to main memory sizes in existing multiprocessors. We also present results which address the issues of optimal cache block size, optimal size of the two-dimensional Multicube, the effect of broadcast invalidations on system performance, and the viability of several hardware techniques for reducing the latency for remote memory requests. Multiprocessors are emerging as an important class of computer architectures. These machines include multiple CPUs, each having some amount of local memory, which are connected through a high-speed interconnect to shared memory and I/O devices. The design of a multiprocessor is complex, and protot~rping the system is expensive. Thus, this class of systems represents an important opportunity for performance models to contribute to the actual design process.. The "Wisconsin Multicube" is a new large-scale multiprocessor architecture [GoWo87] . The two-dimensional version of this architecture is envisioned as scaling to 1024 processors. A key characteristic of the machine is that each processor has a very large cache, comparable in size to the size of main memory on existing machines. Another key characteristic of the machine is that it is based on shared buses and a snooping cache coherence protocol. That is, copies of shared data blocks that reside in caches local to the system processors are kept consistent by hardware that monitors the bus traffic. This greatly simplifies the job of the programmers and/or system compilers, since they need not be concerned with explicitly mapping data to the various local and shared memory modules for efficient execution.. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specfic permission.. The organization of the shared buses and shared memory in the Wisconsin Multicube architecture is unique and non-hierarchical. Issues that must be considered in the preliminary design of the system include the required bandwidth of the system buses, the optimal blocksize for transferring data between shared memory and the caches, and the viability of various hardware techniques for reducing remote memory access latency. This paper presents a performance model we have used to study the feasibility and several design issues for the Wisconsin Multicube. We develop an approximate mean-value analysis to study the effects of bus interference for a machine with up to 1024 (i.e. 32x32) processors. The model assumes first-come first-served service on the buses, and deterministic bus access times. Asynchronous invalidation and memory write-back requests are also modeled. Parameters of the model include system size, mean time between bus requests, estimates of the workload-dependent frequency and type of shared data accesses, bus speed, and block transfer size.. Initial results indicate that a 1024-processor Multicube can operate at 75 -90% of its peak processing power if the mean time between bus requests is larger than 1000 bus cycles (50 microseconds for 20 MHz buses). In addition, the model results indicate that, strictly from a performance perspective, cache block size should be on the order of 16 -32, and no larger than 64 times the width of the data bus. Furthermore, our results show that nearly linear speedups are possible for up to 1024 processors, for miss rates below one per 20130-4000 bus cycles. Additional results that address the effects of broadcast invalidations, and viability of hardware techniques for reducing remote memory request latency, are also presented.. The accuracy of the model relative to the real system is unknown. However, the level of detail in the model is compatible with the level of detail known about the real system at this time. Furthermore, the modeling approach has been shown to be remarkably accurate for evaluating cache coherence protocols for single-bus multis [LaVZ88] . The level of detail in the model is comparable to the level of detail in queueing network models used in capacity planning for uniprocessor systems. We believe that this level of detail is also appropriate for initial design and capacity planning for multiprocessor systems.. The organization of the remainder of this paper is as follows. We describe the architecture and cache coherence protocol of the Wisconsin Multicube in section 2. In section 3, we develop the approximate meanvalue model and discuss the accuracy of the model. Section 4 contains some results of the mean-value analysis that are useful for assessing the feasibility of Wisconsin Multicube and for selecting some initial design parameters. Section 5 contains the conclusions of this study.

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A statistical model for synthesis of detailed facial geometry (a) Original low-resolution face (b) Synthesized detail (c) Aged Detailed surface geometry contributes greatly to the visual realism of 3D face models. However, acquiring high-resolution face geometry is often tedious and expensive. Consequently, most face models used in games, virtual reality, or computer vision look unrealistically smooth. In this paper, we introduce a new statistical technique for the analysis and synthesis of small three-dimensional facial features, such as wrinkles and pores. We acquire high-resolution face geometry for people across a wide range of ages, genders, and races. For each scan, we separate the skin surface details from a smooth base mesh using displaced subdivision surfaces. Then, we analyze the resulting displacement maps using the texture analysis/synthesis framework of Heeger and Bergen, adapted to capture statistics that vary spatially across a face. Finally, we use the extracted statistics to synthesize plausible detail on face meshes of arbitrary subjects. We demonstrate the effectiveness of this method in several applications, including analysis of facial texture in subjects with different ages and genders, interpolation between highresolution face scans, adding detail to low-resolution face scans, and adjusting the apparent age of faces. In all cases, we are able to re-produce fine geometric details consistent with those observed in high resolution scans. Creating realistic models of human faces is an important problem in computer graphics. Face models are widely used in computer games, commercials, movies, and for avatars in virtual reality applications. The goal is to capture all aspects of a person's face in a digital model -i.e., "Digital Face Cloning" [Pighin and Lewis 2005] .. Ideally, a face model should be indistinguishable from a real face, easy to acquire, and intuitive to edit. However, digital face cloning remains a difficult task for several reasons. First, humans are very accustomed to looking at real faces and can easily spot artifacts in computer generated models. Second, capturing the high resolution geometry of a face is difficult and expensive. Finally, editing face models is still a time consuming and largely manual task, especially if changes to fine-scale details are required.. In this work we focus on a small but important source of realism in faces: geometry of small facial features, such as wrinkles and pores. As mentioned in Igarashi et al. [2005] , wrinkles are folds of skin formed through the process of skin deformation, whereas pores are widely dilated orifices of glands that appear on the surface of skin. They are visible to the naked eye, and their appearance is very familiar to us. Small-scale variations in skin color and reflectance, typically caused by freckles and moles, are not the topic of this work.. Acquiring high-resolution face geometry with small features is a difficult, expensive, and time-consuming task. Commercial active or passive photometric stereo systems (e.g., EyeTronics, 3QTech) have fast acquisition times (in seconds) and have become affordable. However, they only capture large wrinkles and none of the important small geometric details that make skin look realistic. Laser scanning systems (e.g., CyberWare) may be able to capture the details, but they are expensive and require the subject to sit still for tens of seconds, which is impractical for many applications. Moreover, the resulting 3D geometry has to be filtered and smoothed due to noise and motion artifacts. The most accurate method is to make a plaster mold of a face and to scan this mold using a precise laser range system (e.g., XYZRGB). However, not everybody can afford the considerable time and expense this process requires. In addition, the molding compound may lead to sagging of facial features [Pighin and Lewis 2005] .. In this paper we present a statistical face model that makes it possible to extract, transfer, and synthesize small facial features. Our approach is based on the analysis of high-resolution face scans. Using a commercial 3D face scanner and a custom-built face scanning dome, we acquire high-resolution 3D face geometry. We use displaced subdivision surfaces [Lee et al. 2000 ] to separate facial detail (the displacement map) from the smooth underlying mesh (the subdivision surface). We then apply a novel tile-based extension of parametric texture analysis/synthesis [Heeger and Bergen 1995] to compile a spatially-varying statistical model for the displacement map. The model is used to synthesize fine geometric details (e.g., new wrinkles and pores) on the same or another base mesh. Our method of generating facial detail can be used in conjunction with methods such as the PCA-based analysis of Blanz and Vetter [1999] , which are well suited for modeling coarse facial deformations.. This statistical model of facial detail, combined with our database of statistics for 149 subjects, opens ground for new applications. Chief among these is adding detail to a low resolution face ( Figure 1b ). This application allows anybody to create plausible, detailed face meshes from low resolution scans by synthesizing detailed displacement maps from statistics that we make publicly available. Second, users can interpolate between multiple faces without blurring high resolution details. Finally, they can age or de-age a face using statistics of older or younger subjects in the database (Figure 1c) .. The main contributions of this work are:. • a statistical model of fine geometric facial features based on an analysis of high-resolution face scans;. • an extension of parametric texture analysis and synthesis methods to spatially-varying geometric detail;. • a database of detailed face statistics for a sample population that is made available to the research community;. • new applications, including introducing plausible detail to lowresolution face models and adjusting face scans according to age and gender. We have presented a method for analyzing and synthesizing facial geometry by separating face meshes into smooth base meshes and displacement images, extracting statistics from the images, and synthesizing new faces with fine details based on extracted statistics. This work takes a small step towards more realistic facial models and suggests several directions for further research. Within the existing framework, several areas of improvement may lead to more plausible facial synthesis. The statistical model we use can be augmented to include correlations within and between steerable pyramid filter outputs [Portilla and Simoncelli 2000] . Moreover, the noise introduced at the first stage of synthesis could be made more realistic by incorporating information about the existing overall geometry, the results of an explicit large-scale wrinkle model, or even image-based information from photographs. These refinements would improve the ability of our synthesis to capture long, continuous features (e.g., long wrinkles across the forehead), at the expense of additional processing and complexity in our statistical representation.. It would be interesting to expand this model into the larger context of face synthesis, rendering, and animation. This involves extending our model to capture not only statistics of geometric detail, but also the correlations between shape, appearance, and movement. For example, we might capture the fact that pores appear in both geometry and reflectance or that the shape of wrinkles depends on expression, and use this information to produce compelling syntheses that are consistent with all available data.. Moving beyond the applications presented here, we would like to explore the use of the statistical model for identifying or classifying faces. The results presented here suggest that the statistics we use offer cues about the age and gender of an individual, and represent a heretofore underutilized source of information that may complement overall shape and appearance for the purposes of identification. More generally, the availability of high-resolution scans of faces from a broad population may enable new classes of applications in comparative analysis and realistic rendering. There has been a wealth of research in capturing and modeling faces in computer graphics and computer vision. In this overview, we focus on the relevant work in statistical modeling and synthesis of face geometry.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Local and Global Recency Weighting Approach to Bug Prediction Abstract To remain competitive in the fast growing and changing world of software development, project managers must optimize the use of their limited resources to maximize the delivery of quality products on time and within budget [1] . Software bug repositories have been recognized as reliable data assets that can provide useful information to assist with the software development and project management process. Therefore, in this work, we have modeled the occurrence of bug patterns through the characterization of information stored within bug repositories. We use the proposed approach is to predict the number of bugs and types of bugs reported for the Eclipse project for a specified period, as required by the proposed challenge in [2] .. The remaining part of the paper is organized as follows. In section 2, we discuss the information source for our input data. The model used for the bug predictions is described in Section 3. The prediction results obtained using our technique, are presented in Section 4. Section 5 concludes the paper. In this work, we have presented an elegant approach to the prediction of bugs for a software project based upon the information contained in open-source bug repositories. We have illustrated the use of a local and global recency weighting technique for the design of a bug prediction algorithm. While we do encounter some extreme cases in the results, the overall results obtained are, in general, promising. In the future, we plan to address predicting for a more distant future by considering issues such as software versioning impacts. We also plan to study related issues such as change and stability analysis.

O O B O O O O O O O O O O O O O O O O O O B O B O O O O B O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O B O O O B O O O B B O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B O O B B O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O B O O O O O O O O O O O O O O O


Affine invariant extended cyclic codes over Galois rings Abstract-Recently, Blackford and Ray-Chaudhuri used transform domain techniques to permutation groups of cyclic codes over Galois rings. They used the same technique to find a set of necessary and sufficient conditions for extended cyclic codes of length 2 over any subring of GR (4 ) to be affine invariant. Here, we use the same technique to find a set of necessary and sufficient conditions for extended cyclic codes of length over any subring of GR ( ) to be affine invariant, for = 2 with arbitrary and for = 2 with arbitrary . These are used to find two new classes of affine invariant Bose-Chaudhuri-Hocquenghem (BCH) and generalized Reed-Muller (GRM) codes over for arbitrary and a class of affine invariant BCH codes over for arbitrary prime . The automorphism/permutation groups of codes over finite fields are known to be useful for decoding (see [1] - [7] , for example). Permutations of F p (where p is a prime) of the form x 7 ! ax + b, where a; b 2 Fp ; a 6 = 0, are called affine permutations. These permutations form a subgroup of the symmetric group of order p m which is denoted as AGL(1; p m ). A code of length p m with components indexed by elements of Fp is said to be affine invariant if it is invariant under the affine permutations.. Clearly, affine invariant codes, after the 0th component is deleted, are cyclic codes. Kasami, Lin, and Peterson [8] found a necessary and sufficient condition on the defining set of any cyclic code over a finite field, under which the extended cyclic code is affine invariant. As corollaries, they showed that the famous extended Bose-Chaudhuri-Hocquenghem (BCH) and generalized Reed-Muller (GRM) codes are affine invariant. Often it is comparatively easier to determine the full permutation groups of affine invariant codes [9] - [13] .. Though coding theorists have for a long time had theoretical interest in codes over integer residue rings [14] , [15] , codes over integer residue rings, and more generally over Galois rings, have received serious attention [16] - [22] after it was shown [23] that some important families of nonlinear binary codes can be obtained via Gray map from linear codes over 4 .. The conditions for extended cyclic codes over integer residue rings to be affine invariant were derived by Abdukhalikov [24] . Blackford and Ray-Chaudhuri [25] used a transform domain approach to characterize affine invariant extended cyclic codes of length 2 m over subrings of GR (4; m) and using this characterization, they found new classes of affine invariant BCH and GRM codes over Galois rings. In this correspondence, we extend their approach to extended cyclic codes of length 2 m over any subring of GR (2 e ; m) and also to extended cyclic codes of length p m over GR (p 2 ; m) for arbitrary prime p. In Section II, we discuss some basic facts about Galois rings and cyclic codes over Galois rings as a preparation to the subsequent section, where the results of this paper are presented. We conclude with few remarks in Section IV. A set of necessary and sufficient conditions were derived for extended cyclic codes of length p m over any subring of GR(p e ; m) to be affine invariant for p = 2 with arbitrary e and for e = 2 with arbitrary p. Classes of affine invariant BCH codes and GRM codes over

O O O B I O B I O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O B I O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B I O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


EWA volume splatting In this paper we present a novel framework for direct volume rendering using a splatting approach based on elliptical Gaussian kernels. To avoid aliasing artifacts, we introduce the concept of a resampling filter combining a reconstruction with a low-pass kernel. Because of the similarity to Heckbert's EWA (elliptical weighted average) filter for texture mapping we call our technique EWA volume splatting. It provides high image quality without aliasing artifacts or excessive blurring even with non-spherical kernels. Hence it is suitable for regular, rectilinear, and irregular volume data sets. Moreover, our framework introduces a novel approach to compute the footprint function. It facilitates efficient perspective projection of arbitrary elliptical kernels at very little additional cost. Finally, we show that EWA volume reconstruction kernels can be reduced to surface reconstruction kernels. This makes our splat primitive universal in reconstructing surface and volume data. Volume rendering is an important technique in visualizing acquired and simulated data sets in scientific and engineering applications. The ideal volume rendering algorithm reconstructs a continuous function in 3D, transforms this 3D function into screen space, and then evaluates opacity integrals along line-of-sights. In 1989, Westover [18, 19] introduced splatting for interactive volume rendering, which approximates this procedure. Splatting algorithms interpret volume data as a set of particles that are absorbing and emitting light. Line integrals are precomputed across each particle separately, resulting in footprint functions. Each footprint spreads its contribution in the image plane. These contributions are composited back to front into the final image.. We introduce a new footprint function for volume splatting algorithms integrating an elliptical Gaussian reconstruction kernel and a low-pass filter. Our derivation proceeds along similar lines as Heckbert's elliptical weighted average (EWA) texture filter [4] , therefore we call our algorithm EWA volume splatting.. EWA volume rendering is attractive because it prevents aliasing artifacts in the output image while avoiding excessive blurring. Moreover, it works with arbitrary elliptical Gaussian reconstruction kernels and efficiently supports perspective projection. Our method is based on a novel framework to compute the footprint function, which relies on the transformation of the volume data to so-called ray space. This transformation is equivalent to perspective projection. By using its local affine approximation at each voxel, we derive an analytic expression for the EWA footprint in screen space. The rasterization of the footprint is performed using forward differ- We present a new splat primitive for volume rendering, called the EWA volume resampling filter. Our primitive provides high quality antialiasing for splatting algorithms, combining an elliptical Gaussian reconstruction kernel with a Gaussian low-pass filter. We use a novel approach of computing the footprint function. Exploiting the mathematical features of 2D and 3D Gaussians, our framework efficiently handles arbitrary elliptical reconstruction kernels and perspective projection. Therefore, our primitive is suitable to render regular, rectilinear, curvilinear, and irregular volume data sets. Finally, we derive a formulation of the EWA surface reconstruction kernel, which is equivalent to Heckbert's EWA texture filter. Hence we call our primitive universal, facilitating the reconstruction of surface and volume data.. We have not yet investigated whether other kernels besides elliptical Gaussians may be used with this framework. In principle, a resampling filter could be derived from any function that allows the analytic evaluation of the operations described in Section 4.2 and that is a good approximation of an ideal low-pass filter.. To achieve interactive frame rates, we are currently investigating the use of graphics hardware to rasterize EWA splats as texture mapped polygons. We also plan to use sheet-buffers that are parallel to the image plane to eliminate popping artifacts. To render non-rectilinear datasets we are investigating fast back-to-front sorting algorithms. Furthermore, we want to experiment with our splat primitive in a post-shaded volume rendering pipeline. The derivative of the EWA resampling filter could be used as a bandlimited gradient kernel, hence avoiding aliasing caused by shading for noisy volume data. Finally, we want to exploit the ability of our framework to render surface splats. In conjunction with voxel culling algorithms we believe it is useful for real-time iso-surface rendering. The EWA resampling filter has a number of useful properties, as illustrated in Figure 8 . When the mapping from camera to ray space minifies the volume, size and shape of the resampling filter are dominated by the low-pass filter, as in the left column of Figure 8 . In the middle column, the volume is magnified and the resampling filter is dominated by the reconstruction kernel. Since the resampling filter unifies a reconstruction kernel and a low-pass filter, it provides a smooth transition between magnification and minification. Moreover, the reconstruction kernel is scaled anisotropically in situations where the volume is stretched in one direction and shrinked in the other, as shown in the right column. In the bottom row, we show the filter shapes resulting from uniformly scaling the reconstruction kernel to avoid aliasing, as proposed by Swan et al. [17] . Essentially, the reconstruction kernel is enlarged such that its minor radius is at least as long as the minor radius of the lowpass filter. For spherical reconstruction kernels, or circular footprint functions, this is basically equivalent to the EWA resampling filter. However, for elliptical footprint functions, uniform scaling leads to overly blurred images in the direction of the major axis of the ellipse. We compare our method to Swan's method in Figure 8 (see colorplate). The images on the left were rendered with EWA volume splats, those on the right with Swan's uniformly scaled kernels. We used a square zebra texture with x and y dimensions of 1024 × 512 in the first row, and 1024 × 256 in the second row. This leads to elliptical reconstruction kernels with a ratio between the length of the major and minor radii of 2 to 1 and 4 to 1, respectively. Clearly, the EWA filter produces a crisper image and at the same time does not exhibit aliasing artifacts. As the ratio between the major and minor radii of the reconstruction kernels increases, the difference to Swan's method becomes more pronounced. For strongly anisotropic kernels, Swan's uniform scaling produces excessively blurred images, as shown on the right in Figure 8 . Each frame took approximately 6 seconds to render on an 866 MHz PIII processor.. In Figure 9 (see colorplate), we compare EWA splatting using volume kernels on the left to surface reconstruction kernels on the right. The texture size is 512 × 512 in x and y direction. Typically, the perspective projection of a spherical kernel is almost a circle. Therefore, rendering with volume kernels does not exhibit anisotropic texture filtering and produces textures that are slightly too blurry, similar to isotropic texture filters such as trilinear mipmapping. On the other hand, splatting surface kernels is equivalent to EWA texture filtering. Circular surface kernels are mapped to ellipses, which results in high image quality because of anisotropic filtering.. In Figure 10 (see colorplate), we show a series of volume renderings of the UNC CT scan of a human head (256 × 256 × 225), the UNC engine (256 × 256 × 110), and the foot of the visible woman dataset (152 × 261 × 220). The texture in the last example is rendered using EWA surface splatting, too. The images illustrate that our algorithm correctly renders semitransparent objects as well. The skull of the UNC head, the bone of the foot, and the iso-surface of the engine were rendered with flattened surface splats oriented perpendicular to the volume gradient. All other voxels were rendered with EWA volume splats. Each frame took approximately 11 seconds to render on an 866 MHz PIII processor. The original work on splatting was presented by Westover [18] . Basic splatting algorithms suffer from inaccurate visibility determination when compositing the splats from back to front. This leads to visible artifacts such as color bleeding. Later, Westover [19] solved the problem using an axis-aligned sheet buffer. However, this technique is plagued by disturbing popping artifacts in animations. Recently, Mueller and Crawfis [14] proposed to align the sheet buffers parallel to the image plane instead of parallel to an axis of the volume data. Additionally, they splat several slices of each reconstruction kernel separately. This technique is similar to slice-based volume rendering [2, 1] and does not suffer from popping artifacts. Mueller and Yagel [15] combine splatting with ray casting techniques to accelerate rendering with perspective projection. Laur and Hanrahan [7] describe a hierarchical splatting algorithm enabling progressive refinement during rendering. Furthermore, Lippert [9] introduced a splatting algorithm that directly uses a wavelet representation of the volume data.. Westover's original framework does not deal with sampling rate changes due to perspective projections. Aliasing artifacts may occur in areas of the volume where the sampling rate of diverging rays falls below the volume grid sampling rate. Swan et al. [17] use a distance-dependent stretch of the footprints to make them act as low-pass filters. This antialiasing method is closely related to EWA volume splatting, and we will discuss it further in Section 7.. Additional care has to be taken if the 3D kernels are not radially symmetric, as is the case for rectilinear, curvilinear, or irregular grids. In addition, for an arbitrary position in 3D, the contributions from all kernels must sum up to one. Otherwise, artifacts such as splotches occur in the image. For rectilinear grids, Westover [19] proposes using elliptical footprints that are warped back to a circular footprint. To render curvilinear grids, Mao et al. [10] use stochastic Poisson resampling to generate a set of new points whose kernels are spheres or ellipsoids. They compute the elliptical footprints very similar to Westover [19] . As pointed out in Section 4, our technique can be used with irregular grids to efficiently and accurately project and rasterize the elliptical splat kernels.. We develop EWA volume splatting along similar lines to the seminal work of Heckbert [4] , who introduced EWA filtering to avoid aliasing of surface textures. We recently extended his framework to represent and render texture functions on irregularly pointsampled surfaces [21] . Section 5 will show the connection between EWA volume and surface splatting. We implemented a volume rendering algorithm based on the EWA splatting equation. Our implementation is embedded in the VTK (visualization toolkit) framework [16] . We did not optimize our code for rendering speed. We use a sheet buffer to first accumulate splats from planes in the volume that are most parallel to the projection plane [19] . In a second step, the final image is computed by compositing the sheets back to front. Shading is performed using the gradient estimation functionality provided by VTK and the Phong illumination model. We summarize the main steps which are required to compute the EWA splat for each voxel: First, we compute the camera coordinates u k of the current voxel k by applying the viewing transformation to the voxel center. Using u k , we then evaluate the Jacobian J as given in Equation (25). In line 4, we transform the Gaussian reconstruction kernel from object to ray space. This transformation is implemented by Equation (20) , and it results in the 3 × 3 variance matrix V k of the Gaussian in ray space. Remember that W is the rotational part of the viewing transformation, hence it is typically orthonormal. Moreover, for spherical kernels, V k is the identity matrix. In this case, evaluation of Equation (20) can be simplified significantly. Next, we project the voxel center from camera space to the screen by performing a perspective division on u k . This yields the 2D screen coordinateŝ x k . Now we are ready to setup the resampling filter ρ k according to Equation (22) . Its variance matrix is derived from V k by omitting the third row and column, and adding a 2 × 2 identity matrix for the low-pass filter. Moreover, we compute the determinants 1/|J −1 | and 1/|W| −1 that are used as normalization factors. Finally, we rasterize the resampling filter in line 7. As can be seen from the definition of the elliptical Gaussian (9), we also need the inverse of the variance matrix, which is called the conic matrix. Let us denote the 2 × 2 conic matrix of the resampling filter by Q. Furthermore, we define the radial index function. Note that the contours of the radial index, i.e., r = const. are concentric ellipses. For circular kernels, r is the squared distance to the circle center. The exponential function in (9) can now be written as e. r . We store this function in a 1D lookup table. To evaluate the radial index efficiently, we use finite differencing. Since r is biquadratic inx, we need only two additions to update r for each pixel. We rasterize r in a rectangular, axis aligned bounding box centered aroundx k as illustrated in Figure 7 . Typically, we use a threshold c = 4 and evaluate the Gaussian only if r(x) < c. Heckbert provides pseudo-code of the rasterization algorithm in [4] .

O O B O O O O O O O O O O B I O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O B I O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O B I O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Analysis of a simple evolutionary algorithm for the multiobjective shortest path problem We present a natural fitness function f for the multiobjective shortest path problem, which is a fundamental multiobjective combinatorial optimization problem known to be NPhard. Thereafter, we conduct a rigorous runtime analysis of a simple evolutionary algorithm (EA) optimizing f . Interestingly, this simple general algorithm is a fully polynomialtime randomized approximation scheme (FPRAS) for the problem under consideration, which exemplifies how EAs are able to find good approximate solutions for hard problems. In the last decade, a lot of effort has been put into the runtime analysis of evolutionary algorithms (EAs). These results are aiming at a well-founded understanding of the working principles of EAs. One of the first papers of this line of research has investigated the runtime of a simple EA, the (1+1) EA, on simple pseudo-Boolean functions [6] . Later, there have been similar investigations of basic EAs for multiobjective optimization [12] . The first runtime analyses have mainly dealt with artificial example functions. In the last few years, the analysis of EAs for classical algorithmic problems has attracted more and more attention since EAs are often applied to solve combinatorial optimization problems.. There is a number of papers showing that EAs are able to solve simple combinatorial optimization problems efficiently where "simple" means that there are problem-specific algorithms solving the problems in polynomial time. One of the first papers that deal with this subject is [21] whose authors analyze the runtime of EAs for sorting and the single-source shortest path problem. The topic of [8] is the maximum matching problem. Furthermore, there is a series of papers analyzing EAs for the Eulerian cycle problem [15, 3, 5, 4] and the minimum spanning tree problem [18, 17] .. Since EAs are often applied to find good solutions for NPhard problems, it would be even more interesting to increase the knowledge of how much runtime EAs require to solve such problems. There are quite a few results attesting simple EAs the ability to find constant-factor approximations for NP-hard combinatorial optimization problems efficiently. Such results are presented in [14] for the biobjective minimum spanning tree problem, in [24] for the partition problem, and in [16] for the minimum multicut problem. While these results point out the capability of EAs to find good approximate solutions, it would be more appropriate if the user of the EA could provide the desired approximation ratio. We refer to the comprehensive survey [19] for further references on this topic. This paper is dedicated to the well-known multiobjective shortest path problem, which consists in finding a set of shortest paths capturing all possible trade-offs in a weighted directed graph with k ≥ 1 weight functions. Figure 1 shows a possible input instance for the problem at hand and Table 1 shows all paths from node 1 to node 7 and which of them are Pareto-optimal. Before we consider EAs, we summarize some general results concerning the problem. The problem is known to be NP-hard if we are confronted with two or more weights per edge [7] . In addition, there are instances with an exponential number of Pareto-optimal paths w. r. t. the input size. The term "input size" refers to the number of bits of a natural binary encoding of the graph and the edge weights. Hence, approximative solutions seem to be the only resort. The first fully polynomial-time approximation scheme (FPTAS) for the biobjective case is presented in [9] . If we restrict ourselves to directed acyclic graphs, an FPTAS for the general multiobjective case is presented in [23] . In [20] it is shown that there is for each multiobjective optimization problem a succinct approximation of the Pareto front of polynomial size. Furthermore, the au- . Figure 1: Example of a weighted directed graph with 2 weights per edge. Table 1 : Overview of all paths from node 1 to node 7 with the corresponding weights. The Pareto-optimal or efficient paths are shown in bold.. thors show how to construct for several multiobjective optimization problems (including the shortest path problem) an FPTAS. An FPTAS with an improved runtime for the single-source multiobjective shortest path problem is presented in [22] . The last-mentioned paper [22] has actually motivated the runtime analysis of a simple EA for this problem since such algorithms can "simulate" the FPTAS, which relies on dynamic programming in combination with scaling and rounding.. All available papers, which investigate the runtime of EAs for the shortest path problem from a theoretical point of view, have concentrated on the single-objective case. The above-mentioned paper [21] has analyzed the runtime of a simple EA for the single-source variant. The authors of [1] have refined this analysis by classifying the input instances according to where is the smallest integer such that any vertex can be reached from the source via a shortest path with at most edges. The all-pairs variant has been presented in [2] as a natural example where the use of an appropriate recombination operator can improve the runtime of an EA.. This paper provides the first runtime analysis of a simple EA for the multiobjective shortest path problem. The analysis shows that the runtime of the investigated EA is competitive with the best known problem-specific algorithm [22] .. We conclude this section with a short overview of the paper. In Section 2 we give a brief introduction into multiobjective optimization and explain how to measure the runtime of an EA. Section 3 is dedicated to the multiobjective shortest path problem. The fitness function and the basic EA, which we will use in this paper, are presented in Section 4 and Section 5, respectively. In Section 6 we analyze the runtime of the EA on the fitness function. We state the used proof technique in Section 7 for possible future applications and conclude the paper with a discussion in Section 8. We have shown that a natural EA operating on a natural model of a practical relevant NP-hard problem is an FPRAS. Comparing the upper bound on the optimization time of the EA (see Theorem 2) with the upper bound O n · m · n · log (n · w max ). on the runtime of the FPTAS provided in [22] , which is the most time-efficient FPTAS available, shows that both bounds coincide up to a factor of Θ n 2 /m . For dense graphs the difference reduces to Θ(1). It appears to be plausible that this difference is close to the true difference of the optimization time and the runtime. While the EA relies on a randomized variation operator to explore the search space, the FPTAS explores the search space in a systematic manner. Hence, although the EA follows a randomized approach, it achieves the optimization goal with similar success. However, a standard implementation of the proposed EA needs an expected number of O n 2 · n · log (n · w max ). steps for the simulation of a single generation, which deteriorates the actual runtime. It would be interesting to set further limits to the optimization time of the EA by showing a nontrivial lower bound. In the end, the upper and lower bound should match. Furthermore it would be interesting to investigate if other general randomized search heuristics beat the runtime of the proposed EA. Another possible topic for future research is the analysis of the average-case behavior of the EA w. r. t. reasonable distributions of the input instances.

O O O O O O O O O B I I O O O O O O O O O O B I I O O O O O B I O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O B I I O O O O O O O O O O O O B I I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Unified framework for fast exact and approximate search in dissimilarity spaces In multimedia systems we usually need to retrieve database (DB) objects based on their similarity to a query object, while the similarity assessment is provided by a measure which defines a (dis)similarity score for every pair of DB objects. In most existing applications, the similarity measure is required to be a metric, where the triangle inequality is utilized to speed up the search for relevant objects by use of metric access methods (MAMs), for example, the M-tree. A recent research has shown, however, that nonmetric measures are more appropriate for similarity modeling due to their robustness and ease to model a made-to-measure similarity. Unfortunately, due to the lack of triangle inequality, the nonmetric measures cannot be directly utilized by MAMs. From another point of view, some sophisticated similarity measures could be available in a black-box nonanalytic form (e.g., as an algorithm or even a hardware device), where no information about their topological properties is provided, so we have to consider them as nonmetric measures as well. From yet another point of view, the concept of similarity measuring itself is inherently imprecise and we often prefer fast but approximate retrieval over an exact but slower one.To date, the mentioned aspects of similarity retrieval have been solved separately, that is, exact versus approximate search or metric versus nonmetric search. In this article we introduce a similarity retrieval framework which incorporates both of the aspects into a single unified model. Based on the framework, we show that for any dissimilarity measure (either a metric or nonmetric) we are able to change the "amount" of triangle inequality, and so obtain an approximate or full metric which can be used for MAM-based retrieval. Due to the varying "amount" of triangle inequality, the measure is modified in a way suitable for either an exact but slower or an approximate but faster retrieval. Additionally, we introduce the TriGen algorithm aimed at constructing the desired modification of any black-box distance automatically, using just a small fraction of the database. In recent years the volume of available multimedia data has been growing rapidly, so the multimedia retrieval systems and multimedia databases (DBs) are becoming more important than ever. As we see the progress in the fields of acquisition, storage, and dissemination of various multimedia formats, the application of effective and efficient multimedia management systems becomes indispensable in order to handle all these formats. The application domains for multimedia retrieval include image/audio/video databases, and CAD databases, but also molecular biology and medicine databases, geographical information systems, biometric databases, and many others. In particular, more than 95% of Web space is used to store multimedia content; other multimedia data is stored in corporate and scientific databases, personal archives, and digital libraries.. Due to the quick growth of multimedia data volumes, the text-based multimedia systems are becoming useless, since the requirements on manual annotation exceed human possibilities and resources. The metadata-based search systems are similar; we need an additional explicit information to effectively describe multimedia objects (e.g., structured semantic description, like class hierarchies or ontologies), which is not available in most cases. 1 The only practical way how to process and retrieve the vast volumes of raw multimedia data is the content-based similarity search, that is, we consider the real content of each particular DB object. Because the multimedia objects have no universal syntactic and semantic structure (unlike traditional strong-type rows in relational database tables or XML with a schema), the most general and feasible abstraction used in multimedia retrieval is the query-by-example concept, where the database objects are ranked according to similarity to a query object (the example). Only those database objects are retrieved that have been ranked as sufficiently similar to the query object. The similarity measure returns a real-valued similarity score for any two multimedia objects on the input. In this article we have proposed a unified framework for exact and approximate search in general dissimilarity spaces by the use of metric access methods (MAMs), which can be utilized especially in the area of multimedia retrieval. We have shown the triangle inequality property is not restrictive for similarity search and can be enforced for every semimetric (modifying it to a metric) in order to achieve exact search. Or, conversely, the triangle inequality can be intentionally violated for the sake of better structuring of the indexed dataset, so we achieve faster, but imprecise, retrieval. Furthermore, we have introduced the TriGen algorithm for the automatic turning of any black-box dissimilarity into an approximation of a metric just by the use of distance distribution in a fraction of the database. Such a "TriGen-approximated metric" can be safely used to search the database by any MAM, while the similarity orderings with respect to a query object (the retrieval effectiveness) are correctly preserved.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Phase reset affects auditory-visual simultaneity judgment Abstract We continuously receive the external information from multiple sensors simultaneously. The brain must judge a source event of these sensory informations and integrate them. It is thought that judging the simultaneity of such multisensory stimuli is an important cue when we discriminate whether the stimuli are derived from one event or not. Although previous studies have investigated the correspondence between an auditory-visual (AV) simultaneity perceptions and the neural responses, there are still few studies of this. Electrophysiological studies have reported that ongoing oscillations in human cortex affect perception. Especially, the phase resetting of ongoing oscillations has been examined as it plays an important role in multisensory integration. The aim of this study was to investigate the relationship of phase resetting for the judgment of AV simultaneity judgement tasks. The subjects were successively presented with auditory and visual stimuli with intervals that were controlled as SOA 50% and they were asked to report whether they perceived them simultaneously or not. We investigated the effects of the phase of ongoing oscillations on simultaneity judgments with AV stimuli with SOAs in which the detection rate of asynchrony was 50 %. It was found that phase resetting at the beta frequency band in the brain area that related to the modality of the following stimulus occurred after preceding stimulus onset only when the subjects perceived AV stimuli as simultaneous. This result suggested that beta phase resetting occurred in areas that are related to the subsequent stimulus, supporting perception multisensory stimuli as simultaneous. When a falling cup smashes on the floor and simultaneously emits a breaking sound, humans generally receive multisensory inputs from the single event. The brain aims to explain which event generates each type of sensory information and integrate them in an appropriate manner in order to construct unified recognition. It is thought that judging the simultaneity of such multisensory stimuli is an important cue when we discriminate whether the stimuli are derived from one event or not. In addition, it is a common hypothesis that the perception of simultaneity depends on the temporal differences of the onsets of the stimuli, which is called stimulus onset asynchrony (SOA) .. In psychological studies, it is known that the detection rates of asynchrony are plotted in a psychometric curve that is related to SOA in auditory-visual (AV) simultaneity judgment tasks (AV SJ task) (Stone et al. 2001; Vroomen and Keetels 2010) . The AV SJ task is a psychological experiment in which auditory and visual stimuli with various SOAs are presented to subjects. In this task, subjects are asked whether they perceive the stimuli as simultaneously presented or not. It is known that SOA 50% exists in which the judgment rate of asynchrony is 50 % even though the stimuli are presented with a constant physical time difference. This indicates that subjective simultaneity judgment is influenced by external factors and the internal states of the subjects. Bushara et al. (2001) have revealed correlations between SOA lengths and post stimulus activations in the right insula by positron emission tomography. In addition, Franciotti et al. (2011) have indicated that the latencies of magnetoencephalogram responses in the auditory cortex after auditory stimuli correlate with the detection rates of asynchrony. Although these studies have investigated the correspondence between AV simultaneity perceptions and the neural responses, there are still few studies of this.. In electrophysiological studies, it has been reported that ongoing oscillations in human cortex affect perception (Babiloni et al. 2006; Hanslmayr et al. 2007; Romei et al. 2010) . In addition to oscillatory amplitude, its phase is involved in visual processing (VanRullen and Koch 2003; Busch et al. 2009; Mathewson et al. 2009; Dugue et al. 2011) . Especially, the phase resetting of ongoing oscillations has been examined as it plays an important role in multisensory integration. This is a phenomenon in which the oscillations in a certain sensory cortex are reorganized into a particular phase by a sensory input. The phase resetting is induced by not only a preferred stimulus but also by another modality stimulus. It has been reported that a sensory input to the somatosensory cortex resets the phase of oscillations in the auditory cortex and amplifies neuronal responses to the auditory stimulus ). Recent studies in primates have described that the phase reset is bidirectional and is both AV and visual-auditory (VA) (Kayser et al. 2008; Lakatos et al. 2009 ). These findings implicate that the phase resetting that is induced in the sensory area, which does not concern the modality of the input stimulus, plays a profound role in multisensory interaction.. The aim of this study was to investigate the relationship of phase resetting for the judgment of an AV SJ task. In this study, the subjects were successively presented with auditory and visual stimuli with intervals that were controlled as SOA 50% and they were asked to report whether they perceived them simultaneously or not. Electroencephalography (EEG) signals were recorded from the subjects, and the phase resetting of EEG oscillations was analyzed by Intertrial Coherence (ITC). In the present study, we investigated the effects of the phase of ongoing oscillations on simultaneity judgments with AV stimuli with SOAs in which the detection rate of asynchrony was 50 %. Our results showed that phase resetting at the beta frequency band in the brain area that related to the modality of the following stimulus occurred after preceding stimulus onset only when the subjects perceived AV stimuli as simultaneous. Therefore, these results suggested that beta phase resetting occurred in areas that are related to the subsequent stimulus, supporting perception of multisensory stimuli as simultaneous.

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Numerical approximation for singular second order differential equations Abstract: We consider numerical approximation of solutions of singular second order differential equations. In particular, we study the backward (or implicit) Euler method. We prove results concerning consistency, global error and stability. We show that the global error is linear with respect to the step size. Numerical results are also given, which demonstrate the linear convergence and compare the numerical results with known approximations. in which p might be singular at t  0. In this paper, we continue our study of this problem by investigating numerical approximation of solutions. In section 2, we consider application of the backward Euler method (also known as the implicit Euler method). That method seems to be ideal for (IVP1), since evaluation of p at t  0 for that method is not needed. Based roughly on the approach in [2], we prove consistency, stability and also a global error estimate. In section 3, we supply numerical results for these theorems. We conclude the article by further discussing the results. For further background, the reader can find that the backward Euler method has been applied in a number of recent papers, including, for 1

B I O O O O O O O O O B I O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O B I I O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O