Dynamic interactions in physically realistic collaborative virtual environments

This work describes our efforts in creating a general object interaction framework for dynamic collaborative virtual environments. Furthermore, we increase the realism of the interactive world by using a rigid body simulator to calculate all actor and object movements. The main idea behind our interactive platform is to construct a virtual world using only objects that contain their own interaction information. As a result, the object interactions are application independent and only a single scheme is required to handle all interactions in the virtual world. In order to have more dynamic interactions, we also created a new and efficient way for human users to dynamically interact within virtual worlds through their avatar. In particular, we show how inverse kinematics can be used to increase the interaction possibilities and realism in collaborative virtual environments. This results in a higher feeling of presence for connected users and allows for easy, on-the-fly creation of new interactions. For the distribution of both the interactive objects and the dynamic avatar interactions, we keep the network load as low as possible. To demonstrate the effectiveness of our techniques, we incorporate them into an existing CVE framework.     

The Semijoin Algebra and the Guarded Fragment

In the 1970s Codd introduced the relational algebra, with operators selection, projection, union, difference and product, and showed that it is equivalent to first-order logic. In this paper, we show that if we replace in Codd’s relational algebra the product operator by the “semijoin” operator, then the resulting “semijoin algebra” is equivalent to the guarded fragment of first-order logic. We also define a fixed point extension of the semijoin algebra that corresponds to μGF.This author has been partially supported by the European Community Research Training Network “Games and Automata for Synthesis and Validation” (GAMES), contract HPRN-CT-2002-00283.     

Microbiological risk assessment models for partitioning and mixing during food handling

To describe the transmission dynamics of a pathogen over a food pathway in quantitative microbiological risk assessment (QMRA), several types of processes need to be modelled. Next to microbial processes like bacterial growth and inactivation, four food handling processes can be identified. Among these are partitioning and mixing of the food product. With these processes, the (sizes of) units of food product are modified and the pathogenic cells are reallocated over the units, so that the prevalence of contaminated units and the number of cells per unit may change. Usually, simple models of these processes are applied in QMRA food chain models. These models assume independence of units, random homogeneous distribution of cells (for partitioning) and equal contribution of units (for mixing), which is often not realistic in food and food handling processes. In this paper, these assumptions are abandoned. The use of multivariate distributions is proposed to include the effect of dependence between units: the Multinomial distribution for partitioning and the Dirichlet distribution for mixing. Effects of cell clustering and/or unequal sizes of units formed by partitioning or contributing to mixing are incorporated. Some algorithms are derived that are easily implemented in spreadsheet models that simulate food production and preparation. Some examples show the effect of more realistic modelling by implementation of these algorithms on the prevalence and the probability distribution of the number of pathogens per unit of food product. In general, cell clustering will result in lower prevalences, but higher levels of contamination in contaminated food units. With the methods presented, these effects can be quantified. Difficulties in estimating the model parameters and the impact of implementation of the proposed methods on risk estimates in QMRA are discussed.     

Modelling bacterial growth in quantitative microbiological risk assessment: is it possible?

Quantitative microbiological risk assessment (QMRA), predictive modelling and HACCP may be used as tools to increase food safety and can be integrated fruitfully for many purposes. However, when QMRA is applied for public health issues like the evaluation of the status of public health, existing predictive models may not be suited to model bacterial growth. In this context, precise quantification of risks is more important than in the context of food manufacturing alone. In this paper, the modular process risk model (MPRM) is briefly introduced as a QMRA modelling framework. This framework can be used to model the transmission of pathogens through any food pathway, by assigning one of six basic processes (modules) to each of the processing steps. Bacterial growth is one of these basic processes. For QMRA, models of bacterial growth need to be expressed in terms of probability, for example to predict the probability that a critical concentration is reached within a certain amount of time. In contrast, available predictive models are developed and validated to produce point estimates of population sizes and therefore do not fit with this requirement. Recent experience from a European risk assessment project is discussed to illustrate some of the problems that may arise when predictive growth models are used in QMRA. It is suggested that a new type of predictive models needs to be developed that incorporates modelling of variability and uncertainty in growth.     

Interactive design of 3D models with geometric constraints

In this paper, an interactive graphical approach for the design of parameterized part-hierarchies is presented. Primitive solids can be grouped into compound objects, and multiple instances of a compound object can be used in further designs. Geometric relations between primitives and instances are specified by geometric constraints between their local coordinate systems. The user can specify and edit a model by direct manipulation on a perspective or parallel projection with a mouse, whereas a procedural model representations is automatically generated via visual programming. The obtained twoview approach offers two concurrent interface styles to the end-user and enables the combination of an intuitive direct manipulation interface with the expressiveness of a procedural modeling language.     

Identification of Volatiles That Are Used in Discrimination Between Plants Infested with Prey or Nonprey Herbivores by a Predatory Mite

Carnivorous arthropods can use herbivore-induced plant volatiles to locate their herbivorous prey. In the field, carnivores are confronted with information from plants infested with herbivores that may differ in their suitability as prey. Discrimination by the predatory mite Phytoseiulus persimilis between volatiles from lima bean plants infested with the prey herbivore Tetranychus urticae, or plants infested with the nonprey caterpillar Spodoptera exigua, depends on spider mite density. In this article, we analyzed the chemical composition of the volatile blends from T. urticae-infested lima bean plants at different densities of spider mites, and from S. exigua-infested plants. Based on the behavioral preferences of P. persimilis and the volatile profiles, we selected compounds that potentially enable the mite to discriminate between T. urticae-induced and S. exigua-induced volatiles. Subsequently, we demonstrated in Y-tube olfactometer assays that the relatively large amounts of methyl salicylate and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene emitted by T. urticae-infested bean plants compared to S. exigua-infested plants enable the predators to discriminate. Our data show that specific compounds from complex herbivore-induced volatile blends can play an important role in the selective foraging behavior of natural enemies of herbivorous arthropods.     

Effects of long-term use of video-communication technologies on the conversational process

This paper examines the conversational process in video-mediated groups as compared to face-to-face groups. In a longitudinal between-groups design, 22 three-person groups were tested in four one-hour test sessions at two-week intervals. In each session, groups had to select and argue the correct answer out of three alternatives from a total of ten questions. The questions were selected from the archives of an annual Dutch television show called The Science Quiz. Results showed that, compared to face-to-face groups, video-mediated groups took more time for turns, required fewer turns to complete the task, interrupted each other less, and were less satisfied. At the end of the experiment, initial differences between groups on all these measures, except for satisfaction, had disappeared, suggesting an adaptation effect. The results are discussed in terms of implications for the design of video-communication technologies and possibilities for future research.Part of this research was funded by the European Commission, Directorate-General Information Society, project IST-1999-10044 VIRTUE.     

A retail and consumer phase model for exposure assessment of Bacillus cereus

An exposure assessment is conducted for psychrotrophic and mesophilic Bacillus cereus in a cooked chilled vegetable product. A model is constructed that covers the retail and consumer phase of the food pathway, using the output of a similar model on the industrial process as input. Microbial growth is the predominant process in the model. Variability in time and temperature during transport and storage is included in the model and different domestic refrigerator temperature distributions are compared. As an end point, probable levels of B. cereus colony forming units (cfu) in packages of vegetable purée are predicted at the moment the consumer takes the product from its refrigerator, that is prior to a cooking process. The psychrotrophic strain is predicted to end up above a threshold level of 10(5) cfu/g in 0.9% to 6.3% of the vegetable purée packages, depending on domestic refrigerator temperature. Accounting for spoilage this reduces to 0.3% to 2.4%. Even if the purée is stored at 4 degrees C in the domestic refrigerator and use-by-date (UBD) is respected, the threshold level may be passed. For the mesophilic strain the threshold level is rarely passed, but in contrast to the total viable count, the spore load at the end point is predicted to be higher than in the psychrotrophic strain. Our study illustrates how an exposure assessment model, which may be used in quantitative risk assessment, can integrate expertise in modelling, food processing and microbiology over the food pathway, and thus evaluate food safety, identify gaps in knowledge and compare risk management measures. As important gaps in knowledge, the lack of sporulation and germination models and data, validated non-isothermal growth models and a spoilage model useful for risk assessment are identified. Knowledge of the dose-response relationship is limited and does not allow a full risk assessment. It is shown that exposure can be lowered by lowering domestic refrigerator temperatures, and less so much by monitoring and withdrawing contaminated products at the end of industrial processing.